ALPRESERV

Sustainable Sediment Management in Alpine Reservoirs considering ecological and economical aspects

Volume 2 / 2006

Sediment Sources and Transport Processes Wilhelm Bechteler

www.alpreserv.eu

Neubiberg 2006

Die Deutsche Bibliothek – CIP-Einheitsaufnahme

ALPRESERV Sustainable Sediment Management in Alpine Reservoirs considering ecological and economical aspects Neubiberg, 2006

Publisher: Institut für Wasserwesen Universität der Bundeswehr München 85577 Neubiberg Germany Tel.: +49-(0)89-6004-3498, - 3493 Fax: +49-(0)89-6004-3858 http://www.unibw.de/ifw/Institut/

Editor: Dr.-Ing. Sven Hartmann Dipl.-Ing. Dr. Helmut Knoblauch Dr.-Ing. Giovanni De Cesare Dipl.-Math. Christiane Steinich

ISSN 1862-9636 Alpreserv (Print) ISSN 1862-9644 Alpreserv (Internet)

© 2006 All rights reserved

Print: Universität der Bundeswehr München (Germany)

Preface

Preface

Sediment transport in running waters with its multitude of practical consequences is among the phenomena that still defy reliable prediction despite substantial progress having been made in fundamental research and the development of numerical methods. Further intensive study will be needed to acquire the capability to provide safe information that will stand the test of practice in the fields of morphological stream development, reservoir sedimentation, sediment deposition in floodplains, breach of dams etc. There are many reasons for this unsatisfactory situation. They begin with the measurement results from nature often being both incorrect and incomplete and, naturally, showing wide dispersion. Such data are difficult to determine, the more so as their magnitude may vary along with streamflow. Furthermore, equations used to calculate morphological processes are capable of describing reality in nothing but a more or less simplified manner. A large number of interacting parameters need to be determined, each body of water is a morphological and hydrological individual, and experience gathered at certain streams or rivers is of limited use in dealing with other water bodies. Simulation of extreme hydrological events often lacks measured data, as heavily unsteady processes are involved, and the hydrological and morphological time scales differ distinctly. Knowledge about morphodynamic models is still unsatisfactory in spite of recent progress and the amount of time and money that must be spent for their installation, calibration and application is substantial. Thus, it is not surprising that simple empirical formulas are still a widespread means of dealing with hydrodynamic phenomena. This publication is an attempt to explain the essential hydrodynamic and sedimentological fundamentals without dwelling too much on the details. But a great number of references to recent literature are given, intended to enable the reader to obtain more detailed information on many special effects of sediment transport. The topical interest of the above problem is reflected in several papers having recently been published by national professional associations in Germany and Austria, such as DWVK, DWA, ÖWAV etc., and presented at technical meetings, where experts have tried to pass on their practical experience. The great number of case studies discussed in this context have illustrated both the range of the problem and the specific dimensions of each particular example. Luckily, the application for establishing a project group for dealing with sediment management in Alpine reservoirs within the framework of the INTERREG III programme has been successful and given rise to pilot studies on the subject as well as the preparation of the following publications; WP 5 "Sediment Sources and Sediment Transport" (Universität der Bundeswehr, Munich, Germany) WP 6 "Reservoir Sedimentation" (École Polytechnique Fédérale Lausanne, Switzerland) WP 7 "Sediment Management – Technical and legal aspects" (Graz College of Technology), Austria There is some content overlap in the above publications, but this is necessary to ensure that each of them can be read and understood in isolation. Thanks are due to project coordinator Sven Hartmann, Dr.-Ing., for his creativity and skill in negotiating this project, which would never have become reality without his untiring efforts and determination. Thanks should also be expressed to the other project partners who have Sediment Sources and Transport Processes always been helpful and ready at any time to contribute their expert knowledge to the success of this work. I should finally like to thank Mrs. S. Weichselgartner, Mrs. C. Langer and Mrs. C. Steinich for undertaking to look after the textual graphic design outside regular work hours, thus enabling the publication of this work.

Neubiberg, April 2006

Wilhelm Bechteler, R.-Ing. Univ.-Prof. i. R. Institut für Wasserwesen Universität der Bundeswehr München

Contents I

Contents

1 Introduction 1 2 Sediment Sources 3 2.1 Forces acting on the earth’s crust 3 2.2 Types of rock - weathering 3 2.3 Denudation Processes 6 2.3.1 Types of Denudation 6 2.3.2 Erosion 7 2.3.3 Mass Movements 9 2.4 Denudation (Denudation Rate) – Erosion 10 2.4.1 Definition 10 2.4.2 Denudation Rate, Erosion Rate 10 2.4.3 Sediment yield – Sediment Delivery Ratio 12 2.4.4 Sedimentation Rate VA 14 2.5 Measuring Sedimentation Rate 19 2.6 Reducing Sediment Delivery 20 3 Sediment transport principles 21 3.1 Introduction 21 3.1.1 Types of sediment transport 21 3.1.2 Terms, parameters and dimensionless quantities 22 3.1.3 Range of application – limitations 23 3.2 Transport media 24 3.2.1 Water 24 3.2.1.1 Properties of pure water 24 3.2.1.2 Effects of ingredients 24 3.2.1.2.1 Types of water-solids movements 24 3.2.1.2.2 Low concentration 24 3.2.1.2.3 High sediment concentrations 25 3.2.2 Sediment 25 3.2.2.1 Sediment properties 25 3.2.2.1.1 Single grain 25 3.2.2.1.2 Sediment sample 27 3.2.2.1.2.1 General classification 27 3.2.2.1.2.2 Grain size distribution, characteristic diameter 28 3.2.2.1.2.3 Sampling 30 3.2.2.1.2.4 Sample splitting 30 3.2.2.1.2.5 Sieve analysis (particle size > 0.06 mm) 31 3.2.2.1.2.6 Sedimentation method / pipette method (grain diameter < 0.06 mm) 32 3.2.2.1.2.7 Sediment density 32 3.2.2.1.2.8 Angle of internal friction 33 3.2.2.1.2.9 Sediment concentration 33 3.2.2.2 Settling velocity 34 3.2.2.2.1 General 34 II Sediment Sources and Transport Processes

3.2.2.2.2 Single sphere 34 3.2.2.2.3 Non-spherical particles 36 3.2.2.2.4 Other influences 37 3.2.2.2.5 Particle swarms – particle concentration 37 3.2.2.2.6 Grain fractions 38 3.2.2.2.7 Flocculation 38 3.2.2.2.8 Measuring settling velocity 39 3.3 Basic hydromechanics 40 3.3.1 Initial remark 40 3.3.2 Flow resistance – roughness 41 3.3.2.1 Fundamentals 41 3.3.2.2 Roughness of gravel beds 41 3.3.2.3 Roughness of steep channels – mountain rivers 42 3.3.2.4 Roughness of bed forms 44 3.3.2.5 Roughness of vegetation 45 3.3.3 Cross sections with uniform roughness 45 3.3.4 Cross sections with non-uniform roughness 48 3.3.5 Cross sections with bank and flood plain vegetation 50 3.3.5.1 Flow conditions 50 3.3.5.2 Partitioning of cross sections 51 3.3.5.3 Cross sections with bed form beds 51 3.3.5.4 Summary 54 3.4 Measuring channel roughness 54 3.4.1 The problem 54 3.4.2 Direct measurement from stream bed parameters 54 3.4.3 Indirect measurement from discharge and river characteristics 55 3.4.4 Literature and experimental values 55 4 Sediment Transport 57 4.1 Incipient motion 57 4.1.1 General 57 4.1.2 Relatively uniform bed material 57 4.1.2.1 Probability of incipient motion 57 4.1.2.2 Definition of incipient motion 58 4.1.2.3 Critical velocity 58 4.1.2.4 Effective critical shear stress 59 4.1.2.4.1 Shear stress for non-uniform flow 59 4.1.2.4.2 Effective shear stress for banks 60 4.1.2.4.3 Critical shear stress – incipient motion 60 4.1.2.4.4 Critical shear stress for rough flow 61 4.1.2.4.5 Critical shear stress for steep channels 63 4.1.2.4.6 Special aspects 63 4.1.2.4.6.1 Biological stabilisation 63 4.1.2.4.6.2 Bed forms 64 4.1.2.4.6.3 Waves 64 4.1.2.4.6.4 Cohesive soils 65 4.1.2.4.6.5 Crushed material 66 4.1.2.4.6.6 Upward seepage 66 Contents III

4.1.2.4.6.7 Estimated values of critical shear stress 67 4.1.3 Non-uniform bed material 68 4.1.4 Armouring – pavement 68 4.1.4.1 Definition 68 4.1.4.2 Criteria for armour development 69 4.1.4.3 Initial grain size distribution – armour layer 69 4.1.4.4 Stability conditions 70 4.1.4.5 Calculation of armour stability 73 4.1.4.6 Example 75 4.1.5 Critical discharge for mountain rivers 76 4.2 Bed forms 76 4.2.1 General 76 4.2.2 Bed forms and banks 76 4.2.2.1 Bed forms 76 4.2.2.2 Banks (bars) 78 4.2.2.3 Criteria for the occurrence and dimensions of bed forms 78 4.2.2.4 Sediment transport in bed forms 80 4.2.2.5 Interaction between bed forms and bed shear stress 81 4.3 Bed load transport 81 4.3.1 General 81 4.3.2 Bed load transport equations 84 4.3.2.1 General 84 4.3.2.2 Einstein (1942, 1950) 84 4.3.2.3 Meyer-Peter/ Müller (MPM, 1949) 85 4.3.2.4 Smart-Jäggi (1983) 87 4.3.2.5 Hunziker (1998) 88 4.3.3 Additional aspects for determining bed load transport 88 4.3.3.1 Unsteady flow – flood 88 4.3.3.2 Armouring 90 4.3.3.3 Sediment sorting 91 4.3.3.4 Fractional transport 91 4.3.3.5 Abrasion, downstream fining 91 4.3.3.6 Other effects 92 4.3.4 Duration curve for bed load – Yearly bed load transport 93 4.3.5 Summary 94 4.4 Suspended sediment transport 95 4.4.1 Introduction 95 4.4.1.1 Measured values 95 4.4.1.2 Particle sizes for suspended sediment 96 4.4.1.3 The significance of suspended sediment transport 96 4.4.2 The mechanism of suspended sediment transport 97 4.4.3 Bed load versus suspended load 98 4.4.4 Diffusion theory 100 4.4.4.1 Definition 100 4.4.4.2 Diffusion equation for steady suspended sediment transport 101 4.4.4.3 Vertical distribution of the diffusion coefficient 101 IV Sediment Sources and Transport Processes

4.4.4.4 Distribution of suspended-sediment concentration 102 4.4.4.5 Rouse number 104 4.4.4.5.1 Settling velocity 104 4.4.4.5.2 Turbulent Schmidt number 104 4.4.4.5.3 Von Kármán constant 104 4.4.4.5.4 Reduced Rouse number after Einstein & Chien 105 4.4.4.5.5 The influence of water temperature 105

4.4.5 Calculating reference concentration Ca 105 4.4.6 Limiting remarks 107 4.4.7 Calculation of the suspended sediment load 108 4.4.8 Erosion – transport – sedimentation for suspended sediment transport 109 4.4.8.1 Erosion 109 4.4.8.2 Transport 110 4.4.8.3 Sedimentation 112 4.4.8.4 Non-uniform transport 113 4.4.9 Discharge / suspended sediment transport relation, sediment rating curve 113 4.4.9.1 Hydrological (stochastic) approach 113 4.4.9.1.1 General aspects 113 4.4.9.1.2 Facts from German rivers 117 4.4.9.1.3 General equations 119 4.4.9.1.4 Example: Relationship between discharge and suspended sediment concentration for the Kempten gauging station on the River Iller 120 4.4.9.2 Hydromechanic approach (deterministic) 123 4.5 Bed material load 123 4.6 Bed evolution equation 123 5 River morphology – Sediment transport and river bed 125 5.1 Introduction 125 5.2 Morphological principles 125 5.2.1 General 125 5.2.2 Plan view 126 5.2.2.1 Criteria of tortuosity and sinuosity 126 5.2.2.2 Straight river 128 5.2.2.3 Braided river 128 5.2.2.4 Meandering river 130 5.2.2.5 Distinguishing criteria 131 5.2.2.6 Sediment transport in river bends 132 5.2.3 Longitudinal profile 133 5.2.3.1 Undisturbed river 133 5.2.3.2 Disturbances in the longitudinal profile 135 5.2.3.3 Base level of erosion 136 5.2.3.4 Backward erosion 136 5.2.3.5 Bed failure - chute formation 136 5.2.3.6 Ripple-pool system – alternate bars 139 5.2.4 Cross-section 141 5.2.4.1 Introduction 141 5.2.4.2 Dimensions of scours, pools and bars – structural variety 141 Contents V

5.2.4.3 Flood plains 142 5.2.4.4 Fluvial terraces 142 5.2.4.5 Alluvial fans 143 5.2.4.6 River mouth – estuary – delta 143 5.3 Fluvial system 144 5.4 Development of river beds 146 5.4.1 Sediment-continuity-equation (Exner) 146 5.4.2 Equilibrium of a river bed (steady state) 146 5.4.3 Influence of bed width 147 5.4.4 Bed forming or dominant discharge 148 5.4.5 Stable channels – regime theory 150 5.5 Possibilities of calculating theoretical bed load transport capacity 150 5.5.1 Discrete method 150 5.5.2 Continuous method 152 5.6 Artifical sediment feeding 152 5.6.1 Fundamentals 152 5.6.1.1 Compensation of bed load deficit through sediment feeding 153 5.6.1.2 Enrichment of coarsematerial fraction 153 5.6.2 Example: River Rhine downstream of Iffezheim barrage 153 6 Models 155 6.1 General 155 6.2 Types of models 155 6.3 Physical Models (PM) 155 6.3.1 Nature 156 6.3.2 Fixed bed models 156 6.3.3 Movable bed models 156 6.4 Mathematical models (MM) 156 6.5 Hybrid and combined models 157 6.6 Model laws for physical models with movable bed 158 6.6.1 Dimensionless parameters 158 6.6.2 Undistorted models – natural materials 160 6.6.3 Distorted models – lightweight materials 161 6.6.4 Comparing distorted and undistorted models 162 6.7 Comparing physical and mathematical models 163 6.7.1 Common features 163 6.7.1.1 Boundary conditions 163 6.7.1.2 Calibration 163 6.7.1.3 Validation 164 6.7.1.4 Verification 164 6.7.1.5 Terrain information 164 6.7.2 Differences 164 6.7.2.1 Extent of area 164 6.7.2.2 Time scales 165 6.7.2.3 Variant studies – sensitivity analysis 165 6.7.2.4 Precision 165 VI Sediment Sources and Transport Processes

6.7.2.5 Cost 165 6.8 Time and space scales 166 6.9 Sediment transport models 167 6.9.1 Introduction 167 6.9.2 Modelling protocol 169 6.9.3 Selection criteria 170 6.9.3.1 Model operators 170 6.9.3.2 Sensitivity analysis 170 6.9.3.3 Variant studies 170 6.9.3.4 Subsequent investigations 171 6.9.3.5 Cost – operating time 171 6.9.3.6 Special effects 171 6.9.3.7 Accuracy 171 6.9.3.8 Presentation of results – persuasive power 172 6.9.4 Outlook 172 6.10 Numerical sediment transport models 172 7 Reservoir Sedimentation 173 7.1 Introduction 173 7.1.1 Definition of reservoirs (German Standard DIN 4048) 173 7.1.2 Objectives of reservoirs 174 7.1.3 Problems caused by impounding facilities 174 7.1.4 Conclusions 174 7.2 Reservoir sedimentation 174 7.2.1 Causes for sedimentation 175 7.2.2 The sedimentation process in general 175 7.2.3 Reservoir surveying 176 7.2.4 Reservoir characteristic and deposition pattern 177 7.2.4.1 River impoundment 177 7.2.4.1.1 General 177 7.2.4.1.2 Type of impoundment 178 7.2.4.1.3 The characteristics of the sedimentation process 179 7.2.4.2 Reservoir lakes 179 7.2.4.2.1 Lifetime of a reservoir 179 7.2.4.2.2 Storage loss due to sedimentation 180 7.2.4.2.3 Turbidity currents as the main reason for sediment transport in reservoirs 182 7.3 Negative consequences of reservoir sedimentation 184 7.4 Measures of reservoir sedimentation control 185 7.4.1 Basic sedimentation control measures 185 7.4.2 Measures in the catchment area 186 7.4.2.1 Erosion protection in the catchment area 186 7.4.2.2 Erosion protection in the tributaries 187 7.4.3 Measures at the dam 190 7.4.4 Measures in the reservoir 191 7.4.5 Reservoir management 191 7.4.5.1 Sediment diversion – bypass 192 Contents VII

7.4.5.2 Sediment pass-through 192 7.4.5.3 Sediment traps – check dams 193 7.4.5.4 Further measures 193 7.5 Evacuation of reservoirs 193 7.5.1 Flushing 194 7.5.2 Mechanical excavation 196 7.5.2.1 Dry excavation 196 7.5.2.2 Dredging 196 7.5.3 Sediment rearrangement 196 7.5.3.1 Sediment rearrangement within the reservoir 196 7.5.3.2 Rearrangement outside the reservoir 197 7.5.4 Combining rearrangement measures 197 7.5.5 Decommissioning of dams 198 7.5.6 Problems caused by excavations 198 7.6 Special problems of reservoirs 199 7.7 Outlook 199 8 Local Scours 201 8.1 General 201 8.2 Scouring at piers 202 8.2.1 Formation of scour 202 8.2.2 Governing parameters 203 8.2.3 Scour depth 203 8.2.4 Reducing scour depth by structural measures 204 8.3 Scouring below low head structures – two-dimensional scour 204 8.4 Further causes for scouring 206 9 Grit chamber design 207 10 Instruments for measuring bed load and suspended load 211 10.1 Introduction 211 10.2 Bed load measuring devices 211 10.2.1 General 211 10.2.2 Direct measurement 212 10.2.2.1 Mobile bed load samplers 212 10.2.2.2 Stationary bed load samplers 213 10.2.3 Indirect measurement 214 10.2.4 Other indirect methods 215 10.2.5 Bed material sampling 216 10.3 Suspended sediment measurement 216 10.3.1 General 216 10.3.2 Single point measurement 219 10.3.3 Multiple point measurement 219 10.3.4 Integration measurement 220 10.3.5 Other methods 221 11 Ecological Aspects of reservoir sedimentation and removal 223 11.1 Introduction 223 VIII Sediment Sources and Transport Processes

11.2 Definition of contaminants 223 11.3 Contaminant sources and charge 224 11.3.1 Sediment formation in lakes 224 11.3.2 Suspended sediment as a potential contaminant carrier 225 11.3.2.1 Geogenic and anthropogenic contaminant supply 225 11.3.2.1.1 Geogenic contaminant supply 225 11.3.2.1.2 Anthropogenic material supply 227 11.3.2.2 Contaminants and other materials according to LAWA 227 11.3.2.2.1 Nutrients 228 11.3.2.2.2 Depleting agents 228 11.3.2.2.3 Disturbing agents 229 11.3.2.2.4 Poisonous substances 229 11.3.2.3 Relation between discharge and sedimentation – contamination 229 11.3.2.4 Suspended sediment as an indicator of contaminant load 231 11.4 Water quality 232 11.5 Sediment quality – investigation methods – parameters 233 11.6 Interaction between water body and sediment 233 11.7 Modelling fine sediment and contaminant transport 233 11.8 Utilisation, treatment and relocation of dredged materials 235 11.8.1 Definition of dredged material 235 11.8.2 Investigation parameters 236 11.8.3 Quantities of dredged materials 238 11.8.4 Regulations and guidelines 239 11.8.5 Dealing with dredged material 240 11.8.6 Utilisation and removal of sediment 245 11.8.6.1 Utilisation of untreated sediment 245 11.8.6.1.1 Use of sediment in agriculture and forestry 245 11.8.6.1.2 Use of sediment in landscape engineering 245 11.8.6.1.3 Sediments for construction works 245 11.8.6.1.4 Recultivation of disused mines 246 11.8.6.1.5 Construction of landfills 247 11.8.6.2 Utilisation of treated sediment 247 11.8.6.3 Landfill disposal of sediment 247 11.8.6.4 Further possibilities of sediment disposal 248 11.9 The ecological impact of desedimentation 248 11.9.1 Perpetuation of evidence in the case of disturbance to biological communities in running waters, with special attention to desedimentation 248 11.9.1.1 Introduction 248 11.9.1.2 Desedimentation and its impact on the fauna and flora of a river 249 11.9.1.3 Ecological perpetuation of evidence 250 11.9.1.3.1 Selection of the method 250 11.9.1.3.2 Surveys 250 11.9.1.3.2.1 Ecological framework 250 11.9.1.3.2.2 Biological situation 251 11.9.1.3.3 Investigation schedule and locations 251 11.9.1.3.4 Interpretation and assessment 251 Contents IX

11.9.1.3.4.1 Ecofactors and uses 252 11.9.1.3.4.2 Biocenotic analysis 254 11.9.1.3.5 Study expenses for perpetuation of evidence 257 11.9.2 Ecological aspects of desedimentation with an emphasis on watercourses in the Alpine region and foothills 259 11.9.2.1 General 259 11.9.2.2 Ecological objectives of sediment management in river systems 260 11.9.2.3 General effects on aquatic organisms from turbidity increase caused by desedimentation measures 260 11.9.2.3.1 Direct damage to organisms 260 11.9.2.3.2 Damage to the habitat 261 11.9.2.4 Types of desedimentation 262 11.9.2.4.1 Flushing 262 11.9.2.4.1.1 Examples of reservoir flushing effects 265 11.9.2.4.1.2 Measures for minimising the ecological impact from flushing 266 11.9.2.4.2 Evacuation 268 11.9.2.4.2.1 Excavation in the reservoir with partial water level drawdown 268 11.9.2.4.2.2 Dredging in the reservoir with or without water level drawdown 269 11.9.2.4.2.3 Disposal of dredged materials: gravel and fine sediment 269 11.9.2.4.2.4 Other measures – relocation 270 11.9.2.5 Criteria for assessing desedimentation measures 270 11.9.2.6 Summary 271 12 Legal aspects 273 13 Case studies 275 14 References 277 15 Miscellaneous 293 15.1 Abbreviations 293 15.2 Definitions 294 16 Contact 295

X Sediment Sources and Transport Processes

Figures XI

Figures

Figure 2.1-1: Main Geomorphodynamic System (Ahnert, 1996) 3 Figure 2.2-1: Cycle of Rocks (GEO kompakt, 2004) 4 Figure 2.3-1: Erosion/denudation as well as deposition phenomena (Vanoni, 1977) 8 Figure 2.3-2: Influence of vegetation on yield factor (%) and erosion (tonnes per hectare). Annual averages calculated from a 50m² test area of sandy loam with a slope of 3.5% (Rapp et al., 1973). 9 Figure 2.4-1: Relationship between the mean values of relief and denudation according to Table 2.4-1 11 Figure 2.4-2: Annual sediment yield versus catchment size, AE, and region (Schröder et al., 1984) 13 Figure 2.4-3: Sedimentation, or erosion, rates in Austrian reservoirs on the River Danube (Habersack, 1996)18 Figure 3.1-1: Schematic diagram of sediment transpor (Bechteler, 2004) 21 Figure 3.2-1: Density and kinematic viscosity of pure water plotted against temperature 24 Figure 3.2-2: Kinematic viscosity and density of saltwater as a function of temperature (the dotted lines are interpolations and extrapolations) 25 Figure 3.2-3: Particle sizes of bed load in various rivers: 1. Upper Rhine / Karlsruhe, 2. Lower Rhine / Emmerich, 3. Elbe / Geesthacht 28 Figure 3.2-4: Comparison of different national grain-size classification scales (Morris et al., 1997) 29 Figure 3.2-5: Cumulative curve of particle size distribution 29 Figure 3.2-6: Schematic drawing of a sieve set 32 Figure 3.2-7: Resistance coefficient cW for spheres plotted against Reynolds number using test results and approximations (Vanoni, 1977) 35 Figure 3.2-8: The effect of carrier-medium temperature on the settling velocity of quartz spheres (Vanoni, 1977) 36 Figure 3.2-9: Sieve diameter plotted against settling velocity for different shape factors SF (quartz particles) (Vanoni, 1977) 37 Figure 3.2-10: Fall behaviour of barge-dumped material (Zanke, 2002) 38 Figure 3.2-11: Schematic drawing of a Macro-Granometer sedimentation system (Schrimpf, 1987) 39 Figure 3.2-12: Result of a settling-velocity analysis by use of a MacroGranometer - cumulative curve p(vs) and density curve q(vs) of distribution (Schrimpf, 1987) 40 Figure 3.3-1: Plan and longitudinal section through a ripple-pool structure, left, and a step-pool structure, right (Schälchi, 1991) 44 Figure 3.3-2: Forces acting on the fluid element 45 Figure 3.3-3: Composition of channel resistance (DVWK, 1993b) 48 Figure 3.3-4: Influence areas after Horton (1933) 49 Figure 3.3-5: Flow conditions in channels with and without vegetation (Bertram, 1985) 50 Figure 3.3-6: Macroturbulence resulting from bank vegetation (DVWK, 1991, Merkblatt 220) 50 Figure 3.3-7: Partitioned cross section (DVWK, 1993b, Mertens) 51 Figure 3.3-8: Bed form development after Gehrig (1981) 52 Figure 3.3-9: Bed shear stress plotted against flow velocity and bed form, τK = particle resistance, τF = shape resistance (Figure 3.3-3) (Engelund et al., 1982) 53 Figure 4.1-1: Probability distribution of the shear stresses at threshold of motion 57 Figure 4.1-2: Mean critical velocities after HJULSTRÖM (Zanke, 2002) 59 Figure 4.1-3: Natural angle of slope plotted against particle diameter and particle shape 60 Figure 4.1-4: Shields Diagram (1936) (after Zanke, 1990) for different motion risks, R 61 Figure 4.1-5: Definitions of critical dimensionless shear stress (Schöberl, 1990) 62 Figure 4.1-6: Incipient motion for biologically stabilised beds (Führböter, 1983) Left:Measured values plotted in the Shields Diagram (Figure 4.1-4) Right:Schematic drawing showing the effect of biological stabilisation on sediment transport 63 Figure 4.1-7: The influence of bed forms on onset of erosion (Höfer, 1984) 64 Figure 4.1-8: Critical bed shear stress τoc as a function of void ratio (DIN 19661/2) 66 Figure 4.1-9: Exposure and hiding effects for single-grain and multiple-grain materials (Zanke, 2002) 68 Figure 4.1-10: Initial mix As, Fuller Curve AF, armour mix D, subscript F = Fuller Curve, x = sample (Günter, 1971) 69 Figure 4.1-11: Erosion rate E& (G in the above Figure) changing during armour and pavement evolution (Jain, 1990) 71 Figure 4.1-12: Sublayer and armour layer from sampling at Isar River Kilometre 1.8 and maximum armour layer determined after Günter 75 Figure 4.1-13: Evolution of the median particle diameter along the Upper Iller (Bechteler, 2002) 75 Figure 4.2-1: Bed forms in alluvial channels (Gehrig, 1981) 77 Figure 4.2-2: Backward erosion during dam overtopping 77 Figure 4.2-3: Classification of banks (after Zarn, 1997) 78 XII Sediment Sources and Transport Processes

Figure 4.2-4: Bed forms for the range 1 < D* < 5000 (Wieprecht, 2001) 78 Figure 4.2-5: Characteristic length of a dune (Hunzinger, 1998) 79 Figure 4.2-6: Soundings along the River Danube (Kilometre 2265.5 to Kilometre 2265.7) for different flows, carried out between April 1988 and June 1989 (bed form material: dm = 21mm) (DVWK, 1992b) 80 Figure 4.2-7: Grain material sorting at the lee slope of a bed form (after Zanke, 1982) 80 Figure 4.2-8: Water depth plotted against flow velocity as a function of bed forms 81 Figure 4.3-1: Left: Schematic drawing showing turbulence-induced secondary flows in open channels and the resulting streambed reactions (after Tsujimoto, 1989) Right: Turbulence-induced secondary flows in a physical model (after Zarn, 1997) 82 Figure 4.3-2: Bed load discharge mG and suspension-load discharge mS as a function of location in a measuring cross-section in the River Rhine at Worms, River Kilometre 444 (DVWK, 1992b) 83 Figure 4.3-3: Bed load discharge plotted against flow (ATV-DVWK, 2003) 83 Figure 4.3-4: Bed load function after Einstein (1942, 1950) and Meyer-Peter/Müller (1949) 85 Figure 4.3-5: Flow proportion effective for bed load transport 87

Figure 4.3-6: Variation of bed load transport, m& G , resulting from variations in the number, shape and position of banks and thalwegs in a ramified river a) for steady flow b) for unsteady flow 89 Figure 4.3-7: Left: Bed load function with flow Q0 for incipient motion, and QD for break-up of the armour

layer. Right: Duration curve of flow Q and bed load transport m& G . The shaded area corresponds to the minimum volume of bed load transport GFmin, or min mFf, for Q < QD, the hatched area is the maximum volume of bed load transport for Q > Q0 (Hunziker, 1995) 90 Figure 4.3-8: The influence of abrasion coefficient aw on bed load mass loss for different distances x. 92 Figure 4.3-9: Determining annual volume of bed load transport (Zanke, 2002) 94 Figure 4.4-1: Grading curves of suspended-load samples 96 Figure 4.4-2: Development of a stationary suspended-sediment concetration profile over depth 97 Figure 4.4-3: S Grading curves for bed load / suspended load transition 98 Figure 4.4-4: Spectrum of particle-size distribution from 15 suspended-sediment samples from Bavarian rivers (after Burz, 1964) 99 Figure 4.4-5: Deriving the two-dimensional diffusion equation (u = vx, w = vz) (Chang, 1987) 101 Figure 4.4-6: Theoretically (Curve 1) and experimentally (Curves 2 and 3) determined distributions of the turbulent diffusion coefficient over depth 102 Figure 4.4-7: Relative concentration distribution of suspended particles for different exponents z 103 Figure 4.4-8: Velocity profile for pure water and suspensions (Vanoni, 1977) 104 Figure 4.4-9: Measured (z1) and calculated (z) Rouse numbers (Einstein et al., 1954) 105 Figure 4.4-10: Determining suspended-sediment discharge 108 Figure 4.4-11: Onset of motion after van Rijn (1984) 110 Figure 4.4-12: Erosion diagram (Roberts et al., 2003) 111 Figure 4.4-13: Erosion, transport and sedimentation of uniform bed material using numerical values (Dittrich, 1998) 112 Figure 4.4-14: Adaptation of the local sediment concentration for transition from clear water to an alluvial bed (van Rijn, 1987) 113 Figure 4.4-15: Discharge – suspended-sediment-concentration curves for the River Dart (GB) (catchment area = 600km²), (Walling, 1977) 114 Figure 4.4-16: Discharge – suspended-sediment-concentration curves (Dart), (Walling, 1977) 114 Figure 4.4-17: Discharge – suspended-sediment-concentration curves (Dart) – different regression curves (Walling, 1977) 115 Figure 4.4-18: Relation between discharge and suspended-sediment transport for different particle diameters (Chien et al., 1999) 116 Figure 4.4-19: Distribution of suspended-sediment concentration at the Burghausen measuring station on the River Salzach on 09-07-1990, discharge 740m³/s (WRS, 2000) 117 Figure 4.4-20: Transport of the suspended-sand fractions, Elbe/Schönau/Schmilka, River Kilometre 2.6/4.4 (BfG, 2000) 118 Figure 4.4-21: Relation between discharge and suspended-sediment-concentration for the River Lech near Füssen (Engelsing, 1988) 118 Figure 4.4-22: Discharge vs. suspended-sediment-concentration, River Bauna (July to November 2000) (Sobirey, 2001) 119 Figure 4.4-23: Rating curve for discharge and suspended-sediment concentration, Whitsun flood 1999, Kempten gauging station / River Iller 120 Figure 4.4-24: Rating curve for discharge and suspended-sediment concentration, August flood 2000, Kempten gauging station / River Iller 121 Figure 4.4-25: Rating curve for discharge and suspended-sediment concentration, September flood 2000, Kempten gauging station / River Iller 121 Figures XIII

Figure 4.4-26: Relation between discharge and suspended-sediment concentration from three flood events, Kempten gauging station / River Iller 122 Figure 4.4-27: Relation between discharge and suspended-sediment concentration for ascending and descending flood wave, Kempten gauging station / River Iller 122 Figure 4.6-1: Vertical split of the water body using exchange term s as shown in the 1D formulation (ATV- DVWK, 2003) 124 Figure 5.2-1: Dimensions for a meandering stream (DIN 4049) 126 Figure 5.2-2: Schematic diagram showing stabilisation of a navigation channel in a straight river section 127 Figure 5.2-3: (a) Braided river system, (b) Meandering river system (after Einsele, 1991) 128 Figure 5.2-4: Transverse channels on gravel bars (lower River Lech) 129 Figure 5.2-5: Natural levées (exposed) along the River Ammer, 1998 (WRS, 2000) 129 Figure 5.2-6: Plan view of the upper River Isar in the years 1925, 1971, 1982 and 1984 (WRS, 2000) 130 Figure 5.2-7: Meander migration (Zeller, 1967) 130 Figure 5.2-8: Distinguishing channel morphologies after de Silva (1991) 131 Figure 5.2-9: Schematic diagram showing flow pattern in a bend 132 Figure 5.2-10: Example of a meander section (Ahnert, 1996) 133 Figure 5.2-11: Characteristic subdivision of a watercourse in plan and longitudinal section (Ahnert, 1996) 134 Figure 5.2-12: Longitudinal profile of the River Rhine (Ahnert, 1996) 135 Figure 5.2-13: Erosion channel in the Urstein conglomerate (WRS, 2000) 137 Figure 5.2-14: Ripple-pool series (WRS, 2000) 139 Figure 5.2-15: Determining the minimum slope, needed for the development of alternate bars, B = channel width, d = particle diameter (Jäggi, 1983) 140 Figure 5.2-16: Longitudinal sections along the Bavarian and Austrian banks of the River Salzach in the Freilassing basin for the year 1953 (WRS, 2002) 140 Figure 5.2-17: Mean and maximum scour depths after Zarn (1997) for the River Salzach in the Freilassing basin (WRS, 2000) 142 Figure 5.2-18: Development of rock and alluvial terraces and their development stages (Ahnert, 1996) 143 Figure 5.2-19: Schematic drawing showing estuary and delta 144 Figure 5.3-1: Overall morphological system (Ahnert, 1996) 145 Figure 5.4-1: The influence of bed width on bed load transport capacity (top: a) and gradient (bottom: b) (Zarn, 1997) 147 Figure 5.4-2: Determining dominant flow (Bechteler, 2004) a) flow duration curve b) bed load transport curve c) product of a) and b) 148 Figure 5.4-3: Determination of dominant flow for the section between River Kilometres 52.8 and 49.0 on the Lower River Salzach (WRS, 2002) 149 Figure 5.5-1: Profilweise Ermittlung des Geschiebetransports für eine Flussstrecke 151 Figure 5.5-2: Theoretical bed load-transport capacity for the actual state (2002) of the Upper Iller 152 Figure 6.5-1: Schematic diagram of hybrid model 157 Figure 6.6-1: Geometrically reduced model grading 161 Figure 6.6-2: Criteria of similitude for distorted models with movable beds (after Gehrig, 1981) 162 Figure 6.8-1: Space and time scales for morphodynamic processes (Habersack, 2000) 166 Figure 6.8-2: Measuring and modelling scale range for sediment transport (Habersack, 2000) 167 Figure 6.9-1: Main factors acting on sediment transport (Bechteler, 2004) 168 Figure 6.9-2: Experiment (laboratory test) and model test for sediment-transport processes 168 Figure 7.1-1: Classification of impounding facilities 173 Figure 7.2-1: Sedimentation pattern for a reservoir basin (Vischer, 1981) 175 Figure 7.2-2: Longitudinal profile of Lake Mead with Hoover Dam, sedimentation between 1937 and 1946 (Vischer, 1984) 176 Figure 7.2-3: Flow and sedimentation patterns (Westrich, 1988) 177 Figure 7.2-4: Final sedimentation stage in a reservoir area without structural measures– plan view (after Baumhackl, DWA, 2006) 178 Figure 7.2-5: Final sedimentation stage in an reservoir area without structural measures – cross section (after Baumhackl, DWA, 2006) 178 Figure 7.2-6: Trap efficiency of reservoirs as a function of mean residence time (Brune, 1953) 180 Figure 7.2-7: Development over time of the worldwide net storage volume based on projection of sedimentation rate (White, 2001) 182 Figure 7.2-8 Annual rate of loss of storage as a function of reservoir size (White, 2001) 182 Figure 7.2-9: Schematic section through a reservoir showing turbidity current (Oehy, 2002)) 183 Figure 7.2-10: Maximum transportable particle diameter vs. turbidity-current flow velocity after Fan (1986)183 Figure 7.3-1: Change in reservoir storage curve due to sedimentation, left for delta-formation, right for sedimentation at the deepest point of the reservoir 185 Figure 7.4-1: Overview of preventive and retroactive sedimentation-control measures (after Schleiss, DWA, 2006) 186 XIV Sediment Sources and Transport Processes

Figure 7.4-2: Possibilities of erosion protection in the catchment area above a dam 187 Figure 7.4-3: Possibilities of erosion protection along tributaries 188 Figure 7.4-4: Method of checking bank protection (boxes are explained in greater detail in the Study) (Cranfield University, 1999) 189 Figure 7.4-5: Sediment routing strategies (after Morris et al., 1997) 192 Figure 7.4-6: Reservoir with forebay that can be flushed through a tunnel ending below the main dam (Vischer, 1981) 192 Figure 7.4-7: Bypass arrangement of reservoirs (Morris et al., 1997) 193 Figure 7.5-1: Long-term reservoir development in the case of flushing, for narrow and wide reservoirs (Morris et al., 1997) 195 Figure 8.1-1: Evolution in time of a clear-water scour at a pier (Case a) as against a live-bed scour (Case b) (Breusers et al., 1991) 201 Figure 8.2-1: Schematic drawing showing flow around a cylindrical pier with developed scour (Raudkivi, 1982) 202 Figure 8.2-2: Laboratory data for determining scour depth for cylindrical piers under a relatively large water depth (Breusers et al., 1991) 203 Figure 8.3-1: Scouring below low-head structures after Franke (1960) 205 Figure 8.3-2: Flow chart for determining scouring and stream stability (US Department of Transportation, 2001) 206 Figure 9.1-1: Design diagram for a grit chamber with triangular concentration distribution increasing towards the bottom at the inlet, and with logarithmic velocity distribution (Schrimpf, 1982) 208 Figure 9.1-2: Sand trap downstream from a water diversion (Gieseke et al., 1998) 209 Figure 10.2-1: Helly-Smith sampler (DVWK, 1992) 212 Figure 10.2-2: Bed load sampler developed by the Federal Institute of Hydrology, Koblenz (DVWK, 1992)213 Figure 10.2-3: Bed load trap in the River Rhine near Wesel (DVWK, 1992) 214 Figure 10.2-4: Evolution in time of the delta formed by the Tiroler Ache streams in Lake Chiemsee between 1869 and 1970 (Mangelsdorf et al., 1980) 215 Figure 10.2-5: VAN VEEN Grab System (DVWK, 1997) 216 Figure 10.3-1: Mean annual suspended load of the River Danube and main tributaries (based on Bavarian State Agency for Water Management, 1992) 217 Figure 10.3-2: System of suspended-sediment concentration measurement after Schemmer (1995) 218 Figure 10.3-3: OTT sampler (DVWK, 1986) 220 Figure 10.3-4: US-Sampler D 49, dimensions in [mm] 221 Figure 11.3-1: Sediment formation in lakes (Sturm et al., 1995) 224 Figure 11.3-2: Types of mass movement in the Alpine region (DVWK, 1993) 226 Figure 11.3-3: Geological map of Bavaria 226 Figure 11.3-4: Man-made contaminant supply to running waters in the Alpine region and foothills (Westrich, 1988) 227 Figure 11.3-5: Principle of self-purification and eutrophication (NAJU, 2004) 228 Figure 11.3-6: Suspended-load transport in backwater reservoirs; (a) flow duration curve, (b) suspended-load storage function (Kern, 1993) 229 Figure 11.3-7: Sedimentation diagram for catchments in the Alpine region and foothills (Westrich, 1981) 230 Figure 11.3-8: Short core drilled from a sediment layer in the reservoir above Unit 15 at Landsberg, Bavaria (BAWAG) 232 Figure 11.7-1: Model components of the programs by Jacoub (2004 235 Figure 11.7-2: Model components of the WASP 5 programs 235 Figure 11.8-1: Legal procedure (Köthe et al., 1995) 239 Figure 11.8-2: Possible ways of disposing of dredged materials (Geowissenschaften+Umwelt, 1999) 241 Figure 11.8-3: Main procedural steps for dealing with dredged materials (Geowissenschaften + Umwelt, 1999) 242 Figure 11.8-4: Project procedure: Sediment relocation (DVWK, 1992) 244 Figure 11.8-5: Simplified schematic overview of utilisation concept (from „Nachhaltiges Niedersachsen – Baggergutmanagement“, S. 29) 247 Figure 11.9-1: Barrage (Faimingen, Danube) 258 Figure 11.9-2: Shell armouring fallen dry after reservoir emptying 258 Figure 11.9-3: Biological investigation of river banks 258 Figure 11.9-4: Sorting a sample 258 Figure 11.9-5: Common in large rivers: colony of a fresh-water fungus 259 Figure 11.9-6: Shells, snails, bryozoa – aspect typical of dammed-up rivers and canals 259 Figure 11.9-7: Head of a caddis-fly larva (anabolia) from tranquil river sections 259 Figure 11.9-8: Mouth tools of a non-biting midge (microtendipes) from fine river sediment 259 Figure 11.9-9: Impact from increased turbidity on fauna and flora (Bruton, 1985) 261 Figures XV

Figure 11.9-10: Free gravel substrate (left) and gravel body clogged through internal and external colmation (Photo: U. Schälchli) 261 Figure 11.9-11: Influencing factors and consequences of flushing operations on the downstream river section (Eberstaller et al., 2000) 263 Figure 11.9-12: Potential factors and effects of flushing in undeveloped downstream sections (Eberstaller et al., 2000) 264 Figure 11.9-13: Turbidity from reservoir flushing 265 Figure 11.9-14: Sediment blocks breaking off suddenly may cause short-term turbidity peaks 267 Figure 11.9-15: Training works for reservoir structuring 268 Figure 11.9-16: Gravel and fine sediment dumped in bank areas (Alpine Rhine) 269 Figure 11.9-17: Reservoir structuring 270 XVI Sediment Sources and Transport Processes

Tables

Table 2.2-1: The principal rocks and sediments in Central Europe, their substrates and bed loads (Briem, 2002) 5 Table 2.3-1: Denudation processes (Ahnert, 1996), regolith = loosened rock material 7 Table 2.4-1: Mean values of relief and denudation for different areas (Ahnert, 1996) 11 Table 2.4-2: Sediment load of rivers in the Canadian Cordillera (Slaymaker et al., 1972) 14 Table 2.4-3: Sedimentation rate in Swiss lakes (Vischer, 1981) 15 Table 2.4-4: Parameters from 19 Swiss reservoirs used in Eq. 2.4-4 (Beyer et al., 2000) 16 Table 2.4-5: Sediment yields for several reservoirs in USA (Vanoni, 1977) 17 Table 2.4-6: Sedimentation rates for Austrian reservoirs (Habersack, 1996) 18 Table 3.2-1: Up-to-date measuring methods for determining grain diameter 27 Table 3.2-2: Particle size distribution according to DIN 4022 28 Table 3.2-3: Sample size for sieve analysis according to DIN 18123 (DVWK, 1992a, Table 6) 30 Table 3.2-4: Sample splitting methods depending on the mass of the basic population 31 Table 3.3-1: Estimating equivalent sand roughness, kS, from the characteristic grain diameter of the bed surface (nach DVWK, 1997) 42 Table 3.3-2: Characterisation of the bed morphology of mountain streams (Rosport, 1997) 43 Table 4.1-1: Safe critical shear stress τcr or.critical velocity vcr according to DIN 19661/ 67 Table 4.1-2: Criteria for armour development (DVWK, 1997) 69 Table 4.1-3: Calculation of armour layer after Günter (DVWK, 1997, Table 4.3c) 70 Table 4.1-4: Ansätze zur Abschätzung der kritischen Sohlenschubspannung (Dittrich, 1998) 72 Table 4.1-5 Determination of critical bed shear stress after Meyer-Peter/Müller (DVWK, 1997) 73 Table 4.1-6: Calculation method after Schöberl (1979, 1991) 74 Table 4.1-7: Calculation method after Chin (1985) 74 Table 4.3-1: Annual bed load-transport values in Bavarian rivers 81 Table 4.4-1: Records of annual means (1971/ 2000) for Alpine rivers in Bavaria (DGJ, 2000)) 95 Table 5.2-1: Possible ways of chute formation (WRS, 2000) 138 Table 7.1-1: Purposes of impounding facilities 174 Table 7.2-1: Estimated average annual loss of storage, in percent 180 Table 7.2-2: Estimated annual loss of reservoir storage due to sedimentation as well as estimated reservoir half-life (White, 2001) 181 Table 7.3-1: Problems from reservoir sedimentation 184 Table 7.5-1: Detrimental downstream impact of reservoir flushing 194 Table 7.5-2: Evaluation of the effects from various processes for removing and avoiding reservoir sedimentation (BUWAL, 1994) 198 Table 8.3-1: Scour magnitude below gates after Franke (1960) 205 Table 10.2-1: Mean annual bed load-transport values for several Bavarian rivers 211 Table 10.3-1: Long-term records (1971 – 2000) of mean annual suspended-sediment transport and suspended load in Bavarian rivers (DGJ, 2000) 217 Table 10.3-2: Comparison of different technologies of suspended-sediment measurement (Wren, 2000) 222 Table 11.8-1: Parameters for general sediment characterisation (DVWK, 1992) (TOC: Total Organic Carbon, SM: Schwermetall (heavy metal), AOS: Absorbable Organic Sulphur compound) 236 Table 11.8-2: Heavy metals to be identified (DVWK, 1992) 237 Table 11.8-3: List of priorities for sediment-relevant contaminants or contaminant groups (DVWK, 1992) 237 Table 11.8-4: Biotests available for assessing water samples (DVWK, 1992) 238 Table 11.8-5: Mean annual dredged quantities classified by origin and particle size (BMV, 1987)) 238

Equations XVII

Equations

⎡ m ⎤ Eq. 2.4-1: d = 0,0001535 ⋅ h - 0,011 ⎢ ⎥ 10 ⎣1.000 a ⎦ ⎡ t ⎤ Eq. 2.4-2: D = E = R ⋅ K ⋅ L ⋅S⋅ C ⋅ P ⎢ ⎥ 12 ⎣ 0,4ha ⎦ m Eq. 2.4-3: D = Fa 100 []% 13 d 3 -9 0,607 0,091 6,042 0,154 ⎡ m ⎤ Eq. 2.4-4: VA = 566,045 ⋅10 ⋅ HSommer ⋅ EB ⋅ OV ⋅ ∆GL − 267 ⎢ 2 ⎥ 15 ⎣km ⋅ a ⎦ ⎡ kg ⎤ Eq. 3.2-1: ρ = ρ W ⋅ ()1− C + ρ F ⋅ C 25 ⎣⎢m 3 ⎦⎥ dmax d 1 Eq. 3.2-2: i 29 d m = ∑ ∆pi ⋅ []m where d i = ⋅ ()d i + d i+1 []m dmin 100 2 d Eq. 3.2-3: U = 60 []− 30 d10 d Eq. 3.2-4: σ = 84,1 []− 30 d15,9

mi Eq. 3.2-5: ∆pi ()d = n ⋅100 []% 31 ∑ mi i=1 n Eq. 3.2-6: p()d = ∑ ∆pi ()d []% 31 i=1 ρ ⋅ Vol ⎡mg kg ⎤ Eq. 3.2-7: C = F F or 33 m ⎢ 3 ⎥ VolF + VolW ⎣ l m ⎦

VolF Cm Eq. 3.2-8: CV = = []− 33 VolF + VolW ρF

ρF ⋅ VolF 6 Eq. 3.2-9: C W = ⋅10 []ppm 33 ρF ⋅ VolF + ρW ⋅ VolW π ⋅ d 2 ρ ⋅ v 2 Eq. 3.2-10: F = c ⋅ ⋅ W S []N 34 W D 4 2 π ⋅ d 3 Eq. 3.2-11: G = ⋅ g ⋅ ()ρ − ρ []N 34 4 F W 4 g ⋅ d ρ − ρ ⎡m⎤ Eq. 3.2-12: F W 34 vS = ⋅ ⋅ ⎢ ⎥ . 3 c W ρ W ⎣ s ⎦ 24 v ⋅ d Eq. 3.2-13: c = for Re()v = S < 0,1 . 34 W Re S ν 1 g ⋅ d 2 ρ − ρ ⎡m⎤ Eq. 3.2-14: F W 35 vS = ⋅ ⋅ ⎢ ⎥ . 18 ν ρ W ⎣ s ⎦ XVIII Sediment Sources and Transport Processes

24 ⎛ 3 ⎞ Eq. 3.2-15: c W = ⋅⎜1+ Re⎟ for Re < 1 35 Re ⎝ 16 ⎠

24 4 5 Eq. 3.2-16: c W = + + 0,4 for Re < 2 ⋅10 35 Re Re c Eq. 3.2-17: SF = 36 a ⋅b 1 ⎡ 3 ⎤ − 1 26 m Eq. 3.3-1: 6 ⎢ ⎥ 41 k St = 5,87 ⋅ 2g ⋅ k S = 1 6 ⎢ s ⎥ k S ⎣ ⎦ 1 21 ⎡m 3 ⎤ Eq. 3.3-2: ⎢ ⎥ 42 k St = 1 6 ⎢ s ⎥ d 90 ⎣ ⎦

Eq. 3.3-3: k S = 3,6 ⋅ d 90 []m . 42 H 2 Eq. 3.3-4: k = 10,5⋅ . 45 S L

Eq. 3.3-5: τ0 ⋅ U ⋅ ∆l = G ⋅ I = ρ W ⋅ g ⋅ A ⋅ ∆l ⋅ I [N] where sin ϕ ≈ tgϕ = I 45 ⎡ N ⎤ A Eq. 3.3-6: τ0 = ρ W ⋅ g ⋅ R ⋅ I where R = 45 ⎣⎢m 2 ⎦⎥ U τ ⎡m⎤ Eq. 3.3-7: * 0 46 v 0 = = g ⋅ R ⋅ I ⎢ ⎥ ρ W ⎣ s ⎦

* ⎡m⎤ Eq. 3.3-8: v m ~ v0 = g ⋅ R ⋅ I 46 ⎣⎢ s ⎦⎥ ⎡m⎤ Eq. 3.3-9: v m = C ⋅ R ⋅ I 46 ⎣⎢ s ⎦⎥ 1 1 ⎡m 2 ⎤ Eq. 3.3-10: C = k ⋅ R 6 ⎢ ⎥ 46 St s ⎣⎢ ⎦⎥ 2 1 3 2 ⎡m⎤ Eq. 3.3-11: v m = k St ⋅ R ⋅ I 46 ⎣⎢ s ⎦⎥ 1 ⎡m⎤ Eq. 3.3-12: v m = ⋅ 8⋅ g ⋅ R ⋅ I 47 λ ⎣⎢ s ⎦⎥ 1 v ⎛ 0,628 k ⎞ Eq. 3.3-13: = m = −2,03⋅ lg⎜ + S ⎟ . 47 * ⎜ ⎟ λ 8 ⋅ v 0 ⎝ Re⋅ f ⋅ λ 14,84 ⋅ R ⋅ f ⎠

1 v m ⎛ R ⎞ Eq. 3.3-14: = = 2,03⋅ lg⎜12,27 ⎟ 47 λ 8⋅ g ⋅ R ⋅ I ⎝ k S ⎠

1 v m ⎛ h ⎞ Eq. 3.3-15: = = 2,03⋅ lg⎜11,0 ⎟ . 47 λ 8⋅ g ⋅ h ⋅ I ⎝ k S ⎠ 1 ⎛ R ⎞ ⎡m 2 ⎤ Eq. 3.3-16: C = 8 ⋅ g ⋅ 2,03⋅ lg⎜12,27 ⎟ ⎢ ⎥ 47 ⎜ k ⎟ s ⎝ S ⎠ ⎣⎢ ⎦⎥

Eq. 3.3-17: G i ⋅ I = τi ⋅ U i ⋅ ∆l 49 ⎡ N ⎤ Eq. 3.3-18: τ = ρ ⋅ g ⋅ R ⋅ I 49 i i ⎢ 2 ⎥ ⎣m ⎦ Equations XIX

* τi ⎡m⎤ Eq. 3.3-19: v o = = g ⋅ R i ⋅ I 49 i ρ ⎣⎢ s ⎦⎥

1 v m ⎛ R i ⎞ Eq. 3.3-20: = = 2,03⋅ lg⎜12,27 ⎟ 49 λ i 8⋅ g ⋅ R i ⋅ I ⎝ k i ⎠ * 1,889 0,2655 0,3034 Eq. 3.3-21: FrS = 0,05604⋅Frg ⋅I ⋅σg 53 ⎛ ν ⎞ Eq. 4.1-1: v cr = α ⋅ ⎜ ρ′⋅ g ⋅d + 5,25 ⋅ c⎟ 59 ⎝ d ⎠ ⎡ N ⎤ Eq. 4.1-2: τ = ρ ⋅ g ⋅ h ⋅ I − I − I ⋅ρ ⋅ v2 59 W m ()bed surface level surface level W m ⎢ 2 ⎥ ⎣m ⎦ 2 2 τB tan α sin α Eq. 4.1-3: = cosα ⋅ 1− 2 ≈ 1− 2 60 τcr tan β sin β

* τcr Eq. 4.1-4: Θcr = Frcr = 61 ()ρF − ρ W ⋅ g ⋅d ⎡ N ⎤ Eq. 4.1-5: τgr = τcr = 0,047 ⋅ ()ρF − ρ w ⋅ g ⋅ d m 62 ⎣⎢m 2 ⎦⎥ 1 ⎛ τ ⎞ Eq. 4.2-1: H < ⋅ h ⋅⎜1− cr ⎟ , 79 6 ⎝ τ ⎠ m Eq. 4.3-1: Φ = G []− 84 3 ρF ⋅ ρ′⋅ g ⋅ d d ⋅ρ′ Eq. 4.3-2: Ψ = []− 84 h ⋅ I 3 Φ = G * ⋅ Fr* 2 Eq. 4.3-3: 1 85 Ψ = Fr* 3 2 ρ 8 1 ⎡ ⎤ kg F ⎢ ' ⎥ ⎡ ⎤ Eq. 4.3-4: mG = ⋅ ⋅ ⋅ ρw ⋅ g ⋅µ ⋅ I ⋅ R s − 0,047 ⋅ρ ⋅ρw ⋅ g ⋅ dm ⎢ ⎥ 85 ρF − ρw g ρw ⎢1424 434 14244 4344 ⎥ ⎣m ⋅s⎦ ⎣ τo τcr ⎦

dmax d i 1 Eq. 4.3-5: d m = Σ ∆pi ⋅ []m mit d i = ⋅ ()d i + d i+1 [m] 85 dmin 100 2

τcr * Eq. 4.3-6: 0,047 = = Frcr = Θcr 86 g ⋅ ()ρF − ρ w ⋅ d m ⎡ N ⎤ Eq. 4.3-7: τ = 760,8⋅ d 86 cr m ⎢ 2 ⎥ ⎣m ⎦

QS Eq. 4.3-8: R S = h ⋅ ~ []m 86 Q b Eq. 4.3-9: R ≈ h ⋅ []m 86 S U Q Eq. 4.3-10: β = r 87 Q XX Sediment Sources and Transport Processes

3 2 ⎛ k St ⎞ Eq. 4.3-11: µ = ⎜ ⎟ []− 87 ⎝ k S ⎠ 1 26 ⎡m 3 ⎤ Eq. 4.3-12: k S = ⎢ ⎥ 87 6 d s 90 ⎣⎢ ⎦⎥ 0,2 4 ⋅ρ ⎛ d ⎞ ⎛ Θ ⋅ρ′⋅ d ⎞ kg F ⎜ 90 ⎟ 1,6 ⎜ cr m ⎟ ⎡ ⎤ Eq. 4.3-13: m G = ⋅⎜ ⎟ ⋅ v m ⋅ R b ⋅ I ⋅⎜1− ⎟ ⎢ ⎥ 87 ρ′ ⎝ d 30 ⎠ ⎝ R b ⋅ I ⎠ ⎣m ⋅s⎦ 3 2 Eq. 4.3-14: Φ = 8⋅ ()Θ′ − Θcr 88 3 2 Eq. 4.3-15: Φ = 5⋅ ()Θ′ − Θcr 88 m Eq. 4.3-16: Φ = G []− 88 3 ρF ⋅ ρ′⋅ g ⋅ d x − a ⋅ W 3 Eq. 4.3-17: d x = d o ⋅ e [mm] 91

− a W ⋅ x Eq. 4.3-18: m Gfx = mGfo ⋅ e [t] 92 v2 Eq. 4.4-1: Fr2 = m = 360 []− 99 g ⋅ dgr v 2 Eq. 4.4-2: d = m []m 99 gr 360 ⋅ g m Eq. 4.4-3: * ⎡ ⎤ 100 v 0 = 0,25 ⋅ vs ⎣⎢ s ⎦⎥ ∂C ∂C ∂C ∂ ⎛ ∂C ⎞ ∂ ⎛ ∂C ⎞ ⎡ kg ⎤ Eq. 4.4-4: + u ⋅ + w ⋅ = ⎜ε ⋅ ⎟ + ⎜ε ⋅ ⎟ 100 ⎜ x ⎟ ⎜ y ⎟ ⎢ 3 ⎥ ∂t ∂x ∂y ∂x ⎝ ∂x ⎠ ∂y ⎝ ∂y ⎠ ⎣m ⋅s⎦ dC kg Eq. 4.4-5: ⎡ ⎤ 101 vs ⋅ C + εsy = 0 dy ⎣⎢m 2 ⋅s⎦⎥ y y ⎡m 2 ⎤ Eq. 4.4-6: * ⎛ ⎞ , 102 ε ty = κ ⋅ v 0 ⋅ h ⋅⎜1− ⎟ ⋅ ⎢ ⎥ ⎝ h ⎠ h ⎣ s ⎦ Eq. 4.4-7: 102 εsy = β ⋅ ε ty y y ⎡m 2 ⎤ Eq. 4.4-8: * ⎛ ⎞ 102 εsy = β ⋅ ε ty = β ⋅ κ ⋅ v 0 ⋅ h ⋅⎜1− ⎟ ⋅ ⎢ ⎥ ⎝ h ⎠ h ⎣ s ⎦ z C ⎛ h − y a ⎞ Eq. 4.4-9: = ⎜ ⋅ ⎟ []− . 103 Ca ⎝ y h − a ⎠ v Eq. 4.4-10: s 103 z = * []− β ⋅ κ ⋅ v 0 1 ρ′⋅ g 3 Eq. 4.4-11: * ⎛ ⎞ 106 D = ⎜ ⎟ ⋅ d 50 ⎝ ν 2 ⎠ 2 2 v*' − v* Eq. 4.4-12: T* = 0 0cr 106 * 2 v 0cr g ⎛12 ⋅ R ⎞ Eq. 4.4-13: *' ′ ⎜ ⎟ (cf. Eq. 3.3-16) 106 v0 = ⋅ vm where C = 18⋅ log⎜ ⎟ C′ ⎝ ks ⎠ Equations XXI

1,5 d T* ⎡ kg ⎤ Eq. 4.4-14: C = 0,015⋅ρ ⋅ 50 ⋅ 106 a F 0,3 ⎢ 3 ⎥ a D* ⎣m ⎦ y=h ⎡ kg ⎤ Eq. 4.4-15: m = C ⋅ v dy 108 s ∫ x ⎢ ⎥ y=a ⎣m ⋅s⎦ m Eq. 4.4-16: * ⎡ ⎤ . 109 v0cr > vs ⎣⎢ s ⎦⎥ ⎛ τ ⎞ ⎡ kg ⎤ Eq. 4.4-17: E& = k ⋅⎜ 0 −1⎟ für τ ≥ τ 109 ⎜ ⎟ 0 cr,E ⎢ 2 ⎥ ⎝ τcr,E ⎠ ⎣m ⋅s⎦ 1,5 0,3 ⎛ τ ⎞ ⎡ kg ⎤ Eq. 4.4-18: E& = 0,00033⋅ρ ⋅ D* ⋅⎜ 0 −1⎟ ⋅ ρ′⋅ g ⋅ d 109 s ⎜ ⎟ ⎢ 2 ⎥ ⎝ τcr,E ⎠ ⎣m ⋅s⎦ 1 ⎛ ρ′⋅ g ⎞ 3 Eq. 4.4-19: D* = ⎜ ⎟ ⋅ d 110 ⎝ ν 2 ⎠ τ ⋅ v ⎡ kg ⎤ Eq. 4.4-20: C′ = 0,0018⋅ 0 112 gr ⎢ 3 ⎥ ()ρF − ρ W ⋅ g ⋅ h ⋅ vs ⎣m ⎦ kg Eq. 4.4-21: ⎡ ⎤ 112 S& = vs ⋅ ()C − Cgr a ⎣⎢m 2 ⋅s⎦⎥ kg Eq. 4.4-22: ⎡ ⎤ 112 S& = vs ⋅ Ca ⎣⎢m 2 ⋅s⎦⎥

Eq. 4.4-23: m s = a ⋅ ()Q − Q 0 119 b Eq. 4.4-24: m s = a ⋅ ()Q − Q 0 119 b Eq. 4.4-25: m s = a ⋅ Q 119

Eq. 4.4-26: m Fs = 0,0864 ⋅ C ⋅ Q [t] 119

Eq. 4.5-1: m Ff = m FG + m Fs [t] 123 ∂ z Eq. 4.6-1: ()1− n ⋅ B + divq = s mit q = q , s = s 124 p s s ∑ si ∑ i ∂t i i

l F − lT Eq. 5.2-1: e L = 126 lT l − c Eq. 5.2-2: e = T 127 F c

lT − c Eq. 5.2-3: eT = 127 lT B h Eq. 5.2-4: Y = F and Z = , 131 h d

∂ z B ∂ q G ⎡m⎤ Eq. 5.4-1: (1− n) ⋅ + − s = 0 ⎢ ⎥ 146 ∂ t ∂ x ⎣ s ⎦

Fluid : kinematic viscosity ν, density ρW Bed material : particle size d, density ρ Eq. 6.6-1: F 158 Channel flow : water level h, bed gradient I Motion initiated by : acceleration g

Eq. 6.6-2: γ F = g ⋅ ()ρF − ρ W 158 XXII Sediment Sources and Transport Processes

* 158 Eq. 6.6-3: v0 = g ⋅ h ⋅ I * Eq. 6.6-4: ν,ρ W ,h,d,ρF , γ F , v 0 158 * Eq. 6.6-5: A = f (ν,ρ W ,h,d,ρF , γ F , v0 ) 159 v* ⋅ d Eq. 6.6-6: Re* = 0 159 ν 2 v* Eq. 6.6-7: Fr* = 0 159 ρ′⋅ g ⋅ d ρ − ρ Eq. 6.6-8: ρ′ = F W 159 ρ W h Eq. 6.6-9: ε = 159 d Eq. 6.6-10: π = ϕ(Re* ,Fr* ,ρ′,ε) 159

Eq. 6.6-11: Tr = L r 163

Eq. 8.2-1: h K ≈ 2,3⋅ D []m 204 ⎡m⎤ Eq. 8.2-2: d = 6 − 3,3⋅ v + 4 ⋅ v 2 []cm v in 204 ⎣⎢ s ⎦⎥ 1 1 ⎛ q 2 ⎞ 3 ⎛ ∆h ⎞ 2 ∆h Eq. 8.3-1: ⎜ ⎟ ⎜ ⎟ 205 h Ges = h K + h u = A ⋅⎜ ⎟ ⋅⎜ ⎟ = A ⋅ h c ⋅ []m ⎝ g ⎠ ⎝ d 90 ⎠ d 90 dv Eq. 9.1-1: τ = ρ ⋅ ε ⋅ x 207 t dy

λ τ τ ⎡m 2 ⎤ Eq. 9.1-2: ε = ⋅ h ⋅ 0 = 0,075⋅ 0 ⋅ h ⎢ ⎥ 207 8 ρ ρ s ⎣⎢ ⎦⎥ ∂C ∂ 2 C ∂C ⎡ kg ⎤ Eq. 9.1-3: v x ⋅ = εs ⋅ + vs ⋅ 207 ∂x ∂y 2 ∂y ⎣⎢m3 ⋅s⎦⎥ Q ⎡m⎤ Eq. 9.1-4: v 0 = 207 O ⎣⎢ s ⎦⎥ 2 λ v m ⎡m ⎤ Eq. 9.1-5: ε = ⋅ κ ⋅ h ⋅ ⎢ ⎥ 207 8 6 ⎣ s ⎦

Concentration()n +1 − Concentration ()n −1 Eq. 10.3-1: Concentrationn = Dischargen 219 Discharge()n +1 − Discharge ()n −1 Introduction 1

1 Introduction Exogenic processes such as precipitation, wind, temperature as well as vegetation are the agents causing the weathering, loosening, fragmentation and, finally, transportation of the materials making up the earth's surface. This starts in the high mountains, then continues at the medium levels and in the river plains, ending in a lake or ocean, where the material carried along by running waters usually settles as a result of the reduced flow velocity, to form such deposits as deltas. The delivered material comes from denudation, landslides, stream banks as well as extensive or local streambed degradation. Over time, these processes tend to flatten the earth's profile above and below seal level, that is, mountains are worn down while basins are being filled. The civil engineer may have a great share in these processes. Weirs constructed in running waters involve upstream sedimentation and downstream streambed erosion. Removal or addition of solids disturbs, or brings about, the transport equilibrium of a stream. The logical consequence is that the engineer dealing with hydraulic structures should study the problem of sediment transport in great detail to do justice to his responsibility of predicting as accurately as possible the impact of the measures he is planning (with support from the descriptive geosciences). Important information on the origin and transport of fluviatile solids has been contributed by such disciplines as geology, mineralogy, hydrology and in particular geomorphology. Water- solids flows have also gained some importance in other engineering disciplines, such as process engineering (solids transport in pipelines, segregation and mixing of fluids and solids), where theoretical treatment of these so-called multi-phase flows is usually easier, mainly because their parameters lend themselves better to accurate measurement. Sediment transport in natural channels involves additional problems arising from the interaction between streambed and flow field. Depending on the flow rate, the streambed may change as bed forms develop or change and lead to the formation of bed armouring, which in turn, by its roughness, affects the flow profile and the moment at which sediment transport begins. Likewise, the lateral channel geometry is subject to variation where adequate bank protection is absent. Another problem is the sediment with its variations in space and time, which needs several parameters – particle size distribution, density, particle shape –to enable classification. It is thus not surprising that thoroughly satisfactory solutions to the various questions relating to sediment transport are still lacking and that the relationships derived so far are still characterised more or less by empirical coefficients. Many of these come from simplified stationary two-dimensional laboratory investigations, and their application to natural three-dimensional conditions, although far from being ideal, often provides the only feasible solution to practical problems. Another difficulty is the fact that sediment transport is made possible, or severely affected, by turbulence, which is a very complex problem to characterise and formulate in terms of physics. The use of hydrodynamic-numerical models, however, is increasingly helping to make allowance for turbulence phenomena even in solids transport simulations. As a rule, natural channels are in a state of critical hydraulic-sedimentological equilibrium. Any disturbance from hydraulic-engineering or water-management measures may imply a multitude of consequences. Sediment transport is thus an important factor in fields such as: • Reservoir sedimentation • Streambed erosion, e.g. downstream from dams • Streambed alteration in the vicinity of structures (local scouring) • Wear and tear of turbines, pumps, screw propellers and fittings 2 Sediment Sources and Transport Processes

• Sand and gravel trap dimensioning, as for hydro power developments, irrigation canals or drinking water supply facilities • Reaction of the morphological environment to the transformation of the streambed and banks, as through near-natural stream development • Destabilisation or reduction of river deltas as a result of sediment retention above upstream dams (e.g. Nile Delta) • Maintenance of navigation channels and harbour basins, as through dredging • Near-coast sediment migration, coast protection Profound knowledge of sediment delivery and discharge is of particular significance for the design and the sustainable management of reservoirs. As mentioned above, sediment produced by the various degradation processes is delivered to running waters, which carry them to the reservoirs, where they end up by filling them up unless prevented more or less successfully by specific management methods. In the worst case, this process may take place within a relatively short period, thus restricting or even making completely impossible the multitude of purposes for which the development was built, such as flood protection, low- flow improvement, drinking and raw water production, electricity generation etc. Avoiding or minimising the risk of such failures already at the design stage is the aim of this publication, which presents in sufficient detail the bases of sediment transport in running waters and the resulting phenomena.

Sediment sources 3

2 Sediment sources 2.1 Forces acting on the earth’s crust Figure 2.1-1 below presents the so-called Main Geomorphodynamic System:

Figure 2.1-1: Main Geomorphodynamic System (Ahnert, 1996)

As demonstrated above, exogenous and endogenous forces act on the earth's crust. The exogenous processes (processes acting from outside, that is, from the atmosphere, on the earth's crust) include weathering, degradation and transport of loose material through water, ice (glaciers) and wind under the action of gravity. They cause the earth's relief to be reduced. By contrast, the endogenous forces tend to increase the relief. These refer to uplift, subsidence and displacement of crustal blocks as well as folding and volcanism (Ahnert, 1996).

2.2 Types of rock - weathering The material making up the earth's crust is termed rock. This consists of chemical elements, mainly (approximately 90 %) of oxygen, silicon, aluminium, iron and calcium. These in turn form minerals (e.g. quartz, mica etc). We distinguish three main types of rock: • Igneous rocks 4 Sediment Sources and Transport Processes

• Sedimentary rocks • Metamorphic rocks Igneous rocks (magmatic rocks) are formed by a molten rock-mass cooling down. While magma solidifies slowly underground, typically cooling as granite, lava, which solidifies above ground, produces fine crystals (basalt). The density of igneous rocks ranges between 2,600kg/m³ and 3,300kg/m³. Sedimentary rocks are formed as deposits of particles produced from rocks which are first weathered and then eroded. Consolidation of such particles (as loose sediment) under pressure and through cementation leads to the formation of sedimentary rocks (sedimentary rock). Loose sediments are subdivided according to particle size, into clay, silt, sand and gravel (Table 3.2-2). Plastic sediments are named according to components they mainly consist of (claystone, sandstone). Conglomerates are composed of fine-to-coarse pebbles, commonly rounded, and usually with a sandy matrix. Metamorphic rocks are formed through alteration of the above rocks under high pressure and/or temperature (schist, quartzite, marble). The cycle of rocks is shown in Figure 2.2-1 below.

Figure 2.2-1: Cycle of Rocks (GEO kompakt, 2004) Sediment sources 5

The main rock types occurring in Central Europe are shown in Table 2.2-1 below.

Table 2.2-1: The principal rocks and sediments in Central Europe, their substrates and bed loads (Briem, 2002)

Rocks and Sediments Substrate Bed load Weathering, Soils Prevailing types Origin and Chemistry Proportion of weathering residue (d Quantities and grain sizes > 2mm) QUARTZITES – from quartz veins in Quasi unweatherable weathering Are conserved even after long bedrock, also as quartz pebbles in residue in soils of the bedrock and transports and very long weathering sandstones coarser sandstones / conglomerates – periods – residual rock debris, residual in Germany: Buntsandstein pebbles, sands GRANITE – feldspars, quartz grains and "Grus" (angular debris material of Boulders, stones, sands – generally dark & light coloured micas – about 71% sand to fine-gravel size, resulting from little bed load SiO2,, sufficient ions and nutrients – bedrock physical weathering of granular rock) with unweathered quartz grains: stable, granular, loamy mass interspersed with boulders

GNEISS - + 60% SiO2,, Soils rich in fine-grained material with Very much extremely hard bed load: high proportions of angular weathering stones, boulders, hardly any sands – quartz veins common, sufficient ions and residue (mostly stones, also blocks) very high resistance to transportation nutrients – bedrock SCHIST: siliceous, Soils rich in fine-grained material with Much platy bed load: stones, boulders, high weathering-residue proportions of hardly any sands, resistance to varying SiO2 contents, sufficient ions and platy fragments transport highly varying but lower than nutrients – bedrock gneiss Hard VOLCANITES: porphyries, diabases, Soils rich in fine-grained materials with Much bed load: stones, boulders, basalts - + 50% SiO2,, rich in ions and high weathering-residue proportions of gravels, hardly any sands, high nutrients angular fragments transport resistance SANDSTONES – here: quartzitic (up to Light sandy soils with lower contents Large amount of sands, residual >98% SiO2) or clay-cemented siliceous of fine-grained materials and varying gravels (quartz gravels) – at high sandstones (Bunter and Middle Keuper), weathering-residue proportions: at levels much platy and blocky bed load, very poor in ions and nutrients - overburden high levels, very high on the slopes minor transport resistance, only and in quartzitic sandstones: platy quartzitic bed load resists longer fragments and blocks (usually quartzitic) CARBONATE ROCKS: limestones, Soils rich in fine-grained materials with Platy bed loads: stones, gravels, dolomites (MgCaCo3) mainly of shell varying, often high, weathering- hardly any sands, few boulders, often limestone and Jurassic (up to >80% residue proportions of angular and great amounts of fine-grained material, CaCO3), rich in ions and nutrients – platy fragments suspended solids, very low transport overburden resistance MARLS & MUDSTONES: up to 30% Deep, heavy soils with low proportions No or very little platy bed load, much CaCO3,, partly gypsic (CaSO4), also of fine materials or very low fine-grained material, suspended saliferous (CL+), very soft rocks of the weathering-residue proportions, platy solids, very low transport resistance Zechstein, Keuper and Tertiary – rich in ions fragments on steeper reliefs and and nutrients – overburden slopes TERMINAL MORAINES: in northern In northern Germany light sand In northern Germany many sands, German sandy, in southern Germany stony substrates, hardly any fine material, gravels, occasional boulders; in loose material, interspersed with coarser or podsols; in southern Germany subsoil southern Germany stones, boulders, extremely coarse rock fragments (erratic rich in weathering residual, mostly few sands boulders); greatly varying ion and nutrient brown earths contents depending on age and origin – glacial deposits GROUND MORAINES: In northern In northern Germany loams with In northern Germany little gravelly, Germany boulder clay, calcareous cohesive gravelly rock fragments (often flints); in sandy bed load; in southern German material, interspersed with rock fragments. southern Germany stony substrates, mainly stony bed load In southern Germany stony substrates – mostly calcareous soils (rendzines) varying, but usually high, ion and nutrient contents depending on age and origin – glacial, mainly calcareous deposits LOESS: aeolian sediment of silt grain size Fine-grained material – very little fine No or very little fine-sand bed load (0.002 – 0.063mm) with very high ion and sand in places nutrient content, up to 30% CaCO3 – glacial deposits 6 Sediment Sources and Transport Processes

Further details on the origins and properties of rocks are found in textbooks on petrography. Weathering refers to the mechanical and chemical alteration of rocks through climatic effects such as temperature, moisture, ice and chemical substances dissolved in rain, soil or ground water. Mechanical, or physical, weathering means that the condition (e.g. grain sizes) of rocks is changed, whereas chemical weathering alters their chemical composition. The two processes normally act simultaneously (Ahnert, 1996). The uppermost layer of weathered rock is called soil. The soil types are classified according to particle size (Table 3.2-2), such as sandy clay, gravelly sand etc.).

2.3 Denudation Processes 2.3.1 Types of Denudation The gravity-induced denudation processes can be classified according to the type of medium involved in the transportation, as shown in Table 2.3-1. Details and examples are given in Ahnert (1996). In this context, greater attention will be given only to "5 Spüldenudation" (wash erosion) below, that is, the wearing away and redeposition of regolith material through the action of running water on the land surface (overland flow). This often accounts for a large proportion of the total denudation rate and can be determined by empirical equations (giving the erosion rate). Actually, the drag force of the flow must be great enough to transport the solid particles. The impact velocity of the rain drops, depending on their size, greatly helps to wash the particles from the matrix (splash effect). Sediment sources 7

Table 2.3-1: Denudation processes (Ahnert, 1996), regolith = loosened rock material

1 Gravity-induced mass movements of rock and debris + Rockfall denudation (Sturzdenudation) + Boulder falls (Blockabstürze) + Minor rockfalls (Felsstürze) + Major rockfalls (Bergstürze) + Slides (Rutschungen) + Large-scale rockslides (Bergrutsche) + Boulder slides (Blockrutschungen) + Debris slides in coarse material (Schuttrutschungen in Grobmaterial) 2 Mass movements of the regolith, usually under the additional action of pore water, ice or snow + Debris flows (Muren) + Degradation through avalanches (Abtragung durch Lawinen) + Earth flow (Erdfließen) + Creep denudation (Kriechdenudation) 3 Regolith movement mainly due to frost action, usually with permanently frozen subsoil + Block glacier (Blockgletscher) + Stone rivers (Blockströme) 4 Removal of dissolved substances within the soil and ground water 5 Degradation and material transport through impinging rain and non-concentrated runoff of precipitation water + Splash ("Splash") + Wash erosion (Spüldenudation) 6 Degradation and transport through wind (deflation or aeolian denudation) 7 Degradation and transport through glacial ice action (glacial denudation)

2.3.2 Erosion Figure 2.3-1 is a schematic drawing showing the various erosion processes. The following types of erosion can be distinguished: • sheet or interrill erosion • rill erosion • gully erosion 8 Sediment Sources and Transport Processes

EROSION 1 Sheet and rill erosion 2 Erosion of minor channels 3 Gully formation 4 Erosion of flood plains 5 Streambed deepening 6 Bank erosion DEPOSITION A. Deposition at the foot of steep slopes (talus slopes) B. Deposition in the valley floor C. Formation of sand bars or banks D. Streambed silting

Figure 2.3-1: Erosion/denudation as well as deposition phenomena (Vanoni, 1977)

Ahnert (1996) explains the different varieties and manifestations as follows: "Moving at moderate flow rates, the water finds its way in linear paths between tufts of grass, stones and other irregularities, washing out small rills. Where new rills are formed or rills at least change their paths during each runoff event, the result is extensive degradation, which may be called sheet erosion. In actual fact, the water depth varies due to the surface irregularities of the soil even under a sheet of running water, which thus may also produce such ephemeral, or transitory, rills. Where a soil surface of major roughness and slope concentrates the runoff in narrow paths, longer-lasting rill systems form, and rill erosion predominates over sheet erosion. Under such conditions, rills once formed become preferred runoff paths during subsequent precipitation events and are increasingly deepened. In this case, each individual rill undergoes a definitely linear erosion process, but closely spaced rills result in sheet erosion. Where the water running off in a rill is overloaded with eroded material, the surplus is deposited in places within the rill itself and beyond its rims as a small fan-shaped sediment body or dispersal fan, which interrupts the line of lowest points. This leads to the formation of discontinuous rills. Sediment sources 9

Deep and wide rills eroded down to the saprolith (chemically-weathered subsoil) or bedrock are termed gullies. Their steep lateral slopes correspond as a rule to the maximum possible angle of slope of the respective material”. Sheet erosion is influenced by several factors, such as the type of rock or soil, the vegetation type and state, slope, length of the area under study as well as amount and intensity of rainfall. Figure 2.3-2 below is an example of how vegetation influences the yield factor of a catchment.

Figure 2.3-2: Influence of vegetation on yield factor (%) and erosion (tonnes per hectare). Annual averages calculated from a 50m² test area of sandy loam with a slope of 3.5% (Rapp et al., 1973).

The above factors can be used to derive erosion-reducing measures. A dense low vegetation cover or mulch may thus protect the soil from the impact of rain drops. Stones of major size on the ground surface may produce a similar result, while however reducing the rate of infiltration and thus favouring surface runoff.

2.3.3 Mass Movements Mass movements such as rock falls or debris slides etc. (Table 2.3-1) may have an important bearing on the sedimentation of reservoirs. These phenomena, however, defy mathematical formulation. Their occurrence and volumes can be described only on the basis of catchment observation and with the aid of geomorphological experience. Landslides become particularly hazardous when falling into in a reservoir lake (Vaiont/Italy, 1963, 260 million m³ of rock mass, about 2,000 death toll) or barring or narrowing valleys so as to risk forming debris dams (Sondrio/Italy, 1987), which rupture easily as a result of overtopping or seepage, thus risking the formation of extreme flood waves.

10 Sediment Sources and Transport Processes

2.4 Denudation (Denudation Rate) – Erosion 2.4.1 Definition In the German-speaking world denudation, or mechanical denudation, is understood to refer to sheet degradation through the weathering of loosened rock material (regolith). Erosion, by contrast, is meant to denote linear degradation, such as by streams or rivers and glaciers. Degradation occurs under the action of gravity (slope inclination), often aided by precipitation water, rarely also by wind. Ahnert (1996) thus differentiates the following land-surface formation systems: • Fluviatile system • Glacial system • Littoral system (the action of current, waves and tides on the shaping of coastal zones) • Aeolian system (wind erosion)

2.4.2 Denudation Rate, Erosion Rate The mean denudation rate, d, is defined as the long-term mean of rock volume (m³ or tonnes) transported from a defined area, related to the area drained and the unit of time. The measuring unit is metres per 1,000 years or millimetres per year, or cubic metres or tonnes per area drained x time. The mean relief, h, is the average of the differences in level, in metres, between the highest and the lowest points in quadrants of 20 km x 20km scattered over the respective catchment area (Ahnert, 1996). Table 2.4-1 below lists the mean reliefs and corresponding denudation rates of selected areas. This has been used to derive the regression relationship

⎡ m ⎤ Eq. 2.4-1: d = 0,0001535 ⋅ h - 0,011 ⎢ ⎥ ⎣1.000 a ⎦ which is shown plotted in Figure 2.4-1. Sediment sources 11

Table 2.4-1: Mean values of relief and denudation for different areas (Ahnert, 1996)

Figure 2.4-1: Relationship between the mean values of relief and denudation according to Table 2.4-1 12 Sediment Sources and Transport Processes

Several equations have been formulated for determining the sheet erosion of solids. The Universal Soil Loss Equation (USLE) by Wischmeier et al. (1960) and the Revised Soil Loss Equation (RUSLE) by Renhard (1994) were formulated for U.S.A. conditions (Vanoni, 1977, Section IV). This regression equation, consisting of factors, is

⎡ t ⎤ Eq. 2.4-2: D = E = R ⋅ K ⋅ L ⋅S⋅ C ⋅ P ⎢ ⎥ ⎣ 0,4ha ⎦ where E = mean annual erosion rate for a specific area. The meaning of the individual factors is briefly outlined below: R = Erosivity of precipitation, expressed as the kinetic energy of the maximum 30- minute intensity of the individual precipitation event K = Erodibility of the soil L = Topographic factor for the slope length S = Topographic factor for the slope inclination C = Influence of vegetation management as compared with fallow land P = Coefficient of soil cultivation as compared with the method of maximum erosivity These factors have to be determined for each specific scenario by empirical methods. The erosion or denudation values obtained from this relationship can, therefore, not simply be transferred to other regions (such as mountain landscapes). But they are a valuable instrument for such purposes as evaluating the impact of a change in land use on erosion behaviour or comparing different types of land. Value ranges for German conditions are found in Schwertmann et al. (1987). Corresponding values for Bavarian conditions have been determined by Auerswald (1992) from 22 catchment areas accounting for 10% of the area of Bavaria. However, the Alpine regions were not included, as the prinicipal causes determining erosion behaviour are different from the factors allowed for in Eq. 2.4-2. Hrissanthou (1987) developed an erosion model for the Lech basin above the Rosshaupten/Füssen reservoir. Comparison of the results with the values measured in the field showed good agreement. Empirical equations also exist for gully formation, but they naturally provide nothing but a rough estimate of the expansion rate of the gully (Morris et al., 1997, Section 6.5). Channel and bank erosion is determined by means of sediment transport formulas (Section 4.3), which will be discussed in greater detail below.

2.4.3 Sediment yield – Sediment Delivery Ratio The sediment delivery ratio is defined as the total sediment yield from a catchment, measured at a transverse or reference profile during a certain time span (Vanoni, 1977) or, according to German Standard DIN 4049, as the quotient of sediment load and above-ground catchment within a certain time span, expressed in terms of tonnes/km² or, for annual values, in tonnes/km²· a. As not the entire mass of eroded solids (denudation rate) is transported to a stream, finally to be deposited in a reservoir or the sea, the sediment yield is generally less than the total Sediment sources 13 volume of eroded material. Part of the eroded solids are deposited in intermediate areas as soon as gravity (slope) or the drag forces (discharge) are no longer sufficient to move the rock material. This material is found in places such as debris cones at the foot of mountains, in deposits in shallow water zones and aggradations in running waters as well as deposits in natural and artificial lakes. The change, or reduction, in sediment mass related to the area of the respective catchment, from the source to a measuring station, is termed the sediment delivery ratio, D, and is calculated using the relationship

m Eq. 2.4-3: D = Fa 100 []% d where 2 2 mFa = sediment yield at the measuring station [t/ km ] bzw. [t/ km · a] d = denudation rate [m/ 1000 · a] Mahmood (1987) compiled a worldwide catchment data collection, specifying size, mean runoff depth and sediment yield. Summer et al. (1996) compared denudation rates calculated by use of USLE with measured suspended sediment volumes for the Alpine catchment of the Austrian Danube and used the result for determining the sediment delivery ratio. This has increased over the past few years, mainly as a result of increasing maize cultivation.

Figure 2.4-2 below is a graph showing annual sediment yield, mFa, derived from a study based on 180 measurement results from Central European rivers with completely diverse catchments (Schröder et al., 1984). This allows ranges to be defined for characteristic regions.

Figure 2.4-2: Annual sediment yield versus catchment size, AE, and region (Schröder et al., 1984) 14 Sediment Sources and Transport Processes

Millmann et al. (1983) stated values of sediment yield for 62 river basins all over the world, down to their respective mouths in the sea. These values range between 5 tonnes for an Arctic region in the USSR and 28,000 tonnes per km² for a small catchment in Taiwan. Slymaker et al. (1972) studied sediment yields in Canadian mountain ranges. The results are shown in Table 2.4-2 below, distinguishing between bed and suspended loads. The table also lists denudation results. Corresponding data for European waters are shown in Table 4.4-1 and Table 10.3-1.

Table 2.4-2: Sediment load of rivers in the Canadian Cordillera (Slaymaker et al., 1972)

2.4.4 Sedimentation Rate VA The sedimentation rate is defined as the relationship between the annual volume of sediment deposition and the area of the respective catchment. Table 2.4-3 below lists a few sedimentation rates for Swiss lakes. Allowance should be made for the fact that, depending on the magnitude of the lake, not all solids are retained. The finer particles may leave especially a smaller lake through the outlet. In this case, the sedimentation rate is less than the volume of delivered sediment (trap efficiency, see Figure 7.2-6). Sediment sources 15

Table 2.4-3: Sedimentation rate in Swiss lakes (Vischer, 1981)

Analysis of the sedimentation data from 19 reservoir lakes in Switzerland has enabled a generally applicable relationship between soil erosion and catchment characteristics to be derived for Alpine river basins. The field investigations are best expressed by an erosion model determined by use of a regression relationship (correlation coefficient r = 0.75). The model is defined by the following equation for the annual erosion volume, VA, per unit of area (Beyer et al., 2000):

3 -9 0,607 0,091 6,042 0,154 ⎡ m ⎤ Eq. 2.4-4: VA = 566,045 ⋅10 ⋅ HSommer ⋅ EB ⋅ OV ⋅ ∆GL − 267 ⎢ 2 ⎥ ⎣km ⋅ a ⎦ where

HSommer = mean rainfall depth in summer (June to September) [mm]

OV = percentage of area free of vegetation [%]

EB = percentage of area covered by erodable soil [%]

∆GL = mean annual relative length change of glaciers [%] The data underlying this study can be seen in Table 2.4-4.

16 Sediment Sources and Transport Processes

Table 2.4-4: Parameters from 19 Swiss reservoirs used in Eq. 2.4-4 (Beyer et al., 2000)

Sedimentation rates for American and Austrian reservoirs are shown in Table 2.4-5 and Table 2.4-6, respectively. Figure 2.4-3 is a graph showing annual sedimentation and erosion rates for the barrages on the River Danube.

Sediment sources 17

Table 2.4-5: Sediment yields for several reservoirs in USA (Vanoni, 1977)

18 Sediment Sources and Transport Processes

Table 2.4-6: Sedimentation rates for Austrian reservoirs (Habersack, 1996)

Figure 2.4-3: Sedimentation, or erosion, rates in Austrian reservoirs on the River Danube (Habersack, 1996) Sediment sources 19

2.5 Measuring Sedimentation Rate Sediment yield can be determined by measuring sediment transport in running waters. Details are found in Section 10 below. The sedimentation rate is established by re-surveying the reservoir geometry and comparing the results with previous surveys. Correcting this by the sediment discharged from the reservoir and converting the deposited volume to mass, via density, gives the sediment yield. Measuring sediment yield by means of reservoir surveys affords several advantages over determining this quantity through sediment transport measurements in streams. • No need for costly and time-consuming sediment-transport measurements; • No need to measure during flood-flow periods; • Occasional reservoir surveys tend to be lower in cost than regular sediment-transport measurement; • Relatively high accuracy attainable thanks to up-to-date instruments (echo sounder, GPS); • Both suspended and bed load delivery, that is, the overall sediment transport, are measured; However, there are also some disadvantages to this method, which relativise its accuracy: • The dry-mass density of sediment is not easy to determine, because it increases over time as the material consolidates, and is not constant over the area of deposition. Suitable results from earlier surveys often not being available, this value must be estimated. • In reservoirs with relatively low sedimentation rates survey data are not accurate enough to enable the determination of sediment deposition over short periods (1 year). • For small reservoir-storage to annual-flow ratios (Figure 7.2-6) and low-level outlet works, sediment discharge from a reservoir can only be determined by continuous monitoring. By contrast, this is of minor importance for large reservoirs with high-level outlet works (trap efficiency). • Relatively short-term changes in sediment load cannot be measured by reservoir re- surveying. • Diversion or removal (by the use of gravel traps) of sediment upstream from the reservoir reduces accuracy of conclusions with respect to sediment yield from the catchment. • Organic sediment may also be produced within the reservoir. Further details are given in such literature as Morris et al. (1997, Section 7.3). For methods of surveying reservoirs, see ATV-DVWK (2001). Sediment delivery to a reservoir can also be determined via the theoretical or empirical determination of the relationship between discharge and sediment transport (sediment rating curve) (Section 4.4.9 below).

20 Sediment Sources and Transport Processes

2.6 Reducing Sediment Delivery There are two fundamental methods of reducing sediment delivery to a reservoir (Section 7.4 below): either by preventing erosion or by retaining sediment already transported by the tributary stream (gravel or sand traps). Morris et al. (1997, Section 12) abstract 10 principles from the innumerable possibilities of reducing erosion: • Any measure should agree with the respective soil, climate and terrain. • Disturbance of the soil matrix should be minimised in terms of magnitude of area involved and duration. • Unprotected soil should be protected by suitable in-situ measures. • The vegetation cover should by maximised. • Infiltration should be maximised to minimise surface runoff. • Sloping terrain should receive appropriate treatment to break up flow concentrations (terracing, drainages). • Drainage systems should be provided to divert concentrated runoff without erosion. • Sediment should be intercepted (sand or gravel traps) before reaching the reservoir. • Buffers in the form of wide vegetation strips should be provided between erosion-prone terrain and river or stream. • Any control measure needs careful planning, supervision and maintenance.

Sediment transport principles 21

3 Sediment transport principles 3.1 Introduction 3.1.1 Types of sediment transport Sediment transport in streams and rivers in terms of total load comprises the entirety of material carried along by the running water and is composed of dissolved load, suspended load including washload and bed load (Figure 3.1-1).

Figure 3.1-1: Schematic diagram of sediment transpor (Bechteler, 2004)

The dissolved load is produced by chemical disintegration (Section 2.2 above) and may even contain dissolved organic substances. As suggested by this term, these substances are dissolved and carried along by the water in quantities related to its flow velocity. The suspended load consists of solid particles kept in suspension by the fact of their density being lower than that of water or, as a rule, by turbulence or diffusion. They are transported at approximately the flow velocity of the water. The particle sizes range within the silt and clay fractions. In the case of higher flow velocities, or steeper gradients, sand is also transported in suspension (sand load). The bed load can include sand as well as gravel and boulders, also depending on flow velocity. Transport velocity is here considerably lower than that of the flow, especially for the larger particles. Bed load transport is intermittent. Whereas rivers in the plains carry the greater part of the solids as suspended load, bed load is moved in immediate contact with the bed and predominates in the upper stream courses. A distinction should also be made between transported bed material, containing particle sizes that also occur in the streambed, and washload, which consists of finer-grained material than that of the streambed and results from denudation, or sheet erosion (Section 2.4). Bed material can be transported both as bed load and in suspension, whereas washload is composed of particles swept in suspension through the cross section without contact with the bed (Figure 3.1-1). Bed-material transport (bed load and suspended bed material) is related to the processes taking place at the channel bottom (erosion, sedimentation, resuspension), and is a 22 Sediment Sources and Transport Processes function of both the hydraulic parameters of the channel (discharge, water depth, flow velocity, gradient) and the particle size and particle-size distribution of the bed material. The washload, by contrast, is not directly dependent on these parameters and is influenced by the processes characterising the catchment (precipitation, vegetation state, slope inclination, soil type etc.). Suspended load is primarily transported at the flow velocity of the stream, without contact with the bed. Bed load travels in contact with the streambed and, hence, moves at a much lower speed than the fluid. This makes it advisable to distinguish between bed and suspended loads and analyse them separately, this being justified by the fact that different measuring techniques are used for the two components.

3.1.2 Terms, parameters and dimensionless quantities Section 15 below lists terms and parameters used in this text. The movement of solid particles in a flow is determined by: • The fundamental physical quantities of the fluid: density D, kinematic viscosity <; • The fundamental physical quantities of the solid material, that is, the nature of the streambed at the micro scale: density ρF, particle diameter d or dch, particle shape, particle distribution, settling velocity vs; • Streambed configuration at the meso scale, with bed forms (ripples, dunes) or as flat bed; • Streambed configuration at the macro scale: banks, scours, bends, changes in cross section, changes in slope;

* • Flow dynamics: bottom shear stress τ0 or bed friction velocity v0 , distribution of flow velocity v and turbulence, or diffusion coefficient εt , or the dispersion coefficient εS across the discharge cross-section; • Flow field, particle movement (bed load, suspended load) and bed configuration (bed forms or flat bed) normally form a delicate dynamic equilibrium. Beyond that, the movement of the individual particles is like that of the water particles: turbulent and, hence, stochastic. The great number and interaction of these parameters as well as the difficulty of finding adequate definitions practically preclude the possibility of finding a universally applicable physical formulation of sediment transport. In fact, it is common practice to use dimensionless numbers as a practicable physical definition of empirical equations on a theoretical basis with the help of measurement results. Dimensionless numbers can express the force relationships governing a certain process. The most important characteristic numbers that have proved useful for representing universally applicable results of sediment-transport measurements are mentioned below by way of introduction:

2 v ⎛ v 2 ⎞ v* v 2 ⎜ ⎟ * 0 S Fr = ⎜= ⎟ Fr = Fr()vS = Froude numbers g ⋅ l ⎝ g ⋅ l ⎠ ρ′⋅ g ⋅ d ch ρ′⋅ g ⋅ d ch

v ⋅ d v* ⋅d v ⋅ d Re = Re* = 0 ch Re()v = S ch Reynolds numbers ν ν S ν Sediment transport principles 23

1 1 2 3 1 3 ⎡ * ⎤ 2 3 * ⎡ρ′ ⋅ g ⎤ Re ⎡Re ()vS ⎤ D = ⋅ d ch = ⎢ ⎥ = ⎢ ⎥ sedimentological diameter ⎢ 2 ⎥ * Fr v ⎣ ν ⎦ ⎣⎢ Fr ⎦⎥ ⎣ ()S ⎦ ρ − ρ ρ' = F W relative sediment density ρ W

vS z = * Rouse number β⋅ κ ⋅ v0 ρ′⋅g ⋅m G* = F sediment transport number *3 ρF ⋅ v0

* τcr Θcr = Frcr = Shields parameter ()ρF − ρW ⋅g ⋅dch

3.1.3 Range of application – limitations Most transport formulas are based on more or less idealised assumptions to account for the complexity of sediment transport and the inaccuracy involved in the measurement of many of the governing parameters and their interactions. These idealised assumptions are briefly defined below: Flow: • more or less steady and uniform, • critical checking is necessary before applying assumptions to tidal zones in order to allow for local conditions; Cross section: • channel with compact cross-sectional shape and only few cross-sectional changes; Flow section: • relatively straight alignment, no acute bends: Streambed: • relatively uniform, non-cohesive bed material, • simplified assumptions for armouring and bed forms; Sediment: • non-organic and non-cohesive, • washload is ignored; Local effects: • no allowance for local effects from pronounced secondary flow such as scours at structures or in stream bends. The above limitations decrease in importance to the extent that two or even three dimensional numerical flow models as well as fractional transport assumptions are used and individual effects are parameterised.

24 Sediment Sources and Transport Processes

3.2 Transport media 3.2.1 Water 3.2.1.1 Properties of pure water

The main physical properties of water in this context are density, ρw, and viscosity, υ, which are functions mainly of temperature and only insignificantly of pressure (Figure 3.2-1).

Figure 3.2-1: Density and kinematic viscosity of pure water plotted against temperature

3.2.1.2 Effects of ingredients 3.2.1.2.1 Types of water-solids movements Graf (1998) distinguished between three types of water-solids movements: • NEWTON's fluid: solids concentration C << 1 % density difference between mixture and pure water, ∆ρ << 16kg/m³. This includes sediment transport in the form of bed load and suspended substances. • Quasi NEWTON's fluid: C < 8%, ∆ρ << 130kg/m³. This includes sediment transport in concentrated suspension and turbidity currents. • Non-NEWTON's fluid: C > 8%. ∆ρ > 130kg/m³. This includes hyperconcentrated suspensions (water courses with very gentle slopes and high sediment concentrations consisting of fine sediments; example: Yellow River), debris flows (all particle sizes participate in the movement, short-term phenomenon on steep slopes S > 15°) and hyperconcentrated turbidity currents.

3.2.1.2.2 Low concentration Factors of particular relevance to the problems discussed here are salt and solids ingredients. Both density and viscosity rise along with an increase in salt concentration, as demonstrated byFigure 3.2-2 below. The density increase caused by the presence of suspended solids with a volume concentration C [%o /1000] can be determined by use of the relationship Sediment transport principles 25

⎡ kg ⎤ Eq. 3.2-1: ρ = ρ W ⋅ ()1− C + ρ F ⋅ C ⎣⎢m 3 ⎦⎥

(cf. Section 3.2.2.1.2.9, C = Cv).

Figure 3.2-2: Kinematic viscosity and density of saltwater as a function of temperature (the dotted lines are interpolations and extrapolations)

The influence of suspended particles on viscosity cannot be computed by mathematical means, but tests have shown that viscosity decrease initially – presumably as a result of turbulence damping – and then rises again for higher concentrations.

3.2.1.2.3 High sediment concentrations High levels of sediment concentration, as occur e.g. in debris flows, do no longer show a linear relationship between the wall shear stress and velocity gradient of a flow (Newton's fluid). These are generally Non-Newton's fluids with a different rheological behaviour. Debris flows can be differentiated into granular debris flows with a high percentage of coarse particles and viscous debris flows with a high percentage (< 10%) of fine-grained sediment (d < 0.04mm). These show a behaviour much like that of a homogeneous flow mass, usually with laminar flow. Flows with an even higher percentage of fine sediments can be termed mud flows, which may feature both laminar and turbulent flow. Greater details are given in Schatzmann (2005).

3.2.2 Sediment 3.2.2.1 Sediment properties 3.2.2.1.1 Single grain The main property characterising a particle is its diameter. As there are various different methods of measuring grain diameter, the results are not necessarily comparable. Usually the grain diameter is held to be equivalent to the mesh size of the last sieve which the particle has passed, a method derived from the sieving method of measuring particle size (Section 3.2.2.1.2.5 below). This, however, provides no information on the geometry (shape) of the grain. 26 Sediment Sources and Transport Processes

A more accurate characterisation is possible by introducing equivalent diameters. This is understood to mean the diameter of a sphere having the same physical properties as the irregularly shaped particle under study, for the same specific density. The main equivalent diameters are:

• sphere of equal volume dn (nominal grain diameter)

• sphere of equal surface do German Standard DIN 4049

• sphere of equal settling velocity dä (equivalent grain diameter) Only where the particle to be described is a smooth globe are the above equivalent diameters all identical. It has proved difficult in practice to determine equivalent diameters for individual particles. Grain diameters for gravelly and sandy material will, therefore, best be established by sieve analysis (Section 3.2.2.1.2.5 below). Various shape factors have been introduced to describe the shape of a particle in greater detail: Shape characteristic or shape factor (SF) (Section 3.2.2.2.3), sphericity (Sph) and roundness (Rou). Actually, as determination of these parameters is costly and time- consuming, this will not be discussed in greater detail here (Baier, 1996, Stückrath et al., 2004). Particle shape and its influence on settling velocity are difficult and complicated to determine in practice. For the purposes of process technology, equipment has been developed for measuring both the number of particles and usually also the equivalent sphere diameter for particles in the range between 1µ and 500µ via conductivity (Coulter Counter), light emission (Satorius), light diffraction (laser diffraction spectrometer) or shading (Table 3.2-1). In this case, a sample needs to be taken and analysed in suspended form at the laboratory, which might involve the risk of errors. Sediment transport principles 27

Table 3.2-1: Up-to-date measuring methods for determining grain diameter

Measuring method Physical principle Application

Image analysis Optical microscopy (electron microscopy) together with d > 1 µm (1 nm) digital photography followed by object recognition. Extinction measurement Optical single-particle measuring system – a particle hit by d > 1 µm light waves produces signal extinction corresponding to its size. Laser diffraction (d > λ) Particles in a parallel laser beam diffract the light at a solid General Mie theory: or angle that is a function of particle diameter 20nm < d < 2mm laser scattering (d < λ) (Mie/Frauenhofer theory).. Frauenhofer theory: d > 1µm Conductivity The resistance change during the passage of a particle 1 < d < 100µm through a capillary traversed by direct current is (Coulter Counter) proportional to the displaced volume.. 3d light scatter Measurement of the intensity of light spots generated, due 1nm < d < 3µm to Brownian movement, by two laser beams intersecting each other in the sample volume and fluctuating in time. Interpretation by use of the cross-correlation method.. Chromatography Separation of a material mix through the different 5nm < d 3µm distributions of its components between a stationary phase and a mobile phase. Ultrasound spectroscopy Different dampings of defined ultrasound frequencies. 20nm < d < 20µm X-ray adsorption A parallel bundle of x-rays measures concentration in a 0.1 µm < d < 300µm fluid, the solid particles settle, the decrease in concentration is continuously measured. The values measured are grain and settling-velocity distribution (Stokes Law).

The methods listed in Table 3.2-1 above yield the number of particles in defined size classes and the grain-size distribution of the particles, which are taken to be spherical. In general, optical/acoustic techniques may be used both in transmission and reflection methods. Reflection methods, which can be used for concentration ranges occurring in nature, are interesting for in-situ measuring equipment, while transmission methods are preferred for laboratory tests, where samples can be adequately processed by enrichment or dilution.

3.2.2.1.2 Sediment sample 3.2.2.1.2.1 General classification The classification practised in Germany in accordance with German Standard DIN 4022 is shown in Table 3.2-2 below. Figure 3.2-3 is a graph showing, by way of example, the grain- size distributions of bed load samples. Figure 3.2-4 compares different national grain-size classification systems. 28 Sediment Sources and Transport Processes

Figure 3.2-3: Particle sizes of bed load in various rivers: 1. Upper Rhine / Karlsruhe, 2. Lower Rhine / Emmerich, 3. Elbe / Geesthacht

3.2.2.1.2.2 Grain size distribution, characteristic diameter The particle size distribution of a sample can be described by the cumulative curve p(d) as plotted in Figure 3.2-5.

Table 3.2-2: Particle size distribution according to DIN 4022

Bezeichnung Kornstufen [mm] Blöcke (Steine) > 60 Grobkies 60 - 20 Kieskorn Mittelkies 20 - 6 (Gravel) Feinkies 6 - 2 Siebkorn Grobsand 2 - 0.6 Sandkorn Mittelsand 0.6 - 0.2 (Sand) Feinsand 0.2 - 0.06 Grobschluff 0.06 - 0.02 Schluffkorn Mittelschluff 0.02 - 0.006 (Silt) Schlämmkorn Feinschluff 0.006 - 0.002 (Clay) Ton (Feinstes) < 0.002 Sediment transport principles 29

Figure 3.2-4: Comparison of different national grain-size classification scales (Morris et al., 1997)

Figure 3.2-5: Cumulative curve of particle size distribution where d50 stands for the median value of this distribution. The definition of the mean particle diameter, dm , can be derived, after Mayer-Peter, Müller, from Figure 3.2-5 as the relationship

dmax d 1 Eq. 3.2-2: i d m = ∑ ∆pi ⋅ []m where d i = ⋅ ()d i + d i+1 []m dmin 100 2

where ∆pi stands for the percentage of particle size di identified by the median value of the respective interval. The mean particle diameter so defined implies that the shaded areas in Figure 3.2-5 are equal for a linear abscissa division. In practice, logarithmic abscissa division is preferred. 30 Sediment Sources and Transport Processes

The steeper the curve, the greater the uniformity of the grain-size distribution. This permits a degree of non-uniformity to be defined for an approximated Gauss distribution (S-curve), using the relationship

d Eq. 3.2-3: U = 60 []− d10 and the geometrical standard deviation

d Eq. 3.2-4: σ = 84,1 []− d15,9

Mainly for calculating bed load transport, but also for characterising bed roughness, it is necessary to determine a characteristic particle diameter from the given or measured particle size distribution. This characteristic particle diameter, dch, may vary according to the transport formula used and may be d50, d65 etc. (Section 3.3.2.2).

3.2.2.1.2.3 Sampling As the characteristic particle diameter is an essential physical parameter, both its definition and practical determination from measurements are of great importance. Rule 127 “Geschiebemessungen” regarding bed load measurement, issued by the German Association for Water Resources Management (DVWK, 1992a), makes relatively detailed allowance for this fact, in its Section 5 “Entnahme von Sohlmaterial” on the removal of bed material, providing special information on the choice of the removal site, removal from bed surfaces including the equipment commonly used for this purpose, removal from gravel banks and removal from bed forms. The required sample size is defined in German Standard DIN 18123 and listed in Table 3.2-3 below.

Table 3.2-3: Sample size for sieve analysis according to DIN 18123 (DVWK, 1992a, Table 6)

Estimated maximum grain size of soil sample Minimum sample size [mm] [g] 2 150 5 300 10 700 20 2.000 30 4.000 40 7.000 50 12.000 60 18.000

3.2.2.1.2.4 Sample splitting A sample needs splitting where it is too large for particle size analysis (screening, wet analysis). The sample-splitting method to be selected depends on the size of the sample. Table 3.2-4 below lists the most common methods and equipment to be used for this purpose. Where the sample is still too large after first splitting, the process must be continued until the required mass is reached. Sampling and sample splitting errors cannot be compensated for by increased efforts during the particle-size analysis (error propagation law). Sediment transport principles 31

Table 3.2-4: Sample splitting methods depending on the mass of the basic population

Mass of the basic population Method / equipment > 100 kg Sampling 100 kg bis 10 kg Cone crushing and quartering 10 kg bis 1 kg Stationary ripple splitters 1 kg bis 1 g Rotating ripple splitters < 10 g Sampling from pastes and suspensions

3.2.2.1.2.5 Sieve analysis (particle size > 0.06 mm) The sieve-analysis procedure is defined in German Standard DIN 18123. Several sieves are arranged on top of one another, their mesh widths decreasing from top down (Figure 3.2-6). The sample is placed on the top sieve, then each sieve is vibrated for a certain time, and the relative proportion of initial mass, mi, is measured. Using the relationships

mi Eq. 3.2-5: ∆pi ()d = n ⋅100 []% ∑ mi i=1 and

n Eq. 3.2-6: p()d = ∑ ∆pi ()d []% i=1 the cumulative curve of weight distribution, p(d), is determined (Figure 3.2-5) The particle-size characteristic in terms of sieve analysis is the mesh aperture of the sieves through which the particles have dropped last and been retained on the next smaller sieve. One problem involved in sieve analyses is the fact that the nominal mesh aperture of the sieves tends not to agree with the actual particle-size boundaries. Thus, the cumulative curves of the mesh widths for sieves of equal nominal mesh width may differ. In addition, the boundary for each individual sieve tends to change towards coarser sizes as screening proceeds. In fact, the sieve analysis, though a simple method, may become time-consuming where high accuracy is required. Small particles clinging to larger ones, and particles baked together call for wet screening. Sieve standards have been introduced in various countries, e.g. for Germany: Wire cloth for test sieves: DIN 4188 Hole screens (round, square) DIN 4187 Textile screen cloth DIN 4195 32 Sediment Sources and Transport Processes

Figure 3.2-6: Schematic drawing of a sieve set

3.2.2.1.2.6 Sedimentation method / pipette method (grain diameter < 0.06 mm) The sedimentation method of analysis according to DIN 18123 is used for particle sizes in the range 1µm < d < 60µm (Table 3.2-2). The criterion used in this method is settling velocity. The relationship between particle size, particle density and settling velocity is described by Stoke's Law (Eq. 3.2-14). The sediment sample is mixed with distilled water, and the density of the suspension is measured by means of an areometer at certain time intervals. The analysis is evaluated with the aid of a nomogram, which is also given in the DIN rule. It should be pointed out that in this case the particle characteristic (particle size), unlike the sieve analysis, is the equivalent diameter of a sphere having the same settling velocity, dä (Section 3.2.2.1.1)

3.2.2.1.2.7 Sediment density

The density, ρF , of a sediment particle (true density ρS according to DIN 1080) (particle density) is one of the most important characteristics of a particle population. The instrument most commonly used for its determination is the liquid pyknometer, whose application is described in standards DIN 12795 to 12797. The weighed dry mass of the sediment is put into a calibrated pyknometer at a given temperature, filled up with distilled water, deaerated and weighed. The quotient of dry mass and sediment volume gives the particle density, ρF, or ρS (DIN 1080).

This is distinct from bulk density, ρf (moist) or ρt (dry), which is the quotient of the mass of a soil sample (including voids) and the soil volume. By way of example, the densities for gravelly sand are: ⎡ kg ⎤ ⎡ kg ⎤ ρF = ρS = 2.500 to 2.600 and ρ t = 1.200 to 1.600 . ⎣⎢m 3 ⎦⎥ ⎣⎢m3 ⎦⎥ The difference results from the void content, n, which is the volume of the voids related to the total soil volume, expressed as a percentage. Sediment transport principles 33

The specific gravity, γF, is the ratio of the weight of a particle to its volume. The relationship between density and specific gravity is expressed as ⎡ N ⎤ γ F = ρF ⋅g ⎣⎢m3 ⎦⎥ The following soil characteristics have been determined for sediment from the Luzzone Dam in Switzerland (Sinniger et al., 2000). These are median values from depths of between 0 and 15 metres: Moisture content 53.6 %

Wet density ρf 1,713 kg/m³

Dry density ρt 1,121 kg/m³

Particle density ρF 2,834 kg/m³ Angle of internal friction β 32°

3.2.2.1.2.8 Angle of internal friction This is the angle the slope of dumped material makes with the horizontal just below the threshold of sliding. This angle differs between wet and dry materials. The particle shape has an important bearing on the angle of internal friction, as is also demonstrated by Figure 4.1-3. Cohesive material contained in dumped material may increase the angle of internal friction. A water-sediment mix behind a bottom outlet, for example, would form a slope corresponding to the angle of internal friction, β (or φ according to DIN 1080) when the outlet is opened and the mix is allowed to escape.

3.2.2.1.2.9 Sediment concentration There are three possible ways of defining sediment concentration in suspension (Morris et al., 1997).

The concentration by mass, Cm, is the quotient of sediment mass and the volume (Vol) of the sediment-water mix, written as

ρ ⋅ Vol ⎡mg kg ⎤ Eq. 3.2-7: C = F F or m ⎢ 3 ⎥ VolF + VolW ⎣ l m ⎦ where Volm is the volume of the water-filled pore space.

The concentration by volume, Cv, stands for the dimensionless quotient of sediment volume and mix volume.

VolF Cm Eq. 3.2-8: CV = = []− VolF + VolW ρF

Finally, the concentration by relative weight, Cw, is the weight of the sediment divided by the total weight of the water-sediment mix:

ρF ⋅ VolF 6 Eq. 3.2-9: C W = ⋅10 []ppm ρF ⋅ VolF + ρW ⋅ VolW 34 Sediment Sources and Transport Processes

3.2.2.2 Settling velocity 3.2.2.2.1 General

The settling velocity, vS, of a particle is the main characteristic for all problems related to suspended-sediment transport. The terminal settling velocity (final value of settling velocity after travelling over a certain acceleration length) of a particle in water at rest is a function of • defined geometrical particle dimensions (diameter)

• particle density ρF • particle shape

• physical charactistics of the carrier medium (viscosity ν, density ρw ) • ambient factors, such as wind, other particles etc. • particle surface (roughness) • particle concentration

3.2.2.2.2 Single sphere Most theoretical and experimental studies on settling velocity have been conducted on smooth spheres, which lend themselves to accurate description and comparison with particles of other shapes. For a fluid at rest, the terminal settling velocity can be derived from the equilibrium of forces between the resistance of a sphere immersed in flowing water

π ⋅ d 2 ρ ⋅ v 2 Eq. 3.2-10: F = c ⋅ ⋅ W S []N W D 4 2 and its weight force under uplift pressure

π ⋅ d 3 Eq. 3.2-11: G = ⋅ g ⋅ ()ρ − ρ []N 4 F W as

4 g ⋅ d ρ − ρ ⎡m⎤ Eq. 3.2-12: F W vS = ⋅ ⋅ ⎢ ⎥ . 3 c W ρ W ⎣ s ⎦

For the range of very slow and steady flow (Re < 0.1) around a sphere, Stokes, by solving the Navier-Stokes equation, was able to express the resistance coefficient as

24 v ⋅ d Eq. 3.2-13: c = for Re()v = S < 0,1 . W Re S ν

3 Using the values Re < 0.1, i.e. d < 0.05 mm for T = 12°C and ρF = 2,650 kg/m , the settling velocity can explicitly be calculated as Sediment transport principles 35

1 g ⋅ d 2 ρ − ρ ⎡m⎤ Eq. 3.2-14: F W vS = ⋅ ⋅ ⎢ ⎥ . 18 ν ρ W ⎣ s ⎦

As already mentioned in Section 3.2.2.1.2.6 above, this equation forms the basis for the sedimentation test. For Reynolds numbers to approximately 1.0 (d = 0.12 mm for otherwise equal properties) the approximation after Oseen

24 ⎛ 3 ⎞ Eq. 3.2-15: c W = ⋅⎜1+ Re⎟ for Re < 1 Re ⎝ 16 ⎠ can be used. For even higher Reynolds numbers (Re > 1), the resistance value must be determined by empirical means from test results. A simple equation has been formulated by Kaskas

24 4 5 Eq. 3.2-16: c W = + + 0,4 for Re < 2 ⋅10 Re Re

The characteristic curve of the resistance coefficient for spheres is shown plotted in Figure 3.2-7.

Figure 3.2-7: Resistance coefficient cW for spheres plotted against Reynolds number using test results and approximations (Vanoni, 1977)

Note the effect of carrier-medium temperature – via viscosity – on settling velocity as demonstrated by Figure 3.2-8. 36 Sediment Sources and Transport Processes

Figure 3.2-8: The effect of carrier-medium temperature on the settling velocity of quartz spheres (Vanoni, 1977)

3.2.2.2.3 Non-spherical particles In practice, particles are never exactly spherical, but show elliptical to platy characteristics. Consequently, their resistance coefficient is larger than that of spheres, which in turn reduces settling velocity. Figure 3.2-9 below is a graph showing, analogously to Figure 3.2-8, particle diameter (from sieving) plotted against settling velocity, for natural quartz particles (ρF = 2,650 kg/m³) using different shape factors

c Eq. 3.2-17: SF = a ⋅b where a, b and c are the particle dimensions in three axes, with c as the smallest axis. For further details on particle shape, see Section 3.2.2.1.1. Sediment transport principles 37

Figure 3.2-9: Sieve diameter plotted against settling velocity for different shape factors SF (quartz particles) (Vanoni, 1977)

An in-depth study on the shape and fall behaviour of natural stones, including a CD with a Rock Data Base, has been published by Stückrath et al. (2004). She et al. (2005) also documented the settling velocity of natural particles by means of an image processing system and used the results for developing an empirical equation.

3.2.2.2.4 Other influences Further factors affecting the settling velocity of individual particles are rotation, nearness to wall and bottom and, for larger particles (Re > 3 • 105), the roughness of the particle surface. Particular mention should be made of turbulence, which various empirical studies have shown to reduce settling velocity. Detailed quantitative results are, however, not available.

3.2.2.2.5 Particle swarms – particle concentration Particles in sedimentation processes occur in increased concentrations rather than in isolation, which implies the probability of mutual effects. The results of investigations on spherical- particle swarms can be transferred, adjusting conditions, to any particle shape. A small number of particles grouped together in a fluid will always sink as a bunch, and this will fall at a higher velocity than isolated particles. Where the spheres are evenly distributed throughout the fluid, the settling velocity, depending on concentration, will be lower than that of isolated particles due to the countercurrent induced by the swarm of spheres (hindered settling). Laboratory studies have proved the settling velocity to decrease by up to 30% for laminar particle flow (Re < 1.3) and concentrations of up to 9% (dry weight). The increased settling velocity assumes practical importance for the dumping of dredged material or the addition of bed load from hopper barges. The initial fall velocity, depending 38 Sediment Sources and Transport Processes on the density of the cone of dumped material and its extent, is much higher than that of an isolated particle. However, as the cluster of particles expands by dispersion along with increasing fall distance, its mean density and, along with it, its fall velocity, decrease (Figure 3.2-10).

Figure 3.2-10: Fall behaviour of barge-dumped material (Zanke, 2002)

Specific information, supported by the results from prototype tests, on the swarm behaviour of stone fills in water, including information on grain-size segregation and dispersion behaviour is given in Meng (2004).

3.2.2.2.6 Grain fractions Study of grain-size distributions (Figure 3.2-5) rather than of isolated particles yields settling- velocity distributions, normally plotted as distribution or cumulative curves p(vs) (Figure 3.2-12). In most cases, it is not possible to assign equivalent particle diameters to settling velocities, except for the diameter of a sphere of equal settling velocity, as the shape factors of particles in a cluster vary and the distribution of such shape factors is usually unknown. The cumulative curve, p(vs), will however be adequate for such practical purposes as sand trap design.

3.2.2.2.7 Flocculation The phenomenon of flocculation (organic) or aggregation (inorganic) should also be mentioned in this context. This means that several small particles (< 10 µm) lump together to fall as a larger particle at a higher velocity. Flocculation may in some cases substantially affect settling velocity as compared to a corresponding fraction of isolated particles (final sedimentation tanks). Sediment transport principles 39

Unlike mineral particles, it is difficult to determine or estimate the physical properties of flocs. The size of a floc tends to be a variable quantity, depending on both the chemical composition and the nature of its organic components as well as on the turbulence, or shear stress, of the fluid. Floc size increases along with concentration, but decreases along with increasing turbulence. The density of flocs, too, is difficult to determine. Flocs in high concentration tend to form fluid mud, which may generate density flows. Another factor affecting settling velocity comes from organic substances clinging to sediment. The fact that flocs are individual micro ecosystems with biological properties and processes that determine their structure and, hence, transport behaviour, has been formulated and presented systematically by Droppo (2003).

3.2.2.2.8 Measuring settling velocity Single particles are studied by taking a fluid-filled glass tube and recording the time taken by the particle to travel a predefined distance. Where the settling-velocity distribution of particle clusters is to be determined, the stratified-suspension (pipette analysis) or stratified- sedimentation method (Macro-Granometer balance) is a good choice. A special type of sedimentation balance, the Macro-Granometer, shown in Figure 3.2-11 below, of the Institut für Wasserwesen at the Munich University of the Armed Forces (Schrimpf, 1987) will be presented here. A very small sediment sample (about 5g), dependent on the particle size and obtained by sample splitting, is evenly distributed over a computer- controlled lamellar shutter arranged at the upper end of the tube. Opening the shutter releases the particles almost simultaneously. These now become distributed vertically according to their settling velocities, in the order of decreasing particle size. After passage through a water- filled sedimentation length of about 2m the particles arrive at a highly sensitive underwater balance. The increase in weight registered there is computer processed to give a cumulative curve p(vS) of settling-velocity distribution (Figure 3.2-12), which in turn can be used to derive the distribution density q(vS). With the aid of micro-sieving results from the sample, this may also serve to calculate the shape factor, SF (Eq. 3.2-17), of the particles. Inversely, if the shape factor, SF, is known, this may be used to calculate grain-size distribution. The sedimentation tube is suitable for studying quartz sands with a grain-size diameter range of 0.05 mm < d < 4 mm. The result of a settling velocity analysis is shown plotted in Figure 3.2-12. The influence of water temperature on settling velocity will be discussed later (Figure 3.2-8).

Figure 3.2-11: Schematic drawing of a Macro-Granometer sedimentation system (Schrimpf, 1987) 40 Sediment Sources and Transport Processes

Figure 3.2-12: Result of a settling-velocity analysis by use of a MacroGranometer - cumulative curve p(vs) and density curve q(vs) of distribution (Schrimpf, 1987)

3.3 Basic hydromechanics 3.3.1 Initial remark Discharge and sediment transport in near-natural running waters have long been and are still the subject of comprehensive research studies. In this way, the traditional unidimensional empirical computation methods have been developed into reliable physical aids for analysing the complex conditions characterising near-natural watercourses. Novel methods, unlike earlier – mainly empirical – methods, are capable of analysing in greater detail and quantifying by means of geometrical quantities the main influencing factors such as vegetation, bed forms and varying roughness. The following paragraphs will give some outline of up-to-date approaches. Their physical principles and context will be discussed to the extent needed to convey some basic understanding of the respective method. This understanding provides the user with the physical basis enabling him to adapt the calculations adequately to the great variety of tasks he or she is faced with in practice. Although hydraulics and sediment transport are closely interacting domains, solids transport should be regarded as a consequence of a flow process; which means that the hydraulic problems should be solved in the first place and only then should sediment transport be calculated, using the hydraulic quantities. In addition to the unidimensional methods mentioned above and discussed in the following, two- or three-dimensional models are used, which are always solved numerically. An up-to- date detailed compilation is given in ATV-DVWK (2003), “Feststofftransportmodelle für Fließgewässer” (sediment transport models for running waters).

Sediment transport principles 41

3.3.2 Flow resistance – roughness 3.3.2.1 Fundamentals A main factor in hydraulic analyses is the adequate determination of channel-wall roughness. This governs flow resistance (continuous head loss), which manifests itself as energy gradient or wall shear stress. Whereas for solving hydromechanics problems it is common practice to use the Prandtl/Colebrook equation (Eq. 3.3-13), where the so-called Nikuradse sand roughness, kS, [mm] (Table 3.3-1), is an equivalent quantity characterising surface condition, the Manning/Strickler formula (Eq. 3.3-11) containing the so-called Strickler roughness, kSt , [m1/3/s] is still in use in engineering hydraulics, especially for running waters. This, however, is permissible only for hydraulically rough conditions. Whereas resistance coefficients λ (Eq. 3.3-13) composed of different single resistances (Figure 3.3-3) can be summed to obtain a total value, this is not permitted for Strickler values kSt. Describing surface roughness is particularly difficult where the equivalent sand roughness alone is no longer an accurate enough expression of roughness structure. This is true of movable beds (bed load transport) and flexible roughness elements (vegetation). The Strickler equation (Eq. 3.3-11 is based on empirical studies and applies only in the -3 “hydraulically completely rough flow range”. Schröder (1990) gives kS/ D > 10 (D = hydraulic diameter = 4R). This allows the relationship between the two roughness values to be determined as

1 ⎡ 3 ⎤ − 1 26 m Eq. 3.3-1: 6 ⎢ ⎥ k St = 5,87 ⋅ 2g ⋅ k S = 1 6 ⎢ s ⎥ k S ⎣ ⎦

Sand roughness, kS, should be introduced with the dimension [m]. Strickler gave 21 as a constant, recent studies suggest 23.5. A survey compiled by Schröder (1990, Table 5.2) lists Strickler coefficients for a great number of characteristic surfaces. In this case, some allowance for the cross-sectional shape is possible via the shape parameter, f (cf. Eq. 3.3-13). In the international literature, the Manning value, n, is used for roughness instead of the Strickler value, kSt, with n being the reciprocal of kSt. Roughness values, n, for a great number of channel types are also given in Morris et al. (1997, Section 9).

3.3.2.2 Roughness of gravel beds Head loss for flow over flat surfaces of cohesionless material, a condition often approached by streambeds, can be analysed empirically by determining the equivalent sand roughness, kS, from the characteristic particle diameter (Section 3.2.2.1.2.2) of the bed surface, as indicated in Table 3.3-1.

It should be noted that the roughness coefficient, kSt , is not a constant, but a function of flow or water level. 42 Sediment Sources and Transport Processes

Table 3.3-1: Estimating equivalent sand roughness, kS, from the characteristic grain diameter of the bed surface (nach DVWK, 1997)

Author Equivalent Scope sand roughness kS

Einstein (1950) kS ≈ d65 Densely packed particles, perfectly flat surface

Meyer-Peter (1948) kS ≈ d90 Coarse-grained but flat natural streambed Garbrecht (1961)

Ribberink (1987) kS ≈ 3·d90 Mainly uniform material, uneven streambed disturbed by sediment transport

Engelund, Hansen (1966) kS ≈ 2·d65 -

Hey (1979) kS ≈ 3,5·d84 Streams and rivers carrying coarse bed load

Kamphius (1974) kS ≈ 2·d90 -

In contrast to Eq. 3.3-1, Strickler expresses streambed roughness as

1 21 ⎡m 3 ⎤ Eq. 3.3-2: ⎢ ⎥ k St = 1 6 ⎢ s ⎥ d 90 ⎣ ⎦ and, by comparing this with Eq. 3.3-1, arrives at

Eq. 3.3-3: k S = 3,6 ⋅ d 90 []m .

The roughness values so determined include not only the influence of the flat streambed consisting of the corresponding grain material, but also the resulting sedimentary structure. Movable beds assume a condition of maximum roughness and maximum absorbable shear stress. Dittrich (1998), by contrast, recommends the following:

kS = 3,5⋅d m for medium-coarse gravel

k S = 3,5⋅ d 85 for coarse gravel. For further information, see Section 3.3.2.4. Where the bed material consists of a wide range of distinctly nonuniform particle sizes, allowance has to be made for potential development of armouring (Section 4.1.4).

3.3.2.3 Roughness of steep channels – mountain rivers Gravel beds with slopes of a magnitude I > 8‰ show very broad grain-size distribution, which is frequently even bimodal in character. The coarse stones then tend to form steps across the direction of flow, followed by shallow pools. Such a structure is referred to as ripple-pool or step-pool sequence (Figure 3.3-1). Sediment transport principles 43

Table 3.3-2 shows the relationship between streambed morphology and the parameters slope and characteristic grain diameter. This also illustrates the transition from the ripple-pool or step-pool sequences to ramps as channel gradient and grain diameter increase.

Table 3.3-2: Characterisation of the bed morphology of mountain streams (Rosport, 1997)

44 Sediment Sources and Transport Processes

Such distinct features generate very complex flow patterns, and the resistance coefficient becomes clearly dependent on flow. Rosport (1997) concluded that where such structures existed the resistance coefficient was influenced not only by the particle-size distribution, but also by the Froude number. Laboratory investigations by Wang et al. (2004) have shown step-pool sequences form only where the sediment supply from upstream is less than the transport capacity of the stream. Such streambed configurations are of particular ecological importance as they ensure almost complete passage of organisms, especially during low-flow periods. Bed friction decreases as discharge increases. In general, step-pool sequences are factors that increase roughness and, hence, water depth, while reducing the bed shear stress as against a largely even bed, thereby reducing the erosion risk in the streambed. The experimental roughness values ranged between 35 < kSt < 15. Further publications regarding this problem come from Aberle (2000), Patt (2001), Schälchi (1991) and Egashira et al. (1991).

Figure 3.3-1: Plan and longitudinal section through a ripple-pool structure, left, and a step-pool structure, right (Schälchi, 1991)

3.3.2.4 Roughness of bed forms Only ripples may, strictly speaking, be regarded as wall roughness, their height being small in relation to water depth. By contrast, dunes do not obey the logarithmic law of resistance. In this case, as demonstrated in Figure 3.3-9, the form resistance should be superimposed on the grain resistance to obtain a total resistance value.

Below is a list of several approximate roughness coefficients, kS, after Zanke (1982):

flat bed, uniform bed material k S = d

flat bed, mixed particle sizes k S ≈ 2,5 ⋅ d 50 or d 90

ripple bed k S ≈ h ripple

dune bed k S ≈ h dune

bank vegetation (skin roughness) kS = f (vegetation and profile geometry)

The bed form heights, hripple and hdune, however, are not constant but functions of flow conditions and bed material (cf. Section 3.3.5.3). The flow resistance of bank and washland vegetation is discussed in greater detail in Section 3.3.5.4. Sediment transport principles 45

With knowledge of the bed form size (height H and length L), it is possible to calculate the equivalent sand roughness by the relation (Höfer, 1984).

H 2 Eq. 3.3-4: k = 10,5⋅ . S L

3.3.2.5 Roughness of vegetation For the effect of bank and washland vegetation on roughness, see Section 3.3.5.

3.3.3 Cross sections with uniform roughness 1 The following is based on the assumption of a steady-uniform flow condition, that is flow constant over time in a channel with a constant cross section. In such a channel the driving forces (weight component G · I) and the friction forces at the channel wall are in equilibrium. This approach is the basis of the Chezy formula, which can be regarded as the basic relationship between channel geometry (including roughness), flow velocity and channel resistance (DVWK, 1993b).

Figure 3.3-2: Forces acting on the fluid element

The equilibirum of forces (Figure 3.3-2) can be written as

Eq. 3.3-5: τ0 ⋅ U ⋅ ∆l = G ⋅ I = ρ W ⋅ g ⋅ A ⋅ ∆l ⋅ I [N] where sin ϕ ≈ tgϕ = I which yields the bed shear stress

⎡ N ⎤ A Eq. 3.3-6: τ0 = ρ W ⋅ g ⋅ R ⋅ I where R = ⎣⎢m 2 ⎦⎥ U or

1 This text is an excerpt from an article published in DVWK Mitteilung 25, of which a second edition is available since 2006, and is reproduced here with the author's approval (von Mertens, 1994). 46 Sediment Sources and Transport Processes

τ ⎡m⎤ Eq. 3.3-7: * 0 v 0 = = g ⋅ R ⋅ I ⎢ ⎥ ρ W ⎣ s ⎦

* v0 because of its dimension [m/s], is termed shear-stress velocity at the bed although it does not in fact characterise velocity, but the channel resistance, or the forces by which the flow acts on the walls (bottom, banks). For the purposes of dimension analysis, Brahms and De Chezy suggest the approach

* ⎡m⎤ Eq. 3.3-8: v m ~ v0 = g ⋅ R ⋅ I ⎣⎢ s ⎦⎥

From this follows the Chezy equation

⎡m⎤ Eq. 3.3-9: v m = C ⋅ R ⋅ I ⎣⎢ s ⎦⎥

The de Chezy coefficient, C [m1/2/s], is not constant, but is a function of channel roughness (height, shape, distance between the roughness elements) and the discharge cross section (shape, size). Out of the great number of approaches for quantifying the Chezy value, only that by Gauckler, Manning und Strickler will be mentioned here. Using

1 1 ⎡m 2 ⎤ Eq. 3.3-10: C = k ⋅ R 6 ⎢ ⎥ St s ⎣⎢ ⎦⎥ we obtain the Manning-Strickler formula

2 1 3 2 ⎡m⎤ Eq. 3.3-11: v m = k St ⋅ R ⋅ I ⎣⎢ s ⎦⎥ which, being easy to use, is the equation most commonly applied for calculating mean flow velocity. 1/3 The so-called Strickler coefficient, kSt, [m /s] (reciprocal n = Manning coefficient) includes the sum of all the resistances acting in the flow cross section under study (Figure 3.3-3), as is also the case for the Chezy coefficient. Use of the great number of values available in the relevant literature (Section 3.3.2.1) is, therefore, subject to the following limitations: • Due to its dimension, the Strickler coefficient is a function of channel size and water depth. • Superimposing individual resistances as is permitted for Eq. 3.3-13 below is not possible here. • The formula includes no allowance for viscosity (Reynolds number). Hence, use of the Manning-Strickler equation is limited to large Reynolds numbers, or the hydraulically rough range (Re* > 70). Unlike the above flow formulas, developed by largely empirical means, the logarithmic laws of flow are based on approaches of turbulence theory. Sediment transport principles 47

These are based on the Darcy-Weisbach equation

1 ⎡m⎤ Eq. 3.3-12: v m = ⋅ 8⋅ g ⋅ R ⋅ I λ ⎣⎢ s ⎦⎥ with the resistance coefficient after Prandtl-Colebrook-White

1 v ⎛ 0,628 k ⎞ Eq. 3.3-13: = m = −2,03⋅ lg⎜ + S ⎟ . * ⎜ ⎟ λ 8 ⋅ v 0 ⎝ Re⋅ f ⋅ λ 14,84 ⋅ R ⋅ f ⎠

This equation has been adopted from pipe hydraulics; the resistance coefficient, λ, can be taken from the Moody diagram as a function of Reynolds number and relative roughness. The shape coefficient, f, is unity for pipes with a circular cross section. Other profiles: wide rectangular channel (width to height > 25) f = 0.60 wide rectangular channel f = 0.74 compact rectangular or trapezoidal channel (width to height < 25) f = 0.83

The roughness coefficient, kS, corresponds to a characteristic particle diameter d, for a flat sand bed; the Reynolds number, Re = v · D/ ν, makes allowance for viscosity effects. In channel hydraulics, it is common practice to use a simpler version of the logarithmic law of flow, the Keulegan equation. This is derived from the Colebrook-White relationship by ignoring viscosity effects (rough channels) and using the above shape coefficients (no influence from the banks). The resulting roughness functions are • for trapezoidal cross sections

1 v m ⎛ R ⎞ Eq. 3.3-14: = = 2,03⋅ lg⎜12,27 ⎟ λ 8⋅ g ⋅ R ⋅ I ⎝ k S ⎠

• for wide rectangular cross section

1 v m ⎛ h ⎞ Eq. 3.3-15: = = 2,03⋅ lg⎜11,0 ⎟ . λ 8⋅ g ⋅ h ⋅ I ⎝ k S ⎠

Transformation then yields an equation analogous to the Chezy formula. The C value (trapezoidal cross section) results as

1 ⎛ R ⎞ ⎡m 2 ⎤ Eq. 3.3-16: C = 8⋅ g ⋅ 2,03⋅ lg⎜12,27 ⎟ ⎢ ⎥ ⎜ k ⎟ s ⎝ S ⎠ ⎣⎢ ⎦⎥ that is to say, the logarithmic laws of flow in fact also describe the equilibrium of forces and the relationship between flow velocity and channel resistance (only the C function being different from that used e.g. in the Manning-Strickler formula). 48 Sediment Sources and Transport Processes

The advantage of the logarithmic laws of flow mainly lies in the wide scope of this approach and in the physical importance of the roughness value, kS , as against the purely empirical Manning-Strickler coefficients, kSt , where the roughness value is not a measurable geometrical quantity following from the roughness structure of the surface on which flow takes place. Also, it should be pointed out once more that individual resistance coefficients (Eq. 3.3-13) from various individual resistances (Figure 3.3-3) can be summed to an overall value, whereas this is not permissible for Strickler values (Section 3.3.2.1).

3.3.4 Cross sections with non-uniform roughness The flow forces acting on a streambed (action) are in equilibrium with the resistance forces of the streambed (reaction). This simple statement was the basis of the flow laws after Manning- Strickler and Keulegan derived in the preceding Section. The underlying assumption was, however, that channel roughness and channel resistance are distributed approximately evenly over the bottom and banks. The overall resistance in a natural watercourse is, however, composed of a number of different fractional resistances. The banks and the bottom of a channel usually show different roughnesses, which in turn may consist of several components (Figure 3.3-3).

Figure 3.3-3: Composition of channel resistance (DVWK, 1993b)

Where bed forms (ripples and/ or dunes) are present at the channel bottom, the bed resistance results from the friction resistance of the rough bed assumed as flat (∆ particle resistance) and the resistance from ripples or dunes (∆ shape resistance). Several other resistances may also be present, e.g. due to bends, vegetation, bars, scours. These, however, have hardly been studied in quantitative terms and, hence, have to be ignored here. They may need to be determined by field measurements or allowed for by means of empirical values. The procedure, in the case of different roughnesses, will be explained by means of a trapezoidal cross section (Figure 3.3-4). Sediment transport principles 49

Figure 3.3-4: Influence areas after Horton (1933)

Horton (1933) suggests subdividing – theoretically – the discharge cross section into influence areas assigned to bank walls and bed. The boundary lines run at right angles to the isotachs (lines of equal velocity), thus preventing impulse exchange (transfer of forces) in terms of turbulence theory between the influence areas. The following statements are important for our further hydraulic discussion: 1 The driving weight component in the individual influence areas (i) and the flow resistance of the wall section assigned to it are in equilibrium, that is (cf. Figure 3.3-2).

Eq. 3.3-17: G i ⋅ I = τi ⋅ U i ⋅ ∆l

As demonstrated in Section3.3.3, this results (with G i = ρ ⋅ g ⋅ A i ⋅ ∆l ) in

⎡ N ⎤ Eq. 3.3-18: τ = ρ ⋅ g ⋅ R ⋅ I i i ⎢ 2 ⎥ ⎣m ⎦

and

* τi ⎡m⎤ Eq. 3.3-19: v o = = g ⋅ R i ⋅ I i ρ ⎣⎢ s ⎦⎥

2 The individual fractional cross sections obey the logarithmic law of flow, e.g. as a Keulegan equation (Eq. 3.3-14).

1 v m ⎛ R i ⎞ Eq. 3.3-20: = = 2,03⋅ lg⎜12,27 ⎟ λ i 8⋅ g ⋅ R i ⋅ I ⎝ k i ⎠

Note: After Einstein-Horton, vm,i ≈ vm and Ii ≈ I. According to Figure 3.3-4, i should be provided with the subscripts l, r, S for the lateral walls and bed, respectively. A relatively simple algorithm now results for the hydraulic analysis. With knowledge of all geometrical quantities of the stream (cross-sectional dimensions, gradient, roughness values of banks and bottom), discharge Q and bottom shear stress τS can be calculated as follows:

1 Estimate the mean velocity, vm.

2 Calculate the hydraulic radii, Ri, of the individual influence areas (banks, bed) by use of the Keulegan equations Eq. 3.3-14). The influence areas follow from Ai = Ri · Ui. 50 Sediment Sources and Transport Processes

3 Check: AG = Σ Ai, that is, the sum total of all influence areas should correspond to the overall cross sectional area. Where necessary, the calculation should be repeated from Item 1 using a new vm. 4 After termination of the iteration calculation the quantities to be found result as Q = vm ·AG resp. τS = ρ g RS I (where RS = AS / US, hydraulic radius of the influence area, AS, assigned to the bed).

3.3.5 Cross sections with bank and flood plain vegetation 3.3.5.1 Flow conditions In the 1980s, Deutsche Forschungsgemeinschaft (DFG) promoted several basic research projects dealing with the hydraulic conditions in streams lined with tree or shrub vegetation (banks, washland). The basic hydraulic processes in such streams will briefly be discussed here, using a trapezoidal channel with simulated slope vegetation (Figure 3.3-5). The above studies have demonstrated that the flow conditions differ perceptibly between streams with vegetation and vegetation-free watercourses. This applies in particular where the ratio of water depth to stream width is relatively large and the flow passes through the vegetation. The isotachs show that vegetation reduces the flow velocity considerably, not only near the slopes covered with plant growth, but also in the vegetation-free central cross section. This is due to distinct energy-dissipating swirl and recirculation flows involving intensive macroturbulent water and impulse exchange (Figure 3.3-6), which substantially reduces the velocity of the main current, while giving little acceleration to the flow along the slopes (due to the high flow resistances within the vegetation). As can be seen from Figure 3.3-5, the discharge capacity of the channel decreased by 60% to 65%, for the same gradient and water depth. Flow reductions of similar magnitude were also found by Felkel from tests with natural willow vegetation (cf. Rouvé, 1987).

Figure 3.3-5: Flow conditions in channels with and without vegetation (Bertram, 1985)

Figure 3.3-6: Macroturbulence resulting from bank vegetation (DVWK, 1991, Merkblatt 220) Sediment transport principles 51

Actually, the flow conditions in a stream lined with vegetation are extremely complex and difficult to analyse by methods of mathematics and physics. Over the past years, however, practical methods have been developed which enable the hydraulic analysis of such streams with a reasonable input of time and money. In the following, such a computation method will be discussed using a simple symmetrical trapezoidal cross section, assuming that the k values of the vegetation fringes are known or can be calculated. For analysing complex cross sections (shape, roughness, plant growth), see DVWK (1991, Merkblatt 220), BWK (1999) as well as the publications by Mertens (1989 and 2004). The paper DVWK (1993b, Mertens) had been revised and was published by DWA in 2006. This suggests a practical method for the following cases • discharge in fractional cross sections without vegetation • discharge in partial cross sections with vegetation • cross sections with bed form beds giving formulas and computation methods for practical use. The last of the above cases will be briefly discussed here (Section 3.3.5.3). A recent list of various studies on discharge analysis for natural channel cross sections is also found in Lehmann (2005, Section 4). 3.3.5.2 Partitioning of cross sections The flow conditions in the zones next to planted slopes differ fundamentally from those in the vegetation-free central cross section. While the vegetation-induced shape resistances dominate in the discharge zones situated in the vicinity of planted slopes, discharge in the central zone is governed by friction resistances (bed) and vegetation-induced macroturbulence. Allowance is made for this difference by partitioning the channel cross section by fictitious interfaces (Tl, Tr) at the vegetation fringes, into planted and non- vegetation subareas. The central cross section represents a "channel" bounded by the bottom and fictitious interfaces (Figure 3.3-7). The interface roughness (kT value) is quantified so that the friction resistance of the area corresponds to the macroturbulence-induced flow resistance.

Figure 3.3-7: Partitioned cross section (DVWK, 1993b, Mertens)

3.3.5.3 Cross sections with bed form beds Streams with movable beds are characterised by a close mutual relationship between flow processes and sediment transport. Very conspicuous manifestations of the close interrelations between water and sediment are the bed forms (ripples, dunes, antidunes) 52 Sediment Sources and Transport Processes developing at the bottom under the action of flow (Figure 3.3-8). These wave-like forms in turn affect the flow conditions by a distinct increase in bed resistance.

Figure 3.3-8: Bed form development after Gehrig (1981)

The close interrelations between bed forms, flow velocity and bed shear stress can be seen from Figure 3.3-9. The streambed (sand) is relatively flat for low flow velocities; the bed shear stress results from the grain roughness of the sediment only (τS = τK). As the flow velocity increases, ripples or dunes develop which considerably increase flow resistance; now the shear stress, τS is composed of a particle component, τK , and a shape component, τF. Then follows a transition zone with flattening bed forms (decreasing shear stresses, τS ≈ τK). Relatively high flow velocities finally generate standing waves and antidunes, which in turn result in an increase in shear stress: τS = τK + τF (see also Figure 3.3-9). Sediment transport principles 53

Figure 3.3-9: Bed shear stress plotted against flow velocity and bed form, τK = particle resistance, τF = shape resistance (Figure 3.3-3) (Engelund et al., 1982)

As suggested in the English technical literature, three flow conditions are defined as functions of bed form shapes (Figure 3.3-3), which are classified below: • lower flow regime (flat bottom, ripples, dunes) • transition regime (flattening bed forms) • upper flow regime (flat bottom, standing waves, antidunes) Analogous to the relationship between flow velocity and bed shear stress as shown in Figure 3.3-9, there is also a relationship between flow velocity and water depth (Figure 4.2-8). Resistance functions describing the variable bed forms and the resulting bed shear stresses have been developed by such authors as Engelund (1966), Brownlie (1983) and Karim et al. (1999). The Brownlie formula for the lower flow regime (ripples, dunes) is given below by way of example. The resistance relationship for a sand bed can thus be written as

* 1,889 0,2655 0,3034 Eq. 3.3-21: FrS = 0,05604⋅Frg ⋅I ⋅σg

* R S ⋅ I where FrS = dimensionless water depth, ρ′⋅ d50

v m Frg = dimensionless flow velocity, ρ′⋅ g ⋅ d50

d84 σg = and d16 54 Sediment Sources and Transport Processes

ρ − ρ ρ′ = F W . ρW (For further formulas and their scopes, see DVWK (1993b, Mertens)). The hydraulic analysis corresponds to the methods described in Sections 3.3.4 and 3.3.5 above, except that the streambed resistance is calculated after Brownlie or similar rather than Keulegan.

3.3.5.4 Summary

As an alternative to the Keulegan equation (using a kS value), streambed resistance can be quantified with the help of a resistance function after Brownlie, Engelund or Karim/ Kennedy, which makes allowance for the variable flow resistance of a streambed with bed forms (ripples, dunes, antidunes etc.) (cf. Mertens, 1994). Examples of analysing discharge through cross sections of various shapes and roughnesses as well as with vegetation and bends are also given in the annex to Dittrich (1998). Further literature in this context includes Millar (2000) and Darby (1999) as well as Mertens (2004). A documentation of numerical simulation models for running waters has been published recently by Bloß et al. (2005).

3.4 Measuring channel roughness 3.4.1 The problem Adequate determination of channel roughness with all its aspects (Figure 3.3-3) is of particular practical importance, being an essential basis for computing such quantities as channel capacity (flood safety) and sediment transport. However, as the preceding Sections have suggested, the problem is extremely complex. To make things worse, the overall resistance does not lend itself to representation as a static quantity but, as a result of the dynamic processes of a natural stream, is a function of such quantities as the flow hydrograph (preceding discharge conditions, flow dynamics) and the state of vegetation within the flow cross section. This is roughly expressed by the great number of parameters governing sediment transport, as shown in a simplified manner in Figure 6.9-1. A few hints will be given in the following paragraphs to help determining channel roughness in practice.

3.4.2 Direct measurement from stream bed parameters As can be seen from Figure 3.3-3, several types of resistance make up the overall resistance value of a streambed. Particle resistance is relatively easy to derive from sediment samples by use of the formulas given in Section 3.3.2, and so is bank resistance without vegetation. A more expensive and time-consuming, but more accurate, method of measuring streambed roughness in nature is plotting accurate digital contour models by laser scanning. By contrast, shape resistance where present can only be estimated by longitudinal streambed depth-sounding (Figure 4.2-6). The same applies to other potential fractional resistances (bends, scours, islands). They can be determined only approximately using empirical formulas, which are not given here. In practice, one will have to content oneself with the determination of bottom roughness from the grain-size distributions of samples taken at characteristic locations in the stream. Where grain-size distributions cover a wide range, account should be taken of the formation or destruction of armouring layers. Sediment transport principles 55

3.4.3 Indirect measurement from discharge and river characteristics Section 3.3.3 above lists different empirical and theoretical flow equations for flow cross sections of uniform roughness. For example, measuring discharge in a channel of known transverse profile as well as water level and gradient using the Manning-Strickler equation (Eq. 3.3-11) enables derivation of the Strickler coefficient, kSt. This includes all types of channel resistance in the section under study. Strictly speaking, however, this roughness value only applies to the discharge underlying this procedure. It is, therefore, advisable to apply this method to several characteristic flows. This is needed in particular when the flow behaviour changes perceptibly as is the case e.g. when the stream overflows its banks and washland is flooded.

3.4.4 Literature and experimental values As mentioned in Section 3.3.2.1 above, the relevant literature offers experimental data for a multitude of characteristic stream sections. This is a simple and practicable help in preliminary investigations.

56 Sediment Sources and Transport Processes

Sediment transport 57

4 Sediment transport 4.1 Incipient motion 4.1.1 General Calculating sediment transport requires knowledge of the conditions under which sediment particles start moving. But the great number of parameters governing incipient motion as well as the fact that their observation is necessarily subjective do not permit exact deterministic definition. As discussed in the preceding sections, not only is computation of the respective flow conditions based on idealised assumptions (e.g. logarithmic velocity distribution), but sediment properties are extremely difficult to describe accurately and include a multitude of elligible coefficients, extending from particle diameter to particle shape, particle-size distribution and degree of compactness. Thus, whilst rivers in plains tend to show relatively homogeneous bed material (Figure 3.2-3, Curve 3), sediment found in mountain streams at medium and high altitudes is usually characterised by a wide range of particle sizes (Figure 3.2-3, Curve 1), which tends to favour the formation of specific features such as armouring (Section 4.1.2). These are factors that have variable effects on onset of motion, depending on the preceding hydraulic history of the respective stream.

4.1.2 Relatively uniform bed material 4.1.2.1 Probability of incipient motion Onset of motion is not an instantaneous process but, as illustrated by Figure 4.1-1below, is in fact a fluid transition. Both the attack of flow, i.e. the effective bed shear stress, τ0 and the resistance the bed particles offer to the flow, i.e. the critical bed shear stress, τcr are quantities characterised by statistical distribution (turbulence as well as particle size, shape, density and compactness etc.).

Figure 4.1-1: Probability distribution of the shear stresses at threshold of motion

The smaller the distance between the two mean values of shear stress, τ0 and τcr , the more intensive is transport. This begins, for the very small overlap of the two distributions shown in Figure 4.1-1 above, with individual particles starting to move, then becomes selective and finally develops into full transport where the two distributions show maximum overlap, or where the bed shear stress, τ0, generated by the flow field distinctly exceeds the threshold for incipient motion, τcr.

58 Sediment Sources and Transport Processes

It is common practice, however, to replace this stochastic phenomenon by deterministic approaches, which calculate a threshold value, τcr between the condition of rest and that of motion.

4.1.2.2 Definition of incipient motion As for many problems encountered in natural and engineering sciences, the laboratory experiment is imperative for the study of the main principles of sediment transport. In fact, the formulas describing incipient motion have in most cases been found through systematic experiments in channels under controllable and repeatable conditions. Vanoni (1964) tried to classify conditions of incipient motion, studying a small bed section and counting turbulent bursts per unit time. He defined the terms: rest, negligible, small, critical, general. Except for “rest”, each condition may represent incipient motion, although the difference in flow velocity between minimum and maximum particle movements exceeds 50%. It is useful, therefore, not to describe incipient motion by an exact numerical value, but define a range of conditions conducive to onset of motion. Recent studies define the motion risk, R, according to the explanation given in Section 4.1.2.1 above (e.g. Figure 4.1-4). The practical importance of incipient motion has engendered a multitude of studies containing appropriate criteria (Hrissanthou et al., 1995). Several parameters may be used for defining the threshold of motion:

• critical velocity vcr

• critical shear velocity τcr

• critical water depth hcr

• critical slope Icr

• critical discharge Qcr or qcr The following paragraphs discuss in greater detail the critical velocity and the critical shear stress for incipient motion. The other threshold values are then easily derived from these.

4.1.2.3 Critical velocity The relationships containing the mean flow velocity and a characteristic particle diameter – both being parameters relatively easy to measure or calculate – lend themselves best to application in practice. The best-known graphical representation is the empirical HJULSTRÖM Diagram (1935) derived for largely flat streambeds. Sediment transport 59

Figure 4.1-2: Mean critical velocities after HJULSTRÖM (Zanke, 2002)

An empirical relationship derived by Zanke (2002) gives values similar to those of Hjulström:

⎛ ν ⎞ Eq. 4.1-1: vcr = α ⋅ ⎜ ρ′⋅ g ⋅d + 5,25 ⋅ c⎟ ⎝ d ⎠ where vcr = critical mean flow velocity for incipient motion [m/s] α = 1.5 (lower limit) or 2.8 (upper limit) The coefficient c is an allowance for cohesion; c = 1 for natural sands. The above formula is valid for water depths of between 0.7m and 2m (approximately 1.4 m on average). For larger depths, the velocity determined by use of the above equation can be multiplied by the factor

1 6 ⎛ h vorh. []m ⎞ ⎜ ⎟ , ⎝ 1,4 []m ⎠ which means that the critical velocity increases along with water depth.

4.1.2.4 Effective critical shear stress 4.1.2.4.1 Shear stress for non-uniform flow

* Bed shear stress, τ0, and velocity, vo , were derived in Section 3.3.3 as Eq. 3.3-6 and Eq. 3.3-7, respectively, for normal flow. For steady non uniform flow and accelerated motion,

⎡ N ⎤ Eq. 4.1-2: τ = ρ ⋅ g ⋅ h ⋅ I − I − I ⋅ρ ⋅ v2 W m ()bed surface level surface level W m ⎢ 2 ⎥ ⎣m ⎦ applies. With reversed signs the formula is valid for decelerated flow. Thus, for accelerated flow the shear stress becomes smaller and, for decelerated flow, it becomes greater relative to the condition of uniform flow. 60 Sediment Sources and Transport Processes

4.1.2.4.2 Effective shear stress for banks Equilibrium can be used to derive the stability of a particle in a bank and, hence, the natural angle of slope, β. Neglecting the lift force and effects of secondary flow, the relationship of the shear stresses acting on the slope, τB and τS, can be derived for an angle of slope α as

2 2 τB tan α sin α Eq. 4.1-3: = cosα ⋅ 1− 2 ≈ 1− 2 τcr tan β sin β

Detailed information on the influence of structures provided in a channel and the resulting forced non uniform flow as well as the shear stresses involved is given in Schiereck (2002, Section 3.4). A relationship between the natural angle of slope, β, and grain diameter for non cohesive material is depicted in Figure 4.1-3 below.

Figure 4.1-3: Natural angle of slope plotted against particle diameter and particle shape

4.1.2.4.3 Critical shear stress – incipient motion As in the case of the relationships derived from the threshold velocity (Section 4.1.2.3) for onset of motion, there are a number of more or less empirical approaches for computing critical shear stress. The most widely used basis universally applicable for studying onset of motion is the diagram developed by Shields (1936), which is shown as modified by Zanke (1990) with indication of motion risk, R, in Figure 4.1-4 below. Sediment transport 61

Figure 4.1-4: Shields Diagram (1936) (after Zanke, 1990) for different motion risks, R

Shields used his extensive laboratory tests (different grain sizes, densities, shapes for a uniform distribution and a flat bed) and theoretical considerations regarding a relationship between flow and friction forces for a single particle to derive a relationship between the solids Froude number, Fr*, and the solids Reynolds number, Re*, for incipient motion. This neglected the so-called lift force. In the hydraulically rough range (strictly speaking, from Re* * * * > 300 × D ≈ 150 → dch ≈ 0,6cm), Fr is no longer a function of Re , which means that in this case only pressure forces are effective (Section 3.1.2). It is common practice to assume the hydraulically rough range to start already at Re* > 70. In the hydraulically smooth range (Re* < 2) Fr* becomes 0,1 Fr* ≈ , Re* which means that the viscosity forces are here the dominant factors. Between the two ranges is a transition zone, which applies in many practical cases. The threshold for fully rough flow, using the solids Froude number, becomes

* τcr Eq. 4.1-4: Θcr = Frcr = ()ρF − ρ W ⋅ g ⋅d

This number is also termed critical Shields parameter or dimensionless shear stress. This is, according to Figure 4.1-4, a function of the particle Reynolds number, Re*, which stands for the relationship between particle diameter and thickness of the laminar sublayer.

4.1.2.4.4 Critical shear stress for rough flow For a fully rough bed, the laminar sublayer becomes practically insignificant. Flow resistance is solely a function of turbulence and hence the Shields parameter assumes a constant value. For the steep particle-size distribution curve Shields used in his tests, the characteristic particle diameter can be replaced by the d50. Various different assumptions exist regarding * this critical parameter (dimensionless shear stress, Frcr ). Figure 4.1-5 below is a diagram illustrating the different types of transport. Selective transport means that only the fine fractions of the bed material are in motion. This implies 62 Sediment Sources and Transport Processes that the grain-size distribution of the bed material differs from that of the moving bed load. The coarser particles at rest accumulate in the upper bed layer, thus augmenting their stability against erosion, which in turn reduces sediment transport (armouring). Partial transport describes a condition where all particles are in motion, but the grain-size distribution of the moving bed load is still finer than that of the bed material. This is the range where motion is initiated (Section 4.1.2.2), and it does not decrease again until the slope changes as a result of erosion or due to a change in roughness. Finally, in the case of full transport, all the fractions of the bed material are in motion. Shields determined onset of motion by extrapolating the relationship between Fr* and bed * load transport m& G to zero. This gave a value Frcr = 0,03. However, for practical applications, * he recommended using the value Frcr = 0,06, which Günter (1971) defined as about 50% of the particles already in motion (cf. Figure 4.1-4).

Figure 4.1-5: Definitions of critical dimensionless shear stress (Schöberl, 1990)

* Using Frcr = 0.047 as recommended by Meyer-Peter/Müller (MPM), the critical bed shear stress in the hydraulic range becomes

⎡ N ⎤ Eq. 4.1-5: τgr = τcr = 0,047 ⋅ ()ρF − ρ w ⋅ g ⋅ d m ⎣⎢m 2 ⎦⎥

Experience suggests that even shear stresses inferior to that calculated after Schields (Eq. 4.1-5) induce some minor transport (cf. Figure 4.1-5). This may partly be a result of neglecting the lift force. The studies by Zanke (2001), however, include these force effects. It follows that Shields’ threshold shear stress for absolute incipient motion is slightly too high. A comprehensive survey of the great number of studies relating to onset of sediment transport was prepared by Buffington et al. (1997). Except for the approach by MPM, the studies mentioned so far have all been conducted for steep grading curves (single-grain material). Calculation of critical shear stresses for particle mixes liable to form armouring should use different approaches as given in Section 4.1.4 (Table 4.1-5 to Table 4.1-7). Sediment transport 63

4.1.2.4.5 Critical shear stress for steep channels

* As recommended by van Rijn (1987), the critical Shields parameter, Frcr , when used for steep channels should be reduced by a factor sin()β − α

sin β where α is the channel slope and β is the angle of internal friction. Other recommendations are given in Schiereck (2001, Section 3). Dittrich (1998, Section 4.4) concluded from studies by other authors that the critical Shields parameter in steep streams (smaller water depth with the different velocity profile involved) was a function of the relative cover depth – water depth / particle diameter – above the armouring layer, the gradient and * the dimensionless head loss due to roughness. Frcr increases substantially from a gradient of about >5%, reaching values of up to 0.11.

4.1.2.4.6 Special aspects 4.1.2.4.6.1 Biological stabilisation Recent investigations have shown that the initial flow velocities required for incipient motion may increase considerably as a result of so-called biological stabilisation. This particularly applies to zones tending to fall dry at regular intervals (tidal mud flats) or also to streams or lakes with water levels low enough at times to allow biological growth to develop at the bottom (Figure 4.1-6).

Figure 4.1-6: Incipient motion for biologically stabilised beds (Führböter, 1983) Left:Measured values plotted in the Shields Diagram (Figure 4.1-4) Right:Schematic drawing showing the effect of biological stabilisation on sediment transport

64 Sediment Sources and Transport Processes

4.1.2.4.6.2 Bed forms Sediment transport starts much later on stream bottoms covered with bed forms. Even without going into the details of the complex resistance behaviour of shape roughnesses (Figure 3.3-3), it is fairly evident that higher initial velocities are needed for a particle on the loo side of a dune to be set in motion (Figure 4.2-5). This means that the critical bed shear stress, characterised by the Fr* coefficient, is higher for a deformed bottom than for a flat bed. This has been established by experiments conducted by Höfer (1984) (Figure 4.1-7).

Figure 4.1-7: The influence of bed forms on onset of erosion (Höfer, 1984)

4.1.2.4.6.3 Waves The conditions conducive to incipient sediment transport in the case of waves, that is short- phase flow oscillations, are about the same as for constant flow conditions. This problem may be approached either by using the maximum orbital bottom velocity, vB, generated by the wave as a characteristic quantity in relation to the sediment particle diameter, or by * developing formulas for calculating shear stress, or shear stress velocity, vo , for expressing this phenomenon by use of dimensionless numbers. Sediment transport 65

Bonnefille & Pernecker et al. (1965) have proved the D*-Re* criterion for flows of constant direction also to apply to incipient transport in the case of waves. The authors used the following approach for the shear-stress velocity

0,25 ⎛ ⎞ ⎜ ⎟ ν ⋅ H 2 ⎡m⎤ Gl. 4.1-1: v* = 2,2 ⋅ ⎜ ⎟ o ⎜ ⎟ ⎢ ⎥ 3 ⎛ h ⎞ ⎣ s ⎦ ⎜ T ⋅sinh ⎜2 ⋅ π ⋅ ⎟ ⎟ ⎝ ⎝ L ⎠ ⎠ where H wave height [m] L wave length [m] T wave period [s] Given the fact that particle diameters of streambeds in the coastal and tidal zones of the oceans tend to be fairly similar, the relevant range in this case is limited to a particle diameter of between about 0.1mm and 0.3mm.

4.1.2.4.6.4 Cohesive soils Cohesive material such as silt and clay differs substantially in erosion behaviour from incipient motion in non cohesive material. Exceedance of the critical speed, rather than mobilising individual particles, tears major lumps from the bed, which either go immediately into suspension (liquid soil) or, in the case of solid soils, are first transported in a rolling motion and do not go into suspension until after adequate comminution. Systematic studies of the transport behaviour in cohesive soils have recently been carried out by Krier (1987): Guide values of critical bed shear stress (threshold drag force) for cohesive soils according to German Standard DIN 19661/2 can be seen from Figure 4.1-8, with the void ratio being defined as V − V e = S Vs where V = total volume of soil sample,

Vs = solid volume of soil sample. 66 Sediment Sources and Transport Processes

Figure 4.1-8: Critical bed shear stress τoc as a function of void ratio (DIN 19661/2)

4.1.2.4.6.5 Crushed material Solids with particle shapes significantly different from natural material, such as crushed material or tailings (mined material), can be used for such purposes as artificial sediment addition or for bed stabilisation where the available gravel is insufficient. Such material may also differ from natural material not only in particle shape (usually angular or platy), but also as to compactness. Thus, the resulting structures at the bed surface are also different (roof-tile pattern), with the corresponding effect on incipient motion and sediment transport. Other points of interest are transport velocity as well as the manner in which such materials mix into the bed. As general conclusions cannot be drawn in this case, we have to rely on appropriate laboratory tests on each specific material. Investigations by Promny (2003) have proved crushed material and tailings to show a higher critical bed shear stress and lower transport rates compared to well-rounded material.

4.1.2.4.6.6 Upward seepage Cheng et al. (1999) undertook both experimental and theoretical studies relating to the effects of upward seepage on the erosion resistance of solid particles with diameters of between 0.6mm and 5.8mm. As expected, they found the critical shear stress velocity to decrease along with increasing seepage velocity.

Sediment transport 67

4.1.2.4.6.7 Estimated values of critical shear stress German Standard DIN 19661, Part 2, gives recommendations both for the critical shear stress (threshold drag force) and for the critical flow velocity (Table 4.1-1).

Table 4.1-1: Safe critical shear stress τcr or.critical velocity vcr according to DIN 19661/

τ Bed structure 0 N/m² Fine sand, 1.0 grain size 0.063 to 0.2mm Medium sand, 2.0 grain size 0.2 o 0.63mm Coarse sand, 3.0 grain size 0.63 to 1.0mm Coarse sand, 4.0 grain size 1 to 2mm Coarse sand, 6.0 grain size 0.63mm to 2mm Gravel-sand mix, 9.0 Single-grain grain size 0.63mm to 6.3mm, structure tightly packed, flooded over prevailing major periods V0 Gravel-sand mix, 12.0 Bed structure grain size 0.63mm to 6.3mm, m/s tightly packed, temporarily flooded Fine sand, 0.20 to 0.35 grain size 0.03 to 0.2mm Medium gravel, 15.0 grain size 6.3mm to 20mm Medium sand, 0.35 to 0.45 grain size 0.2 o 0.63mm Coarse gravel, 45.0 grain size 20mm to 63mm Coarse sand, 0.45 to 0.60 grain size 0.63 to 2.0mm Platy bed load, 50.0 Single-grain Fine gravel, 0.60 to 0.80 10mm to 20mm high, structure grain size 2mm to 6.3mm 40mm to 60mm long prevailing Loamy sand 2.0 Medium gravel, 0.80 to 1.25 grain size 6.3mm to 20mm Sediment containing loam 2.5 Coarse gravel, 1.25 to 1.60 Loose mud 2.5 grain size 20mm to 63mm Slightly colloidal soil Loamy gravel, flooded over 15.0 Stones, 1.60 to 2.00 major periods grain size 63mm to 100mm Loamy gravel, temporarily 20.0 Loose mud 0.10 to 0.15 flooded Loose loam 0.15 to 0.20 Loose loam 3.5 Tightly p sandy loam 0.40 to 0.60 Colloidal soil Tightly packed loam 12.0 Highly Tightly packed loam 0.70 to 1.00 colloidal soil Clay 12.0 Solid clay containing sea 0.90 to 1.30 Tightly packed mud 12.0 silt

Covered Grass, flooded over major 15.0 Grass, flooded over major 1.5 Covered with with dense periods periods dense grass grass Grass, temporarily flooded 30.0 Grass, temporarily flooded 2.0

68 Sediment Sources and Transport Processes

4.1.3 Non-uniform bed material The mobility of sediment with a relatively uniform grading as charactises streams and rivers in the plains (Figure 3.2-3, Sample 3) is aptly described by a characteristic diameter (Section 3.2.2.1.2.2) such as dm according to Eq. 3.2-2. As the degree of non-uniformity, U (Eq. 3.2-3) decreases, sediment mobility loses its uniformity, in that the particles start moving at different times, depending on size and position in the streambed (Section 4.1.2.4.4). Larger particles when exposed on a fine-grained layer are easier to set in motion – the more so as finer material tends to be more easily eroded behind coarser material, which then lacks its support – than where imbedded more or less flush with the streambed. Smaller particles hide behind the larger ones, thus losing their mobility (Figure 4.1-9).

Exposure Hiding

Figure 4.1-9: Exposure and hiding effects for single-grain and multiple-grain materials (Zanke, 2002)

Bed load as well as suspension-load transport for non uniform bed material (Figure 3.2-3, Sample 2) as characterises mountain streams (Figure 4.1-12) is calculated by use of a fractional method. The grain-size distribution is split into several ranges (histogram) and a fractional transport rate is determined for the mean grain diameter, di, of each range. These fractional transport rates – weighted by their respective shares in the total size fraction, ∆pi, (Figure 3.2-5) – are summed up to a total transport rate.

4.1.4 Armouring – pavement 4.1.4.1 Definition “The terms armouring or pavement are applied to layers having the thickness of one particle. Such layers, formed by selective erosional processes, protect the underlying bed material from the attack of the current. The process of armouring or pavement formation may be described as follows: During periods of medium flow and the associated shear stresses, also of medium magnitude, part of the medium-sized particles are eroded and moved downstream. Loss of these particles where bed load delivery from further upstream is very small results in enrichment of coarser particles. As the process continues, the number of stabilising coarser particles in the streambed increases, protecting the underlying material (sublayer from further erosion. The large and medium-sized particles start jamming and interlocking to form a stable surface layer. When the shear stresses exceed the threshold, τgr,, for maximum bed stability, they are unable to deposit sufficient large, stabilising particles at the bed surface and the existing armouring breaks up. In such a case, the process of bed erosion will continue until the high shear stress due to a reduction in discharge or bed gradient has decreased again. The condition in which the shear stress reaches the above mentioned threshold,τgr, is termed maximum bed stability.” (DVWK, 1997).

In the paragraphs which follow, the critical shear stress, or threshold drag force, is termed τgr, τc, τcr or τoc, depending on the respective literature. DIN 4044 stipulates τcrit.

Sediment transport 69

4.1.4.2 Criteria for armour development The ability of a particle mix to develop armouring is mainly a function of its distribution. Table 4.1-2 is a DVWK (1997) list of criteria from the relevant literature.

Table 4.1-2: Criteria for armour development (DVWK, 1997)

4.1.4.3 Initial grain size distribution – armour layer Armouring as described in the preceding paragraphs manifests itself as increasing coarseness in the grading of the upper bed layers as shown in Figure 4.1-10 after Günter (1971).

Figure 4.1-10: Initial mix As, Fuller Curve AF, armour mix D, subscript F = Fuller Curve, x = sample (Günter, 1971) 70 Sediment Sources and Transport Processes

Use of the Fuller Curve as reference grading is justified by the fact that it has been seen to be similar in shape to the particle-size distributions of many natural streambeds, as in Switzerland. The equations employed for calculating the grading of the maximum armour layer from the sub-layer, after GÜNTER, are shown plotted in Table 4.1-3, Equations (4.26) to (4.29).

Table 4.1-3: Calculation of armour layer after Günter (DVWK, 1997, Table 4.3c)

4.1.4.4 Stability conditions The interaction between flow and streambed was described in detail by Dittrich (1998). For a better understanding of the characteristic phenomena involved, some important terms will be briefly discussed in the following paragraphs. A main distinction is made between streambeds of uniform and streambeds of non uniform material, the latter being the dominant type found in the Alpine regions of Central Europe. The stability of uniform material was studied by such authors as Schields (1936) and Hjulström (1935), as discussed in Section 4.1.2 above. Non uniform bed material is much more complex due to segregation and armouring; in this case we have to distinguish between stable and mobile surface layers. As the bed shear stress increases (bed shear stress velocity u* in Figure 4.1-11), a stable surface layer – the armour layer – first develops from a well-mixed gravel layer. This * persists as long as the shear stress is less than the so-called transition shear stress velocity, ut . However, when this value is exceeded (as a result of an increasing flow rate), a mobile surface layer – a pavement – forms. Sediment transport 71

Figure 4.1-11: Erosion rate E& (G in the above Figure) changing during armour and pavement evolution (Jain, 1990)

As demonstrated in the above figure, armouring is also characterised by the fact that the quantity of material eroded from the streambed rises along with an increase in shear stress, but goes back to zero under continuing constant load. However, if the shear stress exceeds the * so-called transition shear stress velocity, u t , the erosion rate, E& , no longer goes back to zero and a mobile surface layer – a pavement – forms. The transition shear stress describes at the same time the maximum degree of coarseness and maximum bed stability. Where bed load transport from upstream is largely prevented, as would result from the presence of transverse structures intercepting sediment, armour layers and their stability assume particular importance in sediment analysis. In practice, however, the condition of maximum bed stability is rarely attained due to the natural flow variations. Hence, as the surface layer tends to remain in an intermediate state (armour evolution region), it will resist smaller stresses than in the case of maximum stability. 72 Sediment Sources and Transport Processes

Table 4.1-4: Ansätze zur Abschätzung der kritischen Sohlenschubspannung (Dittrich, 1998)

Sediment transport 73

Table 4.1-4 above lists the main methods of estimating critical bed shear stress. These are discussed in greater detail below.

4.1.4.5 Calculation of armour stability DVWK (1997, Section 4.4.1.2) contains several methods of calculating the stability of natural armour layers. Table 4.1-3 above shows the analysis after Günter (1971). Analogous approaches by Meyer-Peter/Müller (1949), Schöberl (1979, 1991) and Chin (1985) are given in Table 4.1-5 to Table 4.1-7 (DVWK, 1997).

Table 4.1-5 Determination of critical bed shear stress after Meyer-Peter/Müller (DVWK, 1997)

Using Günter’s approach, the dominant grain diameter, dmD, of the armour layer is determined from Table 4.1-3 (4.26), whereas Schöberl’s approach uses Table 4.1-6 (Bild 4.32). 74 Sediment Sources and Transport Processes

Table 4.1-6: Calculation method after Schöberl (1979, 1991)

Table 4.1-7: Calculation method after Chin (1985)

The above data allow the critical bed shear stress, τgr, at the threshold of maximum bed stability to be computed from the grading curve of the initial mix (material of the sublayer). This corresponds to the upper limit of attainable bed stability (Section 4.1.4.4), a condition that is fairly rare in nature, depending on the local and hydrological conditions, as both slope and discharge vary. As mentioned above, the armour layer forming in practice will correspond to an intermediate state of armour evolution, which is less resistant to bed stresses. Sediment transport 75

4.1.4.6 Example Figure 4.1-12 below is a graph showing the sublayer and armour-layer grading curves determined from samples taken from the Lower Isar, River Kilometre 1.8, together with the armour layer determined after Günter (Table 4.1-3).

Figure 4.1-12: Sublayer and armour layer from sampling at Isar River Kilometre 1.8 and maximum armour layer determined after Günter

The above graph demonstrates that the armour layer present at the time of sampling was not fully developed.

The longitudinal development of the median particle diameters, dm, for the respective layers in the Upper Iller, depicted in Figure 4.1-13, suggests a decrease in diameter in the downstream direction (Sections 4.3.3.3 and 4.3.3.5).

Figure 4.1-13: Evolution of the median particle diameter along the Upper Iller (Bechteler, 2002) 76 Sediment Sources and Transport Processes

4.1.5 Critical discharge for mountain rivers

Rickenmann (1996) established the following equation for the threshold discharge, qcr, for onset of motion in mountain streams:

1,67 q ⎛ ρ − ρ ⎞ cr = 0,065⋅⎜ F w ⎟ ⋅ I []− 3 ⎜ ρ ⎟ g ⋅ d 50 ⎝ w ⎠ where the particle diameter, d50, needs to be determined from the sublayer. A similar equation was formulated by Aberle (2000) for step-pool streams: q ⎛ ρ − ρ ⎞ cr = 0,062 ⋅⎜ F w ⎟ ⋅ I−1,11 []− 3 ⎜ ρ ⎟ g ⋅ dk ⎝ w ⎠ where the characteristic diameter, dk, is the median diameter of the armour layer.

4.2 Bed forms 4.2.1 General The bed of a natural channel tends to lose its flatness once sediment transport has set in, developing more or less distinct surface roughnesses (Figure 4.2-1) termed bed forms. These in turn influence the intensity of sediment transport. Dunes, reaching heights of as much as several metres, are particularly common in navigable rivers in the plains. They present obstacles to navigation and need controlling or, where necessary, dredging. But dredging to keep navigation channels clear is costly. Another practical aspect of the interaction between flow and microscale bed alterations are algae coatings on the walls of long-distance water supply pipelines and the pressure tunnels of hydroelectric developments. Ripple coatings not more than 0.5mm to 0.7mm high and 3mm to 5mm long found on such pipe walls have caused head losses of as much as 40%. Hence, there is interest in finding out the causes for the development of such features. Although it has not been possible yet to find a physical explanation of the relationship between discharge, particle diameter and bed form, there are many criteria helping to estimate the conditions under which bed form development can be expected.

4.2.2 Bed forms and banks 4.2.2.1 Bed forms The technical literature dealing with bed forms and their classification is comprehensive, as this phenomenon (water and wind generated bed roughnesses) is also a subject of geosciences. The classification given below is limited to the terms common in engineering language. The bed forms thus include: a) ripples between several centimetres and tens of centimetres in magnitude b) dunes between several tens of centimetres and metres in magnitude c) antidunes d) banks or bars Figure 4.2-1 below is a simplified drawing of the streambed phenomena found in alluvial channels (cf. also Figure 3.3-8). Sediment transport 77

Figure 4.2-1: Bed forms in alluvial channels (Gehrig, 1981)

Ripples are small bed roughnesses of irregular, often three-dimensional, shape generally several tens of millimetres in magnitude. The surface structure of tidal mud flats (coast) or fine-sand surfaces altered by wind action is composed of ripples. Dunes are usually two- dimensional and, where caused by water or wind action, may be as much as 1m to 60m in height and 50m to several kilometres long. Ripples and dunes move in the direction of flow. Antidunes are bed forms which appear to migrate against the direction of flow. They occur only where very high flow velocities are involved (Fr ≥ 0.8). They are of minor practical importance. The antidune phenomenon may also be compared to backward erosion as may occur where dams and dykes are overtopped, which risks destroying such structures (Figure 4.2-2). The migration process characterising antidunes does not occur in this case, as these are isolated events without material transport from upstream.

Figure 4.2-2: Backward erosion during dam overtopping

78 Sediment Sources and Transport Processes

4.2.2.2 Banks (bars) A special phenomenon in this context are banks or bars. Figure 4.2-3 below is an attempt at classification (Zarn, 1997). The term bed form applies only to (b) and (d), whereas the other banks are dominated by deposition and lateral erosion processes.

Figure 4.2-3: Classification of banks (after Zarn, 1997)

4.2.2.3 Criteria for the occurrence and dimensions of bed forms The physical context for the description of bed forms has yet not been explored to complete satisfaction. However, there are – mostly empirical – criteria permitting an approximate determination of bed form shapes and dimensions. Wieprecht (2001) suggested that the diagram shown in Figure 4.2-4 might be used to define the different bed form types.

Figure 4.2-4: Bed forms for the range 1 < D* < 5000 (Wieprecht, 2001) Sediment transport 79

In practice, however, the boundary lines are less clear than shown in the above figure, as also suggested by the measurement readings entered in the diagram. Whereas onset of motion is accompanied by ripple development for particle diameters d < 0.6mm and D* < 15, sediment transport from d > 0.6mm and D* > 15 on at first does not involve bed deformation, but, as flow velocity increases, rapidly changes into the dune range. The transition between ripple and even bed for d ≈ 0.6mm is a function of temperature. Ripples occur at low temperatures, while transport continues on an even bed at higher temperatures. A physical explanation of this phenomenon has not been found so far. The above criteria provide no information on what magnitudes the bed form will attain. Since ripples (minor forms) develop independently of water depth, they represent no obstacles to practical uses such as navigation, but they do have a bearing on roughness (Section 3.3.2.4). The following paragraphs will, therefore, will be limited to criteria for dune dimensions. Several empirical approaches are available, but these give different results. Yalin expresses height H of dunes as

1 ⎛ τ ⎞ Eq. 4.2-1: H < ⋅ h ⋅⎜1− cr ⎟ , 6 ⎝ τ ⎠ which means that the maximum height is reached for τ >> τcr, hence 1 H < ⋅ h . 6 Generally speaking, the mean variation of maximum bed form heights (dunes) the various authors give for rivers in the plains is H 0,15 < < 0,3 h where h is the mean water depth in the bed form field (Figure 4.2-5).

Figure 4.2-5: Characteristic length of a dune (Hunzinger, 1998)

Yalin expresses the length of bed forms as H 1 ≈ L 30 where H/L is termed steepness. Predicting bed form development is particularly difficult in the case of rivers carrying gravel and for shear stresses only slightly above threshold. Thus, bed forms occur in the Danube 80 Sediment Sources and Transport Processes over the entire section between Straubing and Vilshofen, but a deterministic relationship with discharge cannot be clearly identified as demonstrated by the soundings along the river (Figure 4.2-6). A factor of great importance is obviously the “past history” of the riverbed. A period of constant flow of relatively long duration has been seen to favour bed form development.

Figure 4.2-6: Soundings along the River Danube (Kilometre 2265.5 to Kilometre 2265.7) for different flows, carried out between April 1988 and June 1989 (bed form material: dm = 21mm) (DVWK, 1992b)

4.2.2.4 Sediment transport in bed forms Close observation of bed form migration (Figure 4.2-7) reveals that a sediment particle is only temporarily allowed to travel. This happens whenever it emerges from under its “covering”, whereas the motion intensity of a particle on an even bed is much higher. The forced delays of particles in bed forms are substantial. Führböter (1983) estimated them at a magnitude of 10³ to 104. This does not signify, however, that sediment discharge is reduced by the same magnitude. The combined action of material transport from upstream, bed form geometry and hydrological events results in an overall process that is much more complex than the cycle of an individual particle.

Figure 4.2-7: Grain material sorting at the lee slope of a bed form (after Zanke, 1982) Sediment transport 81

4.2.2.5 Interaction between bed forms and bed shear stress As demonstrated by Figure 3.3-9 above, bed forms exert an additional resistance to flow. Qualitative information is available on the flow resistance of banks or bars (Figure 4.2-3). This suggests that the form drag of alternating banks decreases in relation to particle resistance along with increasing discharge, but may account for as much as between 50% and 75% of the total flow resistance at minor flows. Millar (2000), however, arrived at different conclusions. He established from data found in the relevant literature a perceptible influence of form drag from banks or step-pool channels in the high flow range. It follows that water depth is a function not only of flow but also of bed forms, as suggested in qualitative terms by Figure 4.2-8 below.

Figure 4.2-8: Water depth plotted against flow velocity as a function of bed forms

4.3 Bed load transport 4.3.1 General Bed load, although considerably inferior in volume – about 5% to 15% of the total sediment load – to the suspended fraction, is a decisive factor in river morphology and in the development of a streambed. Table 4.3-1 below lists a few values for Bavarian rivers:

Table 4.3-1: Annual bed load-transport values in Bavarian rivers

River Location Catchment area Recording Mean annual volume Notes interval of bed load transport [km2] [m3/a] Donau Hofkirchen 47.489 1989-1990 28.000 Bed load trap Inn Rosenheim 10.000 1961-1980 104.000 Reservoir dredging Isar Sylvenstein 1.156 1958-1983 53.600 Delta survey Isar Plattling 8.839 1988-1989 57.000 Bed load trap Saalach Reichenhall 940 1969-1984 95.000 Reservoir dredging Salzach Mündung 6.717 1953-1987 112.600 Reservoir dredging Ammer Ammersee 709 1962-1988 24.000 Delta survey Tiroler Achen Chiemsee 952 1869-1965 40.000 Delta survey 82 Sediment Sources and Transport Processes

Classical terms describing the type of movement of a sediment particle travelling as bed load are “rolling, sliding, hopping, bouncing”. These terms convey the idea that, as against the suspended load, the contact with the channel bottom is maintained or is interrupted only for a short time. No exact definition exists as to which interruption time characterises the term bed load or suspension load (cf. Section 4.4.3). The transport of bed load and suspended particles is very much a function of the turbulent flow fluctuations, or bursts, of the transport medium. The movement of suspended sediment is relatively continuous, like that of the water, whereas bed load discharge is discontinuous and often occurs in the form of streaks. This flow-generated mechanism resulting from irregular shear stress distributions, depicted in the schematic drawing of Figure 4.3-1 below (left), is a factor deserving particular attention in bed load measurement. The same effect has been observed in physical model tests. Figure 4.3-1 (right) shows that transport took place almost exclusively (80%) in the region of two longitudinal – catenary – ripples (Allen, 1968), whereas no bed movement was found to occur in the bank regions.

Figure 4.3-1: Left: Schematic drawing showing turbulence-induced secondary flows in open channels and the resulting streambed reactions (after Tsujimoto, 1989) Right: Turbulence-induced secondary flows in a physical model (after Zarn, 1997)

Further information on the influence of coherent flow structures on such factors as sediment transport has been provided by Nezu (2005).

Bed load discharge mG over the width of a measuring cross-section is shown in Figure 4.3-2 and is plotted against flow in Figure 4.3-3 below. In streams and rivers whose flow regimes are episodic in character, as due to snowmelt or extreme floods, sediment transport – usually bed load – is spasmodic. Almost a whole year’s volume of sediment load may in extreme cases be moved through a cross section during a single flood wave. This is then deposited wherever the flow velocity falls below the critical limit. Measuring the process of such sediment pulses is practically impossible for reasons of safety. As stated in the context of incipient sediment motion (Section 4.1), there are transport equations that include purely empirical relationships. Not being dimensional, they should be applied with care. There are also dimensional formulas for calculating the instantaneous sediment discharge or transport with the help of the theshold shear stress for onset of motion, and of the instantaneous shear stress for the respective flow condition. Recent results suggest that the quality of a sediment-transport formula cannot only be assessed against other formulas by comparisons of a momentary condition in a cross section, but that practical questions are better answered, and the quality of a formula better tested, by comparing balances over major periods. Sediment transport 83

The DVWK publications (1988, No. 87) list various transport relationships prepared for easy translation into computer programs. Sediment transport models are discussed in great detail in ATV-DVWK (2003).

Figure 4.3-2: Bed load discharge mG and suspension-load discharge mS as a function of location in a measuring cross-section in the River Rhine at Worms, River Kilometre 444 (DVWK, 1992b)

Figure 4.3-3: Bed load discharge plotted against flow (ATV-DVWK, 2003) 84 Sediment Sources and Transport Processes

4.3.2 Bed load transport equations The above-mentioned DVWK publications as well as DVWK Rule 127, Geschiebemessungen (bed load measurements) (1992), may serve as a detailed basis for practical analysis.

4.3.2.1 General ation, bed load removal etc. Even the highly complex computer programs now available (ATV-DVWK, 2003) do not permit reliable modelling of all these effects and their inter- relationships, although remarkable progress has been made in this respect over the past few years. However, the efficiency of such models depends on the quality of the data used, and these are costly to obtain. Running such models calls for experienced experts and is, moreover, extremely time-consuming. Hence, most practical applications will best be based on the assumption that calculation or estimation of bed load transport is fundamentally possible by use of a formula that may appear suitable. The above effects would then either be ignored or allowed for at least qualitatively by means of calibration or empirical coefficients. Such formulas will be used to determine the theoretical bed load-transport capacity of a stream or river section and this need not necessarily correspond to the actual transport rate (Section 4.3.3.2). The bed load formulas can be classified by basic structure as follows: • proportionality to flow velocity • proportionality to bed shear stress • proportionality to stream power v · I plus formulas derived on a stochastic basis (DVWK, 1988) A few commonly used formulas mainly valid for streams at alpine and medium altitudes will be discussed in greater detail in the following paragraphs.

4.3.2.2 Einstein (1942, 1950) Einstein very realistically considered sediment motion as a probability problem influenced by turbulence. A detailed description is found in Graf (1971). Einstein’s approach develops two dimensionless coefficients whose relation has been found by measurement in a laboratory channel using single-grain mixes. The grading curve is split into several fractions, and weighted superimposition of the constituent fractions (Section 4.3.3.4) gives the total transport. The transport intensity can be written as

m Eq. 4.3-1: Φ = G []− 3 ρF ⋅ ρ′⋅ g ⋅ d and the flow intensity as

d ⋅ρ′ Eq. 4.3-2: Ψ = []− h ⋅ I

The relationships between Φ and Ψ are plotted for the equations after Einstein and after Meyer-Peter/Müller (Section4.3.2.3) in Figure 4.3-4 below. Sediment transport 85

Figure 4.3-4: Bed load function after Einstein (1942, 1950) and Meyer-Peter/Müller (1949)

Both Φ and Ψ can be represented, after appropriate transformation, as functions of the dimensionless numbers mentioned in Section 3.1.2 above:

3 Φ = G * ⋅ Fr* 2 Eq. 4.3-3: 1 Ψ = Fr*

Einstein enlarged his ideas regarding bed load transport in 1950 by introducing a Hiding Factor (Section 4.1.3) and the lift force.

4.3.2.3 Meyer-Peter/ Müller (MPM, 1949) The bed load discharge formula most commonly used at least in the Alpine region goes back to Meyer-Peter/Müller. It was derived from comprehensive tests for single-grain as well as multiple-grain material with particle diameters of between 0.4mm and 29mm, water depths of between 0.1m and 1.2m and slopes of between 0.0004 and 0.02. This formula is written as

3 2 ρ 8 1 ⎡ ⎤ kg F ⎢ ' ⎥ ⎡ ⎤ Eq. 4.3-4: mG = ⋅ ⋅ ⋅ ρw ⋅ g ⋅µ ⋅ I ⋅ Rs − 0,047 ⋅ρ ⋅ρw ⋅ g ⋅ dm ⎢ ⎥ ρF − ρw g ρw ⎢1424 434 14244 4344 ⎥ ⎣m ⋅s⎦ ⎣ τo τcr ⎦

This formula clearly reveals its basic idea – the difference of two shear stresses – in the paranthetical expression. The first term describes the instantaneous shear stress, τo, caused by the flow (Eq. 3.3-6) and the second term corresponds to the threshold shear stress, τcr, (Eq. 4.1-5) for onset of motion of bed load with a mean particle diameter dm (Eq. 3.2-2). This is defined as

dmax d i 1 Eq. 4.3-5: d m = Σ ∆pi ⋅ []m mit d i = ⋅ ()d i + d i+1 [m] dmin 100 2 86 Sediment Sources and Transport Processes

where ∆pi is the percentage of grain fractiondi , which is characterised by the median value of the selected interval. The instantaneous shear stress in the square bracket is supplemented by the so-called ripple factor, µ, (Eq. 3.3-11) and the transport-effective flow proportion, Rs, (Eq. 4.3-8), as explained in the following. Meyer-Peter/Müller assumed the constant for incipient motion, Θcr , (Shields parameter) in the range independent of Re* (Figure 4.1-4) as 0.047, as mentioned in Section 4.1.2.4.4 above:

τcr * Eq. 4.3-6: 0,047 = = Frcr = Θcr g ⋅ ()ρF − ρw ⋅ d m

Using a further assumption, ρF = 2,650 kg/m³,Eq. 4.3-6 gives the critical bed shear stress as a function of the median grain diameter as

⎡ N ⎤ Eq. 4.3-7: τ = 760,8⋅ d cr m ⎢ 2 ⎥ ⎣m ⎦

Selection of the critical Shields parameter 0.047 after MPM means, as mentioned in Section 4.1.4.4 above, that particles are already moving, with the absolute onset of motion ranging around 0.03 (cf. Figure 4.1-4). Shields determined this value to be Θcr = 0.06 from his well- known tests by extrapolation of bed load discharge towards zero.

Rs in Eq. 4.3-4 is the hydraulic radius for the transport-effective flow proportion, Qs, [m³/s],

QS Eq. 4.3-8: R S = h ⋅ ~ []m Q to be determined according to Figure 4.3-5. As the shear stresses are transferred at right angles to the lines of equal velocity (isotachs), only the shaded range is effective in terms of ~ bed load transport. Hence, the quotient Qs / Q describes the ratio of Qs to flow within the rectangular region of the trapezoidal section. As exact calculation by use of measured isotachs will usually not be possible in practice, one will have to content oneself with an ~ approximation according to Figure 4.3-5, computing the flows Q and Qs according to their area proportions by use of the mean flow velocity in the cross section. Another method of sufficient accuracy of calculating the transport-effective hydraulic radius is the approximation

b Eq. 4.3-9: R ≈ h ⋅ []m S U

In wide channels of a width more than 30 times the water depth, Rs can be replaced – accurately enough – by the water depth, h, Sediment transport 87

Figure 4.3-5: Flow proportion effective for bed load transport

Jäggi (1992) used an efficiency factor instead of the transport-effective flow proportion

Q Eq. 4.3-10: β = r Q where Qr is the flow proportion corresponding to that part of the cross section which exerts shear stress on the bottom. For a trapezoidal cross section with little influence from the walls, this factor lies in a range 0.8 < β < 0.9. For uneven beds (presence of bed forms, Figure 4.2-1), the increased transport probability (mobility) is accounted for by multiplying the sediment Froude number, Fr*, by a so-called ripple factor

3 2 ⎛ k St ⎞ Eq. 4.3-11: µ = ⎜ ⎟ []− ⎝ k S ⎠ where kst is the roughness coefficient after Strickler (Eq. 3.3-11) for the channel bottom including bed forms (form resistance + grain resistance, Figure 3.3-3), which may have been determined on the assumption of a constant water level, and ks (not to be confused with the equivalent sand roughness normally termed ks) is the Strickler coefficient for an even bed characterised by a grain diameter d90 in [m] (grain resistance, Eq. 3.3-1).

1 26 ⎡m 3 ⎤ Eq. 4.3-12: k S = ⎢ ⎥ 6 d s 90 ⎣⎢ ⎦⎥

The ripple factor, µ, allows for the roughness produced by the bed forms.

4.3.2.4 Smart-Jäggi (1983) The following formula was developed by Smart-Jäggi especially for steep channels (I > 2%):

0,2 4 ⋅ρ ⎛ d ⎞ ⎛ Θ ⋅ρ′⋅ d ⎞ kg F ⎜ 90 ⎟ 1,6 ⎜ cr m ⎟ ⎡ ⎤ Eq. 4.3-13: m G = ⋅⎜ ⎟ ⋅ v m ⋅ R b ⋅ I ⋅⎜1− ⎟ ⎢ ⎥ ρ′ ⎝ d 30 ⎠ ⎝ R b ⋅ I ⎠ ⎣m ⋅s⎦ 88 Sediment Sources and Transport Processes where Θcr is the dimensionless parameter for incipient motion, according to Eq. 4.3-6. This is here assumed as 0.05. The relationship between flow, or velocity, and water depth needed for this equation can be determined e.g. by use of the Manning/Strickler equation (Eq. 3.3-11). Further bed load-transport studies for steep gradients were conducted by Rickenmann (2005), who included the results from more than 250 laboratory tests using Froude numbers within the range 0.35 ≤ Fr ≤ 2.6. It is recommended to increase the gradient by a factor where steep slopes are involved, so as to arrive at a better agreement between measurement and computation.

4.3.2.5 Hunziker (1998) Recent investigations and evaluations by Hunziker regarding the measurements by Meyer- Peter/Müller have resulted in a reformulation of their equation (Eq. 4.3-4), which may also be written in a dimensionless form

3 2 Eq. 4.3-14: Φ = 8⋅ (Θ′ − Θcr ) respectively

3 2 Eq. 4.3-15: Φ = 5⋅ ()Θ′ − Θcr where Θcr = 0.05 and the coefficient 23.5 is used instead of 26 for determining particle roughness according to Eq. 4.3-12 and Θ′ = Θ ⋅µ . Φ is the dimensionless bed load discharge, or transport intensity after Einstein (Eq. 4.3-1).

m Eq. 4.3-16: Φ = G []− 3 ρF ⋅ ρ′⋅ g ⋅ d

4.3.3 Additional aspects for determining bed load transport 4.3.3.1 Unsteady flow – flood As mentioned earlier, all the bed load discharge formulas have been derived for steady / uniform (normal) flow, the transport rates obtained standing for a long-term mean of the respective readings. The omnipresent turbulence and inhomogeneities in the bed material, however, cause the transport rate to vary more or less distinctly around this median value, as illustrated by Figure 4.3-6. Sediment transport 89

Figure 4.3-6: Variation of bed load transport, m& G , resulting from variations in the number, shape and position of banks and thalwegs in a ramified river a) for steady flow b) for unsteady flow

Figure 4.3-6 also reveals the problems involved in bed load transport measurement in nature. In addition, transport intensity varies perceptibly across the river width, e.g. as a result of secondary flows caused by turbulence and in bends (Figure 4.3-1 to Figure 4.3-3). Mention was already made of sediment pulses in Section 4.3.1 above. Slides, debris flows, bank erosions etc., as a rule in conjunction with heavy rainfall and flood events, lead to unusually high bed load production in rivers and streams. This material is moved in dependence of the bed load-transport capacity of the respective watercourse, this being also a function of such factors as flow and slope. Where the flow rate changes, as during a subsiding flood wave, the transport capacity decreases as well and the surplus material is deposited more or less accidentally in the riverbed, forming isolated accretions (gravel banks), which risk affecting flood safety. The same applies to changes in slope, as at points where a torrent enters a plain to build alluvial fans. The bed load-transport capacity also decreases in case the stream or river overflows its banks, favouring sedimentation in the respective channel section. This risk is aggravated by backwatering above obstacles such as bridges and culverts. In conjunction with floating logs, these factors may even entirely alter the river profile, and bed load may be deposited outside the streambed (Bezzola et al. 1996). 90 Sediment Sources and Transport Processes

Cui et al (2005) developed a numerical model to describe the dispersion of sediment pulses in mountain streams. Numerically simulated scenarios have shown that pulse dispersion as well as abrasion (Section 4.3.3.5) are decisive factors, besides wave translation, for the evolution of a streambed.

4.3.3.2 Armouring The development of armour layers, which in extreme cases may become pavements, and their effects on channel roughness were described in some detail in Section 4.1.4 above. Their impact on bed load transport will be discussed in the following paragraphs. The bed load-transport equations given by way of example in Section 4.3.2 can be used to compute the theoretical transport capacity of a stream or river. This, however, need not necessarily be identical with the actual (effective) bed load transport rate – apart from the general difficulty of calculating bed load transport with adequate accuracy (Section 4.3.2.1). The two quantities are approximately equal as long as the calculated transport capacity is utilised by bed load either delivered from upstream or eroded from the streambed (or from gravel bars or the banks). Where an armour layer forms at the surface of a streambed, bed load material is picked up less easily. This implies that the actual bed load transport will fall below the theoretical transport capacity. In this case, withdrawal of material from the bed will be initiated only when the threshold drag force, τcr, for armouring is exceeded and the armour layer is set in motion or broken up. This happens at a flow QD. This situation is shown qualitatively in Figure 4.3-7 below. The term latent erosion stands for a situation where bed load discharge, although predicted by calculation, does not take place because armouring prevents the streambed from moving.

Figure 4.3-7: Left: Bed load function with flow Q0 for incipient motion, and QD for break-up of the armour layer.

Right: Duration curve of flow Q and bed load transport m& G . The shaded area corresponds to the minimum volume of bed load transport GFmin, or min mFf, for Q < QD, the hatched area is the maximum volume of bed load transport for Q > Q0 (Hunziker, 1995)

The threshold value, τgr or τcr, for break-up of the armour layer can be derived from Table 4.1-3 to Table 4.1-5. The product of bed load-transport function and duration curve finally yields the total bed load, mFf, for the considered period, t. Thus, the difference in incipient transport, depending on whether or not an armour layer is present, results in different bed load volumes, as follows from Figure 4.3-7 right: minimum load with armouring (GFmin) and maximum without armouring (GFmax) (mGf according to DIN 1045). Sediment transport 91

As can further be seen from Figure 4.3-7, selection of threshold bed shear stress, ΘD, and threshold flow, QD (e.g. through different Shields parameter values according to Sections 4.1.2.4.3 and 4.3.2.3) for incipient transport has a great bearing on the transport rate. This, however, decreases as the flow increases to large values.

4.3.3.3 Sediment sorting Natural bed load with a distinct grain-size distribution undergoes sorting on its way downstream. Coarser material is deposited in regions of reduced drag force such as widened channel sections or reduced channel slopes. This effect can naturally not be simulated by a bed load discharge formula, which considers only a median, or characteristic, particle size. In this case we need to introduce the so-called fractional transport, where such formulas are applied separately for individual size fractions. This procedure is contained in recent simulation models (Section 4.3.3.4).

4.3.3.4 Fractional transport The method of fractional transport splits the bed material into different size ranges. Transport capacity is first calculated separately for each size fraction, using an appropriate transport formula for uniform bed material, which may contain a correction factor to allow for the nonuniformity of the material (Section 4.3.3.6 below, Hiding - exposure). The total transport is obtained by weighted summation of the values resulting for the individual fractions. More details are found in ATV-VWK (2003, Section 5.2.2.2). A very good presentation of the formula structure for several bed load transport equations is given in Lehmenn (2005, Section 3.2.3). Hunziker (1995) studied fractional bed load transport in great detail.

4.3.3.5 Abrasion, downstream fining A factor of particular importance to be considered in establishing sediment-transport balances is the reduction in particle diameter in the downstream direction (Figure 4.1-13). In the relevant literature, this phenomenon is named sorting (Section 4.3.3.3) or downstream fining. The bed load particles are worn and mechanically comminuted during transport. This process is called abrasion. The material worn away is usually very fine-grained and tends to go into suspension. Other factors favouring downstream fining include frost action, mechanical breaking as well as chemical or biological weathering. Fining results from alteration in respect of • shape of bed load particles • grain-size distribution • petrographic composition of the grain material Bed load abrasion can be estimated for idealised conditions after Sternberg:

x − a ⋅ W 3 Eq. 4.3-17: d x = d o ⋅ e [mm] where d0 = particle diameter [mm] at the beginning of the section under study

dx = particle diameter [mm] at the end of the section of a length x [km]

aw = abrasion coefficient [1/km] 92 Sediment Sources and Transport Processes

A study conducted by the Bayerische Landesamt für Wasserwirtschaft (LfW, 1974) indicates -1 a value of aw = 0.089km for the Rivers Linder and Ammer. This is very high as compared to the values from the relevant literature (Zarn, 1997). Recent comprehensive investigations into the abrasion phenomenon are given in Mikos (1993, Section 6). The study WRS (2000) recommended an abrasion coefficient of 0.0050 for the Lower Salzach River. Actually this value is significantly dependent on the rock type. Parker (1991) indicated magnitudes of 0.00006 for quartzite and 0.01 for limestone. Wright et al. (2005) developed a numerical model to simulate longitudinal profile and particle-size reduction in large sand-bed rivers.

Figure 4.3-8: The influence of abrasion coefficient aw on bed load mass loss for different distances x.

Analogously to abrasion, the theoretically computed bed load mass loss is

− a W ⋅ x Eq. 4.3-18: mGfx = m Gfo ⋅ e [t]

The significant effect of the abrasion coefficient on bed load is illustrated by Figure 4.3-8. The abrasion coefficient the LfW Study gives for the River Ammer (aw = 0.089) results in a bed load of as little as 17% of its original value after a distance x = 20km, which is far from reflecting reality. Downstream fining is a phenomenon observed in most natural streams over major lengths, above all in the upper courses (streams at high and medium altitudes) (Figure 4.1-13). This effect is composed of two factors: • sorting (Section 4.3.3.3) • abrasion. Parker (1991) developed models to simulate these factors on the basis of both laboratory tests and real-life measurements.

4.3.3.6 Other effects A multitude of further effects exist which in practice have a more or less distinct influence on bed load transport. Attempts are being made to include these in simulation models by use of mathematical/empirical formulas, but this is difficult, especially as appropriate calibration possibilities are lacking, and requires highly experienced staff. Such effects include: Sediment transport 93

Hiding - exposure The individual particles of a sediment mix of non-cohesive material are set in motion as dictated by size and exposure to the flow. The coarsest particles usually present the larger area to the flow (exposure), while also resisting the attack of larger discharges (Figure 4.1-9). Smaller particles are sheltered between the larger ones (hiding), so as to exert less resistance to erosion. These factors with partly opposite effects may result in many fractions of a particle mix being set in motion almost simultaneously (equal mobility), which permits, by way of approximation, a characteristic particle diameter to be used to describe onset of bed load motion. It is common practice to use the range between d50 and d65 of the grading curve and this usually corresponds to the median particle diameter, dm (Eq. 4.3-5).

Loading Sudden changes in channel geometry or bed composition would imply an equally rapid change in theoretical bed load-transport capacity. This, however, will never be encountered in nature as the processes of bed load pick-up or deposition are never instantaneous but proceed more or less continuously. A suitable help in this case is to average this stationary, nonuniform process through the introduction of a Loading Law. The same applies to the transport of suspended sediment.

Mixing Bed load is not only picked up from the uppermost bed layer, but also from underlying layers. As a result, finer material from the lower layers may mix into the flow despite the presence of an armour layer. The same applies to deposited material. Allowance is made for this phenomenon in simulation models by introducing several bed layers with mutual exchange possibilities (DVWK, 2002, Section 5.3.3).

Bank vegetation In Section 3.3.5.4 above, mention was made of the influence of the banks on bed load transport, a phenomenon that loses its importance in wide streams, but is very distinct in compact discharge cross sections with bank vegetation. The macroturbulences (Figure 3.3-6) caused by wooded banks in particular tend to intensify bed load transport. DVWK (1993, Mertens, revised edition published in 2006) provides an empirical equation for this intensification.

4.3.4 Duration curve for bed load – Yearly bed load transport By use of a transport formula appropriate to the respective case, it is possible to calculate a bed load-transport / flow relation. For calculating the annual volume of bed load transport, Figure 4.3-9 is used to determine the bed load duration curve from the flow duration curve for 1 year (cf. Figure 4.3-7, right). Its integration (hatched area) finally gives the annual volume of bed load transport. As can also be seen, this Figure can serve as an easy means of determining e.g. the effect of a change in flow duration curve (diversions) on bed load transport. The area differential between the two transport duration curves corresponds to the erosion or deposition volume of a stream section. 94 Sediment Sources and Transport Processes

Figure 4.3-9: Determining annual volume of bed load transport (Zanke, 2002)

4.3.5 Summary As mentioned in Sections 4.3.1 and 4.3.2.1 above, bed load transport in a natural channel is a highly complex process subject to a great number of factors and can be determined by nothing but approximation. In fact this process is the result of three interacting spheres, flow, sediment and streambed, as demonstrated in a simplified manner in Figure 6.9-1. A multitude of parameters with their errors and scatters as well as their mutual influences act on bed load transport. The number of publications dealing with this problem is huge. Bed load transport is the dominant subject in what are considered the three main international technical journals, IAHR – Journal of Hydraulic Research, ASCE – Journal of Hydraulic Engineering, WASER – Journal of Sediment Research. In addition, there are relevant articles in national journals and an enormous number of grey literature in the form of university-department and authority newsletters. These contain many comparisons of results from different real-life and laboratory measurements using several bed load-transport formulas (Mertens, 2004). The greater part of the publications on bed load transport apply to sand or gravelly material as found in alluvial streams and rivers. For the smaller transport rates as occur in nature over major periods, measured and theoretical values differ substantially, especially where so-called threshold-value formulas are used, which are based on the assumption that bed load transport does not start before a threshold bed shear stress – however calculated – is reached (Section 4.1.2.4). Agreement tends to improve for the higher transport rates – the fully mobile condition – where all the size fractions present in the bed participate in the transport. Stochastic models as developed by Kleinhans et al. (2002) make allowance for velocity variations, as caused by turbulence, and their effects on onset of motion and bed load transport proper. These results are clearly superior to deterministic models in the range around incipient motion, whereas deterministic models show good agreement with real-life measurements at high transport rates. A very easy method of estimating bed load discharge is establishing a rating curve by finding, as in the case of suspended-sediment transport (Figure 4.4-27), a relationship between discharge and bed load transport via a regression relationship. This, however, requires appropriate measurements. Sediment transport 95

A problem that has attracted little attention so far is the interaction between fine and coarse transport material. Ikeda et al. (1988) found the overall mobility to be larger where the fine material is only the predominant fraction rather than the only material present. Finally, it should be noted that the conventional bed load-transport formulas when applied to torrents are unlikely to yield reliable results. All equations (Smart-Jäggi, Section 4.3.2.4) that have been derived for major gradients are based on laboratory studies. Bed load measurements in torrents are extremely difficult, and accurate determination of a characteristic particle size, as well as of slope and flow rate, poses substantial problems and is subject to errors. A study on bed load transport in steep channels has recently been published by Rickenmann (2005). A comprehensive study on bed load transport especially for graded bed material (wide particle- size distribution) was carried out by Scheer et al. (2001).

4.4 Suspended sediment transport 4.4.1 Introduction 4.4.1.1 Measured values Solids carried in suspension by rivers in the plains account for about 85% to 95% of the total sediment load. Table 4.4-1 below lists some of the values characterising suspended-sediment transport:

Table 4.4-1: Records of annual means (1971/ 2000) for Alpine rivers in Bavaria (DGJ, 2000))

River r Measuring Catchment Mean Maximum Suspended Sediment station Area Concentration Concentration load yield

AEo MQ Cm Cmax mFs [km2] [m3/s] [g / m³] [g / m3] [t] [t / km2 a] Donau Vilshofen 47.677 656,0 22 600 557.099 11,68 Iller Kempten 955 46,8 111 12.731 164.206 172,01 Lech Füssen 1.422 60,5 196 8.800 376.637 264,86 Isar München 2.855 92,7 37 1.955 109.554 38,37 Loisach Schlehdorf 639 29,4 78 3.386 54.420 85,16 Ammer Weilheim 601 14,8 177 23.692 82.561 137,60 Inn Oberaudorf 9.712 303,0 179 10.269 1.710.963 176,17 Tiroler Achen Staudach 952 35,4 231 53.196 259.605 275,01 Salzach Burghausen 6.649 250,0 171 6.919 1.350.594 203,13 Saalach Unterjettenberg 940 36,6 206 10.428 238.264 253,47

It is noted that the volume of eroded solids (sediment yield, Section 2.4.3), that is suspended load related to the size of the catchment area and a unit time (normally 1 year), shows substantial variations. In addition to the respective flood events, these variations are caused by the specific features of the various catchments. Such readings are shown plotted for characteristic regions in Figure 2.4-2. As to the origin, suspended load can be explained as being composed of material eroded from the river bottom and banks (suspended bed-material load) and material resulting from surface erosion in the catchment and washed into the channel by the surface runoff (wash load) as well as material carried into the river along with meteoric water and sewage (Figure 3.1-1). Suspended transport of the heterogeneous water-sediment mix is only possible in the turbulent flow condition, especially for low concentrations. Hence, as soon as the stream or river reduces its speed or stops moving at all, part or the entirety of the suspended particles will 96 Sediment Sources and Transport Processes settle. This occurs in such places as widened river sections, tributaries, port entrances and reservoirs, where the resulting sedimentation risks causing operational problems. Streams or rivers entering a lake or sea with little tidal influence form deltas (Section 5.2.4.6).

4.4.1.2 Particle sizes for suspended sediment Solid material carried in suspension consists of very fine particles. For the distinction between bed load and suspended-load particle sizes, see Section 4.4.3. The grading curves for Alpine rivers in Bavaria are shown plotted in Figure 4.4-1 below. As can be seen, part of these show bimodal distributions, as e.g. Sample 1 (Iller) with maximum values at approximately 0.03mm and 0.15mm, respectively. Particle sizes greater than 0.5mm are practically absent (cf. Figure 4.4-4).

Figure 4.4-1: Grading curves of suspended-load samples

4.4.1.3 The significance of suspended sediment transport Suspended-sediment transport is a factor of particular significance in dealing with such problems as reservoirs and flood-retention basins, deposition of material on banks and assessment of wear and tear on pumps, turbines, fittings and marine propellers as well as production of drinking water and industrial water from rivers. In such cases, reliable determination of the suspended-load concentration distribution over the discharge cross section is of substantial practical interest (Table 10.3-1). The multitude of parameters and the complexity of the process make it difficult to establish universally applicable formulas, theoretically well founded and satisfactory in practical application, for determining concentration and velocity distributions especially for suspended sediment in rivers. This is particularly true of the washload proportion (Figure 3.1-1), which is not directly dependent on Sediment transport 97 discharge, but is a function of such conditions as rainfall intensity, vegetation state etc. in the respective catchment area. It should also be noted that suspended load may also be contaminated. Especially the fine fractions (less than 20µm) tend to attract organic and inorganic contaminants. The transport, sedimentation and re-suspension of solids obey the laws of hydromechanics. This is of particular interest for impounded streams and rivers, where a large proportion of the suspended sediment may be deposited as a result of reduced flow velocity or turbulence. Reservoir sedimentation is a factor of substantial significance in the transport balance of contaminants. The contaminant-loaded sediment deposits constitute a latent cumulative risk to watercourses, which is of importance for the assessment of water quality (DVWK, 1985 and 1993). This problem becomes particularly critical where large amounts of (contaminated) solids need to be removed to maintain navigation in rivers, river mouths and ports (dredgings).

4.4.2 The mechanism of suspended sediment transport In tranquil water, there is no reason for particles to separate from the bed material and go into suspension since, apart from Braun’s molecular movement, vertical velocity components and, hence, forces do not occur. But turbulent water movement causes resistance forces to act on the suspended particles in the streambed in and across the direction of flow. As soon as these and potential lift forces exceed the weight of a particle under the action of uplift forces, sediment transport may result. Depending on flow velocity, turbulence and particle properties (density, shape, compactness, size), the result may either be bed load transport in the immediate vicinity of the bed only or suspended-sediment transport over the entire discharge cross section, with the concentration profile C(y) increasing towards the channel bottom.

Figure 4.4-2: Development of a stationary suspended-sediment concetration profile over depth

According to Figure 4.4-2 above, a particle-loaded fluid element at a depth y in a steady flow, on which an upward directed velocity component v′y from turbulence is acting, will, coming from a zone of higher concentration, transport more particles upward than an element of equal size moving from a higher layer y + 1 downward due to a velocity variation v′y acting in the opposite direction. At the same time the particles will sink at a velocity vs as a result of their larger density, which leads to an equilibrium between the upward transport due to 98 Sediment Sources and Transport Processes turbulence (diffusion), ε · MC/My, and the downward transport due to gravity, C · vs, which manifests itself as a corresponding concentration profile. Where the particles are very small, or where the difference between solids density and fluid density is small, concentration will be more or less evenly distributed over water depth.

4.4.3 Bed load versus suspended load Mathematical/physical treatment of sediment transport often distinguishes between bed load and suspended sediment (cf. Section 3.1.1), which is not correct at least in the vicinity of the channel bottom. At first a distinction should be made between the two transport modes. This can be made by measurement, for each specific case, within the transition zone between the grading curves for bed load and suspended material, as demonstrated by Figure 4.4-3 below.

Figure 4.4-3: S Grading curves for bed load / suspended load transition

Burz (1964), by measuring particle-size distributions in suspended load in – mostly Bavarian – rivers, arrived at the distribution shown in Figure 4.4-4. This suggests that 90% on average of all suspended particles are smaller then 0.2mm. Larger particles are transported only near the bottom. Sediment transport 99

Figure 4.4-4: Spectrum of particle-size distribution from 15 suspended-sediment samples from Bavarian rivers (after Burz, 1964)

Based on measurements made in several Austrian rivers, Kresser (1964) found that a Froude number of

v2 Eq. 4.4-1: Fr2 = m = 360 []− g ⋅ dgr holds for the threshold particle diameter, dgr, at a mean flow velocity vm. From this follows the diameter

v 2 Eq. 4.4-2: d = m []m gr 360 ⋅ g

Another distinction is possible via the Rouse number z as derived in Section 4.4.4.4 below. As can be concluded from Figure 4.4-7, above about z > 2 (Eq. 4.4-10) the respective particle sizes can only be transported in the lower portion of the discharge cross section. Taking the product of the two parameters, β · κ = 0.4, three characteristic ranges can be distinguished, e.g. after Raudkivi (1982): 15 > z > 5 bottom load 5 > z > 1,5 bouncing 2 > z > 0 suspension Engelund (1965) thought along similar lines and arrived at the relationship 100 Sediment Sources and Transport Processes

m Eq. 4.4-3: * ⎡ ⎤ v 0 = 0,25 ⋅ vs ⎣⎢ s ⎦⎥ which means that a solid particle can be transported in suspension where the bed shear stress * velocity, v0 , is equal to at least 25% of the settling velocity of this particle, that is z < 10. Zanke (1982) obtained a proportionality constant of 0.4, instead of 0.25, for Eq. 4.4-3, which results in a suspended-sediment coefficient z < 6.25. Hence, with knowledge of the bed shear stress velocity (Eq. 3.3-7), it is possible to calculate the maximum possible diameter for a suspended particle from its settling velocity, as by use of Eq. 3.2-13.

4.4.4 Diffusion theory 4.4.4.1 Definition Diffusion is fundamentally understood to mean the gradual mixing of different gases or fluids, or the compensation of differences in concentration and heat as well as the propagation of the impulse within fluids. This is a result of the thermal motion of individual molecules (molecular diffusion) in tranquil fluids, or fluid clusters in turbulent flow (turbulent diffusion). Where a concentration difference is present, the amount of a property removed from fluid elements coming from a zone of higher concentration exceeds that received from a fluid element coming from zones of lower concentration (cf. Section 4.4.2). Hence, compensation of differences always occurs in the direction of decreasing concentration. Normally, molecular diffusion can be ignored in comparison with that due to turbulence. For a general method of deriving the differential equations for diffusion and dispersion, see the relevant literature (DVWK, 1999). The two-dimensional diffusion equation, Eq. 4.4-4, for unsteady flow is easily derived, by use of Figure 4.4-5, from the continuity of inflows and outflows and the concentration change in a fluid element.

∂C ∂C ∂C ∂ ⎛ ∂C ⎞ ∂ ⎛ ∂C ⎞ ⎡ kg ⎤ Eq. 4.4-4: + u ⋅ + w ⋅ = ⎜ε ⋅ ⎟ + ⎜ε ⋅ ⎟ ⎜ x ⎟ ⎜ y ⎟ ⎢ 3 ⎥ ∂t ∂x ∂y ∂x ⎝ ∂x ⎠ ∂y ⎝ ∂y ⎠ ⎣m ⋅s⎦ Sediment transport 101

Figure 4.4-5: Deriving the two-dimensional diffusion equation (u = vx, w = vz) (Chang, 1987)

Steady nonuniform flow, neglecting longitudinal diffusion, is described by the diffusion equation Eq. 9.1-3.

4.4.4.2 Diffusion equation for steady suspended sediment transport Given a steady uniform flow condition the general diffusion equation can be used to derive, after integration over water depth, the relationship (cf. Figure 4.4-6),

dC kg Eq. 4.4-5: ⎡ ⎤ vs ⋅ C + εsy = 0 dy ⎣⎢m 2 ⋅s⎦⎥ for suspended-sediment transport as originally established by Schmidt (1925) for determining dust distribution in the atmosphere. The two terms stand for the state of equilibrium of suspended-load transported in the y direction (vertical to the flow direction) through a horizontal unit area (Figure 4.4-2). The first term represents the sinking suspended-load proportion, while the second describes the amount of suspended particles moved in the opposite direction under the action of diffusion (cf. Section 4.4.2), with εsy being the dispersion coefficient for suspended particles.

4.4.4.3 Vertical distribution of the diffusion coefficient

The turbulent diffusion coefficient, εsy, is by no means constant over depth, as follows e.g. from measurements by Kalinske & Pien (1943) (Figure 4.4-6).

102 Sediment Sources and Transport Processes

Figure 4.4-6: Theoretically (Curve 1) and experimentally (Curves 2 and 3) determined distributions of the turbulent diffusion coefficient over depth

Curve 2 was obtained from a relatively narrow channel (0.19m high by 0.29m wide, I = 0.004), Curve 3 from a wider channel (0.16m high by 0.69m wide, I = 0.0015). The authors measured the diffusion of dyed drops of a fluid that did not mix with water but had the same density. Theoretical derivation of the impulse exchange coefficient could be as follows: Assuming a shear-stress distribution linear over depth and a logarithmic velocity distribution, and neglecting the laminar shear-stress proportion, the turbulent diffusion coefficient would take the form of the relationship

y y ⎡m 2 ⎤ Eq. 4.4-6: * ⎛ ⎞ , ε ty = κ ⋅ v 0 ⋅ h ⋅⎜1− ⎟ ⋅ ⎢ ⎥ ⎝ h ⎠ h ⎣ s ⎦ which is shown plotted as Curve 1 in Figure 4.4-6. The κ value stands for the von Kármán constant (Section 4.4.4.5.3).

4.4.4.4 Distribution of suspended-sediment concentration Assuming a linear relationship between the turbulent diffusion coefficient for the impulse exchange, εty, and that for sediment exchange, εsy, (dispersion) according to the relationship

Eq. 4.4-7: εsy = β ⋅ ε ty gives the distribution for the dispersion coefficient as

y y ⎡m 2 ⎤ Eq. 4.4-8: * ⎛ ⎞ εsy = β ⋅ ε ty = β ⋅ κ ⋅ v 0 ⋅ h ⋅⎜1− ⎟ ⋅ ⎢ ⎥ ⎝ h ⎠ h ⎣ s ⎦ Sediment transport 103 he reciprocal value, l/β, is also termed turbulent Schmidt coefficient. This function can now be introduced into the initial equation (Eq. 4.4-5), from which, after separation of the variables and integration, follows the relative concentration distribution after Rouse (1937) as

z C ⎛ h − y a ⎞ Eq. 4.4-9: = ⎜ ⋅ ⎟ []− . Ca ⎝ y h − a ⎠

The exponent

v Eq. 4.4-10: s z = * []− β ⋅ κ ⋅ v 0 is also named Rouse number.

The unknown integration constant (concentration is unknown for both near-surface and near-bed zones) has been eliminated, by use of the above equation, to a fictitious height y = a, via the concentration Ca present there (but not known). The characteristic curve of relative concentration distribution for different Rouse numbers, z, is shown plotted in Figure 4.4-7 below. It is common practice to take the height, a, as 5% of water depth h.

Figure 4.4-7: Relative concentration distribution of suspended particles for different exponents z

104 Sediment Sources and Transport Processes

4.4.4.5 Rouse number Considering that, as becomes clear from Figure 4.4-7, the Rouse number, z, exclusively determines the concentration distribution according to Eq. 4.4-9, its parameters and methods of determining them will be dealt with in greater detail below.

4.4.4.5.1 Settling velocity The problems involved in determining settling velocity for natural sediment particles of equal size were discussed in greater detail in Section 3.2.2.2. It was pointed out that this value is probably best determined by measurement. In practice, we are always faced with particle-size distributions, which in turn imply different settling velocities (Figure 3.2-12), which must be used to derive a representative value of settling velocity. Moreover, the increasing concentration near the bottom may cause a reduction in settling velocity, which implies that this quantity is no longer a constant. The same is true of the variable turbulence intensity.

4.4.4.5.2 Turbulent Schmidt number As follows from Eq. 4.4-7, this parameter, or its reciprocal value β, reflects the relation between the turbulent diffusion of the fluid and the dispersion of sediment particles. As little is known yet about this phenomenon, it is common practice to use a simplified value β = 1. Laboratory measurements suggest that β may rise to a value of 1.5, that is εs > εt. This may be explained by the centrifugal force (turbulence eddy) (van Rijn, 1984).

4.4.4.5.3 Von Kármán constant The constant named κ after von Kármán refers to turbulent flow provided the velocity distribution is uniform and logarithmic. Tests conducted by Nikuradse have resulted in a value κ ≈ 0.4, independently of wall roughness. Where the velocity distribution changes for constant shear-stress velocity and water depth, the logarihmic velocity law can be adjusted to measured values only by modifying the von Kármán constant, assuming that the velocity distribution is still logarithmic. Bed forms affect the κ value (Vetter, 1992) towards larger values. As the suspended-sediment concentration increases, this “constant” may, however, decrease to about 0.2 (Figure 4.4-8).

Figure 4.4-8: Velocity profile for pure water and suspensions (Vanoni, 1977) Sediment transport 105

4.4.4.5.4 Reduced Rouse number after Einstein & Chien As the Rouse number increases (z ≥ 0.5), the systematic differences between the calculated and the measured suspended-sediment distribution curves increase as well (Figure 4.4-9). Einstein & Chien (1954) stated several possible reasons for these divergences and developed several models for studying the effect the respective assumption has on the concentration profile.

Figure 4.4-9: Measured (z1) and calculated (z) Rouse numbers (Einstein et al., 1954)

For a comprehensive survey of other possibilities of determining suspended-sediment concentration distribution and comparison with a large number of measured values, see Vetter (1992, Section 5).

4.4.4.5.5 The influence of water temperature Water temperature affects solids transport in two ways. In the first place, viscosity rises as temperature decreases (Figure 3.2-1), which results in a corresponding reduction in settling velocity and Reynolds number. This means that the suspended-sediment concentration is lower at higher temperatures and, hence, that in summer less suspended sediment is transported than in winter, for otherwise equal conditions. Secondly, the onset of sediment motion is also changed as viscosity affects friction, or shear stress.

4.4.5 Calculating reference concentration Ca As mentioned in Section 4.4.4.4 above, with one boundary condition missing, suspended- sediment concentration distribution can be stated only as a relative value, that is to say, it has to be related to a still unknown reference concentration, Ca, and cannot be obtained as an absolute value. This problem has induced authors to develop several methods of computing the near-bed reference concentration in isolation. The reference height, a, is usually assumed to refer to a defined near-bed zone. 106 Sediment Sources and Transport Processes

A fundamental approach was developed by Einstein (1950). He defined the thickness of the near-bed layer in which bed load is moved as a function of the respective particle size as a = 2d. Assuming equilibrium to be present between sediment concentration at the top of the near- bed layer and sediment concentration at the bottom of the suspension zone, he arrived at an equation for the concentration Ca at a distance a from the bottom, assuming however a constant concentration curve in the sediment-laden layer. The idea of calculating sediment concentration at the bottom of the suspension zone from bed load discharge or, speaking more generally, by starting from the bottom, was adopted by several authors (Chang, Simons & Richardson, 1965, Itakura & Kishi, 1980). A recent assumption comes from van Rijn (1984). Van Rijn used two dimensionless parameters for deriving his relationships: Sedimentological diameter (Section 3.1.2)

1 ρ′⋅ g 3 Eq. 4.4-11: * ⎛ ⎞ D = ⎜ ⎟ ⋅ d 50 ⎝ ν 2 ⎠

Transport parameter

2 2 v*' − v* Eq. 4.4-12: T* = 0 0cr * 2 v 0cr

* where the particle diameter, d50, refers to the bed material and v0cr stands for the critical bed shear stress velocity after Shields (1936). Particle roughness related to bed shear stress *' velocity, v0 , is computed as follows:

g ⎛12 ⋅ R ⎞ Eq. 4.4-13: *' ′ ⎜ ⎟ (cf. Eq. 3.3-16) v0 = ⋅ vm where C = 18⋅ log⎜ ⎟ C′ ⎝ ks ⎠

Van Rijn took sand roughness as 3 · d90 of the bed material; R is the hydraulic radius. Bed load discharge is calculated from the product of the velocity and bounding height of the particles as well as from bed load concentration. Furthermore, bed load discharge, assuming constant concentration over the depth of the near-bed layer, is also obtained as a product of this concentration for a layer thickness a and an effective particle velocity. As in Einstein (1950), the concentration in the near-bed zone is assumed to be equal to the suspended- sediment concentration, Ca, at the bottom of the suspension zone. The effective particle velocity is taken as proportional to the velocity of the bed load particles. Comparison of the two bed load-discharge relationships finally gives an equation for the reference concentration

1,5 d T* ⎡ kg ⎤ Eq. 4.4-14: C = 0,015⋅ρ ⋅ 50 ⋅ a F 0,3 ⎢ 3 ⎥ a D* ⎣m ⎦

For the dimensionless value of concentration, the density, ρF, is omitted in Eq. 4.4-14. This value then applies only to total concentration rather than individual size fractions. The reference height, a, was determined by van Rijn as follows: Sediment transport 107

• in the presence of bed forms, a is equal to half their height,

• otherwise a = ks = 3 · d90 is taken, • the minimum value is a = 0.01 h. In order to be independent of measurements, van Rijn developed empirical approaches for the occurrence of bed forms and their dimensions as a function of the parameters D* and T*. All the approaches of this type require knowledge of the grain-size distribution of the bed material. Their disadvantage lies in the fact that they permit calculation of reference concentration only for the suspended-load fractions also present in the bed material. The proportion of wash load (Figure 3.1-1) transported in suspension only, which may account for more than 50% of the entirety of suspended particles, is thus ignored from the outset. In addition, the percentage of a certain size fraction in suspended bed material need not necessarily correspond to the percentage of the same size fraction in bed load and bed material.

4.4.6 Limiting remarks A multitude of approaches are known today for describing concentration distribution for suspended sediment in turbulent channel flows. They are based on various different assumptions. The most common models are those derived from the diffusion equation. It has been demonstrated that the concentration distribution curves so obtained are relatively satisfactory representations of the measured values (Vetter, 1992). Practical application, however, still involves two problems: 1 Reference concentration For computing absolute concentration distribution, it is necessary at first to measure the concentration at one point (usually y = 0.05h) in each cross section, as one boundary condition (at the bottom) for solving the diffusion equation is lacking. Methods of calculating this reference concentration, Ca, were mentioned and the reference concentration after van Rijn (Eq. 4.4-14) was discussed by way of example in the preceding section. 2 Comparing calculated and measured concentration distributions On the other hand, however, the good representation of measured values by the various models presented in the relevant literature has usually been achieved by curve adjustment, which means that the Rouse number was calculated back from measured data, whereas the value determined from sediment and channel data is usually greater. This is mainly a result of the fact that various, partly simplified, assumptions find their way into the various formulas, as with respect to velocity distribution, settling velocity, diffusion coefficient, von Kármán constant, Schmidt number, turbulence parameters etc. whose magnitudes cannot be determined with sufficient accuracy, the more so as these parameters interact.

Van Rijn (1984) presented a closed method for computing suspended-sediment discharge, ms, (Section 4.4.7) in 13 steps with all the necessary equations. As mentioned several times, the methods of calculating suspended-sediment transport allow only for the bed material and cannot be used for determining wash load. Many practical problems come from multi-dimensional effects in a cross-sectional plane, as e.g. secondary flows, which defy treatment by the one-dimensional approaches introduced here. This applies to widened channel sections, bends, groyne fields, distorted cross sections, reservoirs etc., where distinct secondary flows and at least two-dimensional effects 108 Sediment Sources and Transport Processes significantly influence the idealised one-dimensional flow pattern. In this case it is necessary to couple two-dimensional flow models averaged over depth with the diffusion-convection equation for suspended-sediment transport and solve them by appropriate numerical means. Separate allowance must also be made for the effects from steady nonuniform flow or unsteady flow, or sediment transport. In the preceding sections flow was always assumed to be steady and uniform. Such 2D or 3D models do exist and are used to solve practical problems (ATV-DVWK, 2003). These models can also be run under unsteady flow conditions, which makes it possible to simulate the development in time and space of sedimentation in washland, widened channel sections or reservoirs through long-term simulation using actual or synthetic inflow hydrographs and coupling them with an appropriate rate of suspended-sediment transport. It should finally be stressed once more (Section 4.4.1) that the hydraulic approaches presented here for computing suspended-sediment transport applies only to those particle sizes which, as shown in Figure 3.1-1, are present in the bed material of the stream under study (bed- material load). The wash load, which is an important factor in water-quality problems and, in countries having a large erosion potential, also in sedimentation problems, defies such calculation based on physical data. The methods used in these cases describe the process of sediment transport as a hydrological quantity. By measuring flow and the simultaneous suspended-sediment concentration, a relationship can be established between the two quantities using a regression equation (Section 4.4.9), where hystereses are common.

4.4.7 Calculation of the suspended sediment load Suspended sediment discharge, that is the mass of suspended sediment transported per unit time through a strip of cross section 1m wide, can be calculated according to Figure 4.4-10 by integration of the products of concentration C(y) and the associated velocity, vx(y), over water depth as

y=h ⎡ kg ⎤ Eq. 4.4-15: m = C ⋅ v dy s ∫ x ⎢ ⎥ y=a ⎣m ⋅s⎦

This is based on the assumption that the particles of suspended solids move at the same speed as the surrounding fluid, which may not necessarily be true of the near-bed zone.

Figure 4.4-10: Determining suspended-sediment discharge Sediment transport 109

Another problem to be considered is the fact that the calculated concentration distribution only applies to a representative particle diameter. Allowance for the actual particle-size distribution of the suspended load is made by determining the respective concentration distribution and, hence, the magnitude of suspended sediment discharge in isolation for several representative diameters of the total fraction (fractional suspended-sediment discharge). The fractional formulation of suspended-sediment discharge is treated in detail in ATV-DVWK (2003).

Eq. 4.4-15 can only be solved if the reference concentration, Ca, is known. This quantity is obtained either by use of a concentration measurement at the level y = a or by appropriate empirical approaches (Section 4.4.5). Reference is again made to van Rijn (1984). There are also some empirical formulas for suspended sediment discharge, which are all given in DVWK (1988, No. 87, Section 9). These methods, too, require knowledge of the suspended-sediment concentration at a given distance a from the bed. Wu et al. (2000) developed relatively simple formulas for nonuniform fractional transport.

4.4.8 Erosion – transport – sedimentation for suspended sediment transport 4.4.8.1 Erosion The particles set in motion after onset of erosion are transported as bed load or suspended load, the type of transport being mainly determined by the particle size (settling velocity) and the flow or shear-stress velocity. A particle picked up from the upper bed layer is moved by gravity (weight), the hydrodynamic uplift force (lift force) and the resistance force (pressure and friction resistance). Turbulence and its partly upward-directed velocity components (sweeps) naturally play an important part. Suspended-sediment transport is possible only where these components are greater than the settling velocity, which acts in the opposite direction. Studies have shown the magnitude of this vertical turbulent velocity variation to be of the same magnitude as the shear stress velocity, hence

m Eq. 4.4-16: * ⎡ ⎤ . v0cr > vs ⎣⎢ s ⎦⎥

The erosion rate or pick-up rate can be expressed in a simplified form for artificial homogeneous samples as

⎛ τ ⎞ ⎡ kg ⎤ Eq. 4.4-17: E& = k ⋅⎜ 0 −1⎟ für τ ≥ τ ⎜ ⎟ 0 cr,E ⎢ 2 ⎥ ⎝ τcr,E ⎠ ⎣m ⋅s⎦ where k is an empirical coefficient standing, among other things, for the state of consolidation of the ground and τcr,E(z) is the threshold bed shear stress for onset of erosion at a depth z. This simple relationship was given by such authors as Partheniades (1962) for cohesive material. For further information, see ATV-DVWK (2003) and Schweim (2005). An empirical equation developed by van Rijn (1986) for the pick-up rate is written as

1,5 0,3 ⎛ τ ⎞ ⎡ kg ⎤ Eq. 4.4-18: E& = 0,00033⋅ρ ⋅ D* ⋅⎜ 0 −1⎟ ⋅ ρ′⋅ g ⋅ d s ⎜ ⎟ ⎢ 2 ⎥ ⎝ τcr,E ⎠ ⎣m ⋅s⎦ 110 Sediment Sources and Transport Processes

4.4.8.2 Transport Exceedance of the threshold shear stress for onset of erosion does not automatically mean transport in suspension. As pointed out in the preceding section, assistance from turbulence, or diffusion, is needed, as expressed by Eq. 4.4-16. This is depicted in Figure 4.4-11 below, which demonstrates that for D*< 5mm, or d < 0.2mm, transport in suspended condition predominates; the particles, as soon as mobilised, will immediately go into suspension, thus precluding near-bed bed load transport. Larger particles, with D*> 10mm, or d > 2mm, once erosion has set in, need sufficiently large turbulent velocity variations to go into suspension. Such velocity variations may be caused by such factors as bed forms. Although studies like those on onset of erosion (Höfer, 1984, Figure 4.1-7) are not available for this problem, it is justified to assume here that in the presence of bed forms larger shear stresses are needed for suspension to set in than in the case of a smooth bed. The sedimentological diameter, D*, is defined as

1 ⎛ ρ′⋅ g ⎞ 3 Eq. 4.4-19: D* = ⎜ ⎟ ⋅ d ⎝ ν 2 ⎠

Figure 4.4-11: Onset of motion after van Rijn (1984)

The results of recent laboratory investigations by Roberts et al. (2003) on the ratio of suspended load to bed load as a function of particle diameter and bed shear stress (Shields parameter Θ = Fr*, Eq. 4.1-4) are shown plotted in Figure 4.4-12 below. Sediment transport 111

Figure 4.4-12: Erosion diagram (Roberts et al., 2003)

A dimensional representation of the relationship between particle diameter and mean flow velocity is shown in Figure 4.4-13. “The zones determining stability are the two main regions erosion (motion after the particles have been picked up from the bed) and sedimentation (rest). Hjulström called the threshold defined by Curve A erosion velocity. This is defined as the mean velocity at which uniform bed material starts moving. The third region, transport, is the result of subdividing the rest region. This includes velocities of sufficient magnitude to be capable of carrying along already eroded material of a particle size d without picking up material at rest from the bed. Curve C describes the stability behaviour of a bed consisting of the small particle sizes (d < 0.2mm) and not yet destabilised when coarse material is being transported over its surface. Hjulström pointed out, however, that this curve had been difficult to determine and, hence, was inaccurate. Therefore he added, for the purpose of comparison, a curve found by Owen (from: Hjulström, 1935) (Curve D in Figure 4.4-13) to his own interpretation of data. The importance of the different curves in Figure 4.4-13 will be illustrated by an example: Particles with a diameter d = 40mm can be transported over a bed of loosely packed particles of uniform size from a velocity um = 160cm/s (intersection with Curve B at Point 1). Erosion of this bed does not occur before a velocity um = 250cm/s (intersection with Curve A at Point 2) is reached. If such particles with a diameter d = 40mm and um = 160cm/s are transported over a bed of finer material, the bed material starts being unstable where the particles have a diameter d ≤ 18mm (intersection of the horizontal starting from Point 1 with the erosion curve at Point 3). If, however, coarse particles move over very fine material, then Curve C, rather than Curve B, should be selected to determine onset of motion for the streambed. Hence, take the point of intersection of the vertical at d = 40mm with Curve C (Point 4) and then draw a horizontal through this point of intersection and continue this horizontal to its point of intersection with the erosion curve for very fine material (d < 0.2mm) (Point 5). This point of intersection defines the maximum particle diameter bed material is allowed to have in order to remain stable just before starting to move. In this example, this is about d = 0.01mm” (Dittrich, 1998). 112 Sediment Sources and Transport Processes

Figure 4.4-13: Erosion, transport and sedimentation of uniform bed material using numerical values (Dittrich, 1998)

4.4.8.3 Sedimentation From a certain concentration of suspended particles, decrease in velocity, or turbulence, results in sedimentation. Logical considerations led Westrich (1988) to establish that e.g. for an even streambed a threshold concentration, Cgr′ , for onset of sedimentation can be derived from the relationship of the energy per unit time needed for suspension to the rate of turbulent-energy generation averaged over the discharge depth, which can be written as

τ ⋅ v ⎡ kg ⎤ Eq. 4.4-20: C′ = 0,0018⋅ 0 gr ⎢ 3 ⎥ ()ρF − ρ W ⋅ g ⋅ h ⋅ vs ⎣m ⎦

For fixed beds with bed forms, the generation of turbulent energy and, hence, the threshold concentration is perceptibly higher.

The sedimentation rate can be determined from the difference between the actual concentration and the near-bed threshold concentration (subscript a) as

kg Eq. 4.4-21: ⎡ ⎤ S& = vs ⋅ ()C − Cgr a ⎣⎢m 2 ⋅s⎦⎥

For simplification and assuming that the threshold concentration, Cgr,a is small in relation to the near-bed concentration of equilibrium, Ca, the relationship

kg Eq. 4.4-22: ⎡ ⎤ S& = vs ⋅ Ca ⎣⎢m 2 ⋅s⎦⎥ is also used. Sediment transport 113

The determination of the near-bed reference concentration was treated in Section 4.4.5 above. There are further approaches for determining the rate of sedimentation. They are similar in structure to Eq. 4.4-21 and include the threshold shear stress for onset of sedimentation.

4.4.8.4 Non-uniform transport All the above considerations have been based on the assumption of uniform conditions. However, where the streambed composition changes distinctly (transition from a fixed bed to a movable bed, Figure 4.4-14), suspension particles cannot be picked up instantaneously from the bottom under suitable flow conditions. A certain adaptation length is needed for steady conditions to be reached, with diffusion (balancing of the concentration gradient) as an important factor. This is an effect to be allowed for if suspended-sediment concentration distribution is calculated in discrete transverse profiles where these are situated within such an adaptation zone.

Figure 4.4-14: Adaptation of the local sediment concentration for transition from clear water to an alluvial bed (van Rijn, 1987)

4.4.9 Discharge / suspended sediment transport relation, sediment rating curve 4.4.9.1 Hydrological (stochastic) approach 4.4.9.1.1 General aspects A graphical relationship can be established between discharge and sediment transport on the model of stage-discharge, or Q-h, relations, also termed rating curves. This approach is suited for bed load and suspended-load transport as well as for the total transport and is commonly used in hydrology. The different approaches were described by Walling (1977). If e.g. median values of daily discharge are correlated with those of suspended-sediment load, the relation obtained is relatively unsatisfactory, as compared with hourly values, for small catchments, but improves for larger catchment areas. Hourly values are always larger than daily records because the relationship between discharge and suspended-sediment transport is not linear and the large values are of no great consequence due to averaging. The variation is between about 20% and 50%. Measurement of the two quantities during floods has shown, moreover, that the maximum suspended-sediment concentration is often reached before the maximum discharge is Figure 4.4-15). 114 Sediment Sources and Transport Processes

Figure 4.4-15: Discharge – suspended-sediment-concentration curves for the River Dart (GB) (catchment area = 600km²), (Walling, 1977)

This can be explained by the ascending flood wave showing a larger energy-line slope than the descending wave (hysteresis) and, hence, generating larger bed shear stresses. In addition, the fine material deposited in the streambed during preceding low-flow periods usually starts moving much earlier than the coarser bed load (non cohesive material) as discharge increases. This implies that the relations between discharge and suspended-sediment concentration are not entirely clear, as can be seen from Figure 4.4-16. Moreover, this relationship is dependent on the season as such factors as vegetation affect the amount of suspension load washed into a stream.

Figure 4.4-16: Discharge – suspended-sediment-concentration curves (Dart), (Walling, 1977)

Figure 4.4-16 above finally shows all the measured data for the River Dart, which permits derivation of clearly different regression relations (Figure 4.4-17) Sediment transport 115

Figure 4.4-17: Discharge – suspended-sediment-concentration curves (Dart) – different regression curves (Walling, 1977)

Naturally, this example for the River Dart is not representative of other streams, but it may generally be stated that such rating curves should be used only where sufficient measured data are available for the respective watercourse and if allowance is made for the season as well as hysteresis for unsteady flows. In addition, account should at any rate be taken of the way in which measured values have been obtained. It should be pointed out again in this context that there is a difference between suspended sediment from bed material and that from wash load (Figure 3.1-1). Whereas the transport of relatively coarse material (bed material) is dependent on transport capacity, permitting a significant relation to be established between discharge and transport, this does not apply in the case of very fine-grained material washed from the ground surface, as demonstrated by Figure 4.4-18 below. 116 Sediment Sources and Transport Processes

Figure 4.4-18: Relation between discharge and suspended-sediment transport for different particle diameters (Chien et al., 1999)

In Section 4.4.4.4 above, the distribution of suspended-sediment concentration over water depth was studied as a function of particle size and analysed. This unidimensional approach, however, does certainly not reflect the distribution of suspended-sediment concentration in a real stream. This would require knowledge of the actual distribution over the whole cross section of the stream. Figure 4.4-19 depicts, by way of example, the distribution of suspended-sediment concentration over the cross section as obtained from individual measurements over the depth and width of a stream (multi-point measurements) as is practised for discharge measurements. Sediment transport 117

Figure 4.4-19: Distribution of suspended-sediment concentration at the Burghausen measuring station on the River Salzach on 09-07-1990, discharge 740m³/s (WRS, 2000)

Although the above graph demonstrates how particle concentration increases from the surface down to the bottom, there are significant variations from the predicted distribution (location of the maximum).

4.4.9.1.2 Facts from German rivers Elbe The large-scale interdisciplinary project "The Morphodynamics of the River Elbe" included extensive studies on suspended-sediment load (BfG, 2000). This differentiated between fine- grained suspended particles (d < 63µm) and suspended sand (d > 63µm). In Figure 4.4-20, the relationship between suspended-sediment transport and flow is shown plotted separately for three suspended-sediment categories: Bed-forming fraction Bed-material proportion that does not go into suspension until high shear stresses are reached and is deposited when these decrease (suspended bed material). Wash-load fraction Proportion washed into the stream from tributaries and ground surface which is of a small enough particle size to be transported over major distances with almost uniform concentration distribution over depth. Total suspended material Bed-forming plus wash load material 118 Sediment Sources and Transport Processes

Figure 4.4-20: Transport of the suspended-sand fractions, Elbe/Schönau/Schmilka, River Kilometre 2.6/4.4 (BfG, 2000)

Lech/ Füssen Figure 4.4-21 below shows the relationship between discharge and suspended-sediment concentration as determined from measurements by Engelsing (1988).

Figure 4.4-21: Relation between discharge and suspended-sediment-concentration for the River Lech near Füssen (Engelsing, 1988) Sediment transport 119

Bauna/ Baunatal A rating curve showing an entirely different characteristic was obtained by Sobirey (2001) for the River Bauna. The suspended-sediment concentration appears to tend towards a threshold value along with rising discharge (Figure 4.4-22).

Figure 4.4-22: Discharge vs. suspended-sediment-concentration, River Bauna (July to November 2000) (Sobirey, 2001)

4.4.9.1.3 General equations Generally speaking, the relationship presented above (with the exception of the Bauna example) can be expressed by the following fundamental equations:

Eq. 4.4-23: m s = a ⋅ ()Q − Q 0

b Eq. 4.4-24: m s = a ⋅ ()Q − Q 0

b Eq. 4.4-25: m s = a ⋅ Q where Q0 stands for the discharge at the threshold of motion (BfG, 2000). Where a reasonable relationship between discharge and suspended-sediment concentration is present, the volume of suspended-sediment load can be determined by use of the relation

Eq. 4.4-26: m Fs = 0,0864 ⋅ C ⋅ Q [t]

(Morris et al., 1997). C should be introduced in terms of mg/l and Q in m³/s.

120 Sediment Sources and Transport Processes

4.4.9.1.4 Example: Relationship between discharge and suspended sediment concentration for the Kempten gauging station on the River Iller The rating curves for stage, discharge and suspended-sediment concentration using single readings (instead of daily averages) of suspended-sediment concentration obtained during three significant flood events, made available by the Bayerische Landesamt für Wasserwirtschaft, are shown plotted in Figure 4.4-23 to Figure 4.4-25 below. The plots clearly reflect the often-observed fact that the peak of maximum suspended-sediment concentration is reached earlier than the peak of the maximum stage or discharge (cf. Figure 4.4-15). An explanation for this phenomenon was given in Section 4.4.6 above. Plotting discharge against suspended-sediment concentration for the flood events shown in Figure 4.4-23 to Figure 4.4-25 yields the relationship shown in Figure 4.4-26, which, having a correlation coefficient of 0.95, shows good agreement. The discharge-concentration relation is plotted separately for the ascending and descending flood waves in Figure 4.4-27. The difference between the two also demonstrates that the rising flood wave carries a higher suspended-sediment concentration than the falling wave (Section 4.4.9.1.1). Actually, the concentration values that can be read from the annual records 1971 to 1998 from the German Hydrological Yearbook are even higher than the values of suspended-sediment concentration determined from the above three flood events as shown in Figure 4.4-26. The maximum value, obtained on July 10, 1975, was 12.7g/l.

Figure 4.4-23: Rating curve for discharge and suspended-sediment concentration, Whitsun flood 1999, Kempten gauging station / River Iller Sediment transport 121

Figure 4.4-24: Rating curve for discharge and suspended-sediment concentration, August flood 2000, Kempten gauging station / River Iller

Figure 4.4-25: Rating curve for discharge and suspended-sediment concentration, September flood 2000, Kempten gauging station / River Iller 122 Sediment Sources and Transport Processes

Figure 4.4-26: Relation between discharge and suspended-sediment concentration from three flood events, Kempten gauging station / River Iller

Figure 4.4-27: Relation between discharge and suspended-sediment concentration for ascending and descending flood wave, Kempten gauging station / River Iller Sediment transport 123

4.4.9.2 Hydromechanic approach (deterministic) In addition to the empirical methods of determining suspended-sediment concentration or suspended-sediment load from discharge, suspended-sediment values can also be obtained via the physical relations for suspended-sediment transport (Section 4.4.4). In this case it is necessary to define the parameters governing the process, such as particle-size distribution, settling velocity, bed shear stress, Von Kármán constant, diffusion coefficients for sediments, reference concentration etc. This allows the suspended-sediment concentration, or suspended- sediment load, to be determined for selected size fractions or a representative diameter of the particle-size distribution (van Rijn, 1984, Section 4.4.7).

4.5 Bed material load The total transport of bed material (not including wash load, see Figure 3.1-1) is composed of bed load and suspended load according to

Eq. 4.5-1: m Ff = m FG + m Fs [t]

Several formulas exist, the first going back to Einstein (1950), which –as in the case of bed load transport alone (Section 4.3.2) – are based on the assumption of bed material transport being proportional to velocity or bed shear stress, or stream power. Meanwhile stochastic models have been added. A list of formulas is given in DVWK (1988, Section 9).

4.6 Bed evolution equation Variation in bed level due to erosion or deposition can be determined by balancing the sediment flows entering and leaving a measuring section as well as from the mass of suspended particles present in this section. “Since suspended-sediment transport and bed load transport show different characteristic velocities and mechanisms, it is common practice to simulate them in isolation in numerical sediment-transport models. For this purpose, the control volume for deriving the balance equations as shown schematically inFigure 4.6-1 is split into a water body with dominant suspended-sediment transport and a thin near-bed layer with dominant bed load transport. This split can be omitted in large-scale models with a large grid aperture as compared to the evolution length of bed load and suspended load transport. In this case, the mass balance can be established directly by use of the total sediment flows – composed of bed load and suspended load – entering and leaving the section under study. By way of further simplification, a total-transport formula can be used.

124 Sediment Sources and Transport Processes

Figure 4.6-1: Vertical split of the water body using exchange term s as shown in the 1D formulation (ATV-DVWK, 2003)

For the above split into a water body and a bed load-carrying layer, the balance of the suspended-sediment flows entering and leaving the upper control volume yields the variation in time of suspended-sediment concentration in the water body. Attention must here be given to the sediment flows from the control volume, which are either “downward” (sedimentation) or “upward” (erosion). This sink/ source term in the suspended-sediment transport equation appears with inverted sign in the balance equation for the bed load-carrying layer, which yields the variation in bed level. As a balance is here established over the sediment volume, the variation in bed level needs to be multiplied by (1-np), where np stands for the porosity of the soil. The sediment volumes stored in the near-bed layer and their variations in time are ignored. In two-dimensional models, the balance equation for the near-bed layer, also termed bed evolution equation or Exner equation, is written as

∂ z Eq. 4.6-1: ()1− n ⋅ B + divq = s mit q = q , s = s p s s ∑ si ∑ i ∂t i i

r where qs (in [m³ sediment /(m· s)]) stands for the vector integrated over all fractions of bed load discharge. Where suspended-sediment transport is not bed-forming, the source term s can be neglected in the bed evolution equation. Where only suspended-sediment transport is important, e.g. for fine-grained material, the bed load-transport term is omitted. Then the variations in bed level are determined via the exchange term, s, alone. Its modelling is of central importance.” (ATV-DVWK, 2003) For soil formulation, see ATV-DVWK (2003, Section 5.3)

River morphology – Sediment transport and river bed 125

5 River morphology – Sediment transport and river bed 5.1 Introduction The preceding sections have presented the fundamentals to be considered in dealing with sediment transport problems. The greater part of these relationships have been obtained from systematic investigations in laboratory channels, calling for some measure of simplification, such as narrow particle size grading, straight channels and prismatic cross sections. Natural processes are, however, subject to all kinds of influences that cannot always be captured by formulas and, hence, may differ distinctly from laboratory conditions. Important contributions to the study of morphological processes may also come from other disciplines, in particular geology. The assessment of such processes not only includes computing sediment transport at a selected cross section, or striking a balance over a given period (usually one year), but knowledge of the geological conditions and the development of the overall regime within the continuum of the stream or stream section. Only then can any intervention in a watercourse be reasonably planned. Some fundamental comments should here be made regarding the development and maintenance of watercourses. Classical stream engineering as practised in the 19th and half the 20th centuries mainly relied on stream regulations and hard bank stabilisations. Meanwhile this philosophy has been superseded by near-natural development of streams, which aims to allow more freedom to running waters and adapt them more efficiently to their natural environments. There are, of course, limitations to such measures, as the protection of people from floods is still a matter of priority, especially in densely populated regions. Yet, even near-natural development means introduction of elements that alter the hydraulic as well as sedimentological and morphological conditions of a watercourse. Whereas a multitude of studies are available that offer useful recommendations regarding the calculation of water surface levels (e.g. DVWK-Merkblätter 220, 1991), it is still difficult to make general predictions regarding the effects on sediment transport, although relevant studies are under way. First recommendations and results have been published, e.g. in DVWK-Schrift 118 (1997) and Mitteilung 25 (1993b). The above factors do, however, in no way affect the morphological fundamentals found during investigations on running waters, most of them of constant direction. These fundamentals are the subject of this Section.

5.2 Morphological principles 5.2.1 General Plan as well as longitudinal and transverse sections provide important information on the character of a natural watercourse. Attempts have been made to devise rules permitting a universally valid description or classification. But this will always remain incomplete in view of the great variety of natural phenomena involved and the subjectivity of the observer. But it will be attempted in the following to link the rather abstract formulation of transport problems used so far to the features specific to a particular stream. There are two main valley configurations: • V-shaped valleys (erosive) • Floodplain valleys (accumulative) 126 Sediment Sources and Transport Processes

While in V-shaped valleys and their varieties (ravine, gorge, trough-shaped valley) the lateral valley boundaries reach directly to the streambed, the floodplain valley has a flat bottom extending between its lateral boundaries and the stream. The valley bottom has been formed by lateral erosion, fluvioglacial deposits or both. In the latter case, the stream is allowed to develop its course more or less independently of the valley shape, forming loops or meanders or braiding as can be seen very well in a plan view. A detailed classification of running waters was presented by Briem (2002).

5.2.2 Plan view 5.2.2.1 Criteria of tortuosity and sinuosity A few dimensionless numbers are given below to serve as a rough clue for classifying a stream or valley in relation to its surrounding landscape.

Figure 5.2-1: Dimensions for a meandering stream (DIN 4049)

Figure 5.2-1 above is the schematic diagram given in German Standard DIN 4049 showing lengths and designations. This may be used to define three ratios. The expression

lF − lT Eq. 5.2-1: e L = lT stands for tortuosity and is a measure of the extent to which the natural environment allows a stream or river to move sideways and form bends. The smaller eL, the straighter the watercourse. River morphology – Sediment transport and river bed 127

Stream sinuosity is defined as

l − c Eq. 5.2-2: e = T F c

This factor is mainly used for assessing meandering stream sections. Large stream sinuosity values are obtained when the watercourse is allowed to develop pronounced bends (Rhine: source to mouth, eF ≈ 0.9; Bonn to Cologne, eF ≈ 0.38). Finally, valley sinuosity is defined by the relationship

lT − c Eq. 5.2-3: eT = lT

Further terms relative to meander formation are given in Figure 5.2-1 above. In the same way as streambeds are altered mainly by bed forms in the presence of sediment transport, any natural watercourse will tend to form its bed in a sequence of braidings or ramifications and bends. This process is governed by such factors as gradient, discharge, particle-size distribution of bed material and the possibilities for lateral movement. There is no naturally straight river course flowing in its own alluvium. Where a straight course is produced by outer restraints, the thalweg within this section will not run along the centreline of the river or stream channel, but will swing from one bank to the other, thus producing alternate bars. Figure 5.2-2 below shows a regulated river section adapted to mini meandering.

Figure 5.2-2: Schematic diagram showing stabilisation of a navigation channel in a straight river section

Figure 5.2-3 below is a simplified representation of a braided and meandering river system. Several transition forms and varieties between the two are also possible. 128 Sediment Sources and Transport Processes

Figure 5.2-3: (a) Braided river system, (b) Meandering river system (after Einsele, 1991)

5.2.2.2 Straight river Channels referred to as straight rivers as occur in V-shaped valleys show only minor river evolution (Eq. 5.2-2). A straight river course is a result of mainly two parameters: Steep gradient and a watercourse evolution governed by geology. The thalweg does not run in the middle of the riverbed but swings between the two sides. The same phenomenon has been observed in laboratory tests. Rivers passing through their own alluvium do not remain stable in a straight course unless bed load transport is relatively low, and originally braided rivers tend to straighten when bed load transport decreases or is suppressed (Figure 5.2-6).

5.2.2.3 Braided river The characteristic features of ramified or braided rivers are a medium to large gradient and the heavy bed load transport involved. The river splits into several branches of approximately equal size joining in places to ramify again further downstream, forming islands and gravel banks, which change in location and shape during each flood (Figure 5.2-3 a). Three types of river braiding have been observed (Ahnert, 1996): • Erosional ramification, in rocky riverbeds (erosion-resistant section) • Wide ramification, in non-cohesive bed material and with non-stabilised banks • Ramification forming natural levées, in partly cohesive material Wide ramification prevails in Alpine regions. Bank erosion widens the riverbed, yet without necessarily resulting in a larger discharge cross-section, since the deposited islands in turn reduce the overall cross section. Any additional braiding thus reduces bed load-transport capacity, so that the number of possible branches is limited. Minor branches, having a lower transport capacity, have often been seen to shift during major flow and bed load-transport rates. River morphology – Sediment transport and river bed 129

The material eroded from the sides is deposited in the form of gravel bars as soon as transport capacity declines further downstream. The water flowing over such bars slows down due to the rough surface and the small water depth, so that bed load can further be deposited and the gravel bar grows. This process continues until the bar is no longer flooded even in periods of mean flow. Only large flows go over the gravel bar. Their smaller velocity in relation to that of the deeper main channel results in high stages, which in turn generates transverse flows eroding chutes (Figure 5.2-4).

Figure 5.2-4: Transverse channels on gravel bars (lower River Lech)

Figure 5.2-5: Natural levées (exposed) along the River Ammer, 1998 (WRS, 2000)

Natural levées (Figure 5.2-5) develop when floods carry sediments beyond the stream banks and into the flood plain. These materials are deposited behind the bank so as to heighten the bank slope. These, usually of varying height, are submerged in places during floods, which in 130 Sediment Sources and Transport Processes turn leads to the formation of chutes in the washland parallel to the river proper (Section 5.2.4.3).

Figure 5.2-6: Plan view of the upper River Isar in the years 1925, 1971, 1982 and 1984 (WRS, 2000)

Figure 5.2-6 above illustrates how the River Isar changed in the Ascholding flood plain from a braided to a straight river between 1925 and 1994. The original channel showed typical braiding. Then reservoirs were constructed at Sylvenstein and Bad Tölz, trapping considerable quantities of bed load, which then lacked further downstream, so that the riverbed alterations ceased. In addition, flow became more regular due to the large reservoir, the river channel incised into the ground (vertical erosion), which acted to straighten the bed.

5.2.2.4 Meandering river Two types of meandering can be distinguished: • Free meanders (river meanders) • Valley meanders A free meander forms in a valley floor consisting of riverine (alluvial) deposits. A valley meander, by contrast, has been formed through vertical erosion by a meandering river, which means that the valley itself meanders (Figure 5.2-3 b). Another phenomenon of this river type is migration. This is understood to mean both the widening of loops and their downstream movement (Figure 5.2-7).

Figure 5.2-7: Meander migration (Zeller, 1967)

What a river needs in order to develop meanders is a gentle slope, fine-grained (alluvial) material and the possibility of moving freely in its own alluvium.

River morphology – Sediment transport and river bed 131

5.2.2.5 Distinguishing criteria By use of the dimensionless values

B h Eq. 5.2-4: Y = F and Z = , h d where BF = river width, h = flow depth (channel full to bank level) and d = characteristic particle diameter, it is possible to predict channel shapes from Figure 5.2-8 below.

Figure 5.2-8: Distinguishing channel morphologies after de Silva (1991)

It should be noted, however, that the transitions are in fact fluid and the boundaries are not as distinct as suggested by Figure 5.2-8. This diagram is based on the assumption of a bed being in a state of equilibrium, with a balanced bed load regime. The relevant literature offers a number of further diagrams for characterising expected channel shapes. Freely developing river bends of mean-water and flood beds have become rare in populated regions, at least in Central Europe, where flood protection generally takes priority. River banks are usually stabilised by stone revetment. In addition, impoundment of a river acts to alter fundamentally its flow and sediment-transport regime. Renaturing of watercourses as mentioned in Section 5.1 above calls for profound knowledge of the morphological context as a basis for predicting potential effects on the flow and sediment-transport regime prior to launching structural measures, such as riverbed widening or removal of bank stabilisations. The physical causes for the development of braiding and meandering are still not entirely known. There are empirical formulas for predicting meander dimensions, and several reference quantities are available. Out of the great number of equations, the relationships relating to discharge are listed below, using simplified coefficients: 0,5 • Meander length lm ≈ 60 Q 0,5 • Meander oscillation width bM ≈ 10 Q

• Radius of curvature r ≈ 0,2 lM 132 Sediment Sources and Transport Processes

The flow Q to be used in the calculation corresponds to a channel full to bank level with a return period of one year. Furthermore, it is justified to assume that meandering occurs only for a bed slope I < 1 ‰. Detailed theoretical explanations and derivations from mechanics, especially of meander development, are found in Yalin et al. (2001).

5.2.2.6 Sediment transport in river bends Sediment transport in river bends is of extraordinary relevance not only to the formation of meanders, but also for stream-engineering measures (such as diversions).

Figure 5.2-9: Schematic diagram showing flow pattern in a bend

Figure 5.2-9 is a schematic diagram showing the flow pattern and water surface developing in a bend. The water-surface inclination resulting from the centrifugal force generates a spiral flow, resulting in a difference in flow direction between surface and bottom. This in turn creates a tendency to erode either the outer (eroding) bank, unless stabilised, or else the streambed (scour) and to divert the material together with the sediment carried by the stream to the inner bank, where part of it is deposited. This context is depicted once more in Figure 5.2-10 below showing a meander section. River morphology – Sediment transport and river bed 133

Figure 5.2-10: Example of a meander section (Ahnert, 1996)

As the greater part of sediment transport takes place at the convex curvature, lateral widening for the purpose of water extraction, as for once-through water cooling, or port mouths will best be arranged at the outer bank. Where it is absolutely necessary to construct a dam in a bend, the need for good flood discharge (weir at the outer bend) needs to be carefully weighed against the potential sedimentation risk for the powerhouse bay (at the inner bend). Recent experimental tests by Worm-eaten et al. (2005) regarding velocity distribution, water level changes and sediment transport in meanders have also thrown light on the effects of flooded smooth and rough wash lands on secondary flows and channel configuration.

5.2.3 Longitudinal profile 5.2.3.1 Undisturbed river The longitudinal profile of a river provides a good overview of the sediment regime between source and mouth. The classical subdivision is into upper course, middle course and lower course, with the river mouth as a possible additional section (Figure 5.2-11). 134 Sediment Sources and Transport Processes

The ideal river Upper course Middle course Lower course Mouth Plan view Types of mouth such as delta, estuary etc.

Irregular and frequent changes in morphology

Longitudinal section

Scouring

Accretion

Mainly deposition of fine sand and mud (High) mountains Mountains of medium Lowlands, plain relief intensity (foothills), hilly country Distinct valleys, few and Wide valley floors Little developed valleys small valley floors common Many small tributaries Few large tributaries Very few tributaries

Steep, Fairly uniform Flat longitudinal gradient distinctly changing longitudinal gradient longitudinal gradient Irregular cross section Fairly uniform Flat cross section

cross section Irregular course, Straightened course, Large bends, many bends gentle bends meandering

Mainly vertical Lateral erosion / Mainly lateral erosion / erosion accumulation or accumulation steady condition Coarse rubble to Gravel to Sand to ultra-fine particles ultra-fine particles ultra-fine particles

Figure 5.2-11: Characteristic subdivision of a watercourse in plan and longitudinal section (Ahnert, 1996) River morphology – Sediment transport and river bed 135

In an undisturbed river, erosion will prevail in the upper course, the middle course will be more or less in a state of equilibrium, while sedimentation will occur in the lower course. This is an approximate rule. In practice, degradation and accretion in minor sections depend very much of local conditions. The principal factors are the respective transport capacity of the river and the actual amount of sediment delivered to the section under study. Where transport capacity and sediment load remain constant, there is dynamic equilibrium, which means that the mean bed level does not change either over a major period. When transport capacity in the section under study decreases, sedimentation occurs, the material that is not moved on remains settled. In the inverse case, the river will make up for its bed load deficit by picking up material from its bed, so that the bed is eroded. A special variety of transport-capacity increase is what is termed latent erosion, where the riverbed remains unchanged either because it consists of immobile material, such as rock, or because the coarse portion of a particle fraction is left at the surface (paving) and protects the underlying mobile particles (cf. Section 4.1.4). Figure 5.2-12 shows the longitudinal profile of the River Rhine from source to mouth. Unlike the linear scale, the logarithmic height scale offers the advantage of showing very clearly the changes in gradient present in the lower course of the river. Another method would be to split the longitudinal profile into several sections and to use different height scales.

Figure 5.2-12: Longitudinal profile of the River Rhine (Ahnert, 1996)

The longitudinal profile is also very much affected by the fact that the particle diameter of the bed material decreases in the downstream direction (Figure 4.1-5 and Kap. 4.3.3.5).

5.2.3.2 Disturbances in the longitudinal profile Longitudinal profiles also clearly reflect disturbances to the natural watercourse due to such structures as dams. 136 Sediment Sources and Transport Processes

Reservoirs will silt up to a certain extent in the course of time as sediment transport decreases due to the reduced flow velocity within the dammed-up river section. Where a reservoir is an isolated facility or the last of a chain, there will always be erosion downstream from the dam because no sediment is supplied from upstream, so that the transport capacity of the flow must be met from available alluvial material. As a result, the riverbed will erode and deepen. This may be prevented by covering the riverbed with coarse material (stabilisation) so as to generate "latent erosion". Another method would be artificial sediment addition. Backwater lakes above river barrages may tend towards a new state of equilibrium after a certain amount of sedimentation.

5.2.3.3 Base level of erosion The main base level for a watercourse is sea level. The upstream riverbed cannot erode beyond the level of the river mouth. A regional base level for a tributary is its junction with a major stream. A local base level may also be a natural (rock bar) or artificial obstacle (transverse structure), which constitutes the maximum erosion depth for the upstreambed.

5.2.3.4 Backward erosion A change in erosion base level, as e.g. due to streambed deepening, increases the upstream gradient and thus intensifies erosion. This process propagates in the upstream direction – backwards – until a new state of equilibrium is reached, corresponding to the new base level with the larger gradient and the resulting larger bed load transport capacity. This in turn acts to deepen a watercourse including its tributaries, the valley slopes steepen and, hence, denudation (Section 2.4) intensifies, also proceeding uphill (backward denudation). Backward erosion has already been mentioned in the context of dyke or dam overtopping and their resulting destruction (Figure 4.2-2). The inverse effect is achieved by raising the base level, as by providing a transverse structure. This reduces the upstream transport capacity, resulting in deposition and sedimentation in the riverbed (backward sedimentation).

5.2.3.5 Bed failure - chute formation Erosion-prone sublayers, as of lacustrine clay or flinz, may become exposed when a more erosion-resistant gravel surface layer is removed. Then, any major flood event risks causing sudden vertical erosion with very dangerous consequences for the stability of banks and bridge piers. The Water Framework Directive for the River Salzach (WRS, 2000), which includes a detailed description of the different varieties and systematics of chute formation, states that a residual gravel layer of between 1.2m and 1.5m is required. This study is the source of Table 5.2-1 listing the causes of bed failure, as well as of Figure 5.2-13 showing chute formation in the River Salzach near Urstein. River morphology – Sediment transport and river bed 137

Figure 5.2-13: Erosion channel in the Urstein conglomerate (WRS, 2000) 138 Sediment Sources and Transport Processes

Table 5.2-1: Possible ways of chute formation (WRS, 2000)

Possible ways of chute formation

Minor riverbed incision into Chute-shaped riverbed Extensive riverbed incision into lacustrine clay or fine lacustrine clay or fine incision into lacustrine clay sands sands or fine sands

Slow or very fast Extremely fast deepening Gradual deepening deepening process in part process with sudden far- Local scours of limited process with far-reaching of the riverbed in straight reaching consequences extent long-term consequences sections without any (during extreme floods and (most likely variety) appreciable secondary on fine-sand subsoil) and transverse flows The phenomenon The phenomenon

Development of longitudinal chutes cutting into the fine-grained Sufficient normal bed load Bed load supply is sublayer, causing rapid transport to form a thin insufficient to offer efficient deepening of the chutes Self-protection through cover on the fine-grained protection to the fine- due to flow concentration. bed load supply is sublayer. This is gradually grained sublayer. This During minor flow rates, possible where sufficient sucked off by the flow leads to fast vertical the stream flows in the gravel is available (example: below erosion. A flood will result chute and risks causing Bergheimer Schwelle on in a permanently permanent deepening. The process The process the River Salzach) deepened bed. The ensuing process corresponds to the two varieties in the case of extensive incision Consequences as described in the column to the left, plus high damage potential for infrastructure and agricultural areas from a flood event, • Lowering of riverbed and water table, • Destruction of riparian areas, • Riparian areas run dry, affecting the ecology, • Hazard to infrastructure near or in rivers • Temporarily high (Siggerwiesen water maintenance cost treatment plant, cooling- Serious consequences caused by need for Consequences as for the water abstraction from where scouring occurs constant bank two potential varieties due Riedersbach stream, near bridge piers or banks restabilisation, to extensive incision (see in populated areas bridges at Laufen, left) • Rehabilitation measures Tittmoning, that are costly in the Burghausen),

The consequences The consequences long term, without the • Flood damage to prospect of residential areas development towards a (Laufen, Oberndorf, near-natural stream or Tittmoning, riparian area Burghausen) due to damage to bridges from floating logs falling into the river as banks are destroyed, and from damaged flood dykes.

River morphology – Sediment transport and river bed 139

5.2.3.6 Ripple-pool system – alternate bars This phenomenon, illustrated by Figure 3.3-1, was briefly discussed along with the roughness of natural streambeds. Field observations have suggested that especially minor streams are characterised by alternating sections showing higher flow velocity (often rapid), smaller water depth and, hence, larger slope in places, and of lower flow velocity with larger water depth and smaller slope. The latter are referred to as scours or pools, the former as ripples.

Figure 5.2-14: Ripple-pool series (WRS, 2000)

High, often supercritical, flow velocity over a ripple results in increased oxygen intake, which is as desirable in terms of stream ecology, especially fishery, as is speed reduction in the deeper sections. Such sequences, with a wave length about 5 to 7 times the stream width (Figure 5.2-14) as well as alternate bars are among the rhythmic phenomena in stream morphology. In minor streams involving greater slopes and usually larger bed loads, sediment tends to be deposited in the ripples, so as to form actual steps. This results in step-pool sequences (Figure 3.3-1 right). Alternate bars (Sections 4.2.2.2 and 5.2.4.2), distinctly visible in periods of low and medium flows, are regarded as forming the first step towards meandering (Jäggi, 1983), whereas bars in the middle of a stream (Figure 4.2-3 e.) may lead to braiding. While having positive ecological effects, alternate bars generate higher friction levels (Section 3.3.2.3), so as to affect bed load transport, especially during low and medium flows. By contrast, the effect of bars on roughness is small during bed-forming flows. Figure 5.2-15 (Jäggi, 1983) is a graph defining the regions likely to involve the development of alternate bars. 140 Sediment Sources and Transport Processes

Figure 5.2-15: Determining the minimum slope, needed for the development of alternate bars, B = channel width, d = particle diameter (Jäggi, 1983)

The magnitude of bars is defined by wave length, width and height. Normally, alternate bars migrate downstream. Their speed (migration velocity) is not constant, but is very much a function of flow. Jäggi (1987) gave a median migration velocity of 150m p.a., with a maximum value of 400m p.a., for the Alpine section of the River Rhine. This value should not be exceeded by the relationship between grid aperture and time step (adaptation length) of numerical models. Jäggi (1987) also gives equations for computing maximum bar height. Figure 5.2-16 below is a graph showing wave length in the longitudinal sections along the two opposite banks (Bavaria and Austria) of the River Salzach at a given time. This results as approximately 1,400m (WRS, 2002).

Figure 5.2-16: Longitudinal sections along the Bavarian and Austrian banks of the River Salzach in the Freilassing basin for the year 1953 (WRS, 2002)

This study also revealed, for Section 1 (River Salzach in the Freilassing basin), a decrease in wave length from 1,400 m to 1,100 m within the period 1953 to 1992, whereas the gravel bar River morphology – Sediment transport and river bed 141 remained fairly constant at a length of about 600 m, while decreasing in width from 70 m to 40 m. Strikingly, the height of the gravel bar dwindled from about 5 m to as little as 1m. This trend may be explained by the distinct bed load deficit of the section under study. Gravel bars act as bed load buffers and are systematically worn down where bed load delivery from upstream is low or lacking at all while bed load-transport capacity remains constant. At the same time, the thalweg tends to even out. Jang et al. (2005) developed a 2D model to simulate the behaviour of alternate bars, with allowance being made for erodable banks and the effect of bank vegetation.

5.2.4 Cross-section 5.2.4.1 Introduction Plan, longitudinal section and cross section interact closely, but acts of human interference with the stream regime have altered what used to be an undisturbed evolution of forms. Structuring streambed cross sections as has become fairly common corresponds in fact to an artificial development generated by the need for flood protection. The mean-water bed carries the smaller flows (and minor floods), whereas the – usually vegetation covered – washlands are inundated only by major floods. Bed load is transported only at the bottom of the mean- water bed, whereas suspended transport during floods extends over the whole stream cross section. Hence, floods may leave accumulations of formerly suspended material in the flood plains. Under conditions of free play of forces, cross sections will develop as dictated by the plan and longitudinal section of a watercourse as well as by the material in which flow takes place.

5.2.4.2 Dimensions of scours, pools and bars – structural variety Streams of appropriate structural variety develop distinct scours and bars. The depth of such pools is an important factor to be allowed for in the design of bank stabilisations, so that at least assessment of maximum scour depths is of great practical interest. Several authors have developed empirical approaches. That by Zarn (1997) will be outlined here. As in the case of defining characteristic stream shapes (straight, braided, meandering) in Section 5.2.2.5 above, Zarn introduced the three parameters stream width BF, mean water depth h and mean particle diameter dm as factors governing the development of height or depth (i.e. deviation from the mean bed level) of bed forms such as bars or pools in his formulas. They are given in Zarn (1997, Section 10). In Figure 5.2-17 below, scour depths determined after Zarn (1997) are plotted against discharge for several conditions in the River Salzach. Variant 2 stood for an increased width of 200 m as compared to the present state of 87m. This suggests that for the state of 1817 with an assumed mean width between banks of 500 m, maximum scours would have been expected to occur during maximum flow, whereas for the smaller width (and smaller gradient) these occur during lower flows. Zarn (1997) also derived empirical formulas for a parameter σ/h for the structural variety of a stream, where σ stands for the standard deviation of the differential heights of individual points in the transverse profile from their mean value. The ratio σ/h becomes large where water depth is small, which is an indicator of a large structural variety. These considerations become valid when bed load transport sets in, or the threshold bed shear stress is exceeded. Structural variety and, hence, the ecological value of a watercourse, naturally augment as stream width increases. 142 Sediment Sources and Transport Processes

Figure 5.2-17: Mean and maximum scour depths after Zarn (1997) for the River Salzach in the Freilassing basin (WRS, 2000)

5.2.4.3 Flood plains Floods carry substantial amounts of suspended load. When a dyked river overflows its banks during a flood, suspended material is carried into the flood plains and is deposited there. As water depth and flow velocity are distinctly lower than in the main channel, shear stress velocity is also reduced, resulting in an increase in Rouse number for the same particle diameter or settling velocity, as expressed byEq. 4.4-10. This means, according to Eq. 4.4-7, that concentration increases in the near-bed zone. Aided by washland vegetation, suspended material is "combed out", which results in sediment deposition and rising flood plain levels. In the long term, this may reduce the flood safety of the composite discharge cross section. Contaminated suspension material (Section11) may, moreover, add ecological problems to the flood hazard. This knowledge has led to the initiation of the INTERREG IIIB Project SUMAD (Sustainable Use and Management of Alluvial Plains in Diked River Areas). Results can be obtained from Grambow et al. (2006).

5.2.4.4 Fluvial terraces After Ahnert (1996), "a terrace is a flat slope portion bounded above and below by steeper slopes. Fluvial terraces are what remains of former valley floors, while the valley has further deepened. Having been created by the river, they bear witness to the fact that depth erosion was interrupted by a phase of lateral erosion or alluvial deposition during valley development". Terraces are classified, according to whether the valley floor consists of rock or fill material, into (Figure 5.2-18) • rock terraces and • alluvial terraces. River morphology – Sediment transport and river bed 143

Figure 5.2-18: Development of rock and alluvial terraces and their development stages (Ahnert, 1996)

Terraces may form as a result of: • Movements of the earth's crust • Changes in sea water level • Climatic changes • Changes in erosion base level A typical example of terrace formation is the middle course of the River Rhine.

5.2.4.5 Alluvial fans Where a stream coming from the mountains enters a plain, its slope and flow depth are substantially reduced and part of the sediment load is deposited. Once such alluvial cones or fans have formed, the stream starts again incising its own sediment. This may also involve the development of terraces.

5.2.4.6 River mouth – estuary – delta The junction of a minor stream with a major stream may take a great variety of forms, depending on gradient, sediment load and their respective flows. Any structural measure in such a case will have to be preceded by in-depth study of the local conditions. The following paragraphs will deal with the special features of major river mouths into the seas. There are two fundamental mouth types: a) estuary b) delta Figure 5.2-19 below shows schematic drawings of the two. 144 Sediment Sources and Transport Processes

Estuaries or estuarine deltas develop where the range of tide is large (as at the German North Sea coast) and estuarine flow is almost entirely superimposed on river flow from upstream over a major length (this is e.g. 100 km for the River Elbe). Such mouths are funnel- shaped since otherwise relatively large tidal flows (about 800 · 106 m³ for the River Elbe) could not flow in. In general, a profile develops under low tide, in relation to the tidal volume, so that sedimentological equilibrium is reached. Estuaries offer favourable conditions for the maintenance of deepened navigation channels.

Figure 5.2-19: Schematic drawing showing estuary and delta

Deltas normally develop on coasts with a low tidal range (such as Nile Delta, Po Delta). Another important condition is a high sediment transport rate in the river: The rising tide or flood current is no longer capable of driving back the normal flow of the river. River mouths, especially estuaries, are locations where the effects from the flow conditions in the river channel overlap with those directly induced by the sea, such as wind waves und currents parallel to the coast, which are generated either by the tides or by waves. Coastal currents may transport substantial amounts of sediment, causing structural changes in the coastline and, hence, are a factor to be allowed for in the design of coast protection structures. Fairly accurate computation of such transport processes (in particular the overlap between wave and current) is possible by use of morphodynamic models (ATV-DVWK, 2003). Careful field observation and measurements, combined with computations, will remain important requirements for the design of hydraulic structures.

5.3 Fluvial system Discussion of several phenomena governing river morphology in the preceding section will now be followed by an outline of the overall morphological system of a watercourse. Figure 5.3-1 below (Ahnert, 1996) is an attempt to represent the main elements with their interrelationships. River morphology – Sediment transport and river bed 145

Figure 5.3-1: Overall morphological system (Ahnert, 1996)

The exogenous and endogenous ("ex-systemic") forces were discussed already in Section 2.1 above. The "en-systemic" components - those forming an integral part of the system itself - can be subdivided into three groups: 1 Shape components I regional relief II streambed and valley floor and their gradients III slope shape and height 2 Process components: I weathering II denudation III fluvial erosion IV fluvial transport V fluvial deposition 3 Material components: I rocks and geological structures II regolith III stream load 146 Sediment Sources and Transport Processes

When splitting the overall system into its individual aspects, as depicted in Figure 5.3-1 above, allowance should naturally be made for the fact that the components interact, at least partly, and should not be considered in isolation. Moreover, these components proceed at different scales of length and time, which will be discussed in greater detail in Section 6.8 below.

5.4 Development of river beds 5.4.1 Sediment-continuity-equation (Exner) As in the case of the continuity equations for fluid mechanics, it is necessary to allow for the continuity of sediment masses over a control section. Derivation of this relationship (Section 4.6) for the unidimensional case and uniform flow results in the Exner equation:

∂ z ∂ q ⎡m⎤ Eq. 5.4-1: (1− n) ⋅ B + G − s = 0 ∂ t ∂ x ⎣⎢ s ⎦⎥ where n [-] porosity of bed sediment

zB [m] bed level above datum plane

qG [m³/ms] volume of bed load transport s [m/s] exchange velocity between suspended sediment and bed load (erosion or sedimentation) The source term s becomes zero where suspended-sediment transport does not participate in bed formation. For the two-dimensional case, see Eq. 4.6-1.

5.4.2 Equilibrium of a river bed (steady state) For assessing state of equilibrium, it is necessary in a first step to consider the space and time scale of the section under study (Section 6.8). The substantial seasonal flow variations will always cause local changes where no pronounced armouring exists. But these may be balanced over major periods. This situation is referred to as dynamic equilibrium, which suggests that occasional deepening or sediment deposition do occur, but the two processes balance each other in the long term. Even local effects, such as bank failure, risk causing streambed changes in the immediate vicinity, without however affecting a major streambed length. Dynamic equilibrium of a stream section is defined by the fact that the mass of material supplied to the section under study is equal to that discharged from it, and that the granulometry of the transported material is equal to that of the sublayer. As becomes clear from Figure 4.3-7, this condition is reached provided the threshold flow for the armour layer to break open, QD, is exceeded. Where, however, Q < QD, the granulometry of the transported bed load material no longer corresponds to that of the sublayer. In the case of bed load deficit – material supply being inferior to the stream's transport capacity – bed load is picked up from the bed until the transport capacity is met. This is brought about by a reduction in bed gradient and the ensuing new state of equilibrium. Erosion may be reduced by armouring, which, while preventing bed erosion, reaches but an apparent equilibrium (latent erosion). River morphology – Sediment transport and river bed 147

Where bed load supply exceeds the stream's transport capacity over the section under study, bed load will be deposited in the bed until the gradient is large enough to be capable of carrying the material supplied through that section. In practice, dynamic equilibrium will be maintained only for relatively short periods because man-made intervention, such as retention above dams, dredged material, river-basin development) or changes in the hydrological (flood) or sedimentological conditions will always alter the stream's morphology to some extent.

5.4.3 Influence of bed width An essential parameter to be allowed for in stream regulation is bed width, which has a great influence on the regime. The following quantities change along with an increase in bed width, BF: • decrease in water depth • decrease in wall influence • increase in transport-effective width.

Figure 5.4-1: The influence of bed width on bed load transport capacity (top: a) and gradient (bottom: b) (Zarn, 1997)

This relationship is depicted in Figure 5.4-1 above. Starting from a very small bed width, and for constant gradient and flow, the bed load-transport capacity rises along with increasing width until it reaches a maximum value when arriving at what is termed optimal width. This term reminds of the fact that former stream regulation measures were aimed at maximum transport capacity, and that this was the width which best met this aim (Hunzinger, 1998). Decreasing width – to the left of this maximum – is accompanied by a reduction in transport- effective width and, hence, by an increase in wall or bank influence, which in turn involves a 148 Sediment Sources and Transport Processes reduction in transport capacity. Where the streambed width is above optimal, flow depth decreases and, hence, the bed shear stress is reduced; bars and stream ramifications develop, which in turn implies a decrease in transport capacity, tending towards a constant value (threshold value) for large bed widths. On the other hand, when starting from a constant transport capacity (Figure 5.4-1, bottom), the gradient needs to be adjusted to the various widths, reaching a minimum value for the optimal width. Bed width and discharge normally being given facts, it is the gradient that needs to be changed to adjust a stream to different rates of bed load transport. That means that a stream of different width should accordingly have a different gradient to ensure a given transport rate. A change in bed load supply, as due to tributaries or abstraction, involves a change in gradient changes. The adverse consequences of former stream control works – bed deepening, watertable lowering, hazard to bank stabilisations and bridge piers, ecological impoverishment of floodplain vegetation etc. – are now being remediated by taking such measures as mechanical or auto dynamic streambed widening. This has been the subject of a large number of publications (Zarn, 1997, DVWK, 1997, Section 4.5). An outline of experience gathered so far has recently been published by Requena (2005). In their publication, Schmautz et al. (2002) compare results from model tests with numerical simulations of auto dynamic streambed widening.

5.4.4 Bed forming or dominant discharge Long-term modification or adjustment of a streambed to changed conditions can be simulated by means of hydrodynamic sediment-transport models for appropriate flow hydrographs (Section 6.4). A simplified concept, omitting such time-consuming computations, uses a "dominant (bed-forming) flow" instead, which is taken as representative of bed evolution. Uniform definitions of this term do not exist, but there are different approaches by various authors for determining this value. Leopold, to quote an example, takes this as the mean annual flow. Others choose the discharge in a "brimful" channel as being dominant. Another definition came from Schaffernack (1950). His approach is illustrated by the schematic diagram shown below (Figure 5.4-2). The dominant flow in this case is the maximum of the curve obtained from the product of flow duration curve and bed load transport curve.

Figure 5.4-2: Determining dominant flow (Bechteler, 2004) a) flow duration curve b) bed load transport curve c) product of a) and b) River morphology – Sediment transport and river bed 149

This procedure is shown plotted in Figure 5.4-3 below for a section of the lower River Salzach, together with the bed load transport curves obtained from the different approaches. This demonstrates that actually a distinct maximum is not always obtained for the dominant flow as defined according to Figure 5.4-2, but that maximum bed load transport takes place over a certain flow range. Naturally, the selected bed load transport function also affects this value.

Figure 5.4-3: Determination of dominant flow for the section between River Kilometres 52.8 and 49.0 on the Lower River Salzach (WRS, 2002)

The same can be said of the "dominant – bed-forming – water level". Analogous terms used for dominant discharge are formative, representative and characteristic flow or discharge, as well as sediment efficiency.

150 Sediment Sources and Transport Processes

5.4.5 Stable channels – regime theory This is understood to mean an empirically developed description of fundamental data for a watercourse that is not to be changed in geometry, so as to remain stable for a long time (stable channel design). This regime theory was developed for the construction of irrigation canals in Pakistan and India. The regime equations are fundamentally parameters that are functions of discharge subject to coefficients: " • water depth h = Ah · Q $ • bed width b = Ab · Q ( • bed gradient I = AI · Q * • flow velocity v = Av · Q The various non-dimensional coefficients and exponents are extremely variable and dependent on local conditions. Application of the regime theory calls for appropriate experience and means nothing but finding relationships specific to a certain watercourse. In this case, these will normally be straight channels with constant discharge and homogeneous sand beds. For greater details, see Chang (1987) and Raudkivi (1976). This literature also provides information on the so-called rational method, by which stable transverse profiles are computed by use of empirical hydraulics and sediment-transport formulas. A summarising critical discussion of the regime theory is found in Bakker et al. (1989).

5.5 Possibilities of calculating theoretical bed load transport capacity 5.5.1 Discrete method It is still fairly common practice to assess the bed load-transport capacity of a stream by calculating a theoretical bed load transport with the help of a bed load-transport formula as described in Section 4.3.2, and a flow formula, such as that after Manning/ Strickler, for a representative cross section defined by geometrical values (width, flow, gradient, characteristic particle diameter etc.). This is then compared with actually measured bed load data where available, or with a predicted bed load supply. Whilst measurements are usually short-term in character, bed load estimations are possible only for major periods, e.g. one year. Then, by calculating the annual bed load volume by use of the duration curve (Section 4.3.4), it is possible to compare the two values. The complexity of the problem – mainly due to the great number of time-variable parameters often difficult to grasp, such as roughness, armouring, bed forms etc. – implies the risk of major divergences between measurement and computation. As pointed out in Section 4.3.1 above, bed load-transport formulas are always based on simplified assumptions. Actually, all such equations have been derived from laboratory tests for steady flow conditions, which is but one factor responsible for the substantial differences from natural conditions, where the main proportion of bed load transport takes place under highly unsteady flow conditions. Analysis by points (in profiles) then yields transport values for each cross profile, as shown in the schematic diagram of Figure 5.5-1 below. River morphology – Sediment transport and river bed 151

Figure 5.5-1: Profilweise Ermittlung des Geschiebetransports für eine Flussstrecke

The transport capacity as determined according to Figure 5.5-1 is then used for comparing results over a stream section of major length so as to derive sedimentation or erosion trends. To a certain extent this may also serve for predicting the potential effects of planned hydro- engineering measures on stream regime or bed evolution. Based on simplified assumptions for given values of stream width, gradient and particle size, Jäggi (1992, Section 2.2) derived a relationship between the transport-effective fraction of a volume of water flow and a volume of sediment load. "Calculations by use of different flow duration curves yield the influence of a variation in hydrological parameters. Any analysis by individual cross profiles will usually take the form of comparing two or more volumes. Depending on the problem, these will mean comparing only minimum and maximum volumes, or one minimum and one maximum volume each. This will depend on whether there is an erosion trend or a sedimentation trend at the site under study. In order to find out e.g. whether bed erosion in a steep stream section results in sedimentation in the following flat section, it is necessary to compare the maximum transport capacity of the flat section with the maximum transport capacity of the steep section." (Jäggi, 1992) Figure 5.5-2 below is an example of plotting the curve of theoretical bed load-transport capacity, in terms of tonnes p.a., for a stream section using several transport formulas. The boundary conditions used included the mean duration curve for characteristic stream sections (with allowance being made for tributaries) and the sublayer taken as bed material. 152 Sediment Sources and Transport Processes

Figure 5.5-2: Theoretical bed load-transport capacity for the actual state (2002) of the Upper Iller

5.5.2 Continuous method Natural processes such as discharge are rarely steady. In fact, discharge and bed load transport tend to change distinctly over relatively short periods, especially during floods. Also, there is no state of equilibrium in this case , as pointed out in Section 5.4.2. The streambed will rise due to sedimentation or deepen due to erosion when the threshold shear stress for break-up of a potential armour layer is exceeded, which in turn will affect discharge and gradient. This dynamics is amenable to differential treatment for small time steps only, which means that the computation can no longer be carried out for steady conditions – for a characteristic flow however determined – but needs to be based on at least quasi-steady (i.e. for constant, but abruptly changing discharge) or unsteady conditions for an actual flow hydrograph. In addition to the time steps ∆t, discretisation should also be performed for length intervals ∆x, which will normally be much smaller than the usual 200m interval between cross profiles. For each of these sections, differential equations are solved for flow (equations of motion and continuity), bed load transport, and possibly also suspended-sediment transport, and the continuity equation is solved for the sediment (Exner equation, Eq. 5.4-1) for each time interval, so as to make allowance for the dynamics of the process. Morphodynamic models will briefly be discussed in Section 6.4.

5.6 Artifical sediment feeding 5.6.1 Fundamentals It follows from the theory of bed load transport, thoroughly discussed in Section 4.3 above, that artificial bed load addition may serve two purposes: 1 compensate for bed load deficits, 2 stabilise a streambed through coarse-grain enrichment and armouring. River morphology – Sediment transport and river bed 153

5.6.1.1 Compensation of bed load deficit through sediment feeding Every stream has a theoretical bed load-transport capacity which can be calculated or estimated, according to Section 5.5, as a function of discharge for each characteristic section. Where this capacity cannot be met through material supply from upstream, the stream power available for bed load transport will pick up particles from the streambed. Starting from a downstream natural (rock sill) or artificial (transverse structure) "fixed point", bed erosion will proceed along a rotational or wedge-shaped path (for base level of erosion, see Section 5.2.3 above) until the gradient and, hence, the flow velocity have become small enough to restore the state of equilibrium. Artificial addition of bed load material approximately corresponding to the grain mix of the streambed may help to reduce or even entirely remedy this deficit, at best so as to maintain the status quo. Excess material, in case the addition exceeds the stream's capacity, is however deposited in the streambed since the natural transport capability is no longer large enough to carry along the entirety of added gravel. In practice it is not at all easy to add the "correct" amount of material since the theoretical bed load-transport capacity is strongly dependent on discharge, which in turn is impossible to predict in the long term. It has proved good practice to provide for ground storage at one or several points, usually by the side of the respective watercourse, from which the stream will pick up and carry along bed load material to the extent of its transport capacity.

5.6.1.2 Enrichment of coarsematerial fraction Instead of natural bed load material (approximately the same particle-size distribution for added and natural bed materials), it is possible to add coarser material in order to permit the formation of an armour layer of corresponding coarseness, which – as described in detail in Section 4.1.4 above – is capable of resisting higher shear stresses so as to protect the underlying streambed material (sublayer) from erosion. Bed load of finer particle size can be moved over this layer without causing any major bed deformation.

5.6.2 Example: River Rhine downstream of Iffezheim barrage The Iffezheim reservoir and power station (commissioned in 1977) is the last of 10 barrages built to date in the canalised Upper Rhine. Another power station was planned to be constructed at Au/Neuburg, where the gradient is still relatively large and, by the standards of that time, could be controlled only by means of barrages This project was, however, deferred by virtue of a Franco-German agreement of 1982 until bed load addition was feasible as a precaution against bed erosion. This measure was suggested already in 1989 by the German Bundesanstalt für Wasserbau at Karlsruhe. Bed armouring, studied as an alternative in prototype tests, was also rejected. The reason was that the coarse material, while proving stable for flood discharge, was moved by the propeller jets from vessels, and even risked causing damage to the vessels (Rossbach et al., 2000). Preliminary tests were performed below the upstream Gambsheim power station by dumping gravel of about the same particle mix as the bed material from hopper barges of 150 m³ capacity. The layer of added material was between 40m and 220m long, between 7 m and 15 m wide and 0.1 m to 0.6 m thick. An average volume of 200,000 m³ of gravel (minimum 68,000 m³, maximum 339,000 m³) is added every year below the Iffezheim barrage. The barge places the material in 7 to 10 154 Sediment Sources and Transport Processes downstream trips per day. This volume (1,000 m³ to 1,700 m³ per day) is regularly hauled and placed, so that material is deposited during low-water periods and distinct scouring occurs during floods. Especially at low stages, preceding scouring might result in a drop in water level and thus become a hazard to navigation. Refilling the scours helps to restore the water level to its normal stage. "Placement itself is organised according to plans that are constantly updated following bed level surveying by depth-soundings in profiles at the placement sites and immediate interpolation to draw underwater contour maps. In addition to these daily direct depth- soundings, the overall placement area as well as bed load transport over a downstream control section of 2 km length is monitored on a fortnightly basis. After extraordinary hydrological events, such as floods, in any case however once year, the section between the Iffezheim lock and approximately Rhine Kilometre 352.0 is surveyed by depth-soundings and the resulting underwater contour map subjected to comparative assessment. At the same time, the flow-dependent water surface levels should be monitored after bed load addition so as to derive riverbed behaviour in terms of time and space, such as accumulation or – in case of water level drop – scouring. Thirty-six gauges installed in the river section under study monitor the water level evolution. Bed load addition has been seen to cause variations of up to 0.3 m for comparable flows. The water level at the Iffezheim gauge (Rhine Kilometre 336.2) during a low-water flow of 570 m³/s is related to an initial water level of 111.11 m above datum from the year 1978, and an underwater ground sill level of 197.78m above datum for the locks has been taken as the reference level for potential changes. The measure of change, that is a drop in water level for comparable flows beyond a predefined level, is taken to be a signal for taking well-planned extended action. (Rossbach et al., 2000).

Models 155

6 Models 6.1 General Models are a simplified, usually reduced, depiction of (or excerpt from) nature, intended to reproduce the parameters governing a natural process. Such models are used to carry out experiments (model tests, numerical simulations, scenarios). The German dictionary Brockhaus (dtv) defines the term "experiment" as follows: "The artificial generation and permutation of observation conditions in order to obtain scientific data with the aim of establishing, confirming or disproving hypotheses, laws, theories. The experiment is the main empirical method of the natural sciences as well as of other sciences employing empirical means". Models are used where complex processes defy deterministic/analytic description with a reasonable input of time and money. Such processes certainly include unsteady flows in natural channels, especially where study of the interaction between flow and a movable bed is required. In such cases, models are used to simulate processes taking place in nature. Normally these models can consider no more than the principal properties or parameters (Section 6.6.1) governing a process. Also, models are capable of prediction only if calibrated against nature through parameter adjustment and if, where possible, tested (validated) through additional, independent measurements (Section 6.7.1.3) In addition, the quality of model results is dependent on the model operator's experience with both the natural processes and the respective model technique.

6.2 Types of models In the following, the emphasis will be placed on bed load transport as a determinant factor in the morphological evolution of watercourses. Models can be classified into the following principal types: • physical models (PM) (Section 6.3) • mathematical or hydrodynamic-numerical or morphodynamic models (MM) (Section 6.4) • hybrid models (Section 6.5). Model applications have shifted more and more towards the mathematical type over the past few years, as increasing computer capacities and the great progress that has been made in numerical mathematics offer almost unlimited simulation possibilities. Nonetheless, there are still a host of problems where use of a physical model (laboratory model, Section 6.9.1) is necessary or at least of advantage. A detailed up-to-date description of the two model types, including practical examples, is given in the publication ATV-DVWK (2003), of which a new edition is planned for 2007.

6.3 Physical Models (PM) Physical models are fundamentally subject to scale: Translation between model and prototype needs to be supported by so-called model laws.

156 Sediment Sources and Transport Processes

6.3.1 Nature The best physical model is nature itself (scale 1:1). Man-made interventions and structures should deal very carefully with this "model" as otherwise unforeseen and undesirable effects may result. However, experience gathered from comparable situations (reference sections) on other streams with similar conditions (case studies) may be used to advantage for developing solutions to a particular problem, with due account being taken of the characteristics specific to the site under study.

6.3.2 Fixed bed models Such models are now used mainly for optimising a particular structure involving flow processes that are too complex to permit simulation in a mathematical model at reasonable cost (e.g. geometrical discretisation). Fixed-bed models can also be run at different scales for lengths and heights (distorted models) in order to provide reasonable water depths especially for major rivers or low-stage flows (Section 6.6.3). This would imply, however, that certain effects can no longer be modelled with sufficient reliability (jet separation, spreading processes etc.). Such models are relatively easy to build and operate. Their use is limited to situations where bed load transport has no appreciable effect on flow.

6.3.3 Movable bed models Where the dynamic development of movable streambeds or banks, or islands, is to be studied in practice, the models need to be provided with movable beds. Simulation of transport processes including interaction between water and streambed, as characterises almost any natural channel, is usually possible only in a PM provided appropriate simplifications are made. This is due to several reasons: • the input of time and money is relatively large; • translation of model results to nature is not easy since the pertinent physical conditions may not be sufficiently known; • the scale ranges of the hydromechanical and transport-mechanics parameters are very large; • model technology is difficult and calls for great experience. Nonetheless, PMs with movable beds will often be the only means of finding answers to certain practical questions. The main purpose of a movable-bed model is the near-natural simulation of all phenomena of sediment transport (incipient motion, transport rate, bed forms, interaction between bed load and suspended sediment etc.). The other conditions of similitude need to answer this requirement.

6.4 Mathematical models (MM) Most of the extensive, but increasingly also small-scale, flow processes are nowadays studied by use of mathematical models, also referred to as 1D, 2D or even 3D models. The respective dimension is selected according to the flow problem under study. Essential to such models, unlike physical models, are equations that are reliable descriptions of the physical processes involved. Models 157

It is common practice to use the Navier-Stockes, or shallow-water, equations coupled with a loss equation (turbulence model or simple loss equation after Strickler). Empirical-theoretical equations that are adequate descriptions of the complex relationships involved (DVWK, 1988) are available for sediment transport (Section 4). Together with the shallow-water equations, they yield a system of three to eight or more equations, which are solved numerically by use of appropriate diagrams. To this end, a system of cross sections or grid points needs to be optimally adjusted to the area under study. The equations are then coupled or decoupled, as the case may be (fluid motion and sediment transport). Inaccuracies come from simplified, often empirical, assumptions for the flow equations and in particular the transport equations. They also come from the numerical treatment necessary for solving this system and from the the necessity of making assumptions for the great number of parameters, which in many cases do not reflect reality with sufficient truthfulness. Thus, the variation of individual parameters, as roughness, along with discharge and season is often left out of account. Numerical models are discrete models, equations being solved only for the points of the numerical grid. Topographical conditions (failure edges and banklines) and flow effects lying within one grid interval are not allowed for. This demonstrates the importance of well-devised grid generation. As in the case of physical models, mathematical models need calibration against nature. In addition, it is urgently recommended to carry out a plausibility check (verification) as well as validation (analysis of natural phenomena other than used for the calibration), especially where highly complex models are involved. Problems where present are usually due to the absence of record sets of sufficient variance (Section 6.7.1). The advantage of mathematical models lies in the absence of scale effects for what takes place in the computer is a non-reduced depiction of nature, related to the grid. Furthermore, the variation of parameters or boundary conditions (roughness, bank or bed locations etc.) presents hardly any problems. Especially the 1D models have very short computation times, which is of substantial importance for long-term simulations.

6.5 Hybrid and combined models A hybrid model is understood to refer to the direct coupling of mathematical and physical models, so as to make the two interact (Figure 6.5-1). E.g. while extensive flow fields are modelled on the computer, small-scale problems are supplemented by physical models.

Figure 6.5-1: Schematic diagram of hybrid model

A combined model may be the method of choice for a situation like the following: A physical model incapable of answering the conditions of similitude for various reasons (e.g. distortion) is analysed at the same scale in the mathematical model using the measured data from the PM and is then calibrated. Then the MM is corrected for distortion and adjusted to real-life conditions, that is reality is now simulated with allowance being made for experience gathered from the PM. 158 Sediment Sources and Transport Processes

It is also possible to simulate individual phenomena (effects from structures built into the streambed, relocation of flood dykes etc.) in distorted or non-distorted physical models at a relatively large scale and then use a mathematical model to check the results from the physical model for the entire stream section under study. This would also take account of unsteady effects. This helps to identify the limits of mathematical models, e.g. where scaling by means of the adjustment parameters does not yield at least satisfactory agreement for the results from the physical model. This is particularly true of 2D and 3D effects, such as secondary flows, on sediment transport (alternate banks, bed deformation in bends etc.). Combined use of mathematical and physical models substantially enhances the prediction quality for a particular design.

6.6 Model laws for physical models with movable bed 6.6.1 Dimensionless parameters A large number of studies exist on criteria of similitude for physical models with movable beds, as e.g. Yalin (1977 and 1992). According to these, geometrical quantities, such as configuration of discharge cross section and particle shape as well as particle-size distribution (grading curve), are assumed from the outset as being similar to nature, with the aim of minimising the number of governing parameters. In addition, the following considerations hold only for non-cohesive material: Thus, two-phase flow (water and bed load) can be completely characterised by seven quantities:

Fluid : kinematic viscosity ν, density ρW Bed material : particle size d, density ρ Eq. 6.6-1: F Channel flow : water level h, bed gradient I Motion initiated by : acceleration g

This parameter set (Eq. 6.6-1) can, however, be modified by combination of its quantities, which leads to an equivalent system for describing steady bed load transport. Yalin (1977) introduced the following parameter combinations: Specific gravity of particle under uplift pressure:

Eq. 6.6-2: γ F = g ⋅ (ρF − ρ W )

Bed shear stress velocity:

* Eq. 6.6-3: v 0 = g ⋅ h ⋅ I

Thus, according to Eq. 6.6-1, the parameter set is modified on an equivalent basis to yield the following seven quantities:

* Eq. 6.6-4: ν,ρW ,h,d,ρF , γ F , v0

Any mechanical quantity A describing a phenomenon of two-phase flow needs to contain the 7 dependent variables of the equation set Eq. 6.6-4, hence Models 159

* Eq. 6.6-5: A = f (ν,ρW ,h,d,ρF , γ F , v0 )

According to Buckingham's theorem, the seven parameters with three dimensions (mass, length, time) can be used to define four dimensionless quantities: Sediment Reynolds number:

v* ⋅ d Eq. 6.6-6: Re* = 0 ν

Sediment Froude number:

2 v* Eq. 6.6-7: Fr* = 0 ρ′⋅ g ⋅ d

Relative density:

ρ − ρ Eq. 6.6-8: ρ′ = F W ρ W

Relative roughness:

h Eq. 6.6-9: ε = d

The sediment Reynolds number (Eq. 6.6-6) takes account of the influence of near-bed viscosity. The smaller this number, the larger its impact on sediment transport. The sediment Froude number (Eq. 6.6-7) characterises the ratio of flow resistance acting on a particle to its weight. This number thus describes mobility. The larger the sediment Froude number, the larger is the rate of bed load transport. This is why this quantity is of particular importance for studying bed load transport.

The relative density ρ' (Eq. 6.6-8) makes allowance for the influence of sediment density ρF and thus governs the mobility (named "ballistics" by Yalin) of a sediment particle. The relative roughness ε (Eq. 6.6-9) describes the influence of water depth. This quantity is important with respect to the shape (height and length) of bed forms, which in turn governs the flow and, hence, bed load transport. Using the dimensionless numbers of Eq. 6.6-6 to Eq. 6.6-9, Eq. 6.6-5 can be written as

Eq. 6.6-10: π = ϕ(Re* ,Fr* ,ρ′,ε) which suggests that, under the above conditions, all hydraulic-sedimentological processes are functions of these four characteristic numbers. This implies that a process proceeds in the same way in the model and in nature provided these characteristic numbers are equal in both spheres. It follows furthermore that where identical fluids are used (water in model and nature), all scales (ratios between nature and model) become 1, which means that complete similitude is obtained only if the model scale is the same as the natural scale for all properties. 160 Sediment Sources and Transport Processes

The consequence to be drawn for the practice of hydraulic model testing using movable beds (M ≠ 1:1) is that it is impossible to include all factors involved and that no complete similitude can be obtained. Satisfactory results will still be reached on the condition that the criterion expected to have the least influence on the outcome is omitted. The multitude of publications on this problem (such as Yalin, 1977, DVWK, 1984, Mertens, 1987) allow the summarising conclusion that there are two opposed possibilities: 1 sacrifice similitude of sediment Reynolds number → undistorted models, sand as model material; 2 sacrifice similitude for relative density and relative roughness and allow for sediment Froude and sediment Reynolds numbers instead → distorted models, model material of less density than the natural material. These two cases will be dealt with in greater detail below.

6.6.2 Undistorted models – natural materials It has been pointed out above that among the four dimensionless coefficients governing bed load transport the sediment Froude number (Eq. 6.6-7) has the greatest effect on particle mobility. This coefficient should, therefore, always be equal in model and nature, as is also evident from the Shields diagram in Figure 4.1-4, which demonstrates that the influence of the Reynolds number on incipient motion becomes negligible above a certain level (Re* > 70) and ceases entirely for Re* > 300. Hence, where sufficiently high Re* values are involved, similitude between nature and model becomes irrelevant. This even applies to "hydraulically rough" behaviour. Thus, the influence of viscosity (Re number) can here be neglected; the resulting model, built according to the model laws, is an undistorted Froude model with the scale ratios: • length scale = height scale = particle-size scale • sediment density identical in model and nature. Hence, where neglection of the sediment Reynolds number is permitted, the models will achieve a high level of similitude with nature. This is also due to the fact that height distortion is not needed, so that near-natural flow is obtained. Furthermore, the relative roughness (Eq. 6.6-9) can here be considered near natural and hydrological and sedimentological time scales become identical, so that the onset of motion and transport proper are also fairly true to nature in the model. The problem in such models is that high Re* numbers call for particle sizes of at least 1mm in the model, a similitude requirement that is met only where either coarse-grained material is present in nature or where it is possible to adopt a large enough model scale. In addition, the scaled-down model material needs to be transported as bed load, not in suspension, and there should be no cohesion between individual particles. This means that the scaled-down grain- size distribution curve of the model material is usually cut off in the lower region around 0.1 to 0.2mm, so as to become steeper. But the characteristic particle diameter should absolutely be retained (Figure 6.6-1 below). It should also be seen to it that the bed forms developing in the model correspond to those in nature (a requirement hard to meet in distorted models). Models 161

Figure 6.6-1: Geometrically reduced model grading

Undistorted models using natural transport material are mainly used for minor streams with relatively coarse bed material.

6.6.3 Distorted models – lightweight materials Major natural watercourses with minor characteristic particle diameters can in many cases not be considered as hydraulically rough. Geometrical reduction may then risks entering the cohesive range. Another problem comes from the large dimensions of rivers, as the available laboratory space may impose limits on the range of eligible scales. In such cases distortion of the model – different scales for horizontal and vertical dimensions – will be the only feasible solution. This may also be required for hydraulic reasons, as small water depths tend to involve surface tension or subcritical Reynolds numbers (laminar flow), which would jeopardise the quality of representation, not to mention sediment-transport similarity. Of the four dimensionless coefficients (Eq. 6.6-6 to Eq. 6.6-9), similitude of sediment Reynolds number was sacrificed in the preceding section. Sacrificing the similarity of either relative roughness or relative density will yield unrealistically distorted models. This leaves a single solution: Sacrifice the similitude between the two relative values, ε and ρ'. This is justified by the fact that the relative density already forms part of the sediment Froude number (Eq. 6.6-7) and that the effect of relative roughness can be allowed for by a further equation (discharge formula). Thus, Gehrig (1981) introduced the Manning/Strickler formula in addition to Eq. 6.6-6 and Eq. 6.6-7 and assumed Froude similitude. Further details including conversion factors are given in DVWK (1984). In this way distorted models are obtained which have bed materials of less density than the corresponding natural material (lightweight material). 162 Sediment Sources and Transport Processes

Figure 6.6-2: Criteria of similitude for distorted models with movable beds (after Gehrig, 1981)

Figure 6.6-2 is a simplified graph showing the similitude relationship developed by Gehrig (1981). The graph also provides qualitative information on the density required for the model material. Similar relationships are available for determining the scale factor for particle diameter. The main principle to be observed for distorted movable-bed models with lightweight materials is that motion should set in on the model under Froude conditions similar to nature. Since the available density scale of materials smaller than 2,650kg/ m³ does not completely and continuously cover the range down to 1,000kg/ m³, it is necessary to use the material that is nearest to the theoretical scale. Factors such as commercial availability (e.g. particle diameter) or abrasion resistance in tests of long duration impose compromises. While the first step – theoretical selection of scales – involves no problems, building and running the model as well as interpretation of the results make high demands on the knowledge and imagination of test engineers. The hydraulic time scale after Froude is not identical with the morphological time scale. Although this may be computed in advance by use of appropriate relationships, its verification calls for so-called historical records, which means simulation of known morphological variations within certain time intervals in the model. For this purpose, the appropriate hydrological conditions need to be impressed on the model (unsteady flows). There are also limits to the amount of model distortion to be selected. Excessive distortion implies that the morphological processes can no longer be controlled (Yalin, 1977). The Manning/ Strickler roughness coefficient, introduced by Gehrig (1981), is only valid for flat rough beds and includes no allowance for bed forms. Shape resistance can be taken into account by use of a method devised by Mertens (1987).

6.6.4 Comparing distorted and undistorted models The practice of hydraulic model testing rarely offers any choice between distorted and undistorted models for bed load studies. In fact, such a choice will mainly be governed by the characteristics of the natural material and the feasible laboratory scale. Generally speaking, preference will be given to undistorted models which, providing for three-dimensional – Models 163 hence near-natural – flow conditions, depict nature more truthfully, provided, however, that the requirement for the neglection of viscosity flow (Re* > 70) is at least largely met and the water-level gradient is identical in nature and model. The hydraulic and sedimentological time scales thus become identical, so that the simulation of even unsteady processes is fairly true to nature. The disadvantage to this otherwise highly desirable similitude is the fact that due to the time scale

Eq. 6.6-11: Tr = L r where Tr = hydraulic time scale

Lr = model scale all sedimentological processes are slow, too, which involves long testing times for long simulation times (annual hydrograph). This problem is much less acute in distorted models, where the sedimentological time scale is much larger than the hydraulic one, resulting in comparatively shorter testing times. As soon as the Re* coefficient is substantially below the threshold value 70, the model will have to be distorted, which offers the advantage of obtaining comparatively large water depths in the model. Thus, relatively small excerpts from nature with coarse bed material as normally applies to Alpine regions will usually lend themselves to testing in undistorted models with sand as bed load material, whereas large phenomena with, moreover, finer-grained bed material will best be reproduced in distorted models using lightweight materials to simulate bed load. Care should above all be taken, especially in distorted models, to reproduce natural processes correctly. This makes a model capable of prediction, at least to a certain extent, which means that potential consequences of variations in boundary conditions, such as geometry, bed load delivery and discharge, can be predicted, at least as trends.

6.7 Comparing physical and mathematical models 6.7.1 Common features 6.7.1.1 Boundary conditions Both model types call for previous definition of boundary conditions in terms of water levels and/or flows. It is common practice to define velocities (flows) in the inlet region and water levels at the outlet. The less accurate these data, the more important it is to extend the model area in the upstream and downstream directions in order to allow it to develop the desired regime by the time the project area is reached. Since the effect of backwatering above dams tends to reach a long way upstream, tranquil flow as the lower boundary condition is of particular importance. A sensitivity analysis will in this case minimise the effect inaccurate data may have on the simulation result. Unsteady flows call for control of boundary conditions in both model types.

6.7.1.2 Calibration As mentioned above, both model types need calibration against measured results from nature,. observing constant water-level to flow relationships. Where these are lacking or have become partly or entirely outdated as a result of changes in cross section from bed load transport or relocation, it is necessary to estimate roughnesses (Section 3.3.2) from experience or literature. Since such constant water-level to flow relationships will rarely be available for extreme discharges, although these are of particular practical importance, roughness will 164 Sediment Sources and Transport Processes normally need to be estimated, which is easier where it has been possible to calibrate the model over its lower flow range by suitable measurements. Calibration is also possible via measured velocity profiles. Where the roughness values obtained from calibration differ substantially from experience or the relevant literature, it is absolutely necessary to check whether these discrepancies are due to modelling errors or other factors acting on water level. In multi-dimensional models, the roughness values obtained by calibration usually show better agreement with empirical results than in the case of 1D models, where channel geometries are usually modelled with a higher resolution level, so that friction values refer only to the components physically associated with them, but do not include other energy loss, such as impulse loss from changes in cross section.

6.7.1.3 Validation This is understood to mean checking a calibrated model with the aid of additional measured results that are independent of the calibration values. This measure is indicated for both model types to support the quality of simulation results. It says in the DVWK (1999): "Calibration is not complete until the addition of further independent results brings no further changes in parameters. Such additional measurements then validate the model." In practice, an appropriate number of records are usually not available or have ceased to be comparable due to changes in cross section. In such cases, it is advisable to carry out at a least a sensitivity analysis in an effort to assess the effect of inaccurate assumptions on the result by variation of the flow parameters.

6.7.1.4 Verification This step is needed for numerical models only and is used to find out whether or not a model is fundamentally correct. This may be done e.g. by comparison with analytical solutions. Unfortunately such analytical solutions are available only for simple systems with usually linear behaviour, otherwise the numerical models would not be needed at all. Yet, this check is a necessary condition of applicability for numerical models.

6.7.1.5 Terrain information Truthful modelling of the terrain including all its flow-relevant structures is essential for both model types. Even the best model could never compensate for errors in simulating terrain. Photogrammetric images or laserscan air photos for large landforms combined with terrestrial check measurements for highly structured small-scale surface contours will yield optimal terrain information, which will best be supplemented by air photos. In spite of this in-depth information, careful personal field reconnaissance is absolutely necessary.

6.7.2 Differences 6.7.2.1 Extent of area There are, of course, limits to the magnitude of area that can be reproduced in physical models. Either the areas will be too large to be reproduced at reasonable cost or, if a small scale is selected, the results will not be accurate enough. Such models will, therefore, best be used for small-scale flow problems with complex two- or three-dimensional effects. Provided the model laws characterising flow pattern and transport are obeyed, the processes will proceed to scale without calling for exact knowledge of the physical processes involved. This Models 165 means that the physical-empirical equations needed in mathematical models for describing system behaviour are not required. By contrast, hydrodynamic-numerical models require description of all relevant processes via physical or empirical equations. The extent of area to be dealt with is practically unlimited in numerical models, the more so as the calculating and storage capacities of computers are steadily increasing, while the price of hardware is on the decrease.

6.7.2.2 Time scales Where it is not possible to run a physical model within the hydraulically rough range (Re* > 70) and, hence, lightweight material is used, the hydraulic and sedimentological scales will differ distinctly (Section 6.6.4). This has a special bearing on the operation of movable-bed models under unsteady conditions, which is practically the only eligible operating mode for obtaining realistic results. Mathematical models, too, may involve some time displacement of unsteady sediment transport process in relation to reality. This comes from the fact that a bed load-transport formula is not a correct simulation of nature and, hence, is often inaccurate, which may lead to incorrect conclusions (prediction of relocation, changes in bed configuration, sedimentation etc.)

6.7.2.3 Variant studies – sensitivity analysis This is where numerical models are definitely superior. Any change in geometry, or topography, roughness and boundary conditions is usually easy to introduce in the model, while this is both costly and limited in size in physical models.

6.7.2.4 Precision Provided the boundaries for physical models (area extent) are observed and insufficient water depths are avoided, a model will require no distortion, especially in the case of Alpine streams (Section 6.6.3). Thus, the two model types will promise results of about the same quality. The essential point is always the experience of the test engineer (PM) or model operator (MM) and the care he takes over this work. The possibility of conducting sensitivity analyses as a relatively easy means of obtaining quantitative information on precision is an advantage of mathematical models and would be more difficult in physical models. In addition, simple comparison of several variants in numerical models can reveal trends with great accuracy, which means that the influence of a change in parameters is here easy to identify, but is difficult in physical models, the more so as the measurements are not accurate enough.

6.7.2.5 Cost Due to the steady increase in computer capacity and the substantially reduced staff requirements involved, numerical models generally offer a price advantage over physical models. Another reason is the fact that in general software is purchased and established but once and will then be available for other uses, whereas a physical model needs to be built for each specific project, to be demolished and disposed of afterwards. Movable-bed models cause additional cost due to their sediment requirements, especially so where the bed material cannot be recirculated. Laboratory measuring techniques have made much progress over the past few years, probes are becoming ever more efficient and more accurate, which in turn boosts the cost of data measuring and processing. 166 Sediment Sources and Transport Processes

6.8 Time and space scales Morphological and biological processes take place at different time and space scales. These extend from continental long-term changes, such as plate tectonics, to momentary phenomena, such as movement of individual streambed particles due to turbulence. This principle, as presented by Habersack (2000), is illustrated in Figure 6.8-1 below.

Figure 6.8-1: Space and time scales for morphodynamic processes (Habersack, 2000)

Habersack (2000) developed this approach to create the so-called River Scaling Concept (RSC), providing for up- or downscaling depending on the point of view. Thus, morphodynamic models are usually derived from laboratory results at relatively small scales, but are extended to apply to natural phenomena which may be as large as catchment areas, which corresponds to upscaling. In this case it is generally accepted that model assumptions can easily be transferred to the large scale, with some parameters, as roughness or particle size etc., however, being upscaled on the basis of experience or measurements in nature. In this as well as the opposite procedure, it is necessary to see whether the fundamental physical processes retain their validity when up- or downscaled or whether new effects appear which e.g. would play no or a lesser role at the smaller scale. Figure 6.8-2 below is a schematic representation of the River Scale Concept for sediment transport. Models 167

Figure 6.8-2: Measuring and modelling scale range for sediment transport (Habersack, 2000)

6.9 Sediment transport models 6.9.1 Introduction Water or wind flow acts on a more or less deformable boundary surface (land, streambed). This results in deposition (sedimentation), equilibrium or degradation (erosion) of this boundary surface, depending on the transport capacity of the fluid. This extremely complex field of problems is studied by both engineering sciences (civil engineers) and geo sciences (hydrologists, geologists, geographers), but cooperation between the two with their different approaches could often be better. Sediment transport has been a special discipline for more than 100 years, involving a multitude of aspects that are increasingly becoming subjects of scientific study. The number of publications is growing. Yet the present state of knowledge is far from satisfactory, although renowned scientists, such as Du Boys, Schields, Meyer-Peter, Einstein, Rouse, Yalin etc. have made valuable contributions towards a better understanding of the various phenomena making up sediment transport, such as incipient motion, bed load and suspended-load transport and bed forms. It is due to the complexity of multi-phase flow that this subject has not been fully explored so far. The multitude of phenomena involved is demonstrated by the following graph: • Interaction of the great number of phenomena (Figure 6.9-1) and parameters taking part in the process 168 Sediment Sources and Transport Processes

Figure 6.9-1: Main factors acting on sediment transport (Bechteler, 2004)

• Stochastic (chaotic) effects It becomes clear from the factors listed in Figure 6.9-1 above that many parameters involved in the process, such as turbulence and sediment properties, need to be described by stochastic rather than deterministic means, that is with the aid of a probability distribution function. Strictly speaking, the results cannot be obtained deterministically either, but should be found stochastically with the help of the reliability theory, using an expected value and appropriate confidence limits. Determination of the statistical magnitudes of individual parameters, such as grading curves, tends to be difficult in practice and the result is often unsatisfactory due to the inhomogeneities of nature. • Laboratory studies Almost all knowledge on sediment transport has been obtained from experiments and physical laboratory tests, since only these offer the possibility of any desired number of repetitions. Figure 6.9-2 below illustrates the difference between experiment and model test.

Figure 6.9-2: Experiment (laboratory test) and model test for sediment-transport processes

Laboratory investigations on sediment transport are subject to a multitude of limitations as simultaneous study of all the interacting parameters participating in this complex phenomenon is hardly feasible. Various simplifications are, therefore adopted, such as: + It is common practice to use single-grain material instead of a natural grain-size distribution. Models 169

+ Most of the tests are run in chutes, which rarely reproduce 3D effects (secondary flows) correctly. + Unsteady effects, such as flood waves and scouring, are often ignored. Models are usually run with water and sediment discharge rates that are variable, but constant over time. The results are used to derive analytic, or empirical, relationships, which are introduced into MMs, and these in turn simulate unsteady phenomena. Studying each of the various parameters of sediment transport in isolation in an effort to compose a whole from the individual results is certainly not satisfactory. The automatic laboratory measuring systems, which are becoming more and more accurate, rapid and miniaturised, give rise to some hope that the understanding of the interaction of the various parameters will improve.

6.9.2 Modelling protocol Morris et al. (1997) suggested that the following steps should be observed in dealing with a particular sediment-transport problem: 1 Project definition The first step in any model study is a definite purpose, which should be made the subject of a legally binding agreement with the client. Subsequent additions or even changes in purpose after a model type has been decided upon tend to be difficult to implement or may produce unsatisfactory compromises. 2 Model concept Any concept needs to be based on an accurate and profound knowledge of the site to be studied. This includes a data collection on all parameters relevant to the project and available measuring stations. Lacking data should be secured, which may involve a substantial input of time and money. This is where the project engineer's experience from similar projects comes in. This applies in particular to the acquisition of sediment data specific to the necessary sampling sites, for determining grain-size distribution. Here, too, the engineer responsible for the model test should personally be present at the site and accompany the sampling procedure. Also, residents, fishermen and river superintendents should be questioned. They often possess information documented nowhere else, and they often have a good memory. 3 Definition of modelling purpose, extent and methods The special purposes to be attained by the model should be defined on the basis of a clear concept. This would include the extent of the study as well as the events to be simulated, such as floods, long-term phenomena etc. In addition, the appropriate internal and external boundary conditions (Section 6.7.1.1) should be determined. This forms the basis for selecting the type of model to be used, which will also be a function of the available time and cost budget. 4 Model construction and calibration On the basis of Step 3 above, a physical model is constructed at the scale agreed upon, or a numerical model is adjusted to the specific situation under study and provided with the required input data. What the two model types have in common is the need for calibration (Section 6.7.1.2). 5 Model validation This has already been mentioned in Section 6.7.1.3 above. A common problem in this context is a lack of data sets where the available data have already had to be used for calibration. 170 Sediment Sources and Transport Processes

6 Model results After the model has been built, calibrated and, where possible, validated, the simulations (future development, effect of variations, change in operating mode etc.) can be run. Sensitivity analyses are imperative, especially where the available data imply uncertainties. This is, however, costly in physical models (Section 6.9.3.2). 7 Results and recommendations Model results should be evaluated and interpreted before definite recommendations regarding future developments are made.

6.9.3 Selection criteria 6.9.3.1 Model operators Some tend to believe that, whilst physical models should be run only at a laboratory possessing appropriate equipment and experience, mathematical models can be operated by civil engineering firms or planning and design authorities as well, simply by acquiring the appropriate software. This needs contradicting in the light of the present state of knowledge. Mathematical sediment-transport models are extremely complex and should be handled with great care where the operating staff lacks long-term experience and skill. Many parameters ("adjustment screws") need to be derived or assumed with account being taken of the natural conditions, and their effects may not be as simple to predict as e.g. selecting a representative roughness value. Reality often forbids representation of the natural material by a single particle size. The important point will then be the distribution of the particle diameters involved in the process, with their largest and smallest sizes. This particularly applies to problems such as the transition between bed load and suspended load or the simulation of armouring effects. Other phenomena, such as hiding or the development and influence over time of bed forms, are not easily assessed with the required accuracy by non-experts in the field of sediment transport. Competent treatment of such problems calls for well trained expert staff having appropriate experience. The same is true of the construction and operation of physical models.

6.9.3.2 Sensitivity analysis As already mentioned several times, most of the parameters taking part in the process can be defined but inaccurately. They are difficult to derive from nature and need some limitation due to their large fluctuation range. This is where the sensitivity analysis comes in. Greater or smaller variation of one (sensitivity analysis) or several parameters at a time (reliability analysis, Monte Carlo Method) is a systematic means of studying the impact of such variations on the overall result. This helps to identify those parameters which essentially act on the process and, on the other hand, to assess the magnitude of the fluctuation range of the final result. In this case, mathematical methods are superior to physical methods, in that such studies are much easier to perform than in the case of physical models, where e.g. use of several particle sizes would involve a substantial input of time and money.

6.9.3.3 Variant studies A model study will rarely aim at formulating a single proposal. What is usually needed is a number of variants whose effects on the sediment-transport capacity of a stream or river is to be studied. Here, too, it is important that agreement be sought, possibly prior to the start of the studies, on the number and type of feasible solutions to be investigated, whatever the selected model type. This may affect the model type and structure and, naturally, the cost and contract period, especially for physical models, but to some extent also for mathematical models. The Models 171 latter offer the advantage of making subsequent changes – as e.g. in channel geometry with the help of a digital terrain model – much easier than in the case of physical models.

6.9.3.4 Subsequent investigations Depending on the purpose of a study, subsequent investigations may be needed, e.g. in the case of public objection to a proposed solution, so that new possibilities need to be devised and studied, perhaps many years after completion of the original study. Physical models take much space in a laboratory and, thus, cannot be maintained for an unlimited time. It is much less difficult to resume a mathematical model even years afterwards, especially where the original operator is still present. Conservation of the model itself and the required data, e.g. on a DVD, is simple and cheap. Problems may only arise where newly introduced operating systems or expired licences are involved.

6.9.3.5 Cost – operating time Sediment-transport models are usually expensive, requiring experienced and skilled staff and laboratories equipped with up-to-date measuring equipment. Whereas one user is usually sufficient for a mathematical model, physical models call for a large amount of infrastructure with the appropriate staff. Physical models require almost continuous supervision, since operating troubles may occur and important intermediate conditions may not always be automatically recorded. This problem has remained much the same, despite the great progress that has been made in measuring methods with automatic data collection and evaluation. By contrast, a time-intensive mathematical model could well remain without any serious supervision for major periods. A physical model is usually unique. It is designed, built, run – and then demolished – exactly for the tasks to be dealt with, whereas mathematical models tend to remain basically the same for different tasks, and call only for adjustment of boundary conditions as well as of initial and final values to the natural conditions of a particular project. Hence, physical models will usually be more expensive and time-consuming than numerical ones.

6.9.3.6 Special effects An outstanding feature of physical models, especially where non-distorted, is that flow and its turbulent, three-dimensional character – except for the boundary layer (Reynolds number) – can be simulated very truthfully. This is of particular advantage in respect of local secondary flows and their effects on sediment transport. Modelling such effects by mathematical means would require complex 3D models with turbulence formulas which – at least so far – make long-term simulation of morphological changes difficult. Generally speaking, morphological studies will preferably be based on mathematical models, especially for the smaller particle sizes.

6.9.3.7 Accuracy It is difficult to decide which of the two model types is more accurate. This depends on the envisaged task, on the scale in the case of physical models and, for both model types, on what calibration and validation possibilities are available.

172 Sediment Sources and Transport Processes

6.9.3.8 Presentation of results – persuasive power Where the results of a study on sediment transport phenomena are to be presented to the public, the physical model will usually be superior, as true water and sediment are moving within a geometry accurately modelled to nature, and non-experts are rarely fully aware of the problems involved in translating results from model to nature. However, by virtue of up-to-date computer graphics combined with video animation, which is capable of conveying images much like those of a physical model, mathematical models have gained in persuasive power. Use of time-lapse techniques enables vivid presentation of long- term processes. In fact, studies on physical models, too, would use videos in such a case.

6.9.4 Outlook It is clear from the criteria listed in the preceding Section that any preference for one or the other model type can only be derived from the particular task at hand. Both models still have their merits. Physical models or experiments are particularly suited for fundamental research and especially complex small-scale problems. The great advantage of mathematical models lies in being readily available for use (except in the case of novel developments), which also implies a cost advantage. These models lend themselves to large-scale problems and long-term simulations. In combination with the graphical possibilities now available, they are increasingly suited for presentation and interpretation of results to non-experts. The trend is clearly towards numerical models, but it should be borne in mind that their basic elements – transport formulas – are usually obtained from physical models or laboratory tests. In-depth study of the great number of aspects characterising sediment transport will continue to call for fundamental laboratory studies. Great advantage lies in combining the two model types. Numerical models will best be used for making a first decision and selecting or reducing the number of variants to be studied, whereas model tests might serve solving small- scale problems and optimisation.

6.10 Numerical sediment transport models It would be too much here to present and assess a selection of existing sediment-transport models. But reference will be made to fundamental literature on this subject. Wang et al. (2005) published a status report on numerical sediment-transport models, discussing such subjects as the fundamental mathematical equations for the 1D, 2D and 3D cases of flows and sediment transport, the equations for turbulence, roughness, adjustment length as well as cohesive transport, scouring, sediment transport in steep channels, secondary flows in stream bends and near the banks. Morris et al. (1997, Section 11.6) briefly described mainly the sediment-transport models available in the American market, such as HEC-6. Peviani (2005) presented his model GIS MORIMOR with an application to downstream consequences of reservoir flushing. ATV-DVWK (2003) contains a brief description and comparative study regarding the models SEDICOUP (Holly, 1990) and MORMO (Hunziker, 1995), using a case study for the Salzach water framework study. The models mentioned so far have mainly been developed for one-dimensional flow. Meanwhile 2D models have been designed, some of which are already being applied, while others are still in the process of development at university institutes. These include CCHE 2d (Wu, 2001), TIMOR (Mevis, 2002), MORPHSIM (Farshi Hagro, 2003), HYDRO_ST_2D (Nujic et al., 2006). 3D models are available for various individual phenomena. As most of them are still in the process of development, they will not be discussed here. Reservoir sedimentation 173

7 Reservoir sedimentation Since this problem has already been dealt with in detail in working papers submitted by colleagues taking part in the EU Project ALPRESERV, a brief outline will suffice here. These papers are: • „Reservoir Sedimentation“ (WP 6, EPFL, Lausanne) • “Sediment Management – Technical and legal aspects” (WP 7, TU Graz)

7.1 Introduction 7.1.1 Definition of reservoirs (German Standard DIN 4048) Impounding a watercourse creates a reservoir. This, like a natural lake, silts up more or less rapidly. In actual fact, a reservoir may completely fill with sediment even within just a few years Table 7.2-2), whereas natural lakes, as in our northern Alpine foreland, may persist as stable features of our landscape for as much as 10,000 or 20,000 years after they were formed during the last Ice Age. Impounding structures include large dams, in the form of fill or concrete dams, as well as river barrages comprising weirs, power plants, locks, impounding dams and dykes (Figure 7.1-1). The artificial lakes formed by such closure structures may be called reservoir lakes and backwater reservoirs, respectively. In addition, there are flood-retention basins, pumped- storage basins and sedimentation basins.

Figure 7.1-1: Classification of impounding facilities

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7.1.2 Objectives of reservoirs The intended purposes of impounding facilities are listed in Table 7.1-1 below, multiple functions being the general rule, especially for large dams.

Table 7.1-1: Purposes of impounding facilities

• Flood protection • Irrigation (agriculture) • Drinking water • Electric power generation including pumped storage • Water level increase during low-flow periods (navigation, cooling water) • Groundwater enrichment • Sedimentation basins (suspended sediment, bed load, silts) • Diversions (including trans-basin diversions) • Stabilisation of stream or streambed (gradient reduction) • Leisure, recreation

7.1.3 Problems caused by impounding facilities There are, however, not only advantages to the above uses of impounding facilities. They also involve problems, mainly due to the interruption of the flow continuum of the stream. As a stream enters a reservoir, its velocity decreases depending on the type of reservoir, so that it starts depositing the solids it is carrying along. This may go as far as a reservoir filling completely with sediment. In addition, the properties of a stream are affected by the reduction or elimination of its flow characteristic, which in turn risks having detrimental effects on e.g. stream ecology, especially for fisheries. Other negative consequences may come from rising water tables. Negative effects are also likely to result for the sections immediately upstream and downstream of a reservoir (Table 7.3-1).

7.1.4 Conclusions Construction of impounding structures will always involve substantial costs, which are justified by its various possible uses (Table 7.1-1 above). But sedimentation of such a facility reduces its merits or may even result in the loss of this investment. The originally intended use will only be sustainable or serve as a renewable resource where sedimentation is controlled by appropriate measures (sediment management).

7.2 Reservoir sedimentation Reservoir sedimentation is caused by the reduction in flow velocity and the resulting decrease in sediment-transport capacity. Sediment carried into the reservoir is partly or completely deposited.

Reservoir sedimentation 175

7.2.1 Causes for sedimentation The origin of sediment as well as degradation processes and the rate of sediment delivery or deposition were discussed in greater detail in Section 2 above.

7.2.2 The sedimentation process in general Reservoirs differ from natural lakes by the morphological peculiarity that their lowest point is almost always situated at the dam and, thus, near the outlet gates. In addition, it is possible to some extent to control discharge from a reservoir and, hence, the reservoir water level. In many reservoirs, the outlet gates permit almost complete water level drawdown or reservoir emptying. These special morphological and hydrological features of reservoirs, as compared with natural lakes, enable increasingly efficient sedimentation-control (Vischer, 1984). The deposition process of sediment carried into a reservoir can be described, in hydraulic- sedimentological terms, by the relationship between discharge, flow velocity or bed shear stress and particle diameter or settling velocity, both for bed load and suspended sediment. It thus becomes clear that the sedimentation pattern is a function of the type and rate of sediment delivery and of discharge and reservoir geometry as well as of the type of reservoir management. Figure 7.2-1 below is a graph illustrating the sedimentation pattern characteristic of a reservoir lake.

Figure 7.2-1: Sedimentation pattern for a reservoir basin (Vischer, 1981)

Whereas coarse sediment settles as bed load directly in the inflow zone, suspended particles are carried farther into the reservoir. They may travel as far as the dam itself to be deposited there, which is mainly a result of density currents (Section 7.2.4.2.3). Figure 7.2-2 below is a longitudinal section through Lake Mead, U.S.A., demonstrating sedimentation within a period of only 9 years. 176 Sediment Sources and Transport Processes

Figure 7.2-2: Longitudinal profile of Lake Mead with Hoover Dam, sedimentation between 1937 and 1946 (Vischer, 1984)

Operation of an impounding facility may also significantly affect the sedimentation pattern.

7.2.3 Reservoir surveying Checking reservoir geometry is by profile surveying at appropriate time and space intervals. The variation in time of these profiles allows conclusions to be drawn about the reaction of the system (stream, reservoir). Integration of these variations over the section under study reflects the erosion or sedimentation volume over two or more time intervals. Accurate knowledge of the morphometric data of a reservoir, in particular its storage, is essential to the efficient management of reservoirs according to their respective uses (Table 7.1-1 above). This is why routine checking of reservoir geometry is required. Additional surveying is recommended after major floods. Where solids (bed load and suspended particles) tend to be deposited within the active storage of a reservoir, depending on the geometry and type of management of a reservoir basin, this will reduce the volume and possibly also the legally secured use of the reservoir. Substantial progress has been made over the past few years in the development of profile-surveying equipment and positioning techniques as well as in the numerical interpretation of terrain data with the aid of digital elevation models. The Workgroup on Reservoir Sedimentation in DVWK Technical Committee 2.6, "Sedimenttransport in Fließgewässern" ("Sediment transport in running waters"), has presented up-to-date methods in its publication "Volumenermittlung von Stauräumen" ("Determining reservoir volume") (ATV-DVWK, 2001). The considerable progress that has been made in measuring technology is discussed in a recent publication by Wagner et al. (2006). The sonar sensors used there permit precise surveying of the dam structure and reservoir volume, including determination of the geotechnical properties of the lake bed.

Reservoir sedimentation 177

7.2.4 Reservoir characteristic and deposition pattern 7.2.4.1 River impoundment 7.2.4.1.1 General A backwater reservoir is not so much a natural or artificial lake as an impounded stream. These backwater reservoirs can be classified into various basic types according to their flow- through characteristic (Figure 7.2-3).

Figure 7.2-3: Flow and sedimentation patterns (Westrich, 1988) 178 Sediment Sources and Transport Processes

Flow velocities vary substantially depending on discharge and cross-profile configuration. During floods, narrow, tube-like backwater areas may develop flow velocities approaching those in unimpounded rivers.

7.2.4.1.2 Type of impoundment The character of an impoundment is primarily governed by its site and design. An isolated reservoir may differ from an impoundment forming part of a series, where the hydrostatic head from an upstream unit is often increased by artificial riverbed deepening, which will then produce the corresponding reservoir widths. In the course of its service life, a wide reservoir may need some structuring by such measures as groynes or levées, so as to avoid braiding in a partly or entirely sediment-filled reservoir area, where the discharge capacity would be insufficient and the water level unacceptably high for dealing with potential floods (Figure 7.2-4 and Figure 7.2-5 below).

Figure 7.2-4: Final sedimentation stage in a reservoir area without structural measures– plan view (after Baumhackl, DWA, 2006)

Figure 7.2-5: Final sedimentation stage in an reservoir area without structural measures – cross section (after Baumhackl, DWA, 2006)

With flow velocity reduced in places, any bank irregularity, any vegetation, any structure may cause local changes in flow pattern, which in turn affect the sedimentation and erosion processes. Reservoir sedimentation 179

By careful design of reservoir geometry, it is possible to avoid detrimental effects from sedimentation or even delay the sedimentation process.

7.2.4.1.3 The characteristics of the sedimentation process The parameters governing reservoir sedimentation are the rate of sediment delivery to the reservoir, reservoir geometry and discharge. Along with decreasing flow velocity the water starts depositing the sediment load it is carrying on its path through the reservoir. Starting from the head of the reservoir, flow velocity and bed shear stress decrease as the discharge cross section increases, and the solids begin to settle. The coarse bed load material is deposited at the head of the reservoir, whereas the finer particles settle further downstream, depending on the respective boundary conditions. Fine material can settle as a function of flow velocity wherever the shear stress velocity falls below the threshold for onset of sedimentation. Natural remobilisation of fine sediment will normally occur only within the main channel as soon as the shear stress exceeds the required threshold under the action of flood flow (Section 4.1). Shallowly flooded washland will tend to silt up in the course of plant service life. In combination with vegetation, this may even end by becoming land again. An important factor affecting the sedimentation process is reservoir geometry. In a narrow elongated reservoir the bed load bar will gradually push from the entrance to the reservoir in the direction of the weir. In wider reservoirs, sedimentation tends to proceed sporadically. Even minor bed load bars, sudden enlargements in cross section (lakes) etc. may cause unexpected sedimentation. Sedimentation processes are also affected by changes over time of inflow, water level and sediment delivery. The result is a constant alternation between sedimentation, state of equilibrium and erosion. A special problem is cohesive inorganic fine sediment. Sedimentation is followed by a major consolidation phase, during which the erosion shear stress may increase by more than an order of magnitude. Man-made substances may intensify this effect. Such sedimentation zones will then be difficult to evacuate even during major floods (Section 4.1.2.4.6). In addition, density currents may occur, in particular in reservoirs with relatively high bed gradients and/or cold tributaries rich in suspended load. This will, however, be less acute in backwater areas of rivers, due to their substantially higher flow velocities and, hence, larger turbulences, as compared with reservoir lakes (Section7.2.4.2.3). Furthermore, potential development of ripples and dunes (bed forms) may adulterate the results of such measurements as echosoundings.

7.2.4.2 Reservoir lakes 7.2.4.2.1 Lifetime of a reservoir The average annual loss in storage volume due to sedimentation in the world's reservoirs has come to exceed the annual increase in storage through the construction of new facilities for such purposes as irrigation, drinking-water supply and hydro power (Figure 7.2-7). Sustainable use of the reservoirs is thus not always ensured in the long term. Although the sedimentation rate in Alpine reservoirs is not more than about 0.2%, which is much below the world-wide average (Table 7.2-2), sedimentation constitutes a serious hazard, as turbidity currents sporadically transport large sediment volumes like avalanches down to the dam, where concentrated deposits jeopardise the operation of gates, power intakes and bottom outlets. In general, however, an impounding facility can be used over long periods provided adequate maintenance is carried out, but sedimentation may restrict or even completely prevent this 180 Sediment Sources and Transport Processes use. Murthi (1977) defined the usable life of a plant as the period during which a reservoir may be operated for the intended or a modified use, even if this use no longer offers any commercial advantage. Under the aspect of sustainability, which should dictate any future action, reservoirs should be designed and operated so as to function as sustainable resources. This requirement will be met in the long term only by combining appropriate reservoir design with sediment management and/or desedimentation.

7.2.4.2.2 Storage loss due to sedimentation The reduction in flow velocity relative to the inflow as well as turbulence cause deposition of solids carried into the reservoir, which thus acts as a sand or gravel trap. Practical design recommendations based on hydraulic principles for dealing with this phenomenon have been prepared by Bechteler et al. (1984). The greater part of sediment transport is suspended load which is, hence, the dominating factor in reservoir sedimentation. A simple estimate of the suspended-sediment retention rate was made by Brune (1953) by studying 40 reservoirs in the U.S.A, identifying as the main parameter the ratio of storage to the volume of mean annual inflow, corresponding to the mean residence time. The result of this study is illustrated in Figure 7.2-6 below.

Figure 7.2-6: Trap efficiency of reservoirs as a function of mean residence time (Brune, 1953)

There are more estimation methods of similar type, which should however be regarded as yielding nothing but reference values. The retention capacity of a certain reservoir can be determined relatively accurately by use of hydrodynamic-numerical models including an allowance for suspended-sediment transport. This would show that the increase in flow velocity as sedimentation proceeds reduces the deposition effect. In some cases, a sedimentological-hydraulic state of equilibrium may be reached in this way. As mentioned above, reservoir sedimentation is a serious worldwide problem as also demonstrated by Table 7.2-1 below: The average annual loss in storage amounts to about 1 or 2 %. The highest sedimentation rate is recorded in China, which may mainly be explained by the extremely high erosion rates and suspended-sediment concentrations mainly of the Yellow River.

Table 7.2-1: Estimated average annual loss of storage, in percent Reservoir sedimentation 181

Country Annual loss [%] Sources China 5-70 IRTCES, 1985 China 2,3 Morris et al., 1997 Turkey 0,7 – 1,5 Gögüs, 1992 U.S.A 0,7 IRTCES, 1985 U.S.A 0,2 Morris et al., 1997 Worldwide ~ 1 Mahmood, 1987

Recent studies (White, 2001) based on data from worldwide reservoirs totalling 6,000km³ in capacity have arrived at different sedimentation values, as shown in Table 7.2-2.

Table 7.2-2: Estimated annual loss of reservoir storage due to sedimentation as well as estimated reservoir half-life (White, 2001)

The worldwide development of net storage, assuming that no major dams will be constructed from 2010, is shown in Figure 7.2-7 below. Furthermore, it is clear from Figure 7.2-8 that small reservoirs are much more subject to sedimentation than large ones. It is clear from the above table that the greater part of the world's net storage will be lost by the end of the century unless effective action is taken. Naturally, sedimentation rates vary considerably from reservoir to reservoir, depending on such factors as climatic zone, nature of catchment area and design of dam and outlet structures. 182 Sediment Sources and Transport Processes

Figure 7.2-7: Development over time of the worldwide net storage volume based on projection of sedimentation rate (White, 2001)

Figure 7.2-8 Annual rate of loss of storage as a function of reservoir size (White, 2001)

7.2.4.2.3 Turbidity currents as the main reason for sediment transport in reservoirs The greater part of the sediment load carried into a reservoir lake is normally made up of suspended particles (80 to 90 % in small and medium-sized, 90 or almost 100 % in the larger reservoirs). Bed load is of lesser importance. The large suspended-sediment loads are transported mainly during flood flow in running waters. Reservoir inflow loaded with great amounts of fine-grained sediment from the catchment has a higher volume weight than the still water in the reservoir. The turbid water mass pushes forward from the mouth until an equilibrium of impulses is achieved. Then the denser water loaded with suspended sediment starts plunging into the lighter lake water. An underwater current forms, called turbidity current, consisting of a mix of water and fine sediment in suspension. In terms of physics, this current may be compared to a powder-snow avalanche racing down a valley slope. The turbidity current moves on the sloping bottom at considerable speed in the direction of the lowest point in front of the dam (Figure 7.2-9). Reservoir sedimentation 183

Depending on the gradient of the thalweg, turbidity currents may reach high velocities, sometimes whirling up already deposited sediment and carrying it in the direction of the dam. Addition of further fine sediment in suspension raises the density of the turbidity current and accelerates it. On the other hand, the current slows down in flat sections, which causes sediment to settle and may end in the death of the turbidity current.

Figure 7.2-9: Schematic section through a reservoir showing turbidity current (Oehy, 2002))

Turbidity currents are often responsible for relocation of sediment within a reservoir. The following conditions are conducive to the development of a turbidity current (Oehy et al., 2002): • High suspended-sediment concentration in the inflow • Great water depths at tributary mouths • Almost still water in the reservoir • Steeply sloping reservoir bottom • Tube-like, straight reservoir geometry These conditions apply to almost all Alpine reservoirs, so that even minor annual floods may trigger turbidity currents. The maximum transportable particle diameter is shown plotted against turbidity-current flow velocity in Figure 7.2-10 below (Fan, 1986).

Figure 7.2-10: Maximum transportable particle diameter vs. turbidity-current flow velocity after Fan (1986) 184 Sediment Sources and Transport Processes

7.3 Negative consequences of reservoir sedimentation Table 7.3-1 lists possible negative consequences of reservoir sedimentation. A point of particular concern is the reduction in usable reservoir storage and, hence, restriction of the intended purposes of a plant (Table 7.1-1).

Table 7.3-1: Problems from reservoir sedimentation

Upstream • Backwatering through delta formation in the reservoir, hence reduction in flood safety • Rising water table

Reservoir area • Reduction of discharge cross section and, hence, reduction in flood safety (river barrages) • Reduction of flood-retention space (large dams) and net storage • Impairment of abstraction possibilities • Interference with navigation, sedimentation of outer and main ports • Wear of turbines and gates • Rising water tables (river barrages) • Deterioration of water quality • Restricted leisure uses • Accumulation of contaminants in the sediment • Reduction in the reaction space of upstream check dams • Reduction in silicium supply to the oceans

Downstream • Erosion due to sediment deficit • Stability problems for stream banks and structures • Change in flow regime – problems of residual flow • Lowering water table • Delta erosion due to sediment deficit • Great turbidity through flushing • Armouring – detrimental to fish spawn • Impairment to navigation • Riverbed sedimentation • Closure of interstices in the river bed – detrimental to fish spawn • Retention of minerals

Reservoir sedimentation 185

The consequences vary according to whether sedimentation occurs as delta development (bed load) or deposition in the dead-storage zone (suspended load), or as a combination of the two, as demonstrated in Figure 7.3-1. It is mainly delta formation that affects reservoir utilisation, such as flood retention and usable storage.

Figure 7.3-1: Change in reservoir storage curve due to sedimentation, left for delta-formation, right for sedimentation at the deepest point of the reservoir

Surface waters carry enormous quantities of dissolved minerals, such as silicium, into the oceans. These find their way into such organisms as diatoms. When these algae die, silicium sinks to deeper layers. In uplift zones, where cold water rises from great depths, part of the minerals are washed to the surface again. Appropriate measures are needed to reduce or avoid altogether the great number of negative consequences of reservoir sedimentation.

7.4 Measures of reservoir sedimentation control 7.4.1 Basic sedimentation control measures Action against reservoir sedimentation can be classified into preventive and retroactive measures. The former are intended as a precaution intended to prevent sedimentation from the outset, the latter are to remove at least part of the sediment once deposited. Another distinction can be made between measures in the catchment, in the reservoir and at the dam (Figure 7.4-1). 186 Sediment Sources and Transport Processes

Figure 7.4-1: Overview of preventive and retroactive sedimentation-control measures (after Schleiss, DWA, 2006)

7.4.2 Measures in the catchment area 7.4.2.1 Erosion protection in the catchment area Soil erosion is a highly critical problem of world-wide concern, as illustrated by the figures below: • Loss in land under agriculture since the beginning of systematic agriculture, 530 million hectares, • Annual loss in land under agriculture due to soil erosion, about 3 million hectares, • Annual soil loss, about 33 billion tonnes, • Annual loss in soil resources, 0.7%, • Total annual damage from eroded sediment in the U.S.A., 6 billion US $. Reservoir sedimentation 187

This enormous damage to economy demands that possibilities be sought to reduce or even avoid soil erosion. This may be done through • agriculture, • forestry, • hydrotechnical measures (UNESCO, 1982). Figure 7.4-2 below depicts the first two of the above possibilities. Hydrotechnical measures are understood to include the construction of ditches, retention basins, dewatering, prevention of erosion slides etc.

Figure 7.4-2: Possibilities of erosion protection in the catchment area above a dam

The most effective precaution against reservoir sedimentation is erosion protection in the catchment. Climatic conditions permitting, surfaces should be planted as a protection against erosion. Afforestation in the catchments above the great number of reservoirs with a sedimentation hazard, especially in Asia, will be one of the main tasks to be faced by mankind in this century. Unfortunately, afforestation will take long to show its effect against sedimentation. Yet this is essential for conserving valuable cultivated land for agricultural uses and as a protection against floods, debris flows and landslides. In vegetation-free catchments, such as found at high levels in the Alps, erosion-control is possible only by technical means, such as stabilisation of valley slopes as well as of stream beds and banks.

7.4.2.2 Erosion protection in the tributaries Figure 7.4-3 below is a graph depicting possibilities of erosion protection in the tributaries to a reservoir basin. 188 Sediment Sources and Transport Processes

Figure 7.4-3: Possibilities of erosion protection along tributaries

The following measures could be considered for this purpose; combinations tend to be the most economical: • Stabilise streambanks • Decelerate flow • Training structures • Stabilisation of transition between stream bottom and banks • Energy-dissipation structures • Revetments • Ramps • Streambed widenings • Gravel and sand traps • Upstream check dams A compilation of near-natural measures of stream stabilisation ("Naturnahe Massnahmen zur Gewässerstabilisierung") can be found in DVWK (1997). Bank protection in particular was the subject of a comprehensive documentation by Cranfield University (1999). Figure 7.4-4 below presents a basic method of status assessment and solution strategy. Reservoir sedimentation 189

Figure 7.4-4: Method of checking bank protection (boxes are explained in greater detail in the Study) (Cranfield University, 1999) 190 Sediment Sources and Transport Processes

Bed load traps and gravel traps are provided in torrents and mountain streams in order to avoid unwanted sediment deposition in downstream sections. These traps capture almost all bed load arriving after first filling during a flood and need to be evacuated. Bed load retention affects, however, the transport capacity of a stream but locally and in general acts to capture only small amounts of suspended solids. In addition, such basins are too small to play any appreciable part in the reservoir-sedimentation problem. In fact, larger forebays are needed for this purpose. These are created by building a check dam in the delta of the main tributary to a reservoir. Such forebays retain almost all bed load and also capture part of the sand. Only relatively large check dams are capable of achieving a high trapping efficiency for fine sediment or suspended solids. A trapping efficiency of 90% e.g. is reached where the ratio of lake capacity to volume of annual inflow is 1:10 (Figure 7.2-6). A forebay needs constant clearing or flushing in order not to fill up with sediment itself. The material flushed through a bottom outlet could be diverted through a flushing gallery by-passing the reservoir, to be restored to the stream downstream from the main dam (Figure 7.4-6 below). The sedimentation tendency of a reservoir is a direct function of the size and character of the immediate catchment area. Reservoirs having a small catchment and fed through trans-basin diversions silt up at a much slower rate, provided however that the diversion intakes admit and divert only water poor in sediment. It can generally be said that erosion protection within the catchment area should be preferred to any other measure.

7.4.3 Measures at the dam A method commonly practised worldwide for preserving the usable storage of a reservoir lake is overdimensioning its volume. This provides for a certain amount of space where sediment is allowed to collect for a given period, typically 50 years. Where this space is not amenable to utilisation, it is termed dead storage. When a large proportion of the usable storage has been lost, this may be compensated for by heightening the dam where feasible. This has been practised at several dams in North Africa (Cornut, 1992) and in Bavaria on the Sylvenstein reservoir in 1997. Where only the outlet works of a dam have been affected by sedimentation, and if moreover efficient flushing is not feasible, these structures need to be relocated at a higher level in order to ensure continued operation. A recent project of this kind is the reconstruction of the outlet and power intake structures at the Mauvoisin Dam (Hug et al., 2000; Schleiss et al., 1996). Pressure flushing to clear the outlets will normally evacuate but a cone in front of the outlet, with slopes corresponding to the angle of internal friction of the deposited material (Figure 4.1-3). In the case of fine sediment, the feasible cone angle will not reach more than about 30° at best (Sinniger et al., 2000). Where an outlet is already entirely covered up with sediment, opening of the gate may cause the material to consolidate altogether and prevent the erosion of a cone. This may be remedied by sinking an injector shaft to admit sufficient water during the initial flushing phase (Krumdieck et al., 1981). At least sporadic evacuation of a cone in front of a power intake is possible by combining this structure with a flushing outlet located immediately below (Hug et al., 2000; Schleiss et al., 1996). Bottom outlets of sufficient capacity are theoretically capable of "sucking in" and passing density currents during floods. The outlet capacity of an Alpine reservoir, however, is normally too small to permit this type of sediment passage. Moreover, frequent operation of outlets against the extremely high pressures they are subject to involves certain risks, such as vibration and jamming of the gates. Finally, Alpine reservoirs often serve for flood protection, which implies that opening the bottom-outlet gates during floods is undesirable. Reservoir sedimentation 191

7.4.4 Measures in the reservoir Once sediment has entered a reservoir, only retroactive (Figure 7.4-1 above) or passive measures are possible, which remove sediment or at least mitigate their adverse effects. Sedimentation can be delayed or prevented by routine evacuation of the deposits. This may be achieved by dredging (Section 7.5.2 below) with a full reservoir as well as with the water level drawn down, from the shore or from barges. Depending on the granulometric grading of the sediment and dredging depth, suction dredgers or purely mechanical, conventional dredging equipment may be used for this purpose. A special application of hydraulic clearing is sucking sediment from the reservoir through piping placed at the bottom of the lake. This is provided with special openings on the underside through which sediment is sucked in as soon as a gate is opened at the end of the pipe (SPSS – Slotted Pipe Sediment Sluicer) (Jacobsen, 2000). An extremely efficient measure of clearing a reservoir is by flushing (Section 7.5.1), which – where possible – should completely empty the basin. But this may involve ecological problems and sedimentation downstream from the dam (Section 11.9). The potential effects of such a measure will depend on the amount of sediment charge passed during the relatively short flushing process (Boillat et al., 2000a and 2000b). Suspended sediment is an important factor in reservoir silting. Where it is possible to prevent suspended particles from settling, they could be continuously discharged through the outlets. Passage of a certain level of sediment concentration through the turbines is in fact acceptable. Thanks to novel materials, turbines are becoming increasingly abrasion resistant (Grein et al., 1992). For wear of hydraulic equipment, see Ortmanns (2006, Section 4). Fine sediment can be kept in suspension permanently by providing for sufficient turbulence. This may be achieved in Alpine reservoirs by taking advantage of the energy coming from stream diversions. Another conceivable method would be by purely mechanical turbulence produced from a "large-scale mixer". Another method of minimising the adverse effects of sedimentation is by controlling the turbidity currents, provided these constitute the dominant process as is the case for a large proportion of the Alpine reservoirs (cf. Section 7.2.4.2.3).

7.4.5 Reservoir management After Morris et al. (1997), this is understood to refer to any method intended to influence flow through the reservoir or the reservoir geometry itself, or both, so as to pass sediment through or around the basin, thus minimising sedimentation. An important point in this respect is identification of the particularly sediment-laden inflow, which will then be given closest attention. This method is distinct from reservoir flushing, where deposited sediment is remobilised, whereas reservoir management is aimed at reducing or preventing the deposition of solids within the reservoir from the outset. Sediment routing is a method of maintaining as far as possible the natural sediment flow in a stream, whereas flushing alters it significantly. These methods require, however, large amounts of water for carrying sediment around or through reservoirs. At any rate, they correspond to the Chinese strategy of "discharging the muddy water and impounding the clear water" (IRTCES 20, 2005, No. 2). Sediment routing can be classified according to Figure 7.4-5 below: • Sediment bypass • Sediment pass-through • Off-stream reservoir 192 Sediment Sources and Transport Processes

Figure 7.4-5: Sediment routing strategies (after Morris et al., 1997)

7.4.5.1 Sediment diversion – bypass A distinction must here be made according to whether mainly bed load or mainly suspended sediment is to be discharged. Bypassing bed load certainly involves the greater problems. This will be successful, however, where the reservoir is not too long and where the gradient is sufficiently steep to ensure transport over the whole length of the bypass. Typical flow velocities – depending on the particle size – range around 10m/s. An undesirable side effect is the high wear of the tunnel invert (Vischer, 1997). Figure 7.4-6 below is a schematic diagram showing an upstream check dam and bypass tunnel ending downstream from the main dam.

Figure 7.4-6: Reservoir with forebay that can be flushed through a tunnel ending below the main dam (Vischer, 1981)

A bypass will generally be located as near as possible to the bottom, where suspended- sediment concentration is highest, whereas the upper portion of the flow carries less and finer material.

7.4.5.2 Sediment pass-through This is classified after Morris et al. (1997) as: • Seasonal water-level drawdown (partial or complete) • Water-level drawdown prior to flood based on upstream gauge readings or hydrological prediction • Density current The seasonal partial water-level drawdown is done during the period of flood flow in order to use the high flow velocities for minimising or entirely preventing sediment retention above Reservoir sedimentation 193 the dam. This is practised in many Chinese reservoirs, including the Three Gorges Basin. Complete emptying of the reservoir would have fairly the same consequences as flushing. The greater portion of the sediment load reaches the reservoir during the period of flood-flow. Short-term water-level drawdown during that period is an attempt to pass the highest possible proportion of the sediment load through the reservoir. Drawdown can be controlled by means of upstream gauges for reservoirs of minor magnitude and with the help of hydrological prediction based on rainfall-discharge models for larger reservoirs. The coarse sediment particles will, however, always be deposited in the reservoir.

7.4.5.3 Sediment traps – check dams Another method of sediment diversion is by providing sand or gravel traps above the inlet to the lake, or check dams. This serves to capture at least the coarse sediment fraction, which then needs to be mechanically removed, possibly to be restored to the stream downstream from the main dam.

7.4.5.4 Further measures Further methods of reducing or preventing sedimentation would be by finding a favourable reservoir location, as e.g. in a tributary valley rather than the main valley. Water is diverted from a sediment-free intake in the main valley to the reservoir in the tributary valley. Additional intakes of this type may intensify the effect. This means that the reservoir itself has a small catchment with a low rate of sediment delivery. Such lay-out possibilities are depicted in Figure 7.4-7 below.

Figure 7.4-7: Bypass arrangement of reservoirs (Morris et al., 1997)

The design of an accordingly sized dead storage volume may avoid limitations for the operation of a reservoir due to sedimentation also, at least until it is filled with accumulated material, too (Section 7.4.3).

7.5 Evacuation of reservoirs This will become a requirement where sediment has already accumulated in the reservoir and cannot be flushed off at regular intervals by such management methods as discussed in Section 7.4 above.

194 Sediment Sources and Transport Processes

7.5.1 Flushing The reservoir surface is drawn down and the gates are opened in order to create near-natural stream flow with the higher velocities involved, which (partially) washes deposited sediment from the basin. This is distinct from sediment routing as mentioned in Section 7.4.5 above, in that flushing remobilises sediment already deposited and involving a time lag between sediment delivery and discharge. Sediment concentration downstream from the dam is much higher than in the case of an unimpounded stream. Also, flushing needs to be continued for a major length of time as otherwise the desired emptying effect is not achieved. Flushing is particularly efficient for relatively small impoundments, especially where these are located in narrow valleys. In very large reservoirs, opening the bottom-outlet gates will flush only a relatively small region within the immediate range of the outlets. Bottom outlet design should consider the risk of choking from high sediment concentrations, e.g. by providing for an injector shaft (Vischer, 1981). High sediment concentrations are likely to have detrimental effects in tailwater sections, such as the phenomena listed in Table 7.5-1 below.

Table 7.5-1: Detrimental downstream impact of reservoir flushing

• Damage to or death of aquatic organisms, especially fish • Benthos covered with sediment • Local sediment accretions and extensive depositions • Choking of diversions • Obstruction to navigation • Reduction in flood safety • Increased wear of turbines and gates • Deterioration of water quality – especially for abstractions • Reduction in leisure uses • Sedimentation in downstream reservoirs • Damage to water intakes

Flushing strategies need to be tested and tailored to the shape and size of a particular reservoir as well as to the particle size of the sediment to be removed. Preliminary studies based on sedimentological-hydraulic-numerical models as well as experience from other reservoirs will be a great help. Where no valley channel marks the original stream course in the reservoir, such a channel will develop and widen in the course of time by lateral bank failures, especially in the presence of fine-grained material. Development of an armour layer during flushing is a cause for interrupting the flushing process. Flushing efficiency can be enhanced by lateral or longitudinal auxiliary channels within the reservoir, either fed from diversion channels or produced mechanically, although such channels risk choking themselves. It is always difficult to flush relatively coarse material. Where it is not possible to mobilise such material completely, the reservoir will increasingly fill with sediment, although much later than without flushing (Figure 7.5-1). Reservoir sedimentation 195

(A) Flushing can remove all sediment fractions deposited in the reservoir

(B) Coarse load cannot be removed and accumulate

Figure 7.5-1: Long-term reservoir development in the case of flushing, for narrow and wide reservoirs (Morris et al., 1997)

Special attention should be given to the bottom or flushing outlets with respect to structural design, size and location. Successful flushing depends on the following criteria, after White (2001): • Near-natural streamflow conditions need to be achieved over an appropriate length of time. • The available flushing flow rate should be large enough to carry the sediment load to be removed. It has proved more efficient to provide for high short-term water feed rates than the other way round. • The mobility of the sediment to be flushed should be sure to be attained by the selected flow. • Reservoir geometry is an important factor for the success of a flushing process. Long and narrow reservoirs are easier to flush. • Flushing should result in a substantial increase in useful reservoir life. About one-third of the originally planned life should be reactivated by the flushing process. This means that in general the reservoirs situated in the upper or middle course of a stream will lend themselves to flushing. 196 Sediment Sources and Transport Processes

Examples of actual flushings are described e.g. in Morris et al. (1997) and White (2001) for large dams and in DVWK (1993) for river barrages. Flushing of reservoirs serving for electricity generation will always be associated with generation loss. Further details on successful reservoir flushings are found in Atkinson (1996).

7.5.2 Mechanical excavation Generally, a distinction is made between dredging and excavation. In any case, mechanical desedimentation compares unfavourably with flushing, as large quantities of sediment are recovered, which need hauling and dumping. In addition, fine material often calls for complicated dewatering. Further problems come from contaminated sediment, which needs either processing or transportation to a dumpsite for hazardous materials.

7.5.2.1 Dry excavation Previous reservoir-level drawdown is required to provide access to the excavation site. Removal of very fine material involves the risk of heavy earthworks machinery getting stuck in the mud, and dewatering takes a long time. Sediment removal from a reservoir may produce similar effects as from flushing (Table 7.5-1). They may be avoided or at least reduced by appropriate action (Morris et al., 1997). Allowance needs naturally to be made also for the ecological implications of reservoir-level drawdown (Section 4)

7.5.2.2 Dredging Unlike excavation, dredging does not call for water-level drawdown. This measure can be implemented during low or higher flows, and the dredging area may cover the whole reservoir. The required – hydraulic or mechanical – earthmoving equipment is usually custom built to suit the intended purpose. Hydraulic equipment (suction dredgers) convey sediment together with water and are capable of dealing with large pumping distances through floating pipelines. Mechanical dredgers usually pick up reservoir sediment by means of buckets emptied onto barges for transportation to the shore. These dredger types are described with their advantages and disadvantages e.g. in Morris et al. (1997) or in DVWK (1993). The former discusses in greater detail the design of drainage and storage sites as well as transport in slurry pipelines.

7.5.3 Sediment rearrangement 7.5.3.1 Sediment rearrangement within the reservoir This refers to removal of sediment and transportation to – or possibly only intermediate storage at – a different location in the reservoir. Such rearrangement may be produced by natural processes, such as a flood, or by mechanical means, such as dredging, or through water-level drawdown. Special problems may arise where sediment accumulated over major periods is mobilised within a short time and in large quantities, because contaminants that may have collected within the sediment risk being released suddenly and in high concentration (DVWK, 1993).

Reservoir sedimentation 197

7.5.3.2 Rearrangement outside the reservoir Mechanical removal of sediment from the reservoir and its addition downstream from the dam is another possibility of restoring sediment transport interrupted by an impoundment. This serves to compensate for the downstream sediment deficit, although this will not meet the requirement of time continuity, as such measures are carried out at major time intervals. Such sediment rearrangement has been and is still carried out e.g. in the reservoirs on the River Isar – Sylvenstein, Bad Tölz and Oberföhring – and on the River Saalach (Kibling). In such cases, the reservoir should be followed by a major free river stretch.

7.5.4 Combining rearrangement measures Combining different measures will be considered where one of them is not capable alone of reaching the desired goal or would be uneconomical. Thus, the finer sediment in the lower portion of a reservoir will best be removed by flushing, whereas the coarse fractions near the reservoir head will preferably be removed by mechanical means. Simultaneous implementation of these different clearing methods is practically impossible, as their respective requirements are mutually exclusive. Even if the coarse fractions could be mobilised by flushing, their transport down to the dam would take too long, especially in long reservoirs, and would require too much flushing water (ÖWAV, 2000). Another conceivable method is combining mechanical measures where e.g. the upper portion of a reservoir can be drained by water-level drawdown so as to permit mechanical sediment removal. As draining the lower portion of the reservoir would require complete emptying, that part could be cleared by means of a suction dredger with the reservoir partly full. Table 7.5-2 below gives an evaluation of the effects of the various methods of removing and avoiding deposition of sediment in reservoirs. 198 Sediment Sources and Transport Processes

Table 7.5-2: Evaluation of the effects from various processes for removing and avoiding reservoir sedimentation (BUWAL, 1994)

Flushing/ Suction Suction Flushing emptying dredging, dredging, methods through Excavation Dredging sediment sediment Precautions with bottom added not added bypass outlet downstream downstream Operational aspects(a)  - ‚ ‚‚ - ‚‚‚ ‚‚ - ‚‚‚ ‚‚ - ‚‚‚ ‚‚ - ‚‚‚ ‚‚‚(1) structural adaptations, staff expenses, material expenses, energy expenses, …

Transport (a)  ‚‚ - ‚‚‚ ‚‚ - ‚‚‚ ‚ ‚‚ - ‚‚‚  structural adaptations, vehicles, fuels, …

(a) Production loss ‚ - ‚‚ ‚ - ‚‚‚  - ‚‚    - ‚

Effects on the Long-term effects, environment(b) - stream, ‚‚ - ‚‚‚  - ‚  - ‚ ‚ - ‚‚‚  - ‚ ‚ - ‚‚ not quantifiable in advance - population ‚‚ - ‚‚‚  - ‚‚  - ‚‚ ‚ - ‚‚‚  - ‚‚  - ‚‚ • noise, air, traffic • non-material values(2) - expense of energy ‚ - ‚‚ ‚‚ - ‚‚‚ ‚‚ - ‚‚‚ ‚‚ ‚‚ - ‚‚‚ ‚‚

Cost(a) of effects(3) ‚ - ‚‚ - ‚‚  - ‚‚ ‚ - ‚‚  - ‚‚  - ‚

Expense (a) / negative consequences (b)  none (1) one-off expense (+ maintenance) ‚ small (2) fishery, bathing etc. ‚‚ medium (3) prevention / remediation ‚‚‚ large

7.5.5 Decommissioning of dams In the extreme case, decommissioning of a dam will need to be considered. This may involve the complete or partial demolition of the structure, or the dam may be left as it is. Greater details are found in Morris et al. (1997). It should be borne in mind, however, that the original stability analysis of the dam was based on water head rather than earth pressure. Moreover, a dam below a sediment-filled reservoir will age, too. Where its breach or failure may constitute a risk, monitoring or even repair becomes a necessity. But the spillway may still be in functioning order in such a case.

7.5.6 Problems caused by excavations The great number of potential problems that may arise in desedimentation will be treated here only by supplementary references to the relevant literature. Ecological aspects of reservoir sedimentation are treated in DVWK (1993) and of reservoir flushing in BUWAL (1994). A documentation on the reuse of dredged materials is found in DVWK/LAWA (1992). The legal implications are summarised in DWA (2006). See also "Sediment Management – Technical and Legal Aspects" (ALPRESERV, WP 7, TU Graz). In the light of the great public interest in environment-related measures, it is recommended that any possible conflict potential be reduced or even prevented by means of open and Reservoir sedimentation 199 competent technical information before a project is launched. According to ÖWAV (1998), this should include: • Information prior to the measure – on its type, extent, duration and possible impact, • Information on the type and extent of the perpetuation of evidence, • Information on accompanying measures and their results, • Information on stream regeneration.

7.6 Special problems of reservoirs The sediment-transport models discussed in Section 6.9 above are not yet generally capable of simulating characteristics specific to reservoirs. These include • density or turbidity currents (Section 7.2.4.2.3), • effects of wind and waves, • surface ice, • layering. Appropriate formulas are available, but will not be discussed here. The problems involved in density or turbidity currents and their effect on reservoir sedimentation were discussed in Section 7.2.4.2.3 above. See also "Reservoir Sedimentation" (ALPRESERV, WP 6, EPFL, Lausanne).

7.7 Outlook A large amount of literature is available on reservoir sedimentation and desedimentation, which demonstrates the practical importance of this subject. Meanwhile much experience has been gathered regarding the problems arising both in backwater reservoirs and reservoir lakes, but this can rarely be directly applied to other facilities. The number of governing parameters is simply too large. Every reservoir is an individual with its specific inflow characteristics (water and sediment), geometry, intake structures and management. The introduction and intensive development of hydrodynamic sediment-transport models (Section 6.10), however, have provided practicable tools for the satisfactory simulation of these conditions. Aided by practical experience, such models are capable of providing at least qualitative information on the various sedimentation probabilities and desedimentation possibilities. By way of winding up, the experience gathered by Nordin (1991) regarding the problem of reservoir sedimentation will be quoted here: • Theoretical and empirical bases have become available for developing management strategies for avoiding problems caused by sediment, e.g. in the form of mathematical models for the long-term preservation of storage volume. • Reservoirs are still being designed without sufficient knowledge of the sediment problem. This, however, cannot be solved, but can only be "managed". Management strategies have meanwhile become available, and these should be exchanged among the decision-making agencies. • Flow in running waters is a renewable resource. But this does not apply to energy from hydro power where this relies on reservoirs subject to sedimentation. The conditions under which long-term storage in reservoirs is possible without time limitation are 200 Sediment Sources and Transport Processes

restrictive. Where such conditions exist, the project should be planned so that the storage volume is preserved without time limitation, provided the project is economical at all. In the light of the great relevance of the problems involved in reservoir sedimentation and desedimentation, several international technical associations have founded an initiative ICCORES in order to combine the state of the art and agree on an international basis on the impetus to be given to further development (DVA, 2006. Section 9).

Local scours 201

8 Local scours 8.1 General Unter Kolk versteht man eine lokal begrenzte Eintiefung in einem Gewässerbett, verursacht Scour is generally understood to mean a local deepening in a streambed caused by certain flow processes, such as a local increase or change in flow characteristics. This may be due to structures such as bridges, weir piers, abutments, training walls etc., where a change in flow characteristics (horseshoe vortex, secondary flow) governs scouring, or to a contraction in discharge cross section (spillways, discharge under gates etc.). Further causes are horizontal or vertical jet deflection and jet junctions (tributary). Such scours, when reaching a certain depth, may become a risk to structural stability.

Scours are distinguished according to sediment transport into the scour, m& G′′ , and out of the scour m& G′ . If both quantities are zero, a scour remains stable. A clear-water scour is present where m& G′ > 0 , whilst m& G′′ ≈ 0 , that is to say, there is no sediment transport in the stream itself. Where, however, both bed load-transport values are m& G′ ≥ m& G′′ ≥ 0 , there is live-bed scour. Figure 8.1-1 is a graph showing the different evolutions in time for these two types. Whereas a clear-water scour reaches its final depth, hK, relatively slowly along an asymptotic path, the other type is characterised by oscillation around the final scour depth and by faster deepening.

Figure 8.1-1: Evolution in time of a clear-water scour at a pier (Case a) as against a live-bed scour (Case b) (Breusers et al., 1991)

The evolution in time of scouring as shown in Figure 8.1-1 may be further shortened due to abrasion of the material forming the scour bed. Laursen (1963) summarised the results from a multitude of studies on scouring as follows • Scour depth corresponds to the capacity difference between bed load-transport out of

the scour ( m& G′ ) and into the scour ( m& G′′ ). • Scouring decreases as the discharge cross section increases. • The magnitude of scour tends towards a limit, for given conditions. • This limit is reached asymptotically after a certain time (Figure 8.1-1). All these conditions apply to non-cohesive bed material. Little is known about the influence of waves on scouring (Raudkivi, 1982). 202 Sediment Sources and Transport Processes

Despite recent in-depth studies on the various scour problems, the present knowledge, especially about the possibilities of quantitative local computation of scours in terms of evolution in time, is still unsatisfactory. This is mainly due to the following reasons: • Flow around the structure causing scouring is three-dimensional; jet separation, turbulence and secondary flows occur, the water surface may be substantially deformed. • Sediment transport with its multitude of governing parameters and interaction with flow is also subject to 3E effects and is unsteady. Failure of structures due to scouring occurs normally under conditions of unsteady flow with the sediment-transport dynamics involved. The great variety of structural shapes likely to favour scouring as well as their effects on flow and sediment transport make it difficult to apply experience gained elsewhere (Breusers et al., 1991). The greater part of fundamental studies rely on laboratory experiments and more or less idealised assumptions (e.g. single- grain material). Translation to natural conditions is not entirely satisfactory in such cases. This applies in particular to strongly unsteady or shooting flow.

8.2 Scouring at piers 8.2.1 Formation of scour The best-studied problem in this context is flow around a cylinder on a flat bottom. As demonstrated by Figure 8.2-1 below, the flow structure caused by the pier is complex and can be split into the following schematic components (Raudkivi, 1982): • vertical flow in front of the cylinder resulting from approach flow (jet generated by dynamic pressure), • boundary-layer and horseshoe vortex, • separation vortex and wake vortex. The downward-directed jet generated by dynamic pressure changes into a horseshoe vortex, which is responsible for erosion upstream from the pier.

Figure 8.2-1: Schematic drawing showing flow around a cylindrical pier with developed scour (Raudkivi, 1982) Local scours 203

8.2.2 Governing parameters Only the principal parameters governing scour formation will be listed here so as to demonstrate the complexity of the problem. The evolution in time of scour depth was mentioned in Section 8.1 (Figure 8.1-1) above. Substantial time is needed in laboratory tests to reach the final scour depth. Naturally, particle size and particle-size distribution have a direct bearing on scour depth. Thus, a particle mix of appropriate coarseness in areas subject to scour risks may prevent scouring altogether (cf. Section 8.2.4). Likewise, flow and bed shear stress affect scouring. This may moreover be influenced by pier shapes and sizes. Finally, mention should be made of the potential presence of bed forms and the effect of waves. Results from laboratory tests are shown plotted in Figure 8.2-2, where

ys = scour depth (hk) b = pier diameter (D) u = flow velocity (v)

uc = critical flow velocity (vcrit)

Figure 8.2-2: Laboratory data for determining scour depth for cylindrical piers under a relatively large water depth (Breusers et al., 1991)

8.2.3 Scour depth It has become clear from what has been said so far that the scouring process is extremely complex. It is not surprising, therefore, that an equation of general validity for determining final scour depth does not exist. But there are several empirical formulas derived from the results of laboratory measurements made under special conditions. A simple estimation 204 Sediment Sources and Transport Processes formula for the depth of a clear-water scour for uniform bed material and for a cylindrical pier of diameter D can be written as

Eq. 8.2-1: h K ≈ 2,3⋅ D []m

This relationship can also be derived from Figure 8.2-2 above. A comprehensive overview is found in Breusers et al. (1991, Section 5), where a multitude of individual parameters (particle size and distribution, water depth, pier shape and direction of approach flow etc.) are taken into account. Another contribution regarding scour at piers and abutments came from Melville (1997) and Dey (1997).

8.2.4 Reducing scour depth by structural measures Shen (1971) specified four possibilities of reducing scour depth or preventing scouring altogether: a) Change pier configuration: Changing the design of piers rectangular in plan from a blunt to a rounded front acts to reduce or almost entirely suppress horseshoe vortices (Figure 8.2-1). However, this effect is nullified as soon as the approach flow is not exactly parallel to the pier axis, a fact that cannot entirely be avoided due to irregularities in channel geometry and bed form development. The direction of flow tends to be a function of discharge and water depth. b) Provide horizontal apron: Scouring is safely reduced by providing a horizontal slab beneath the original riverbed. c) Provide upstream auxiliary piers: Small auxiliary piers provided upstream from the structure to be protected reduce scouring at the main pier by at least 20%. d) Armouring in the area subject to scouring: The commonest method, which may also be applied subsequently, is armouring in the area subject to scour around the pier, by means of armour stones or mattresses. The minimum required particle size of the stones as a function of flow velocity should be selected as

⎡m⎤ Eq. 8.2-2: d = 6 − 3,3⋅ v + 4 ⋅ v 2 []cm v in ⎣⎢ s ⎦⎥

The thickness of armouring should be equal either to the pier width or three times the particle diameter of the stones. Further information can be obtained from the paper "Bemessung von Sohlendeckwerken (künstlich) unter starkem Strömungsangriff" (Dimensioning streambed armouring (artificial) subject to strong flow attack) in DVWK (1997, Section 4.4.4.2) and Lim et al. (2001).

8.3 Scouring below low head structures – two-dimensional scour This process fundamentally resembles that of scouring behind piers as discussed above, except that the flow in this case is two-dimensional. Local scours 205

Figure 8.3-1: Scouring below low-head structures after Franke (1960)

Studies by Franke (1960) for the gate positions shown in Figure 8.3-1 above led to the formula

1 1 ⎛ q 2 ⎞ 3 ⎛ ∆h ⎞ 2 ∆h Eq. 8.3-1: ⎜ ⎟ ⎜ ⎟ h Ges = h K + h u = A ⋅⎜ ⎟ ⋅⎜ ⎟ = A ⋅ h c ⋅ []m ⎝ g ⎠ ⎝ d 90 ⎠ d 90 for a rough estimation of scour depth below gates. The factor A and the distances l1 and l2 from the outlet can be read from Table 8.3-1:

Table 8.3-1: Scour magnitude below gates after Franke (1960)

Case according to A Discharge per m lengths 3 Figure 8.3-1 [-] q1 or q2 [m /s m] l1 [m] l2[m]

q1 a 1,0 ≤ A ≤ 2,4 0 ≤ ≤ ∞ - - q 2

b 2,4 q2 = 0 0,5 hges 1,8 hges

c 1,0 q1 = 0 3,0 hges 6,0 hges

Recent and more comprehensive studies on this subject are reported in Breusers, Raudviki (1991, Section 7), where mention is also made of the impressive scatter of the results. Case studies are described in BAW (2002, No. 85). An in-depth study was conducted by the US Department of Transportation (2001), using programs HEC 18, HEC 20 and HEC 23. The flow chart used is shown in Figure 8.3-2 below. 206 Sediment Sources and Transport Processes

Figure 8.3-2: Flow chart for determining scouring and stream stability (US Department of Transportation, 2001)

8.4 Further causes for scouring There are still more causes for scouring, which deserve brief mention here. Further details are given in the relevant literature. Scours in rivers, especially in bends and junctions, are important (see Section 5.2.2.6). For further details, see such literature as Breusers et al. (1991, Section 3), Graf (1998, Section 9) and Schöberl (1989). This literature also deals with scours caused by submerged jets and mentions riverbed deepening below culverts and dropping jets. Further practical hints regarding the treatment of various scour types, including case studies, were published by Annandale et al. (2002). Nagata et al. (2005) developed a three-dimensional numerical model for flows and streambed deformation to simulate scour development. Examples of groynes and cylindrical piers show good agreement with laboratory results.

Grit chamber design 207

9 Grit chamber design 9.1 Camp (1946) applied the analytical solution of the simplified general diffusion equation, found by Dobbins (1944) to the sinking motion of particles in grit chambers. Assuming a velocity profile with a parabolical distribution over depth and using the equation for turbulent shear stress

dv Eq. 9.1-1: τ = ρ ⋅ ε ⋅ x t dy a constant value is obtained for the turbulent diffusion coefficient

λ τ τ ⎡m 2 ⎤ Eq. 9.1-2: ε = ⋅ h ⋅ 0 = 0,075 ⋅ 0 ⋅ h ⎢ ⎥ 8 ρ ρ s ⎣⎢ ⎦⎥

This has been based on a resistance coefficient λ = 0.045. Assuming the velocity distribution over depth to be constant and ignoring diffusion in the direction of flow, ,x , the diffusion equation so simplified

∂C ∂ 2 C ∂C ⎡ kg ⎤ Eq. 9.1-3: v x ⋅ = εs ⋅ + vs ⋅ ∂x ∂y 2 ∂y ⎣⎢m3 ⋅s⎦⎥ for constant initial concentration and the given boundary conditions can be solved by use of series-expansion methods. The result is the retention factor of a grit chamber, which can be read from a diagram (of the type shown in Figure 9.1-1 below) provided the surface loading

Q ⎡m⎤ Eq. 9.1-4: v 0 = O ⎣⎢ s ⎦⎥ with surface O and discharge Q of the grit chamber is known. The above limiting assumptions can largely be omitted where the problem is solved by numerical means (Schrimpf, 1987). In this case, the diffusion coefficient averaged over depth,

2 λ v m ⎡m ⎤ Eq. 9.1-5: ε = ⋅ κ ⋅ h ⋅ ⎢ ⎥ 8 6 ⎣ s ⎦ holds, where vm is the median flow velocity. Figure 9.1-1 shows, by way of example, the design diagram obtained for logarithmic velocity distribution, as a basis for determining the retention factor for a grit chamber, with triangular concentration distribution at the inlet. This model was expanded by Schrimpf (1987) so as to permit determination of suspension- load concentration distribution in a stream, given suitable boundary conditions (resuspension factor at the channel bed). 208 Sediment Sources and Transport Processes

Figure 9.1-1: Design diagram for a grit chamber with triangular concentration distribution increasing towards the bottom at the inlet, and with logarithmic velocity distribution (Schrimpf, 1982)

Information on the design and operation of grit chambers (and settling tanks) are found in the ATV Handbook "Mechanische Abwasserreinigung" (Mechanical sewage treatment) (1997). The above design equations hold for prismatic channels with uniform flow as found e.g. as grit chambers in sewage treatment plants. The grit chamber length so computed needs to be increased by appropriate inlet and outlet lengths (effective space) where the velocity distribution in these areas is not yet uniform. Care should also be taken to avoid exceeding the threshold velocity (Section 4.1.2.3) or shear stress (Section 4.1.2.4) for separation of the desired particle size. Sand and gravel traps provided at intakes in streams are intended to keep sediment from entering such structures as turbines (Figure 9.1-2 below). Gravel traps in streams usually consist of nothing but lateral streambed widening to reduce the flow velocity. The size of the trapping space should be dimensioned as a function of the expected annual bed load. Here, too, it is important to remember that the effective space is smaller than the volume of the widened streambed. An empirical and theoretical study on sand-traps in hydroelectric facilities was recently conducted by Ortmanns (2006). Grit chamber design 209

Figure 9.1-2: Sand trap downstream from a water diversion (Gieseke et al., 1998) 210 Sediment Sources and Transport Processes

Instruments for measuring bed load and suspended load 211

10 Instruments for measuring bed load and suspended load 10.1 Introduction This Section deals with various types of measuring devices and their uses. The DVWK has prepared two rules in an effort to standardise the practice of measuring and interpretation methods: "Schwebstoffmessungen" (Suspended-sediment measurement, 125, 1986), and "Geschiebemessungen" (Bed load measurement, 127, 1992, under revision). Observance of the above rules is recommended not only for measuring practice, but for dealing with sediment-transport problems in general. The relevant literature provides detailed information on bed load-measuring techniques in general as well as special measuring methods and error treatment procedures, thus e.g. Morris et al. (1997, Section 8), Graf (1971, Section 13.2), WMO (1981, Sections 4 and 6), IAHS (1981) and Shen (1971, Sections 14 and 15).

10.2 Bed load measuring devices 10.2.1 General Direct measurement of bed load (volume and particle-size distribution) is more time- consuming and holds greater inaccuracies than in the case of suspended sediment. This is why major records are rare, and where such information is not available, bed load volumes are often determined from dredged material and delta surveys. A few mean annual bed load volumes are shown for Bavarian Rivers in Table 10.2-1 below.

Table 10.2-1: Mean annual bed load-transport values for several Bavarian rivers

River Place Catchment Area Period of Mean annual Note observation bed load volume [km2] [m3/a] Donau Hofkirchen 47.489 1989-1990 28.000 Bed load trap Inn Rosenheim 10.000 1961-1980 104.000 Reservoir dredging Isar Sylvenstein 1.156 1958-1983 53.600 Delta growth survey Isar Plattling 8.839 1988-1989 57.000 Bed load trap Saalach Reichenhall 940 1969-1984 95.000 Reservoir dredging Salzach Mündung 6.717 1953-1987 112.600 Reservoir dredging Ammer Ammersee 709 1962-1988 24.000 Delta growth survey Tiroler Achen Chiemsee 952 1869-1965 40.000 Delta growth survey

Bed load motion takes place in the immediate vicinity of the streambed. The individual particles move rolling, sliding, hopping and bouncing. The maximum leap height attained by a bouncing particle does not exceed a finite multiple of its diameter. It should be attempted, therefore, to capture the entire material, with account being taken of the leap parameters. This should be borne in mind when selecting the dimensions for the inlet of a mobile bed load sampler. A bed load sampler must be long enough to prevent particles from leaping over it. Attention needs to be given to bed forms when using mobile samples or, in fact, also stationary bed load traps. That means that measuring stations should not be located behind the crest of a bed form. Bed load motion does not spread continuously across the channel width, but is concentrated in longitudinal streaks due to secondary flows (Figure 4.3-1). The consequences to be drawn for measurement are the following: 212 Sediment Sources and Transport Processes

• The time intervals between single measurements should not be too short. • The time interval between single measurements should be varied. • Measurement results with mG = 0 should not generally be regarded as measuring errors. Bed load measurement should not only be distributed over the channel width (Figure 4.3-2), but should be made at time intervals selected according to a random scatter pattern in order to obtain representative mean values. Allowance should also be made for the dependence of bed load transport on discharge and its variation over streambed width as shown in Figure 4.3-3 above. Bed load measurement should be combined with measurement of the particle-size distribution in armouring and sublayer of a channel, as well as with determination of a velocity profile. Moreover, echosoundings should be made along the streambed in order to obtain information on bed form locations.

10.2.2 Direct measurement 10.2.2.1 Mobile bed load samplers Mobile bed load samplers are usually customer made, developed and tailored to the respective conditions by hydrological and hydraulic engineering institutes. The commonest bed load samplers are similar in design. The commercial Helly-Smith sampler (Figure 10.2-1) will be discussed below by way of example. This device has performed very well in sandy to gravelly streambeds and is easily modified and adapted to local conditions.

Figure 10.2-1: Helly-Smith sampler (DVWK, 1992)

An advanced version of this device has been presented by the German Federal Institute of Hydrology (Figure 10.2-2). Combined with an underwater camera fixed to the inlet, the device can be used to check position on the streambed and bed surface (armouring). Instruments for measuring bed load and suspended load 213

Figure 10.2-2: Bed load sampler developed by the Federal Institute of Hydrology, Koblenz (DVWK, 1992)

10.2.2.2 Stationary bed load samplers Bed load traps or slots are stationary samplers sunk into the streambed. They capture bed load and need emptying at certain intervals (Figure 10.2-3). Designed to the respective stream size, they can be used to obtain bed load-transport balances (load). The result can be used to calculate the mean rate of bed load transport provided the trap covers the entire bed load- carrying width of the stream. Bed load traps are simple and almost maintenance-free. The disadvantages lie in the disturbance to the bed load regime and the uncertainties involved in calculating large bed load volumes. Bed load traps that do not cover the entire channel width, or several traps arranged over the width of the streambed, may give rise to systematic measuring errors caused by secondary flows with increased bed load transport rates (Figure 4.3-3). 214 Sediment Sources and Transport Processes

Figure 10.2-3: Bed load trap in the River Rhine near Wesel (DVWK, 1992)

10.2.3 Indirect measurement Sound Samplers The acoustic method of measuring bed load uses underwater microphones (hydrophones), which either directly record the noise from bed load transport or measure the noise produced by particles hitting a metal body connected to a microphone.

Pigmented bed load – grain tracking Bed load material from elsewhere having the same grain-size distribution and density and pigmented to a colour different from the river bed load can be used as a marker in the river section under study. This does not permit direct measurement of bed load transport, but provides concrete hints on the transport distance of the marked grain fraction under natural conditions. Finding the particles may pose problems, especially as they risk sinking to deeper layers during flows of high transport intensity.

Luminophores Luminescent dye particles are fixed to bed load grains by use of a strong bonding agent and added as markers to the river bed load. "In situ" checks being not possible, a great number of samples need to be taken, dried and manually counted under UV light in order to find the marked particles.

Instruments for measuring bed load and suspended load 215

Tracers - transponders Successful attempts have been made since 1981 to measure bed load transport by recording magnetic bed load (material with natural magnetic rock components or gravel particles provided with iron cores) at a measuring station equipped with induction coils. Transponder (small transmitters) can also be installed in bed load particles, to be located by antennas.

10.2.4 Other indirect methods For details, see DVWK Rule No. 127 "Geschiebemessungen" (Bed load measurement, DVWK, 1992).

Dredging Long-term registration of material quantities dredged from a river section, e.g. at the head of a reservoir, may be used to determine mean annual bed load volumes as well as extreme values.

Delta growth surveying By far the largest proportion of the sediment load delivered to lakes or reservoirs comes from tributaries, whereas the contribution through direct supply from rock falls, slides, avalanches and debris flows is usually insignificant. Bed load and coarse-grained suspended sediment are usually deposited where the stream enters the lake so as to form a delta (Figure 10.2-4). The fine-grained fractions are passed on as dictated by the reservoir configuration (Section 7.2.2). Bed load is determined by first making a delta growth survey to establish the total volume of deposited sediment. Then the silt, sand and gravel proportions are determined by borings (Section 7.2.3).

Figure 10.2-4: Evolution in time of the delta formed by the Tiroler Ache streams in Lake Chiemsee between 1869 and 1970 (Mangelsdorf et al., 1980)

216 Sediment Sources and Transport Processes

10.2.5 Bed material sampling Knowledge of the particle-size distribution of bed material is essential for assessing the collection behaviour of the sampler (leap distance), for calculating bed load volumes and for identifying the present regime. Any bed load-transport measurement should, therefore, be accompanied by bed load sampling. Particular care is indicated when using grabs for sampling (as shown in Figure 10.2-5) or borings, in order to obtain reliable results. Detailed knowledge of the river section to be studied is, therefore, needed for the selection of sampling sites.

Figure 10.2-5: VAN VEEN Grab System (DVWK, 1997)

The sample volume required is difficult to obtain, especially where coarse bed material is concerned. The most reliable method is direct sampling by well-trained divers.

10.3 Suspended sediment measurement 10.3.1 General Unlike bed load-transport, suspended sediment is carried in streams throughout the year. Even clear stream water tends to contain minor quantities detectable only by measurement. A suspended sediment concentration smaller than 15 mg/l is no longer discerned as turbidity. Suspended sediment transport is subject to pronounced seasonal variations, the greater proportion of the annual volume being transported within a few months of major flow, in small streams even only in periods of flood flow. Figure 10.3-1 below depicts the mean annual suspended-sediment load of the River Danube and its principal tributaries as a suspended-solids band based on records from the 1971 to 1989 series. Note the two collecting channels, one is the Danube receiving the northern tributaries poor in suspended sediment and the other one is the southern tributaries richer in suspended sediment, Iller, Lech, Isar and, above all, the River Inn, which is the main suspended-sediment collector of a great proportion of the Eastern Alps and gives the River Danube its special character downstream from the town of Passau. Table 10.3-1 lists suspended-sediment data from several Bavarian measuring stations. When comparing this with Figure 2.4-2, it is necessary to deduct the bed load. The purpose of a measurement is to determine suspended-load concentration in a given water volume. This may be accomplished by filtering a water sample and then weighing the filter residue.

Instruments for measuring bed load and suspended load 217

Table 10.3-1: Long-term records (1971 – 2000) of mean annual suspended-sediment transport and suspended load in Bavarian rivers (DGJ, 2000)

River Measuring Catchment Area Suspended-sediment Annual Specific station concentration suspended sediment load erosion [km2] [g / m3] [103 t] [t / (km2/ a)] Mittel Maximum

Donau Neu Ulm 5.460 44 1.767 76,1 13,9 Donau Ingolstadt 20.008 44 3.786 446,9 22,3 Iller Kempten 955 111 12.731 164,2 172,0 Iller Wiblingen 2.115 64 2.209 148,0 70,0 Lech Füssen 1.422 196 8.800 376,6 264,9 Naab Duggendorf 5.426 25 923 40,1 7,4 Isar München 2.855 37 1.955 109,6 38,4 Isar Plattling 8.839 36 1.909 200,5 22,7 Ammer Weilheim 601 177 23.692 82,6 137,6 Amper Inkofen 3.043 25 1.210 38,7 12,7 Inn Oberaudorf 9.712 179 10.269 1.711,0 176,2 Tiroler Achen Staudach 952 231 53.196 259,6 275,0 Traun Stein 378 94 12.220 37,0 97,7 Saalach Unterjettenberg 940 206 10.428 238,3 253,5 Salzach Burghausen 6.649 171 6.919 1.350,6 203,1

Figure 10.3-1: Mean annual suspended load of the River Danube and main tributaries (based on Bavarian State Agency for Water Management, 1992) 218 Sediment Sources and Transport Processes

Figure 10.3-2: System of suspended-sediment concentration measurement after Schemmer (1995)

Figure 10.3-2 above shows the principal possibilities of determining suspended-sediment concentration. Detailed information on a multitude of suspended-sediment measuring instruments is found in Schemmer (1995). The literature on bed load-measuring equipment mentioned in Section 10.1 also gives information on devices for measuring suspended sediment. Measuring problems are discussed in great detail in WMO (1981, Section 3). Graf (1971) suggests that the following points should be observed in order to obtain reliable results: • Suspended-sediment concentrations are subject to temporal variation due so such factors as turbulence. This is why measurements should cover a major period. Instantaneous measurements (bucket samples) should be assessed with care. • It is difficult to distinguish correctly between suspended sediment and bed load in the near-bed (unsampled) zone. • Besides suspended-sediment concentration, velocity distribution and total discharge should also be measured. • Samplers naturally also take washload (Figure 3.1-1). • Depending on accuracy requirements, measurements will be needed at several verticals (Figure 10.3-2), whose locations should be selected as follows: + one vertical in the middle of the river + one vertical in the thalweg + verticals at the quarter, half and three quarters points of the river width + four or more verticals a the centrepoints of discharge cross sections of equal width + verticals at the centres of areas of equal discharge. The annual suspended-sediment load is calculated by determining instantaneous suspended- sediment concentration through the maximum possible number of individual measurements. As in the case of discharge measurement, these are of particular interest when the stage changes. Hourly measurements are needed during the passage of floods so as to capture all the partial maxima and minima. Daily values determined from one or several samples are of no great informative value. It is only the results from a long-term series of measurements that provide reliable mean values of suspended-sediment concentration and suspended load (Section 4.4.9). Experience has shown the period of record should not be shorter than 15 years.

Instruments for measuring bed load and suspended load 219

10.3.2 Single point measurement Although the distribution of suspended-sediment concentration may vary considerably over the discharge cross section, the constant multiple-point measurements taken since the beginning of official suspended-sediment measurements in Bavaria, deemed too expensive technologically, have been superseded by a much simpler method. This consists in sampling from a near-surface zone within the line of maximum velocity by use of a 10-litre bucket. The number of samples taken at a measuring station, depending on the respective discharge, extends from a single sample per week to eight samples a day and mainly makes allowance for the respective stage. An average of 300 samples should be taken per year per measuring station (DVWK, 1986), which practice has proved a reliable means of determining suspended-sediment concentration and load. Concentration averaged over time can be calculated in two ways: a) without allowance for discharge, i.e. via concentration [kg/m³] b) with allowance for discharge, i.e. via transport [kg/s] Alternative b) is preferable especially where the discharge shows major variation over time, as in Alpine rivers. The Bayerische Landesamt für Wasserwirtschaft calculates mean values for the Hydrological Yearbook with allowance for discharge, using the following procedure: Transport is calculated using the daily mean of discharge in the case of one single-point measurement per day, and by use of the simultaneous discharges in the case of several concentration results. Where no single-point measurement is available during low-flow periods or at week-ends, the nearest loads on the time axis are interpolated linearly. Where a major record of daily single-point measurements is lacking, computed concentrations (identified by "Status I", instead of "Status M" for real measured value) at a time n can be introduced manually.

Concentration()n +1 − Concentration ()n −1 Eq. 10.3-1: Concentrationn = Dischargen Discharge()n +1 − Discharge ()n −1

The desired time-averaged concentration is now calculated from the integration of the linearly linked discrete transport values followed by division by the total discharge over the desired period (e.g. mean daily or monthly values). This is why it is not possible to calculate monthly averages from daily means. In the case of individual measurements, bucket sampling may be replaced by scoop sampling (Schemmer, 1995). Greater sampling depth than is feasible by means of simple sampling buckets is achieved by devices designed to keep closed before the desired depth is reached.

10.3.3 Multiple point measurement Multiple-point measurements are carried out occasionally, especially during floods, to check and calibrate single near-surface measurements. They consist in dividing the measuring cross section into several verticals, in much the same way as for discharge measurement, and taking one sample at each of various depths selected in advance. One of the special devices used for this purpose is presented below by way of example:

220 Sediment Sources and Transport Processes

OTT sampler OTT has developed a suspended-sediment sampler holding six 2-litre bottles and travelling along a cable crane, from which it is sunk along the vertical to the desired measuring depth (Figure 10.3-3). Behind the inlet nozzle is a revolving distributor which, electrically controlled, connects to the respective next of the bottles arranged in series in a case. This device offers the advantage of permitting up to six samples to be taken one after the other without having to haul the device home each time for changing bottles. A disadvantage is its great weight (about 80 kg).

Figure 10.3-3: OTT sampler (DVWK, 1986)

10.3.4 Integration measurement According to Figure 10.3-2 above, there are two types of integration measurement: 3 point integrating one bottle each 3a depth integrating } Case 3a is illustrated by an example below:

US Sampler D 49 This is a streamlined device (Figure 10.3-4) with a hollow containing a bottle of about 0.5 litre capacity. Exchangeable inlet nozzles of different diameters serve to control filling time. This device permits integration measurement, in that the bottle fills gradually while passing through the measuring vertical at a constant sinking and lifting speed, from the water surface to the bottom and back. Due to its low sample capacity and the somewhat tedious selection of the suitable inlet-nozzle diameter, this device is now rarely used. Instruments for measuring bed load and suspended load 221

Figure 10.3-4: US-Sampler D 49, dimensions in [mm]

10.3.5 Other methods Determination of the turbidity of a stream using a light source and a photocell may convey some idea of the magnitude of suspended-sediment concentration. Photogrammetric measurement is distinguished into absorption and light-scatter methods. Turbidity can be measured instantaneously or continuously at one or several points of the stream section under study. Continuous turbidity measurement consists in pumping in water continuously and conveying it to a turbidity meter. This can be located on a boat or in a measuring station and may be connected to recording instruments to obtain a turbidity graph. There are also probes working "in situ", obviating the need for sampling. Conversion of a turbidity value to suspended- sediment concentration depends on particle size, which imposes limits on the application of this method. Hengl et al. (2004) studied the suitability of turbidity probes. The authors investigated very fine particles (0.1 ÷ about 10µm) under laboratory conditions. Depending on the type of device, the results obtained under these idealised conditions were rather disappointing. Similar results were obtained from a study comparing a turbidity probe with suspended-sediment samplers at the Luzzone reservoir. Correlation was less than 0.3 (ALPRESERV, WP6 "Reservoir Sedimentation", Section 2.5.2). A comprehensive up-to-date report on suspended-sediment measurement using the techniques at present under discussion, plus assessment, was prepared by Wren (2000). Table 10.3-2 below is a brief list of the results. 222 Sediment Sources and Transport Processes

Table 10.3-2: Comparison of different technologies of suspended-sediment measurement (Wren, 2000)

It becomes clear from the above that acoustic methods are promising, in that they simultaneously yield spatial and temporal flow velocity as well as suspended-sediment concentration including the discharge and suspended-sediment transport values derived from them. The backscattered values, however, are dependent on shape, size and condition of the sediment particles. The measuring signal is only a relative value (Maushake, 2005)

Ecological aspects of reservoir sedimentation and removal 223

11 Ecological aspects of reservoir sedimentation and removal 11.1 Introduction Beside those solid particles resulting from erosion and abrasion processes of rock material in the catchment area a lot of further substances are transported by the water which are introduced into the water body intended or unintended. For example solved and dissolved substances from sewage treatment facilities are released into the rivers and may be found in reservoirs. Additionally increasing amounts of contaminants are transported into reservoirs via the atmosphere. Therefore sediments of reservoirs may contain a certain quantity of contaminants. As a consequence evacuation measures of a reservoir are often intrinsically tied to environmental concerns, too. If an operator plans an evacuation of a reservoir or is forced to do so, he has to taken into account that he might also extract contaminants along with the sediments. According to the present legal regulations contaminated sediments are generally regarded as waste which has to be treated. An evacuation can be therefore extremely costy. Part of the environmental concerns about evacuation measures of reservoirs are not only possible contaminants but also the fact that aquatic sediments are often valuable habitats of animals and plants. An evacuation and deposition of sediments outside of the water body is resulting in an extraction of the residing animals and the destruction of their natural habitat. If a relocation of sediments by means of technical measures is applied, e.g. flushing operations, a significant quantity of sediments is transported downstream within a short period of time resulting in further risks for the aquatic ecosystems. Beside the difficulties regarding possible contaminants more burdens are added to the planning process of evacuation measures due to nature preservation as habitats and living animals should not be affected or destroyed by technical procedures. Therefore for sediment removal activities also the ecological situation has to be taken in account beside the specific conditions of the sediments itself. Consequently investigations of possible contaminants and an ecological inventory is, beside the volumetric survey and the geotechnical specification of the material, part of the planning process prior to an evacuation campaign. Along with the extraction of sediments from the water body complex legal regulations are applied which can be very different in different countries. The ALPRESERV publication “Sediment Management - Technical and Legal Aspects” gives detailed information for the alpine nations. The regulations explained in chapter 11.8 of this publication are applicable to Germany only. However, presently a common administrative application of the legal framework for the treatment of excavated sediments is not effective in Germany due to the federal structure.

11.2 Definition of contaminants Förstner (1990) differentiated between • environmental chemicals and • environmental contaminants. Environmental chemicals are substances introduced into the environment through human action, such as fertilisers, detergents, solvents, fungicides and herbicides. Environmental contaminants are naturally occurring substances likely to have detrimental effects on creatures or material goods. 224 Sediment Sources and Transport Processes

Contaminants may cause acute or long-term damage. Their dangerousness comes from a high level of adsorption capacity, long decomposition times as well as great stability and mobility. For further details on the various contaminants, their occurrence, availability and effects, especially heavy metals and asbestos, see Förstner (1990, Section 2).

11.3 Contaminant sources and charge 11.3.1 Sediment formation in lakes The following is based on studies on natural lakes by Sturm et al. (1995); the results apply analogously to major reservoirs. Tributaries are responsible for the greater part of the allochthonous (external) sediment load carried into a lake, where the solids settle in layers. Other paths of sediment introduction are the atmosphere through wind and rainfall as well as groundwater, through which substances washed from the surface find their way into the lake. Further paths are water treatment plants and stormwater overflow, waste water from industrial plant as well as quarries and mines and finally navigation and accidents (Figure 11.3-1).

Figure 11.3-1: Sediment formation in lakes (Sturm et al., 1995)

In addition to the substances carried into a lake from outside (alochthonous), particles may also form in the lake itself (autochthonous load), through such factors as algae production and precipitation (calcite). Such sedimentation often proceeds in something like annual rings (varvae), which form a cyclic sequence of light and dark co loured layers. Clastic varvae tend to form in oligotrophic (nutrient-poor) lakes controlled by tributaries, whereas biochemical varvae are found in eutrophic (nutrient-rich) lakes. Such sediment may serve for the precise dating of extraordinary events such as volcanic eruptions, extreme floods, radioactive fall-out etc.). This forms archives capable of providing information on climatic change over thousands of years.

Ecological aspects of reservoir sedimentation and removal 225

11.3.2 Suspended sediment as a potential contaminant carrier 2 Man-made contaminant sources are fairly rare in Alpine regions and foothills. Potential sources are underground or surface mines as well as tunnelling sites with their overburden and spoil dumps. Although the introduction of industrial contaminants at these altitudes is not a characteristic risk, it is nevertheless important to be aware of the great sorption potential of these sensitive ecosystems. This is determined by concentration [mg/l] and particle diameter [d] and represents an important factor in the ecology of an aquatic system. Especially in reservoirs above river barrages, this fact may produce environmental problems. Construction of artificial barriers, while capable of evening out discharge to a large extent, interrupts the transport of part of the suspended sediment and of almost the entire bed load. Fine sediment then settles in the reservoir. Contamination of the suspended sediment leads to the formation of depots which the self-cleaning potential of the local riverine habitat alone is incapable of neutralising.

11.3.2.1 Geogenic and anthropogenic contaminant supply Clear information on contaminant supply to reservoirs in the Alpine region and foothills can only be obtained through knowledge of the wide range of local influences from industry, agriculture and civilisation as well as natural sources and local anomalies (e.g. arsenic deposits in the Mur river basin) within the respective catchment, and by in-depth study of the various factors concerned. The list of geogenic and anthropogenic contaminant sources mentioned below is certainly not complete and should be regarded as mere examples and basis for further study.

11.3.2.1.1 Geogenic contaminant supply Natural paths of contaminant supply to the aquatic system are those of general sediment supply through erosion from soil, banks and streambed. In the Alpine region and foothills, where phenomena such as (a) earthflows (mudflows, forest collapse), (b) weathering and (c) rock falls and rock slides occur within a catchment, it is mainly rocks that are eroded (Figure 10.3-2).

2 The greater part of this section has been taken from Klitschmüller (2004) 226 Sediment Sources and Transport Processes

Figure 11.3-2: Types of mass movement in the Alpine region (DVWK, 1993)

In many regions (e.g. Bavaria) almost all rock types may be present, as shown on geological maps (Figure 11.3-3). These include limestone (from b, c), flysch (from a), schistose rocks (from a, b, c), sedimentary rocks (from a, c) and magmatic rocks (from b, c) (Section 2.2).

Figure 11.3-3: Geological map of Bavaria Ecological aspects of reservoir sedimentation and removal 227

This mainly inorganic sediment supply from the Alpine catchments especially causes increased metal concentrations, due to geochemical reasons, in streams (fluid phase) and in sediment transport (solid phase). Metals such as zirconium, rubidium and strontium are thought to derive from local weathering processes. In regions where bedrock is made up of magmatic rocks, this may cause an increase in the natural contents of iron, cobalt, nickel and chromium (Reichert et al., 1982).

11.3.2.1.2 Anthropogenic material supply The main man-made sources of contamination for the aquatic system in the Alpine region and foothills through civilisational influences include introduction of solids via water treatment plants and stormwater overflows (Westrich, 1988) as well as via the air (DVWK, 1993). The anthropogenic contaminant load usually corresponds to a multiple of the geochemical base load. Heavy rainfall, mainly in small catchments in the Alpine region and foothills, may wash substantial quantities of dissolved and adsorbed contaminants into a river. The responsible sources are the stormwater overflows of sewerage systems, which carry unpurified stormwater or combined sewage into the stream. The initial flush caused by a heavy rainstorm may moreover introduce organic particles suspended in sewage. This increases not only the tendency towards mud silting, but also the sorption potential of the solids in a stream. Furthermore, contaminants such as heavy metals from industrial plant, air contaminants from atmospheric rain and agrarian chemicals from agriculture may be adsorbed to an increasing degree (Figure 11.3-4), so as to add to the contaminant load and finally contaminate the reservoir itself (Westrich, 1988).

solid + dissolved solid

Sewerage Underground and Tunnelling surface mines site Dredging Treatment plant

Disposal by Stromwater hopper barge overflow River

Tributary diffuse Sources Rainfall Flushing

Agriculture Bed load, suspended load and dissolved load (geochemical background) Navigation (Tourism)

Figure 11.3-4: Man-made contaminant supply to running waters in the Alpine region and foothills (Westrich, 1988)

11.3.2.2 Contaminants and other materials according to LAWA The preceding Section 11.3.2.1 differentiated between geogenic and anthropogenic contaminant sources. The German Länderarbeitsgemeinschaft Wasser moreover subdivides 228 Sediment Sources and Transport Processes the organic and inorganic substances settling in dissolved or particular form in a backwater reservoir into four groups (LAWA, 1990).

11.3.2.2.1 Nutrients The main nutrients present in running waters are nitrogen and phosphorus from the discharge of purified and unpurified water as well as substances washed from agricultural areas. They are the principal reason for the eutrophication (Figure 11.3-5) of reservoirs. They govern the magnitude of primary production of phytoplankton (algae) and macrophytes (aquatic plants). Decomposition of dead organic substances and the resulting oxygen depletion greatly reduce the self-purification capacity (Figure 11.3-5) of backwater reservoirs. Large quantities of nitrogen compounds and phosphates are present mainly in sediment. Relocation of sediment through dredging and flushing may release them suddenly and cause substantial damage to the aquatic ecosystem in slowly flowing stream sections, as in backwater reservoirs, as well as nutrient accumulation below a barrage (LAWA, 1990).

11.3.2.2.2 Depleting agents These substances, as mentioned above, decompose dead organic compounds through physical, chemical and/ or biological action (DIN 4049; Part 2, 1990). Such decomposition processes risk depleting the oxygen reserves (Figure 11.3-5 below) of water in reservoirs. The depleting agents include organic and inorganic substances such as detritus (organic decomposition products) and coarser particular organic substances, carbohydrates, protein compounds and ammonium as well as bacteria and fungi (LAWA, 1990). Their decomposition products are again the nitrogen compounds and phosphates responsible for the eutrophication (Figure 11.3-5) of the reservoir as well as carbon dioxide. Where more biomass is produced than is decomposed, or where more oxygen is offered through photosynthesis than can be consumed by decomposition processes, there is accumulation of depleting agents with their detrimental impact on the ecosystem of the reservoir (DVWK, 1993).

Figure 11.3-5: Principle of self-purification and eutrophication (NAJU, 2004)

Ecological aspects of reservoir sedimentation and removal 229

11.3.2.2.3 Disturbing agents Inflow of cold water and great streambed gradients in the Alpine region may cause turbidity currents rich in suspended particles (Section 7.2.4.2.3 above), which may contain mineral, organic and organismic substances (LAWA, 1990). The turbidity so produced minimises the photosynthesis of algae and water plants. In fact, the algae themselves may become a nuisance when providing the reservoir water with an unpleasant odour from their decomposition and metabolic products (DVWK, 1993).

11.3.2.2.4 Poisonous substances Poisonous substances include heavy metals, pesticides and halogenated hydrocarbons. What they have in common is the fact that even small concentrations have toxic effects on the biocenoses (LAWA, 1990). The degree of damage to the riverine ecosystem is a function of pH value, water hardness as well as oxygen and salt concentrations (Wachs, 1988). These poisonous substances find their way to reservoirs mainly via man-made sources (Figure 11.3-4) in the catchment. Accumulation of suspended particles in the impounded flow, in combination with the high sorption potential for the above substances, increases the adsorption capacity for poisonous substances. This in turn would lead to an increase in contaminant loads/sources and to the contamination of the streambed through the formation of contaminant sinks in the reservoir (Westrich, 1988).

11.3.2.3 Relation between discharge and sedimentation – contamination Reservoir sedimentation is generally governed by a great number of extraordinarily complex factors (Section 7), among which discharge plays a dominiant role and thus has a direct influence on the contamination level of sedimentation zones. After Kern (1993), discharge may be regarded as resulting from a great number of parameters and conditions within the catchment area and as the “motor” of suspended-load transport (Figure 11.3-6).

Figure 11.3-6: Suspended-load transport in backwater reservoirs; (a) flow duration curve, (b) suspended-load storage function (Kern, 1993)

Figure 11.3-7 below is a schematic diagram showing the above factors for a backwater area in the Alpine region in analogy to the general sedimentation diagram after Westrich (1981) for reservoirs in the Alpine region and foothills. 230 Sediment Sources and Transport Processes

CATCHMENT AREA IN ALPINE REGION + FOOTHILLS

Size Hydrology Topography Geology

Flow Sediment supply

Sediment transport

DISCHARGE Sewage (including combined sewage) Stormwater

Dam & Reservoir

Water level control Hydraulic structures

Power station Weir Lock/ Bypass Groyne field Fish ladder

Main flow Dead storage

Constant transport Variation in space and Suspended and sedimentation time fo transport and sediment supply rate sedimentation

Sedimentation/ Erosion

Service life - Commissioning year - Position of the reservoir in a series (head station, further stations, most downstream station) - Flushing - Dredging/ Excavation

Sedimentation Sedimentation

Equilibrium Dredging/ Excavation

Figure 11.3-7: Sedimentation diagram for catchments in the Alpine region and foothills (Westrich, 1981) Ecological aspects of reservoir sedimentation and removal 231

As can be seen from Figure 11.3-6, medium and, especially, slightly raised flows involve an increased supply of suspended matter and, thus, deposition of fine sediment in a reservoir. Floods, by contrast, act to remove part or the entirety of accumulated sediment. Studies have shown erosion rates are 100 to 1000 times larger than sediment-deposition rates for low flow rates. The transition between accumulation of matter and release of matter is characterised by the critical or threshold flow Qc. In view of the fact that construction of a hydraulic closure structure is always a compromise of flood protection versus sedimentation, the probability of Qc being exceeded is very small indeed. Field observations have shown that in large catchment areas and in the long term there is in fact something like interaction between flow and reservoir sedimentation. But it should also be mentioned that change – as caused by seasonal cycles, preceding wet/dry periods, modified vegetation conditions (deforestation) or enhancement of water-treatment plant and retention-space efficiencies (storage/ retention basins) – is a potential cause for instationarities (Kern, 1993). These manifest themselves by variations in sediment concentration at a single observation cross-section of a river. Catchments of minor size rarely exhibit such interaction, which allows the conclusion that in such cases there is no appreciable correlation between flow and contaminant accumulation in sedimentation zones (Müller et al., 1980).

11.3.2.4 Suspended sediment as an indicator of contaminant load There is now general consensus that the particles suspended in water are the main transport medium for contaminant substances. The main idea behind the use of suspended matter as an indicator of contaminant load is the “fingerprint” of the contaminants, which they retain unchanged from the point of introduction. By thus identifying suspended-sediment properties it is possible to determine the origin of contaminants. Unfortunately, there are still some inconsistencies with regard to the assignment of such “fingerprints” in theory and practice. In the Alpine region and its foothills, this problem mainly comes from the dynamics of suspended sediment (Symader, 1993). What is needed here is in the first place up-to-date analytics in the form of fine-sediment and contaminant transport modelling. Inconsistencies usually develop as a result of the difficulties involved in the attempt to reproduce the significant transport processes and flows in the model completely and correctly, both qualitatively and quantitatively and according to the state of the art. The “fingerprints” tend to become blurred especially in undeveloped stream reaches, as a result of the thinning effect. Also, reference material may sometimes not be available. Thus it may be difficult to find the best site for water quality monitoring. This is where backwater reservoirs may be a great help. Bottom mud as well as cores drilled from the sedimentation and stillwater zones (Figure 11.3-8 below) may provide information on past water pollution levels when compared with a dated reference layer (Westrich, 1988). 232 Sediment Sources and Transport Processes

Figure 11.3-8: Short core drilled from a sediment layer in the reservoir above Unit 15 at Landsberg, Bavaria (BAWAG)

In the presence of barrages, the particles sizes of suspended matter will be finer and more evenly distributed than in comparable unimpounded rivers. In addition, suspended matter below barrages is distributed fairly evenly over the discharge cross section due to the good mixing effect, so that single-point sampling may be regarded as sufficient. However, the potential validity of sampling needs careful examination in each particular case (Kern, 1993), since otherwise assessment of potential contaminant sources would involve great uncertainties (Westrich, 2004).

11.4 Water quality After Hamm (1991), impounded rivers are characterised by: • reduction in flow velocity; hence: + increase in sedimentation and sediment-retention tendency, + intensification of “self-purification capacity” (decomposition and elimination of allochthonous substances); but at the same time: • intensification of eutrophication tendency; hence: + increase in secondary contamination (autochthonous production of organic substances through plant biomass) and thus load on the oxygen regime; + changing biocenotic structures and biocenoses (stillwater species instead of rheophile species); prevention of migration behaviour); + changing relations between water body and environment. Adsorption of contaminants by sediment leads to accumulation of substances in a reservoir . This effect has been termed “self-purification”, which in fact refers to downstream river sections rather than the reservoir itself and may engender a long-term quality problem. Redissolution processes risk causing contaminants to go into solution, usually through sediment relocation, even long after contaminant supply may have been cut as e.g. by a reduction in sewage introduction. Even though contamination from sewage is on the decrease, a growing amount of organic and inorganic contaminants as well as nutrients are entering the lakes. Ecological aspects of reservoir sedimentation and removal 233

Reservoirs, by virtue of their increased residence time, afford better living conditions for phytoplankton than rivers do. Hamm (1991) published quality targets for nutrients in running waters: • Just tolerable: 0.160 – 0.200 (total P) mg/l • Desirable quality target: 0.050 – 0.150 (total P) mg/l These values apply for the vegetation period and low flow. Much greater amounts of organic substances than those resulting from outside sources may be produced by the phytoplankton. Decomposition of such organic matter leads to oxygen deficiency, to be met by such remedial measures as aeration. Hamm (1991) stated 4mg/l was the lowest acceptable limit for the oxygen concentration of a water body. With regard to the water biocenosis, it should be pointed out that reduced flow velocity, organic contamination and reduced oxygen concentration combined tend to produce biocenoses poorer in species but possibly richer in individuals than in undeveloped streams. The biocenosis in a reservoir is thus classified as being of lesser quality. This also applies to the fish population, worsened by the fact of their reduced migration possibilities. For further information on the impact of reservoirs on water quality, see Wolf et al. (1986).

11.5 Sediment quality – investigation methods – parameters Sediment is generally made up of a dominant proportion of mineral components mixed with a certain amount of organic matter. The fine particle fractions (mainly d < 20µm), due to their above-mentioned affinity to the substances dissolved in the water body, accumulate in the fine sediment. Thus extensive analytics is required to study and assess the substances contained in sediment samples for their ecotoxological effects. Extensive documentation on the methods of investigating, studying and assessing sediment deposits and suspended matter in waters was published by DVWK (1999b). Thus this subject will not be treated in any greater detail here. ATV Instruction Leaflet M 362, Part 3, "Umgang mit Baggergut und Mindestuntersuchungsprogramm"(Dealing with dredged materials and minimum study programme) provides practical information on this subject. Mention of the contamination of soils in the floodplains of the River Rhine was made in Müller et al. (1992).

11.6 Interaction between water body and sediment The interaction between dissolved and particular phases is an extremely complex problem to which no entirely satisfactory solution has yet been found, due to the host of different microbial and biochemical processes taking place between a water body plus pore water and sediment with the great number of substances it contains. This was the reason for founding a combined project on fine-sediment dynamics and contaminant mobility in running waters ("Feinsedimentdynamik – Schadstoffmobilität in Fließgewässern" - SEDYMO) by the German Ministry of Education and Research, as well as a DWA workgroup WW 9 "Die Bewirtschaftung kontaminierter Sedimente" (contaminated-sediment management). These activities have given rise to several workshops (thus at the Hamburg-Harburg University of Technology, in March 2006), so that this subject will not be treated any further here. For further references, see Evans et al. (1997) and Calmano et al. (1998).

11.7 Modelling fine sediment and contaminant transport 234 Sediment Sources and Transport Processes

Fine-sediment dynamics and contaminant mobility are modelled on the basis of the causal chain erosion – transport – sedimentation (ETS Cycle), also known under the more differentiated term erosion – transport – deposition – consolidation (ETDC Cycle). The complexity of the problem of dynamic fine-sediment transport is further aggravated by the above-mentioned microbial and biochemical processes. Thus the problem can be considered and modelled at different scales. At the microscale level, high-resolution 3D models can allow for density gradients, high particle concentrations and aggregating fine particles. For simulating contaminant transfer and determining the precise settling velocity, it is necessary to analyse aggregation and segregation behaviour. At the mesoscale level, the sorption specific to particles and contaminants will be modelled for a suspension mix. At the macroscale level, finally, 2D or 3D transport models will be used to describe the relationship between emission and immission including residence times in the water body. The results from such research projects were published in the above-mentioned SEDYMO workshop (Section 11.6 above). Jacoub (2004) developed a 2D model, on the basis of the TELEMAC Code, for simulating contaminant transport. This model simulates the transport of dissolved and particular phases in the water body. CTM-SUBIEF-2D describes the transport of dissolved and particular phases in the water body and in the mixing layer at the stream bottom. The sorption kinetics is described by a first-order reaction. The new module describes the processes for a total of five variables: suspended sediment as well as the dissolved and particular phases of a substance both in the water body and in the mixing layer, with transport differential equations having been formulated for the suspended phase. The above module includes eight parameters: sorption-kinetic parameters, equilibrium-distribution coefficient in the water body, or mixing layer, and contaminant decomposition coefficients. Furthermore, it considers diffusion between the dissolved phases in the water body and the upper sediment layer. Two case studies will be presented below: Wodrich et al. (2005) applied the simulation model WASP 5, developed by the U.S. Environmental Protection Authority (EPA), to sediment transport in a backwater reservoir. Figure 11.7-1 and Figure 11.7-2 below convey a fairly good idea of the two model concepts. Ecological aspects of reservoir sedimentation and removal 235

Figure 11.7-1: Model components of the programs by Jacoub (2004

Figure 11.7-2: Model components of the WASP 5 programs

11.8 Utilisation, treatment and relocation of dredged materials 11.8.1 Definition of dredged material The affinity of very fine sediments (mineral fraction < 20µm and organic components) towards the adsorption of substances dissolved in water (heavy metals, organic compounds) risks engendering major environmental problems in contaminated water bodies. Adsorption leads to concentration of such substances and may, in the extreme case, correspond to as much as one to several thousand times the amount of dissolved substances. This implies a high risk potential for the water body with its biocenoses, especially in the case of river sections where fine sediment is deposited. In special deposition sections, such as reservoirs, 236 Sediment Sources and Transport Processes ports, bays etc., such contaminated sediment may present a problem as it needs to be removed – or in fact dredged – in the case of operating troubles such as sedimentation in front of gates or obstruction to navigation. This poses the problem of having to decide whether to see to the appropriate disposal of the removed material or to risk resuspension of these particles through the various removal and relocation measures. Inundations during floods may carry substantial amounts of contaminated sediment into the floodplains, so as to damage agricultural areas. Navigation, through its bow waves and propeller jets, causes whirls sweeping deposited material to lateral riverbed zones and groyne fields (DVWK, 1992). Changes in the environmental (redox potential, salt content, pH value etc.) may remobilise contaminants adsorbed by fine sediment.

11.8.2 Investigation parameters The study scope required for assessing a particular case should be tailored to the needs of the respective boundary conditions. Use of standardised study programmes is not recommended in view of the large range of boundary conditions involved. Such decisions are up to the discretion of technical authorities and experts and should be made for each individual case.“ (DVWK, 1992) There are four fundamental categories of study parameters: • General sediment parameters (Table 11.8-1) • Heavy metals (Table 11.8-2) • Organic contaminants (Table 11.8-3) • Biotests (Table 11.8-4). For details, see the DWVK Study (1992) and other relevant literature. Due to the application of the European Water Framework Directive (WFD) further investigations might be necessary. Where required it has to be examined whether and which contaminants are going to be introduced to water bodies regarding the Annexes VIII, IX and X of the WFD. For material listed in Annex VIII quality standards have to be defined on catchment area level and therefore are not commonly valid.

Table 11.8-1: Parameters for general sediment characterisation (DVWK, 1992) (TOC: Total Organic Carbon, SM: Schwermetall (heavy metal), AOS: Absorbable Organic Sulphur compound)

• Particle size distribution (DIN 19 683) • Content of solid matter (DIN 38 414 T 2) • (Momentary) oxygen-consumption rate (DIN 38 414 T 6) • Nutrient contents: ammonium content in pore water, total nitrogen content, phosphate content in pore water, total phosphorus content (DIN 38 414 T 12) • Loss on ignition (DIN 38 414 T 3); TOC • Carbonate content (in the fraction < 63µm relevant to the heavy-metal content • Sulphur/sulphide content, AOS • Aluminium content (in the same sample as the heavy metals; for particle size correction) • pH value (DIN 38 414 T 5) Ecological aspects of reservoir sedimentation and removal 237

• Buffering: pH value reduction after addition of 5mmol of acid per gram of solids in a 10% sediment suspension

Table 11.8-2: Heavy metals to be identified (DVWK, 1992)

• Mercury • Copper • Cadmium • Chromium • Arsenic • Zinc • Lead • Nickel

Table 11.8-3: List of priorities for sediment-relevant contaminants or contaminant groups (DVWK, 1992)

Group I: Standard studies • Adsorbable halogens adsorbed by organic matter (AOX, DIN 38 414 T 18) • Mineral-oil hydrocarbons (analogously to DIN 38 409 T 18) • Polychlorated biphenyls (PCB) • Polycyclic aromatic hydrocarbons (PAH) with leading substances benzo[a]pyrene (BaP) and fluoranthene Group II: Specific studies • Oligo-chlorobenzene-toluenes ("ugilec") • DDT/DDD/DDE (only for River Elbe) • Hexachlorobenzene (HCB) and other chlorobenzenes • Phtalates (DEHP, DBP) • Polydimethylsiloxanes (“silicones”) • Chlorophenols • Organic tin compounds, especially tributiltin compounds (TBT) • Sulphur / nitrogen heterocycles • Polychlorated dibenzo-p-dioxins and polychlorated dibenzofuranes (PCDD/PCDF) • Alkylbenzenes • Gamma-hexa-chlorocyclohexane (“lindane”) and other HCH isomers • Acridine • Atracine • Linear alkylbenzene sulphonate (LAS) and other tensides • Trialkyl and triphenyl phosphates • Diphenyl ether, ditolyether 238 Sediment Sources and Transport Processes

Table 11.8-4: Biotests available for assessing water samples (DVWK, 1992)

• Pseudomomonas cell growth inhibition test (DIN 38 412 T 8) • Scenedesmus cell growth inhibition test (DIN 38 412 T 9; T 33) • Luminous-bacteria waste water test (E DIN 38 412 T 34) • Dapthnia short-term test (DIN 28 412 T 11; T 30) • Fish test (DIN 38 412 T 15; T 20;T 31) • Tubificoides test • Asellus test • Mussel test • UMU genotoxicity test using bacteria • SOS chromotest • DNA-synthesis inhibition test using eukaryonts

Dredged sediment may come exclusively from the streambed, as in the case of maintenance measures. Development measures, however, involve removal of material beyond the existing streambed profile, and this usually shows an entirely different physical and chemical composition. Dredged material (Geowissenschaft + Umwelt, 1999) may thus consist of • sediment or subhydric soils from the streambed • subsoils and their parent material in the immediate vicinity of the streambed, • top soils from banks and floodplains.

11.8.3 Quantities of dredged materials The rough estimation from the DVWK Study (1992) is shown in Table 11.8-5 below.

Table 11.8-5: Mean annual dredged quantities classified by origin and particle size (BMV, 1987))

Silt, fine sand Sand, gravel Total (all fractions) in million m³ in million m³ in million m³ Tidal and coastal zone 21,0 28,2 49,2 Inland waterways 0,8 1,2 2,0 Total 21,8 29,4 51,2

Recent surveys for Germany’s 16 Länder assume a quantity of 8.1 million m³ of dredged materials for inland regions, of which 5 million is contaminated, and 40 million m³ coming from the Federal Waterways. These data refer to Item 1 of the above list (streambed) and do not include development measures. The annual amount of dredged materials given for the Hamburg port is about 2 million m³.

Ecological aspects of reservoir sedimentation and removal 239

11.8.4 Regulations and guidelines Germany has not yet issued any regulations applicable to all its Länder regarding ways of dealing with dredged materials, but there are relevant recommendations in some of the Länder. The German Abwassertechnische Vereinigung - ATV (German Association for the Water environment) has developed its own idea (ATV Instruction Leaflet 362 Part 1 - “Umgang mit Baggergut”, 1997). Relevant rules have also been prepared in U.S.A., Canada and the Netherlands, with an emphasis on the aspect of recycling. This first provides for • separation • destruction • immobilisation. This recycling process leaves contaminant-reduced material, which may be used as silt, sand or gravel for landscaping, road construction or as construction material (brick, expanded clay, pellets). The publication “Recycling requirements for mineral waste material” of the LAGA (“Länderarbeitsgemeinschaft Abfall” the official working committee of the waste management authorities of the states of the federal republic of Germany) can be regarded as a common framework. The coarser particle fractions (d > 60µm) are usually uncontaminated and lend themselves to further utilisation. Figure 11.8-1 is a schematic diagram showing the legal procedure:

Figure 11.8-1: Legal procedure (Köthe et al., 1995) 240 Sediment Sources and Transport Processes

Abbreviations used in Figure 11.8-1: AbfG Abfallgesetz (Waste Act) KrW-/AbfG Kreislaufwirtschafts- und Abfallgesetz (Cicular Economy and Waste Act) WaStrG Bundeswasserstraßengesetz (Federal Waterways Act) WHG Wasserhaushaltsgesetz (Water Management Act) Details regarding Figure 11.8-1 are found in ATV Instruction Leaflet M 362 (1997, Part 1). Recent developments in the field of dredged materials management are discussed in a publication on a workshop organised by the BfG (German Federal Institute of Hydrology, 2002). Further information on conditions in the water right procedure regarding the monitoring of reservoir sedimentation and sediment removal is given in DVWK Publication 105 (1993). DWA (German Association for Water, Wastewater and Waste, 2006, Section 8) deals with the legal basis for the implementation of desedimentation measures in Austria, Switzerland and Germany.

11.8.5 Dealing with dredged material The above-mentioned DVWK study (1992) differentiated between two fundamental methods of dealing with dredged materials: • Reintroduction into or relocation in the respective stream, • On land storage with or without utilisation, ponds, silt island. The choice of the suitable relocation method is a question of the contamination level of the dredged materials as well as of the local conditions. Figure 11.8-2 below presents potential paths of dealing with dredged materials (Geowissenschaften + Umwelt, 1999).

Ecological aspects of reservoir sedimentation and removal 241

Figure 11.8-2: Possible ways of disposing of dredged materials (Geowissenschaften+Umwelt, 1999)

Figure 11.8-3 below shows the fundamental methods of dredged materials management. 242 Sediment Sources and Transport Processes

Figure 11.8-3: Main procedural steps for dealing with dredged materials (Geowissenschaften + Umwelt, 1999)

Figure 11.8-4 below, from the DVWK study (1992), shows the procedurals steps for the implementation of sediment management projects. The figures shown in square brackets are explained in greater detail in Attachment H to the DVWK Materials (1992) as quoted below: [1.0] The input information available on the materials to be relocated, to be used as a basis for the implementation of the project, is limited to the above data. If the sediment is classified as clearly uncontaminated, relocation is basically possible from the contamination point of view, and the procedure will be continued under [1.1]. If no sufficient information on sediment contamination is available, proceed to [2.0] as the next Decision Making Step. [1.1] If utilisation of the dredged materials, e.g. as construction material, is planned, no further analyses are needed. Since the material is uncontaminated, it can be utilised [II] after treatment according to [1.1.1]. If the material is to be deposited in the stream, this plan should be evaluated in Decision Making Step [1.2]. [1.1.1] Treatment (e.g. classification, dewatering) is generally required where dredged materials are to be subjected to further use. [1.2] At this point of the decision-making process it is necessary to decide whether dumping dredged materials at the planned site is basically justifiable. The decision should be made according to the criteria listed in Section 4.2.2/ 4.2.3 (DVWK, 1992). If relocation is basically justifiable, proceed to Decision Making Step [1.4]. If this site is precluded from consideration for materials relocation, an alternative site may be specified instead. Continue the decision-making path under [1.3]. [1.3] Decision-making at this point proceeds in analogy to [1.2]. If relocation at the alternative site is not justifiable, subject the sediment to treatment [in analogy to 1.1.1]. If deposition at the alternative site is justifiable, proceed to Decision Making Ecological aspects of reservoir sedimentation and removal 243

Step [1.3.1]. The treatment to be carried out is tailored to the requirements of the planned subsequent use [1.3.2]. A utilisation possibility should be sought for the uncontaminated dredged materials. [1.4] This decision making point means assessing whether limitations should be imposed on the basic justifiability according to [1.2]. This decision is based on the assessment concept described under Section 4.2.2/ 4.2.3 (DVWK, 1992). If no such limitations exist, the materials can be deposited in the water body. [1.4.1] The requirements imposed on the relocation project are listed under Section 4.3. (DVWK, 1992) Relocation is possible subject to these requirements. [2.0] The investigations specified under Section 4.2.1 (DVWK, 1992) should be performed to extend the knowledge of the sediment properties. The results of these investigations will provide information on whether or not relocation of the sediment under study is justifiable. If relocation is not justifiable, the dredged materials should be removed for treatment. Removal of such contaminated materials is subject to special conditions ([2.1]). If relocation is justifiable, proceed according to [2.1]. [2.1] Dredging of sediment precluded from relocation on the grounds of their contaminant content should make allowance for the risk potential involved in this sediment. [2.2] Find out whether the dredged sediment can be separated into a contaminated fine fraction and a usually uncontaminated coarse fraction. If such separation is possible, proceed to Step [2.3]. Where no such separation is possible, the entire dredged materials should be subjected to treatment ([3.5.1]). [2.3] At this point, the dredged materials are separated into a fine fraction and a coarse fraction. [2.4] The coarse fraction can as a rule be considered as uncontaminated. [2.4.1] Decide whether the coarse fraction should be relocated or utilised. In the case of relocation, continue the decision making processes shown in Diamond [1.2]. Otherwise, the decision-making path ends with the utilisation (e.g. as construction material) of the coarse fraction. [2.5] Subject the contaminated fine fraction of the dredged material to treatment. [2.5.1] Treatment of the fine fraction of the dredged materials includes in any case dewatering, with the contaminated water having to be treated in a purification process. An overview of methods of treating dredged materials is given in Attachment A. (DVWK, 1992) [2.5.2] Further treatment may make utilisation of the material possible, and the decision making path ends with the utilisation [II] of the material. Material that is unsuitable for utilisation is to be disposed of at a landfill site.

244 Sediment Sources and Transport Processes

Figure 11.8-4: Project procedure: Sediment relocation (DVWK, 1992) Ecological aspects of reservoir sedimentation and removal 245

As shown in Figure 11.8-2 above, ATV Instruction Leaflet M362 – Part 1 describes in major detail the possibilities of dealing with dredged materials. Also, the revised version of ATV Instruction Leaflet M362 – Part 1, to be published soon, together with a Part 2 on practical examples provides up-to-date decision-making guidance.

11.8.6 Utilisation and removal of sediment The ATV-DVWK Working Group on Reservoir Desedimentation deals in great detail with this problem in its recent publication DWA (2006, Section 6). An outline will be given here.

11.8.6.1 Utilisation of untreated sediment 11.8.6.1.1 Use of sediment in agriculture and forestry Sediment may be used for soil improvement (as fertiliser) by virtue of its composition and nutrient content. Such use in Germany should conform to the Federal Soil Protection Law (BBodSchG, 1998), the Federal Soil Protection and Hazardous Waste Order (BBodSchV, 1999) and to the legislation in force in the various Provinces. Further important legal instruments are the Sewage Sludge Order and the Fertiliser Law. Sediment used as fertiliser on fields should have a pH value of not less than 5.5. Material with a lower pH value should be upgraded. Use of sediment as a fertiliser is permitted every three years and in an amount of 5 tonnes per hectare and, where the contamination is 50% or more below the statutory limits, of 10 tonnes per hectare. Technology makes it possible at present to spread sediment with a solid-matter content of more than 30% or to apply in a liquid state sediment with a solid- matter content of up to 10%. The range between 10 and 30% is covered by special methods.

11.8.6.1.2 Use of sediment in landscape engineering Being a mineral material, sediment can be used in landscape engineering. The sediment should correspond to the requirements of each particular project, such as in respect of particle size and moisture content. The German Law for the Promotion of Circular Economy and the Environmentally Compatible Waste Disposal (KrW-/AbfG, 1994) provides that sediment should be used in due form and free from harmful effects (DWA, 2006). That means that it should conform to the pertinent regulations and must not interfere with public welfare. Reintroduction of noxious materials into the cycle of materials should be safely precluded. A differentiation is made between soil layers likely to be rooted, which are subject to the LAGA regulations (Länderarbeitsgemeinschaft Abfall – working committee of the waste management authorities of the states of the federal republic of Germany) (2004), and the topsoil, which is subject to the BBodSchV (1999).

11.8.6.1.3 Sediments for construction works The Technical Regulations of the LAGA (2004) allow an unlimited use of soils with assignment value Z0 and a reuse of soils with assignment values Z1 and Z2 in technical constructions under certain restrictions. The classification of solid material and eluates due to LAGA (2004) is based on specified assignment values. In those states of Germany not adopting parts II and III of the LAGA Guideline 20 different regulations are applied. The following remarks refer solely to the states where part II had been implemented.

246 Sediment Sources and Transport Processes

Unlimited use Assignment value Z0 (for solid material only) and for mixtures of different soils additionally assignment value Z0 for eluat concentrations, too. If regionally limited geogenic background values exist higher assignment values maybe applied for single parameters. Relevant subjects of protection are not affected.

Limited use Assignment values Z1 for the solid material and Z1.1 resp. Z1.2 for the eluate. Upper limit for the use as top soil in technical constructions.

Limited use with defined technical protection measures Assignment value Z2 for solid material and eluate. Infiltration of contaminants into the ground and groundwater must be anticipated.

Considering the regulations and limits of LAGA (2004) sediments may generally be reused for • recultivation of mining pits, • road construction and collateral earth structures, • industrial areas and stacking grounds, • parks as long as they are fully covered by vegetation, • derelicted areas3 if no restrictions due to the protection of biotopes are applied, • noise protection walls, • road side earth dams, • base layers of roads, • dams at waterways, • wide ranged landfills (e.g. airport extensions). Generally such a limited reuse will be handled as a particular case considering the appropriate approval procedures which can be different in the German states.

11.8.6.1.4 Recultivation of disused mines Another potential field of application is disused mines where piles and destroyed areas need restoring. Being subject to the Mining Act, such measures require approval by the competent mining authority. At present, Classes Z 0, Z 1.1 and, in exceptions, Z 1.2 are admitted for placement. When used as top material, the sediment needs to contain 5 to 10 % organic materials. Coarse sediment has proved particularly recommendable.

3 Devastated areas Ecological aspects of reservoir sedimentation and removal 247

11.8.6.1.5 Construction of landfills Requiring large amounts of soil for construction, landfills provide a suitable application of sediment. Sediment can be placed at the fringes of landfill sites or to even out the ground surface. These materials can also be used for the impervious, levelling or recultivation courses provided they possess the required properties and quality. The legal principles and requirements are based on different laws and regulations.

11.8.6.2 Utilisation of treated sediment Sediment may need treatment. This will be the case where sediment is too wet or dirty, where the contaminant load exceeds the statutory limits or where only a certain particle fraction is needed. Treatment should be considered only where the cost involved is not prohibitive and where this is more environmentally compatible than removal. Utilisation of treated sediment is subject to the same criteria as untreated sediment. The contaminant levels after treatment must fall below the respective statutory limits. The contaminant-loaded remnants must be disposed on a landfill or treated by “destruction” or “immobilisation” methods. A simplified overview of sediment-utilisation planning and the respective regulations is given in Figure 11.8-5 below:

Figure 11.8-5: Simplified schematic overview of utilisation concept (from „Nachhaltiges Niedersachsen – Baggergutmanagement“, S. 29)

11.8.6.3 Landfill disposal of sediment Landfill dumping is a very costly method of sediment disposal and, moreover, does not conform to the KrW-/AbfG (1994). Still, this may represent the only possible solution if no other utilisation method is applicable. The legal instruments applicable in such cases are the regulations AbfAblV (2001) and DepV (2002). Sediment should be dewatered to some extent 248 Sediment Sources and Transport Processes in order to become suitable for placement at landfill sites. Especially fine-grained material tends to contain a substantial amount of organic materials. The permitted proportion of such material is limited to an ignition loss less than 3 percent by weight for construction-waste landfills (Landfill Class DK I), and to less than 5 percent by weight for waste landfills (Landfill Class DK II). In 2001 (DK I) and 2005 (DK II), a possibility was created to dump sediment with less than 10 percent by weight at landfill sites for waste needing special monitoring (Technical Directive on Waste – TA Abfall, 1991).

11.8.6.4 Further possibilities of sediment disposal Gravel pits and quarries represent a further destination for sediment disposal. Due to territorial, geological and land use differences as well as different legislative regulations in the German states no general guideline is applicable for the disposal of sediments in gravel pits or quarries irrespective of dry or flooded installations. Generally it must be distinguished between two cases: Where such sites are still in operation, sediment disposal is governed by the Mining Law, which permits sediment with a low contamination level of up to LAGA Class Z 1.2. In the case of disused sites, sediment disposal is subject to a plan approval procedure. In general it is extremely difficult to dispose sediments in gravel pits or quarries especially if the site had been decommissioned long time ago already and a new nature-like situation had established in the meantime. A sediment disposal might be regarded as a disturbance in such a case. Gravel pits or quarries representing a risk may be filled with sediment. Thus, slides may occur unless a pit is refilled. Sediment may here be used for rehabilitation and stabilisation, in exceptional cases sediment up to over Z 2. Large-scale sediment placement can also be practised for refilling surface-mining holes as in the Lausitz region. But it should be seen to it that contaminated material must be placed above the – potentially rising – water table. Uncontaminated or slightly contaminated sediment up to Z 1.2, which usually shows a high organic proportion and large quantities of living biomass, may have positive effects on water quality. Likewise, such material may be spread on barren land, so as to help to repopulate such areas with plants by virtue of its nutrients.

11.9 The ecological impact of desedimentation4 Mention should also be made of DVWK/LAWA (1991), Phase C1, dealing with this subject and including a detailed assessment of sediment relocation – "waterways" – with respect to its environmental relevance. DVWK (1993, Section 7) discusses in great detail the environmental aspects especially of backwater reservoirs.

11.9.1 Perpetuation of evidence in the case of disturbance to biological communities in running waters, with special attention to desedimentation 11.9.1.1 Introduction

4 This text is an unabridged quotation from the DWA publication "Entlandung von Stauräumen" (reservoir desedimentation) (2006, Section 7), except that the numbering and layout have been adapted to this publication. The authors are Erik Mauch and Jürgen Eberstaller. Ecological aspects of reservoir sedimentation and removal 249

Reservoirs are sediment traps. The greater the sedimentation rate, the greater the decrease in space available for storing water. Thus, preservation of storage volume needs desedimentation. Where the only purpose is raising the water level, rather than the creating storage space, sedimentation above a closure structure is ecologically desirable in that it reduces the discharge cross section and raises the flow velocity; a stepped longitudinal section will develop in the ideal case. But this is not to be discussed here. Reservoir desedimentation means dredging with heavy equipment, and this involves severe interference with the ecosystem of the stream. Another desedimentation method is reservoir flushing. Such measures affect both the reservoir proper and downstream river sections.

11.9.1.2 Desedimentation and its impact on the fauna and flora of a river Dredging sediment from a reservoir causes local interference with the organisms living there, through mechanical destruction and drifting as well as through parts of the lake bottom falling dry. Repopulation after termination of the measure is from unaffected zones and through drift from the upstream river section. The ecological damage being caused to the reservoir defies control and cannot but be accepted. Flushing means that sediment is carried away by the evacuating effect of the flowing wave. This occurs already during major natural flows. What will be discussed here, however, is operationally supported reservoir flushing taking advantage of major flows. This includes opening bottom outlets and partial or complete water-level drawdown. Artificially enhanced sediment transport, the primary aim of such measures, at the same time counteracts bed load deficits. The merits and demerits in terms of stream biology of this method of dealing with sediment is very much dependent on particle size and the material components involved. Evidence from mountain streams with natural bed load transport during flood events shows the invertebrate fauna is adapted to such conditions in that it escapes to the interstitial of the bed. Flushing followed by addition of the coarser bed load to downstream river sections is regarded as positive in terms of stream biology. The greater problem is the fine sediment. Turbulence produced by flushing causes fine sediment deposited in the reservoir to go into suspension and then find its way downstream as suspension load. This may even remobilise contaminants and cause oxygen depletion. As flow velocity decreases further downstream, the fine sediment is redeposited and finally covers the entire riverbed. As a result, repopulation of the solid substrate is practically prevented. In addition, colmation may clog interstitial spaces, which form a biotope for aquatic invertebrates. Frequent reservoir flushing is nearer to natural processes and should be preferred over rarer and, hence, more massive transports. Flushing frequency is a function of operational requirements as well as sedimentation intensity and natural flow regime. Desedimentation affects to some degree all the biocenoses of a river; these include 1 Organisms living at the surface of the river bottom: + macroinvertebrates + microscopic growth + underwater vegetation 2 Fishes 3 Organisms living in the interstitial system of sediment. The interstitial is a fringe biotope, where there is interchange between surface water and ground water as one of the main components of the natural water cycle: + juvenile stages of surface-water organisms + interstitial population proper 250 Sediment Sources and Transport Processes

+ groundwater organisms. Disturbance through desedimentation extends over the period of the measure itself as well as the following period of recovery and varies according to the type of river and the other uses, such as sewage pollution, navigation etc. The various ecological aspects are dealt with in Section 11.9.2 below.

11.9.1.3 Ecological perpetuation of evidence 11.9.1.3.1 Selection of the method Disturbance resulting from desedimentation is complex and unspecific. Analysis as for water pollution on the basis of a prevailing factor through biological indicators (e.g. saprobia) is not possible. There is no such as thing as a "desedimentation factor". This would be the wrong approach, since the organisms themselves, rather than what they may indicate, are the objects of protection efforts. In fact, the entire biocenosis should be examined for potential changes. Disturbance is of ecological relevance only in so far as it manifests itself through the biocenotic structure. Such examination should include:

• type and magnitude of changes in biocenosis • affected river section • period of restoration after termination of the measure. This forms the basis for selecting the suitable investigation method:

• Survey of the ecological framework, i.e. the abiotic (physiographic) factors as well as the biological situation by use of the standard methods of analysing running waters (cf. Breitig et al., 1982, DIN 38410, Mauch, 1986, Schwoerbel, 1994, Tümpling et al. 1999). • Interpretation of the data obtained in the form of a biocenotic analysis by means of parameters describing the biocenotic structure. • Assessment of the results with due attention to the need for protection and the sensitivity of the respective biocenosis. Biological analysis of a water body followed by interpretation of the results yields numerical and possibly also quantitative data which provide a comprehensible basis for assessing disturbance levels.

11.9.1.3.2 Surveys 11.9.1.3.2.1 Ecological framework Assessment of eco-factors and uses along with their hierarchy of action calls for knowledge or survey of the ecological framework. This includes information on • environs / floodplain / banks, • stream morphology / substrate, • hydrology / hydraulics, • habitat mosaic, Ecological aspects of reservoir sedimentation and removal 251

• chemistry, water quality and • affluents and other river uses

11.9.1.3.2.2 Biological situation The macroinvertebrates, or macrobenthos, characterise the biotope and, hence, the type of water body better than other groups do, whereas the other lifeforms mainly characterise the – potentially very small – subhabitats as well as short-term changes. Assessment of impacts from reservoir desedimentation can primarily rely on the biocenotic structure of macroinvertebrates. The main groups are water insects, crustaceans, molluscs and various "worms", with insects accounting for the greater part of the species. The survey is carried out both qualitatively and quantitatively at the level of species and in isolation for each subhabitat. The unspecific, multifactorial and possibly also weak impact of reservoir desedimentation implies the need for surveying the entire macroinvertebrate community. This includes groups exclusively amenable to treatment by specialists. In contrast to the above approach, water quality analysis using the saprobia system is less sensitive to the width and depth to which organisms are identified; in this case a limited number of reliable indicators will often be sufficient to permit safe assessment.

11.9.1.3.3 Investigation schedule and locations The upstream/downstream principle is applied where disturbance already exists; "upstream" serves as a reference location. Where changes are planned – disturbance or elimination of disturbance as part of rehabilitation or renaturing projects – the before/after principle comes in; "before" refers to the present condition. Three or four dates, distributed over the year, are needed to survey the biocenosis satisfactorily. Perpetuation of evidence for desedimentation measures should provide for analyses • prior to • during and • after termination of the work. The number of test locations should be selected according to the population differences in respect of current and substrate within the section under study. In the optimal case manual sampling from the bank will suffice. Where the interstitial as a biotope is affected by potential colmation, more sophisticated sampling techniques will be needed, such as production of freeze cores or underwater sampling from a barge by use of heavy grabs or sampling in diving bells as practised along national waterways. Seasonal effects and the flow situation are of great biocenotic importance and should govern both sampling schedules and the assessment of the results.

11.9.1.3.4 Interpretation and assessment The physiographic and biological field data are interpreted in three stages: 1. Determine the general ecological status (degree of naturalness) of each sampling location. 2. Determine biocenotic differences for the sampling locations and sample analyses. 252 Sediment Sources and Transport Processes

3. Check whether the differences identified may be attributed to human interference, in this case to the impact of desedimentation. The universal category for ecological assessment of water bodies is the degree of naturalness. This is a nominal or ordinal parameter denoting the distance of an ecosystem from a state typical of the natural habitat. The interpretations of the results from all the analyses at the abiotic and biotic levels are then pooled to yield an overall judgement that can be checked via individual data. The degree of naturalness can also be determined by use of a numerical method. Such a technique, supported by a list ("ecogram"), was presented by Mauch (1990).

11.9.1.3.4.1 Ecofactors and uses The physiographic data, i.e. ecofactors, are best presented in the form of lists which may be used for deriving their ranking order by checking • which are the man-made ecofactors, • which are the factors (uses) that clearly affect the biocenosis, • which are the dominant factors and • which is the rank of the factor or complex of factors under study – in this case, this would be the factors modified as a result of desedimentation. Experience has shown "construction-site situations" tend to reduce the macroinvertebrate biocenosis severely, which makes this complex of factors rank high on the list during the period of action.

Example of ecofactor ranking: Lower Main River This river section had Water Quality Class III-IV 30 years ago. Water pollution was by far the dominating ecofactor at that time. The impact of other factors such as river development, impoundment and navigation were subordinate in importance. The quality class was a sufficiently satisfactory means of describing the status of a watercourse, obviating the need for other checks. Today sewage pollution of rivers has become comparatively low, and it is ecology, formerly neglected, that now provides the limiting factors for the biocenosis. These refer to the construction of barrages and, above all, to navigation which, producing repeated surges in the narrow bed, causes substantial stress to aquatic organisms, and this in turn makes repopulation with higher-order organisms impossible – the biocenosis remains poor in species despite the fairly good water quality.

Ecological aspects of reservoir sedimentation and removal 253

Test matrix, ecological framework (downstream) Hydrological regime / hydraulics (impoundment, short impounding cycles) Type of substrate • Particle-size grading, geochemistry • Hazardous waste in the substrate Sedimentation, colmation (clogging refugial zones) Morphology, or riverbed development; river straightening (Water quality) Navigation

254 Sediment Sources and Transport Processes

11.9.1.3.4.2 Biocenotic analysis The biocenotic analysis is carried out according to the following steps: 1 Biocenotic status as compared with a reference status typical of the natural habitat Knowledge of the original ecological conditions is of great importance for assessing disturbance. This refers in fact to the degree of naturalness of the ecosystem and the extent to which the naturalness and completeness of the biocenosis were affected already prior to or upstream from the point of disturbance. A degraded biocenosis containing few and tolerant species may perhaps suffer little from further disturbance. Inversely, biocenoses typical of a natural habitat containing a high proportion of delicate species will clearly reflect such disturbances. Note: Reference sections or reference water bodies for large rivers have ceased to exist in Central Europe, they are all more or less heavily modified as defined in the European Water Framework Directive. Not only are water individuals affected, but the type itself is extinct. No river is left in the Alpine foothills or the plains that might serve as a reference, this could at best be reconstructed from old data. 2 Comparison of the results Based on the ecological reference condition, the results from the various tests carried out at one location, or the data from the various test locations, are compared in respect of the disturbance; comparisons are made between before and after as well as between upstream and downstream.

Overall population Various parameters are available for characterising the total population. They will be discussed in the following. Given the substantial scatter of biocenotic parameters, it is not easy to decide whether or not differences between two tests (e.g. before/after) are significant. The indicated significance limits are empirical values.

Number of species / biocenotic completeness Species richness characterises structured biotopes with a habitat mosaic of great diversity, whereas monotonous as well as extreme biotopes are poor in species.

Significance limit in respect of disturbance More than 20 % deficit (species deficit) means significant impoverishment.

Species identity This is the proportion of common species (intersection) obtained from comparison of test results. Various computation methods are available, or the absolute number is assessed, the assessment being dependent on the individual case, i.e. on the respective biocenotic structure.

Significant difference • Non-agreement for at least three species of at least medium abundance • Non-agreement for a highly abundant species

Ecological aspects of reservoir sedimentation and removal 255

Abundance In most water investigations, abundance of a species is expressed as an estimated value on an ordinal scale between 1 (single find) and 7 (abundant). The individual levels on the scale are classes forming an exponential series (cf. Mauch et al., 1994). Abundance can also be determined in absolute terms by counting or weighing (biomass) relative to a unit area. Based on the abundance of one species, it is possible to consider the abundance conditions of a group or in the entire population (mean value, maximum value, distribution of the values). Comparison of abundance for individual species or groups should be done at the habitat level, as e.g. comparison of populations on stones in a strong current. With respect to a single species, any difference from an abundance class should be considered as significant in view of the exponential scale and the numbers of individuals assigned to the classes. As individual abundance values do not show whether they are situated in the centre or at the fringe of a class, the following principle holds: • A difference of at least two abundance levels is significant. The following principle holds for the absolute abundance of individuals or weight: • A decrease of at least 50% or doubling is significant.

Dominance Dominance is relative abundance, or the degree of prevalence or subordinance of a species. This can be indicated as a percentage for absolute abundance values. Where estimates are used for abundance, the dominance ranking should be studied already during the field survey, considering only species of at least medium abundance (abundance level 4 or higher). This yields a ranking for these species. Dominance is subject to seasonal variations corresponding to the life cycles of the species, and this is also a factor to be allowed for in comparing abundance. The aspect of a biocenosis as determined by the highly dominant species is of great informative value. Several test sites or tests can be compared with respect to dominance identity. Example of assessing dominance identity with restriction to the first 5 ranks (1st to 5th dominances): No more than three or less of these dominant species in common = significant difference.

Test matrix, biocenosis, total population • Number of species • Population density • Dominance conditions: great informative value These are of great informative value in respect of the biocenotic implications of disturbance, both as individual data and together

Analysis of the range of species The analysis yields groups classified according to various criteria. These are: • Kin group (e.g. "mayfly") • Life form (e.g. nutrition types, types of locomotion) and 256 Sediment Sources and Transport Processes

• Requirements (e.g. species preferring currents, limited to upper courses, preferring heat). Ecological classifications (habitat, region, life form, current) for a great number of macroinvertebrate species are found in Schmedtje et al. (1996).

Significance limit for a disturbance An increase of at least 50 % and a decrease of at least 30 % in the number of species in a group can be regarded as significant.

Saprobic and trophic indicator values Species can be assigned to saprobia classes by their association with a certain availability degree of dissolved and particular organic substances in the water (= saprobia = organic pollution) (e.g. Mauch et al., 1990) and this may be used to determine the saprobic quality of a water body. Trophia refers to the supply of plant nutrients. Saprobic quality and trophic quality also form part of the ecological status of a biocenosis and are an aid for the overall ecological assessment of a water body (cf. Section 11.9.1.3.4). Where these parameters are not known, they should be determined for the purpose of perpetuation of evidence in the case of disturbance.

Tolerance index The individual species forming a biocenosis differ as to range of adaptation to ecofactors (= ecological valence) or to the habitat. Undemanding species, the ubiquists, are less tied to a certain habitat; they occur in different types of water bodies (they are eurytopic) or show a high tolerance to several ecofactors (they are euryoecious). Hence, tolerant species are better capable of dealing with disturbance than more demanding (stenotopic, stenoecious) species, which tolerate only a narrow range of milieus. The proportion of tolerant species (ubiquists) in an overall population can be used as a basis for formulating a tolerance index to characterise a type of water body. The lower the value (as a percentage or a decimal fraction between 0 and 1), the higher the sensitivity of the biocenosis. Species to be regarded as ubiquist should be marked accordingly on the list. Beginning with its source, a stream shows an increasing amplitude of ecofactors on its way downstream and, along with it, an increase in the number of tolerant species. Still, there is a series of species that are specific to certain river types. These are species that do not occur ubiquitously. Man-made disturbance may accelerate this natural evolution in the downstream direction: Upper courses assume the character of lower courses (potamalisation). Heavily degraded biocenoses are characterised by a dominance of ubiquist species and a high tolerance index, even without additional disturbance, whereas near-natural sections in the same river region have a comparatively lower tolerance index which, when rising, indicates disturbance. A similar approach is by use of the Potamon Type Index (Schöll et al. 2001) as a parameter to be used for the ecological assessment of potamal cenoses and, hence, of rivers in connection with the EU Water Framework Directive. The basis is a taxa list classifying biotope specificity between 5 (stenoecious) and 1 (ubiquist).

Significance of changes Ecological aspects of reservoir sedimentation and removal 257 is dependent on the status of the reference station or the reference data.

Rarity This mainly refers to the Red List status of individual species. Endangered species also provide the grounds for classifying a stream section as needing protection.

Naturalness This characterises the degree to which a biocenosis differs from the near-natural condition through change in fauna. The naturalness of a biocenosis is another measure of the need for protection of the ecosystem in the river section affected by disturbance.

Examples of assessing need for protection: 1 The River Main as a barrage-controlled waterway + The river is severely modified. + The naturalness and, hence, completeness of the biocenosis are severely reduced. + The need for protection is regarded as fairly low (rehabilitation is indicated). 2 Wertach gorge in the Allgäu region, downstream from the Grüntensee reservoir + The section is quasi natural in terms of morphology, but its hydrology is affected by the reservoir. + The biocenosis is typical of a running water, although not typical of a natural habitat (restricted naturalness). + The need for protection is regarded to be high.

Test matrix, biocenosis, range of species • Major systematic groups • Life forms • Ecological types: preference/limitation • Ecological tolerance • (Water quality) All criteria are of great informative value.

11.9.1.3.5 Study expenses for perpetuation of evidence The expenses are a function of the number of sampling sites and the number of visits. The following items of work should be included in the cost estimate: • Survey of the general situation and physiography. • Inclusion of foreign data: chemistry, water quality, hydrology. • Survey of macrobenthos. • Field work. 258 Sediment Sources and Transport Processes

• Sorting and preservation of samples. • Determination work at the laboratory, including preparation. • Consultation of specialists. • Data documentation. • Interpretation and evaluation of the results. • Preparation and production of a report. The amount of time to be devoted to a sampling site is governed by the principle that rich biocenoses call for more time than degraded biocenoses. Any minimum test schedule should be tailored to the needs of the case under study. It can generally be said:

Few in-depth investigations provide better information than a large number of merely informative surveys.

Figure 11.9-1: Barrage (Faimingen, Danube) Figure 11.9-2: Shell armouring fallen dry after reservoir emptying

Figure 11.9-3: Biological investigation of river Figure 11.9-4: Sorting a sample banks Ecological aspects of reservoir sedimentation and removal 259

Figure 11.9-5: Common in large rivers: colony of a Figure 11.9-6: Shells, snails, bryozoa – aspect fresh-water fungus typical of dammed-up rivers and canals

Figure 11.9-7: Head of a caddis-fly larva (anabolia) Figure 11.9-8: Mouth tools of a non-biting midge from tranquil river sections (microtendipes) from fine river sediment

11.9.2 Ecological aspects of desedimentation with an emphasis on watercourses in the Alpine region and foothills 11.9.2.1 General Desedimentation measures, especially reservoir flushing, and the resulting deterioration in the ecological status of water bodies have met with heated public debates and response in the media over the past years. And the new water pollution control requirements under the EU Water Framework Directive have added to the relevance of this problem. Any reservoir will need desedimentation in the long run, as bed load and fine sediment deposited along with the decrease in drag force reduces the available reservoir space und may even become a risk to the operating reliability of reservoirs. The impact of desedimentation measures, however, varies substantially according to the situation. In the following paragraphs attention will be drawn to the ecological aspects of such measures and possible solutions will be suggested for mitigating the impact on the ecology of the water body concerned. The emphasis will here be on watercourses in the Alpine region and foothills in Austria, Switzerland and Bavaria. As a result of the special character of the catchments in this region, the fine sediment mobilised by desedimentation measures usually consists of inorganic components. Organic turbidity and the resulting chemical pollution (such as O2 260 Sediment Sources and Transport Processes depletion, NH3 etc.) as well as contamination of the sediment with such substances as heavy metals are clearly subordinate in importance.

11.9.2.2 Ecological objectives of sediment management in river systems Impoundment modifies the sediment regime in a river system by retaining bed load and, hence, causing bed erosion in downstream sections as a result of the bed load deficit. Their not only ecological but multiple consequences have been the subject of much relevant literature (such as Rhine, Danube …). And, where an armour layer forms so as to prevent or delay soil erosion, there is a lack of ecologically desirable sediment relocation. On the other hand, fine sediment is retained in the reservoirs over major periods, to be remobilised in concentrated form by desedimentation. The following objectives may thus be defined for sediment management in river systems from the ecological point of view: 1 Provide for a sediment regime as near as possible to natural conditions, in particular maintain bed load transport as a basis for a satisfactory ecological condition or the good ecological potential as defined by the Water Framework Directive. 2 Appropriate reservoir design to encourage sediment transport and minimise the need for management, with the aim of ecological enhancement. 3 Great care in dealing with the ecoystem.

11.9.2.3 General effects on aquatic organisms from turbidity increase caused by desedimentation measures In addition to natural turbidity in running waters, which may vary considerably depending on the type of water body (e.g. glacial influence) or flood events, the greater part of sediment management measures cause turbidity which, however, greatly varies as to concentration and duration. Since all organisms are adapted to the natural turbidity regime of their habitat, assessment of the potential impact from desedimentation should always be based on the natural characteristic of the respective stream, with due allowance for seasonal aspects (e.g. glacial turbidity, "clear-water phases" etc.) Attention should also be given to both the direct and indirect effects of turbidity through modification of the habitat (e.g. clogging of the gravel interstitial through deposition of fine sediment).

11.9.2.3.1 Direct damage to organisms Direct damage caused to organisms by a turbidity increase from desedimentation comprises a large range, extending from increased stress (such as through reduced visibility) to direct mechanical damage to flora and fauna (Bruton, 1985). In the case of fish, damage mainly to mucoses around the gills has been reported (Figure 11.9-9). The damage level is mainly a function of turbidity concentration and duration as well as of particle shape (such as sharp- edged glacial silt) and the proportion of organic substances (Newcombe et al., 1996). The individual species and ages show different resistance, which in turn is dependent on the natural turbidity of the respective water body. Ecological aspects of reservoir sedimentation and removal 261

Figure 11.9-9: Impact from increased turbidity on fauna and flora (Bruton, 1985)

The increased flow velocities during flushing may also cause benthos organisms and fish to drift to downstream regions. Damage may also result from a high proportion of organic substances in fine sediment, through oxygen deficiency, hydrogen sulphide, ammonia or other remobilised contaminants (e.g. heavy metals). This problem is of lesser importance in the reservoirs of the Alpine region and foothills, where the greater part of fine sediment is inorganic.

11.9.2.3.2 Damage to the habitat Damage may occur in the reservoir itself as removal of sediment may mean a loss of habitat; particularly sensitive zones are the reservoir head and shallow-water zones near the banks. Likewise, water-level drawdown, especially when extending over major periods (especially for the purpose of flushing) may cause shore zones or fish ladders to fall dry and thus cause damage to organisms.

Figure 11.9-10: Free gravel substrate (left) and gravel body clogged through internal and external colmation (Photo: U. Schälchli) 262 Sediment Sources and Transport Processes

In downstream undeveloped and residual-flow sections in the trout to barbel region (epirhithral to epipotamal), colmation (Figure 11.9-10 above) of the gravel interstitial in the bed causes massive damage, as this is the main habitat for bottom fauna and serves as spawning ground and larva habitat for a large proportion of the local fish species (for details, see Section 11.9.1.2 above).

11.9.2.4 Types of desedimentation There are two main types of desedimentation measures: • flushing • evacuation Flushing means that the increased drag force due to water level drawdown is used for eroding sediment. Evacuation means that sediment is removed or relocated by means of dredging or excavation. Excavation calls for water-level draw-down in the reservoir, dredging is possible with or without drawdown.

11.9.2.4.1 Flushing When assessing the impact of flushing, a distinction should be made between the indirect effects on the biocenoses through interference with the habitat conditions and direct damage through mechanical injury, stress etc. The effects within the reservoir itself can clearly be differentiated from those in undeveloped downstream river sections. Water level drawdown not only causes shallow-water zones to fall dry, but raises the flow velocity within the reservoir, which in turn leads to erosion of its fine sediment deposits. The potential consequences for organisms are presented in Figure 11.9-11 below. Ecological aspects of reservoir sedimentation and removal 263

Figure 11.9-11: Influencing factors and consequences of flushing operations on the downstream river section (Eberstaller et al., 2000)

Downstream flow sections (Figure 11.9-12 below) suffer from flushing effects not only by direct damage but above all by their gravel interstitial being clogged through sedimentation (colmation). This stops flow through the interstitial as is needed for its suitability as a habitat, as by supplying sufficient oxygen to fish eggs and larvae. 264 Sediment Sources and Transport Processes

Figure 11.9-12: Potential factors and effects of flushing in undeveloped downstream sections (Eberstaller et al., 2000) Ecological aspects of reservoir sedimentation and removal 265

Figure 11.9-13: Turbidity from reservoir flushing

The impact from reservoir flushings (Figure 11.9-13 above) is heavily dependent on the season in which they take place. Many bottom-fauna species have stages outside the stream or highly resistant stages during the period of natural flood flow, usually in spring. The consequences of flushing in winter (low-flow period) are much severer than during the natural flood period. The same is true of the fish fauna, especially in streams of the trout region (epi/metarhithral) with brook trout as the main species. This fish species lays its eggs, late in autumn, in the gravel interstitial, where these remain through the winter. The fish larvae hatching from the eggs remain in the gravel interstitial for about a fortnight. Colmation of the interstitial during that period, as through reservoir flushing, causes the death of the fish eggs or larvae (cf. Section 11.9.2.4.1.2). The severity of the effects from flushing is very much a function of the proportion of organic substances as well as potential colmation through fine sediment (see Section 11.9.2.3.1).

11.9.2.4.1.1 Examples of reservoir flushing effects Bodendorf – River Mur On May 13, 1996, the reservoirs above the Bodendorf, St. Georgen and Murau power stations on the River Mur were flushed at a flow rate of 120m³/s (HQ1 = 130m³/s) over a period of 24 hours. The River Mur is a fairly near-natural river in the grayling region in Styria/ Austria. The amount of fine sediment washed off was 80,000m³, downstream turbidity reached a maximum concentration of 11g/l. A fortnight after flushing, fine sediment deposits were still seen in bank zones and above the water line. Increased medium flows/ flood flows that followed eroded the greater part of the material in the medium-flow bed within six weeks after flushing. Analysis of the bottom fauna from a first qualitative sampling two weeks after flushing showed a total absence of the fauna typical of that section of the River Mur and a 80 % reduction in the total number of species. One month later, the bank zones of the medium-flow bed, having been washed free from fine sediment by natural processes, again showed a relatively heterogeneous fauna rich in individuals and characteristic of the River Mur. The biomass along the river course two months after the flushing showed a reduction in relation to the reference level by 8 % to 39 % (Graf et al., 1997). Also two months after the flushing, the fish population in the undeveloped downstream river section amounted to only 50 % to 70 % as compared with the April 1995 level (Kainz et al., 266 Sediment Sources and Transport Processes

1996). Exact quantification of the damage to the fish population is impossible due to uncertainties inherent in the method. It was above all the proportion of one year old graylings that showed a sharp drop in population in July 1996. The picture conveyed by the available data proves that the flushing caused damage to the biocenosis of the River Mur. The large repopulation potential of the fairly near-natural section of the river, combined with the fact that the bed was soon "washed free", acted to compensate for the damage.

Weissenbach Flushing in the Weissenbach stream (discharge from Lake Weissensee in the Province of Carinthia, Austria), a stream in the trout region, caused turbidity over a period of two months, with peaks of 6g of solid matter per litre. After the flushing no fish population was detected about 1km downstream from the dam, and a loss of 85% was measured about 4.5km further downstream and of 75% at 9km (just upstream from the junction with the River Drau) (Friedl, 1994). The heavy damage to the ecosystem despite the turbidity peak being lower than in the preceding example supports what is already known from the relevant literature: The amount of damage to aquatic biocenoses is mainly a function of sediment concentration combined with duration (Newcombe et al., 1996).

11.9.2.4.1.2 Measures for minimising the ecological impact from flushing The above mentioned adverse effects on aquatic biocenoses should be minimised, also in order to satisfy the requirements of the EU Water Framework Directive. The following paragraphs will be dedicated to measures intended to help minimise ecological impacts.

Continuous discharge of fine sediment through the turbines except during low-flow periods Continuous discharge of fine sediment through the turbines may prevent the deposition of large quantities in the reservoir. But care should be taken to avoid low-flow periods, when, normally in winter, there is little natural turbidity. In the rhithral (trout region), this is the reproduction period for book trout and thus holds a high damage potential (Eberstaller et al. 2001).

Adapting the flow regime to the season The flow conditions during flushing should largely correspond to the natural flood regime. This is normally in spring and early summer in the Alpine region and foothills. A differentiation should be made between the various fish regions to account for their differing requirements: • Trout region Spring and early summer are the best flushing periods in the trout region with brook trout as the main species. Having hatched from the substrate, the brook-trout larvae are swimming in near-bank zones. The bottom fauna as well has largely adapted to natural flood events. Moreover, sedimentation (especially in the medium-flow channel) is lowest during floods. • Grayling and barbel region Flushing in spring or early summer involves much greater problems in the grayling and Ecological aspects of reservoir sedimentation and removal 267

barbel region (grayling and barbel being the main species). This phase coincides with the main spawning period, or the eggs and larvae are still in the substrate and thus extremely vulnerable to colmation. Flushing should be carried out when the larvae are already swimming freely.

Flushing frequency and control • Regular and frequent flushing (once or twice a year where possible) acts to reduce the level of turbidity concentration and, on the other hand, to reduce the total amount of sediment. Such a periodicity is recommended from the ecological point of view. • Flushing, along with water-level drawdown, should start already during minor floods in order to arrive at the desired flushing frequency (the work group on desedimentation of the ÖWAV –Austrian Water and Waste Water Union, (2000) –recommends flushing should commence at 50% HQ1 in case a major flood is predicted). This does however not apply to first flushings. • The increase and decrease of the flushing wave should be gentle rather than abrupt. This prevents fish and bottom fauna from drifting off along with the first surge and falling dry during flow reduction. • Especially in by-pass and residual-flow sections, gradual flow reduction in the residual- flow section when the flow is directed back to the headrace after flushing is recommended in order to prevent bank zones from falling dry and enable the formation of a low-flow channel. • Flushing should be controlled as a function of turbidity concentration, using a standard value depending on the type of water body and on the region. This calls for continuous suspended-sediment measurement downstream from the reservoir and for measuring gate openings.

Figure 11.9-14: Sediment blocks breaking off suddenly may cause short-term turbidity peaks

Secondary flushing Secondary flushing with low turbidity concentrations are recommended after flushings with high sediment concentrations or major sedimentation levels in the streambed. Such "clear- water flushings" wash away the fine sediment deposited during the main flushing process and also aid decolmation in the gravel interstitial. Such action substantially reduces the ecological damage and enables ready repopulation of the affected sections (cf. the example of the River Mur). 268 Sediment Sources and Transport Processes

Training structures at the reservoir head and within the reservoir / provision of shelters Training structures at the reservoir head, intended to ensure bed load transport (including gravel) through the reservoir, are recommended from the ecological point of view. At the same time, training works may structure (Figure 11.9-15) the reservoir so as to ensure habitat diversity. Moreover, such structures may serve as shelters during floods and flushings.

Figure 11.9-15: Training works for reservoir structuring

Provision of fish ladders Fish ladders help fish carried along by the flushing wave migrate back to upstream regions. Fish can also migrate to tributaries, which offer essential reproduction areas.

11.9.2.4.2 Evacuation When assessing potential impacts from sediment evacuation, excavation with at least partial water-level drawdown should be differentiated from dredging in the reservoir with or without water-level drawdown.

11.9.2.4.2.1 Excavation in the reservoir with partial water level drawdown Partial water-level drawdown causes shore and shallow-water zones to fall dry and dry out. As the drawdown is usually of long duration, remaining ponds and the substrate dry out completely, so that not only the remaining fish and bottom fauna die, but also water plants (submerse macrophytes) in the shallow-water zones dry. Large important shore structures and, thus, the main spawning substrate for many plant-spawning fishes are lost especially in reservoirs with pronounced shallow-water zones at the transition between the grayling and barbel regions (e.g. Inn, Drau …), the more so as aquatic plants occur almost exclusively in shallow-water zones where they find the appropriate light conditions. Excavation also directly removes animals from the water body and shallow-water zones. Turbidity during such a measure is substantially lower than in the case of flushing, or even dredging provided only dry sediment is dredged. Great attention should be given to a well structured near-natural design of the reservoir head, which is a zone of particular ecological value. The best possible time for such a measure is autumn and winter to allow for the development phases of a large proportion of the aquatic organisms. But in water bodies of the trout region (epi/ metarhithral), it should be remembered that this coincides with the reproduction and egg- Ecological aspects of reservoir sedimentation and removal 269 development period of brook trout, and reservoir heads are often the only spawning grounds left (see Section 11.9.2.4.1).

11.9.2.4.2.2 Dredging in the reservoir with or without water level drawdown Dredging causes much less turbidity than flushing. But turbidity in this case often occurs during periods of low or medium flow and lasts longer. Low-flow periods, which are normally clear-water phases, should if possible be avoided. The ideal time is outside the spawning and egg and larva development phases. Additionally the chemical composition of the sediments has to be taken into account.

11.9.2.4.2.3 Disposal of dredged materials: gravel and fine sediment Before re-introducing gravel dredged from the reservoir head in downstream river sections, studies should be conducted to find out whether such action is wise from the river-engineering and ecological points of view. At first the chemical properties should be analysed as only uncontaminated material should be re-introduced into the riverbed. Contaminated material must be properly dumped or disposed of. If there is a bed load deficit in undeveloped downstream river sections, re-introduction of the gravel is recommended in order to reduce potential scours and enhance the overall ecological condition. Where the gravel is mixed with fine sediment, it should be placed at suitable bank locations in dry areas above the medium water line (Figure 11.9-16), to become eroded during natural floods. The following points should be considered in the case of sediment dumping: Suitable sediment dump configuration with a sufficiently high proportion of gravel should help avoid too rapid erosion and the turbidity pulse involved. The entire sediment supplied to the reservoir by natural means should be re-introduced downstream, as the ecological enhancement through the addition of gravel is the only means of making up for detrimental effects from fine-sediment erosion. On the whole, this measure can serve to create something like a near-natural sediment regime. The erosion process, especially turbidity, should be monitored and recorded and the results used for planning further measures.

Figure 11.9-16: Gravel and fine sediment dumped in bank areas (Alpine Rhine)

Measurement of turbidity from erosion during floods did not show any significant increase in turbidity load 200m further downstream (personal communication from T. Kindle, AfU, FL).

270 Sediment Sources and Transport Processes

11.9.2.4.2.4 Other measures – relocation Relocation of sediment within a reservoir may to some extent compensate for adverse effects, as structuring creates new habitats (Figure 11.9-17 below).

Figure 11.9-17: Reservoir structuring

11.9.2.5 Criteria for assessing desedimentation measures Prediction of potential impacts from desedimentation measures should consider the following main points:

Type of reservoir • Reservoir, dam, high-head or low-head development • Series of power stations or undeveloped section, or downstream residual-flow section

Reservoir size and configuration • Flat, wide flood plains (Lower Inn) • Compact profile

Type of water body – region • Overall characteristic (mountainous, foothill, lowland river etc.) • Natural flow and turbidity regime (distribution of flow over the year, flood characteristic, glacial stream ….) • River morphology (straight, braided, meandering) • Biocenotic or fish region: biocenoses typical of the respective water body

Condition and amount of sediment • Predicted downstream sediment concentration or quantities Ecological aspects of reservoir sedimentation and removal 271

• Particle size and shape

• Organic proportion with respect to potential oxygen depletion, H2S and NH3 formation etc. • Deposition of hazardous substances (e.g. heavy metals…)

Time and duration of the desedimentation measure • With allowance for the natural flow and turbidity regime, present flow regime, spawning and egg development periods etc.

Pooling all the above factors forms the only reliable basis for the prediction of the ecological impact from planned desedimentatin measures and for selecting the ecologically best suited solution.

Monitoring Desedimentation measures meet with extremely complex conditions and should be closely monitored to capture data on the real consequences to be expected and to select suitable further measures. The abiotic conditions should be measured prior to, during and after desedimentation (substrate properties, channel morphology, flow, turbidity, O2, chemical conditions) in the reservoir and downstream river sections, and bottom and fish fauna before and after the flushing.

11.9.2.6 Summary Generally speaking, there is no arguing about the necessity of desedimentation to preserve the operating reliability and the usable storage capacity of a reservoir. At the same time it is important to minimise the ecological damage caused by flushing or evacuating the deposited bed load and fine sediment. The provisions of the EU Water Framework Directive are adding to the importance of this aspect. Interference from desedimentation measures may directly affect the flora and fauna in the reservoir itself as well as in downstream river sections, through mechanical injury, chemical damage etc. Additional damage is caused to habitats when parts of the reservoir fall dry or through clogging of the gravel interstitial etc. The consequences of desedimentation may vary substantially according to the respective situation. Organic contamination and pollution of sediment through heavy metals and the damage involved are of minor importance in the Alpine region and foothills. In such cases, the amount especially of direct damage is mainly a function of sediment concentration combined with the duration of the operation. The main criteria for predicting the impact from desedimentation measures comprise type of reservoir, reservoir size and configuration, type of water body, sediment properties and time/ duration of the desedimentation measure. The ecological effects involved can be substantially reduced by selecting the most appropriate method and a suitable desedimentation design. The complex processes involved in desedimentation should at any rate be monitored.

Legal aspects 273

12 Legal aspects In addition to what is said in DWA (2006, Section 8), reference is made to the publication "Sediment Management – Technical and legal aspects" (WP 7, TU Graz) issued as part of the ALPRESERV project. 274 Sediment Sources and Transport Processes

Case studies 275

13 Case studies Here, too, reference is made to the publications DVWK (1993) and DWA (2006). Workpackage 8 in particular presents a detailed documentation of action actually taken. This includes the following pilot measures: • Bodendorf run-of-river station, Styria • Margaritze check dam, Carinthia • Sylvenstein flood-retention basin, Bavaria • Tourtemagne reservoir, hydro-power development, Wallis • Lago di Barcis water-management reservoir, Friuli-Venezia • Forni reservoir, hydro-power development, Lombardy • Lago di Centro Cadore water-management reservoir, Veneto 276 Sediment Sources and Transport Processes References 277

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Beneficial uses of dredged material http://www.wes.army.mil/el/dots/budm/budm.html BMU: http://www.bmu.de/de/1024/js/sachthemen/gewaesser/wasserrahmenrichtlinie/main.htm CEDA: Central Dredging Association http://www.dredging.org DOER: Dredging Operations Technical Support Program des US Corps of Engineers http://www.wes.army.mil/el/dots/dots.html Hydrotox GmbH, Freiburg: http://www.hydrotox.de/fischtest.html Hafenbautechnische Gesellschaft (HTG): http://www.htg-baggergut.de IGB-Berlin: http://www.igb.berlin.de/abt2/mitarbeiter/kozerski/teller.htm Interreg III B (EFRE) http://www.alpinespace.org Naturschutzjugend (NAJU) http://www.hochwasser- special.de/aifkbilderhochwasser/fischqualitaeten/selbstreinigungvseutrophierung2.gif Net-Lexikon http://www.lexikon-definition.de/bildinfo/Karte einzugsbereich isar.png PIANC: International Navigation Association http://www.pianc-aipcn.org RWTH-Aachen http://www.rwth-aachen.de/iww/about/research/wquality/gfx/erosimess.jpg SEDNET. Demand-driven European Sediment Research Network. http://www.sednet.org SEDYMO: Sedimentdynamik und Schadstoffmobilität in Flussgebieten http://www.tutech.de/sedymo http://www.iws.uni-stuttgart.de UTG GmbH, Eberswalde http://www.utg-online.de/english/197000.htm

292 Sediment Sources and Transport Processes

Miscellaneous 293

15 Miscellaneous 15.1 Abbreviations VAW: Laboratory of Hydraulics, Hydrology and Glaciology of the Swiss Federal Institute of Technology Zurich (Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie der Eidgenössischen Technischen Hochschule Zürich9 ASCE: American Society of Civil Engineering, www.pubs.asce.org ATV-DVWK: German Association for Water, Wastewater and Waste (since 2000) (Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e.V., Hennef (ab 2000)) BAW: German Federal Waterways Engineering and Research Institute, Hamburg (Bundesanstalt für Wasserbau, Karlsruhe, Hamburg) BAW: Austrian Federal Agency for Water Management, Vienna (Bundesamt für Wasserwirtschaft, Wien) BfG: German Federal Institute of Hydrology, Koblenz (Bundesanstalt für Gewässerkunde, Koblenz) BUWAL: since 2006 BAFU: Swiss Federal Office for the Environment, Bern (seit 2006 BAFU: Bundesamt für Umwelt, Bern) BWK: German Association of Water Resources, Waste Management and Land Reclamation Engineers (Bund der Ingenieure für Wasserwirtschaft, Abfallwirtschaft und Kulturbau) DVWK: German Association for Water Management (until 2000) (Deutscher Verband für Wasserwirtschaft und Kulturbau e.V. Bonn (bis 2000)) DWA: German Association for Water, Wastewater and Waste (since 2005) Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e.V., Hennef (ab 2005), www.dwa.de EAWAG: Swiss Federal Institute of Aquatic Science and Technology (Eidgenössische Anstalt für Wasserversorgung, Abwasserreinigung und Gewässerschutz) IAHR: International Association of Hydraulic Engineering and Research, Madrid, www.iahr.org IAHS: International Association of Hydrologic Sciences www.iahs.info IAWQ: International Association on Waters Quality IRTCES: International Research and Training Center on Erosion and Sedimentation, Beijing LAWA: German Working Group on water issues of the Federal States and the Federal Government (Länderarbeitsgemeinschaft Wasse)r ÖWAV: Austrian Association for Water and Waste Management, Vienna (Österreichischer Wasser- und Abfallwirtschaftsverband, Wien) 294 Sediment Sources and Transport Processes

WASER: World Association for Sedimentation and Erosion Research, Beijing (ab 2004), www.waser.cn WMO: World Meteorological Organisation, Genf WRS: Water Management Master Study Salzach (Wasserwirtschaftliche Rahmenuntersuchung Salzach)

15.2 Definitions Standardised symbols and definitions for scientific and commonly used terms are still not applied in international literature. Additionally, explanations of symbols and terms in relevant publications are often incomplete, including the pertinent units. Some remarkable compilations of important terms related to hydro sciences have been published in Germany by the Water Practice Standards Committee (NAW) within the German Institute for Standardization (DIN). As these DIN-compilations are covering a wide range of technical issues in most cases an extraction for specialised fields, like sediment transport phenomena, would be too laborious. Most of the terms with relevance to sediment transport problems, increasingly supplemented with English expressions, can be found in DIN 4044 and 4049.

Contact 295

16 Contact

UBM Universität der Bundeswehr München www.unibw.de/ifw/HYDRO (Lead Partner) (University of the German Armed Forces) Institute of Hydroscience Sven Hartmann (Project Manager) Christiane Steinich (Assistant Project Manager) Werner-Heisenberg-Weg 39 Werner-Heisenberg-Weg 39 85577 Neubiberg 85577 Neubiberg Germany Germany +49 (0)89 6004 2618 +49 (0)89 6004 3498 [email protected] [email protected]

TUG Graz University of Technology www.hydro.tugraz.at Institute of Hydraulic Engineering and Water Resources Management Helmut Knoblauch Günther Heigerth Stremayrgasse 10 Stremayrgasse 10 8010 Graz 8010 Graz Austria Austria +43 (0)316 873 8362 +43 (0)316 873 8360 [email protected] [email protected]

EPFL École Polytechnique Fédérale Lausanne (EPFL) lchwww.epfl.ch Laboratoire de constructions Hydrauliques Giovanni De Cesare Anton Schleiss LCH-ENAC, Bât. GC LCH-ENAC, Bât. GC EPFL EPFL 1015 Lausanne 1015 Lausanne Switzerland Switzerland +41 (0)21 693 25 17 +41 (0)21 693 2382 [email protected] [email protected]