Smart Nourishment of the Frisian Inlet
Morphodynamic analysis of an innovative way of nourishing
Report
May, 2006
Z3873/Z3912
Smart Nourishment of the Frisian Inlet
Thesis Maarten Kluyver
Supervising Committee: Prof. Dr. Ir. M.J.F. Stive Dr. Ir. Z.B. Wang Dr. Ir. M van Koningsveld Ir. E.P.L. Elias Ir. G.J. de Boer
Report
May, 2006
Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
Preface
This report presents the results of my MSc thesis study on the interaction of a smart nourishment with the morphodynamics of the Frisian Inlet. The assignment has been performed as part of the completion of my study Civil Engineering at Delft University of Technology. The study has been carried out in cooperation with WL | Delft Hydraulics and is part of the Delft Cluster 2 project “North Sea and Coast”, as well as the WINN project “More sand, less effort”.
A quick scan of the research and its results can be obtained by reading Chapter 1 and the conclusions and recommendations in Chapter 5. People who are looking for a broader introduction to the subject or who are not familiar with –tidal basins and coastal inlets– morphodynamics are advised to first read Appendix A (The Wadden Sea: a system of tidal basins).
I would like to thank my supervisors and members of my graduation committee at WL | Delft Hydraulics and Delft University; Prof. Dr. Ir. M.J.F. Stive, Dr. Ir. Z.B. Wang, Dr. Ir. M. van Koningsveld, Ir. E.P.L. Elias and Ir. G.J. de Boer for sharing their knowledge, criticism and necessary support during the research.
Furthermore, I am grateful for the opportunity given by WL | Delft Hydraulics to finish my study at their institute and I would like to thank my temporary colleagues and fellow graduate students for showing their interest and making my stay a very pleasant one.
Finally, I want to thank my parents, brothers, sister and Ester, for their interest and support during the years I spent in Delft.
Delft, 1st of May 2006
Maarten Kluyver
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Summary
The Dutch coastal zone is a vulnerable area, which suffers from a persistent loss of sand. Also the integrity of the Dutch Wadden Sea is at threat by the effects of sea level rise and subsidence due to gas extraction leading to sand deficits. The innovative idea has been put forward to mitigate these effects for the Wadden Sea by nourishing the ebb•tidal deltas and promoting sedimentation of the tidal basin by letting natural processes take over. This idea is referred to as “smart nourishment” of the ebb•tidal delta.
This thesis studies the behaviour of smart nourishment of the ebb•tidal delta of the Frisian Inlet. This is done in order to answer the following questions: § How do the hydrodynamic forcings influence the morphological behaviour of the nourished sand, and how do the morphodynamics of the nourished ebb•tidal delta and the tidal basin interact? § Where should a smart nourishment be placed in such a way that natural processes redistribute the sand relatively quickly to the benefit of the basin?
The research objectives are addressed as follows: A model of the Frisian Inlet is set up in the process based modelling environment of Delft3D. Three situations are studied: (1) A situation with a “fixed bottom” and the hydrodynamic forcing of the tide, (2) a “fully” morphological situation with the hydrodynamic forcing of the tide and (3) a “fully” morphological situation with the forcing of tides and waves. For these situations the morphological evolution of the Frisian Inlet with the presence of a nourishment is investigated by means of scenario studies (seven smart nourishment alternatives).
In order to compare the seven nourishment alternatives, first their efficiency of redistributing the nourished sand is investigated. With the means of a morphological inactive underlying bathymetry the behaviour of only the nourished sand under the forcing of the tide is made comprehensible. These simulation results indicate that the alternatives, 3, 5, 6 and 7 have the potential of increasing sedimentation benefiting the basin.
The results of the fully morphological simulations (tide•only) for these four alternatives however, show that effects triggered by the alternatives in a fully morphological simulation are hardly comparable to their “direct” effects on a fixed bottom. This is explained by the fact that the tidal flows in the fixed bottom situation are unsaturated and pick up sand “too easily”. The selection of two alternatives located in the inlet seems to be a good choice, as their effect on the morphodynamics is significant under the forcing of only the tide. The selection of the two alternatives at the west side of the ebb•tidal delta is less satisfactory, as deposition of sediment within the basin is only marginally.
Subsequently the four selected alternatives are analysed on their morphological behaviour under the forcing of tide and waves (morphological period of one year). For the alternatives located in the channels, the inclusion of waves did not significantly contribute to the initial interaction of the nourished sand. Tidal flows have a far more distinct effect on the alternatives within the channels. Constriction of tidal flows within the channel accounts for an increase in bed shear stress and the stirring up of sediment. Subsequently sediment is
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transported to the basin and ebb•tidal delta during flood and ebb. On the other hand, the inclusion of waves within the model did show that the alternatives at the ebb•tidal delta are dependent on wave action for a fast interaction with the morphodynamics of the tidal inlet. Absolute effects of the alternatives located at the ebb•tidal delta showed to be comparable to the effects after three years for the tide•only model. Although a faster interaction is established, effects remain to act on a local scale (no significant import).
Overall we need to conclude that the time span of one year is too short to extrapolate the longer term effects of the nourishment alternatives i.e.( it is impossible to say whether an amount of X m3 sand results in Y m3 sand within the basin on the long term). Within the time span of one year we have mainly observed the spreading out of nourished sand. Although no quantitative conclusions can be drawn, it is possible to address the different physical processes behind the diffusion of the nourishment. This assists in drawing preliminary conclusions.
For a fast redistribution of nourished sand it is advised to nourish the channels. This results in a direct profit during flood (import of sediment with flood flows). Exported sediments may be imported in a later stadium when they are brought back into the sediment transport paths (e.g. by wave action). The alternatives nourishing the ebb•tidal delta depend on wave action. First they need to be stirred up by wave action before they are transported by tidal flows and redistributed over the ebb•tidal delta. Apart from the fact that the redistribution of sand is still very local after one year, the alternatives at the ebb•tidal delta lack the profit of a direct import component, while they behave the same as the exported sediments from the channel nourishments.
The natural dynamics of a coastal inlet tidal basin system, arrows indicate sediment exchange. It is the expectation that an abundance of sand in one element (the ebb•tidal delta) will be distributed to the basin by natural processes driving the sediment exchange.
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Contents
1 Introduction...... 1— 1
1.1 Framework of the smart nourishment research...... 1— 1
1.1.1 1990 Dutch coastline preservation and the sand balance...... 1— 1
1.1.2 Research as part of the Delft Cluster 2 Project...... 1— 3
1.2 The Frisian Inlet, problem description...... 1— 3
1.2.1 The Frisian Inlet as study example ...... 1— 3
1.2.2 Synopsis of expected threats for the Frisian Inlet...... 1— 4
1.3 Hypothesis ...... 1— 5
1.4 Objectives ...... 1— 6
1.5 Readers Guide...... 1— 6
2 Research approach and model setup...... 2— 1
2.1 Introduction...... 2— 1
2.2 Objectives translated into research approach...... 2— 1
2.3 Research means: Delft3D ...... 2— 3
2.4 Grid and Bathymetry ...... 2— 3
2.5 Boundaries and boundary conditions...... 2— 4
2.6 Water level boundary conditions ...... 2— 6
2.6.1 Tide...... 2— 6
2.6.2 Water level Setup...... 2— 6
2.7 Wind and Waves ...... 2— 7
2.8 Numerical implementation of sediment transport due to tide and waves 2— 9
2.8.1 WAVE•Online and Sediment•Online...... 2— 9
2.8.2 Morphogical scale factor ...... 2— 11
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2.9 Numerical and Physical parameters ...... 2— 11
2.10 Model limitations ...... 2— 13
2.11 Implemented model tool: T•Tile Analysis ...... 2— 13
3 Phase A; nourishment behaviour under the forcing of tidal flow only...... 3— 1
3.1 Introduction ...... 3— 1
3.2 Nourishment placing strategy...... 3— 3
3.3 Behaviour of nourishment alternatives on a “fixed bottom”...... 3— 5
3.3.1 Setup nourishment alternatives on a “fixed bottom”...... 3— 5
3.3.2 Results of alternatives on a “fixed bottom”...... 3— 5
3.3.3 Selection of high potential alternatives ...... 3— 7
3.4 Analysis of morphological behaviour autonomous situation ...... 3— 9
3.4.1 Sediment transport in the Frisian Inlet (tide•only) ...... 3— 9
3.4.2 Model behaviour and field observations ...... 3— 13
3.5 Morphological behaviour of the nourishment alternatives...... 3— 15
3.5.1 Setup of morphological simulations...... 3— 15
3.5.2 Nourishment alternative 3 ...... 3— 17
3.5.3 Nourishment alternative 5 ...... 3— 19
3.5.4 Nourishment alternative 6 ...... 3— 21
3.5.5 Nourishment alternative 7 ...... 3— 23
3.5.6 Analysis of morphological behaviour of the nourishment alternatives ...... 3— 23
3.6 Conclusions on nourishment behaviour under the forcing of the tide... 3— 25
4 Phase B; nourishment behaviour under the forcing of tidal flow and waves 4— 1
4.1 Introduction ...... 4— 1
4.2 Setup of one year morphodynamic analysis...... 4— 1
4.3 Analysis of morphological behaviour of the autonomous situation...... 4— 3
4.3.1 Sediment transport in the Frisian Inlet (tide and waves)...... 4— 3
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4.3.2 Model behaviour and field observations...... 4— 5
4.4 Morphological behaviour of the nourishment alternatives ...... 4— 7
4.4.1 Nourishment Alternative 3...... 4— 7
4.4.2 Nourishment Alternative 5...... 4— 11
4.4.3 Nourishment Alternative 6...... 4— 13
4.4.4 Nourishment Alternative 7...... 4— 15
4.5 Conclusions on nourishment behaviour under the forcing of tide and waves ...... 4— 16
4.5.1 Favourable sedimentation...... 4— 16
4.5.2 Direct effects on nourished sediments...... 4— 17
4.5.3 Indirect effects of nourished sediments and the morphodynamics of the ebb•tidal delta ...... 4— 17
4.5.4 Conclusions on one year morphodynamic behaviour...... 4— 18
4.5.5 Extension of model calculations...... 4— 19
5 Conclusions and Recommendations...... 5— 1
5.1 The morphological interaction...... 5— 1
5.2 The “smartest” nourishment...... 5— 2
5.3 Conclusions on the model behaviour and model applicability ...... 5— 3
5.4 Recommendations...... 5— 4
A The Wadden Sea: a system of tidal basins ...... A–1
A.1 Introduction...... A–1
A.2 Characteristics of the Wadden sea ...... A–1
A.3 Tidal Basin Elements ...... A–2
A.3.1 Ebb•tidal delta...... A–2
A.3.2 Tidal inlet...... A–4
A.3.3 The tidal basin...... A–4
A.4 Hydrodynamic processes and factors ...... A–4
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A.4.1 Tidal range and flow...... A–5
A.4.2 Seasonal winds and waves ...... A–6
A.4.3 Geometry of the basins...... A–7
A.5 Equilibrium relations of tidal basins ...... A–8
A.5.1 Introduction to equilibrium relations and morphological balance...... A–8
A.5.2 Relationship between inlet cross•section and tidal prism...... A–9
A.5.3 Relationship between channel volume and tidal prism...... A–10
A.5.4 Relationship between volume of ebb•tidal delta and tidal prismA–10
A.5.5 Relationship between volume of tidal flats and basin size...... A–10
A.6 Climatic and anthropogenic factors...... A–11
A.6.1 Sea level rise...... A–11
A.6.2 Relative sea level rise and human contributions ...... A–12
A.6.3 Human interventions ...... A–13
A.7 Morphological modelling of coastal inlets & tidal basins...... A–13
A.7.1 Introduction...... A–13
A.7.2 Cascade of scales...... A–14
A.7.3 ASMITA...... A–15
B The sand balance reviewed...... B–1
B.1 Introduction to a morphodynamic system ...... B–1
B.2 The sand balance ...... B–1
B.2.1 The balance area...... B–1
B.2.2 The sand balance in equilibrium ...... B–2
B.3 Ins en outs...... B–3
B.3.1 Absolute sea level rise ...... B–3
B.3.2 Subsidence...... B–5
B.3.2.1 Tectonic movement ...... B–5
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B.3.2.2 Gas mining...... B–5
B.3.3 Boundary shifts...... B–6
B.3.3.1 Manmade interventions ...... B–6
B.3.3.2 Subsidence of the NAP reference level...... B–6
B.3.3.3 Watershed movement...... B–6
B.3.4 Discussion; Demand for sand and “sand hunger”...... B–7
B.4 Sand balance for the Frisian Inlet ...... B–8
B.4.1 Data availability...... B–8
B.4.2 Data Oost and Haas 1992 ...... B–9
B.4.3 Data including watershed movement...... B–10
B.4.4 Data Delft3D model calculations tide•only ...... B–10
B.5 Comparison of sediment import ...... B–11
B.6 Citation of Eysink et al. (1998)...... B–12
C Tidal Boundary Conditions...... C–1
C.1 Nesting the Frisian Inlet model into the ZUNO•Grof model ...... C–1
C.1.1 The ZUNO•Grof model ...... C–1
C.1.2 Decomposition and nesting...... C–2
C.2 Validation of tidal boundary conditions...... C–2
C.2.1 Validation water level time series Frisian Inlet model with ZUNO•Grof model...... C–2
C.2.2 Validation time series with harmonic boundary conditions...... C–3
C.2.3 Validation water level time series between Frisian Inlet and measured data ...... C–5
D Derivation of the Morphological Wave Climate...... D–1
D.1 Listing of SON Wave Buoy Data...... D–1
D.2 Derivation of Morphological wave Climate ...... D–1
D.2.1 Method...... D–1
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D.2.2 Example for a medium wave height in the {0º — 15º} directional sector ...... D–2
D.2.3 Cross check for one wave class by model calculations...... D–3
D.3 Delft3D FLOW grid vs. Delft3D WAVE grid...... D–3
E Model parameters...... E–1
E.1 Calibration of the Morphological scale factor...... E–1
E.2 Time step sensitivity...... E–2
E.3 Physical and numerical parameters ...... E–2
E.3.1 Physical parameters...... E–2
E.3.2 Morphological parameters...... E–2
F Sediment distribution for the Frisian Inlet ...... F–1
G Summary of research phases and model settings ...... G–1
H Reference computation Tide•only ...... H–1
I Model simulations of nourishment alternatives on a “fixed bottom”...... I–1
J Model simulations of tide•only (Phase A)...... J–1
J.1 Introduction ...... J–1
J.2 Autonomous•simulation tide•only...... J–2
J.2.1 Transports and current velocities...... J–2
J.2.2 Averaged transports (aggregated)...... J–3
J.3 Nourishment alternative 3 tide•only...... J–4
J.3.1 Effects after 1 year (left column), effects after 3 year (right column)...... J–4
J.3.2 Transports and current velocities...... J–5
J.4 Nourishment alternative 5 tide•only...... J–6
J.4.1 Effects after 1 year (left column), effects after 3 year (right column)...... J–6
J.4.2 Transports and current velocities...... J–7
J.5 Nourishment alternative 6 tide•only...... J–8
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J.5.1 Effects after 1 year (left column), effects after 3 year (right column)...... J–8
J.5.2 Transports and current velocities...... J–9
J.6 Nourishment alternative 7 tide•only ...... J–10
J.6.1 Effects after 1 year (left column), effects after 3 year (right column)...... J–10
J.6.2 Transports and current velocities...... J–11
K Model simulations of tides and waves (Phase B)...... K–1
K.1 Introduction...... K–1
K.2 Autonomous simulation tide and waves ...... K–3
K.2.1 Transports and current velocities (tide and waves)...... K–3
K.2.2 Averaged transports (aggregated) ...... K–4
K.3 Nourishment alternative 3 – tide and waves...... K–5
K.3.1 Effects after 1 year...... K–5
K.3.2 Transports and current velocities (tide and waves)...... K–6
K.3.3 Nourishment alternative 3 ebb and flood (tide and waves) ...... K–7
K.4 Nourishment alternative 5 • tide and waves...... K–8
K.4.1 Effects after 1 year...... K–8
K.4.2 Transports and current velocities (tide and waves)...... K–9
K.4.3 Nourishment alternative 5 ebb and flood (tide and waves) ...... K–10
K.5 Nourishment alternative 6 • tide and waves...... K–11
K.5.1 Effects after 1 year...... K–11
K.5.2 Transports and current velocities (tide and waves)...... K–12
K.5.3 Nourishment alternative 6 ebb and flood (tide and waves) ...... K–13
K.6 Nourishment alternative 7 • tide and waves...... K–14
K.6.1 Effects after 1 year...... K–14
K.6.2 Transports and current velocities (tide and waves)...... K–15
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K.6.3 Nourishment alternative 7 ebb and flood (tide and waves)...... K–16
K.7 Nourishment of the entire ebb•tidal delta...... K–17
K.8 Wave energy dissipation during strong wave conditions ...... K–18
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1 Introduction
1.1 Framework of the smart nourishment research
1.1.1 1990 Dutch coastline preservation and the sand balance
The Dutch coastal zone is a densely populated area in which large investments are being done. Besides that, the coastal zone is a vulnerable area which is under the constant pressure of natural dynamics (water level fluctuations, wind and waves). To resolve this conflict and conflicts among a variety of coastal users and to determine the most appropriate use of coastal resources, coastal zone management has been applied. An aspect of coastal zone management that deals with the preservation of the beach and protection of the hinterland is coastline management.
1990 coastline preservation
In 1990 the Dutch government implemented a dynamic form of maintenance of its coastline. After 1990 structural coastal erosion is fought from the moment it is noted. To define whether erosion is present, the coastline of 1990 is used as reference point; this coastline is the so called “basal coastal line” (in Dutch: BasisKustLijn; BKL). Every year coastal profile measurements are compared with the BKL. If these measurements show that a part of the coastline exceeded the BKL in landward direction, immediate action is taken. These actions are usually in the form of beach nourishments, that are planned accordingly with the annual book of coastline profiles1, in which the differences between actual profile and the coastline profile of 1990 are graphically presented. By now this form of dynamic maintenance has proven its applicability in practice.
Sand balance in the Dutch coastal zone
Although the coastline is successfully maintained at its 1990 position, the Dutch coast still suffers from a loss of sand. According to Mulder(2000) the Dutch coastal sand balance to a depth of •20 meters with correction for nourishments was on average •6,5 Mm3/yr for the period 1965•1995 (Figure 1•1, Figure 1•2). It is possible that these losses could even be a factor 2 higher, this follows from a comparison of sedimentation figures for the Wadden Sea and losses from the nearby sectors for the same period.
1 In Dutch: kustlijnkaartenboek
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Figure 1•1: Sand balance in different depth zones for the nine sectors in Figure 1•2: Nine sectors of the Dutch the Dutch coastal zone, as defined in Figure 1•2, for the period 1965• coast (Mulder, 2000) 1995, accounted for nourishments. For a comparison of the different sectors, which differ in area, the loss is expressed in change in bottom height (mm/yr) averaged over the independent depth zones. (Mulder, 2000)
Mulder (2000) also presents recommendations for future coastline preservation using different scenarios. These scenarios cover the variability of the two most uncertain parameters, viz.: the amount of sand loss from the sand balance and the amount of sea level rise per year. Outcomes result in recommendations regarding how to fulfil sand balance deficits in the future. It is advised to compensate these by artificial nourishments, independent of the amount of losses from the sectors. For the execution the following guidelines are presented: 1) If necessary, and only if necessary nourish the beach, else, if possible execute a shore face nourishment; 2) Execute the shore face nourishment in the shallow coastal zone (above NAP •8 m.) and let natural processes redistribute the sand within the coastal zone.
It is this latter notion that has been an inspiration for coming up with a new nourishment concept for fulfilling the sand balance deficits in the Wadden Sea2. Apart area from aforementioned deficits, the Wadden Sea is also threatened by sea level rise and gas extraction leading to additional sand losses3. The innovative idea now is to mitigate these effects by nourishing the ebb•tidal deltas and promoting sedimentation of the tidal basin by letting natural processes take over. This idea (“smart nourishment” of the ebb•tidal delta) will be the topic of research.
2 See Appendix A for an elaboration of the Wadden Sea (a tidal basin system), its elements, morphodynamic characteristics and its behaviour under intrinsic and external forcings. 3 See Appendix B for an elaboration of the sand balance for the Wadden Sea and Frisian Inlet.
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1.1.2 Research as part of the Delft Cluster 2 Project
The work reported in this thesis was financed by the Delft Cluster 2 project “North Sea and Coast” (DC 05.20). Part of the work of Workpackage 2 of that project is co•financed by the WINN (WaterINNovation) framework, as part of the project “More sand, less effort”. Together with partners from Delft University of Technology, Unesco IHE and Rijkswaterstaat RIKZ, WL | Delft Hydraulics develops a number of innovative technical• morphological adaptation scenarios to deal with the effects of sea•level rise. The work has focused on three scenarios mainly, viz.:
· “Broad beach, Better surfing”, investigating a more efficient way of shoreface nourishment in a context of coastline management. · “Plenty of Sand, Naturally safe”, investigating a more natural approach to dune strengthening in a context of protection from flooding · “Sand for the Wadden sea, Preservation of the flats”, investigating an innovative approach to increase the robustness of tidal inlets in a context of dealing with the adverse effects of sea•level rise.
The work reported in this thesis is a contribution to the third item on the above list: “Sand for the Wadden sea, Preservation of the flats”. Main objective is to analyse the efficiency of an innovative way of nourishing, with respect to previously used more traditional methods.
In the year 2004, Stive and Wang came up with the concept of nourishing the ebb•tidal delta or ebb•channel with sand of different grading. After some preliminary calculations they found that this should be investigated in more detail for the Wadden Sea area. Note that they did this independently of Eysink who suggested a similar idea before. It was already in 1998 that Eysink introduced the idea of nourishing the ebb•tidal delta to compensate coastal erosion present at the island coasts and reduce the combined effects of subsidence due to gas mining and sea level rise (citation of Eysink in Appendix B.6).
This research regarding smart nourishment of the Frisian Inlet is more similar to the form as Eysink suggested it, since sand of different grading is not yet taken into account.
1.2 The Frisian Inlet, problem description
1.2.1 The Frisian Inlet as study example
Earlier studies pointed out the changes that the Wadden Sea has to overcome in the near future (Eysink, 1998 and Mulder, 2000). The main problem is the loss of sand from the coastal sections along the entire Wadden Sea due to both natural processes and anthropogenic interventions.
The Wadden Sea is influenced by sea level rise and subsidence due to gas mining. The tidal inlet between Ameland and Schiermonnikoog, The Frisian Inlet (Figure 1•3), forms a perfect study example since the area is influenced by both processes. Gas mining deserves extra attention as planned developments involve further exploitation of the present gas fields.
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Furthermore, the Frisian Inlet is an area that received attention in earlier research conducted, after the closure of the Lauwers Sea in 1969, and is therefore well documented.
Figure 1•3: Three Wadden Sea ebb•tidal deltas, the Frisian Inlet depicted in the middle. The Frisian Inlet is a double inlet system; Pinkegat is the smaller inlet in the West, the Zoutkamperlaag drainage basin is roughly twice as large as that of the Pinkegat and situated in the East. (from: Cleveringa, 2004)
1.2.2 Synopsis of expected threats for the Frisian Inlet
For the Frisian Inlet sea level rise and gas mining leads to a loss of intertidal area. Furthermore the impacts of human interventions,e.g. closure of the Lauwers Sea in 1969, still have effect on the development of the system (Van der Valket al., 2004). It is expected that the system will seek a new equilibrium and import sand to keep up with the relative rise of the sea level. However the impact on the system is uncertain and depends on several factors: · The rate of sea level rise; · The amount of subsidence due to gas extraction; · The amount of sediment available at the seaward boundary for deposition in the basin.
Given the most likely sea level rise scenario, 56 centimetres per century, and the uncertainty in the availability of sediment at the seaward boundary, one should prepare for negative impacts in the coming century (van Goor, 2001). This includes the uncertainty in the capability of the system to distribute the sediment internally. Naming a few of these impacts: · Loss of intertidal area (drowning of flats) due to sea level rise; · Subsidence due to gas mining and natural ongoing subsidence; · Further erosion of the barrier island coasts.
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1.3 Hypothesis
To mitigate the pre•mentioned negative impacts, the basic hypothesis for this study is that:
An abundance of sand on the ebb•tidale.g. delta, induced by an artificial nourishment, causes sedimentation in the tidal basin.
The above hypothesis is an innovative idea which is captured in the following phrase and title of this research: Smart Nourishment of the Frisian Inlet (Figure 1•4).
Figure 1•4: The natural dynamics of a coastal inlet tidal basin system, arrows indicate sediment exchange. It is the expectation that an abundance of sand in one element (the ebb• tidal delta) will be distributed to the basin by natural processes driving the sediment exchange.
The hypothesis is divided into three sub hypotheses, where a distinction is made between two dominant hydraulic forcings resulting in: Sub 1: Tidal flow transports an abundance of sand from the ebb•tidale.g. delta a ( nourishment) partly into the basin; Sub 2: Waves enhance this effect by their capability of stirring up the sediment. Thereby sediment is picked up easier by tidal flows and transported inwards. The above mechanisms are considered asdirect effects, moving the nourished sand directly inward. Apart from this indirect effects on the morphodynamics of the system are expected, leading to: Sub 3: The nourished sediment interacts with the existing morphodynamics and may thereby change sediment transports trough the inlet in a favourable way. This does not necessarily mean that sediment from the nourishment itself is transported inward.
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1.4 Objectives
The idea of artificial nourishment to a sand demanding basin somewhere along the sediment transport path, from ebb•tidal delta to basin, has been introduced before. As explained in section 1.1.2. it could be an effective and environmentally friendly solution to reduce the sand demand of the basin. Literature supports the theory that an abundant amount of sediment on the outer delta is likely to be imported by a basin suffering a shortage of sand (e.g. Van der Valk et al. 2004; Van Goor, 2001; see also Appendix B). The question remains where to nourish the ebb•tidal delta in such a way that natural processes redistribute the sand relatively quick (order of years), in a way beneficial to the basin. To answer this question research should provide the answer to:
§ How do the hydrodynamic forcings influence the morphological behaviour of the nourished sand, and how do the morphodynamics of the nourished ebb•tidal delta and the tidal basin interact? § Where should a smart nourishment be placed in such a way that natural processes redistribute the sand relatively quickly to the benefit of the basin?
1.5 Readers Guide
The research objectives are worked out as follows: A model of the Frisian Inlet is set up in the process based modelling environment of Delft3D (discussed in Chapter 2). Three situations are studied within this model: 1. A situation with a “fixed bottom” and the hydrodynamic forcing of the tide; 2. A “fully” morphological situation with the hydrodynamic forcing of the tides4 and; 3. A “fully” morphological situation with the forcing of tides and waves. For these situations the morphological evolution of the Frisian Inlet with the presence of a nourishment is investigated by means of scenario studies (seven smart nourishment alternatives). The first two situations are attended in Phase A of this research and elaborated in Chapter 3. The goal of Phase A is to get insight in the flow and sediment transport patterns in the Frisian Inlet under the forcing of the tide only. The third situation (Phase B of this research) aims to determine the distinct contribution of waves. The research approach is visualised in Table 1•1.
4 The term “fully” is only added to make it clearly distinct from the first situation, where only the nourished sand is morphologically active.
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Table 1•1: Graphical representation of thesis set•up
Research Phase Attended in thesis Model setup Chapter 2
Research Simulation type Boundary Studied Modelled Analysed Attended Phase conditions alternatives time span time span in thesis A “Fixed bottom” tide•only 1 • 7 9.5 years 9.5 years Chapter 3
A “Fully” tide•only 3, 5, 6 and 7 9.5 years autonomousChapter 3 morphological situation: 9.5 years the alternatives: 3 years
B “Fully” tides and 3, 5, 6 and 7 1 year 1 year Chapter 4 morphological waves
Research Phase Attended in thesis
Conclusions and Recommendations Chapter 5
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2 Research approach and model setup
2.1 Introduction
Regarding the research a distinction should be made between short•term and long•term morphological processes. To deal with this problem our interest should be looked at on a cascade of scales (De Vriend, 1999, see also Appendix A). The mixture of scales and types of forcing, complicates the modelling and predicting of large•scale morphological behaviour of tidal inlet systems.
The feared negative impacts, the relevant processes and the scales on which they are acting can be categorized. Most of these processes are elaborated in Appendix A, and are briefly recalled in Table 2•1. Table 2•1: relevant processes and their accompanying scales
Relevant processes and their accompanying scales Sea level rise (viz. SLR and natural bottom subsidence) and the loss mega / macro scale of intertidal area Bottom subsidence due to gas mining and the loss of intertidal area mega / macro scale Sediment availability in the ebb•tidal delta and accumulation in the macro scale “sediment•starving” basin The morphological behaviour of the nourished sand volume, which meso scale physical processes result in a net inward flux of sediment?
On the macro•scale, which matches most of the scales of interest above, one typically reverts to more behaviour based modelling approachese.g. ASMITA), ( while the process•based modelling (e.g. Delft3D) is typically relevant on the micro• and meso•scale. Topic of research for this study is the question whether physical processes indeed cause a increase of sediment influx when there is an abundance of sand available on the ebb•tidal delta. And if so, which processes that are. The interest in the physical processes and the need to study the interaction of the nourishment with the tidal•inlet•morphodynamics results in the choice for conducting this research with the use of process•based modelling.
In the last two decades several model studies of tidal inlets have been made using the Delft3D software (e.g. Steijn and Hartsuiker, 1992; Wang, 1993; Hibma, 1999; Van Ledden, 2002; Elias, 2005). Results and recommendations from these studies in combination with the ever ongoing development of the software itself is used to set up a new model for the Frisian Inlet. The model is not completely new since a part of the model is already available (cf. Hibma, 1999 and Van Ledden, 2002). This chapter elaborates the translation of the objectives into a research approach and gives an overview of the model schematisations.
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2.2 Objectives translated into research approach
Emphasizing once more the main objective, one first has to gain more insight into the relevant physical processes. When more insight has been gained this could be used for setting out a nourishment strategy. By taking into account the dominant hydraulic conditions, tidal flow and waves, the choice is made to divide the research into two phases.
Phase A Modelling the Frisian Inlet taking into account tidal flow only; Phase B Modelling the Frisian Inlet taking into account tidal flow and waves.
Corresponding with the objectives, secondly the interest lies with the nourished sand on the ebb•tidal delta and sediment transport into the tidal basin. To investigate this a numerical model has to be set up for the Frisian Inlet which incorporates its hydrodynamic and morphological development for each phase. It should be noted that the objective is not to model as accurately as possible actual hydraulic and sediment transport conditions for the Frisian Inlet. The goal is to get more insight in the residual flow and transports patterns at the ebb•tidal delta and relative effects caused by the nourishment alternatives. This approach allows stating conclusions based on relative differences.
Phase A; Modelling the Frisian Inlet taking into account tidal flow only
The goal of phase A is to get insight in the flow and sediment transport patterns in the Frisian Inlet under the forcing of the tide only (for short: tide•only simulation). Subsequently this insight should be used to come up with a nourishment placing strategy. Finally model results of the different alternatives should make a comparison possible on the effectiveness of the different alternatives.
To study the influence of a nourishment on the ebb•tidal delta taking into account the hydrodynamic forcing of the tide several stages need to be passed:
1. Set up of a model in Delft3D for the Frisian Inlet; 2. Modelling the Frisian Inlet with tide only; 3. What are the results from model runs with tide•only? Is there transport into the basin and how realistic is this? In order to determine this a comparison should be made between model results and sediment import figures derived from an analysis of historical data (see Appendix B); 4. A hydrodynamic analysis of the first model results (tide•only). Information on residual transport patterns is used to design nourishment alternatives; 5. Define nourishment alternatives and implement them in the model environment; 6. To compare the effectiveness of the different alternatives an analysis is made of the sedimentation and sediment transport out and into the basin by the nourished alternative only. This is done by modelling the inlet with a so called “fixed bottom” with the nourishment on top. In this way only the nourished sediment can erode and is thereby possible to trace.
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7. Subsequently a set ofhigh potential alternatives will be subject to a fully morphodynamic analysis. Which alternative increases the sediment transport through the tidal inlet the best and thereby increases sedimentation in the basin? This secondly allows assessing their indirect effect on the morphology, is this effect comparable to effects studied in Stage 6?
At the end of phase A it is explained how residual transport patterns and active morphological zones are used to determine a nourishment strategy. Furthermore several nourishment alternatives have been studied in a morphodynamic analysis on their effect on the sediment transport through the tidal inlet. A set ofhigh potential alternatives, based on their ability to increase sediment transport through the inlet, is selected and will be subject of research in Phase B
Phase B; Modelling the Frisian Inlet taking into account tidal flow and waves
The goal of phase B is to get insight into the flow and sediment transport patterns in the Frisian Inlet under the forcing of tide and waves. Alternatives selected on their effectiveness in Phase A will be studied for their behaviour influenced by the combined forcing of tide and waves. Finally model results of the selected alternatives should determine the effectiveness of each nourishment and result in a final recommendation for an effective “smart nourishment”.
1. Adapt the Delft3D model from Phase A to be able to include sediment transport due to waves; 2. To gain insight in the effects of waves a set of wave conditions5 is applied on the fixed bottom configuration; 3. To model the effects of waves as realistically as possible a morphological wave climate is set up; 4. Assessment of the simulation period for model runs with waves 5. Modelling a set of alternatives taking the influence of waves and tidal flow into account; 6. Determine the effectiveness of the nourishment alternatives and select the most effective alternative. Which alternative increases sediment transport through the inlet the best and results in favourable sedimentation within the basin? 7. Did the set ofhigh potentials, based on sediment transport patterns due to tidal flow proved to be the correctly chosen ones.
The result of Phase B will be the selection of one alternative as “smart nourishment” on the criterion of increasing the sedimentation in the basin the best. Furthermore results are used to comprehend the way natural processes redistribute the sand.
5 Moderate and high wave conditions from 1.25 m. and 2.75 m. for the directions 285°, 315°, 245°, 15°.
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2.3 Research means: Delft3D
The software package which will be used to forecast the effects of the smart nourishment on the hydrodynamics and morphodynamics of the inlet and the adjacent barrier island coasts is Delft3D.
The Delft3D modelling system is developed by WL | Delft Hydraulics to simulate two• dimensional or three•dimensional hydrodynamic and/or morphological development of coastal, river and estuarine areas. Flows, sediment transport, waves, water quality, morphological development and ecology all fall within its field of applicability.
The Delft3D modelling system consists of several modules representing the different steps to come to the simulation of morphologic development. One of the modules is Delft3D• WAVE where the evolution of wind generated waves can be simulated. The FLOW module is a hydrodynamic simulation program, which calculates non•steady flow and transport phenomena resulting from tidal and/or meteorological forcing on a curvilinear, boundary fitted grid. The numerical model of the program solves the unsteady shallow water equations in two or three dimensions. Delft3D•FLOW has an add•on which concerns simultaneous computation of flows and transports and simultaneous feedback to bottom changes called the Sediment Online Module (Lesser et al., 2004).
Figure 2•1: Principles of morphodynamic process based modelling with Delft3D (from Sun, 2004)
For a more extensive elaboration of Delft3D (cf. WL | Delft hydraulics, 2005). The way the Delft 3D model for the Frisian Inlet is set•up is explained in the next sections.
2.4 Grid and Bathymetry
One of the first needed schematisations is the implementation of the bathymetry of the study area onto a computational grid. The grid should provide a high resolution in the areas of interest. The use of a curvi•linear grid makes it possible to focus on important areas and avoiding unnecessarily high resolutions in unimportant areas.
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A grid which covers the entire basin –including the former Lauwers Sea– a part of the island of Ameland and the whole island of Schiermonnikoog is used (available from Hibma, 1999). One of the advantages is that the focus of this grid is already set to the inlet and ebb•tidal delta (see Figure 2•2). The grid size at the ebb•tidal delta is in the order of 200x2002 m while the grid size at the deep•sea side is in the order of 800x8002. mThe grid therefore only needs modification by means of removing the unneeded part which covered the former Lauwers Sea.
The bathymetry was generated using a data set of RIKZ, which contained bathymetry measurements from the year 2000. These depth points were first down•sampled evenly in two directions by 20 meters and were then averaged onto the computational grid.
2.5 Boundaries and boundary conditions
All boundaries of the computational grid should be set to either an open or a closed boundary. For this model three open boundaries were prescribed at the sea side of the inlet. All boundaries in the basin were defined as closed boundaries. At the western part of the basin the closed boundary coincides with the tidal water shed of the tidal basin while in the east the boundary lays some grid cells beyond the physical watershed to allow local erosion and sedimentation.
For the open boundaries a boundary condition has to be prescribed. These could be set to the following: · Water level boundaries; · Current boundaries; · Flux boundaries; · Riemann boundaries. The choice of boundary conditions depends, among other things, on the phenomena to be studied. For the modelling of a tidal inlet under the hydrodynamic forcing of only the tidal flow, prescribing only water levels at the boundary is usually sound. If these boundary conditions are not satisfactory, combinations of water levels and velocities may be prescribed. The criteria to determine which conditions are satisfactory may be extracted from measurements or other models. Corresponding with the phased research approach the types of boundary conditions used are listed below.
Phase A: boundary conditions: tide (water levels) Phase B: boundary conditions: tide (water levels) and waves
The exact implementation of the imposed boundary conditions will be elaborated in the next sections.
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Figure 2•2: (above) Grid of the ZUNO•Grof model. In the yellow box the refined decomposed area around the islands Ameland and Schiermonnikoog is depicted. In the green box the blue marks indicate the observation points used for nesting the boundaries for the new Frisian Inlet model. (bellow) Computational grid for the Frisian Inlet
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2.6 Water level boundary conditions
2.6.1 Tide
For both research phases a tide or schematised tide is necessary. From the study of Hibma boundary conditions are available (spring•neap tidal cycle in harmonic components). Unfortunately, a problem arises when these boundary conditions are applied. In the model used by Hibma the boundary conditions are imposed along 3 sections; the entire, west, north and eastern boundary. Boundary conditions are determined by linear interpolation between two points (beginning and end of each section). Applying conditions over such a great length results in circular residual currents along the boundary (caused by inaccuracy of the amplitude and phase of the tidal constituents). The occurrence of such side effects does not form a problem as long as they do not influence hydrodynamic processes in the area of interest. Unfortunately the circular residual current patterns at the boundaries still influence the currents at the ebb•tidal delta. Therefore a finer set of boundary conditions has to be derived.
For the derivation of a new set of boundary conditions the following has been done: The hydraulic model for the southern part of the North Sea, the so calledZUNO•Grof model (Figure 2•2), has been run for seven months. This model is driven by boundary conditions consisting out of astronomic tidal components at the northern and southern boundary. By means of refining the grid locally (domain decomposition) around the islands of Ameland and Schiermonnikoog enough observation points can be placed at locations which coincide with the boundaries of the Frisian Inlet model (see Appendix C). With the nesting module of Delft3D water level time series from the observation points can be interpolated to boundary conditions for the boundary sections of the Frisian Inlet model. Note that the new boundary sections do not span the entire length of the east, north or western boundary but are cut into smaller segments. In this way the probability of encountering large residual circular currents in the new model is minimized (by means of reducing the errors in amplitude and phase of the tidal constituents).
The newly obtained water level boundary conditions are in the form of a time series file for the period October 2002 – June 2003. The newly obtained boundary condition result in a realistic residual current pattern which matches with theZUNO•GROF residual current pattern. To reduce the computational time, the time series are harmonically analysed and converted to a set of harmonically prescribed water level boundaries.
2.6.2 Water level Setup
The occurrence of large scale water level setup due to seasonal events is not accounted for in the Frisian Inlet model.
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2.7 Wind and Waves
Waves are the second dominant hydraulic process and topic of research in phase B. To model the morphological development in a coastal area due to the influence of waves the wave climate has to be schematised. There are a few options one could attend to for schematising the influence of waves;
1. modelling with one wave 2. modelling with a wave climate
The main goal is to schematise the hydrodynamics in such a way that an accurate prediction of the morphological development can be made. For the Frisian Inlet the influence of the waves on the morphodynamics is mainly visible on the ebb•tidal delta and the adjacent barrier island coasts. As stated in Appendix B the morphodynamics of the ebb•tidal delta are of a complex interacting nature which is hard to understand completely. But for this research the interest regarding waves lies with the inducement of long•shore transports in the surf zone of the islands and over the ebb•tidal delta. Furthermore their “stirring•up capability” of sediment and especially nourished sediment, on the ebb•tidal delta should be well represented. When the wave climate is schematised into a representative set of conditions these transport mechanisms should be kept in mind:
1. Stirring up sediment and especially stirring up nourished sediment by wave action; 2. Transport over the shoals at the ebb•tidal delta; 3. Longshore transport in the surfzone of the islands; 4. Sheltering effect of the ebb•tidal delta for wave related sediment transport at its lee side; 5. Transports in deeper waters (NAP •10 m.).
Considering the above transport mechanisms; the option of modelling with one morphological wave for the Frisian Inlet, is ruled out.E.g. the sheltering effect of the ebb• tidal delta which occurs for a wave from one direction results in a poor representation of the sediment transport at its lee side. The influence of waves is therefore simulated with the use of a morphological wave climate. The above transport mechanisms are used as soft criteria for the needed schematised conditions.
The measured wave conditions for the Frisian Inlet are provided by the SON wave buoy. The SON (Schiermonnikoog north) buoy lies approximately 10 kilometres offshore from the middle of the island Schiermonnikoog. Its location is just outside the area which is covered by the computational domain. The data from the SON buoy are therefore considered to be representative for the boundaries. The yearly statistics for the probability of occurrence of the significant wave height for 12 directional sectors can be found in Appendix D. The wave heights are grouped in bins of 10 centimetres, the directions in sectors of 30 degrees (Figure 2•3).
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Figure 2•3: (a) Probability of occurrence for wave heights up to 4 meters (Hs on radial axis) for 12 directional sectors (from the SON wave buoy data). (b) Location of the SON wave buoy.
By choosing a wave climate which covers more than one direction it is guaranteed that both sides of the ebb•tidal delta will be exposed to waves, and thereby fulfilling criteria four. Secondly it is important to realise that the depth at which sediment motion is induced by waves is related to the height of the wave. If the nourished sediment needs to be stirred up from a depth of approximately 4 meters below mean sea level, relatively high waves are needed. But for a good representation of the wave induced transport over the shallow parts of the ebb•tidal delta and the representation of the yearly averaged conditions, low and medium wave heights are required. The representative set of conditions therefore should include low, medium and high waves apart from different directional sectors.
From the wave buoy data the five most relevant directional sectors are chosen by their probability of occurrence. The sector 30 – 60 degrees is favoured above the 240 – 270 degrees sector since its mean wave direction is nearly parallel to the islands shoreline. For these five directional sectors the wave heights are classed into a low, medium and high bin, respectively {0 m. – 1,5 m.}, {1,5 m. – 3,0 m.} and {3,0 m. – 10,0 m.}. For each wave class in each sector a morphological representative wave height has to be determined. Thereby the following has been assumed: · Within the 10 centimetres bin the wave heights are uniformly distributed; · Within a class (low, medium and high) the wave heights are distributed according to their probability of occurrence; · The morphological effect of a wave class is proportional to the wave heights in that class to the power 2.5.
The proportionality power 2.5 is derived from CERC the •formula for wave induced sediment transport. The derivation of the morphological representative wave heights for each class in each sector can be found in Appendix D. The five main sectors cover 77.6 % – equal to 283 days– of the yearly wave action. The remaining days the waves approach the area mainly from the {240° – 270°} and {60° – 90°} directional sector while the rest alternates over the remaining directions. For the schematisation of the wave climate three additional directions are included, namely: 255°, 85° and 175°. For these directions no wave classes are distinguished and the morphological wave height is determined in the same way as it is done for the classes of the main directional sectors (elaborated in Appendix D). The determined yearly morphological wave climate is shown in Table 2•2.
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Table 2•2: yearly averaged wave climate
For each wave condition the corresponding wave period and wind speed is derived from a wave climate used in an earlier conducted Delft3D Frisian Inlet study (Steijn and Hartsuiker, 1992). Multiplying the probability of occurrence for each wave condition with 365 days shows the occurrence of a wave condition as days per year. The 18 conditions are subjectively put in order to form the yearly wave climate used as input for the Delft3D model. Wave classes and direction are varied steadily to create a smooth transition between the different conditions.
2.8 Numerical implementation of sediment transport due to tide and waves
2.8.1 WAVE•Online and Sediment•Online
Now that the bathymetry and boundary conditions are known, hydrodynamic and morphological development of the model can be simulated with Delft3D. The Frisian Inlet model is a depth averaged model, which means that computations are only executed in one layer. This significantly reduces the computational time compared to 3D•modelling. The velocity is distributed logarithmical over the depth for the computation of sediment transport.
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Table 2•3: Yearly morphological wave climate
wave Hmorf [m] Tp [s] number of wave & wind wind speed condition days direction [°] [m/s]
1 1,14 5,5 35,16 255 5 2 3,75 8,7 2,41 285 19 3 0,94 4,5 50,38 285 5 4 2,09 6,4 21,54 315 10 5 3,69 8,7 1,92 345 19 6 2,03 6,4 2,47 15 9 7 0,80 4,0 21,64 45 4 8 1,02 5,0 20,61 85 5 9 3,14 8,0 0,05 45 15 10 1,90 6,2 2,66 45 9 11 3,62 8,6 0,18 15 18 12 0,91 4,5 58,85 345 5 13 0,80 4,0 28,16 15 4 14 2,04 6,4 18,14 345 9 15 0,92 4,6 52,20 315 5 16 2,04 6,4 17,62 285 9 17 3,88 9,0 4,99 315 20 18 0,80 4,0 26,00 175 4
S 365 days
For the computation of waves the online wave module is activated, which makes use of the SWAN (simulating waves nearshore, Holthuijsen et al., 1993) model. This is a third generation model with the main advantage that it makes use of the same grid as the FLOW• Module. The wave grid is larger than the flow grid in order to ensure that distortions in wave height and direction are minimal on the flow grid (Appendix D).Online stands for an online coupling of WAVE with Delft3D•FLOW: the WAVE model has a dynamic interaction with the FLOW module of Delft3D i.e.( two way wave•current interaction). Through this coupling, both the effect of waves on current and the effect of flow on waves are accounted for (cf. WL | Delft Hydraulics, 2005).
The Sediment•Online version of Delft3D is used for the simultaneous computation of flow, sediment transports and bathymetrical changes. The bathymetry is dynamically updated every time step. The major advantage of this method, over an offline morphological computation, is that the hydrodynamic flow computations are carried out using the actual bathymetry.
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2.8.2 Morphogical scale factor
It is important to note that morphological developments take place on time scales several times longer than typical flow changes. In case of morphological prediction this would lead to relatively long simulation times. To shorten the simulation time a morphological time scale factor can be used. This factor scales up the speed of morphological changes to a state where the morphology has significant influence on the hydrodynamics. For the calibration of the morphological scale factor see Appendix E.
The morphological scale factor is in fact a numerical parameter of the Delft3D model, in the next section remaining numerical and physical parameters will be elaborated.
2.9 Numerical and Physical parameters
Time scale and step
Taking the high morphodynamic nature of the ebb•tidal delta into account it might be questioned whether a nourishment with a volume of 2 Mm3 can be traced after a period of 5 years. According to the research set•up, first only the effects due to tidal flow are investigated. Without the stirring•up effect of waves it might take a substantial amount of time to get any significant sediment transport into the basin. Modelled time scales for Phase A therefore differ significantly form those investigated in Phase B. A comparison with results for a year should indicate whether a year is enough to get an indication of the behaviour of the nourishment. Moreover, to asses the basic model settings, a run over even 20 years is made in accordance with Stage 2 of Phase A. To reduce computational time morphological changes are accelerated with the use of a morphological scale factor.
All simulation periods are preceded by a spin•up period in which spurious oscillations, caused by initial settings and boundary conditions, are dimmed out. The spin•up period is set to 720 minutes. During spin•up the bathymetry is not updated, reducing the effective simulation time with 720 minutes.
To model the effects over the described periods a time step should be chosen in such a way that results are accurate and the computational time is not excessive. Time step limitations in Delft3D depend on several parameters: required accuracy, stability, grid size and water depth. An indication for the required time step is the courant number for wave propagation. This courant number (CFL) gives the relation between wave propagation, time step and the smallest grid size in either x• or y• direction.
g = acceleration due to gravity [m/s2] ght×D h = local waterdepth [m] CFL =£10 {DDxy','} Dt = time step [s] DDxy','= smallest grid size in x• and y•direction [m]
In practical conditions the courant number should not exceed 10. For a water depth of 16 meters and a grid size of 200x200 meters, in the inlet throat, this results in a time step of 160
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seconds. However in case of large variations in depth through space, which is the case for the inlet channels, it should be checked whether this time step gives accurate results. The time step used in all simulations is 1 minute6. Different morphological scale factors are used to cover the simulation period. An overview is given in Table 2•4.
Table 2•4: Time scale and time step for phases of the research
Phase simulation time time step morphological scale factor A order 1 year 1 min 12 A order 10 years 1 min 24 B order 1 month 1 min 1•10 B order 1 year 1 min 1•10
Sediment characteristics
In the Frisian Inlet a wide range of grain sizes can be found. The mean diameter varies between 120 and 240 micrometers in the inlet and at the ebb•tidal delta. A detailed map of sediment distribution around the islands of Ameland and Schiermonnikoog is found in Appendix F. For the model a mean diameter of 180 micrometers has been applied (similar to Hibma, 1999). Sediment characteristics are summarized in Table 2•5.
Table 2•5: Sediment characteristics
Parameter Delft3D Parameter name Value
Mean sand diameter (D50) SedDia 180 mm
Reference density for hindered settling LSed 1600 kg/m3 Specific density RhoSol 2650 kg/m3 Dry bed density CDryB 2650 kg/m3 Initial sediment mass at bed per unit area SdBUni 32000 kg/m3
Apart from the sediment characteristics other physical parameters are used as input for the model (see Appendix E.3).
6 For the results of the computations for the time step sensitivity see Appendix E.
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2.10 Model limitations
Regarding the model applicability some notions are made: · For the sand balance of the Frisian Inlet a preliminary simulation is done to investigate the long term bed level behaviour of the basin. The bed level behaviour did not correspond well with reality on the long term for the tide•only simulation (1970•1990). Long term•bed level behaviour should therefore be interpreted with care (see Appendix B). · The tidal range within the model is overestimated at the observation station “Wierumergronden” (compared to measured data; order 10%). An overestimation of the tidal range will cause discrepancies between modelled and observed morphological behaviour of the inlet. · The boundary conditions are harmonically analysed but remain to keep theirseasonal character (spring neap cycles are not the same over all months). This inequality subsequently also shows in the sediment transport over one cross section To lose this seasonal character one should compose a morphological tide. E.g. A daily morphological tide which induces the same residual sediment transport as that of the daily averaged residual transport during a spring neap tide cycle. In this way morphological developments can easily be accelerated with the use of a morphological scale factor and reduce simulation times.
Model limitations are overcome by focussing on initial relative changes in sediment transport and bed level behaviour. This makes it possible to distinguish the relative effects triggered by the nourishment alternatives (relative to the modelled autonomous development). This approach is independent of the autonomous model behaviour (on the short term) and is used to gain insight in the physical processes which force the interaction of the smart nourishment.
2.11 Implemented model tool: T•Tile Analysis
To asses the effects of the different nourishment alternatives an analysis method has been set up which makes use of implementing a surplus of cross•sections in the Delft3D model. The basics for this method are: · An aggregation of local sedimentation and erosion patterns onto rectangular areas (referred to as tiles), and; · Keeping record of sediment transport over the sides of each tile.
The regular approach to compare the effects of the alternatives would be to compare the cumulative sediment transports through the inlet (or the cumulative change in bed level of the basin) and sedimentation•erosion patterns in the basin. The best alternative then would increase the sediment transport through the inlet the most. But with this approach several problems will arise. (1) The fact that only one cross•section is being monitored makes it sensitive to the time span being analysed.E.g. an alternative further away from the inlet needs more time to increase sediment transport through the inlet compared to an alternative in or close by the inlet. To compare both with the use of a cross•section at a certain point in
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time one needs to be certain that all, or at least most, of the morphological changes triggered by the alternatives have occurred. (2) It is furthermore desirable that the nourished sediment can be traced. Is the nourished sediment itself transported into the basin by natural processes or does it trigger morphological effects elsewhere resulting in increased sediment transport through the inlet. Monitoring the effects with just a few cross•sections does not allow to state which sediment volumes moved where or point out the morphological active zones. Morphological active zones can be pointed out with the use of sedimentation and erosion patterns. Unfortunately with the use of a comparison of these patterns for different alternatives, it is still hard to state which alternative induced the most favourable sedimentation in the basin (e.g. comparing a few fragmented sedimentation spots at different locations).
To overcome the above mentioned problems, the area of interest has been divided into rectangular sections. The main area of interest, the inlet, is covered by even smaller sections. The sides of these rectangular sections consist of cross•sections and coincide with the grid lines of the computational grid. Each section enclosed by four cross•sections is called atile, for which thetransports over its sides are recorded during the simulation (T•Tile is a combination of the words transport and tile).
The advantages for this method are: § Easiness to compare the effects of alternatives mutually.E.g. comparison of alternatives can be based on the amount of sedimentation in the basin, the tiles can more easily be compared than fragmented spots of sedimentation and erosion; § Arrows indicate direction of transport, especially relative differences show whether an alternative has the potential to increase sediment transport into the basin; § Active zones can be distinguished by the size of sediment transport arrows and the dark blue and red tiles corresponding to sedimentation and erosion (this also holds for relative differences); § By comparison of size and direction of the sediment•transport arrows at half and at the end of the simulation, it can easily be determined to what extent the simulation period influences the results. E.g. if mutual size and direction do not differ significantly it can be assumed that the triggered morphodynamic effects of the alternative stopped acting.
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3 Phase A; nourishment behaviour under the forcing of tidal flow only
3.1 Introduction
This chapter presents the results of Phase A. Phase A of this study considers the interaction of nourished sediment with the morphodynamics of the Frisian Inlet, due to tidal flow only. First a nourishment placing strategy is determined. Next two situations are studied: (1) a situation with a “fixed bottom” whereby only the nourished sand is free to move and (2) a “fully” morphological situation. Results of both situations will be presented, analysed and discussed. Preceding Phase B and Chapter 4 of this research all simulations and model settings are summarized in Table 3•1 (also found in Appendix G).
Table 3•1: Summary of research phases and model settings
Research Phase A A B
Simulation type “Fixed bottom” “Fully” morphological “Fully” morphological with tide•only with tides and waves Autonomous situation no yes yes modelled Studied alternatives 1 • 7 3, 5, 6 and 7 3, 5, 6 and 7 Type of boundary water levels water levels water levels conditions (harmonics) (harmonics) (harmonics), morphological wave climate* Time span used water 1st of Jan. 2003 – 1st of Jan. 2003 – 1st Jan. 2003 – levels 27th of May 2003 27th of May 2003 27th March 2003 Morphological scale 24 24 1•13 factor Modelled time span 9.5 years 9.5 years 1 year Analysed time span 9.5 years autonomous situation: 1 year 9.5 years. the alternatives :3 years Attended in thesis see section 3.3 see section 3.5 see section 4.4 Documented results Appendix I Appendix J Appendix K
(* See Appendix D)
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Figure 3•1: Residual currents, vectors on the left, magnitude on the right [m/s].
Figure 3•2: Residual total transports, vectors on the left, magnitude on the right [m3/m/s].
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3.2 Nourishment placing strategy
To determine a nourishment placing strategy the model is run for a morphological period of a year. The bathymetry of the year 2000 is used and only water levels are prescribed at the boundaries. For this so called reference computation7 residual flow and transport patterns are depicted in Figure 3•1 and Figure 3•2.
The residual current and transport patterns from the reference computation are used as a basis for the nourishment strategy. For a relatively quick redistribution of sand, we aim to identify zones with high transport rates. Despite the innovative character of the smart nourishment concept, not all locations are suitable for nourishment. Practical limitations during placement should be taken into accounte.g. ( in The Netherlands shore face nourishments are difficult to execute for a trailing suction hopper dredger at a water depth less than 4 meters). Keeping the tidal amplitude and the experimenting character of this study into account some flexibility is allowed and nourished sediment is not placed above the •3.5 m. NAP level.
Figure 3•3 shows the locations of the seven nourishment alternatives (labelled:SUPL001, SUPL002, … , SUPL007). All nourishment alternatives have a volume of 2 million m3, and composition of the sediment is identical to the existing bed material.
Figure 3•3: Locations of the nourishment alternatives
7 One additional simulation in Phase A is carried out for a morphological time of one year (1 month with morphological scale factor of 12, see Appendix H). This is done in order to determine the Nourishment placing strategy.
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Figure 3•4: The Frisian Inlet with names of its beaches, shoals and flats.
Figure 3•5: Sediment transport 3[10m3] and sedimentation and erosion [m] for alternative 1 on a fixed bottom. The green line indicates the nourishment location. Arrows scaling is linear vs. the square root of the transport for respectively length and width.
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3.3 Behaviour of nourishment alternatives on a “fixed bottom”
3.3.1 Setup nourishment alternatives on a “fixed bottom”
In accordance with Stage 6 of Phase A, the nourishment alternatives are first investigated on their direct effects viz. ( what happens if only the nourished sand is free to move). To compare the alternatives on their capability of increasing sediment fluxes through the inlet and deposition of sediment in the basin, model runs with a so called “fixed bottom” are executed. The bathymetry of the model is kept morphologically inactive except for the sand of the nourishment itself. These results should give insight in the behaviour of the nourished sand under the forcing of the tide and answer the question: Is the nourished sand being picked up by tidal flows and transported into or in the direction of the basin? It should be noted that this method does not asses the indirect effects of a nourishment, but makes a selection of the potential effective alternatives possible.
3.3.2 Results of alternatives on a “fixed bottom”
The nourishment alternatives are implemented in the model environment by means of altering the bathymetry input file. All simulations are executed with the tidal boundary conditions presented in Chapter 2. The simulations cover a morphological period of 9.5 years (125 neap•spring cycles) whereby morphological developments are accelerated with the use of a morphological scale factor of 24. All alternatives will be shortly discussed with the use of a graphical presentation of their effects resulting from the T•Tile analysis (see section 2.11). Moreover the names of both inlets and shoals are given in Figure 3•4 and used to point out site specific results. The results of alternative one (see Figure 3•5) will be used to explain the interpretation of the figures in more detail. More detailed information on the simulation results can be found in Appendix I.
Behaviour of nourishment alternative 1 on a fixed bottom
Figure 3•5 shows the results of alternative 1. The colours red and blue respectively indicate sedimentation and erosion. The arrows indicate the amount and direction of the cumulative sediment transport over the sides of the tiles at the end of the simulation. The blue tile clearly indicates the erosion of nourished sediment. Erosion in the light blue coloured tile is milder and sediment transport is more in southern direction. Tidal currents transport the sediment in eastern direction (in accordance with residual currents over the ebb•tidal delta). Sedimentation spreads out over the three adjoining tiles east of the nourishment location. The bright red tile shows where most of the sediment is deposited.
For alternative 1 the following can be stated: the sediment is transported by tidal currents in eastern direction. There is very little transport into the basin due to nourishment alternative 1. Nourishment alternative 1 is seems to be by•passed by the system and does not result in sedimentation within the basin.
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Figure 3•6: (a) Results for alternative 2 (SUPL002) (b) Results for alternative 3 (SUPL003). (c) Results for alternative 4 (SUPL004). (d) Results for alternative 5 (SUPL005). (e) Results for alternative 6 (SUPL006). (f) Results for alternative 7 (SUPL007). [transports in 103 m3 at end of simulation]. The green line indicates the nourishment location. Arrows scaling is linear vs. the square root of the transport for respectively length and width.
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Behaviour of nourishment alternatives (2•7) on a fixed bottom
Alternative 2 is a nourishment placed on the central part of the ebb•tidal delta. The results for the simulation with a fixed bottom show that there is little sedimentation in the basin (see Figure 3•6.a). There is some transport into the basin but most sediment is by•passing the system, as was the case for alternative 1.
The behaviour of alternative 3 on a fixed bottom is depicted in Figure 3•6.b. The nourishment is located at the marginsof the ebb•tidal delta close to the Pinkegat Inlet. Figure 3•6.b shows a clear increase of sediment transport into the basin and deposition of sediment within the basin. Under the influence of tidal currents alternative 3 has the potential of resulting in a favourable deposition of sediment in the basin.
The effects of alternative 4 (see Figure 3•6.c) are comparable to those of alternative 2. The nourished sediment at the centre of the ebb•tidal delta is mainly being by•passed. A small part of the sediment enters the basin via the Zoutkamperlaag inlet and results in sedimentation in the basin.
Alternative 5 is situated downdrift of alternative 3 and has a more compact shape. The erosion of alternative 5 is clearly visible in the four blue tiles (Figure 3•6.d). The nourished sediment is picked up by the tidal flows and transported through the Pinkegat Inlet into the basin. The yellow tiles show favourable sedimentation. It should be noted that a part of the sediment also by•passes the ebb•tidal delta and is deposited at the eastern side of the ebb• tidal delta. Moreover a small increase in import can be noted for the Zoutkamperlaag inlet
Alternative 6 (see Figure 3•6.e) is located in the ebb channel of the Zoutkamperlaag. Tidal flows transport the sediment out of the inlet. Ebb flows seem to have a more distinct effect on the alternative as sediment transport is more out of the basin instead of into the basin. Sedimentation within the basin is established but marginal.
Alternative 7 is situated in the ebb•channel of the Pinkegat Inlet. The nourished sediment is picked up by the tidal currents and transported into two directions: outwards to the ebb•tidal delta and inwards into the basin. Compared to alternative 6 the increase in import is dominant and deposition of sediment within the basin is clearly visible.
3.3.3 Selection of high potential alternatives
The preceding results show that some alternatives have the desired effect of sediment deposition in the basin. There are however still some additional remarks to be made. It could be questioned whether the simulation period is long enough to asses the effects of all alternatives correctly. According to the main objective a relatively quick redistribution of sand is needed. And although alternative 1, 2 and 4 could prove to cause a stronger increase of sedimentation benefiting the basin on the long term, they are not classified as potential options. The alternatives 3, 5 and 7 show a quicker redistribution of sand benefiting the basin. Furthermore it is remarkable that six out of the seven results show a sedimentation spot at the eastern tip of the ebb•tidal delta. All sediment passing the region seems to meet favourable conditions for settlement and is deposited in this tile and thereby extracted from
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residual sediment transport paths. This suggests that the used bottom topography and hydraulic boundary conditions (only tide instead of tide and waves) are not in equilibrium. Apart from this, changes in the morphodynamics are not included. It can not be said in advance to what extent this interaction influences the behaviour of the nourished sediments. Clearly there are some shortcomings of the fixed bottom simulation, but for the effective management of available computational power a selection of alternatives is needed. Based on the criterion of a relative quick redistribution of nourished sand favouring the basin, alternative 3, 5 and 7 are chosen as high potentials. Alternative 6 is also included in this list in order to compare its behaviour with the other “channel alternative” (alternative 7) for a full morphological simulation.
3.4 Analysis of morphological behaviour autonomous situation
Before modelling the four selected alternatives a computation is made which models the autonomous development of the inlet (for a period of 9.5 years under the forcing of the tide).
3.4.1 Sediment transport in the Frisian Inlet (tide•only)
The sand balance for the Frisian Inlet showed the continuous net influx of sediment. The Delft3D tide•only model calculations could reproduce this importing behaviour on a modest scale (Appendix B). In order to study the sediment transport patterns and model behaviour the correlation with the residual current pattern will be investigated. Van de Kreeke and Robaczewska (1993) suggest that the net long•term tidal transport through inlets essentially depends on: 1: Stirring of sediment by the leading tidal constituent M2 and subsequently transport by the residual current M0 defined as time independent flow duee.g. to tide• topography interactions; 2: Tidal asymmetry in the form of the generation of overtides by tide•topography interaction.
As it is quite comprehensive to separate sediment transport forced by residual currents and tidal asymmetry from Delft3D model results, the analysis of sediment transport at the ebb• tidal delta confines to sediment transport induced by current velocities. It is assumed that the residual current field induces the residual sediment transport (including both tidal residual current and tidal asymmetry driven transport). Changes in the residual current velocity pattern are used to analyse the initial effects caused by the alternative. Subsequently changes in sediment transports are used to analyse the morphologic behaviour of the nourished sand and the ebb•tidal delta.
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Figure 3•7: (a) Residual depth averaged velocities averaged over three years. (b) Magnitude of residual depth averaged velocities. (c) Residual total transports averaged over three years. (d) Magnitude of residual total transports. (e) Yearly averaged wave energy dissipation. (f) Yearly averaged sediment concentrations.
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For the model simulation of the autonomous development Figure 3•7.a.b depicts the mean depth averaged velocities. Tidally averaged flows from west to east at the North Sea•side of the islands are disturbed by the protrusion of the ebb•tidal delta and accelerate around the edge of the delta. Within the basin the ebb flow dominates the ebb channel of the Zoutkamperlaag and the Pinkegat. The Pinkegat channel is partly flood dominated at the east side while the western branch is more ebb dominated. For the Zoutkamperlaag a distinct flood dominated area can be distinguished at the western side of the island Schiermonnikoog. The upstream part of the Zoutkamperlaag main channel is flood dominated.
During each tidal cycle enormous amounts of sand are transported in and out of the Frisian Inlet. Figure 3•7..c.d shows the residual total transports for the autonomous situation. The total sediment transport is the summation of bed load and suspend sediment transport. Bed 3 load transport is directly dependent on the current velocity (Sbed~u ) and therefore follows the residual velocity pattern. Transport of suspended sediment does not directly follow the residual current pattern and is influenced by different processes.E.g. the exchange of sediment between the bed and the flow and the settling velocity of sediment under the action of gravity (settling•lag effect, was described by Van Straaten & Kuenen (1957) and later elaborated by Postma (1961)).
The main channels of the Pinkegat and Zoutkamperlaag are ebb dominated and transport sediment to the periphery of the ebb•tidal delta. This mechanism of outward building of the delta by tidal currents is also referred to as “sprinkling” (Steijnet al., 1992). The residual sediment transport (Figure 3•7.c.d) shows export of sediment through the main channel of the Zoutkamperlaag and import at the west side of Schiermonnikoog. Furthermore noticeable transport occurs at the eastern tip of Ameland, where tidal flows erode the shallow area and form a small new flood channel. This feature strikes well with reality since similar observations have been made in the field. The migration of the Pinkegat is extensively elaborated in Oost (1995) 8.
8 Cyclic development of the Pinkegat Inlet: (1) A single inlet shifting to the east and development of a sandy shoal at the down drift end of Ameland. (2) Formation of a multiple inlet configuration; erosion of the sandy shoal (new flood channel). (3) Abandonment and merger of inlets; formation of a new single inlet and the development of the sandy shoal.
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Figure 3•8: (a) Mean total transports for the autonomous situation and (b) corresponding magnitudes. (c) Mean suspended transports for the autonomous situation and (d) corresponding magnitudes. (e) Mean bed load transports for the autonomous situation and (f) corresponding magnitudes.
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Figure 3•8.a.b shows the total residual transport over the sides of the T•Tiles. Aggregation reveals some of the main features of the sediment transport in the FrisianE.g. Inlet. bypassing of sediment over the ebb•tidal delta, ebb dominant transport in the main channel of the Zoutkamperlaag, import at the flood dominant area (west side of Schiermonnikoog) and import of sediment within the basin. To determine the contribution of suspended and bed load transport to the total transport, both are shown in Figure 3•8.c•f. Figure 3•8.c.d shows that suspended transport is clearly dominant. Note that the scale used in Figure 3•8.e.f is even four times smaller. Tidal flows seem to induce little motion at the bed and consequently do not induce any significant bed load transport.
Figure 3•9: (a) Schematisation of the overall pattern of residual velocities and (b) Schematisation of the overall pattern of residual transports. Where indicates an ebb dominated area, indicates a flood dominated area, indicates littoral drift.
3.4.2 Model behaviour and field observations
One morphological feature was already related to observations in the field (development of a new flood channel in the Pinkegat, in the previous section). If we continue to look at the bathymetrical features of the whole inlet, first the time span for which bathymetrical changes are realistic needs to be determined. The sedimentation and erosion figures of the Frisian Inlet after 1, 3, 4 and 9.5 years model simulation are presented in Appendix J. The bathymetry of the model after 9.5 years simulation shows the pronounced development of a multi channel inlet (see Figure 3•10). Although the formation of some additional flood channels (e.g. in the Pinkegat inlet) can be explained, the multi channel configuration which is present after 9.5 years model simulation does not stroke with reality at all. The formation of multiple channels over the ebb•tidal delta, a second smaller channel parallel to the Zoutkamperlaag main channel and an additional channel over the beaches of Schiermonnikoog, are features which have not been observed before (based on Oost, 1995).
Intermediate bathymetries (between initial and 9.5 years) show the gradual development of multiple channels and deepening and narrowing of the Zoutkamperlaag main channel. The decision is made to consider the morphological development until three years as realistic. The bathymetry after 3 years still shows a good resemblance to the bathymetry after one year (which is considered to be a realistic development from the initial bathymetry).
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Figure 3•10: Bathymetries of the Frisian Inlet model after: 0 (initial bathymetry), 1, 3 and 9.5 years.
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Although the bathymetry after four years could be acceptable as well, from a model point of view, results for the nourishment alternatives are distinct enough to be mutually compared (see next sections).
3.5 Morphological behaviour of the nourishment alternatives
3.5.1 Setup of morphological simulations
In this section all model results of the high potential alternatives are evaluated. The four selected alternatives (3, 5, 6 and 7) are simulated with a morphologically “fully active” bottom. A comparison of the results of the simulation of the autonomous development and the simulation of the alternatives shows the relative effects induced by the nourished sand. Results are presented using figures following from the T•Tile analysis and vector plots of induced differences in sediment transport and current velocity. It should be noted that in these figures relative effects are depicted. The yellow arrows indicate increased sediment transport and its direction (relative to the autonomous situation). Figures of induced sedimentation and erosion are obtained by subtraction of the bathymetry of the autonomous situation (end result) from the bathymetry of the alternative. In this way the nourished sand remains visible. Additional documentation of model results for all alternatives (including induced changes after one year) can be found in Appendix J.
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Figure 3•11: (a) Residual depth averaged velocities averaged over three years. (b) Residual total transports averaged over three years. (c) Induced changes compared to the autonomous situation in residual depth averaged velocities. (d) Induced changes compared to the autonomous situation in residual total transport. (e) Induced changes in bathymetry after three years [m]. (f) Absolute changes in total transport after three years [103 m3]. The initial location of the nourished sand is indicated with a green dashed line.
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3.5.2 Nourishment alternative 3
The presence of alternative three at the west side of the ebb•tidal delta causes an increase in residual tidal currents north of the nourishment location. West of the nourishment location residual currents are slightly reduced (Figure 3•11.a.c). Changes in residual transports are also mainly present at the west side of the ebb•tidal delta. Figure 3•11.d depicts the difference in residual transports for alternative three and shows changes mainly occur at the shallow area between the nourishment and the main channel of the Pinkegat (northern part). The magnitude of changes is one order smaller than the magnitude of residual current velocities and sediment transports.
The depth and location of alternative three account for the small effect it has on tidal flows. Transport patterns change at the northern tip of the alternative and cause transport in onshore direction, aggregation of the differences in mean transports over the sides of the T• Tiles shows that changes in residual transport are marginal overall9. From here it can also be concluded that a large part of the sedimentation at the shallow area between the channel and nourishment location comes from erosion of the channel. An explanation could be that the nourished sand obstructs tidal currents and thereby reduces the interaction of these currents with ebb currents in the channel.Viz. ebb currents spread more freely out as they leave the Pinkegat ebb•channel. This results in a more enhanced sediment transport component to the west (lee side of the nourishment).
The absolute effects after three years (shape of nourished sand) are comparable to the effects after the first year10. After three years effects are more pronounced and show the ongoing onshore movement of nourished sand. The sand at the shallow area flanking the Pinkegat area causes flow patterns to alternate slightly during ebb and flood. This causes the northern part of the Pinkegat ebb•channel to erode faster and change to a more westerly orientation compared to the autonomous development.
The relative effects on sedimentation and erosion patterns are small, on the ebb•tidal delta as well as within the basin. Sediment transports are slightly altered over the ebb•tidal delta. Through the inlet and within the basin effects can be distinguished but are negligible. Overall it should be noted that the effects observed for the “fixed bottom” configuration are not reproduced at all. The import of sediment for alternative 3 (Figure 3•6.b) disappeared and resulted in a reduction of import in the first three years.
9 Based on documented results for the alternatives in Appendix J 10 Based on documented results for the alternatives in Appendix J
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Figure 3•12: (a) Residual depth averaged velocities averaged over three years. (b) Residual total transports averaged over three years. (c) Induced changes compared to the autonomous situation in residual depth averaged velocities. (d) Induced changes compared to the autonomous situation in residual total transport. (e) Induced changes in bathymetry after three years [m]. (f) Absolute changes in total transport after three years [103 m3]. The initial location of the nourished sand is indicated with a green dashed line.
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3.5.3 Nourishment alternative 5
Effects on residual velocities and transports patterns are comparable, but due to the more humplike shape of the alternative (increase in height compared to alternative 3) the interaction is established on a smaller time scale. Changes in residual currents show a circular pattern around the nourishment. This indicates that tidal currents increase north of the nourishment location during flood while during ebb, currents increase south of the nourishment location. Differences in sediment transport do not show the circular pattern, because at deeper waters less additional sediment is stirred up. Changes are mainly present at the shallow area between the Pinkegat inlet and the nourishment location.
The changes in the residual transport patterns cause the nourished sand to move in onshore direction. Apart from the onshore movement the sediment is also spread out over the ebb• tidal delta (along with the littoral drift direction). It should be noted however that part of the deposited sediment also comes from additional erosion of the Pinkegat which is exported to the ebb•tidal delta.
The export of sediment from the Pinkegat to the ebb•tidal delta was not very well visible after the first year11 but Figure 3•12.f clearly shows how sediment leaves the Pinkegat (arrows in export direction). This is a remarkable difference with the nourishment behaviour on the “fixed bottom”, where there was import into the Pinkegat. Next to the export from the main channel, some import is established at the eastern side of Ameland. The presence of the nourishment results in a small increase of sediment transport at the end of Ameland into the basin. Local increased sediment transports help the development of the new flood channel.
The direct effects of the nourishment are concentrated around the nourishment itself and thereby do not result in sedimentation within the basin. The indirect effect of erosion at the tip of Ameland is the only contribution to favourable sedimentation within the basin. Although sediment transport is increased, sedimentation in the basin is still marginal. The effects of alternative 5, after three years, still seem to be concentrated at the ebb•tidal delta (see Figure 3•12.e).
11 Based on documented results for the alternatives in Appendix J
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Figure 3•13: (a) Residual depth averaged velocities averaged over three years. (b) Residual total transports averaged over three years. (c) Induced changes compared to the autonomous situation in residual depth averaged velocities. (d) Induced changes compared to the autonomous situation in residual total transport. (e) Induced changes in bathymetry after three years [m]. (f) Absolute changes in total transport after three years [103 m3]. The initial location of the nourished sand is indicated with a green dashed line.
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3.5.4 Nourishment alternative 6
Alternative 6 is located in the ebb channel of the Zoutkamperlaag. Most striking are the changes in residual currents. The pattern of changes in residual current velocities (Figure 3•13) depicts how ebb currents increase. Nourishing the ebb•channel reduces the cross• sectional. This is a relative larger reduction of cross•sectional area at low water than at high water. The same holds for the influence on ebb and flood currents and explains why ebb current velocities show a stronger increase at the shallow area next to the nourishment location.
Figure 3•13 shows that residual sediment transport is not directly related to the residual current velocity. As differences in residual currents (at the shallow area next to the nourishment) show an increase in ebb direction, differences in residual transports evidently show an increase in flood direction (import). Tidal asymmetry in the form of duration of flood and ebb at the North Sea (duration of ebb is longer) cause flood currents to be larger than ebb currents in the inlet. Although the ebb period is longer, the flood period contains larger velocities and thereby larger instantaneous transports, due to the non•linearity between flow and transport. This accounts for the effect that although residual currents are directed outward, import can be present. Alternative 6 does not cause the tidal asymmetry to change, but enlarges the net influx of sediment at the Westershand shoals.
Figure 3•13.e presents the effect of alternative 6 on the bathymetry. It shows that effects are significantly larger than those of the alternatives 3 and 5. The yellow band in the inlet shows the remaining sediment in the ebb•channel of the Zoutkamperlaag. This indicates that the nourished sediment is spread out in two ways. Sediment is transported to the ebb•tidal delta and into the basin. In the first year mainly thedirect effects were visible (direct export and import of nourished sediments during ebb and flood)12. Figure 3•13.f shows the absolute effects after three years. Thedirect import and export components remain whileindirect effects develop in the form of import at the Westershand shoals.
The import of sediment increased after the first year, but most of the sediment is exported benefiting the ebb•tidal delta. Favourable sediment transport into the basin is established, which does result in a fair amount of deposition within the basin. Alternative 6 shows that the behaviour on a fixed bottom is not representative for the morphological behaviour of the inlet with the presence of a nourishment. The “fixed bottom” configuration showed marginal import (Figure 3•6.e) in contrast to Figure 3•13.f which shows that a channel nourishment is a promising option.
12 Based on documented results for the alternatives in Appendix J
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Figure 3•14: (a) Residual depth averaged velocities averaged over three years. (b) Residual total transports averaged over three years. (c) Induced changes compared to the autonomous situation in residual depth averaged velocities. (d) Induced changes compared to the autonomous situation in residual total transport. (e) Induced changes in bathymetry after three years [m]. (f) Absolute changes in total transport after three years [103 m3]. The initial location of the nourished sand is indicated with a green dashed line.
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3.5.5 Nourishment alternative 7
Nourishment 7 located in the ebb•channel of the Pinkegat has an influence on the residual current pattern up to 5 cm/s (Figure 3•14.c). Distortions mainly occur at and north of the nourishment location. Although a distinct pattern is hard to distinguish, the effect it has on residual currents is significantly larger than for the other alternatives.
Figure 3•14.d depicts the difference in residual transports at the ebb•tidal delta and inlet for alternative 7 compared to the autonomous situation. Noticeable changes can be distinguished north of the alternative in the Pinkgat channel. Furthermore, parallel to the alternative and at the tip of Ameland changes in sediment transport are in inward direction. Figure 3•14.d shows that the effect of the alternative on residual sediment transports is an order larger than those induced by the alternatives 3 and 5.
As ebb currents and transports are dominant at the nourishment location, most of the sediment is transported outward, although during flood a smaller part of the nourished sediment also finds it way into the basin. Placing a nourishment into the channel has a distinct effect on the tidal currents (as was the case for alternative 6). Tidal currents stir up the sediment blocking their path it into suspension. This can also be seen from the averaged increase in bed shear stress13.
Interesting to see is how induced changes evolve after approximately one year. The nourished sediment first causes an increase in sediment transport away from the inlet (after the first year)14. The sediment however does not leave the inlet, but is transported in western direction where it is deposited in front of the inlet again. From here sediment is picked up again by tidal flows and transported inwards. Furthermore the alternative reduces the cross• sectional area of the inlet and causes flood flows to increase at the tip of Ameland. This results in a small increase in current velocities during flood in the new formed flood channel at the tip of Ameland. Erosion of this channel results in import of sediment.
Figure 3•14.f shows changes in sediment transport (aggregated) and differences in bathymetry due to alternative 7 after three years. The effects are of the same order of those triggered by alternative 6 and almost an order larger than those of the alternatives 3 and 5. The effects of this alternative are characterized by direct effects due to the placement of sediment in the channel and indirect effects due to the interaction of the altered bathymetry with the hydrodynamics (e.g. sediment at the ebb•tidal delta being re•imported).
3.5.6 Analysis of morphological behaviour of the nourishment alternatives
The distinct effects of alternative 3 and 5 (for the fixed bottom simulation see section 3.3) are smaller when their interaction with the morphodynamics is included. Taking only the tidal currents into account has a small effect on the alternatives 3 and 5. Alternative 6 and 7, both located in an ebb•channel, show to have a significant effect on the amount of sediment transport in the inlet and at the ebb•tidal delta. For a simple comparison of the effects of the
13 Based on documented results for the alternatives in Appendix J 14 Based on documented results for the alternatives in Appendix J
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alternatives first the absolute numbers for sedimentation volumes benefiting the basin are shown in Table 3•2. Table 3•2: Favourable sedimentation within the basin due to the alternatives for 1st and 3rd year [103 m3] 15.
Alternative 3 5 6 7 Gain in relative sedimentation loss of 18 6 1851 1118 within the basin after the first year year st
1 Correction* 1550 500 Gain (corrected) loss of 18 6 300 600 Gain in relative sedimentation 0 61 1665 1168 within the basin after three years year rd
3 Correction* 1200 400 Gain (corrected) 0 61 500 800
* Approximated correction for remaining nourished sediment in the channel Alternative 7 shows to have a prominent impact. Since alternative 6 and 7 are located partly within the basin, the gain is partly caused by the alternatives themselves and can therefore not be seen as a profit. Corrected numbers show that the alternatives 6 and 7 are the most efficient in establishing sedimentation within the basin after 3 years. Alternative 6 and 7 establish a gain of respectively c. 0,5·106 and 0,8·106 m3 which is equal to roughly 25 and 40 percent of the initial nourished volume. Nourishing the channels of the tidal inlet evidently shows to be a rewarding option.
Foregoing sections showed that placing sand at the ebb tidal delta hardly influences the tidal currents and sediment transport. In the channel however, nourished sand reduces the cross• sectional area. This causes an increase in bed shear stress which account for turbulence and the stirring up of sediment. Current velocities increase at the nourishment location which result in additional export and import.
Within the time span of three years, only the nourishment alternatives within the ebb• channels showed to have significant impact on the morphodynamics of the Frisian Inlet. Alternative 7 even showed that next to a direct interaction also an indirect interaction is possible. Exported sediment form the Pinkegat channel is re•imported in a later stadium.
The tide•only simulations indicate that nourishing the channel is the most effective measure. This is based on the following: · From a quantitative point of view nourishing the channel results in the most favourable deposition within the basin; · A channel nourishment results in direct import with flood currents; · Exported sediments (to the ebb•tidal delta) are possibly re•imported; · A channel nourishment is “morphologically active”. It causes a quick redistribution of nourished sand due to the large effect it has on tidal currents. Furthermore, the reduction
15 Based on documented results for the alternatives in Appendix J
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of the wetted area of the channel accelerates the autonomous development of a new flood channel at the tip of Ameland.
3.6 Conclusions on nourishment behaviour under the forcing of the tide
A nourishment placing strategy has been established based on active zones. Active zones are defined as zones where residual currents and sediment transports are relatively high. Placing a smart nourishment in these zones should make it easy for the nourished sand to interact with the existing morphodynamics.
In order to compare the seven nourishment alternatives, first their efficiency of redistributing the nourished sand is investigated. With the means of a morphological inactive bathymetry the behaviour of only the nourished sand under the forcing of the tide is made comprehensible. These simulation results indicate that the alternatives, 3, 5, 6 and 7 have the potential of increasing sedimentation benefiting the basin.
The results of the fully morphological simulations for these four alternatives however, show that effects triggered by the alternative in a fully morphological simulation are hardly comparable to their “direct” effects on a fixed bottom. The selection of the two alternatives located in the inlet seems to be a good choice, as their effect on the morphodynamics is significant. The selection of the two alternatives at the west side of the ebb•tidal delta is less satisfactory, as deposition of sediment within the basin is only marginally.
The research objectives are set out to determine the physical behaviour of the nourished sand at the ebb•tidal delta. The alternatives in the channel proved to show a strong interaction with the existing morphodynamics. It is the expectation that the alternatives at the ebb•tidal delta are dependent on wave action for their interaction. The alternatives 1, 2 and 4 can not be ruled out with 100% certainty, when optimal favourable sedimentation within the basin is considered. For research of the physical interaction under the forcing of tide and waves only the alternatives 3, 5, 6 and 7 will be studied. In this way the model results for the tide•only simulation can be compared to the tide and waves results. Furthermore due to the available computational power not all alternatives can be modelled, which makes the choice for the four selected alternatives reasonable.
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4 Phase B; nourishment behaviour under the forcing of tidal flow and waves
4.1 Introduction
The set alternatives selected in Phase A (Chapter 3) are studied on their behaviour under the combined forcing of tides and waves. The different stages of Phase B and corresponding model results are consecutively discussed in this thesis16. After a brief introduction of the model changes, needed to simulate waves, the modelled autonomous development of the Frisian Inlet is discussed in section 4.3. Hereafter the model results of the alternatives are subject to a morphodynamic analysis. This includes a comparison with their behaviour in Phase A to determine the distinct contribution of waves and the contribution of the interaction of tides, waves and nourished sand (section 4.4).
4.2 Setup of one year morphodynamic analysis
The Online•WAVE module of Delft3D is included to simulate the effects of waves cf.( WL | Delft Hydraulics, 2005). In addition to altering the input by means of specifying the modelled wave conditions, the wave module also requires a new grid for the calculation of wave propagation. Wave conditions are prescribed at the boundaries.
Model simulations with a set of chosen wave conditions show that the morphological time scale is significantly smaller than the time scale of Phase A (1 to 3 years). Including waves within the model results in significant changes within a month. Distinct relative changes induced by the nourishment alternatives are expected to show within a year when waves are included. For this reason and to easily compare the results with the tide•only model (using one year as typical time unit) the simulation period for the morphodynamic analysis with waves and tide is set to one year.
16 For a brief summary of research phases and model settings one is referred to Appendix G.
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Figure 4•1: (a) Residual depth averaged velocities. (b) Magnitude of residual depth averaged velocities. (c) Residual total transports. (d) Magnitude of residual total transports. (e) Yearly averaged wave energy dissipation. (f) Yearly averaged sediment concentrations.
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4.3 Analysis of morphological behaviour of the autonomous situation
4.3.1 Sediment transport in the Frisian Inlet (tide and waves)
Similar to the analysis of sediment transport for the tide•only case results, first residual depth averaged velocities are presented. Figure 4•1.a.b shows the yearly depth averaged velocities at the ebb•tidal delta of the Frisian Inlet for the autonomous situation (tide and waves). Compared to the tide•only model (Figure 3•7.a.b) the following can be seen: (1) enhanced flood dominance at the eastern tip of Ameland, due to wave induced currents, (2) the overall pattern within the basin does not change, apart from a small increase of residual currents at the flats of Smeriggat, south of the Engelsmanplaat. These result from local induced waves during moderate and strong wave and wind conditions. Although the pattern remains comparable, the residual depth averaged velocities are slightly higher (0.15 m/s instead of 0.1 m/s at the margins of the ebb•tidal delta) compared to the tide•only case. The presence of waves results in a set•up over the shoals at the ebb•tidal delta and increase of residual currents due to water level gradients. Furthermore, wave presence and breaking of waves causes an increase of current velocities over the shoals and beaches of the islands.
Figure 4•1.c.d show the residual transports for the autonomous situation. Compared to the tide•only model some significant differences can be noted. Taking waves into account causes an overall increase of longshore transport and transport around the periphery of the ebb•tidal delta. The inclusion of waves makes ebb and flood transport dominant areas more distinct, e.g. the main channels fulfil the export function while the shoals and marginal flood zones are clearly flood dominant and cause an import of sediment. Furthermore the development of a new flood channel at the eastern tip of Ameland takes place on a smaller time scale when waves are included in the model. Quite striking is the decrease of ebb• dominance and increase of flood•dominance for the shallow channels north of the Engelsmanplaat. Due to the presence of waves, flood flows find it easier to cross over these shallow areas and enter the Zoutkamperlaag channel and basin from this side. A comparison of Figure 4•1.c.e clearly shows how wave energy dissipation drives residual currents and transports in shallow areas.
Stirring up of sediment due to wave presence and wave breaking causes a strong increase of suspended sediment. Viz. wave energy dissipation causes an increase in bed shear stress which accounts for an increase in turbulence and thereby the stirring up of sediment (Figure 4•1.e.f). Sediment from the islands margins is transported by longshore currents to the ebb• tidal delta and bypassed or deposited in the basin. Wave driven currents at the ebb•tidal delta and tidal flows transport the sediment over the ebb tidal delta (bypassing of sediment). While during strong wave conditions part of the sediment is brought back to the islands (see appendix K.8). Moreover, the presence of waves causes the magnitude of residual transports to increase with a factor 5 to 10 compared to the tide•only model (compare Figure 4•2 with Figure 3•8).
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Figure 4•2: (a) Mean total transports for the autonomous situation and (b) corresponding magnitudes. (c) Mean suspended transports for the autonomous situation and (d) corresponding magnitudes. (e) Mean bed load transports for the autonomous situation and (f) corresponding magnitudes.
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The tide•only case showed how sediments are brought to the periphery of the ebb•tidal delta by ebb•dominant flows. This outward•building•mechanism of the ebb•tidal delta is typically contributed to tidal flows. Including the effect of waves in the simulation shows how this behaviour is balanced. The increase of onshore bed load transport is purely wave driven and is one of the main factors which keeps the ebb•tidal delta together (Figure 4•2.c.d.). Although bed load transport is still a factor five smaller on average than suspended transport, bed load transport is always directed onshore.
The increase in sediment transport (including waves) at the same time accounts for altered morphological changes. The bathymetry after one year for the autonomous situation (Figure 4•3) shows how tide and waves have eroded the shallow area north of the Engelsmanplaat and formed two small channels. Overall it can be noted that all bed forms are more distinct than at the beginning of the simulation. Both the Pinkegat and Zoutkamperlaag ebb channel have become narrower and showed an erosive trend. Moreover, the northern part of the Pinkegat ebb channel has changed orientation form north•west to north. Consequently the location (where ebb flows spread out as they leave the channel and deposit sediment) shows a progression in seaward direction. The most striking feature is the sedimentation of the second (smaller) branch of the Pinkegat channel and the development of a new flood channel at the tip of Ameland. This development could also be seen in the tide•only model but was less pronounced.
4.3.2 Model behaviour and field observations
The Pinkegat is a channel which is morphologically quite active and shows a cyclic behaviour in its development17. The migration of the Pinkegat channel is elaborated in Oost (1995) and shows similarities with the model behaviour (formation new flood channel, sedimentation. other channel). Although the formation of the flood channel can be placed in the Frisian Inlet’s morphological context, the narrowing and deepening of both channels does not match observed historical trends.
After the closure of the Lauwers Sea, sedimentation has dominated the Zoutkamperlaag main channel. In the period 1970•1987 the maximum amount of sedimentation is in the order of 2 meters. The sedimentation and erosion rates given by the model are 1 to 2 meters/year. These are rather high values but can be addressed to the high rate of morphodynamics of a tidal inlet. Comparable values have been found in earlier studies (Steijn and Hartsuiker, 1992). These high values and locally even higher values (up to 5 meters erosion or sedimentation) partly resemble channel migration. High erosion rates (deepening of the channel) are addressed to the fact that the model only contains one fraction of sediment. The choice of mean grain diameter (180mm ) appears to be too small, as schematisation of channel sediments, as channel erosion is not decelerated. In reality the channels contain heavier sediments which limit the amount of erosion.
17 Cyclic development of the Pinkegat Inlet: (1) A single inlet shifting to the E and development of a sandy shoal at the down drift end of Ameland. (2) Formation of a multiple inlet configuration; erosion of the sandy shoal (new flood channel). (3) Abandonment and merger of inlets; formation of a new single inlet and the development of the sandy shoal.
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Figure 4•3: (a) Initial bathymetry of the Frisian Inlet. (b) Bathymetry after one year (tide and waves included in the model)
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The presence of two flood dominant channels at the shoals north of Engelsmanplaat is a feature which was present in the late sixties and early seventies (see Oost, 1995). This indicates that it is not a completely new feature of the system and can be excepted as realistic. However, the configuration of the Pinkegat (distinct shoal at the end of the ebb• channel and orientated to the north) is a feature which has not been observed in recent history.
Apart from reviewing the large scale morphological behaviour of the model and comparing it to field data, there is one other parameter which needs to have a realistic value,viz: the annual sediment import. The amount of import for the tide•only model was almost an order of magnitude smaller than numbers published in earlier studies (Appendix B). The sediment transport significantly increased overall with the inclusion of waves. The same is true for the amount of import. For the cross•section in the inlet throat the residual transport is 3,5 million m3/yr inwards (approximately the same is found for the summation of the bed level changes within the basin). It should be noted that this value is rather high and is strongly influenced by the formation of the new flood channel (1,4 million m3/yr at the eastern tip of Ameland). This amount is only locally transported (sediment which crosses the throat from north to south). Choosing the cross•section behind the inlet throat, the amount of sediment transport into the basin is 1,7 million m3/yr. This is in the correct order of magnitude and shows the model behaviour (sediment•import•wise) is valid.
4.4 Morphological behaviour of the nourishment alternatives
In this section all model results of the selected alternatives are evaluated. The initial location of the nourished sand is indicated with a green dashed line for each alternative. Results are mainly presented by means of induced changes relative to the autonomous situation. An analysis is made of the initial response of flood and ebb flows due to the presence of the nourishment. Furthermore changes in residual current velocities and sediment transport – and their contribution to the net effect after one year – are studied. Sedimentation and erosion figures are obtained by subtraction of the bathymetry of the autonomous situation (end result) from the bathymetry of the alternative (end result). In this way the nourished sand remains visible. Additional documentation of model results for all alternatives can be found in Appendix K.
4.4.1 Nourishment Alternative 3
Compared to the autonomous situation, flood flows increase over the nourishment due to the reduction in depth (see Appendix K). In addition it obstructs the flood flow and increases currents along side of the nourishment and decreases currents behind the nourishment. During ebb a similar pattern is observed, whereby ebb flows are enhanced between the nourishment and the marginal shoals of De Hon.
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Figure 4•4: (a) Residual depth averaged velocities. (b) Residual total transports. (c) Induced changes compared to the autonomous situation in residual depth averaged velocities. (d) Induced changes compared to the autonomous situation in residual total transport. (e) Induced changes in bathymetry after one year. (f) Absolute changes in total transport after one year [103 m3]. The initial location of the nourished sand is indicated with a green dashed line.
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Differences are an order smaller than the instantaneous depth averaged velocities and the characteristics of the overall flow pattern therefore do not change (Figure 4•4.a). Changes in yearly residual velocities show two patterns (Figure 4•4.c): (1) an increase in eastward residual currents north of the nourishment and a decrease south of the nourishment location. (2) The spreading of the Pinkegat ebb•jet is enhanced at the western side of the channel and reduced at the eastern side of the channel (less distinct than pattern one). Both patterns result from the obstruction of tidal flows, as described above.
The initial effects on residual transports during ebb and flood concentrate at the marginal shoals of De Hon and at the shoals of the Pinkegat. Within the channel there is a reduction of transport during ebb while flows increase the transport away from the channel at the edge of the ebb•tidal delta (next to the nourishment). During flood a small increase of transports over the nourished sand can be noted while primarily a reduction of inward transport occurs at the shallow area flanking the Pinkegat18.
The characteristics of the yearly residual transport pattern, which also includes changes due to the feedback of bottom changes on the sediment transports, do not show any distinct differences (Figure 4•4.b). The difference in residual transport (compared to the autonomous situation) shows the induced effects are of an order smaller. Next to current velocity, sediment concentrations (among other factors) determines the magnitude of the residual transports. The increase in wave energy dissipation and the increase of bottom shear stress due to the nourishment alternative results in significantly increased stirring up of sediment19. This subsequently results in an increase of transports over the nourishment and at the shallow area behind it (south of the nourishment location). Reduction of flood flows however also results in a decrease of inward transport behind the nourishment. Combined effects result in a gradual onshore movement of the nourished sand as a whole (Figure 4•4.e).
Aggregated transports (Figure 4•4.f) show that the absolute effects after one year are small but spread out over the entire ebb•tidal delta. The majority of nourished sediments stayed at the margins of the ebb•tidal delta, however changes in hydrodynamics resulted in the export of sediment from the basin (mainly from the Pinkegat, but from the Zoutkamperlaag as well). Note that the absolute changes in sediment transport are about 5% (100 vs. 2000 m3/m/yr) from the annual sediment transports (compare with Figure 4•2). Although the effects are still small for alternative 3, the pattern of relative sedimentation and erosion after one year is different compared to the tide•only model results. For the tide•only run changes in bathymetry were marginal (Appendix J.3). The results of the model including waves shows alternative three is dependent on wave action for the interaction with the existing morphodynamics. The inclusion of waves accounts for a more rapid and diffusive onshore movement of the nourished sediment. On the other hand, effects are still not the equivalent of the “fixed bottom” results.
18 Based on documented results for the alternatives in Appendix K 19 Based on documented results for the alternatives in Appendix K
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Figure 4•5: (a) Residual depth averaged velocities. (b) Residual total transports. (c) Induced changes compared to the autonomous situation in residual depth averaged velocities. (d) Induced changes compared to the autonomous situation in residual total transport. (e) Induced changes in bathymetry after one year. (f) Absolute changes in total transport after one year [103 m3]. The initial location of the nourished sand is indicated with a green dashed line.
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4.4.2 Nourishment Alternative 5
Alternative 5 is located at the northern tip of alternative 3, but further offshore at the 6 m. NAP contour. The shape of alternative 5 is more compact (humplike) and forms an obstacle for tidal flows. The absolute effects are however an order smaller and overall patterns therefore do not change. The same holds for the yearly averaged flow pattern. Compared to the autonomous situation residual currents are accelerated north of the nourishment and decelerated south of the nourishment location (Figure 4•5.a.c).
Differences in residual sediment transports show a similar behaviour (Figure 4•5.b.d). An important additional factor for the onshore transport of sediment for alternative 5 is the bed load transport20. Increased bed velocities due to wave presence at the nourishment location cause a strong increase in bed load transport. Furthermore the increase of wave energy dissipation due to wave breaking at the nourishment location causes a local increase of sediment concentrations due to the stirring of sediment.
Aggregated transports (Figure 4•5.f) reveal the local scale on which the effects due to alternative 5 act. Suspended matter is transported to the east along the ebb•tidal delta as well as onshore due to wave action and tidal flows. Deposited sediment at the ebb tidal delta causes a feedback on the hydrodynamics. The ebb•jet of the Pinkegat channel is constricted by the deposited sediment and is partly diverted to the east. The result is an increase of erosion at the northern end of the ebb channel. The autonomous development of the reorientation of the Pinkgat channel to the east is thereby accelerated. Furthermore, a small increase in wave energy dissipation21 decreases the deceleration of tidal flows as they leave the channel mouth (tidal flows opposite to the wave direction). This effect might contribute to a stronger erosion of the northern end of the Pinkegat ebb•channel.
The overall effects of alternative 5 are moderate but far more distinct compared to the tide• only model. Waves are crucial for the interaction of the nourished sand with the morphodynamics of the tidal inlet. The stirring up of sediment and bed load transport due to waves play an essential role. Tidal flows fulfil the role of transport medium for the stirred up sediment. There is however no significant accretion within the basin after one year.
20 Based on documented results for the alternatives in Appendix K 21 Based on documented results for the alternatives in Appendix K
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Figure 4•6: (a) Residual depth averaged velocities. (b) Residual total transports. (c) Induced changes compared to the autonomous situation in residual depth averaged velocities. (d) Induced changes compared to the autonomous situation in residual total transport. (e) Induced changes in bathymetry after one year. (f) Absolute changes in total transport after one year [103 m3]. The initial location of the nourished sand is indicated with a green dashed line.
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4.4.3 Nourishment Alternative 6
Nourishment 6 is located in the ebb channel of the Zoutkamperlaag. Reduction of the cross• sectional area of the channel results in a restriction of the tidal flows during ebb and flood22. During flood and ebb tidal flows increase at the Westershand shoals (flood dominant). During flood this results in an increase in inward transport over the flood dominant area. During ebb the effects on instantaneous transports are concentrated within the channel and result in additional export. The difference in residual transports with the autonomous situation over the year show that ebb transports over the nourishment are enhanced (Figure 4•6.d). The increase of inward transport over the flood dominant area is less distinct. Furthermore some changes in the sediment transport vectors can be noticed at the ebb•tidal delta, these can be contributed to the feedback of morphologic changes to the water motion and sediment transports.
Changes in mean sediment transport are larger than those triggered by the alternative 3 and 5 and operate on a greater scale (Figure 4•6.f). However, compared to the mean annual transports the effects remain an order smaller. The aggregated transports depict two effects: (1) increase in export north of the alternative and (2) increase in import south of the alternative location. Both are purely tide induced as waves do not influence sand placed at the bottom of the ebb•channel. The overall increase of transports and velocities, due to waves, cause the effects (spreading out of the nourishment) to take place on a smaller time scale but are comparable to those of the tide•only model (see Figure 3•13.e and Figure 4•6.e). Exported sand deposited at the ebb•tidal delta is influenced by wave and tidal flow action and results in slightly altered sediment transport patterns. Figure 4•6.f shows the increase in export and import as well as some small changes in sediment transport over the ebb•tidal delta (to compare with tide•only see Appendix J.5).
The direct effects of nourishment 6 are purely tide (suspended transport) related and show little changes in the bed load transport. Because ebb flows are dominant at the location of the nourishment, redistribution of the nourished sediment mainly occurs to the ebb•tidal delta. Due to the overall increase of sediment transport (compared to tide•only) sediments spread out more rapidly. Positive effects, in the form of sedimentation favouring the basin, are realized by increased transports during flood over the Westershand shoals (west of Schiermonnikoog) as well as increased transports over the nourishment itself.
22 Based on documented results for the alternatives in Appendix K
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Figure 4•7: (a) Residual depth averaged velocities. (b) Residual total transports. (c) Induced changes compared to the autonomous situation in residual depth averaged velocities. (d) Induced changes compared to the autonomous situation in residual total transport. (e) Induced changes in bathymetry after one year. (f) Absolute changes in total transport after one year [103 m3]. The initial location of the nourished sand is indicated with a green dashed line.
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4.4.4 Nourishment Alternative 7
Placing a nourishment in the channel results in a restriction of the tidal flows (as was the case for alternative 6). Changes in residual velocities do not show any distinct patterns and mainly concentrate around the nourishment and at the ebb•tidal delta (west side). The following patterns can be recognized: (1) increase of residual velocities over the nourishment and along the nourishment, (2) increase of ebb flow at the tip of Ameland and (3) two circular patterns north of the nourishment. Changes in residual velocities are an order smaller than the actual residual velocities, the overall pattern therefore does not change (Figure 4•7.a.c).
Aggregated transports (Figure 4•7.f) show a distinct increase of import south of the nourishment and an increase in export north of the nourishment location. It should be noted that a similar pattern occurred for the tide•only model but that this time waves refrain the sediment from moving outward and “steer” it in eastern direction (Figure 4•7.e). The import as a result from alternative seven is twofold: direct effects due to the import of sediment during flood and indirect effects due to a small increase in erosion of the newly formed flood channel at the eastern tip of Ameland. This eroded sediment is partly deposited at the edge of the ebb•tidal delta as well as within the basin. At the edge of the ebb•tidal delta additional deposited sediments are transported onshore (partly as bed load transport) due to the wave presence. The overall contribution of bed load transport remains small23.
The contribution of waves on the direct effects of alternative seven are marginal. Due to the depth and location of alternative 7, local wave dissipation and the corresponding contribution to stirring up of sediment or bed load transport are marginal (Appendix K shows no significant increase in bed•shear stress compared to the tide•only model results). However the secondary effects of waves are noticeable. The changes in morphology cause a feedback on the wave propagation and energy dissipation24. E.g. erosion at the edge of the ebb•tidal delta causes some waves to break just a fraction later and shift the peak of wave energy dissipation tens of metres onshore.
The overall behaviour of induced effects of alternative 7 are comparable with those of the tide•only model (compare to Appendix J.6). Waves enhance the effects and spread out the eroded sediment more rapidly. The main difference is that the positive effect for the tide• only model of bringing back sediment in the tidal inlet circulation can not be identified. On the other hand, after one year this effect was not clearly recognizable for the tide•only model.
23 Based on documented results for the alternatives in Appendix K 24 Based on documented results for the alternatives in Appendix K
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4.5 Conclusions on nourishment behaviour under the forcing of tide and waves
4.5.1 Favourable sedimentation
In the foregoing section the effects of the alternatives were presented. A simple comparison on induced sedimentation and erosion showed that the alternatives located in the channels resulted in a more extensive redistribution of the nourished sediment. Moreover the alternatives showed favourable sedimentation within the basin while the alternatives at the ebb•tidal delta (alternative 3 and alternative 5) caused some small additional erosion within the basin (Table 4•1)25.
Table 4•1: Favourable sedimentation within the basin due to the alternatives for the 1st year [103 m3] 26.
Alternative 3 5 6 7 Gain in relative sedimentation loss of 129 loss of 152 1681 1118 within the basin after the first year year st
1 Correction* c.1400 c. 550 Gain (corrected) loss of 129 loss of 152 c.300 c.600
* Correction for remaining nourished sediment in the channel
The sedimentation within the basin did not always resulted directly from the nourishment itself. Adding sand to the morphodynamics of the tidal inlet system does not mean that only the nourished sand needs to be imported. The empirical relations (Appendix A) consider the ebb•tidal delta and basin as interacting elements who balance their volumes according to these relations. An addition to the volume does not mean only the addition (nourished sediments) is redistributed. The reason why most attention was paid to the nourished sand itself was because the objectives required a quick redistribution of nourished sediment. Furthermore, within the time span of a year effects are still very local, while the empirical relations govern time spans of decades.
Although the volume of the nourishment is marginal compared to the total volume of the ebb•tidal delta (2 million vs. 170 million m3), indirect effects were established (most distinct for alternative 7). Before discussing these indirect effects, first a review of the direct effects and the physical processes which caused these effects is given.
25 Based on documented results for the alternatives in Appendix K 26 Based on documented results for the alternatives in Appendix K
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4.5.2 Direct effects on nourished sediments
For alternatives 6 and 7, both situated within an ebb•channel, the inclusion of waves did not significantly contribute to the initial interaction of the nourished sand with the morphodynamics. These alternatives primarily depend on the tidal flows. The nourishment reduces the cross•sectional area of the channel and thereby increases bed shear stress which accounts for turbulence and the stirring up of sediment. Secondly tidal flows fulfil the role of transport medium and transport the sediment to and from the basin during flood and ebb, whereby a small amount of particles remains in the basin. Sediment deposited on the ebb•tidal delta is influenced in a different way. It either becomes part of the predominant littoral drift (directed from west to east) and migrates slowly to the downdrift island or is directly reworked by wave action. During extreme wave conditions sediments at the edge of the ebb•tidal delta may be transported directly to the islands shore. During moderate wave conditions sediment is stirred up again and becomes available for the tidal currents. From here the cycle starts all over again.
Concluding for alternative 6 and 7, the tide primarily redistributes the nourished sediments to the basin and to the ebb•tidal delta. Within the basin only the tidal flow is of significant influence, while at the ebb•tidal delta a combination of wave and tidal action rework the sediments.
The alternatives 3 and 5 are primarily dependent on wave action. The inclusion of waves within the model significantly increased the erosion of the nourished sediments for both alternative 3 and 5. The direct effect of waves at the nourishment location results in a strong increase of suspended sediments, which subsequently become available for the tidal currents. Furthermore, waves increased the bed load transport at the nourishment location, which resulted in an onshore movement.
4.5.3 Indirect effects of nourished sediments and the morphodynamics of the ebb•tidal delta
Once the sediments moved from their original location different processes may take over. In the case of the alternatives in the ebb•channel it was already mentioned that the sediments transported to the ebb•tidal delta are reworked by wave action. For the alternatives at the ebb•tidal delta it is just the other way around; they are first influenced by wave action and brought into suspension after which tidal flows take over.
In general tidal flows act as the predominant transport mechanism. Waves mainly contribute to the amount of sediment in suspension and thereby making it available for the tidal flows (overview of effects in Table 4•2). Considering second order effects, we have seen that both tidal flow and waves can have their distinct indirect effects on moved nourished sediments and the altered morphodynamics of the ebb•tidal delta.E.g. (1) waves bring the exported sediments back into suspension, or (2) refrain the sediment from moving away from the ebb tidal delta (by means of bedload transport), or (3) exported sediments are (re)suspended and brought back into the tide dominated sediment transport paths.
Also the accelerated development of a new flood channel is seen as an indirect effect. Alternative 7, located in the ebb•channel of the Pinkegat, constricts the tidal flows and
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causes an increase in current velocity and sediment transport at the shallow area flanking the channel. As this process repeats itself it accelerates the autonomous development of the flood channel located at the eastern tip of Ameland. Erosion of the channel then causes additional import. Table 4•2: Physical processes and their direct and indirect effect on the alternatives and morphodynamics of the ebb•tidal delta
physics tide 1st order (positive) direct import when ebb•channel is nourished 1st order (negative) direct export when ebb•channel is nourished 2nd order (positive) re•import of exported sediments 2nd order (positive) transport further up the basin 2nd order (positive) accelerated development of new flood channel waves 1st order (positive) stirring up of sediments at the ebb•tidal delta (bringing sediment into sediment transport paths) 1st order (positive) onshore bedload transport 1st order (negative) bypassing 2nd order (positive) bringing exported sediment back into sediment transport paths by stirring up 2nd order (positive) keeping exported sediments at ebb•tidal delta tide and waves littoral drift
4.5.4 Conclusions on one year morphodynamic behaviour
Overall we need to conclude that the time span of one year is too short to extrapolate the longer term effects of the nourishment alternatives i.e.( it is impossible to say whether an amount of X m3 sand results in Y m3 sand within the basin on the long term). Within the time span of one year we have mainly observed the spreading out of nourished sand. Although no quantitative conclusions can be drawn, it is possible to address the different physical processes behind the diffusion of the nourishment. This assists in drawing preliminary conclusions.
For a fast redistribution of nourished sand it is advised to nourish the channels. This results in a direct profit during flood (import of sediment with flood flows). Exported sediments may be imported in a later stadium when they are brought back into the sediment transport paths (e.g. by wave action).
The alternatives at the ebb•tidal delta depend on wave action. First they need to be stirred up by wave action before they are transported by tidal flows and redistributed over the ebb•tidal delta. Apart from the fact that the redistribution of sand is still very local after one year, the alternatives at the ebb•tidal delta lack the profit of a direct import component, while they behave the same as the exported sediments from the channel nourishments.
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4.5.5 Extension of model calculations
The inclusion of waves showed a small increase in residual velocities and a strong increase in the averaged transports. Compared to the tide•only model this increase resulted in a more rapid redistribution of nourished sediment.
With the use of the Delft3D Frisian Inlet model it is now possible to assess the effects of placing sediment within the ebb•channel (spreading out of sediment to the basin and to the ebb•tidal delta). The simulation of one year is however too short to assess the effects after the initial redistribution. This is especially true for the alternatives at the ebb•tidal delta.
Having the spreading•out effects identified, additional model calculations whereby the sediment is distributed over the entire ebb•tidal delta, are recommended. This in order to “pre•fabricate” the spreading out effects and asses whether nourishing the ebb•tidal delta results in sedimentation within the basin (in accordance with the hypothesis). Moreover the following questions should be answered: · Which processes can be observed after the initial spreading out of nourished sand? · Which forces drive these processes? · To what extent do the volume of the nourishment and the time scale of morphological changes of the inlet interrelate?
Preceding to a follow•up study a model simulation is carried out, whereby the entire ebb• tidal delta is nourished with 10 million3 m sand27. The preliminary results show that a decrease in water depth of 10 centimetres over the entire ebb•tidal delta has a small effect on the residual transports. Although alterations spread out over a larger area, the impact is comparable to that of the nourishment alternatives. Although the nourished volume is significantly larger, again we mainly observe the spreading and rearranging of nourished sand (Figure 4•8).
Figure 4•8: (a) Induced changes compared to the autonomous situation in residual total transport. (b) Difference in bathymetry after one year [m] and absolute changes in total transport after one year [103 m3].
27 The nourishment has a volume of 10 million m3 and is spread out over the entire ebb•tidal delta (equal to 10 cm. additional sand over the ebb•tidal delta). Model calculations cover a year with the forcing of tides and waves.
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Sedimentation within the basin after one year is approximately 0.5 million m3 (see Appendix K.7). Compared to the channel nourishment the effectiveness is even smaller, since a relatively smaller part of the nourishment entered the basin.
Without studying the distinct contribution of tide and waves, it is possible to discuss the dependency of morphological changes on the nourishment volume. The marginality of observed changes in the morphodynamics of the Frisian Inlet due to a smart nourishment can be contributed to two factors: (1) The modelled time span is too short or (2) the nourishment volume is too small, or a combination of the two. It was already concluded that the time span of one year is too short to observe distinct changes (see preceding section). The additional simulation however also shows that the volume of the nourishment (order 10 Mm3) is too small to observe pronounced sedimentation within the basin.
This suggests that it is better to look at the smart nourishment as a part of the integral state variable “ebb•tidal delta” when considering the morphological development of the whole system. The ebb•tidal delta of the Frisian Inlet has a volume of approximately 170 million m3. The addition of a nourishment of 10 million3 m (6% of the whole volume) is only “absorbed” into the whole volume during the first year. This takes place by redistributing the sand over the ebb•tidal delta. During this process the basin may partly be benefited by the spread out sand. For a nourishment at the ebb•tidal delta this is more seen as a side•effect than a direct effect.
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5 Conclusions and Recommendations
This thesis deals with the interaction of a smart nourishment with the morphodynamics of the Frisian Inlet. A model of the Frisian Inlet is set up in the process based modelling environment of Delft3D. Three situations are studied: (1) A situation with a “fixed bottom” and the hydrodynamic forcing of the tide, (2) a “fully” morphological situation with the hydrodynamic forcing of the tide and (3) a “fully” morphological situation with the forcing of tides and waves (overview in Appendix G). For these situations the morphological evolution of the Frisian Inlet with the presence of a nourishment is investigated by means of scenario studies. Conclusions will be drawn by means of reviewing the sub hypotheses posed in Chapter 1.
5.1 The morphological interaction
Sub 1: Tidal flow transports an abundance of sand from the ebb•tidal delta (e.g. a nourishment) partly into the basin. Simulations of the nourishment alternatives for the “fixed bottom” situation showed how nourished sand is transported into the tidal basin. These simulation results however are not representative or indicative for the interacting morphological behaviour of the nourished sand and the ebb•tidal delta. This is explained by the fact that the tidal flows in the fixed bottom situation are unsaturated and pick up sand “too easily”. The morphological tide•only simulation showed that the effects of the nourishments at the ebb•tidal delta after three years are still very local. Sub hypothesis 1 is therefore rejected (defining partly in Sub.1 as >10% of nourished volume within three years). Tidal flows have a far more distinct effect on the alternatives within the channels. Constriction of tidal flows within the channel accounts for an increase in bed shear stress and the stirring up of sediment. Subsequently sediment is transported to the basin and ebb• tidal delta during flood and ebb. When the channels are considered as part of the ebb•tidal delta Sub hypothesis 1 can be regarded as valid.
Sub 2. Waves enhance this effect by their capability of stirring up the sediment and thereby sediment is picked up easier by tidal flows and transported inwards. The inclusion of waves within the model caused a significant increase in sediment transport over the ebb•tidal delta. Waves account for an increase in turbulence and suspended sediment and the tidal currents predominantly fulfil the role of transport medium. Furthermore an increase in wave energy dissipation (compared to the autonomous situation) is observed at the nourishment locations. The alternatives at the ebb•tidal delta proved to be dependent on wave action for a fast interaction with the morphodynamics of the tidal inlet. Absolute effects of the alternatives located at the ebb•tidal delta (forcing of tide and waves 1 year) showed to be comparable to the effects after three years for the tide•only model. Although a faster interaction is established, effects remain to act on a local scale (no significant import). For the alternatives located in the channels, the inclusion of waves did not significantly contribute to the initial interaction of the nourished sand with the morphodynamics. Due to the depth of the nourishment at the bottom of the channel, wave influence is minimal.
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Overall it is concluded that in general tidal flows act as the predominant transport mechanism. But wave presence significantly contributes to the sediment transport over the ebb•tidal delta. Sub hypothesis 2 is regarded as valid, although no significant import is established for the alternatives at the ebb tidal delta. This is attributed to the fact that only a period of one year is simulated.
Sub 3: The nourished sediment interacts with the existing morphodynamics and may thereby change sediment transports through the inlet in a favourable way, this does not necessarily mean sediment from the nourishment itself is transported inward. With regard to the alternatives several indirect effects have been observed. These were the most distinct for alternative 7. Nourishing the Pinkegat channel constricts the tidal flows and causes an increase in current velocity and sediment transport at the shoals next to the channel. This process contributes to the autonomous development of a new flood channel. Erosion of this channel results in import of sediment.
In addition, once sediment moved from its original location, different processes may take over. Considering second order effects, both tidal flow and waves can have their distinct indirect effects on moved nourished sediments and the altered morphodynamics of the ebb• tidal delta. E.g. (1) waves bring the exported sediments back into suspension, or (2) refrain the sediment from moving away from the ebb tidal delta (by means of bed load transport), or (3) exported sediments are (re)suspended and brought back into the tide dominated sediment transport paths. The latter of the three is a form of interaction whereby the nourishment “blends” into the autonomous morphological behaviour of the inlet and could cause additional import. Overall it is concluded that sub hypothesis 3 is valid, although the mechanism has not always been observed in a favourable way. Sub hypothesis 3 is therefore not rejected.
5.2 The “smartest” nourishment
In Chapter 1 we posed the following hypothesis: Hypothesis: An abundance of sand on the ebb•tidal delta, e.g. induced by an artificial nourishment, causes sedimentation in the tidal basin. For a fast redistribution of nourished sand it is advised to nourish the channels. This results in a direct profit during flood (import of sediment with flood flows). Exported sediments may be imported in a later stadium when they are brought back into the sediment transport paths (e.g. by wave action). The alternatives at the ebb•tidal delta are dependent on wave action. First they need to be stirred up by wave action before they are transported by tidal flows and redistributed over the ebb•tidal delta. Apart from the fact that the redistribution of sand is still very local after one year, the alternatives at the ebb•tidal delta lack the profit of a direct import component, while they behave the same as the exported sediments from the channel nourishments.
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Discussion
The model simulation period of one year (for tide and waves) is too short to determine whether the observed favourable sedimentation has a permanent character. Within the time span of one year we have mainly observed the spreading out of nourished sand.
With respect to the channel as element of the inlet system one additional note should be made. The hypothesis posed the option of nourishing the ebb•tidal delta, while the main finding of this research is that a channel nourishment is the most effective measure on the short term. Although both elements are highly related, they are usually treated as separate elements, whereby the channel is the connecting element between the flat area and the ebb• tidal delta. Establishing favourable sedimentation within the basin on the short term, by means of a channel nourishment, is from this point of view reasonably the most effective way. Viz. the sediment is already in the mouth of the basin.
Moreover, the additional simulation (Section 4.5.5) points out that it is better to look at the smart nourishment as a part of the integral state variable “ebb•tidal delta” when considering the morphological development of the whole system. The ebb•tidal delta of the Frisian Inlet has a volume of approximately 170 million m3. The addition of a nourishment of 10 million m3 (6% of the whole volume) is only “absorbed” into the whole volume during the first year. This takes place by redistributing the sand over the ebb•tidal delta. During this process the basin partly benefits from the spread out sand.
Additional research is needed to asses the long term effects of channel nourishments and ebb•tidal delta nourishments (exceeding one year up to a decade). In this way it can be ascertained whether the initial redistribution of sand from a channel nourishment to the basin has a positive effect on the longer term. Furthermore it is interesting to know how ASMITA assesses the redistribution process of a channel nourishment. Especially the time scale of this process dependent on the nourishment volume is of our interest, since the additional model simulation suggests that relative large amounts of sand are needed to realise a prominent impact.
5.3 Conclusions on the model behaviour and model applicability
Regarding the model behaviour a few comments are made: · For the tide•only simulations the bed level behaviour did not match with reality on the long term. Water motion and bathymetry are not in dynamic equilibrium. This is contributed to several used model schematisations: § The tidal range within the model is overestimated, compared to measured data (order 10%); § This causes the magnitude of current velocities to be higher within the channels, which consequently results in stronger erosion of the inlet; § This may also be contributed to the use of only one sediment fraction within the model. In reality heavier sediments are deposited at the bottom of the channel which decelerate the erosion;
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§ Furthermore the realistic long term behaviour of deep channels in combination with shallow flats is very hard to reproduce with the use of Delft3D. A known model artefact is the continuing deepening of the channel. · The above model limitations have lead to the decision to use the model only on the short term and thereby focus on relative changes. For the tide•only model distortions stayed within acceptable margins for a time span of three years (bathymetry after three years still showed a good resemblance with the initial bathymetry). For the wave and tide model the availability of computational power restricted the simulation period (one year). Here too the end•bathymetry showed a good resemblance with the initial bathymetry. · The focus on initial relative changes in sediment transport and bed level behaviour makes it possible to distinguish the relative effects triggered by the nourishment alternatives. This approach is independent of the autonomous model behaviour (on the short term) and is used to gain insight in the physical processes which force the interaction of the smart nourishment.
5.4 Recommendations
The previous section confirms the formulated basic hypothesis is valid for a nourishment within the channel. However the simulated period of one year is too short to quantify the effects. This quantification is further made difficult by the unrealistic bed level behaviour of the model on the long term. The recommendations for a follow•up study are twofold: I. Improve the real world case study by improvement of the Frisian Inlet model by means of: a. Improvement of the boundary conditions; i. Calibrate the ZUNO•Grof model for a 2D computation; ii. Run the ZUNO•Grof for a year to derive water level times series for the boundaries of the Frisian Inlet model; iii. Run the Frisian Inlet model with new boundary conditions and compare measured water levels with modelled water levels at observation stations by means of a harmonic analysis; iv. Calibrate and validate the tidal constituents at boundaries of the Frisian Inlet model, by means of adding the difference (between observed and modelled amplitude and phase) of the main constituents to the corresponding constituent at the boundary until observed and modelled amplitude and phase of the main constituents match; v. Tidal boundaries can be furthersimplified by means of generating a morphological spring neap tide or morphological daily tide. b. Use of different sediment gradings; i. Heavier sediment could be used to schematise the composition of the bed of the channel, this will reduce the channel erosion; ii. A fixed bottom could be applied at a •25 m. NAP which prevents the channel to deepen beyond this level. c. Additional case studies; i. Investigate the behaviour of a smart nourishment for a morphological period of 5 to 10 years taking tidal flow and waves into account;
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ii. Simulate the behaviour of nourishment alternatives with a larger volume (> 10 Mm3) and relate their impact on the system to the volume of the ebb•tidal delta; iii. The initial effects of channel nourishments can be studied with only the use of tidal forcing; iv. Due to the smaller size of the Pinkegat Inlet the time scale of morphodynamic changes is expected to be smaller than that of the Zoutkamperlaag. Does this behaviour also show for favourable sedimentation within the basin on the time scale of decades due to a smart nourishment? v. The available sediment at the North Sea with a mean diameter of 120 mm (widely available) could be used for a case study. II. Disregard the real world case and study the interaction of a smart nourishment in a simplified model. a. An option would be to set up a simple rectangular model of a tidal basin with an inlet. With only the forcing of the tide, water motion and morphology should come to a dynamic equilibrium. b. Secondly the effects of a smart nourishment can be studied relative to the (stable) morphological development of the simplified model. c. Asses the effects relative to the size of the impact (nourishment volume relative to the size of the ebb•tidal delta) and compare these with ASMITA results.
At last it is recommended to execute a pilot project in the form of a nourishment of the Pinkegat channel.
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A The Wadden Sea: a system of tidal basins
A.1 Introduction
In this appendix an introduction to a tidal basin system will be given. Its elements, morphodynamic characteristics and its behaviour under intrinsic and external forcings will be elaborated.
A.2 Characteristics of the Wadden sea
The Wadden Sea is a shallow sea separated from the North Sea by barrier islands, stretching along the northern coast of the Netherlands, Germany and Denmark. This system forms one of the major nature reserves in western Europe and is one of the major intertidal areas on earth. The Wadden Sea consists of a series of tidal basins. Figure A•1 shows the present layout of the Dutch Wadden Sea. The Roman numbers refer to the tidal basins mentioned in Table A•1: (separated by a red line).
Figure A•1: The Dutch Wadden Sea, roman numbers refer to Table A•1
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Table A•1: Characteristic data of the Dutch tidal basins (Eysink, 1993)
basin area (km2 ) tidal range (m) tidal prism 6(10 m3 ) I Marsdiep 656 1.65 1015 II Eijerlandse Gat 161 1.65 205 III Vlie 719 1.9 1190 IV Borndiep 269 2.15 475 V Pinkegat 52 2.15 80 VI Zoutkamperlaag: present 123 2.25 195
before closure 230 • 305 Frisian Inlet VII Eilanderbalg 35 2.4 48 VIII Lauwers 128 2.45 210 IX Schild 31 2.45 42
A.3 Tidal Basin Elements
Before schematizing a tidal basin into its elements a further restriction is made regarding the definition of a tidal basin in this case concerning the Wadden Sea. Here tidal basins are defined as to consist of tidal lagoons with approximately equal influx and outflux of water (e.g. the Wadden Sea basins) and of estuaries with negligible influence of river run•off e.g.( western section of the Western Scheldt, Eastern Scheldt and Haringvliet). In this way it is emphasized that there’s negligible fresh•salt water mixing (DeVriend et al.1998). The Wadden Sea of a series of tidal basins. For each system one can distinguish the following elements: ebb•tidal delta, inlet, adjacent coasts, the channels and the tidal flats. The ebb•tidal delta, tidal inlet and basin area are briefly reviewed.
A.3.1 Ebb•tidal delta
After De Vriendet al. (1998) a typical ebb tidal delta can be divided into a main ebb channel, channel margin linear bars, a terminal lobe, swash platforms and bars and marginal flood channels (see Figure A•2).
The channel margin linear bars flank the ebb tidal channel and are deposits built up by the interaction between flood• and ebb tidal currents with wave•generated currents. In the Dutch inlets these distinct bars are not recognisable; instead, there are wide flats. The terminal lobe is a rather steep seaward•sloping body of sand, which forms the outer end of the ebb tidal delta. The main ebb channel is flanked by swash platforms, which are broad sheets of sand, On these swash platforms, isolated swash bars can be recognised, built up by swash action of waves. Marginal flood channels usually occur between the barrier islands coast and the swash platforms. The boundaries of the ebb tidal delta can be found via the no•inlet bathymetry: where the differences in bottom height is nil between the actual and no•inlet bathymetry.
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Figure A•2: Morphological units of tidal inlets The occurrence of ebb• and flood dominated tidal channels is of prime importance to the morphology of the ebb tidal delta. The overall morphology of the ebb tidal delta depends on the interaction of tidal currents and waves. Generally speaking, the ebb tidal delta morphology is determined by the (dynamic) balance between a net offshore directed sediment flux induced by the inlet currents (ebb dominance) and a net onshore directed sediment flux induced by offshore waves. Although, these principles can not solely explain the ebb tidal delta morphology, it gives a basic explanation for the apparent mechanism that "holds the ebb tidal delta together". WL | Delft Hydraulics A–3 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
A.3.2 Tidal inlet
The inlet can appear in many forms. Essentially many inlets are the mouth of rivers where the discharge of fresh water interacts with the tidal flows. But in case of the Wadden Sea fresh water run•off is negligible and the inlet forms the key element for exchange of water and sediment between sea and basin. Due to the influence of waves and tidal currents, sediment is transported through the inlet in and out of the tidal basin. In the basin the sediment can be deposited in the form of a flood•tidal basin and outside of the basin the deposited sediment forms an ebb•tidal delta. The influence and strength of waves and tide determine whether an inlet is tide or wave dominated. Strong wave influence and weak tide influence usually result in the formation of a distinct flood•tidal delta. The opposite: strong tide and weak wave influence result in a distinct ebb•tidal delta. In case of the Wadden Sea one may speak of tide dominated coasts with stable inlets and basins which are characterized by a large number of inlets having ebb• and flood•tidal deltas, giving access to tidal basins.
A.3.3 The tidal basin
The characteristic topography of a tidal basin area is that of a meandering braided and/or branched channel system, ebb• flood chutes, inter•tidal sand and mud flats and marshes. Along barrier coasts of the Dutch Wadden Sea, the basins are often rectangular or near square and the channel structure is often more branched than braided. In the tidal basin a distinction can be made between two morphological elements: tidal flats and channels.
The channels may be defined as the water volumes between the Mean Low Water (MLW) and the bed level. The flats can be defined as the sediment volumes above MLW. Although it is easy to schematize the morphological elements in this way, one must keep in mind that they form a system which is highly interactive. The channels in the basin form the veins of the system; sediment and water is transported through these channels, from and to the flats. Near the inlet the depth and width of the channels is the largest, away from the inlet, into the basin, the channels decrease in size.
The tidal flat area or intertidal zone is the area that normally inundates and dries during one tidal cycle. It is of great importance as feeding ground for many birds and it can be a resting•place for seals.
A.4 Hydrodynamic processes and factors
The large scale development of a specific tidal inlet system can be described with time scales from decades to centuries and space scales in the order of the size of the entire basin. In the last section it became clear that the tide and waves somehow have an influence on the tidal inlet and basin characteristics. The morphological development and state of a tidal basin was analyzed by Hayes (1979). Hayes made a hydrodynamic classification in which he distinguishes five classes of tidal inlets with respect to tidal and/or wave dominance. · High tide dominated; · Low tide dominated’ · Mixed•energy tide dominated; · Mixed energy•wave dominated; · Wave dominated inlets.
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With this classification (see Figure A•3) Hayes indicated a empirical correlation between the integrated characteristics of tide and the wave climate, and the morphological shape of inlets.
The Dutch Wadden Sea inlets are mixed energy inlets, with tide dominance. These inlets are characterized by abundant inlets, relative large ebb•tidal deltas and usually drumstick barriers.
Figure A•3: Hydrodynamical classification of tidal inlets (* indicates the Frisian inlet)
Analogously Eysink (1993) concluded that the morphology of the Wadden Sea or (barrier islands and back barrier area in general) is determined by various mechanisms and factors, among others, below listed the most important ones: · Tidal range and flow; · Seasonal winds and waves; · Geometry of the basins.
The dependency hardly shows on a small time scale (days to years) but manifests itself in changes in the Wadden landscape that become significant over a time span of decades up to centuries. As such, the mentioned factors steer the large•scale morphological changes in the Wadden Sea tidal inlets.
A.4.1 Tidal range and flow
Outside the Wadden Sea the tidal range depends primarily on the ocean tides and their interaction with the continental shelf, the tidal motion in the North Sea is determined by the tide in the Atlantic Ocean. Along the Dutch coast the tide propogates from south to north. The tidal range is minimum at Den Helder, approximately 1 meter, and increases in eastward direction to approximately 3 meter at Rottumerplaat. Due to the limited depth in the tidal basins the tidal energy and celerity decreases after the tidal wave has entered the basin. In the Wadden Sea this results in a complex interaction pattern of incoming and
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reflecting tidal wave components. In the relatively short tidal basins of the Wadden Sea (relative short compared to the tidal wave length), the tidal wave is reflected and has a more standing character.
Approximately in the middle behind the islands two flood currents meet. Where these currents meet, watersheds can be recognized. Here current velocities are very low and conditions are favorable for settlement of sediment which results in ridges. These watersheds form the boundaries of the different tidal basins. It is because of the presence of these watersheds that the Wadden Sea can be looked at as a series connection of coastal basins and inlets.
The propagation of the tidal wave into the tidal basin means that a substantial amount of water has to pass the inlets each tidal cycle. For small basins, compared to the tidal wave length, the tidal prism approximates the complete volume of water that enters the basin during one tidal period (12 hours and 25 minutes). Where the tidal prism has the definition of wet storage area between Mean Low Water (MLW) and Mean High Water (MHW).
Apart from the vertical water motion one should also consider horizontal gradients in waterlevel. The aforementioned difference in tidal range from west to east in the Wadden Sea results in a long•shore water surface gradient. As a consequence a tide driven long•shore current is generated. The interaction between the tidal currents in the inlet and the tidal currents along the North Sea coast determine (in combination with the wave influence) the geometry of the ebb•tidal delta.
A.4.2 Seasonal winds and waves
Besides the considerable influence of the tidal range/tidal prism on the morphology of the tidal inlets in the Wadden Sea, seasonal winds and waves have their own distinct impact on the morphological elements of the system.
Usually wave action is considered to act as a bulldozer on the seaward side elements of a tidal basin system (viz. ebb•tidal delta). Waves induce shear•stress and therefore stir up the sediment which is subsequently transported by wave or tidal currents. In case waves approach the coast under an angle, they will generate a long•shore current in the breakerzone. These currents transport a lot of sediment, which is by•passed over the ebb• tidal delta or enters the basin with the flood current or is transported seawards with the ebb current. It is safely to state that the interaction between the complex bed•topography of the ebb•tidal delta and the wave induced• and tidal• currents in combination with sediment transport is highly complicated and variable.
It is the outer delta which offers some protection to the basin and it is found that the effect of sea waves and swell from the North Sea remains limited to the area near the inlets. Inside the basin waves are primarily generated by local winds. Only during severe storm surges waves penetrate the Wadden Sea and break on the first tidal flats within the basin.
Wind may have an important effect on the water motion in the Wadden Sea. It generates a shear stress at the water surface which causes drift currents and wind waves. If a drift current is directed to or from a coast, the flow component perpendicular to the coast ultimately is compensated by a return flow generated by a slope in the water surface. The net result is a vertical circulation where the flow at the bed opposites the cross•shore wind component. Towards the coast, this slope results in a wind set•up or set•down. In a complex A–6 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
geometry with flats and channels, like the Wadden Sea, the return flow is likely to concentrate in the relatively deep areas. This creates a complicated flow pattern of tidal flow with wind induced drift currents and return flow. In general tidal flow will dominate but on the tidal flats and at the watersheds the opposite may occur during strong wind conditions.
A.4.3 Geometry of the basins
As mentioned before the geometry of the ebb•tidal delta influences the large•scale morphology of a tidal inlet system. The lay•out of the ebb•tidal delta largely depends on the offshore wave climate and the characteristics of the tidal regime. This is described by Sha and Vand den Berg (1993), who used tidal inlets in the South Western part of the Netherlands and the Wadden Sea as study material. According to Van den Berg and Sha the phase difference between the horizontal and vertical tide can have significant influence on the orientation of the outer•delta and its ebb• and flood•channels.
In case of the Wadden Sea basins there is no phase lag between the horizontal and vertical tide, the tidal currents reach their maximum around the mid•tide water level. At the same time the tidal discharge through the inlet is maximal. The interaction between the inlet tidal currents and offshore tidal currents at flood and ebb in this case are illustrated in Figure A•4. At ebb the tidal currents are concentrated on the west side of the inlet and at flood the tide enters the basin from the west side as well. This is the main reason that the main ebb channels in the ebb tidal delta’s of the Wadden Sea are directed slightly to the west. For the same reason the seaward end of the deltas generally bends towards the west.
Figure A•4: interaction of inlet tidal currents with off•shore tidal currents at ebb and flood (Sha, 1989).
The interaction as depicted above in the figure in combination with the geometry of the basin results in a net current pattern which can be roughly sketched as in Figure A•5.
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Figure A•5: Tidal and wave driven currents in ebb•tidal delta (after: Cleveringa 2004)
A.5 Equilibrium relations of tidal basins
A.5.1 Introduction to equilibrium relations and morphological balance
Several morphological elements in a tidal inlet system have been distinguished in the foregoing section. These elements are the ebb•tidal delta, the channel and the tidal basin. With respect to the ebb•tidal delta one should bear in mind that the adjacent coastal stretches are closely related to the ebb•tidal delta when it comes to the morphodynamic behaviour of the outer delta. Furthermore within the basin a distinction has been made between the channels and the flats.
Moreover, it was derived that the morphological processes in an element are mainly related to the hydrodynamic forcings e.g.( tidal prism and wave climate) and morphometrice.g. ( tidal basin area) characteristics (see Figure A•6). This correlation has been noted before and a great deal of research has been done to determine the relation between the hydraulic and morphological parameters of an element.
Figure A•6: Schematization of morphodynamic behavior
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When in state of morphological balance, relationships excist between the element equilibrium state (volume) and the present morphometric and hydrodynamic values. These relationships enable us to predict new morphological equilibrium states of the element if changes occur in morphometric or hydrodynamic values, independent whether these are caused by human interference or natural processes. The basics for these relations are founded in the United States by O'Brien (1931,1967), Jarret (1976)and Walton and Adams (1976). For the Dutch deltas adaptations where made, among others, by Gerritsen (1990). Broad literature surveys of these empirical relations for both Dutch and foreign inlets are given for instance by Biegel (1991), Van Kleef (1991) and Eysink and Biegel (1992).
One of the key forces behind morphological changes in a tidal area is the tidal volume or tidal prism. First two basic definitions are given regarding the tidal volume and the tidal prism:
The tidal volume is the volume of water entering the tidal basin during flood (flood volume) plus the volume of water that is flushed out of the tidal basin during ebb (ebb volume).
The mean tidal prismis equal to the water volume between Mean Low Water (MLW) and Mean High Water (MHW) in the tidal basin. It is based usually on systematic bathymetric surveys of the tidal basin. In relatively short basins (relative to the length of the tidal wave) the tidal motion is that of a standing wave which results in a tidal volume of about twice the tidal prism.
Numerous empirical relationships between tidal inlet characteristics have been studied and proven to exists. In the following sections a description is given for a number of empirical relations between the morphological and hydrodynamic parameters for different elements of a tidal inlet.
A.5.2 Relationship between inlet cross•section and tidal prism
A typical unit of measure for an inlet is the narrowest cross•section at the opening between two barrier islands (the throat of the inlet). A clear connection has been established between the size of this cross•section and the tidal prism. Empirical study has shown that the cross•section of an inlet increases almost linearly with the tidal prism. According to Van Kleef (1991) this is one of the best•investigated empirical relationships between hydraulic and morphological parameters. Eysink (1990) stated that for ‘throat area•tidal prism’ relationship a fairly good description is presented by:
APMSLA=×a Equation A•1
With: Area below MSL (Mean Sea Level) [m2] AMSL Empirical coefficient for the equilibrium flow area [m•1]28 a A P Mean tidal prism [m3]
•1 28 For the Wadden Sea a A is equal to 70·10•6 [m ] (relative to MLW), for the Western Scheldt a A is equal to 80·10•6 [m•1] (relative to MSL) and for tidal inlets on the East Coast of the United States •1 a A is equal to 85·10•6 [m ] (relative to MSL).
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A.5.3 Relationship between channel volume and tidal prism
The following relation is valid for the channel volume of a tidal basin with minor upland discharge (meaning channels are getting narrower farther from the inlet away) (Eysink, 1990).
1.5 VPchannel=×a c Equation A•2
Where: Channel water volume below MSL (Mean Sea Level) [m3] Vchannel Empirical coefficient for the equilibrium flow area [m•1.5] ac P Tidal prism [m3]
A.5.4 Relationship between volume of ebb•tidal delta and tidal prism
The ebb•tidal delta is a ‘storehouse' for a large quantity of sediment. Walton and Adams (1976) found that the volume of the ebb•tidal delta depends on the tidal prism and on the total wave energy available for the transport processes. They derived their relations for inlets on sandy coasts in the United States.
Walton and Adams defined the sand volume of the ebb•tidal delta as the volume above the bed level, which would be there if there were no tidal inlet and thus no ebb•tidal delta. They identified three classes of wave energy: low, moderate and high and found that:
1.23 VPdelta=×a wa Equation A•3
Where: Volume of sand in the ebb•tidal delta [m3] Vdelta P Tidal prism [m3] Proportional constant with dimensions [m•1.23], its value depends on local awa wave conditions.
Eysink (1990) found that this relationship is also valid for the Wadden Sea inlets with an •3 •1.23 awa equal to 6.57·10 [m ].
A.5.5 Relationship between volume of tidal flats and basin size
The relative flat area ( Aflat / Abasin ) depends on the size of the basin and, less importantly, on the tidal range H. According to Eysink (1990) the relative flat area for the Wadden Sea with its mesotidal range, appears to be distinctly dependent on the size of the basin. Renger and Partensky found for the German Bight the following relation:
A Equation A•4 flat =-×1 2.5 A0.5 basin Abasin
Eysink (1991) included the effect of the relative tidal flat area on the tidal prism:
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æöAflat Equation A•5 PAH=×-×basina feç÷ ×× AH basin èøAbasin
Where: Area of the tidal flats, measured at MLW [m2] Aflat Area of the tidal basin, measured at MHW [m2] Abasin P Tidal prism [m3] H Mean tidal range [m] Empirical coefficient for the average tidal flat level. a fe
A.6 Climatic and anthropogenic factors
Apart from the hydrodynamic processes which influence the morphology of a tidal inlet and basin there are two other process which have their distinct influence; sea level rise and human interference. The sea level has been rising over hundred of centuries and is an exogenous process. All man made interventions and works fall within the term human interference, (e.g. closure works or subsidence due to gas mining and sand mining).
A.6.1 Sea level rise
It was after a drop in the late Middle Ages that around the year 1850, the average temperature began rising again. Mountain glaciers shrank again and the sea level along the Dutch coast rose by approximately twenty to thirty centimetres per century. Over the past 150 years the figures reveal a fairly constant increase of 14 to 17 centimetres per century. This rise is probably the ongoing effect of the last Ice Age.
However, recent studies by the KNMI (Royal Netherlands Meteorological Institute) and the IPCC (Intergovernmental Panel on Climate Change) indicate that a dramatic change in global climate is to be expected (Figure A•7). According to the estimates of IPCC the sea level rise in the year 2100 will be between 9 and 99 centimetres higher than in 1990. This rise is caused by:
· Expansion of seawater (about 75%); · Melting of glaciers and ice caps on land (about 25 %); · Melting of Greenland’s ice caps (10%); · Increased snowfall on Antarctica will decrease sea level rise by 20%.
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Figure A•7: Estimate for expected world wide sea level rise (from IPCC rapport (2001)) The absolute sea level rise in the north•eastern part of the Atlantic Ocean (including the North Sea) is expected to be equal to the worldly averaged increase in sea level rise (Katsman 2005). The relative sea level rise will be even larger because of subsidence of the seabed.
A.6.2 Relative sea level rise and human contributions
The summation of sea level rise and subsidence of the sea floor is called relative sea level rise (see Figure A•8).
Figure A•8: Graphic representation of relative sea level rise
Subsidence of the seabed is caused by different processes. In the first place there are natural movements in the tectonic plates which can causes land subsidence. And since these are a result of large•scale geological processes one may assume that, these steady and of little contribution, ground shifts will continue in the future. Next to natural subsidence, human activities like sand mining, gas mining or salt mining can cause subsidence of the sea floor as well. The difference with natural subsidence is that subsidence due to mining arises on a local scale compared to the overall scale of sea level rise and the time scale is much shorter. The behaviour of the subsidence is exponential; just after the start the subsidence is largest and towards the end the subsidence gets smaller and smaller.
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In the Wadden Sea sand mining, gas mining and salt mining activities do occur and contribute to the relative sea level rise. Regarding sand mining one should note that it is only a very local scale while gas mining results in effects on a regional scale. Subsidence of the seabed in the Wadden Sea will eventually result in sooner noticeable effects of absolute sea level rise. The total contribution of subsidence to relative sea level rise is expected to be between 10 an 20 cm resulting in a relative sea level rise for the year 2100 of somewhere between 20 and 110 centimetres (Katsman, 2005). Independent of the uncertainty of the above factors, it is already observed that the development of the system influenced by sea level rise causes erosion of the adjacent barrier islands due to the transportation of sediment into the basin (Tanczos, 1996) and could cause the tidal flats to drown (Van Goor, 2001).
A.6.3 Human interventions
Apart from the contribution to relative sea level rise mankind also influenced the morphodynamics of a tidal basin by empoldering sections of the basin or closing off entire channels. Here the closure of the Lauwers Sea can function as a clear example29. In 1969 an area of 90·104 m2 was reclaimed from the Lauwers Sea. The system responded to this intervention and according to the empirical relationships seeks a new equilibrium. The relative flats area is expected to increase after closure. In reality, a rapid narrowing of the inlet gorge has taken place, by the formation of a large sandy hook at the east side of the inlet. The actual amount of sediment import into the basin was initially 7·106 m3/yr, and between 1970 and 1987 it is estimated at 34·106 m3 (Oost and De Haas, 1992). Clearly, the adjustment process has not yet come to an end. Although the delta evolution is rather dependent on long•term meteorological variations, its volume has decreased by 26·106 m3 and its seaward protrusion has decreased roughly by 1.5 km in these 17 years (de Vriendet al., 1998). So the volume decrease is of the same order of magnitude as the sediment demand in the basin. Apparently, the delta serves as a buffer for the basin. Yet, the equilibrium value of basin volumeis proportional to P1.5 and that of the delta volumeis proportional to P1.23, which means that the basin ultimately demands more sediment than the delta can supply. This sediment has to be provided by the sea bed, the adjacent coasts or could innovatively be provided in the form of an artificial nourishment on the ebb•tidal delta.
A.7 Morphological modelling of coastal inlets & tidal basins
A.7.1 Introduction
As presented in the previous sections; the morphological evolution of coastal inlets and tidal basins is governed by a complex network of mutually interacting processes. Each process acting on its own characteristic time• and space scale. The forcings of these processes may be natural e.g. ( tide, waves), of climatic origine.g. ( sea level rise) or due to human interference e.g. ( gas mining, closure works). Apart from that the forcings may be deterministic or random. This mixture of scales and types of forcing substantially complicates modelling and predicting the inlet morphodynamics. This implies that large•scale behaviour cannot be derived in a deterministic way from small•scale processes.
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A.7.2 Cascade of scales
In order to tackle the problem of mixture of scales and types of forcing, complicating the modelling and predicting of large•scale morphological behaviour of tidal inlet systems, De Vriend (1999) introduced the concept of dealing with our interest on a cascade of scales. With the purpose of dealing with time• and space•scales from large scale to small•scale level, the following cascade of scales is derived by De Vriend (1999):
The mega•scale level, representing the level at which the principal elements of the entire system (barrier islands, outer deltas, inlets) evolve and interact, involves many kilometres in space and centuries in time. The principal forcing are mean sea level rise, climate change, long•term tidal variations, subsidence.
The macro•scale level, representing the level at which the (one order smaller) meso•scale features interact. This level is associated with space•scales that comprise meso•scale features as a whole (e.g. outer delta) and time•scales of decades. The principal forcings are the longterm cycles in the tide e.g.( the 18,6 year lunar•related cycle), long•term variations in the wave climate and long•term human interference (like the construction of dykes and land reclamation).
The meso•scale level, which represents the level of the principal morphological features, such as channels and shoals. At this level, developments have a time•scale of years and a spacescale of hundreds of metres. The principal forcings are seasonal and inter annual variations in the tide and the weather conditions. Also human activities and extreme events have their impact on this scale.
The micro•scale level, which represents the level of the smallest scale (time and space) of morphological processes (e.g. ripple and dune formation). The principal forcings are the tide and the day to day weather and wave conditions.
The highly dynamic nature of the non•linear, stochastically forced systems considered here makes it not very likely that one single model will be able to cover all these scale levels at one time. According to De Vriend (1999), one must expect to run into intrinsic or practical limits of predictability when ascending from small•scale (relatively well known) processes to large•scale (relatively unknown) processes, see Figure A•9. This means that brute•force computing with process based models is probably not a viable approach for predicting the macro• and mega•scale behaviour of the system.
In general, predictability limits can be overcome by aggregation. Based on what is known of the systems behaviour at scales below and above this limit, an aggregated scale model is formulated at the higher scale level without attempting to describe every detail of what happens at the lower scale level. Hence, we should aim at a cascade of models at different levels of aggregation (De Vriend, 1999).
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Figure A•9: Cascade of Scales (modified after De Vriend et al. 1998) According to the present insights, the transitions in the above mentioned scale cascade approximately correspond with the points where predictability limits are to be expected. Hence this cascade should be reflected in the cascade of models. On the macro•scale one typically reverts to more aggregated modelling approachese.g. ( ASMITA), while the process•based modelling (e.g. Delft3D) is typically relevant on the micro• and meso•scale.
A.7.3 ASMITA
In this section ASMITA (Aggregated Scale Morphological Interaction between a Tidal inlet and the Adjacent coast) will be briefly discussed. Although not part of this research, the hypotheses used for the ASMITA modelling concept form the foundations of the hypothesis for this research. In ASMITA, the degree of schematisation is determined by the element of the system, which delivers the lower boundary to the relevant spatial scale. This concerns typically the ebb•tidal delta, for which there is presently no other option than to consider its volume as an integral state variable (Dean and Walton, 1975). Hence the ebb•tidal delta is modelled as a single element, which makes it not very sensible to model the tidal basin and/or the adjacent coast in more detail.
The most important hypothesis used in the model concept is that a morphological equilibrium can be defined for each element depending on the hydrodynamic conditions (e.g. tidal prism, tidal range) and morphometrice.g. ( basin area) it is subjected to. The existence of such an equilibrium has been supported by various field investigations, which have resulted in empirical relations between state variables and parameters of the governing hydrodynamic and morphometric conditions (see Section A.5).
A second hypothesis used in the model concept is based on studies of De Vriend (1999), where it is shown that the different elements cannot be isolated from each other when their morphological development is considered. The interaction between the different elements through sediment exchange plays an important role for the morphological development of the whole system as well as of the individual elements. It is assumed that a long•term residual sediment exchange occurs between the tidal flat and the channel, the channel and the ebb•tidal delta and between the ebb•tidal delta and the adjacent coast. The sediment
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exchange with the surrounding zones,e.g. the foreshore and the coastal stretches further away, is assumed not to play a key role in the morphodynamic interactions considered.
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B The sand balance reviewed
B.1 Introduction to a morphodynamic system
In Figure B•1 the four main morphodynamic elements of a tidal inlet are distinguished; the ebb•tidal delta including the inlet, the tidal basin and the two coasts of the barrier islands. These components play a major role in the sediment flow throughout the system. The morphology of the Wadden Sea is determined by various mechanisms and factors, such as: tidal range and flow, wind and waves and the geometry of the basin. Changes are being established by redistribution of sand.
Figure B•1: Four main elements of a tidal inlet system Figure B•2: Schematisation of wave and tidal current (the sediment exchange is indicated by arrows).induced transport of sand in the morphodynamic (modified after Cleveringa, 2004) system of a tidal inlet. (modified after Cleveringa, 2004) Long•shore currents (mainly due to waves) transport sediment along the coasts of the islands to the ebb tidal delta. Part of the sediment by•passes the inlet system along the margin of the ebb•tidal delta (Figure B•2). The other part will be transported into the tidal basin by flood tidal flows. In the basin the sediment partly settles, the majority however is washed out again by ebb•currents. Concluding; a tidal inlet is a highly complicated system where large quantities of sediment are being exchanged between the morphological elements.
B.2 The sand balance
B.2.1 The balance area
An effective measure to evaluate the changes in a morphodynamic system is to draw up a sand balance for the area of interest. Such a sediment budget study comprehends the measured volumes of sand within a certain area at different times. When the volume of sand is increased or reduced compared to the last time it was evaluated; the area of interest respectively gained or lost sand.
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Regarding the placement of the boundaries of the area of interest standards have been developed. It is for example a far more logical choice to let the vertical boundaries coincide with the watersheds of the tidal basin. At the watersheds the morphodynamics are low and sediment depositions form a physical boundary with the adjoining tidal basins. At the seaward side, usually the middle of the islands is chosen or the vertical boundaries in the inlet are extended in the direction of the sea. For the Dutch coastal zone (not the basins) the horizontal boundary is located at a depth of NAP •20 meter (see Figure B•3). It is assumed that fluxes through and morphological changes below this level are of minor importance for the morphodynamics of the coastal zone.
B.2.2 The sand balance in equilibrium
If there are no net changes in the morphodynamics of a tidal inlet system the system is in equilibrium state. This state is a dynamic equilibrium, since there is still transport of sediment to, from and within the system. In the dynamic equilibrium state the amount of sand transported in equals the amount of sand transported out.
Earlier studies showed that the entire Wadden Sea can be looked at as one morphodynamic system for which it is possible to define a dynamic equilibrium situation (Oostet al., 2000). The Wadden Sea can be schematised to a series of tidal basins which all can be looked at independently. Assuming they are independent makes it possible to review the sand balance for each tidal inlet separately (in reality sediment is exchanged between the basins). When one studies the sand balance, attention should be paid to the definition of the boundaries and processes which influence the balance.
Figure B•3: Schematisation of cross sections (one cross section through inlet and one through barrier island). (modified after Cleveringa, 2004)
Two cross•sections of the tidal inlet are drawn in the same figure (Figure B•3). In case of morphodynamic changes in the system the shaded area in these cross sections can be used to give an indication of the effects on volumes in the sand balance. All sand above NAP •20 meters is included in the sand balance. With the use of similar figures the next section will elaborate relevant aspects and processes which influence the sand balance for a Wadden Sea inlet.
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Figure B•4: Schematisation of the boundaries used for the sand balance in the case of a tidal inlet (boundaries of the balance area are indicated by a solid orange line, water sheds are not completely depicted).
B.3 Ins en outs
B.3.1 Absolute sea level rise
The dynamic lower boundary of the Dutch coastal zone is located at NAP •20 meter. It is assumed that for the adaptation of the coastal profile sediment is used from the area between the dunes and 20 meters below MSL. In case of a rise in sea level over several decades the MSL has changed and so has the active profile. In the new situation a large part of the sediment in the active profile is not available anymore for the coastal morphodynamic processes (see Figure B•5). Loss of sand from the dynamic coastal profile however does not influence the sand balance. The volume of sand above NAP •20 meter has remained the same. The only difference is that the mean water level has gone up.
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Figure B•5: Loss of sand available in the dynamic coastal profile due to sea level rise (hatched area). (modified after Cleveringa, 2004)
Indirect sea level rise may initiate changes in the sand balance. Due to the rise in sea level the morphodynamics of the system change in search of a new morphodynamic equilibrium adapted to the new situation with a higher MSL. Based on geological surveys and earlier studies it is concluded that in case of sea level rise the Wadden Sea will import sediment (Van der Valket al., 2004). Wave and current induced sediment transport will cause a redistribution of the sediment throughout the system. For the sand balance this is not necessarily a loss, since only redistribution takes place. Problems however may arise when there is not enough sand, or sand is not directly available within the system in combination with the time it takes for the hydrodynamic forcings to redistribute the sediment. This may cause a shortage of sand in the basin since the sedimentation of the basin can not keep up with the sea level rise.
Another development which causes the sediment to be less easily available for redistribution is the fact that the barrier islands are, more or less, fixed in position. Dutch maintenance policies with the use of the Basal Coastline30 definition asures that there is hardly any coastal regression. The fact however remains that a flexible barrier island coastline is the natural way for a tidal inlet system to adapt to changes in sea level. A rise in sea level causes the coast to regress due to erosion of the coastal profile. The eroded sediment becomes available for redistribution within the system and can be deposited in the basin (see Figure B•6) (cf. Stive and De Vriend, 1995; Bruun, 1988).
Figure B•6: The height of the flats have the natural tendency to follow the sea level rise, coastal regression allows sediment to become available for sedimentation in the basin. (modified after Cleveringa, 2004)
With the use of nourishments the coastline is artificially kept at is 1990 position. An artificial nourishment is a contribution to the sand balance and partly compensates the losses of sand from the dynamic coastal profile due to sea level rise (Figure B•7). It should be
30 In Dutch: Basiskustlijn (BKL) B–4 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
emphasised that sea level rise itself does not cause a loss of sand from the balance, but only from the dynamic profile. Indirect effects however may explain losses or contributions.
Figure B•7: A nourishment is an addition to the volume of sand in the balance area. (modified after Cleveringa, 2004)
B.3.2 Subsidence
B.3.2.1 Tectonic movement
Another process which should be bared in mind is the subsidence of the land. Long term geological processes cause the Wadden Sea area to subside. As sketched in the figure below (Figure B•8) the shaded area has reduced a little which corresponds with a volume reduction. The reference points for the Dutch NAP system are embedded deep into the Pleistocene sand and are expected not to be influenced by tectonic movement, hence the volume of sand above the NAP •20 meter has reduced. Indirect subsidence also causes changes and can be compared with the consequences of sea level rise. Subsidence is in fact nothing more than an addition to the absolute sea level rise (effects combined are known as relative sea level rise).
Figure B•8: Geological processes can cause subsidence of the land and a loss of sand from the balance area. (modified after Cleveringa, 2004)
B.3.2.2 Gas mining
On a more local scale subsidence can also be caused by human activities such as; sand mining, gas mining, salt mining or seashell mining. In the Wadden Sea area all these forms of mining exist. Gas mining however is the most noticeable one and has the most extensive effects. Due to the mining of gas the bottom subsides and causes a loss of sand from the balance area (Figure B•9).
WL | Delft Hydraulics B–5 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
Figure B•9: Subsidence due to gas mining in the Wadden Sea, causing a loss of sand from the balance area. (modified after Cleveringa, 2004)
B.3.3 Boundary shifts
B.3.3.1 Manmade interventions
When evaluating the sand balance over a period of several years up to decades one should also pay attention to remaining physical interventions made by man in the area of interest. A part of the balance area is cut off in case of enclosure of a part of the tidal basin. Such a radical change in the balance area results in the start of a new balancing period with new boundaries. The impact of such an intervention is described in section 2.6; closing off a part of the basin is likely to result in sedimentation of the basiradicaln. Again this might explain changes in the morphodynamics but is not a direct loss or gain for the sand balance.
B.3.3.2 Subsidence of the NAP reference level
As earlier discussed the lower horizontal boundary for the sand balance is the NAP •20 meter level. The NAP level is based on underground marks which are supposed to be absolutely stable. This means that they do not subside and are not influenced by large scale geological processes. Absolutely stable, until recent measurements showed that there are significant differences between the actual heights of these marks and the documented heights. E.g. the underground mark in Rotterdam was documented at 1 meter above the NAP zero•level in Amsterdam, as it was measured in 1926, today it was assumed to still mark the NAP +1 meter, but actually the mark subsided by about 3 millimetres (AGI, 2003). For the sand balance this means that the horizontal lower boundary of NAP •20 meter was not as fixed as assumed. As a consequence corrections should be applied. Subsidence of the land, creating a loss of sand from the balance, is less than first calculated. Because the NAP• mark followed the subsidence partly.E.g. for the Frisian Inlet (170 km2) a volume of 0,5 million m3 sand can be contributed to the subsidence of the NAP reference level. This is a relative marginal effect on the annual sedimentation within the basin averaged over 20 years (<1%).
B.3.3.3 Watershed movement
A similar boundary problem is the choice of the vertical boundaries in the tidal basin. These boundaries are situated at the line where sediment fluxes are negligible. For the investigation of the sand balance over a couple of years this approach seems satisfactory. On the time scale of decades however these watersheds could vary in location. Ongoing morphological processes in the tidal basin may cause a gradual shift of the watershed. If the
B–6 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
balance area boundaries are not correspondingly shifted a part of the morphological active basin area may fall out of the balance area. Since the areas furthest away from the inlet contain a large part of the total flat area, and consequently contribute significantly to the volume of sand, leaving out just a fraction may already result in a noticeable loss (Figure B•10).
Figure B•10: Movement of the watershed in a tidal basin. a) before and b) after
B.3.4 Discussion; Demand for sand and “sand hunger”
In the previous sections different processes have been discussed which could explain losses or contributions to the sand balance. Table B•1: Processes and events and their influence on the sand balance.
process or event addition or loss of sand from sand balance sea level rise no direct loss or addition (could initiate changes in balance) artificial nourishment addition large scale geological processes loss gas mining loss man made interventions no direct loss or addition (could initiate changes in balance) changes in NAP reference level addition (or a reduction of the loss due to subsidence) watershed movement loss or addition
Due to sea level rise and man made interventions the morphodynamics of the Wadden Sea alter. This has resulted in the import of sediment, also being referred to as its “hunger for sand”31. This sediment can only be delivered to the basin by the ebb•tidal delta and barrier island coasts. The hunger for sand of the basin therefore results in a demand of sand at the ebb•tidal delta and barrier island coasts.
31 in Dutch: Zandhonger. WL | Delft Hydraulics B–7 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
Figure B•11: “Hunger for Sand” within the basin due to natural processes and human interventions (whereby sea level rise is the dominant contribution). (after Cleveringa, 2004)
For the Frisian Inlet several studies have been carried out to determine its “hunger for sand”.
B.4 Sand balance for the Frisian Inlet
B.4.1 Data availability
Within literature there are four main documents which attend to the sand balance for the Frisian Inlet: Oost and De Haas (1992), Biegel en Hoekstra (1995), Eysink et al. (1998) and Mulder (2001). The study of Oost and de Haas is the most extensive one, part 2 covers the period 1970•1987 (part 1 covers the period 1927•1970). The study of Biegel and Hoekstra uses slightly different definitions for the spatial extent of the basin and ebb•tidal delta but resulting differences are small compared to Oost and Haas (same period as Oost and Haas part 2). The study of Eysink is an extension of the study of Oost and De Haas and additionally covers the years 1987•1995. Moreover it pays attention to losses from the balance due to gas minning. The study of Mulder is the most recent one but only treats losses from the ebb•tidal delta.
The decision is made to refer only to the data from the study done by Oost and De Haas (1992). This data set is the most extensive one available and covers the decades of interest (Viz. 1969•1987). Part of this research is set out to compare model results (sediment import) with historical data. The closure of the Lauwers Sea in 1969 caused the most distinctive impact in the morphology of the Frisian Inlet in the last century. The question is to what extent this impact can be reproduced by the Delft3D model.
Apart from the data set of Oost and De Haas (1992) this thesis study also provided a data set resulting from an analysis of the bathymetries in the period 1970•2000. This was done because the basin area did not vary for the period 1970•1987 in the study of Oost and Haas (1992). In reality a gradual shift to the east can be noted for the eastern watershed. A check was done to see whether with a Delft3D data interpolation tool (QUICKIN) values for bed level changes could be reproduced. And if so, to what extend the movement of the watershed influences these numbers. For the method above; flats are defined as the dry area between MHW and MLW32 and are, equal to Oost and De Haas, kept at the same level. Channels are defined as the wet volume
32 MHW equals +1 m. NAP, MLW equals •1.3 m. NAP for the Frisian Inlet B–8 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
below MLW. Subsequently sedimentation within the basin is calculated by a reduction in channel volume and an increase in flat volume.
B.4.2 Data Oost and Haas 1992
Deposition rates for the Frisian Inlet, sedimentation (+), erosion (•), according to Oost and Haas (1992) is listed in the table below. All data are corrected for sand extraction.
Sedimentation Sedimentation Sedimentation Sedimentation [1·106 m3] [cm/yr] [1·106 m3] [cm/yr]
Area total per year per year total per year per year
Pinkegat Basin
1971•1975 +3.7 +0.93 +1.75
1975•1978 •0.3 •0.11 •0.21
1978•1981 •1.2 •0.40 •0.75
1981•1987 +3.3 +0.55 +1.04
1971•1987 +5.5 +0.34 +0.65
Zoutkamperlaag Basin
1971•1975 +14.8 +2.95 +2.33
1975•1978 +10.7 +2.68 +2.12
1978•1981 +4.3 +1.43 +1.13
1981•1987 +3.8 +0.76 +0.60
1971•1987 +33.6 +1.98 +1.56
Frisian Inlet Frisian Inlet ebb•tidal delta Basin
1971•1975 +18.5 +3.88 +2.16 1971•1975 •9.8 •1.96 •2.08
1975•1978 +10.4 +2.57 +1.44 1975•1978 +0.4 +0.10 +0.11
1978•1981 +3.1 +1.03 +0.57 1978•1981 •100 •3.33 •3.53
1981•1987 +7.1 +1.31 +0.73 1981•1987 •6.6 •1.32 •1.40
1971•1987 +39.1 +2.32 +1.30 1971•1987 •26.0 •1.53 •1.62
WL | Delft Hydraulics B–9 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
The main conclusions for the observed bed level changes according to Oost (1995) are: § Net erosion of the ebb•tidal delta occurred after 1950, except for the period 1975• 1979. The total net erosion was equal to 26 · 106 during the period 1970•1987. The erosion in that period was mainly between •4 and •12 m. NAP, whereas sedimentation was found above and below this zone. § Strong sedimentation occurred in the period 1966•1970, probably as a result of the gradual enclosure of the Lauwers Sea. Between 1970•1987, a net sedimentation of 39,1 · 106 m3 occurred in the basin, with the highest sedimentation in the 1970’s. The strongest sedimentation was observed in the deeper channels between •5 and • 15 m. NAP.
B.4.3 Data including watershed movement
Deposition rates for the basin of the Frisian Inlet, sedimentation (+), erosion (•), calculated with the Delft3D data interpolation tool QUICKIN, are listed in the table below.
Sedimentation Sedimentation Basin Area [1·106 m3] [cm/yr] [1·106 m2]
Area total per year
Frisian Inlet Basin
1970•1979 +28.0 +3.11 1.83 170
1979•1982 +5.0 +1.67 0.95 176
1982•1987 +4.0 +0.80 0.46 175
1987•1995 +6.0 +0.50 0.28 177
1995•2000 +7.0 +1.40 0.77 181
1970•1987 +37.0 +2.18 1.23 177
B.4.4 Data Delft3D model calculations tide•only
After the model set•up a long term simulation has been carried out to check whether the model could reproduce the net import behaviour of the Frisian Inlet. This has been done by implementing the bathymetry of the year 1970 into the model, and running the model with tide•only. Note that the used water level boundary conditions are equal to those used by van Ledden (2002). The time span which is modelled is the period 1970•1990. The morphological development is accelerated with the use of a morphological scale factor of 50.
Deposition rates for the basin of the Frisian Inlet, sedimentation (+), erosion (•), for the long term simulation (calculated with the Delft3D data interpolation tool QUICKIN), are listed in the table below.
B–10 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
Sedimentation Sedimentation Basin Area [1·106 m3] [cm/yr] [1·106 m2]
Area total per year
Frisian Inlet Basin
1970•1975 +2.5 +0.50 0.30 164
1975•1979 +0.6 +0.15 0.09 171
1979•1982 +3.9 +1.30 0.78 167
1982•1987 +0.1 +0.02 0.01 174
1987•1990 •0.1 •0.03 •0.02 174
1970•1987 +7.1 +1.01 0.58 174
B.5 Comparison of sediment import
The data including watershed movement matches the data from Oost and Haas rather well. The approximated increase of basin area however is not of significant importance for the yearly averaged sedimentation rates within the basin. The addition of 10 2km contributes approximately 1% to the averaged sedimentation rate of 1 cm/year. With respect to the long•term simulation; results show that the used model, boundary conditions and 1970•bathymetry give an import of sand over the years. The amount of import however is significantly lower than numbers calculated by Oost and Haas. This difference may be contributed to the absence of waves in the used model.
Sedimentation within the time period Amount basin (research) Oost and Haas (1992) 1970•1987 +2.32 Mm3/yr This thesis study 1970•1987 +2.18 Mm3/yr Delft3D long•term simulation 1970•1987 +1.01 Mm3/yr
Apart from the poor representation of sedimentation numbers within the basin the bed level morphology too did not match field observations. In reality a strong deposition of sediment was observed at the deeper parts of the Zoutkamperlaag. In the model however the channels only showed marginal sedimentation.
The decision is made not to model the long•term behaviour of the Frisian Inlet accurate, as this requires a significant amount of additional research and time. The data analysis shows that at present time the Frisian Inlet is still a tidal basin with a net influx of sediment (the influx is in the order of 1 Mm3/yr). For the purpose of this research the net import should be in the same order of magnitude for Phase A and B.
WL | Delft Hydraulics B–11 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
B.6 Citation of Eysink et al. (1998)
With respect to the “hunger for sand” from the Wadden Sea basin in combination with the option to artificial feed the basin sand, Eysink et al. (1998) stated the following:
4.4 Zandsuppletie in geulen in buitendelta33
Kustafslag als gevolg van de zandhonger van de Waddenzee kan theoretisch alleen worden gestopt door afsluiting van de Waddenzee. Deze mogelijkheid is voor het laatst uitgebreid bestudeerd door de Waddenzeecommissie (Mazure et al., 1974) en om economische en waterhuishoudkundige redenen verworpen. Sinds die tijd is tevens het besef rond de natuurwaarde van dit soort gebieden en van de Waddenzee in het bijzonder sterk gegroeid en is afsluiting om die reden alleen al ondenkbaar geworden. Vanuit dit laatste standpunt en de wettelijke status van de Waddenzee op dit moment, is het ook ongewenst om de negatieve effecten van bodemdaling in de Waddenzee bij de bron te bestrijden. De schade aan de natuur zou hierdoor groter zijn dan de geringe effecten van de bodemdaling zelf. Een goed alternatief om het effect op de Noordzeekust te bestrijden is wel het toepassen van zandsuppletie ergens in de transportweg van de kust naar de bodemdalingskuil in de Waddenzee. Een gunstige plek hiervoor is vlak voor of in een diepere vloedschaar in de buitendelta van de vloedkom waarin de bodemdaling optreedt. Daar is het storten van zand niet schadelijker dan op het strand of op de vooroever. De extra zandhonger kan zo effectief worden gestild door de vloedkom het zand als het ware in de mond te leggen. Hierdoor wordt voorkomen dat de extra zandhonger van de vloedkom effect heeft op de Noordzeekust. Een voorwaarde voor deze wijze van suppleren is wel dat de suppletie zelf de geul niet te veel verstoort, waardoor bijvoorbeeld de scheepvaart of de ecologie gehinderd gaat worden. Dit kan worden voorkomen door een juiste keus in suppletievolume en • frequentie en het seizoen te bepalen. Doordat zich in de bodemdalingskuilen in de Waddenzee ook wat slib afzet kan het suppletievolume wat minder zijn dan het volume van de bodemdalingskuil (zie Paragraaf 2.7.3).
Het is niet helemaal uitgesloten dat, met name hij kleine vloedkommen, door beperkingen in de waterdiepte op de stortlokatie noodgedwongen de zandwinning met relatief kleine sleepzuigers meet worden uitgevoerd. Dit is nadelig voor de eenheidsprijs van het te suppleren zand. Het verdient dan ook aanbeveling om per geval de mogelijkheden en de kosten per kubieke meter suppletiezand na te gaan en deze te vergelijken met die voor strandsuppletie.
33 Citation is copied from Eysinket al., W.D., Integrale bodemsalingstudie Waddenzee: geomorfologie en infrastructuur, WL | Delft Hydraulics, Grondmechanica Delft, Alkyon. Assen: NAM, [1998] (section 4.4.) B–12 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
C Tidal Boundary Conditions
C.1 Nesting the Frisian Inlet model into the ZUNO•Grof model
C.1.1 The ZUNO•Grof model
To obtain new boundary conditions for the Frisian Inlet model the ZUNO•Grof model has been run. This Delft3D model covers the southern part of the North Sea. The model has the following schematizations: § Grid size along the Dutch coast is in the order of 1 to 2 km; § Coastal bathymetry (max 10 km. offshore) composed out of measurement data of the year 1999 (or as close by as possible); § Bathymetry of the North Sea derived from a terrain model (1990); § Model is depth averaged and computes only the tidal flows. (wind and waves are not included). The model is originally 3D and calibrated, the 2D model is the 3D version simplified to one layer (but not calibrated); § Astronomic boundary conditions (water levels) are imposed at northern and southern boundary; § Time frame of the ZUNO•Grof model is set to 15th of October 2002 to 15th of October 2003. The used time step is 2 minutes; § The discharges of rivers are included but are of minor influence for the Frisian Inlet region;
Figure C•1: Grid and bathymetry of the ZUNO•Grof model
WL | Delft Hydraulics C–1 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
C.1.2 Decomposition and nesting
The goal of the nesting procedure is to obtain water level data at locations which coincide with the boundaries of the Frisian Inlet model. A single grid point of theZUNO•Grof model however coincides with almost half of the Frisian Inlet Model boundary. Water level data for one point in the ZUNO•Grof model can not assumed to be valid for an entire boundary. This problem is solved by means of local decomposition of the grid ofZUNO•Grof the model around the islands of Ameland and Schiermonnikoog. This means the grid is refined a factor five (Figure C•2). The refined grid allows the nesting procedure of Delft3D to place enough observation points at locations which are close by the boundaries of the Frisian Inlet Model. Subsequently the ZUNO•Grof model is run.
The water level data from the observation points is converted to boundary conditions for the Frisian Inlet model. This is done with the use of the nesting tool of Delft3D. This tool interpolates the water level data from the observation points to water level data for the beginning and end of each boundary segment of the Frisian Inlet model. Along each boundary segment the water level is linearly interpolated.
Figure C•2: Refined grid (red) with observation points (light blue crosses) around the boundaries of the Frisian Inlet Model (yellow).
C.2 Validation of tidal boundary conditions
C.2.1 Validation water level time series Frisian Inlet model with ZUNO• Grof model
The nesting procedure delivered new boundary conditions for the Frisian Inlet model in the form of time series for the period of 15th of October 2002 to 4th of June 200334. To validate the new boundary conditions the water level time series of ZUNO•Grof the model and Frisian Inlet model are compared for two points in space. For the Frisian Inlet model two observation points are chosen which lay close to the boundaries. Figure C•3 shows that the water motion within the Frisian Inlet model follows the water motion of theZUNO•Grof model. The small discrepancies which can be noted are contributed to the fact that the Frisian Inlet water level data set contained 2•hourly observations while ZUNO•Grof the contained hourly observations.
34 due to a premature stop of the ZUNO•Grof model not completed until 15th of October 2003 C–2 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
Figure C•3: Match of modelled water levels between the Frisian Inlet model and theZUNO•Grof model
C.2.2 Validation time series with harmonic boundary conditions
To reduce the computational time of the model, the boundary conditions (time series) are converted to harmonic boundary conditions. Since water levels have not been verified with measurement data a harmonic analysis is not valid. The regular approach would filter out only the significant constituents e.g. ( M2, S2, N2, K1, O1, M4, MS4,etc.). This would however cause the water motion to be different than the water motion of theZUNO•Grof model, and is so far our only point of reference. Therefore an “over dimensioned” harmonic analysis has been done, whereby as much as possible constituents are included in the analysis (185 constituents). This results in prescribed harmonic water levels which match the time series conditions (Figure C•4). The Delft3D model processes harmonic boundary conditions faster than a time series file. In this way the computational time of the model has some what been reduced.
WL | Delft Hydraulics C–3 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
Figure C•4: Match of modelled water levels for the Frisian Inlet with time series boundary conditions and harmonic boundary conditions (only water levels for wave condition 1).
Overall it should be noted that the length of the obtained time series determines the amount of resolvable constituents for a harmonic analysis. The amount of resolvable constituents is determined by the Raleigh Criterion, given by:
1 ||ff-= Equation C•1 21T
Where f1 and f2 are frequency components, and T is the record length of the data set. This criteria requires that only constituents separated by at least one complete period from their neighbouring constituents over the length of the data record be included in the harmonic analysis of a given time series. For example, to resolve the constituents M4 and MS4 (respectively frequencies equal to 1/6.210301 hr. and 1/6.103339 hr.) the record length of the time series needs to be 354.37 hours (14.77 days) at least. To get a sound prediction of the tide for the Dutch coastal waters Rijkswaterstaat uses 94 components. This includes the components K1 and S1 (respectively frequencies equal to 1/23.93447 hr. and 1/24 hr.), for which a record length of 365.2 days is mandatory. The obtained tidal constituents are subsequently only valid for the period of the underlying time series. To make them independent of the time•series period a nodal correction should be applied which corrects the constituents for the 18.6 year lunar cycle.
C–4 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
C.2.3 Validation water level time series between Frisian Inlet and measured data
Figure C•5 shows measured35 and modelled time series for the observation point Wierumergronden. Modelled water levels within the Frisian Inlet model deviate from measured conditions. This is contributed to model schematisations of ZUNO•Grof the model.
Figure C•5: Comparison of modelled water levels for the Frisian Inlet with measured water levels for the first three months of 2003.
The ZUNO•Grof model (3D version) has been calibrated on water level motion within the North Sea for the year 2003. The obtained water level data are from the simplifiedZUNO• Grof model (2D) which is not calibrated (e.g. on bed shear stress) and thereby overestimates the tidal range at Wierumergronden. The characteristics of measured and modelled water levels are shown in the table below.
Amp. M2 [m.] Phase M2 [°] Amp. M4 [m.] Phase M4 [°]
meas. model meas. model meas. model meas. model
Wierumer• 0,94 1,04 •150,94 •159,03 0,09 0,09 •99,32 •88,89 gronden
The amplitude of the diurnal constituent is 10 cm. larger in magnitude. Moreover, the mean water level is measured at •0.06 m. NAP while the modelled water level is +0.14 m. NAP.
The relative phase difference (phaseM4 – 2phaseM2) equals for the measured water levels 211° and for the modelled water levels at Wierumergronden 218°. The water level rise is faster than its fall for a relative phase difference between 180° and 360°. Although the amplitude of the main tidal constituent is not well represented, the relative phase difference is. The relative phase difference determines the tidal asymmetry and is therefore of great importance for the sediment transport in the Frisian Inlet.
35 In fact water levels are not measured but predicted by means of a harmonic analysis of measured conditions from previous years. WL | Delft Hydraulics C–5 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
C–6 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
D Derivation of the Morphological Wave Climate
D.1 Listing of SON Wave Buoy Data
To get a listing of the probability of occurrence for the significant wave height for 12 directional sectors visit the website: http://www.golfklimaat.nl/ direct link http://www.golfklimaat.nl/cgi• bin/gks2004?outputtype=table1&loc=son&period=year&par1=H1_3&cs1=c4&par2=TH1_ 3&cs2=c4&direction=wave&directionclass=c2&language=dutch&output=table&subject=di stributions_simdir
D.2 Derivation of Morphological wave Climate
D.2.1 Method
For the derivation of three morphological waves which represent the morphological effect of all the waves within a directional wave sector the following has been done. The morphological effect of one wave is assumed to related to the its height to the power 2.5 and linear dependent on its probability of occurrence. Morphological effect of one wave: 2.5 Morph. effect of one wave = PHH()ii× The mean morphological effect of alln waves is the summation of each wave height to the power 2.5 times its probability of occurrence divided byn. The morphological effect ofn waves then equals: n 2.5 Morph. effect of n waves = å(PHH()ii× ) i=1 This morphological effect can hypothetically be reproduced by one wave with a wave height of n 2.5 å(PHH()ii× ) i=1 Hmorph =2.5 n åPH()i i=1 And a probability of occurrence equal to n PH(morphi )= å PH () i=1
WL | Delft Hydraulics D–1 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
D.2.2 Example for a medium wave height in the {0º — 15º} directional sector
The probability of occurrence for the wave heights for the so called medium bin (directional sector {0º — 15º}) are listed in Table D•1. For these waves the morphological wave is determined at 2.03 m.
Table D•1: Listing of the probability of occurrence for waves with a height between 1.5 m. and 3 m. (medium wave class) from the directional sector {0º — 15º}. It furthermore shows the derivation of the morphological medium wave for this directional sector.
wave height H P(H) morphological medium bin [m] [m] [%] effect [m2.5]
1.5•1.6 1.55 0.114 0.3409841 1.6•1.7 1.65 0.104 0.3636999 1.7•1.8 1.75 0.063 0.2552323 morph. effect medium 1 1.5831
1.8•1.9 1.85 0.07 0.3258572 P(Hmorph_medium1) 0.407
1.9•2.0 1.95 0.056 0.2973545 Hmorph_medium1 1.7217 2.0•2.1 2.05 0.048 0.2888191 2.1•2.2 2.15 0.037 0.2507829 2.2•2.3 2.25 0.031 0.2354063 morph. effect medium 2 1.3522
2.3•2.4 2.35 0.036 0.30477 P(Hmorph__medium2) 0.181
2.4•2.5 2.45 0.029 0.2724666 Hmorph_medium2 2.2353 2.5•2.6 2.55 0.026 0.2699752 2.6•2.7 2.65 0.02 0.228636 2.7•2.8 2.75 0.023 0.2884427 morph effect medium 3 1.0587
2.8•2.9 2.85 0.01 0.1371236 P(Hmorph_medium3) 0.088
2.9•3.0 2.95 0.009 0.1345233 Hmorph_medium3 2.7046
Morphological 3.9940736 effect of all waves
PH()morph 0.676%
Hmorph 2.03 m.
.
D–2 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
D.2.3 Cross check for one wave class by model calculations
The schematisation of all waves within the {1.5 m. – 3.0 m.} bin to one morphological wave is quite rough. Table D•1 also lists the morphological wave height of sub•bins within the medium wave height bin.E.g. the morphological wave height for the waves with a height ranging from 1.5 m. to 2.0 m. is determined at 1.72 m. This subdividing is done in order to make a small cross•check on the used approach.Viz. does the used medium morphological wave of 2.03 m. result in the same sediment transport as the summation of the 3 sub•medium morphological waves?
Figure D•2 shows the results of both simulations36. The similarity of both results with respect to the sedimentation and erosion pattern is satisfactory (Figure D•2.a.b) although the absolute numbers show some deviations (Figure D•2.c.d). Most striking is the similarity in sediment transport in the inlet (zone of interest) induced by one and the three waves. Overall it is concluded that the morphological wave is not able to reproduce the exact effects of all waves within the wave class. It is however a fair schematisation and a time•saving modelling approach. The above used cross•check could be used to calibrate the morphological waves. As this is an elaborate and time consuming procedure it falls out of the scope of this research.
D.3 Delft3D FLOW grid vs. Delft3D WAVE grid
Figure D•1 shows the flow and wave grid of the Frisian Inlet model.
Figure D•1: Flow and wave grid for the Frisian Inlet model (wave grid not completely depicted)
36 Simulation 1: 3 waves (1.72 m., 2.24 m. and 2.70 m. ) each wave 9 days (with morphological scale factor of respectively 4.65, 2.06, and 1), total morphological time 77 days. Simulation 2: 1 wave (2.03 m.) 10 days (with morphological scale factor of 7.7), total morphological time 77 days. Tidal boundary conditions in form of time series from January 2003. WL | Delft Hydraulics D–3 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
Figure D•2: Left column morphological effects after 2.5 months for the medium morphological wave. Right column morphological effects after 2.5 months for the summation of the three medium morphological waves. (a and b) Sedimentation and erosion patterns averaged over the T•Tiles [m]. (c and d) Values for sedimentation and erosion averaged over the T•Tiles [m]. (e and f) Absolute transports over the sides of the T•Tiles after 2.5 months simulation [103 m3].
D–4 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
E Model parameters
E.1 Calibration of the Morphological scale factor
Figure E•1: (a) Erosion at one grid cell [m]. (b) Difference in erosion for different morphological scale factors [m] during the simulation period of 1440 morphological days (compared to erosion with the use of a morphological scale factor of 12).
The figure above shows the erosion at a grid cell with the use of different morphological scale factors (12, 24 and 48 ). The modelled morphological time span is equal for all simulation (1440 days). The spring neap variation in water levels and velocities can also be seen in the erosion at this point. Subsequently the difference in erosion for the use of different morphological scale factors varies in time. The trend in this difference is used as criterion for the determination of suitable morphological scale factor. The lower plot shows that the difference in erosion increases more rapidly with the use of a factor of 48. It furthermore shows that, when results are compared at the intervals not equal to a full spring neap tide (of the underlying water motion) differences can be significant. For a morphological scale factor of 24 this difference is at the utmost 10% within the first 4 year. This is regarded as acceptable in contrary to the 20% for a factor 48 (which additionally shows a more rapidly increase).
WL | Delft Hydraulics E–1 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
E.2 Time step sensitivity
Figure E•2: Erosion at observation station “Wierumergronden” for simulations with a time step of 0.5, 1 and 2 minutes.
The time step used for all simulations is set to 1 minute. The use of a time step of 1 or 0.5 minutes shows little variation for the erosion at Wierumergronden. The use of a time step of 2 minutes already shows a deviation within the first month (compared to the result with a time step of 0.5 minutes).
E.3 Physical and numerical parameters
E.3.1 Physical parameters
Apart from the parameters discussed in Chapter 2 other physical parameters are used as input for the model
Table E•1: Physical parameters
Parameter Delft3D Parameter name Value Earths gravity Ag 9.81 m/s2 Water density Rhow 1000 kg/m3 Roughness (Manning coefficient) Ccofu \ Ccofu 0.021 Horizontal eddy viscosity Vicouv 1.0 Horizontal eddy diffusivity Dicouv 1.0
E.3.2 Morphological parameters
For the computation of the morphological development several model parameters need to be given a value. Most parameters are set to their default value. Due to the relatively large grid cell area the AlfaBn is rather high to account for possible large gradients in bottom depth within one cell. Although the value of some parameters can have significant influence on the model results, it is expected that their influence on relative results is negligible.
E–2 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
Table E•2: Morphological parameters
Parameter Delft3D Parameter name Value Update bathymetry during flow run MorUpd true Equilibrium concentration at inflow boundaries EqmBc true Include effect of sediment on density gradient DensIn false Van Rijn's reference height AksFac 0.5 Wave related roughness RWave 2.0 Longitudinal bed gradient for bed load transport AlfaBs 1.0 Transverse bed gradient for bed load transport AlfaBn 25.0 Suspended transport factor Sus 0.5 Bedload transport factor Bed 0.5 Wave•related suspended transport factor SusW 1.0 Wave•related bedload transport factor BedW 1.0 Minimum depth for sediment computations SedThr 0.25 [m] Fraction of erosion to assign to adjacent dry cells ThetSD 0.0 Tuning parameter for wave streaming FWFac 1.0 Only for waves in combination with k•epsilonEpsPar true turbulence model
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F Sediment distribution for the Frisian Inlet
source: sediment atlas for Wadden Sea (from: www.waddenzee.nl)
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G Summary of research phases and model settings
Research Phase A A B Simulation type “Fixed bottom” “Fully” morphological “Fully” morphological with tide•only with tides and waves Autonomous situation no yes yes modelled Studied alternatives 1 • 7 3, 5, 6 and 7 3, 5, 6 and 7 Type of boundary water levels water levels water levels conditions (harmonics) (harmonics) (harmonics), morphological wave climate Time span used water 1st of Jan. 2003 – 1st of Jan. 2003 – 1st Jan. 2003 – levels 27th of May 2003 27th of May 2003 27th March 2003 Morphological scale 24 24 1•13 factor Modelled time span 9.5 years 9.5 years 1 year Analysed time span 9.5 years autonomous situation: 1 year 9.5 years. the alternatives :3 years Attended in thesis see section 3.3 see section 3.5 see section 4.4 Documented results Appendix I Appendix J Appendix K Goal asses the direct effects get insight in the determine the (only nourished sand flow and sediment distinct contribution is free) transport patterns in of waves the Frisian Inlet under the forcing of the tide only
Additional simulations reference computation (Appendix H) Long term computation 1970• 1990 (Appendix B)
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H Reference computation Tide•only
To determine a nourishment placing strategy the tide•only model is run for a morphological period of a year. Specifications are listed in the table bellow.
Research Phase A Simulation type “Reference computation” Autonomous situation modelled yes Studied alternatives none Type of boundary conditions water levels (time series) Time span used water levels 1st of January 2003 – 1st of February 2003 Morphological scale factor 12 Modelled time span 1 year Analysed time span 1 year Goal Determination of Nourishment Placing Strategy Attended in thesis Section 3.2
WL | Delft Hydraulics H–1
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I Model simulations of nourishment alternatives on a “fixed bottom”
Figure I•1: Mean total transports 3[m/m/yr]. (a) For alternative one. (b) For alternative two. (c) For alternative three. (d) For alternative four. (e) For alternative five. (f) For alternative six. Nourishment locations marked by green dashed line.
WL | Delft Hydraulics I–1 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
Figure I•2: Mean total transports [m3/m/yr]. For alternative seven. Nourishment location marked by green dashed line.
These figures show the mean total transport (for the “fixed bottom” simulations). In accordance with the residual currents over the ebb•tidal delta, nourished sediments are mainly transported in western direction. This causes the nourished sediment of alternative 1 to barely reach the basin. Alternative 3 and 5 clearly find their way into the Pinkegat. While the channel nourishments 6 and 7 both spread out to the ebb•tidal delta and basin. Alternative 2 and 4 are partly being by•passed, partly benefiting the basin.
I–2 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
J Model simulations of tide•only (Phase A)
J.1 Introduction
This Appendix presents additional figures of results obtained for the tide•only simulation. First the autonomous development of the Frisian Inlet will be elaborated. After which the alternatives will be elaborated. For each alternative their absolute effects after 1 and 3 years will be presented and subsequently the effects on averaged transports and current velocities (averaged over the first three years). Some figures are identical to those presented in the main text of this thesis. This is done in order to make the results of the tide•only and tide and wave simulation (respectively Appendix J and Appendix 1.1.1.1A) easily comparable.
Figure J•1: Sedimentation and erosion after 1, 3 and 9.5 years. Corresponding to Figure 3•10.
WL | Delft Hydraulics J–1 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
J.2 Autonomous•simulation tide•only
J.2.1 Transports and current velocities
Figure J•2: (a) Residual depth averaged velocities averaged over three years. (b) Magnitude of residual depth averaged velocities. (c) Residual total transports averaged over three years. (d) Magnitude of residual total transports. (e) Yearly averaged bed shear stress. (f) Yearly averaged sediment concentrations.
J–2 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
J.2.2 Averaged transports (aggregated)
Figure J•3: (a) Mean total transports for the autonomous situation and (b) corresponding magnitudes. (c) Mean suspended transports for the autonomous situation and (d) corresponding magnitudes. (e) Mean bed load transports for the autonomous situation and (f) corresponding magnitudes.
WL | Delft Hydraulics J–3 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
J.3 Nourishment alternative 3 tide•only
J.3.1 Effects after 1 year (left column), effects after 3 year (right column).
Figure J•4: Difference in bathymetry with autonomous situation [m] after: (a) one year (b) three years. Difference in bathymetry with autonomous situation aggregated over T•Tiles [103m3] after: (c) one year, (d) three years. Difference in bathymetry aggregated over T• Tiles [103m3] and difference in total sediment transport over sides of T•Tiles [103m3/yr] after: (e) one year, (f) three years.
J–4 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
J.3.2 Transports and current velocities
Figure J•5: (a) Induced changes compared to the autonomous situation in residual depth averaged velocities. (b) Induced changes compared to the autonomous situation in residual total transport. (c) Magnitude of changes in residual depth averaged velocities. (d) Magnitude of changes in residual transport. Difference in averaged transport (aggregated over sides of T•Tiles) compared to autonomous situation (J.2.2) for (e) suspended transport and (f) bed load transport. The initial location of the nourished sand is indicated with a green dashed line.
WL | Delft Hydraulics J–5 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
J.4 Nourishment alternative 5 tide•only
J.4.1 Effects after 1 year (left column), effects after 3 year (right column).
Figure J•6: Difference in bathymetry with autonomous situation [m] after: (a) one year (b) three years. Difference in bathymetry with autonomous situation aggregated over T•Tiles [103m3] after: (c) one year, (d) three years. Difference in bathymetry aggregated over T• Tiles [103m3] and difference in total sediment transport over sides of T•Tiles [103m3/yr] after: (e) one year, (f) three years.
J–6 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
J.4.2 Transports and current velocities
Figure J•7: (a) Induced changes compared to the autonomous situation in residual depth averaged velocities. (b) Induced changes compared to the autonomous situation in residual total transport. (c) Magnitude of changes in residual depth averaged velocities. (d) Magnitude of changes in residual transport. Difference in averaged transport (aggregated over sides of T•Tiles) compared to autonomous situation (J.2.2) for (e) suspended transport and (f) bed load transport. The initial location of the nourished sand is indicated with a green dashed line.
WL | Delft Hydraulics J–7 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
J.5 Nourishment alternative 6 tide•only
J.5.1 Effects after 1 year (left column), effects after 3 year (right column).
Figure J•8: Difference in bathymetry with autonomous situation [m] after: (a) one year (b) three years. Difference in bathymetry with autonomous situation aggregated over T•Tiles [103m3] after: (c) one year, (d) three years. Difference in bathymetry aggregated over T• Tiles [103m3] and difference in total sediment transport over sides of T•Tiles [103m3/yr] after: (e) one year, (f) three years.
J–8 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
J.5.2 Transports and current velocities
Figure J•9: (a) Induced changes compared to the autonomous situation in residual depth averaged velocities. (b) Induced changes compared to the autonomous situation in residual total transport. (c) Magnitude of changes in residual depth averaged velocities. (d) Magnitude of changes in residual transport. Difference in averaged transport (aggregated over sides of T•Tiles) compared to autonomous situation (J.2.2) for (e) suspended transport. (f) Increase in mean sediment concentrations compared to autonomous situation.
WL | Delft Hydraulics J–9 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
J.6 Nourishment alternative 7 tide•only
J.6.1 Effects after 1 year (left column), effects after 3 year (right column).
Figure J•10: Difference in bathymetry with autonomous situation [m] after: (a) one year (b) three years. Difference in bathymetry with autonomous situation aggregated over T•Tiles [103m3] after: (c) one year, (d) three years. Difference in bathymetry aggregated over T• Tiles [103m3] and difference in total sediment transport over sides of T•Tiles [103m3/yr] after: (e) one year, (f) three years.
J–10 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
J.6.2 Transports and current velocities
Figure J•11: (a) Induced changes compared to the autonomous situation in residual depth averaged velocities. (b) Induced changes compared to the autonomous situation in residual total transport. (c) Magnitude of changes in residual depth averaged velocities. (d) Magnitude of changes in residual transport. (e) Difference in mean bed shear stress compared to autonomous situation 2 [N/m]. (f) Difference in mean sediment concentration compared to the autonomous situation [kg/m3].
WL | Delft Hydraulics J–11
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K Model simulations of tides and waves (Phase B)
K.1 Introduction
This Appendix presents additional figures of results obtained for the tide and waves simulation. First the autonomous development of the Frisian Inlet will be elaborated. After which the alternatives will be elaborated. For each alternative their absolute effects after 1 year will be presented and subsequently the effects on averaged transports, current velocities, bed shear stress and wave energy dissipation. Additionally, flood and ebb flows during the first wave condition and induced differences in current velocities due to the alternative (compared to the autonomous situation) are presented. Some figures within this appendix are identical to those presented in the main text of this thesis. This is done in order to make the results of the tide•only and tide and wave simulation (respectively Appendix J and Appendix 1.1.1.1A) easily comparable.
WL | Delft Hydraulics K–1 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
Figure K•1: Cumulative sedimentation and erosion for the autonomous situation after 1 year
K–2 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
K.2 Autonomous simulation tide and waves
K.2.1 Transports and current velocities (tide and waves)
Figure K•2: (a) Residual depth averaged velocities averaged over three years. (b) Magnitude of residual depth averaged velocities. (c) Residual total transports averaged over three years. (d) Magnitude of residual total transports. (e) Yearly averaged wave energy dissipation [W/m2]. (f) Yearly averaged mean bed shear stress [N/m2].
WL | Delft Hydraulics K–3 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
K.2.2 Averaged transports (aggregated)
K–4 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
K.3 Nourishment alternative 3 – tide and waves
K.3.1 Effects after 1 year
Figure K•3: Differences compared to autonomous situation (Appendix K.2) (a) Difference in bathymetry [m] after one year. (b) Absolute difference in mean wave energy dissipation. (c) Difference in bathymetry aggregated over T•Tiles after one year [103m3]. (d) Difference in mean bed shear stress. (e) Difference in bathymetry aggregated over T•Tiles 3[10m3] and difference in total sediment transport over sides of T•Tiles [103m3/yr] after one year. (f) Difference in mean concentrations.
WL | Delft Hydraulics K–5 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
K.3.2 Transports and current velocities (tide and waves)
Figure K•4: (a) Induced changes compared to the autonomous situation in residual depth averaged velocities. (b) Induced changes compared to the autonomous situation in residual total transport. (c) Magnitude of changes in residual depth averaged velocities. (d) Magnitude of changes in residual transport. Difference in averaged transport (aggregated over sides of T•Tiles) compared to autonomous situation (Appendix K.2.2) for (e) suspended transport and (f) bed load transport. The initial location of the nourished sand is indicated with a green dashed line.
K–6 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
K.3.3 Nourishment alternative 3 ebb and flood (tide and waves)
(a) Depth averaged velocities during flood flow. (b) Total transports during flood flow. (c) Induced changes compared to the autonomous situation in depth averaged velocities during flood flow. (d) Induced changes compared to the autonomous situation in total transports during flood flow. (e) Depth averaged velocities during ebb flow. (f) Total transports during ebb flow. (g) Induced changes compared to the autonomous situation in depth averaged velocities during ebb flow. (h) Induced changes compared to the autonomous situation in total transports during ebb flow.
WL | Delft Hydraulics K–7 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
K.4 Nourishment alternative 5 • tide and waves
K.4.1 Effects after 1 year
Figure K•5: Differences compared to autonomous situation (Appendix K.2) (a) Difference in bathymetry [m] after one year. (b) Absolute difference in mean wave energy dissipation. (c) Difference in bathymetry aggregated over T•Tiles after one year [103m3]. (d) Difference in mean bed shear stress. (e) Difference in bathymetry aggregated over T•Tiles 3[10m3] and difference in total sediment transport over sides of T•Tiles [103m3/yr] after one year. (f) Difference in mean concentrations.
K–8 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
K.4.2 Transports and current velocities (tide and waves)
Figure K•6: (a) Induced changes compared to the autonomous situation in residual depth averaged velocities. (b) Induced changes compared to the autonomous situation in residual total transport. (c) Magnitude of changes in residual depth averaged velocities. (d) Magnitude of changes in residual transport. Difference in averaged transport (aggregated over sides of T•Tiles) compared to autonomous situation (Appendix K.2.2) for (e) suspended transport and (f) bed load transport. The initial location of the nourished sand is indicated with a green dashed line.
WL | Delft Hydraulics K–9 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
K.4.3 Nourishment alternative 5 ebb and flood (tide and waves)
(a) Depth averaged velocities during flood flow. (b) Total transports during flood flow. (c) Induced changes compared to the autonomous situation in depth averaged velocities during flood flow. (d) Induced changes compared to the autonomous situation in total transports during flood flow. (e) Depth averaged velocities during ebb flow. (f) Total transports during ebb flow. (g) Induced changes compared to the autonomous situation in depth averaged velocities during ebb flow. (h) Induced changes compared to the autonomous situation in total transports during ebb flow.
K–10 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
K.5 Nourishment alternative 6 • tide and waves
K.5.1 Effects after 1 year
Figure K•7: Differences compared to autonomous situation (Appendix K.2) (a) Difference in bathymetry [m] after one year. (b) Absolute difference in mean wave energy dissipation. (c) Difference in bathymetry aggregated over T•Tiles after one year [103m3]. (d) Difference in mean bed shear stress. (e) Difference in bathymetry aggregated over T•Tiles 3[10m3] and difference in total sediment transport over sides of T•Tiles [103m3/yr] after one year. (f) Difference in mean concentrations.
WL | Delft Hydraulics K–11 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
K.5.2 Transports and current velocities (tide and waves)
Figure K•8: (a) Induced changes compared to the autonomous situation in residual depth averaged velocities. (b) Induced changes compared to the autonomous situation in residual total transport. (c) Magnitude of changes in residual depth averaged velocities. (d) Magnitude of changes in residual transport. Difference in averaged transport (aggregated over sides of T•Tiles) compared to autonomous situation (Appendix K.2.2) for (e) suspended transport and (f) bed load transport. The initial location of the nourished sand is indicated with a green dashed line.
K–12 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
K.5.3 Nourishment alternative 6 ebb and flood (tide and waves)
(a) Depth averaged velocities during flood flow. (b) Total transports during flood flow. (c) Induced changes compared to the autonomous situation in depth averaged velocities during flood flow. (d) Induced changes compared to the autonomous situation in total transports during flood flow. (e) Depth averaged velocities during ebb flow. (f) Total transports during ebb flow. (g) Induced changes compared to the autonomous situation in depth averaged velocities during ebb flow. (h) Induced changes compared to the autonomous situation in total transports during ebb flow.
WL | Delft Hydraulics K–13 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
K.6 Nourishment alternative 7 • tide and waves
K.6.1 Effects after 1 year
Figure K•9: Differences compared to autonomous situation (Appendix K.2) (a) Difference in bathymetry [m] after one year. (b) Absolute difference in mean wave energy dissipation. (c) Difference in bathymetry aggregated over T•Tiles after one year [103m3]. (d) Difference in mean bed shear stress. (e) Difference in bathymetry aggregated over T•Tiles 3[10m3] and difference in total sediment transport over sides of T•Tiles [103m3/yr] after one year. (f) Difference in mean concentrations.
K–14 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
K.6.2 Transports and current velocities (tide and waves)
Figure K•10: (a) Induced changes compared to the autonomous situation in residual depth averaged velocities. (b) Induced changes compared to the autonomous situation in residual total transport. (c) Magnitude of changes in residual depth averaged velocities. (d) Magnitude of changes in residual transport. Difference in averaged transport (aggregated over sides of T•Tiles) compared to autonomous situation (Appendix K.2.2) for (e) suspended transport and (f) bed load transport. The initial location of the nourished sand is indicated with a green dashed line.
WL | Delft Hydraulics K–15 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
K.6.3 Nourishment alternative 7 ebb and flood (tide and waves)
(a) Depth averaged velocities during flood flow. (b) Total transports during flood flow. (c) Induced changes compared to the autonomous situation in depth averaged velocities during flood flow. (d) Induced changes compared to the autonomous situation in total transports during flood flow. (e) Depth averaged velocities during ebb flow. (f) Total transports during ebb flow. (g) Induced changes compared to the autonomous situation in depth averaged velocities during ebb flow. (h) Induced changes compared to the autonomous situation in total transports during ebb flow.
K–16 WL | Delft Hydraulics Smart Nourishment of the Frisian Inlet Z3873/Z3912 May, 2006
K.7 Nourishment of the entire ebb•tidal delta
Figure K•11: (a) Induced changes compared to the autonomous situation in residual total transport. (b) Magnitude of changes in residual transport. (c) Difference in bathymetry [m] after one year and difference in total sediment transport over sides of T• Tiles [103m3/yr] after one year. (d) Difference in bathymetry aggregated over T•Tiles after one year [103m3].
WL | Delft Hydraulics K–17 May, 2006 Z3873/Z3912 Smart Nourishment of the Frisian Inlet
K.8 Wave energy dissipation during strong wave conditions
Figure K•12: Results for the fifth wave conditions equal (H to 3.69 m. during two days). (a) Mean total wave energy dissipation[103m3]. (b) Mean total transport during this wave condition.
K–18 WL | Delft Hydraulics