Biogeomorphology Small activities with large effects?

Harriëtte Holzhauer

Biogeomorphology Small activities with large effects?

3-2-2003 Harriëtte Holzhauer

University of Twente Department of Civil Engineering Group of Water Engineering & Management

Biogeomorphology Small activities with large effects?

Preface This report is the result of my graduation project at the University of Twente at the department of Civil Engineering, group of Water Engineering & Management in the Netherlands. This research is carried out in the framework of the Delft Cluster project: Eco- morphodynamics of the seafloor, which is a part of the Theme 3: Coast and River.

I would like to thank my supervisors ir. M.J. Baptist (WL Delft), drs. M.B. de Vries (WL Delft), ir. M. van Ledden (TU Delft), Prof. dr. S.J.M.H. Hulscher (UT Twente) dr. ir. M.A.F. Knaapen (UT Twente).

Cover photo: Harriette Holzhauer; Beach along the North Sea near ‘Paal 10’

i Biogeomorphology Small activities with large effects?

Samenvatting De term biogeomorfologie staat voor de interactie tussen biologie en (geo)morfologie. De bodem van een ondiepe zee zoals de Noordzee of een estuarium zoals de Westerschelde wordt niet alleen bepaald door fysische processen. In en op het bed leven allerlei soorten macro- en micro-organismen. Ook wel benthos genoemd. De dichtheid van deze organismen is op sommige plaatsen erg hoog, bijvoorbeeld op de Molenplaat een getijdeplaat in de Westerschelde en op andere plaatsen juist erg laag. De aanwezigheid van benthische organismen kan de karakteristieken van het sediment veranderen, wat tot ruimtelijke verschillen in een gebied kan leiden. Om de Noordzee duurzaam en ecologisch gezien goed te beheren, is een goede integratie de kennis van biologische processen en morfologische processen van groot belang. De interactie tussen morfologie en benthische organismen is nog niet ver genoeg onderzocht. Het meeste onderzoek heeft zich tot nu toe gericht op de invloed van de morfologie op de benthische organismen. Maar er zijn meerdere aanwijzingen dat deze relatie twee kanten heeft. Organismen zijn in staat om bijvoorbeeld de bodemkarakteristiek, erosie en depositie van het sediment te beïnvloeden. In dit onderzoek wordt een eerste stap genomen om het inzicht in deze relatie te vergroten door te kijken naar de effecten van benthische organismen op een bed karakteristiek zoals het slibpercentage en de invloed van macrozoobenthos op de parameters van een bodemtransportmodel.

De activiteiten van de organismen zijn zeer verschillend. Alle activiteiten zijn gecombineerd tot twee basis effecten. 1) De stabilisatie van het sediment. phytobenthos zoals diatomeeën produceren een plakkerige film op het sediment, die erosie tegen gaat. 2) De destabilisatie van het sediment. Macrozoobentische organismen verplaatsen zich in of door de bodem op zoek naar een schuilplaats of eten. Deze bewegingen zorgen ervoor dat de bodem losser wordt. Dit wordt de destabilisatie van de bodem genoemd.

Deze activiteiten beïnvloeden de sedimentkarakteristieken en bovendien het moment van begin van beweging. De kritische bodemschuifspanning is de minimale wrijving die nodig is om sediment te verplaatsen welke eerst overschreden moet worden voordat er sediment transport optreed. Daarom is de relatie tussen de kritische bodemschuifspanning, de dichtheid van macrozoobenthos en de hoeveelheid aanwezige diatomeeën gebruikt als de verbinding tussen biologie en morfologie.

Er zijn twee modellen gebruikt, een zand-slib model voor de Molenplaat en een bodemtransportmodel voor een zandgolfgebied in de Noordzee. Dit bodemtransportmodel is een onderdeel van een tweedimensionaal model voor ondiepe zeeën. In beide modellen is het effect van benthische organismen meegenomen door middel van de relatie tussen de kritische bodemschuifspanning, de dichtheid van het macrozoobenthos en de aanwezigheid van diatomeeën.

ii Biogeomorphology Small activities with large effects?

De modellen zijn gebuikt in twee aparte casus studies. In de eerste casus zijn metingen van de Molenplaat gebruikt om de sedimentkarakteristieken, de gemiddelde dichtheid van het macrozoobenthos en de gemiddelde hoeveelheid diatomeeën te bepalen. De Molenplaat is op basis van de sedimentverdeling opgedeeld in drie delen, een westelijk, midden en oostelijk deel. In het westelijke en oostelijke deel is een lager percentage slib gemeten dan in het midden van de plaat. De dichtheid macrozoobenthos is veel hoger in het midden van de plaat dan in de westelijke en oostelijke delen. De hoeveelheid diatomeeën daarentegen is het hoogst in het oostelijke deel van de plaat.

Een relatie tussen de kritische stroomsnelheid en de aanwezige dichtheid macrozoobenthos, op een getijde plaat is gebuikt om de destabilisatie van het sediment te beschrijven. Deze relatie is uitgewerkt naar een destabilisatiefactor. Deze factor verandert de ontwikkeling van de kritische bodemschuifspanning bij een oplopende dichtheid macrozoobenthos. Een zelfde soort relatie is afgeleid voor de stabilisatiefactor. Deze factoren samen vormen de ‘Benthos-factor’. Deze factor beschrijft de totale invloed van de aanwezige benthische organismen. Dit komt tot uitdrukking in de kritische bodemschuifspanning welke wordt vermenigvuldigd met de Benthos-factor.

De ontwikkeling van het slibpercentage in een jaar op de Molenplaat, met een start percentage van 20 procent slib, is gesimuleerd met het zand-slib model. Met tijdstappen van 10 seconden is de kritische bodemschuifspanning berekend voor de, op dat moment, aanwezige hoeveelheid diatomeeën en dichtheid macrozoobenthos. Het sediment op de Molenplaat wordt gedestabiliseerd over het gehele jaar en vertoont een verlaging van het slibpercentage van 30 tot 60 procent ten opzichte van het slib percentage zonder de invloed van benthische organismen.

In de Noordzee casus zijn metingen van het macrozoobenthos in een zandgolfgebied gebruikt. Aan de hand van deze metingen zijn de parameters van het bodemtransportmodel voor dit gebied berekend. De Benthos-factor, welke gemaakt is voor een getijde plaat, is ook hier gebuikt om de mogelijke verandering in de kritische bodemschuifspanning te berekeningen als gevolg van benthische invloeden. Omdat er geen organismen leven in het zandgolvengebied met dezelfde eigenschappen als diatomeeën is alleen het destabiliserende effect van het macrozoobenthos meegenomen in de berekeningen. Wat als gevolg heeft dat het destabiliserende effect niet afgeremd wordt door een stabiliserend effect. Dit betekend dat het effect van het macrozoobenthos op de parameter waarden van het bodemtransportmodel wat groter kan zijn dan waarschijnlijk werkelijk het geval is.

Het is mogelijk dat benthische organismen de kritische bodemschuifspanning beïnvloeden. Een verlaging van de kritische bodemschuifspanning zorgt voor een verhoging van de erosie en het transport van sediment. Gezien de gegevens van de Molenplaat is er een verlaging van de kritische bodemschuifspanning opgetreden. In de Noordzee kan de aanwezigheid van benthische organismen het verschil betekenen tussen een gebied met zandgolven of een vlak bed. Dit betekend dat de

iii Biogeomorphology Small activities with large effects? invloed van benthische organismen niet verwaarloosd worden. Vooral in gebieden met een lage stroomsnelheid en een hoge macrozoobenthos dichtheid zijn biologische invloeden belangrijk. Het is wenselijk dat in de zandgolfgebieden meer aandacht wordt besteed aan de mogelijke biologische invloeden. Daarnaast is er nog geen duidelijke relatie beschreven tussen de aanwezigheid van macrozoobenthos die het sediment stabiliseren en de kritische bodemschuifspanning. Verder onderzoek hierin zou een beter beeld kunnen geven van de invloed van benthische organismen in gebieden met een zandig bed. Tot slot is de implementatie van de biologische invloeden in een morfologisch wenselijk.

iv Biogeomorphology Small activities with large effects?

Summary The term Biogeomorphology is describes the interaction between biology and (geo)morphology. The bed of shallow seas like the North Sea or estuaries like the Western Scheldt probably is not fully determined by physical processes only. In and on the seabed live all kinds of macro- and mirco organisms. They are called benthic organisms. At some places the density of these organisms is very high, for example at the Molenplaat, a tidal flat in the Western Scheldt estuary; at other places the density is much lower. The presence of benthic organisms may influence the characteristics of the sediment, leading to spatial differences. For an ecological and sustainable management of the North Sea a good integration of biological processes and morphodynamic processes is important. The interaction of the morphology with benthic organisms is scarcely investigated. Most of the available research has been focused on the influence of the morphology of the bed on benthic organisms. Several indications are given of a relationship between benthic organisms and morphological parameters like bed characteristics, sediment erosion and deposition. In this research a first step is taken, to give insight in the relation between activities of benthic organisms and bed characteristics, like the mud content of the bed and the influence of macrozoobenthos on the parameters of a sediment bedload transport model.

There are a lot of different activities performed by benthic organisms. These activities are combined into two major effects. 1) Stabilisation of the sediment. phytobenthos like diatoms produce a ‘sticky’ film on the sediment. This protects the sediment against erosion. Therefore diatoms are called stabilisers. 2) Destabilisation of the sediment. Macrozoobenthic organisms move through or on the bed in search of food and shelter. These activities make the sediment looser and easier to erode. Therefore macrozoobenthos are called destabilisers. The activities of benthic organisms influence the sediment characteristics and moreover the initial moment of sediment transport. The critical bed shear stress is the value of the shear stress, which must be exceeded before sediment transport occurs. Therefore the relation between the critical bed shear stress, the macrozoobenthos density and the amount of diatoms is used as a connection between biology and morphology.

Two models have been used a sand-mud model for the Molenplaat and a bedload transport model for a sand wave area in the North Sea. This bedload transport model is part of a two-dimensional shallow water model. In both models the influence of benthic organisms is implemented, by using a relation between the critical bed shear stress and the macrozoobenthos density or the amount of diatoms. Both models are applied in a case study. The sand-mud model is used in Molenplaat case and the bedload transport model is used in the North Sea case. In the first case, measurements of the Molenplaat are used to describe the sediment characteristics, the average macrozoobenthos density and the average amount of diatoms present at the tidal flat. The Molenplaat is divided in three parts, west,

v Biogeomorphology Small activities with large effects? centre and east part according to the mean sediment distribution. A lower mud content is measured at the west and east part compared to the centre part. The macrozoobenthos density is much higher in the centre than at the west and east part. The largest amount of diatoms can be found in the east part of the tidal flat.

A relationship between the critical flow velocity and the macrozoobenthos density at a tidal flat is used to describe the destabilisation of the sediment. This relation is expressed in a destabilisation factor, which alters the development of the critical bed shear stress for an increasing density of macrozoobenthos. An analogous relation is derived for diatoms to describe the stabilisation factor. These factors together form the ‘Benthos-factor’. This factor gives the overall influence of benthic organisms. The benthos-factor is multiplied with the initial critical bed shear stress for different combinations of macrozoobenthos densities and diatom amounts.

The development of the mud content in the bed of the Molenplaat, with an initial mud content of 20 percent is simulated with the sand-mud model for one year. For each time step, with increments of 10 seconds, the critical bed shear stress is calculated for the macrozoobenthos density and diatom amount. The sediment at the Molenplaat is mostly destabilised by the organisms and the mud content shows a decrease of 30 up to 60 percent compared to the mud content calculated over one year without biological influence.

In the North Sea case sediment en macrozoobenthos measurements from a sand wave area are used. The parameters of the bedload transport model are calculated for this specific area. The Benthos-factor created for the tidal flat is used to describe a possible change in critical bed shear stress due to biological activities. Because no diatom-like species live in the sand wave area, only the destabilisation of the sediment by a density of macrozoobenthos is taken into account.

Benthic organisms are capable of affecting the critical bed shear stress. A decrease in the critical bed shear stress causes an increase in the erosion and transport of the sediment. The Molenplaat showed a decrease in the mud content of the bed. In the North Sea, the presence of benthic organisms can make the difference between a sand wave area and a flat bed. This means that the influence of benthic organisms can be neglected. In particular in areas with high macrozoobenthos densities and low flow velocities biological activities can be important. It is recommended that in sand wave areas more attention is given to the possible biological influences. Besides this, still there is no clear relation between macrozoobenthos stabilising the sediment (tube construction) and the critical bed shear stress. Further research would give a better vision of the influence of benthic organisms in sand environments. Finally is the implementation of biological influences in a morphological model recommended.

vi Biogeomorphology Small activities with large effects?

Table of contents Preface i Samenvatting ii Summary v Table of contents vii List of figures ix List of symbols xi 1 Introduction 12 1.1 Background 12 1.2 Research approach 13 1.3 Report layout 15 2 Macrozoobenthos and sediment dynamics 16 2.1 Introduction 16 2.2 Destabilisation 17 2.3 Stabilisation 19 3 Area description 21 3.1 Offshore North Sea 21 3.1.1 Macrozoobenthos in the North Sea 22 3.2 Western Scheldt estuary 23 3.2.1 Benthos in the Western Scheldt 24 4 The role of bed shear stress in morphodynamics 25 4.1 Bed shear stress and critical bed shear stress 25 4.1.1 Bed shear stress 25 4.1.2 Critical bed shear stress 26 4.2 Morphodynamic models 29 4.2.1 Water motion 30 4.2.2 Sediment transport 31 4.2.3 Bed topography 34 5 Case: Molenplaat 35 5.1 Area description and data availability 35 5.2 Benthos densities at the Molenplaat 36 5.3 Quantification, biological effects on critical bed shear stress 38 5.3.1 The “Benthos-factor” 38

5.3.2 Destabilisation of the sediment (Bd) 39

5.3.3 Stabilisation of the sediment (Bs) 41 5.3.4 The Benthos-factor at the Molenplaat 42 5.4 Application with sand-mud model 45 5.4.1 Specification model parameters 45

vii Biogeomorphology Small activities with large effects?

5.4.2 Results for the Molenplaat 47 5.4.3 Discussion and conclusions 51 6 Case: North Sea 53 6.1 Description sand-wave area and data availability 53 6.2 Sediment characteristics and benthos densities 54 6.3 Parameters of the sediment transport model 54 6.3.1 Bed-slope coefficient 55 6.3.2 Proportionality coefficient 57 6.3.3 Non-linearity parameter 57 6.4 Results for the sand wave area 58 6.4.1 Bed-slope coefficient λ 58 6.4.2 Proportionality coefficient α 59 6.4.3 Non-linearity parameter b 60 6.4.4 Conclusion and discussion 61 7 Discussion 62 8 Conclusions 64 9 Recommendations 65 10 Literature 66 Appendix 1: Macoma balthica I Appendix 2: Grain size distribution Molenplaat IV Appendix 3: Distribution Chlorophyll a Molenplaat VIII Appendix 4: Distribution macrozoobenthos density at the Molenplaat X Appendix 5: Determination α and b XII

viii Biogeomorphology Small activities with large effects?

List of figures Figure 1: Macrozoobenthos: number of species on the NCS (Netherlands Continental Shelf) in the North Sea 12 Figure 2: Sand tubes protruding from the bed 13 Figure 3: Research approach 14 Figure 4: Benthos remaining on a sieve after sampling of an intertidal flat 16 Figure 5: Schematised drawing of zoobenthos activity in a subtidal North Sea bottom, after de Wolf (1990) 17 Figure 6: The mussel (Mytilus edulis) 17 Figure 7: Bioturbating organisms 18 Figure 8: Suspension feeding and deposit feeding 19 Figure 9: Brackish water marshes in the Western Scheldt 19 Figure 10: Benthic diatom Caloneis africana 20 Figure 11: Benthic diatoms occurring in the Wadden Sea 20 Figure 12: Algae film on intertidal flat 20 Figure 13: North Sea with areas of high natural value 21 Figure 14: Species found in the North Sea, from left to right: Lunatia alderi, Macoma balthica, Owenia fusiformis, Urothoe poseidonis, Amphiura filiformis 22 Figure 15: Satellite photo of the Western Scheldt estuary 23 Figure 16: Intertidal zonation in an estuary 24 Figure 17: Kiezelwieren in the Western Sheldt 24 Figure 18: Morphodynamic cycle with the influence of biological activities. The solid lines give interactions used in this research. The dashed lines give possible interactions. 25 Figure 19: Forces acting on a sediment particle 27 Figure 20: Range of average flow velocity at which sediment particles of different sizes are eroded. The curves for sediment finer than about 0.1 mm is for relatively uncompacted silts and muds 28 Figure 21: Morphological elements 29 Figure 22: Scheme numerical sand-mud model 29 Figure 23: Sketch of the model geometry 30 Figure 24: Modes of sediment transport in water 31 Figure 25: Scheme of the cohesive and non-cohesive regimes 32 Figure 26: Molenplaat in the Western Scheldt; The Netherlands 35 Figure 27: Subdivision Molenplaat; mud percentage in September; with Parisian coordinates 36 Figure 28: Average mud percentage at the Molenplaat for four measuring periods in 1995, March, June, September and December 38 Figure 29: Critical bed shear stress according to (Eq. 27) for a Macoma balthica density ranging from 10 to 35000 40 Figure 30: Relationship between the Critical bed shear stress and the Chlorophyll a content of the sediment according to Widdows 41 Figure 31: Destabilisation and stabilisation factors for densities of Macoma balthica and Chlorophyll a, as given by (Eq. 28) and (Eq. 30) 42 Figure 32: Benthos-factor for the average density of macrozoobenthos and average concentration of Chlorophyll a at the whole Molenplaat for June (diamond), September (hexagram) and December (square); 1995 43 Figure 33: Benthos-factor (B=Bs⋅Bd) for 3 divisions of the Molenplaat in June (diamond), September (hexagram) and December (square) 44 Figure 34: Input sand-mud model; modelled year cycle of the average macrozoobenthos density 46 Figure 35: Input sand-mud model; modelled year cycle of the Chlorophyll a concentration 46 Figure 36: Critical bed shear stress at the Molenplaat for each division with and without benthic organisms over one year. The basic (red) line is without the influence of benthic organisms 47

ix Biogeomorphology Small activities with large effects?

Figure 37: Mud percentage at the Molenplaat with and without benthic organisms over one year. The basic (red)line is without the influence of benthic organisms 47 Figure 38: Benthos-factor for the annual cycles of the average density of Macrozoobenthos and average concentration of Chlorophyll a at the divisions of the Molenplaat 48 Figure 39: Maximal range of critical bed shear stress for the west part of the Molenplaat. The ranges are derived with a minimal and maximal Benthos-factor. 49 Figure 40: Maximal range of mud percentage for the west part of the Molenplaat. The ranges are derived with a minimal and maximal Benthos-factor. 49 Figure 41: Maximal range of critical bed shear stress for the centre of the Molenplaat. The ranges are derived with a minimal and maximal Benthos-factor. 50 Figure 42: Maximal range of mud percentage for the centre of the Molenplaat. The ranges are derived with a minimal and maximal Benthos-factor. 50 Figure 43: Maximal range of critical bed shear stress for the east part of the Molenplaat. The ranges are derived with a minimal and maximal Benthos-factor. 50 Figure 44: Maximal range of mud percentage for the east part of the Molenplaat. The ranges are derived with a minimal and maximal Benthos-factor. 50 Figure 45: Location of three study area A, B, C. 53 Figure 46: Low resolution multibeam image and sample location for area 3 (sand waves) Water depth 26-30m 53 -1 Figure 47: The bed shear stress (τb) and C2. for a flow velocity u=0.6 ms and viscosity -3 -3 v0=7·10 kgm 56 Figure 48: Bed-slope coefficient for a sand wave area with and without biological influence; September 2001 58 Figure 49: Bed-slope coefficient for a sand wave area for a flow velocity u = 0.5 ms-1; September 2001 59 Figure 50: α for different values of τcr with and without influence of macrozoobenthos. 60 Figure 51: b for different values of τcr with and without macrozoobenthos 60 Figure 52: Macoma balthica I Figure 53: Silt percentage Molenplaat March IV Figure 54: Silt percentage Molenplaat June IV Figure 55: Silt percentage Molenplaat September V Figure 56: Silt percentage Molenplaat December V Figure 57: Medium sand percentage Molenplaat March VI Figure 58: Medium sand percentage Molenplaat June VI Figure 59: Medium sand percentage Molenplaat September VII Figure 60: Medium sand percentage Molenplaat December VII Figure 61: Distribution of Chlorophyll a at the Molenplaat, June VIII Figure 62: Distribution of Chlorophyll a at the Molenplaat, September VIII Figure 63: Distribution of Chlorophyll a at the Molenplaat, December IX Figure 64: Distribution of the macrozoobenthos density at the molenplaat, March X Figure 65: Distribution of the macrozoobenthos density at the Molenplaat, June X Figure 66: Distribution of the macrozoobenthos density at the Molenplaat, September XI Figure 67: Distribution of the macrozoobenthos distribution at the Molenplaat, December XI

x Biogeomorphology Small activities with large effects?

List of symbols

τ -2 b Bed shear stress Nm τ -2 cr Critical bed shear stress Nm τ -2 d Shear stress for deposition Nm ρ Water density kgm-3 ρ -3 s Sediment density kgm fc Friction factor of Darcy Weisbach - u Depth averaged flow velocity ms-1 -1 u* Shear velocity ms -1 ucr Critical flow velocity ms g Gravitation acceleration ms-2 C Chézy coefficient m1/2s-1 ks Roughness height of Nikuradse m D Particle diameter m F Force N H Water depth m θ Shields parameter - θ cr Critical Shields parameter -

Re Reynolds number - ν Kinematic viscosity m2s-1 α Proportionality coefficient m-2s2 λ Bed-slope coefficient m2s-2 E Erosion rate ms-1 D Deposition rate ms-1 M Macrozoobenthos m-2 C Chlorophyll a µgg-1 σ Tidal frequency s-1 n Manning coefficient sm-1/3

xi Biogeomorphology Small activities with large effects?

1 Introduction

1.1 Background The aim of the project Eco-morphodynamics of the seafloor is to increase the knowledge of the eco-morphology of the North Sea. For an ecological and sustainable management of the North Sea, a good integration of field observations and knowledge of eco- morphodynamic processes of the North Sea are vital (Laban, 2002). The morphology of a seabed is not fully determined by physical processes only; the presence of macrozoobenthos1 at the seafloor may influence sediment characteristics, leading to spatial differences (Eckman, 1981). Variations in the distribution of macrozoobenthos (Figure 1) in the North Sea not only depend on abiotic parameters such as depth, temperature, sediment composition, organic content of the sediment, tidal range and current speed, but also on biotic parameters such as competition and predation. High diversity of macrozoobenthos in the North Sea is encountered in physically stable environments such as the central Oyster ground and north of the Dogger Bank Figure 1: Macrozoobenthos: number of species on the NCS (Figure 13). Samples taken in turbulent (Netherlands Continental Shelf) in the North Sea erosive areas such as the southern Bight yield (Source: ICONA, 1992) the lowest numbers of species (Holtmann, 1996). The interaction of the morphology of the North Sea with the macrozoobenthos is scarcely investigated. Most of the available research has focused on the influence of the morphology of the seabed on the macrozoobenthos distribution and not the other way around. However, there are indications that macrozoobenthos are capable of affecting the sediment characteristics. This interaction between benthic organisms and (geo)morphology is called biogeomorphology. Morphological processes may affect benthic organisms and benthic organisms may in turn affect morphological processes. In this research there is tried to gain insight in the relation and influence of benthos on the morphology. Although the effects of benthos are small, they can be important because of the non-linearity of dynamics of rhythmic bedforms like sand banks and sand-waves (Hulscher, 1996). The shallow water model could not explain smaller-scale variations in the sand wave area. It is shown that one of the

1 Macrozoobenthos is a term used to refer to a group of invertebrate animals, larger than 1 mm, living in or on top of the seabed

12 Biogeomorphology Small activities with large effects?

possible factors causing these variations is the type of bed deposit. Although perhaps obvious, it was quantified that in the North Sea, sand waves only occur at locations where sand is the dominating bed material. Even small fractions of gravel and/or mud in the bed lead to absence of sand waves (Hulscher, 2001). Consequently, a small change in the bed roughness due to worms or an increased cohesion of the sediment due to algae may change the bed topography considerably. 1.2 Research approach The aim of this research is to gain insight in the relation and influence of benthic organisms on morphology. In this research there is a focus on one important parameter for morphology and ecology, the critical bed shear stress. The activities of benthic organisms influence the sediment characteristics and moreover the initial moment of sediment transport. The critical bed shear stress is the value of the shear stress, which must be exceeded before sediment transport occurs. Therefore the relation between the critical bed shear stress, the macrozoobenthos density and the amount of diatoms is used as a connection between biology and morphology.

Based on this aim, three research questions are addressed in this research: 1. Is it possible that abundant activity of macrozoobenthos affect the critical bed shear stress? 2. What is the effect of the change in bed shear stress on the seabed morphology? 3. Is it acceptable to ignore biological activity in morphodynamic modelling?

The research approach given in Figure 3 is used to answer the research questions. First a literature study on the activity of macrozoobenthos at the sea floor is carried out. Up to now, most studies to the activities of macrozoobenthos are carried out in the estuarine environment. Therefore, the literature study focuses on the estuarine environment instead of offshore. Despite the different environments, the basic activities such as feeding, building and burrowing of benthic organisms are similar. Only the abundance of species which depend on the physical characteristics of the environment, differ between offshore and estuarine areas. The effects of activities of the benthic organisms are related to the critical Figure 2: Sand tubes protruding from the bed bed shears stress of the bed. (Source: www.ucmp.berkeley.edu)

Second the effect of a change in the critical bed shear stress is investigated with the help of two different models. The first model is an analytical two-dimensional morphological model (Hulscher, 1996) and can be used in offshore areas. The model describes the interaction mechanisms between tidal currents and bedform changes. In this research, the parameters of the bedload transport model, which is a part of the two-dimensional morphological model, will be investigated for a

13 Biogeomorphology Small activities with large effects? varying critical bed shear stress due to activity of the macrozoobenthos. The second model is a numerical process-bases sand-mud model (Van Ledden, 2002). The model is mainly developed for estuaries and tidal lagoons. In these areas the alternation of sand and mud is very important. This 1D-point model describes the changes in the bed level and composition of a bed with two sediment fractions, sand and mud. The critical bed shear stress is an important parameter in the erosion and deposition process of the two sediment fractions. In the final part the literature study and the model study are combined in two cases, a Molenplaat case and a North Sea case. The critical bed shear stress is in both cases the connection between morphology and biology. Measurements from the Molenplaat, an intertidal flat in the Western Scheldt estuary in The Netherlands, are used to gain insight in the possible effect of macrozoobenthos on changes in the seabed, in particular the mud percentage of the bed. Measurements from a sand wave area in the North Sea are used to describe the effect of macrozoobenthos on the model parameters of the bedload transport model. Morphological and biological measurements from the intertidal flat and the North Sea are used to determine the presence of specific benthic species and their role in the sediment transport process.

Literature study on the influence of ecology on sediment transport

Study of two morphological models Activities of benthos • • Analytical two-dimensional Feeding shallow water model • Building • Numerical sand-mud model • Burrowing

Important parameters of Important parameter of Alterations in the “Shallow water model” the “Sand-mud model” • Sediment properties Sediment bedload Sediment transport • Initiation of movement transport parameters parameters - Coefficient (α) - Critical bed shear stress - Power of transport (b) for cohesive and non- -Bed-slope coefficient (λ) cohesive sediment

Critical bed shear stress Model application in different situations

Molenplaat; Western Scheldt Offshore; North Sea • Field data • Field data

Is it acceptable to ignore biological activity in the morphological modelling with regard to the critical bed shear stress? Figure 3: Research approach

14 Biogeomorphology Small activities with large effects?

1.3 Report layout In chapter 2 a description of benthic organisms and their effect on the sediment dynamics is given. The different activities of benthos are combined into two major effects, the destabilisation of the sediment and the stabilisation of the sediment. In chapter 3 the offshore and estuarine environment are described. Some specific parameter values for sand wave areas in the North Sea are given. For both areas benthic species common in the area are shortly described. In chapter 4 the role of bed shear stress and critical bed shear stress in morphodynamic models is described. The influence of the critical bed shear stress is given a closer look for two morphodynamic models: a sand-mud model and a two-dimensional shallow water model. The effect of the critical bed shear stress on the sediment transport and the bed topography is discussed shortly. In chapter 5 a case study is given for the Molenplaat a tidal flat in the Western Scheldt estuary. Here the effect of the abundance of benthic organisms is given for the mud content of the bed. In chapter 6 a second case is investigated. A sand wave area in the North Sea is considered. For this specific area the effect of a change in critical bed shear stress due to the presence of macrozoobenthos on the parameters of the bedload transport model is calculated. The final three chapters are used for discussion, conclusions and recommendations for further research.

15 Biogeomorphology Small activities with large effects?

2 Macrozoobenthos and sediment dynamics

2.1 Introduction Biogeomorphology is the interaction between biology and morphology. Biological activity mainly affects the sediment structure and the sediment dynamics. The effects on hydrodynamics are smaller and mainly act through the change in bed roughness (Crosato, 2002). This research is focused on the biological activities affecting the sediment dynamics.

Figure 4: Benthos remaining on a sieve after sampling of an intertidal flat (Source: www.antwerpennoord.be)

The term benthos refers to all organisms living on, in, or near the bottom of water bodies. "Phytobenthos" is used fore the primary producers (i.e., various algae), whereas "zoobenthos" applies to all consumers (i.e., benthic animals). The benthos may be further subdivided based on their size. Large benthic animals (> 1 mm) are collectively referred to as macrozoobenthos (Figure 4) or macro- invertebrates. Representatives include clams, snails, worms, amphipods, and crayfish. All macrozoobenthos affect their environment to some degree. The characteristic of sediment is sometimes strongly influenced by the activity, because the majority of sediment consists of particles small enough to be ingested or manipulated.

According to Lee & Swartz (1980), the benthic organisms can be grouped to three groups. The first division is based on the place where the zoobenthos lives in the sediment. The infauna species live in the sediment and the epifauna species live on the sediment. The second division is based on the difference in mobility. Some species move through the sediment on a regular basis, thereby disrupting the sediment structures, they are called mobile. Some move only during times of stress or reproduction. These species are considered to be immobile. The final division is based on the different feeding strategies. The main feeding systems are suspension feeding and deposit feeding. An illustration of the different behaviour of some benthic species is given in Figure 5.

16 Biogeomorphology Small activities with large effects?

1 Sea potato (Echinocardium cordatum) 2 Parchment (Chaetopterus sp.) 3 Mud Shrimp (Calianassa sp.) 4 Black Clam (Arctica sp.) 5 Brittle Star (Amphiura sp.) 6 Polychaete worm (Gattyana sp.). 7 Polychaete worm (Glycera sp.) 8 Rag worm (Nereis sp.) 9 Polychaete worm (Notomastus sp.) 10 Spoon worm (Echiurus sp.)

Figure 5: Schematised drawing of zoobenthos activity in a subtidal North Sea bottom, after de Wolf (1990) (Source: Crosato, 2002)

Once the benthic species are grouped according to the way they live, move or eat the effect of these activities to the sediment can be grouped as well. Namely: Biodeposition, Bioturbation and Biostabilisation. In a first attempt to give a quantification of the effect of The mussel is macrozoobenthos there is a focus in this an example of research on two effects of benthic an epifaunal activities on sediment transport. Namely: suspension feeder. The the destabilising effect of the bed due to mussel is a reef bioturbation and the stabilising effect of builder, and can the bed due to biostabilisation. These two be found in large banks. A effects are described in more detail, first mussel bed will Figure 6: The mussel (Mytilus edulis) biodeposition is described shortly. influence the (Source: www.waddenzee.nl) sediment composition in its surroundings. Due to the intensive filtering, many fine particles will stick Biodeposition is a sedimentary process together and excreted by the mussel. Mussels pump caused by suspension feeders. Suspension 2 liters of water per hour. In some estuaries mussels feeders ingest particles from the water (both natural beds and culture lots) may filter the column and bring them to the bottom as whole water mass within one week. faeces. Epifaunal filter feeders, which live on the sediment, have a greater biodepositon rate than most infaunal filter feeders, which live in the sediment. The faeces produced by suspension feeders are aggregates of small particles that would otherwise remain in the water column. Large densities of suspension feeders can have a significant effect on deposition rate. It has been suggested that Cardium (Cockles) and Mytilus (Mussels) populations annually deposit 100.000 and 25.000- 75.000 metric tons, respectively, in the Wadden Sea (Lee & Swartz, 1980). 2.2 Destabilisation Bioturbation is the destabilization of the bottom caused by various biological activities of macrozoobenthos such as moving or burrowing through the bed in search for food or shelter. It can modify patterns of sediment stratification and expose deeper material to re-suspension. Almost all bioturbation activities have a destabilizing effect on the sediment in the seabed. In chapter 5 the destabilizing

17 Biogeomorphology Small activities with large effects? effect of Macoma balthica on the sediment is used to gain some insight in the effect of macrozoobenthos on bed characteristics. Bioturbation comprises more than the activities of the Macoma balthica alone but it is still not possible to give an overall explanation of the effect of bioturbation because the knowledge of the various, most important, processes of bioturbation activities is not sufficient. In Figure 7 a representation of some bioturbating organisms is given. The most important activities are burrowing, tube construction and feeding.

From left to right: Arenicola marina, Heteromastus filiformis, Pectinaria sp., Macoma balthica, Corophium volutator, Hydrobia ulvae. Below right: errant polychaete crawling through sediment. After Cadée (1984).

Figure 7: Bioturbating organisms (Source: Crosato, 2002)

Burrows are small holes formed by macrozoobenthos in soft sediments. They may occur either on the surface or deeper in the seabed. The influence of burrowing on sediment transport depends on the cross sectional area of the burrower, distance burrowed and, perhaps, the velocity of burrowing. It appears however, that burrowing does not affect particle transport as much as feeding or perhaps tube construction (Lee & Swartz, 1980). Some worms construct tubes. These tubes stick partly out of the sediment. Laboratory flume experiments with varying densities of the tube-building polchaete worm Owenia fusiformis have conducted that dense populations of tubicolous fauna can stabilize sediment and increase biodeposition through filtering of fine sediment (Eckman, 1981). The main feeding systems of macrozoobenthos are suspension feeding and deposit feeding.

Suspension feeders ingest particles located in the water column; most suspension feeders are filter feeders. Deposit feeders feed on material on or slightly below the surface, ingesting selectively or a-selectively particles of variable size and composition. An explanation of these two ways of feeding is given, using the feeding system of the Macoma balthica as example (Figure 8). The Macoma balthica feeds in several different ways depending on the environment it lives in. On sandy bottoms, they filter food from the water. On muddy and sheltered bottoms they sweep the muddy surface with their long inhalant siphons to gather small particles. This is called deposit feeding.

18 Biogeomorphology Small activities with large effects?

Suspension feeding Deposit feeding

Figure 8: Suspension feeding and deposit feeding (Source: http://www.vattenkikaren.gu.se)

Deposit feeders are important in changing bottom characteristics. Most deposit feeders alter the sediment in order to assimilate only the most useful part of the material. Mobile deposit feeders move both laterally and vertically, thus transporting and mixing bottom particles. 2.3 Stabilisation The stabilisation of sediment by biota is called biostabilisation and can occur in different situations. First, the stabilisation of the sediment can occur due to the binding effects of roots (like Salt Biostabilisation of Salt marshes. They marshes, Figure 9), anchorages or by a prosper in relatively cover of shells anchored to each other sheltered coastal and to the soil protecting the sediment areas, with enough sediment available to from erosion. The presence of high stimulate deposition. densities of these organisms decreases They start to appear the bottom erodibility: a higher shear at the highest bottom levels, with short stress is needed to erode the bottom. inundation times. Lower densities may have the opposite The distribution is effect (Eckman, 1981). patchy at the edges and gets denser and Figure 9: Brackish water marshes Secondly, benthic diatoms can form uniform closer to the in the Western Scheldt extensive mats and excrete EPS mucus, land. The presence (Source: Crosato, 2002) which is a sticky substance, made of of vegetation generally results in enhanced sediment trapping and enhanced accretion rates. polysaccharides that glues the sediment together and therefore protects the sediment against erosion. It has been observed that biopolymers formed by microphytobenthos (diatoms) increase the sediment stability and increase erosion thresholds (Widdows, 2000b). Benthic diatoms are unicellular algae (Figure 10 and Figure 11).

19 Biogeomorphology Small activities with large effects?

Figure 10: Benthic diatom Caloneis africana Figure 11: Benthic diatoms occurring in the Wadden Sea (Source: Crosato, 2002)

The group of diatoms can be divided into two main groups. The first group are the epipsamic species, who attach themselves to the sediment particles and are dependent on sediment reworking. The second group is the epipelic species; they emit an extracellular polymeric substance (EPS). This substance forms a thin film on the surface that increases the stability of the sediment. During a bloom period, large densities of diatoms are known to form algae mats. These mats sometimes are visible as a brownish jelly- like layer (Figure 12). During storm periods, parts of the film can be eroded. The total replacement of a diatom community takes about one week, which indicates that a diatom population adapts very quickly to changes in environmental conditions. On an intertidal flat the algae and bacteria films are mainly found on the less exposed and parts with more mud of the flat. The more exposed sandy edges of the flat are less suitable habitats. Diatoms do not actively contribute to removal of particles from the water column. Therefore, sediment particles must first settle onto the flat surface. Thus, hydrodynamic conditions need to be favourable for sediment deposition before trapping can occur. In the next chapter; an attempt is made to Figure 12: Algae film on intertidal flat quantify the stabilising effect of diatoms (Source: Crosato, 2002) on an intertidal flat in the Western Scheldt.

20 Biogeomorphology Small activities with large effects?

3 Area description There are two areas of interest, i.e. offshore and estuarine. Both areas are very different according to the physical parameters such as for example the water depth and flow velocity. The activities and effects of benthos on the sediment characteristics and seabed described in chapter 2 occur in offshore areas and estuaries. Therefore a short general description of both areas is given. 3.1 Offshore North Sea The Dutch part of the North Sea is a shallow sea. This part is just a small part of the whole North Sea and is called the Dutch Continental Shelf (DCS). The depths at the DCS are ranging from 20-30 meter and decreases in the southern direction. The sediment consists mainly of sand. The grain size decreases from south to north. Very fine sands and mud occur mainly in the northern part of the DCS. Large grains and boulders are only found at some places such as the Klaverbank. Only close to the coast local mud patches are found which are derived from input by rivers, harbour sludge and the Wadden Sea. The present distribution of surface sediments in the Southern North Sea is for a major part governed by the tidal currents. The maximum current velocities decrease in northward direction, consequently the median grain size. The bed of the North Sea shows a variety of regular patterns. The largest-scale phenomena are tidal sandbanks and sand waves. Outside the 20-meter depth line in the southern part of the DCS a lot of sandbanks can be found Figure 13: North Sea with areas of high natural value (www.noordzee.org). Tidal sandbanks are (Source: www.noordzee.nl) characterised by strong tidal current s (0.5-2.3 ms-1)in shallow seas. In general, their crests are oriented slightly anti-clockwise with respect to the principal tidal flow direction. The amplitude of sandbanks is between 2 and 20 m (Hulscher, 1993). The centre part is characterised by sand waves (www.noorzee.org). Sand waves have a smaller spatial scale than sandbanks. Their typical wavelength is between 100-800 m, typical amplitude in the order of 5 m. In most observed cases, the sand wave crests are perpendicular with respect to the major tidal current (Hulscher, 1993). The northern part is relatively flat except in the northwest part where large banks appear such as the Klaverbank en de Doggersbank (Figure 13).

21 Biogeomorphology Small activities with large effects?

The tidal current transports the sediment at the bed. This transport can be described bedload transport model, which will be described in more detail in paragraph 4.2.2. This transport model makes use of three important parameters, the proportionality coefficient (α= 0.3 s2m-1), the non-linearity parameter (b) and the bed-slope coefficient (λ= 0.0085 m2s-2). Without further description of the model some values of the parameters specific for the North Sea are given. The parameter values are taken from Komarova (2000) and Nemeth (2002). Due to different formulations of the bedload transport model the values are given for de sediment balance given in (Eq. 11) (Komarova, 2000), for this reason the dimensionless parameter values for α, b and λ used in Nemeth are (2002) adapted to Komarova (2000). The flow velocity, water depth and kinematic viscosity are according to the specific author. This gives the following dimensionless parameter values for α, b and λ.

Parameter [-] Komarova (2000) Nemeth (2002) [-] ’ − − α 1.4⋅10 4 3.5⋅10 4 λ’ 24.3 10.2 1 1 b 2 2

Table 1: Specific parameter values for α, b and λ. The values given by Nemeth (2002) are adapted to Komarova (2000) to make it possible to compare two different model formulations of the bedload transport model.

3.1.1 Macrozoobenthos in the North Sea The species found in the North Sea can be divided in four groups the Molluscs (clams, mussels, and snails), Polychaeta (segmented worms), Crustaceans (crab, lobster, and shrimp) and the Echinodermata (brittle stars and sea urchins). The Molluscs consists of eight individuals classes, two of these classes are present in the North Sea: Gastropods (snails) and Bivalves (two shells) (Figure 14). Most bivalves are suspension or deposit feeders. Species of the Bivalves and Gastropods are present everywhere in the North Sea.

Molluscs Polychaeta Crustaceans Echinodermata Gastropods Bivalve

Figure 14: Species found in the North Sea, from left to right: Lunatia alderi, Macoma balthica, Owenia fusiformis, Urothoe poseidonis, Amphiura filiformis (Sources: www.waddenzee.nl; www.marlin.ac.uk; www.victorbos.com; www.nioz.nl; ICONA, 1992)

The number of species is the highest at the Oyster Ground and at the north side of the Dogger Bank (Figure 13). The major part of the species is Polychaeta or Crustacean, while the number of Molluscs and Echinoderms are lower in the southern North Sea. Further offshore the number of organisms of one species is less. Close to the coast suspension feeders are dominant and offshore there are more deposit feeders. The distribution of the macrozoobenthos is influenced by

22 Biogeomorphology Small activities with large effects?

water temperature, water depth, nutrition and the composition of the sediment. The North Sea is a shallow sea with small temperature differences for this reason plays temperature a minor role in the distribution of the macrozoobenthos. 3.2 Western Scheldt estuary The Western Scheldt is the Dutch part of the Scheldt estuary. An estuary is a tidal inlet at the mouth of a river, which consists of one or more channels, and intertidal flats, which are alternately submerged by the rise and fall of the tides. In Figure 15 a satellite picture of the Western Scheldt estuary is given.

Figure 15: Satellite photo of the Western Scheldt estuary (Source: www.antwerpennoord.be)

The estuary constructed of a couple of zones, the main channel, the intertidal flat, the high tidal flat and the salt marshes. The different zones are given in Figure 16. The main tidal channel is the deepest part of the estuary, submerged for all or most of the tidal cycle and subjected to strong tidal currents and minor wave action. The intertidal flat commonly forms the widest zone. They are usually submerged during mid-tide, when tidal currents tend to reach their maximum speed; these currents mainly control sediment movement. The sediment of this zone often consists of alternations of sands or muddy sands and fine mud, in which the layering is commonly disturbed by dense populations of burrowing organisms such as lugworms and – where sands predominate – cockles and other bivalve molluscs. The high tidal flats are mud flats submerged only at high tide when current speed falls close to zero, and are generally reached by waves of only low amplitude. Little bedload transport occurs, but during periods of slack water tidal currents are smallest, at the turn of the tide, mud settles out of suspension onto the tidal mud flat. Ultimately the high tidal flat is exposed for sufficiently long periods for colonization by salt-tolerant higher plant species, leading to the development of a salt marsh.

In twenty-four hours the river transports ten million cubic meter of water to the sea. In this same period an amount of seawater hundred times larger has flowed in and out of the estuary. The ensemble of fresh water and salt water determines the form and content of the Western Scheldt estuary.

23 Biogeomorphology Small activities with large effects?

Figure 16: Intertidal zonation in an estuary (Source: The open university, 1999)

The sediment of the bed are continuously picked up, transported and deposited by the water flow. Due to different sediment properties, the flux and direction of transport of non-cohesive and cohesive sediments are often not equal The flow of water required to erode fine sediment on the outgoing tide is larger than required for deposition on the incoming tide. The result is a strongly varying mud content in the bed in both horizontal and vertical directions. An additional factor that promotes sediment accretion on tidal mud flats is binding of sediment particles by diatoms mats.

3.2.1 Benthos in the Western Scheldt In the Western Scheldt live a kinds of species such phytobenthos and the most of the benthic species living offshore. The animals in Figure 14 exist in the Western Scheldt as well. Besides zoobenthos there are algae (diatoms) and bacteria in the bottom. The diatoms mainly consist of unicellular organisms. Sometimes the amount is large enough to give the tidal flats a brown colour. This is described earlier in chapter two. A resent research in the Western Scheldt indicated that there is a strong relation between Kiezelwieren (Figure 17) and the erosion of the mudflat. Kiezelwieren are unicellular organisms and protect the flat against erosion, due to tidal flows and waves, Figure 17: Kiezelwieren in the Western Sheldt by sticking the sediment together. (Source: www.nioo.knaw.nl) (www.nioo.knau.nl).

24 Biogeomorphology Small activities with large effects?

4 The role of bed shear stress in morphodynamics The morphological process is described by the hydrodynamics of the water flow, the sediment transport and the seabed changes. The interaction of these processes results in a dynamic system called morphodynamic cycle (Figure 18). This cycle consists of a number of non-linear processes, with strong feedback mechanisms. These processes take place on different time scales. The response of water motion to bed form changes is almost instantaneous, whereas the bed form changes due to sediment transport are slow. Hence, small perturbations in the sediment motion, or the initial state, can have a huge impact on the morphology of the seabed (Knaapen, 2001). These perturbations can be formed by biological activities and therefore influence sediment characteristics, leading to a change in the morphological process.

Flow

Biological activities Sediment transport - Destabilisation - Stabilisation

Bed level

Figure 18: Morphodynamic cycle with the influence of biological activities. The solid lines give interactions used in this research. The dashed lines give possible interactions.

The activities of benthic organisms influence the sediment characteristics and moreover the initial moment of sediment transport. The critical bed shear stress is the value of the shear stress, which must be exceeded before sediment transport occurs. Therefore the relation between the critical bed shear stress and benthic organisms is used as a connection between biology and morphology. 4.1 Bed shear stress and critical bed shear stress The processes in the near-bed region dominate the sediment transport process. Therefore, the shear stresses near the bed are given a closer look.

4.1.1 Bed shear stress Bed shear stress is the stress caused by the movement of water over the bed. The

bed shear stress (τb) depends on the characteristics of the sediment and the flow velocity. The overall time-averaged bed shear stress can be parameterised by (Van Rijn, 1993):

1 τ = ρf u 2 (Eq. 1) b 8 c

25 Biogeomorphology Small activities with large effects?

ρ -3 where is the fluid density (kgm ), fc the friction factor of Darcy-Weisbach and -1 u the depth-averaged velocity (ms ). The friction factor fc is (Van Rijn, 1993):

8g f = (Eq. 2) c C 2 where g is the gravitational acceleration (ms-2) and C the Chézy-coefficient (m1/2s-1). For hydraulic rough flows the Chézy-coefficient according to White-Colebrook (Van Rijn, 1993) yields:

12h  C =18log  (Eq. 3)  ks 

where h is the water depth (m) and ks the roughness height of Nikuradse (m). The roughness height is given by (Van Rijn, 1993):

= α ⋅ ks d90 (Eq. 4) α α ≥ α were is a coefficient ( = 1 for stones d50 0.1 m and = 3 for sand and gravel material) and d90 a grain size diameter (m).

4.1.2 Critical bed shear stress When the water is flowing over a bed of loose grains, there will be a certain velocity at which the bed shear stress exceeds a critical value, the critical bed shear

stress (τcr), and sediment transport occurs. The critical bed shear stress depends on the characteristics of the sediment. A decrease of the critical bed shear stress means an increased erodibility of the bed.

There are a couple of fluid forces acting on a sediment particle on a flat bed. These forces consist of skin friction forces and pressure forces (Figure 19). The skin friction forces act on the surface of the particle by viscous shear. The pressure

force, consisting of a drag (FD) and a lift force (FL) is generated by pressure differences along the surface of the particle. At a high Reynolds number the pressure force will be much larger than the (viscous) skin friction force and the

resulting fluid force FD will act through the centre of the particle. Particle movement will occur when the moments of instantaneous fluid forces FD and FL are larger than the stabilizing moment of the submerged particle weight G, yielding:

+ ≥ FDa FLb Gc (Eq. 5)

where a, b and c are distances of the force to the contact point of the grain with the bed.

26 Biogeomorphology Small activities with large effects?

Lift force (FL) b F

a Drag (FD) Bed c

Particle weight (G)

Figure 19: Forces acting on a sediment particle

The resulting fluid forces are related to the bed shear stress, yielding: ∝ τ 2 F bd50 (Eq. 6)

For the gravity force this gives: ∝ ρ − ρ 3 G g( s )d50 (Eq. 7)

From substitution of (Eq. 6) and (Eq. 7) in (Eq. 5) follows the ratio between fluid forces and particle weight. This is a dimensionless measurement of the mobility of sediment; the Shields parameter (θ).

τ θ = b ()ρ − ρ (Eq. 8) g s d50 ρ where s is the sediment density and d50 mean the grain size diameter.

There is a critical Shields parameter below which no sediment transport occurs.

The critical Shields parameter (θcr) depends on the hydraulic conditions near the bed, the particle shape and the particle position relative to the other particles. This critical value for initiation of motion depends on the grain size characteristics and the flow regime. For a hydraulic rough regime the range of the critical shields parameter is 0.04 < θcr ≤ 0.06. The critical Shields parameter is given by:

τ θ = cr cr ρ − ρ (Eq. 9) g( s )d50

In the sea, the bed is almost invariably rippled. A threshold for grains on a rippled bed is proposed by Bagnold, but without experimental verification (Dyer, 1986). As a consequence of the form drag of the ripples, which must be partly offset by the acceleration of the water flow towards the ripple crest, it is the curve is virtually parallel to the flat bed threshold curve, but with a bed shear velocity value about double the flat bed value. Later experiments showed a reasonable agreement with the threshold curves (Dyer, 1986).

27 Biogeomorphology Small activities with large effects?

Influence of cohesive material When the bed consists of cohesive sediments, cohesive forces between the sediment particles become important. These forces cause a distinct increase of the strength of the soil against erosion. Depending on the type of clay mineral, this effect may be more or less pronounced (Figure 20). Sediment particles smaller than 63 µm are usually referred to as mud particles. Fine grains can be found in low energy conditions like tidal flats. Coarse grains are found in high-energy conditions near breaker bars along the coast and in the deeper channels of rivers and estuaries, where the finer grains are flushed away. The relationship between grain size and critical bed shear stress is not straightforward in a cohesive regime. Cohesive forces are important when the bed consist of appreciable amounts of clay particles (5-10%) (Open university, 1999). A sandy bed with a small percentage of mud or clay already shows a distinct increase in resistance against erosion (Dyer, 1986). The critical shear stress required to set cohesive sediments in motion is much greater than it might be predicted from its small particle size. Figure 20 shows the range of average flow velocity at which sediment particles of different sizes are eroded. Biological activity at the bed may also influence the critical values for initiation of motion, especially in muddy and silty environments.

In general, for muddy sediments in marine environments, the shear stress to move the sediment ranges from 0.5 to 5 Nm-2 (Open university, 1999), depending partly on the particle size and partly on the degree of sediment compaction and binding by organisms. Values larger than 5 Nm-2 may occur in older sediments of estuaries or tidal flats, which tend to be both drier and more compacted than ‘fresh’ muds.

Figure 20: Range of average flow velocity at which sediment particles of different sizes are eroded. The curves for sediment finer than about 0.1 mm is for relatively uncompacted silts and muds (Source: Open university, 1999)

28 Biogeomorphology Small activities with large effects?

4.2 Morphodynamic models The first model considered, is a two-dimensional shallow water model (Hulscher, 1996 and Németh, 2002). This model can be used to describe tidal sand banks and sand waves (Figure 21). The model consists of three parts: a description of the water motion, the sediment motion and sediment balance. The Coriolis force only slightly affects sand waves. Therefore the behaviour of sand waves can be described with the help of two-dimensional vertical shallow water equations (Németh, 2002). For simplicity the boundaries are taken infinitely far away and no wind stress is taken into account. In this research the parameters of the sediment transport model to describe the sediment motion are expressed in the critical bed shear stress. This is done to give an idea Figure 21: Morphological elements of the effect of biological activities (Source: ICONA, 1992) to the sediment transport.

The second model is a numerical process-based sand-mud model (Van Ledden, 2002), which predicts the bed level and the bed composition. The model is a one- dimensional point model and can be used in estuarine environments. The incorporated processes are deposition, erosion and mixing of the sediment. A scheme of the model is given in Figure 22.

U = Water motion h = Water depth U zb = Bed level p = Mud content in cout cmud, csand h m,0 exchange layer E Di i cout = Outside concentration cmud = Mud concentration zb pm,0 csand = Sand concentration j=0 Ei = Erosion of fraction i pm,1 z j=1 Di = Deposition of fraction i δ j = Layer number j=0 = Exchange layer pm,J j=J δ = Max bed layers Figure 22: Scheme numerical sand-mud model (Source: Van Ledden, 2002)

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Two sediment fractions are taken into account, a sand and a mud fraction. As in the first model, this model has three parts: a description of the water motion, the sediment transport and the bed level and composition. The tide is assumed to be the dominant forcing for water motion. Short waves and density currents are neglected. Deposition, erosion and mixing processes, for the behaviour of non- cohesive and cohesive sediments are described. Flocculation (= binding of the mud particles) and consolidation are not taken into account.

4.2.1 Water motion The water motion in the two-dimensional shallow water model is driven by tides and can be described with the two-dimensional vertical shallow water equations. The shallow water equations are used because in a coastal sea the tidal wavelength is long compared to the depth (Hulscher, 1996). The two-dimensional vertical shallow water equations are given by an equation of motion (Eq. 10) and a continuity equation (Eq. 11):

∂u ∂u ∂u ∂ζ ∂  ∂u  + u + w = −g +  A  (Eq. 10) ∂t ∂x ∂z ∂x ∂z  v ∂z  ∂u ∂w + = 0 (Eq. 11) ∂x ∂z Here u and w are the velocity components in the x and z direction, z = ζ is the free surface elevation and h is the bottom level with respect to the undisturbed water dept H (Figure 23).

z =ζ z =0

z = −H +h(x) z z =−H x

Figure 23: Sketch of the model geometry

The boundaries in the horizontal plane are located infinitely far away. The boundary conditions at the water surface are defined as follows:

∂u ∂ζ ∂ζ = 0 , w = + u (Eq. 12) ∂z ∂t ∂x At the bottom (z=-H=h) a partial slip condition is used to describe the horizontal flow components. The vertical velocity component at the bottom is given by the kinematic condition:

∂h ∂h ∂u + u = w, A = Su (Eq. 13) ∂t ∂x v ∂z

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With the help of the partial slip condition, one can model shear stress at the bottom without including explicitly the complicated processes o n the thin bottom boundary layer. In this layer the shear stress is approximately constant and the flow adjusts to the no-slip condition at the roughness height above the seabed. (Hulscher, 1996)

The description of the water motion in the numerical process-based sand-mud model is simpler. The water depth h is assumed constant and the varying depth-averaged flow velocity U during the tide is given as a function of time (Eq. 14):

U = Uˆ sin(ωt) (Eq. 14) where Uˆ is the amplitude of the velocity and ω the annular frequency of the tidal period. Note that this state of a tidal flow over a flat bed is a valid solution of the shallow water equations. This is often referred to as the basic state.

4.2.2 Sediment transport Sediment transport is caused by seawater motion. The total sediment transport is usually divided into three types: bedload, saltation and suspended load. Bedload: The grains remain in direct contact with the bed. The transport takes place by rolling and sliding along the bottom. Saltation: The grains are lifted from the bed but are too heavy to remain in the water column. Suspended load: Grains are lifted from the bed and move along with the flow. The grains transported in suspension end up much further away than grains transported as bedload. Since the weight of finer grains is less, they are brought into suspension more easily than coarser grains.

Figure 24: Modes of sediment transport in water

In the two-dimensional shallow water model it is assumed that, the sediment is quite large (50 µm – 2 mm) and the velocities are moderate (0.5 - 1.5 ms-1). The sediment transported is assumed to have a non-cohesive character; this is because cohesive sediment is not supposed to be essential for the initiation of large-scale rhythmic features like sandbanks and sand waves (Hulscher, 1993). The sediment transport q (m2s-1) is concentrated on the bedload sediment. This is not a real shortcoming, as at offshore locations the bedload transport is usually

31 Biogeomorphology Small activities with large effects? dominant (Komarova, 2000). The bedload sediment transport at the seabed is modelled by:

 ∂h  q = α |τ |b τ − λ  (Eq. 15) b  b ∂x 

2 -2 Here, τb denotes the volumetric bed shear stress (m s ). The formula is a generic expression that reflects the influence of the two most important forces which act on bed sediment grains. The first term shows the scraping effect of the drag force. The second term represents the gravity component along the bed profile; the effect is weighted by the down-slope coefficient λ (m2s-2). Finally, the proportionality coefficient α (s2m-1) in combination with the non-linearity parameter b describes how efficient the particles of sand are transported by the bed shear stress (Komarova, 2000). Many laboratory experiments have been performed to provide estimates of α, b and λ (Hulscher and Ribberink, 2000). In this research, these parameters are expressed in terms of the critical bed shear stress.

The numerical process-based sand-mud model uses two sediment fractions: a sand fraction and a mud fraction. The erosion and deposition flux for sand and mud are described for two regimes: a non-cohesive and a cohesive regime (Figure 25).

Cohesive

Erosion τe,c Non-cohesive

τe,nc

τd Bed shear stress stress shear Bed Deposition

Figure 25: Scheme of the cohesive and non-cohesive regimes

The critical bed shear stress for erosion in a cohesive regime is larger than in a non- cohesive regime. In the non-cohesive regime sand and mud are assumed to behave independently during erosion and deposition. In the cohesive regime the sand and mud particles behave only independently during deposition. During erosion the mud and sand particles behave in the same way.

For the description of the sand and mud transport in suspension, the depth- averaged advection-diffusion equation for local approach is used with an extra term for horizontal advection and diffusion (van Ledden, 2002). This is done to make the depth-averaged advection-diffusion equation realistic for the mud faction. The mud concentration equation becomes:

32 Biogeomorphology Small activities with large effects?

∂hc m = E − D + k (c − c ) (Eq. 16) ∂t m m m out m where cm is the depth-averaged volumetric mud concentration Dm the mud deposition rate, Em the mud erosion rate, cout is the mud concentration outside and km is a transport coefficient (Figure 22).

The last term is a transport term by which de mud concentration cm is affected by the concentration outside the model. When the inner concentration is lower than the outside concentration, it is assumed that by advective and diffusive transport mud is imported from outside and vice versa.

Non-cohesive regime In the non-cohesive regime, sand and mud are assumed to behave independently during erosion and deposition. The exchange of sand between the bed and the water column is given by:

− = γ − Es Ds ws (ce,s cs ) (Eq. 17) where γ is a form coefficient and takes into account the vertical distribution of the velocity and concentration profile, ws the settling velocity of the sand fraction and ce,s the depth-averaged equilibrium concentration of the sand fraction given by Engelund-Hansen. The erosion process of mud is described by the erosion formula of Partheniades where the mud content is added to the traditional formula. For mud deposition the formula of Krone is applied. The exchange of mud is given by (Van Ledden, 2002):

 τ   τ   τ   τ  E − D = p M  b −1H  b −1 − w c 1− b H 1− b  m m m,0 τ τ m m τ τ (Eq. 18)  e,nc   e,nc   d   d 

Where pm,0 is the mud content in the exchange layer, M the erosion rate, τe,nc the critical erosion shear stress for non-cohesive regime, H the Heavyside function2, wm the settling velocity of mud and τd the critical deposition shear stress.

Cohesive regime In the cohesive regime, the erosional behaviour of sand and mud is not independent. Van Ledden (2002) assumes that sand and mud particles are eroded in the same way, but once in the water column behave independently again. For erosion of mud and sand particles the Partheniades formula is used again with a parameter for sand and mud content in the exchange layer. The deposition formulations for sand and mud remain the same as for the non-cohesive regime. The exchange of sand and mud in the cohesive regime are given by:

 τ   τ  E − D = p M  b −1H  b −1 −γw c s s s,0 τ τ s s (Eq. 19)  e,c   e,c 

2 1 The Heaviside function is defined as follows H(x)=0 for x<0, H(0)= 2 and H(x) = 1 for x>0

33 Biogeomorphology Small activities with large effects?

 τ   τ   τ   τ  E − D = p M  b −1H  b −1 − w c 1− b H 1− b  m m m,0 τ τ m m τ τ (Eq. 20)  e,c   e,c   d   d 

respectively, where ps,0 is the sand component in the exchange layer and τe,c is the critical erosion shear stress for the cohesive regime.

4.2.3 Bed topography The local bed topography, which is determined by the size of the sediment grains, the presence of bedforms, vegetation and biological formations, plays an important role for the movement of water and sediment. It determines the bottom roughness, which influences the friction between the bottom and the moving water. When excluding biological factors, it can be stated that in general the increase of the concentration of mud on the bottom surface decreases the bottom roughness (Van Ledden, 2002).

The behaviour of the bed in the two-dimensional shallow water model is based on mass conservation of the sediment equation (Eq. 21). This means that changes in the local depth are caused by sediment transport gradient; a decrease in total sediment transport leads to local erosion, and conversely, an increase to accretion of the bed. The formulation of this consideration is as follows:

∂h ∂  b  ∂h  = −α '  τ τ − λ'  = 0 (Eq. 21) ∂t ∂x  b  b ∂x  in which (.) represents the sediment transport, and h denotes the variation in the local mean water depth H. When the sediment transport is changed by the activities of macrozoobenthos, the behaviour of the bed will change as well.

In the numerical process-based sand-mud model, the bed level changes are determined by the fluxes of sediment to and from the bed, for both sand as well as mud. The bed density is assumed to be constant in time and space. The bed level change per unit is given by (van Ledden, 2002):

∂z ρ b = s []D − E + D − E ∂ ρ m m s s (Eq. 22) t b ρ ρ where zb is the bed level, s the sediment density and b the bed density. The bed composition is given by an advection-diffusion equation:

∂p ∂  ∂z ∂p  s + b −ε s =  ps z  0 (Eq. 23) ∂t ∂z  ∂t ∂z 

where ps is the sand content, z the distance from the bed surface (positive downwards) and εz a mixing coefficient within the bed.

34 Biogeomorphology Small activities with large effects?

5 Case: Molenplaat

5.1 Area description and data availability The Molenplaat is a 1,5 km2 intertidal flat in the turbid, nutrient-rich Western Scheldt estuary, The Netherlands (Figure 26). Most of the flat is located between – 1 and +1 m relative to mean tidal level. Mean tidal range is approximately 5 m (Herman, 2001). The mean flow velocity and the mean water depth are given by de Vries3. They are respectively 0.8 ms-1 and 3 m

Figure 26: Molenplaat in the Western Scheldt; The Netherlands (source: www.nioo.knaw.nl)

A survey, carried out at the Molenplaat in 1995, is used to get an idea of the biogeomorphological processes taking place on the intertidal flat. This survey encompassed 92 sampling stations on a rectangular grid where macro fauna density and biomass per species were determined, as well as grain size distribution in the top ten centimetres. It was surveyed four times in the months: March, June, September and December (BEON, 1997). Profiles of radionuclides are used by Herman (2001) to determine long- and short- term sedimentation rates on the Molenplaat in the year 1997. The tracer 7Be is used to measure recent mud accretion. This analysis showed a seasonal build-up of fine material in the top 10 cm of the sediment. Between March and September 1997,

3 A. Crosato, de Vries, M.B. and Kuijper, C., (1998). A tool for mudflat Classification. EU MAST3, INTERMUD program, Delft Hydraulics Report Z2037.50, December 1998

35 Biogeomorphology Small activities with large effects?

the 7Be inventory increased by one order of magnitude. According to Herman (2001) this result was consistent with the field observation that the sediment is sandy over the entire intertidal flat in winter, but becomes increasingly muddy in the central part between March and June. Since the whole intertidal flat shows net accumulation, and mud percentages in the deeper layers are consistently low, it is clear that the mud deposition is temporary (Herman 2001).

Based on the grain size distribution, the Molenplaat is divided into three parts. Despite the seasonal dependency of deposition of fine material, the subdivision remains visible in all four seasons. In Figure 27 the subdivision of the Molenplaat is given. The figure shows the spatial distribution of the mud percentage in September. In Appendix 2: Grain size distribution Molenplaat, the mud and medium sand percentages for March, June, September and December are given.

384600.00 75. 70.

384400.00 65. [%] percentage Mud 60. 384200.00 55. 50. 45. 384000.00 40. Par y East 35. 383800.00 30. 25. 383600.00 Centre 20. 15. 383400.00 10. West 5.0 383200.00 0.0

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 27: Subdivision Molenplaat; mud percentage in September; with Parisian coordinates

The sediment in the centre of the Molenplaat consists of fine material, whereas the sediment at the west corner of the intertidal flat is coarser and features such as ripples and mega ripples can be formed (Herman 2001). Medium sand can be found in the east part of the flat. In areas with a muddy bed, no ripples were found. 5.2 Benthos densities at the Molenplaat The benthos densities at the Molenplaat are measured four times for 93 samples points. The benthos is divided in macrozoobenthos and Chlorophyll a. For each division given in Figure 27 there is an average is taken form the samples point present in the division. This is done for all four measuring periods. The result is three combinations of average macrozoobenthos densities with an average amount of Chlorophyll a for four measuring periods.

The diatom is a very common bio-stabiliser in the Western Scheldt. The diatom is present all over the Western Scheldt. They are found in the top centimetres of the

36 Biogeomorphology Small activities with large effects? bed. These diatoms use Chlorophyll a (leaf green) for photosynthesis, which means they depend heavily on light availability. To measure the amount of diatoms the concentration of Chlorophyll a in the top of the sediment is measured and used as the indicator for diatom biomass (Nederbragt, 2001). In Table 2 the average concentration of Chlorophyll a for each division is given. There were no measurements available in March. In the centre of the intertidal flat the concentration of Chlorophyll a is very high which means a high amount of diatoms. In Appendix 3: Distribution Chlorophyll a Molenplaat, the distribution for all three measuring events is given.

Assuming that all macrozoobenthos have a destabilising effect on the sediment, the sum is taken of all macrozoobenthic species (Suspension feeders, deposit feeders and Surface deposit feeders) for each of the 93 measuring points. The average density of macrozoobenthos in the three parts of the Molenplaat is given in Table 2. The macrozoobenthos is found mostly in the central part of the flat. All benthos seem to prefer muddy areas with low shear stresses (BEON, 1997). In the western part of the flat hardly any macrozoobenthos is found. The distribution is given in Appendix 4: Distribution macrozoobenthos density at the Molenplaat. Seventy percent of all macrozoobenthos present at the Molenplaat is found in the centre part of the flat. At the eastern part of the intertidal flat the occurrence of macrozoobenthos is very low again.

Chlorophyll a Macrozoobenthos Mud [µgg-1] %[m-2] %[%] % West March - 5351.00 ± 45 2.68 ± 13 June 3.34 ±35 3414.00 ± 71 7.41 ± 122 September 2.57 ±57 6806.74 ± 58 8.58 ± 35 December 3.45 ±101 6204.38 ± 37 7.07 ± 16 Centre March - 14070.33 ± 56 8.70 ± 61 June 9.69 ±54 32199.23 ± 88 38.24 ± 56 September 10.74 ± 50 34821.69 ± 50 34.12 ± 49 December 10.76 ± 69 26519.56 ± 51 29.85 ± 50 East March - 3165.21 ± 74 3.64 ± 41 June 10.57 ±43 3090.79 ± 73 10.90 ± 66 September 12.33 ± 40 6992.92 ± 106 10.68 ± 66 December 11.98 ± 59 4831.38 ± 106 13.05 ± 61

Table 2: Average concentration Chlorophyll a in the sediment and densities of macrozoobenthos for each measuring period for each division of the Molenplaat

As can be seen in Table 2 is de difference between the sample points in one division very high for all three parameters measured. Some points vary more than 80 percent from the average value. This means that the model calculations which are carried out with these average values have to be used with care.

The presence of benthic organisms is influenced by temperature and light availability. Small size organisms like diatoms responded faster to temperature changes than larger organisms. Light availability and temperature show seasonal behaviour. The abundance of macrozoobenthos is not completely seasonal

37 Biogeomorphology Small activities with large effects?

(Widdows 2000b). For example the amount of Macoma balthica shows more inter- annual than seasonal variation, with high recruitment years tending to follow cold winters. High Macoma balthica values tend to be associated with lower diatom values, due to increased grazing pressure (Widdows, 2000b). For the year 1995 the average macrozoobenthos density at the Molenplaat is low in March (spring) and high in June till September (summer) (Figure 34). There is a large difference in density between the centre of the Molenplaat and the western and eastern parts. The difference between the eastern and western part is rather small. According to Widdows (2000b) follow diatoms a seasonal pattern, with higher densities in spring and (sometimes) autumn. The average Chlorophyll a content for the year 1995 shows a seasonal (rather flat) pattern, with a higher amount in September, in the western and centre part (Figure 35). The western part does not follow this pattern. This is probably due the fact that the flow velocity in the western part larger is than the eastern part. Here the diatoms are not capable of creating a large algal film. In the centre of the Molenplaat there is Mud percentage besides a high average density of % 45 macrozoobenthos a high average 40 West concentration of Chlorophyll a. The mud 35 Centre percentage is highest in the centre of the 30 East tidal flat as well. The east and west part 25 show a striking difference in Chlorophyll a 20 concentration for an almost similar 15 density of macrozoobenthos. A look at 10 the mud percentage for the east and west 5 part shows that the mud percentage in 0 the east part has a tendency to increase mrt jun sept dec and in the west part the mud percentage Figure 28: Average mud percentage at the Molenplaat for four declines at the end of the year. measuring periods in 1995, March, June, September and December 5.3 Quantification, biological effects on critical bed shear stress The effect of biological activity is quantified with a “benthos-factor” consisting of a destabilising factor and a stabilising factor. The benthos-factor and the destabilising and stabilising factor will be described in the next paragraphs.

5.3.1 The “Benthos-factor” The critical bed shear stress is altered by the presence of biological activity by macrozoobenthos and diatoms. The change in the critical bed shear stress is expressed in the benthos-factor. The new critical bed shear stress (Nm-2) with biological influence can be expressed as:

τ = ⋅τ cr,b B cr (Eq. 24)

Where τcr,b is the bed shear stress without biological effects and the benthos-factor = ⋅ is given by B B d B s , where Bd and Bs are respectively the destabilising factor and the stabilising factor.

38 Biogeomorphology Small activities with large effects?

The benthos-factor consists of two different factors because both factors depend on the amount of macrozoobenthos or diatoms present in the bed. Most macrozoobenthos feed on diatoms. The abundance of macrozoobenthos implies a low amount of diatoms and the other way round. The precise relation is not known. For this reason all possible combinations of macrozoobenthos and diatoms are taken into account. This means that stabilisation and destabilisation of the sediment can occur simultaneously. It is possible that both effects neutralise each other.

5.3.2 Destabilisation of the sediment (Bd) The destabilisation of the sediment caused by macrozoobenthos is expressed in the

destabilisation factor (Bd). This factor is derived from results from flume experiments within the laboratory and the field. These experiments were carried out by Widdows (1998a, 1998b, 1998c, 2000b). Widdows studied the bioturbation activities of the Macoma balthica. The Macoma balthica is a deposit feeder living in the Western Scheldt estuary and one of the key organisms for destabilising the sediment. With in situ flume experiments, Widdows (2000b) quantified the impact of the clam Macoma balthica when added to upper shore sediments of the Humber estuary (UK) at various stages during the spring-neap tidal cycle. Sediment erodibility was

quantified in terms of critical erosion velocities (ucr), in relation to increasing current velocities. Widdows (2000b) demonstrates that when sediment is exposed to a constant velocity, exceeding the critical threshold, increasing the density of Macoma balthica leads to larger amounts of sediment being eroded. Increasing Macoma balthica density further has a progressively smaller effect, until an asymptote is approached, which corresponds to the whole surface to the sediment being within siphoning range of at least one Macoma balthica individual.

Widdows (2000b) gives a relationship between the critical erosion velocity and Macoma balthica density. This relation is based on measurements from the Skeffling upper shore in the Humber estuary (UK) between April 1996 and May 1997. In spring 1996 there was low sediment erodibility due to a low Macoma balthica density and a well-developed algal film. On the other hand in spring 1997 there was high sediment erodibility due to high Macoma balthica density and absence of visible algal film. The relationship derived by Widdows (2000b) is based on both these measuring periods in 1996 and 1997. Therefore gives Widdows (2000b) a relation for both algal films and Macoma balthica density.

A new relation is derived between the critical erosion velocity (ucr) and Macoma balthica density using only critical flow velocity for Macoma balthica densities larger than 500 individuals m-2 and an initial flow velocity of 0.17 ms-1. It is assumed that with smaller densities algal films can easily be formed to stabilise the sediment. The new critical flow velocity derived from Widdows (2000b) is then given by:

= − ⋅ + ucr 0.0063ln(M A) e (Eq. 25) -1 -1 where ucr is the critical erosion velocity (ms ) and e a constant (e =0.17 ms ). The Macoma balthica density is represented with M (m-2) for a surface A (A =1 m2).

39 Biogeomorphology Small activities with large effects?

A significant positive correlation has been recorded in the Western Scheldt estuary during the period 1996 to 1997 by Widdows (2000b). An increase in the Macoma balthica density gives a decrease in critical erosion velocity and therefore an increase in the erodibility of the sediment.

The critical bed shear stress for a given Macoma balthica density is derived from the bed shear stress estimated by Widdows (1998a) and the critical erosion velocity for the Macoma balthica density (Eq. 25). Widdows (1998a) generated current velocities and bed shear stresses by rotating an annular drive plate of adjustable height in the annular flume. Widdows (1998a) related the events to free-stream current velocities rather than to bed shear stress, as the current velocity is the parameter, which can be determined with greatest accuracy. The bed shear stress cannot be measured directly but only estimated from the log boundary layer velocity profiles (Dyer, 1986). The relationship between the free-stream current velocity and estimates for bed shear stress (Nm-2) for smooth turbulent flow over fine-grain cohesive mud within the flume is described by the following equation (Widdows, 2000b).

τ = β 2 − β + β b 1u 2u 3 (Eq. 26) -1 Where u is the flow velocity (ms ) and β1, β2 and β3 are constants with values -3 -2 -1 -1 -2 respectively β1=8 kgm , β2 =0.06 kgm s and β3 = 0.0052 kgm s .

The critical erosion velocity (Eq. 25) is filled in (Eq. 26) to derive the critical bed shear stress as a function of Macoma balthica density. The critical bed shear stress (Nm-2) is then given by:

τ = ⋅ 2 − ⋅ + cr 0.00032ln(M A) 0.017ln(M A) n (Eq. 27) Where n is a constant (n=0.2 Nm-2). This is the critical bed shear stress for the Humber estuary (UK).

] -2 (M) [Nm cr τ

Macoma balthica [m-2] Figure 29: Critical bed shear stress according to (Eq. 27) for a Macoma balthica density ranging from 10 to 35000

40 Biogeomorphology Small activities with large effects?

The critical bed shear stress in the Western Scheldt estuary is assumed to have the same relation between critical bed shear stress and macrozoobenthos density. The

critical bed shear stress without the influence of macrozoobenthos (M=0) is τcr≈0.2 Nm-2. When the bed is completely covered with Macoma balthica individuals there is no further effect of more individuals. The Macoma balthica has a size ranging from 8 mm to 15 mm. This gives a maximum of 4500 to 15000 individuals for one square meter. There are measurements of larger densities. In the Western Scheldt the maximum density of macrozoobenthos measured at the Molenplaat is -2 approximately 35000 individuals m . This gives a critical bed shear stress τcr≈0.06 Nm-2 (Figure 29).

A decrease in the critical bed shear stress means an increased erodibility of the bed.

The destabilising factor (Bd) for densities of Macoma balthica is derived by dividing the relation of critical bed shear stress and the Macoma balthica density (Eq. 27) by its last term, this gives:

= γ ⋅ 2 − γ ⋅ + Bd 1 ln(M A) 2 ln(M A) 1 (Eq. 28) γ γ where M is the Macoma balthica density, 1 = 0.0016 and 2=0.085 are constants derived from the relation between Macoma balthica density and the critical bed shear stress.

5.3.3 Stabilisation of the sediment (Bs) Diatoms present in the sediment of the bed improve the stability of the sediment. As mentioned before, Chlorophyll a is an indicator for diatom biomass. The relationship between critical bed shear stress (Nm-2) and the concentration Chlorophyll a in the sediment in the Western Scheldt estuary is studied by Widdows. In Figure 30 the relationship is given.

Figure 30: Relationship between the Critical bed shear stress and the Chlorophyll a content of the sediment according to Widdows

This relationship is measured in the ECOflat campaigns 1-4, sites 1-4 and has a positive correlation of R2=0.6032 according to Widdows. The formulation for the

41 Biogeomorphology Small activities with large effects?

critical bed shear stress in the Western Sheldt estuary is based on this relationship. The equation for the critical bed shear stress is than given by:

τ = + cr 0.014C n (Eq. 29) where C is the concentration Chlorophyll a (µgg-1) in the sediment and n is a constant (n=0.2 Nm-2).

-2 The critical bed shear stress for a bed without Chlorophyll a (C=0) is τcr≈ 0.2 Nm . An increase in the critical bed shear stress of the bed means a decrease in

erodibility of the bed. The stabilising factor (Bs) for the concentration of Chlorophyll a in the sediment is derived by dividing (Eq. 30) by its last term, this gives:

= + β Bs 1 (C) (Eq. 30) where β is a constant β = 0.014/0.2 = 0.07.

5.3.4 The Benthos-factor at the Molenplaat

In Figure 31 the destabilisation factor Bd and stabilisation factor Bs are given for different values of respectively Macoma balthica density and Chlorophyll a. concentration. The density of Macoma balthica is taken from 1 to 40000. The amount of Chlorophyll a is taken from 1 to 40 µgg-1. Note that the Macoma balthica density is represented with a log scale and the amount of Chlorophyll a with a linear scale.

d B

Macoma balthica [m-2]

s B

Chlorophyll a concentration in sediment [µgg-1] Figure 31: Destabilisation and stabilisation factors for densities of Macoma balthica and Chlorophyll a, as given by (Eq. 28) and (Eq. 30)

42 Biogeomorphology Small activities with large effects?

Both factors are derived from field data. This means that these factors depend very strong on the sediment characteristics present at the measuring point. For this reason one has to be careful when these factors are used in other environments. The average benthos-factor at the Molenplaat is derived from the measured density of macrozoobenthos present at the tidal flat in 1995. All kinds of macrozoobenthos have some destabilising effects and therefore the macrozoobenthos density is assumed to behave in the same manner as the Macoma balthica density. For this reason the average macrozoobenthos density at the

Molenplaat is used in the destabilisation factor (Bd) for the Molenplaat. The amount of Chlorophyll a is measured at the Molenplaat and is used in the stabilisation factor (Bd) of the Molenplaat. A benthos-factor larger than one causes an increase in the critical bed shear stress and therefore a decrease in the erodibility of the bed. If the benthos-factor is smaller than 1 the erodibility of the sediment will increase. The benthos-factor is given for three periods, June, September and December (Figure 32).

Benthos-factor; Molenplaat Benthos-factor B ] -1 [µgg a B=1

Chlorophyll Chlorophyll

Macrozoobenthos [m-2] Figure 32: Benthos-factor for the average density of macrozoobenthos and average concentration of Chlorophyll a at the whole Molenplaat for June (diamond), September (hexagram) and December (square); 1995

The sediment of the Molenplaat is destabilised by biological activities in each measuring period. The average densities of macrozoobenthos and amounts of Chlorophyll a used in Figure 32 for the three measuring periods are given in Table 3. There is a very small shift during the year to more stabilised sediment for June to December. This can be explained by the fact that the average amount of macrozoobenthos in December is lower than in June and the average amount of Chlorophyll a is larger in December than in June.

43 Biogeomorphology Small activities with large effects?

Macrozoobenthos [m-2] Chlorophyll a [µgg-1] June 15532 7.84 September 18731 8.58 December 14458 8.77

Table 3: Average density of macrozoobenthos and concentration of Chlorophyll a in the sediment of the Molenplaat in June, September, and December (1995)

The benthos-factor is also estimated for the three divisions separately. The average density of macrozoobenthos and Chlorophyll a concentration for each division is already given in (Table 2). The three divisions of the intertidal flat (Figure 33) show a difference between the density of macrozoobenthos and amount of Chlorophyll a from place to place. Note that the Molenplaat has a size of 1.5 km2.

Benthos-factor; 3 parts Molenplaat Benthos-factor B ] -1

[µgg Centre a B=1

Chlorophyll Chlorophyll East

West

Macrozoobenthos [m-2]

Figure 33: Benthos-factor (B=Bs⋅Bd) for 3 divisions of the Molenplaat in June (diamond), September (hexagram) and December (square)

The benthos combination in the centre part of the Molenplaat gives an almost equal Benthos-factor as in the benthos combination in the east part. The difference is that in the east part the Benthos-factor shows a large decrease during the year than the Benthos factor in the centre part. In the east part there is a relative high amount of Chlorophyll a compared to the west part. This is why the east part lies above the west part in Figure 33 and is less destabilised.

44 Biogeomorphology Small activities with large effects?

5.4 Application with sand-mud model The method for calculating the effect of Chlorophyll a and macrozoobenthos on the critical bed shear stress is built in the process-based sand-mud model. First the mud percentage is calculated with specific values for the Molenplaat, without benthic influence. This mud percentage is called the basic mud percentage. Second, the macrozoobenthos densities and Chlorophyll a amounts are simulated for one year. These simulations are used in the calculations with benthic influence.

5.4.1 Specification model parameters The model parameters are as far as possible specified with values for the Molenplaat. With these values, a basic mud percentage in the first layer (10 cm) of the bed is calculated. Two sediment fractions are taken; a mud fraction and a sand -2 fraction. The critical deposition shear stress for mud is τd = 0.15 Nm (Van Rijn, 1993). The erosion rate in the Western Scheldt is approximately 1·10-8 ms-1 and the bed roughness is expressed with a Manning-coefficient of n = 0.013 (BEON, 1997). The expression of the Chézy-coefficient in the Manning-coefficient (Van Rijn, 1993):

1 h 6 (Eq. 31) C = n Where h is the water depth (m) and n the Manning-coefficient (sm-1/3). For a water depth of 3 m and a Manning-coefficient of 0.013 sm-1/3 this gives a Chézy- coefficient of C=92 m1/2s-1. This value is within the range for muddy environments (Van Rijn, 1993). This value is kept the same for each division of the Molenplaat. The initial bed composition is assumed to consist for 80% of sand this is based on information given by de Vries4 and on the average mud content of the complete

Molenplaat according to the measurements. The initial critical bed shear stress (τcr) for a non-cohesive bed is 0.18 Nm-2. When the mud percentage is larger than 30% the regime becomes cohesive. The cohesive regime is assumed to have a critical -2 bed shear stress 1.6*τcr,nc. The gravitational acceleration is g=9.81 ms , the ρ -3 ρ -3 sediment density is s =2650 kgm and the water density is =1000 kgm . The -6 2 -1 -5 kinematic viscosity ν =1·10 m s and the outside mud concentration is cout=4·10 (≈100 mgl-1).

The benthic influence is calculated from the measurements from March, June, September and December. These measurements are translated in two year cycles using a sinus approximation. For the macrozoobenthos density this gives:

= + ω −φ M (t) M mean M var Sin( t m ) (Eq. 32) -2 Where Mmean is the average macrozoobenthos density (m ) for one year, Mvar is the variation between the average densities for every measuring period, ω is the φ frequency (1 year) and m is the phase shift used to fit the sinus function with the measurements.

4 A. Crosato, de Vries, M.B. and Kuijper, C., (1998). A tool for mudflat Classification. EU MAST3, INTERMUD program, Delft Hydraulics Report Z2037.50, December 1998

45 Biogeomorphology Small activities with large effects?

The translation of the Chlorophyll a measurements in a year cycles gives:

= + ω −φ C(t) Cmean Cvar Sin( t c ) (Eq. 33)

Where Cmean is the yearly average amount of Chlorophyll a (µg/g), Cvar the variation between the average concentrations for each measuring period, ω is the frequency φ (1 year) and c is the phase shift used to fit the sinus function with the measurements. In Table 4 the model input values are given for the benthic year-cycles. The average macrozoobenthos density in one division over the year 1995 is calculated. The phase shift of the sinus function is derived from the best fit of the average macrozoobenthos density for the four measuring periods to the sinus function. This is done for the Chlorophyll a concentration as well and for each division of the Molenplaat.

Macrozoobenthos Chlorophyll a Phase (φ ) Phase(φ ) Mean Var m Mean Var c Western part 5444 1479 -2.05 3.1 0.48 -0.11 Central part 26903 9230 2.87 10.4 0.61 -2.13 Eastern part 4520 1834 -2.71 11.6 0.93 -2.32

Table 4: Model input; average macrozoobenthos density and average Chlorophyll a concentration with the variation from the average value and phase shift to fit the year cycle with a sinusoidal function.

In Figure 34 and Figure 35 the cycles for one year based on the measurements from 1995 and (Eq. 32)and (Eq. 33) are given. The solid lines are the modelled benthic cycles. The dots are the average macrozoobenthos densities and Chlorophyll a concentrations derived from the field data (Table 2).

M [m-2] Macrozoobenthos C [µgg-1] Chlorophyll a 40000 14 12 30000 10 20000 8 6 10000 4 2 0 0 123456789101112 123456789101112 time (month) time (month)

West data West model West data West model Centre data Centre model Centre data Centre model East data East model East data East model

Figure 34: Input sand-mud model; modelled year cycle of the Figure 35: Input sand-mud model; modelled year cycle of the average macrozoobenthos density Chlorophyll a concentration

46 Biogeomorphology Small activities with large effects?

5.4.2 Results for the Molenplaat The development of the critical bed shear stress is calculated with the numerical sand-mud model for the Molenplaat for one year with and without the influence of benthic organisms. The choice to run the model only for one year is based on the fact that the benthic cycles change from year to year. The macrozoobenthos for example have large densities after a strong winter (Widdows, 2000b). The change in critical bed shear stress is calculated with (Eq. 24) for time steps, t is 10 seconds during the year.

There are four runs with the sand-mud model. The runs consist of one run without benthic organisms this is called the basic run and three runs with benthic organisms for each division. For each run a mud content and a critical bed shear stress is derived.

The critical bed shear stress (Figure 36) for a bed with an initial mud content of 20 -2 percent without the influence of benthic organisms is τcr= 0.2 Nm . This value indicates that the bed is non-cohesive. The basic mud percentage increases from an initial bed composition of 20 percent mud without the influence of benthic organisms up to almost 30 percent at the end of the year (Figure 37).

-2 Nm Critical bed shear stress % Mud percentage 0.25 35 0.2 30 25 0.15 20 0.1 15 10 0.05 5 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 year year

Basic West Centre East Basic West Centre East

Figure 36: Critical bed shear stress at the Molenplaat for each division Figure 37: Mud percentage at the Molenplaat with and without with and without benthic organisms over one year. The basic (red) benthic organisms over one year. The basic (red)line is without the line is without the influence of benthic organisms influence of benthic organisms

The critical bed shear stress with benthic organisms in each division is below the basic value, which is without benthic organisms. This result is very comprehensible because the Benthos-factor for every part is smaller than one Figure 38. The mud percentage is decreased as well. The decrease is caused by the decrease in critical bed shear stress, which is caused by the destabilisation of the sediment by benthic organisms.

The differences between the divisions are rather small. The east part has the largest

τcr compared to the west and centre part, which means less erosion due to a smaller destabilising effect of the benthic organisms present in this part. The critical bed

47 Biogeomorphology Small activities with large effects? shear stress in the centre and Benthos-factor the west part show just a small 0.8 difference. This is caused by 0.75 the Benthos-factor, which is 0.7

almost equal for both places, 0.65s *B despite the fact that the d 0.6 B combination of macro- 0.55 zoobenthos density and 0.5 Chlorophyll a concentration is 0.45 not the same for both parts. 0.4 The decrease in mud 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 year percentage is much larger in the west and centre than in the West Centre East east. In the east part the mud Figure 38: Benthos-factor for the annual cycles of the average density of Macrozoobenthos and average concentration of percentage remains almost Chlorophyll a at the divisions of the Molenplaat constant. The other places show a decrease of 10 percent compared to the initial concentration.

Minimal and maximal Benthos-factor As mentioned before, are the variations in the average densities of the macrozoobenthos and the concentration of Chlorophyll a rather large. To get an idea of the maximal and minimal change that can occur in the critical bed shear stress and the mud content caused by the influence of benthic organisms, the minimal and maximal Benthos-factor is calculated and taken as model input for the sand- mud model. The possible maximal and minimal Benthos-factor is calculated for each division for the year 1995.

The combination of benthic organisms with the largest Benthos-factor and thus with the most stabilising effect on the sediment is obtained by taking the smallest measured density of macrozoobenthos together with the highest measured concentration of Chlorophyll a found in one part of the Molenplaat. The smallest Benthos-factor and therefore the most destabilising effect on the sediment by benthic organisms is obtained by taking the larges measured density of macrozoobenthos together with the smallest measured concentration of Chlorophyll a found in one part of the Molenplaat. The combinations for the minimal and maximal Benthos-factor for each division are given in Table 5.

Combinations West Centre East Min B: Macrozoobenthos [m-2] (max) M = 41563 M = 99018 M = 29148 + Chlorophyll a [µgg-1] (min) C = 0.24 C = 1.25 C = 1.39 Max B: Macrozoobenthos [m-2] (min) M = 105 M= 946 M= 421 + Chlorophyll a [µgg-1] (max) C = 13.01 C= 39.67 C= 25.82

Table 5: Combinations for a minimal and maximal possible Benthos-factor. These combinations of Macrozoobenthos and Chlorophyll a are model input in the sand-mud model to obtain the possible range of critical bed shear stress and mud content.

48 Biogeomorphology Small activities with large effects?

The model output for these combinations is given in the next six figures. The red line in each figure indicates the basic run, which is the critical bed shear stress with corresponding mud content without the influence of benthic organisms. Note that these combinations are hypothetical. The model output shown in the figures Figure 39 to Figure 44, show only the largest possible range of critical bed shear stress and the mud content for each part of the Molenplaat.

Model output; western part The model gives in the first 2.5 months of the year a critical bed shear stress ranging from 0.05 to 0.24 Nm-2 (Figure 39). The Benthos-factor for the west part

for a maximal destabilisation effect on the sediment is Bmin=0.28 and for a maximal stabilisation effect Bmax=1.14. After 2.5 months the τcr with the minimal Benthos- factor remains the same. The sediment with a maximal Benthos-factor becomes -2 cohesive and the τcr increases to 0.4 Nm . These changes are visible in the development of the mud percentage (Figure 40). The mud percentage after one year for the minimal and maximal Benthos-factor is respectively 5.5 and 70 percent. This is respectively a decrease of 81 percent and an increase of 133 percent compared to the mud percentage (30 %) after one year without benthic influence.

τ Critcal bed shear stress cr for Bmin and Mud percentage for Bmin and Bmax; West B ; West part part max 100 0.5 90 0.4 80 70 -2 0.3 60 Nm 0.2 % 50 40 0.1 30 0 20 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 year 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 year

Bmin yearBmax Basic Bmin yearBmax Basic

Figure 39: Maximal range of critical bed shear stress for the west part Figure 40: Maximal range of mud percentage for the west part of the of the Molenplaat. The ranges are derived with a minimal and Molenplaat. The ranges are derived with a minimal and maximal maximal Benthos-factor. Benthos-factor.

Model output; centre part In the centre of the Molenplaat the change from a non-cohesive to cohesive bed occurs earlier in the year. The critical bed shear stress (Figure 41) in the centre ranges from 0.05 to 0.37 Nm-2 in the first month of the year. With a benthos factor

for maximal destabilisation Bmin=0.24 and maximal stabilisation Bmax=1.84. The Bmin is larger than in the western part and gives a larger mud percentage after one year (Figure 42). The mud percentage after one year for the minimal and maximal Benthos-factor is respectively 5 and 90 percent. This is respectively a decrease of

49 Biogeomorphology Small activities with large effects?

83 percent and an increase of 200 percent compared to the mud percentage (30 %) after one year without benthic influence. τ Critical bed sear stress cr for Bmin and Mud percentage for Bmin and Bmax; Centre Bmax; Centre part part 0.7 100 90 0.6 80 0.5 70

-2 0.4 60 50 0.3 % Nm 40 0.2 30 20 0.1 10 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 year year

Bmin yearBmax Basic Bmin yearBmax Basic

Figure 41: Maximal range of critical bed shear stress for the centre of Figure 42: Maximal range of mud percentage for the centre of the the Molenplaat. The ranges are derived with a minimal and maximal Molenplaat. The ranges are derived with a minimal and maximal Benthos-factor. Benthos-factor.

Model output; eastern part In the eastern part changes the sediment from non-cohesive to cohesive after 1.2 months. This is earlier than the in the western part but later than the centre part. The critical bed shear stress (Figure 43) ranges in the first 1.2 months of the year -2 from 0.06 to 0.3 Nm . The Benthos-factor for maximal destabilisation Bmin=0.29 and for maximal stabilisation is Bmax=1.51. The critical bed shear stress for Bmin remains the same and gives a mud percentage of 6.5 percent. The mud percentage (Figure 44) for maximal stabilisation after one year is 80 percent. This is respectively a decrease of 78 percent and an increase of 167 percent compared to the mud percentage (30 %) after one year without benthic influence

τ Critical bed shear stress cr for Bmin and Mud percentage for Bmin and Bmax; East Bmax; East part part 0.6 100 90 0.5 80 70 0.4 60 -2

0.3 % 50

Nm 40 0.2 30 0.1 20 10 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 year year

Bmin yearBmax Basic Bmin yearBmax Basic

Figure 43: Maximal range of critical bed shear stress for the east part Figure 44: Maximal range of mud percentage for the east part of the of the Molenplaat. The ranges are derived with a minimal and Molenplaat. The ranges are derived with a minimal and maximal maximal Benthos-factor. Benthos-factor.

50 Biogeomorphology Small activities with large effects?

5.4.3 Discussion and conclusions The decrease in mud content shown in the model outputs (Figure 37) do not match with the measured mud content (Table 2). This is probably caused by a couple of factors. First the Benthos-factor itself is considered. The relation for critical bed shear

stress (Eq. 27) used in the destabilising factor Bd is derived by altering the relation given by Widdows (2000b). According to Widdows (2000b) the initial critical bed shear stress would be approximately 1.1 Nm-2, which is rather large. A more common value for the critical bed shear stress is approximately 0.2 Nm-2. A second

point concerning the Benthos-factor is that the destabilisation factor Bd is derived with measurements from the Skifflet mud flat in the Humber estuary (UK). A more accurate Benthos-factor for the Molenplaat would be derived with macrozoobenthos measurements and critical flow velocities from the Molenplaat itself. The relation for the critical bed shear stress, used in the stabilisation factor

Bs, is derived from measurements from the Western Scheldt by Widdows and is not altered. Secondly, the Molenplaat is divided in only three parts based on the sediment composition and an average is taken from the measured macrozoobenthos density in each division. By taking the average value small scale variations are neglected. Table 2 showed that the variation in one division is sometimes more than 80 percent. The reason to take the average density is to make it possible to implement a benthic year cycle in the one dimensional sand-mud model. Smaller divisions would probably give a decrease in the variation of the amount of benthic organisms within one division. And a more accurate year cycle of the benthic organisms. A more accurate year cycle of the benthic organisms and a ‘better’ Benthos-factor and would lead to a description of the influence of benthic organisms on the mud content which is closer to the reality.

The initial mud percentage of 20 percent is based on the total Molenplaat. For the centre part separately this value is low. Therefore some extra runs are carried out for the centre with an initial mud percentage of 80 percent. The extra model run with benthic influence showed a decrease and in the mud content from 80 to 10 percent. This value of 10 percent is only reached after three years instead of one year. This indicates that there is some kind of equilibrium mud percentage for the centre part with the annual benthic cycles given in Figure 34 and Figure 35. To check this equilibrium of 10 percent of mud, a control run is made with an initial mud content of 10 percent and the biological influence. The run showed a mud percentage varying around 10 percent for one up to three years. The influence of benthic organisms caused the decrease in mud content and created a new equilibrium, different from the mud content without the influence of benthic organisms.

The results of the mud percentage show that there is a significant effect of benthic species on the mud content of the bed at the Molenplaat in the Western Scheldt estuary. At the end of the year the mud content with the influence of benthic

51 Biogeomorphology Small activities with large effects? organisms is 30 to 60 percent less compared to the mud content at the end the year without benthos (Figure 37). The results from the maximal and minimal Benthos-factor show that the range of the mud content and critical bed shear stress can be very large. The presence of benthic organisms can make the difference between a mud flat and a sandy flat.

52 Biogeomorphology Small activities with large effects?

6 Case: North Sea

6.1 Description sand-wave area and data availability In the Delft Cluster project: Eco-morphodynamics of the seafloor, three North Sea sites were chosen Figure 45. These sites are representative for large areas on the North Sea floor. Each site is chosen for different reasons. The first area is a sand- wave area on a shoreface-connected ridge. The second area forms a transition from the lower shoreface to the inner continental shelf. The third area is a sand-wave area on the inner continental shelf. This third area is used for the linking of morphodynamics, macrozoobenthos and fish. Measurements from this area are used to get an idea of the sea floor and the macrozoobenthos living on this part of the sea floor. The area measures 1 km x 5 km and is oriented approximately parallel to the present coastline. It is located 55 km offshore Bergen aan Zee, in water depths between 25 and 30 m. It is characterized by sand waves that are 1-3 m in height, which are superimposed on a flat seafloor sloping at less than 1:1000. These sand waves have wavelengths of approximately 200 m. The sand waves are covered by megaripples. Digital terrain maps are made of the research site Figure 45: Location of three study area A, B, C. with a multibeam echo sounder. Source: (Delft Cluster, 2002)

Figure 46: Low resolution multibeam image and sample location for area 3 (sand waves) Water depth 26-30m (Source: Delft Cluster, 2002)

53 Biogeomorphology Small activities with large effects?

The black dots in Figure 46 are bottom samples. The samples are taken with cylinder-shaped box cores to collect macrozoobenthos and sediment samples. The box corer has a diameter of 32 cm and the penetration varied between 0.2 and 0.3 m. In area three 32 sample locations were selected. The samples were selected on the basis of their position on prominent morphological units. 6.2 Sediment characteristics and benthos densities The sediment measurements of the sand wave area showed that the bed consists of sand with a median grain size (274-304 µm) for March 2001 with no systematic variation between sand wave crests and troughs. However, in September 2001 the median grain size of sand wave troughs (257-271µm) were significantly lower than those of crests (273-286 µm). Passchier (Delft Cluster, 2002) concluded a sediment composition in area 3 with a general fining during the summer 2001 and an increase of bioturbation. The troughs are finer-grained than the crests and show most evidence of bioturbation. The analysis of a selection of the macrozoobenthos samples from area 3 showed a small but distinct difference in the macrozoobenthos composition between the crests and troughs. Van Dalfsen (Delft Cluster, 2002) found an indication that in areas with bedforms on the scale of only a few meters, morphological features as sand waves may result in differences in macrozoobenthic communities. The most abundant species are the Mollusc, the Crustaceans and the Polychaete worms. According to Baptist, there are relative high densities found of Spiphanes bombyx. This worm inhabits a stiff sandy tube which usually protrudes slightly above the surface. The worm is capable of stabilising the sediment when it occurs in high densities. These worms are found with an average density of 1086 individuals m-2. There is a great difference between the density at the crest and the trough of a sand wave. In the trough the density is 553 individuals m-2. The densities of species capable of destabilising the sediment are much lower. The density is estimated by Baptist on 500 individuals m-2. This is a high density compared with other macrozoobenthos measurements in the North Sea (ICONA 1992). These results, from the Delft cluster project, are used to give an idea of the effect of biological activities on the parameters of the bedload transport model. In the sand wave area only the destabilisation effect is taken into account. The relationship between the tube building worms and the stabilisation of the sediment is not (yet) known and diatoms does not live in areas with a large water depth (26 m) such as the sand wave area. The effect of the macrozoobenthos density may be overestimated due to the fact that no stabilisation is taken into account. 6.3 Parameters of the sediment transport model The bedload sediment transport model described in paragraph 4.2 has a couple of important parameters. These parameters are the bed-slope coefficient λ, the proportionally coefficient α and the non-linearity parameter b. The parameters of the bedload sediment transport model are expressed in the critical bed shear stress.

54 Biogeomorphology Small activities with large effects?

6.3.1 Bed-slope coefficient The bed-slope coefficient (λ) takes the down-slope effects into account. A small λ,

gives a short wavelength of a sand wave. There is a critical bed-slope coefficient λc. If the bed-slope coefficient is larger than λc all waves are damped due to the gravity term. As soon as λ becomes smaller than λc, an exponentially growing mode appears (Komarova, 2000). The critical value λc, can be calculated specific for the sand wave area but this goes beyond the scope of this research. A dimensionless ’ value for λc in a sand wave area is given by Komarova (2000) λ c=21.7.

The bed-slope coefficient (m2s-2) is according to Komarova (2000) given by:

3θ g()s −1 d [τ ] λ = c0 + γ~ φ φ (Eq. 34) 2 tan s tan s

With: ()|τ | − 3 C γ~ = b 2 2 H()|τ | −C τ b 2 (Eq. 35) b

where = θ ()− C2 c0 g s 1 d (Eq. 36)

In which θc0 is the critical Shields parameter above which sediment starts to move, -2 = ρ ρ g is the acceleration due to gravity (ms ), d the grain diameter (m), s s / ρ where the relative density of the sediment is ( s ) with respect to water density ρ φ ( ), s is the friction angle, the angle of no response and [τ] is the bed shear stress (m2s-2).

The bed shear stress is given by Komarova (2000):

ν U [τ ] = 0 (Eq. 37) δ 2 -1 -1 where ν0 is the kinematic viscosity (m s ) and U the flow velocity (ms ) and δ, the unperturbed turbulent boundary layer thickness (m). This boundary layer is given by:

2ν δ = 0 (Eq. 38) σ where σ is the tidal frequency (s-1).

This approximation for the bed-slope correction is only valid for a small bed

gradient and a flow far beyond critical (C2<<τb). Despite the flow has to be far beyond critical the use of this approximation will still be appropriate because the critical Shields parameter is still present in the approximation.

55 Biogeomorphology Small activities with large effects?

These constrains make that the approximation is not always valid. This is explained in Figure 47. In this figure the grain size diameter d and the critical Shields parameter θc0 are taken variable. Usually the critical Shields parameter is taken to be constant, but we have seen before that the presence of biological activities due to organisms changes the critical bed shear stress. The effect depends on the kind of organism and the density of the organism.

The C2 in (Eq. 36) can be seen as the critical bed shear stress. If the bed shear stress is larger than the critical bed shear stress sediment transport occurs. In

Figure 47 C2 and τb are given for a varying θc0 and d. Specific parameter values of the North Sea are the tidal frequency σ=1.4·10-4 s-1, the gravitational acceleration -2 ρ -3 g=9.81 ms , the density of sediment s =2650 kgm and the density of water ρ =1000 kgm-3. The flow velocity is taken u=0.6 ms-1 and the viscosity is -3 2 -1 taken ν0=7·10 m s (Komarova, 2000). -4 2 -2 The flat plane is the bed shear stress (τb≈4.2·10 m s ) and the curved plane is C2. The curved plane cuts through the flat plane. Valid combinations of grain size and the critical Shields parameter are located on the left side of this cut.

τb ] -2 s 2 m [

2 , C b τ

C2

Figure 47: The bed shear stress (τb) and C2. for a flow velocity u=0.6 ms-1 and viscosity v0=7·10-3 kgm-3

This gives a valid range of combinations for the grain size diameter d ranging from 50 µm to 600 µm and a critical Shields ranging from parameter from 0 to 0.047. A smaller range of the grain size diameter gives a larger range of the critical Shields parameter

In paragraph 6.4 the bed-slope coefficient is calculated for typical values of a sand wave area in the North Sea.

56 Biogeomorphology Small activities with large effects?

6.3.2 Proportionality coefficient The proportionally coefficient (α) in combination with the non-linearity parameter (b) describes how efficiently the particles of sand are transported by the bed shear

stress (τb). The bed shear stress is given by (Eq. 1)-(Eq. 3). The morphological time scale is affected by α as well. The hydrological time scale is much faster than the morphological time scale. This follows from the small value of α in (Eq. 15). This means physically that the ratio between the tidal period and the morphological time scale is small. A measure for the morphological time is (Komarova, 2000).

1 ∆T ≡ (Eq. 39) α'σ Where α’ is the dimensionless proportionality coefficient.

The proportionally coefficient is expressed in the critical bed shear stress. This is done to be able to take biological activities into account. The bedload transport formula of van Rijn (1984) with a bed shear stress parameter is treated as equivalent with the bedload transport formula used in the morphological model for shallow water (Eq. 15). Here the bed-slope coefficient λ is neglected for simplicity reasons. The complete determination of α is given in

Appendix 5: Determination α and b. The expression is given for α is valid for: τcr < τb and T<3, where T is the dimensionless bed shear stress parameters given by:

τ −τ T = b cr τ (Eq. 40) cr

This gives the next expression for the proportionality coefficient:

τb τ −τ 0.1 1 (−4.2 ) τ −τ 2 ()b cr ()τ 2 τ b −τ cr K( b cr ) τ b c α = cr (Eq. 41) 1 2 τ cr 2 -1 1/2 3/2 where K is a constant (m s ), c is a constant (kg m ), τb the bed shear stress -1 -2 -1 -2 (kgm s ) and τcr the critical bed shear stress (kgm s ). A scaling according to (Eq. 42) gives α (m2s-1) for different values of the critical bed shear stress.

§ τ · ¨ b ¸ 1 ¨ −4.2 ¸ 2 τ −τ α = α ()τ © b cr ¹ (Eq. 42) 1 / b c

6.3.3 Non-linearity parameter The non-linearity parameter is determined in Appendix 5: Determination α and b. The results for the dimensionless bed shear stress parameters T<3 is given by:

τ b = 4.2 b τ − τ (Eq. 43) b cr

57 Biogeomorphology Small activities with large effects?

6.4 Results for the sand wave area In this paragraph the bed-slope coefficient, proportionality coefficient and the non- linearity parameter are calculated for different values of the critical bed shear stress specific for the sand wave area.

6.4.1 Bed-slope coefficient λ First the bed-slope coefficient is given for the sand wave area in September 2001. The sediment grain size d is 271 µm (Delft Cluster 2002). The flow velocity u is taken to be 0.6 ms-1. The flow velocity has a strong influence on the development -3 2 -1 of the bed-slope coefficient. The viscosity is ν0=7·10 m s (Komarova, 2000). This -4 2 -2 2 -2 gives δ≈10 m τb≈4.2·10 m s . The critical bed-slope coefficient λc=0.009 m s .

In paragraph 6.3.1 the bed-slope coefficient is given for a variable critical Shields

parameter. Generally the θc is taken to be 0.047. Note that this value is actually derived for rivers. At the sea bed the Shields parameter is probably slightly larger.

With a critical Shields coefficient of θcr = 0.047 the critical bed shear stress is -4 2 -2 2 -2 τcr≈2.06·10 ms . The bed slope coefficient is λ≈0.0053 m s , indicated by the little box in Figure 48.

λc ] -2 s 2 Bed-slope coefficient [m

Critical Shields parameter [-] Figure 48: Bed-slope coefficient for a sand wave area with and without biological influence; September 2001 □ = bed slope coefficient without biological influence, θc=0.047, τcr≈2·10-4 m2s-2, λ≈0.0053 m2s-2. ○ = bed-slope coefficient with biological influence, θc=0.024, τcr≈1·10-4 m2s-2, λ≈0.0022 m2s-2.

The grain size distribution at the crest is a little bit larger than the grain size distribution in the trough but this difference is too small to influence λ. A larger grain size (for example d=500 µm) gives a larger bed-slope coefficient for lower values of the critical Shields parameter.

58 Biogeomorphology Small activities with large effects?

A density of macrozoobenthos of 500 -2 ] -2 individuals m gives a destabilisation s 2 factor of Bd=0.5 according to the expression for the destabilising of the sediment used at the Molenplaat (Eq. λ 28). This gives a new critical bed shear c -2 stress τcr≈ 0.1 Nm and a critical Shields parameter for this value of θcr≈ 0.024. The bed-slope coefficient with

de influence of macrozoobenthos, Bed-slope coefficient [m indicated with the circle, is λ ≈0.0022 2 -2 m s (Figure 48). For a flow velocity of Critical Shields parameter [-] -2 0.5 ms the bed-slope coefficient Figure 49: Bed-slope coefficient for a sand wave area for a flow without biological influence is located velocity u = 0.5 ms-1; September 2001 above the critical bed-slope coefficient. □ = bed slope coefficient without biological influence, θc=0.047, τcr≈2·10-4 m2s-2, λ≈0.01 m2s-2. The bed-slope coefficient with the ○ = bed-slope coefficient with biological influence, θc=0.024, influence of macrozoobenthos is τcr≈1·10-4 m2s-2, λ≈0.002 m2s-2. located below the critical bed-slope coefficient (Figure 49).

As can be seen a low θc generates a low λ. The stabilisation of the sediment due to for example high densities of tubes results in a larger λ. A larger λ results in a larger wavelength of the sand waves. If the sediment is strongly stabilised the bed-slope coefficient could reach the critical bed-slope coefficient If the critical bed-slope coefficient is reached all sand waves will be suppressed (Komarova, 2000). With a destabilising effect of the benthic organisms the opposite effect takes place and the wavelength will shorten.

6.4.2 Proportionality coefficient α The proportionality coefficient α for the sand wave area in September 2001 depends on the bed shear stress. The same parameter values are used as for the bed-slope coefficient. The viscosity is taken to be v=1·10-6 m2s-1 and the grain size diameter is d=271 µm. This is the grain size in the trough of a sand wave in -2 September 2001. This gives for the bed shear stress τb≈0.9 Nm , Chézy-coefficient C≈102 m1/2s-1 and K≈5.3·10-7 m2s-1. The result is given in Figure 50.

The proportionality coefficient does not change very much for different grain sizes as given for the troughs en crests in the sand wave area. The box indicates the -2 critical bed shear stress τcr=0.2 Nm for θcr=0.047 which is a value mostly taken for the critical Shields parameter. The proportionality coefficient for this value of the Shields parameter is α≈8·10-6. A destabilisation of the sediment causes an increase in α. A macrozoobenthos density of 500 individuals m-2 gives, according to the destabilisation factor used in at the Molenplaat, a destabilising factor of 0.5. This -2 results in a new critical bed shear stress of τcr= 0.1 Nm . The proportionality coefficient for this new critical bed shear stress (α’≈4·10-5) is indicated with a circle in Figure 50

59 Biogeomorphology Small activities with large effects?

] -1 s 2 Proportionality coefficient Proportionality [m

Critical bed shear stress [Nm-2]

Figure 50: α for different values of τcr with and without influence of macrozoobenthos. □ = bed slope coefficient without biological influence, τcr =0.2 Nm-2, α≈8·10-6 ○ = bed-slope coefficient with biological influence, τcr= 0.1 Nm-2, α’≈4·10-5

6.4.3 Non-linearity parameter b τ b The parameter b depends only on the ratio τ . The physical range of b (b > 1) is cr given in the next graph. The box indicates the Shields parameter θcr = 0.047 where -2 without the influence of macrozoobenthos which gives a τcr = 0.2 Nm and b=5.3. The circle indicates a destabilising effect of a factor 0.5 by a density of 500 -2 -2 individuals m . This gives τcr =0.1 Nm and b =4.3.

Non-linearity parameter b [-] parameter b [-] Non-linearity

-2 Critical bed shear stress τcr [Nm ]

Figure 51: b for different values of τcr with and without macrozoobenthos □ = bed slope coefficient without biological influence, τcr=0.2 Nm-2, b=5.3 ○ = bed-slope coefficient with biological influence, τcr =0.1 Nm-2, b=4.3

60 Biogeomorphology Small activities with large effects?

The parameter b is larger than 4.2 because the boundary for the critical bed shear

stress is τcr<0. For a critical bed shear stress almost equal to the bed shear stress the value for b increases very fast.

6.4.4 Conclusion and discussion The results just give an indication of the possible effect macrozoobenthos can have on the parameters of the bedload transport model. This is for two reasons.

First the destabilisation factor Bd used in this sand wave area is actually derived for an intertidal flat in the Western Scheldt estuary. This Benthos-factor is used because the precise relationship of the effect of macrozoobenthos on the critical bed shear stress in sand is not known (yet). And second a stabilisation could not be determined because the relationship of these tube building worms and the critical bed shear stress is not known (yet).

In spite of it all the results do show that sand waves areas are influenced by the presence of macrozoobenthos. For low flow velocities the bed-slope coefficient becomes near critical and the destabilisation of the sediment due to macrozoobenthos can make the change between a sand wave area and a flat bed. The other bedload transport parameters are influenced by macrozoobenthos as well. The values found for the parameters with and without biological influence are within the physical ranges given by other authors.

61 Biogeomorphology Small activities with large effects?

7 Discussion In this research two completely different areas, i.e. North Sea sand wave field and Westerschede tidal flat, are considered. One has to be careful to compare these two areas. There are a lot of physical differences for example flow velocity, sediment characteristics, water depth and salinity and biological differences like macro- and microzoobenthos densities, predation and competition. At the Molenplaat for example the water depth is 3 m and the bed has an average mud content of 30 percent. In the sand wave area the bed consists of sand and the water depth is approximately 30 m. Therefore the conclusions for the sand wave area has to be treated with care.

The distribution of the macrozoobenthos at the Molenplaat shows spatial differences between the three divisions. Note that the Molenplaat is approximately 1.5 km2. This indicates that the macrozoobenthos distribution is not homogeneous. The macrozoobenthos samples are taken with a small box. The number of individuals in the box is extrapolated to a larger area. Therefore it must be taken into account that the extrapolation might lead to an overestimation or underestimation of the macrozoobenthos density for one measuring point. The measurement data from the Molenplaat are taken in one year (1995). To get a better idea of the lifecycle of macrozoobenthos and diatoms a larger data set would be preferred. The seasonal cycle of the macrozoobenthos and diatoms is influenced by light availability, temperature and predation. For this reason one year of measurements is not enough to create a lifecycle.

The benthos-factor is derived from experiments carried out on two different places. The destabilisation factor is derived from measurements at the Skefling flat at the Humber estuary (UK) in 1996 and 1997. The stabilisation factor on the other hand is derived for measurements at the Western Scheldt estuary. The measurements used for the destabilising factor are based on the relation between the critical flow velocity and an increasing Macoma balthica density for the year 1996 and 1997. In 1996 there the sediment was stabilised due to high amounts of Chlorophyll a and low densities of Macoma balthica. The second year the sediment was destabilised due to a low amount of Chlorophyll a and a high density of Macoma balthica. Both these measuring periods are used in the relation derived by Widdows (2000b). For this reason the relation does not only show the effect of an increasing amount of Macoma balthica on the critical flow velocity but the effect of a decreasing amount of Chlorophyll a is taken into account as well. This results in very high values of critical flow velocity for low densities of Macoma balthica due to the stabilising effect of the diatoms. To create a relation between the Macoma balthica and the critical flow velocity without the influence of diatoms, a new relation is made by hand assuming an initial flow velocity of 0.2 ms-1 and using the critical flow velocity given for densities larger than 500 individuals m-2. The effect of the Macoma balthica feeding on the diatoms is not taken into account.

In the sand wave area are only a destabilising effect of a density of macrozoobenthos is taken into account. The destabilisation effect in the sand wave

62 Biogeomorphology Small activities with large effects? area caused by a density of macrozoobenthos is calculated with the same benthos- factor. Although the parameters of the bedload transport model change due to the biological influence it has to be taken into account that the benthos-factor is derived for completely different sediment. The bed used to derive the benthos- factor consists of large amounts of mud, which generates cohesive effects in the sediment. Cohesive sediments have a larger critical bed shear stress than non- cohesive sediments. In cohesive sediment strong forces keep the small particles together. In a sandy bed the critical bed shear stress is smaller which, means that the critical flow velocity is lower than for cohesive beds. The sediment of a non- cohesive bed is more loose and therefore is the effect of macrozoobenthos on destabilising the bed is smaller.

Furthermore it must be taken into account that the destabilising or stabilising effect of benthos is not the only biogeomorphological process changing the morphodynamics of the seabed. Other effects of the presence of zoobenthos are an increase in the erosion rate of the sediment and a change in porosity of the bed.

63 Biogeomorphology Small activities with large effects?

8 Conclusions

Is it possible that abundant activity of macrozoobenthos affect the critical bed shear stress?

It is possible that abundant benthic organisms increase or decrease the critical bed shear stress. There are species that are capable of stabilising the sediment by covering it with a ‘sticky film’ (e.g. diatoms) or built sticky sand tubes (e.g. Owenia fusiformis or the Spiphanes bombyx). This research meanly focuses on the stabilisation of the sediment by the diatom. Species, which are capable of destabilising the sediment, move through the sediment making burrows or feed themselves. These species are mainly larger than 1 mm and called macrozoobenthos. The measurements at the Molenplaat in March, June, September and December show that the densities of macrozoobenthos can be very large, up until almost 40000 individuals m-2. These large densities of macrozoobenthos are capable a destabilising the sediment and lower the value for the critical bed shear stress form approximately 0.2 Nm-2 to 0.06 Nm-2. The diatoms are measured at the Molenplaat in concentrations up to 30 µgg-1. They stabilise the sediment and can increase the critical bed shear stress from 0.2 Nm-2 up to 0.3 Nm-2.

What is the effect of the change in the critical bed shear stress on the seabed morphology?

Estuary The results of the mud percentage show that there is a significant effect of benthic species on the mud content of the bed at the Molenplaat in the Western Scheldt estuary. At the end of the year the mud content with the influence of benthic organisms is 30 to 60 percent less compared to the mud content at the end the year without benthos. The results from the maximal and minimal Benthos-factor show that the range of the mud content and critical bed shear stress can be very large. The presence of benthic organisms can make the difference between a mud flat and a sandy flat.

Sand waves The presence of macrozoobenthos at the sand wave area causes a destabilisation of the sediment. This destabilisation gives a reasonable change in the values of the parameters of the bedload transport model. The results do show sand waves areas are influenced by the presence of macrozoobenthos. For low flow velocities the bed-slope coefficient becomes near critical and the destabilisation of the sediment due to macrozoobenthos can make the change between a sand wave area and a flat bed.

Is it acceptable to ignore biological activity in the morphodynamic modelling?

Biological activities can not be ignored in the morphodynamic modelling. In the estuary as offshore there are benthic organisms capable of influencing the morphology of the bed.

64 Biogeomorphology Small activities with large effects?

9 Recommendations There are a lot of assumptions made to create the Benthos-factor. It would be recommended to create a benthos-factor for each specific environment. Assuming the species with the highest densities have the most influence first the domination species has to be found in the area. Next, experiments are required to derive the relation between this species (for example the Macoma balthica) and the critical bed shear stress for the specific bed characteristics.

From the biological point of view more research to the precise activities of benthos would make it possible to pin-point species, which cause destabilisation and stabilisation of the sediment. Also knowledge of the cumulative effect of the different activities carried out by one organism would give more understanding of the effect the specific organism can have on the sediment transport or sediment bed. The Macoma balthica for example is a deposit feeder and a suspension feeder. The two different methods to obtain food have a different effect on the sediment present on the bed and in the water column.

In the North Sea in areas where the bed-slope coefficient is close to the critical bed-slope coefficient the density of the benthic species should be measured. These are the areas where the influence of benthos can become important.

The effects of tube building organisms in sand environments are very important to the understanding of de stabilisation of sand by organisms in the offshore areas and further research would be recommended.

Further research and model implementation of the destabilising and stabilising effects of benthic organisms is recommended. This would give insight in the effect of small-scale biological activities on the morphology of the bed.

65 Biogeomorphology Small activities with large effects?

10 Literature Baptist M.J. Short introduction on coastal biogeomorphology: with examples for water systems of the Netherlands (www.biogeomorphology.org) Brouwer J.F.C, Bjelic S., Deckere de E.M.G.T. and Stal L.J. (2000) Interplay between biology and sedimentology in a mudflat (Biezelingse Ham, Western Scheldt then Netherlands); Cintinental shelf Research vol 20 pag. 1159-1177 BEON (1997), Thoolen P., Baptist M. and Herman P. Habitat macro micro; Comparing patterns in macro-fauna structure at different scales: within tidal flats, between tidal flats and between estuaries. BEON rapport nr. 98-14 BEON (1998), Bergman M.J.N. et al. The distribution of benthic macrofauna in the Dutch sector of the North Sea in relation to the micro distribution of bean trawling; BEON rapport nr. 98-2 Cadée G.C. (1976) Sediment reworking by Arenicola marina on tidal flats in the Dutch Wadden Sea; Netherlands Journal of Sea Research vol 10 pag. 440-460 Cadée G.C. (1979) Sediment reworking by the polychaete Heteromastus filiformis on a tidal flat in the Dutch Wadden Sea; Netherlands Journal of Sea Research vol 13 pag. 441-456 Cadée G.C. (2001) Sediment Dynamics by Bioturbationg Organisms; In: Reise K. (ed) Ecological Comparisons of Sedimentary Shores, Berlin, Springer pag. 127- 148 Carter R.W.G. (1998) Coastal environments, an introduction to the physical, ecological and cultural systems of coastline; London, Academic Press Crosato A., Tanczos I. and Vries de M. (2002) Quantification of biogeomorphological variables for Dutch tidal systems; WL delft, The Netherlands Dyer K.R. (1973) Estuaries, a physical introduction. 2nd ed; Chichester, Wiley Dyer K.R. (1986) Coastal and estuarine sediment dynamics; Chichester, Wiley Eckman J.E., Nowell A.R.M. and Jumars P.A. (1981) Sediment destabilisation by animal tubes; Journal of Marine Research vol 39 pag. 361-374 Fox R.W. and A.T. MacDonald (1994) Introduction to fluid mechanics; New York, Wiley Fredsøe J. and Deigaard R. (1992) Mechanics of coastal sediment transport; Singapore, World Scienctific Herman P.M.J., Middelburg J.J. and Heip H.R. (2001) Benthic community structure and sediment processes on an intertidal flat: results from the ECOFLAT project; Continental Shelf Research vol. 21 pag. 2055-2071 Holtmann S.E. et al. (1996) Atlas of the zoobenthos of the Dutch continental shelf; Rijswijk, Ministry of Transport, Public Works and Water Management, Directorate-General of Public Works and Water Management, North Sea Directorate Hulscher S.J.M.H. (1993) The generation of offshore tidal sand banks and sandwaves; Continental Shelf Research vol. 13 pag. 1183-1204 Hulscher S.J.M.H. (1996) Tidal-induced large-scale regular bedform patterns in a three- dimensional shallow water model; Journal of Geophysical Research vol. 101 pag. 20727-20744 Hulscher S.J.M.H. and Ribberink J.S (2000) Mariene Dynamica II en morfologie (in Dutch); Universiteit Twente

66 Biogeomorphology Small activities with large effects?

Hulscher S.J.M.H. and Brink van den G.M. (2001) Comparison between predicted and observed sand waves and sand banks in the North Sea; Journal of Geophysical Research vol. 106 pag. 9327-9338 ICONA (1992) North Sea atlas; for Netherlands policy and management; Amsterdam, Stadsuitgeverij Komarova N.L. and Hulscher S.J.M.H. (2000) Linear instability mechanisms for sand wave formation; Journal of Fluid Dynamics vol. 413 pag. 219-246 Knaapen M.A.F. (2001) Predicting large waves in an erodible sand bed; PhD dissertation, University of Twente Laban C. et al (2001) Progress report 2000; Theme 3: Coast and River, Eco- morphodynamics of the seafloor, Delft Cluster Laban C. et al (2002) Progress report 2001; Theme 3: Coast and River, Eco- morphodynamics of the seafloor, Delft Cluster Ledden van M. (2002) A process-based sand-mud model; In: J.C. Winterwerp and C. Kranenburg (eds) Fine sediment Dynamics in the Marine Environment; Elsevier Sience B.V. pag. 577-594 Lee. H. II. and Swartz R.C. (1980) Biological processes affecting the distribution of pollutants in marine sediments. Part II. Biodeposition and bioturbation; In: R.A. Baker (ed) Contaminants and sediment, Science publication pag. 555-606. Nederbragt G. (2001) Seasonal variations of suspended sediment and diatoms in the Western Scheldt; Theme 3: Estuaries and Coasts Eco-morphology, Delft cluster (Z2837) Nemeth A.A, Hulscher S.J.M.H., Vriend de H. (2002) Modelling sand wave migrationin shallow shelf seas; Continental Shelf Research vol 22 pag. 2795- 2806 Open university (1999) Waves, Tides and shallow-water processes. Oxford, Buttereworth-Heinemann. Pender G., Meadows P.S. and Tait T. (1994) Biological impact on sediment processes in the coastal zone. Proceedings of the Institution of Civil Engineers vol 106 pag. 52-60 Ribberink J.S. and Buijsrogge R.H. (1999) Transportverschijnselen en morfologie (in Dutch); Universiteit Twente Ribberink J.S., Hulscher S.J.H.M. and Buijsrogge R.H. (1999) Ondiepwaterstromingen (In Dutch); Dictaat, Universiteit Twente Rijn van L.C. (1993) Principles of sediment transport in rivers, estuaries and coastal seas; Amsterdam, Aqua Publications Spencer T. (1988) Coastal biogeomorphology; In: H.A. Viles, Biogeomorphology, Oxford, Basil Blackwell pag. 255-318 Viles H. and Spencer T. (1995) Coastal problems: geomorphology, ecology and society at the coast; London, Arnold Widdows J., Brinsley M.D., Bowley N. and Barret C. (1998a) A benthic annular flume for In Situ measurements of suspension feeding-biodeposition rates and erosion potential of intertidal cohesive sediments; Estuarine, Coastal and Shelf sience vol 46 pag. 27-38 Widdows J., Brinsley M. and Elliott M. (1998b) Use of in situ flume to quantify particle flux (biodeposition rates and sediment erosion) for an intertidal mudflat in relation to changes in current velocity and benthic macrofauna; In: K.S. Black et al.

67 Biogeomorphology Small activities with large effects?

(eds) Sedimentary Processes in the Intertidal Zone, London, Geological Society vol. 139 pag. 85-97 Widdows J., Brinsley M.D., Salkeld. P.N. and Elliott M. (1998c) Use of annular flumes to determine the influence of current velocity and bivalves on material flux at the sediment-water interface; Estuaries vol 21 pag. 552-559 Widdows J., Brown S., Brinsley M.D., Salkeld P.N. and Elliott M. (2000b) Temporal changes in intertidal sediment erodibility: influence of biological and climatic factors; Continental Shelf Research vol 20 pag. 1275-1289 Wood R. (2000) A model of biotically-influenced sediment transport over an intertidal transect; EMPHASYS Consortium, (E.U. MAFF ProjectFD1401)

Websites www.ucmp.berkeley.edu/annelida/annelidalh.html www.antwerpennoord.be/schorren/nederlands/slik_en_schor_nl.html www.antwerpennoord.be/schorren/natuurtalent/p22.htmlwww.waddenzee.nl www.vattenkikaren.gu.se/fakta/arter/mollusca/bivalvia/macobalt/macobae.html www.noordzee.nl/natuur/ www.noordzee.org/rws/dnz/ www.nioo.knaw.nl/cemo/ecoflat/help/hs2.htm www.waddenzee.nl/dutch/navigatie/fr_index.html?/dutch/ecomare/NED0769.HTM www.marlin.ac.uk www.victorbos.com/images/texel_index.html www.nioz.nl/en/deps/mee/projects/longterm/uro-pos.jpg

68 Biogeomorphology Small activities with large effects?

Appendix 1: Macoma balthica

Scientific names5: Macoma is thought to be combination two words: Ma, a goddess from Kappadocian in present day Turkey and the Greek word koma, meaning deep sleep. The Latin balthica means baltic. The species is characteristic for the Baltic Sea.

Appearance: The shell is a plump almost circular shell, up to 25 mm in length, with ambones close to the midline. The posterior of the shell may be very slightly tapered. The shell from the Macoma balthica that has lived on sandy bottoms is thin and white, pink or yellow in colour, while those that have lived on clayey bottoms have thicker shells and are blue or dark red in colour. There are no lateral teeth by the hinge. Global distribution From temperate to arctic Figure 52: Macoma balthica coastal waters in both the North Atlantic and North Pacific oceans. Depth: 0 - ca 30 m, but lives deeper in the Baltic. Environment: Lives a few centimetres below the surface of sand, mud and muddy sand. It is found from the upper regions of the intertidal into the sublittoral, particularly in estuaries and on tidal flats. Abundance: The species can be found in large concentrations, from a few hundred, and up to a couple of thousand per m2. Classification: Member of the bivalve group under the molluscs. Growth Form: Bivalved; A shell of two calcareous valves joined by a flexible ligament. Mobility: Burrower mobile species; able to burrow upward and surface from a depth of 5 - 6 cm Characteristic feeding method: Suspension feeder and surface deposit feeder. Typically feeds on: Diatoms, deposited plankton, suspended phytoplankton and detritus. Migration Pattern: Non-migratory / Resident; Remaining within the same area

Explanation of sensitivity and recoverability ranks Substratum Loss Macoma balthica inhabits the upper layers of sandy and muddy substrata in physiographic locations where activities causing substratum loss occur e.g. channel dredging. Consequently, removal of the substratum would remove the population of Macoma balthica from the area affected. Direct evidence of recovery by Macoma balthica following substratum loss is given by Bonsdorff (1984).

5 Source: http://www.marlin.ac.uk/demo/Macbal.htm and http://www.vattenkikaren.gu.se/fakta/arter/mollusca/bivalvia/macobalt/macobae.html

I Biogeomorphology Small activities with large effects?

Increase in suspended sediment Macoma balthica is known to practice two alternative modes of feeding. It either holds its feeding organ, the siphon, at a fixed position just above the sediment surface to filter out food particles suspended in the overlying water, or extends and moves its siphon around on the sediment above it to vacuum up deposited food particles (Peterson & Skilleter, 1994). Facultative switching between the modes of feeding in Macoma balthica is directly affected by food availability in the overlying water (Lin & Hines, 1994). In turn, changes in feeding mode from suspension to deposit feeding directly affects burial depth. Burrowing in the sediment is one of few defensive mechanisms Macoma balthica has against predators. In the laboratory, Lin & Hines (1994) observed specimens of Macoma balthica kept in estuarine water supplemented with 75 µg L-1of algae to maintain a deeper burial position whilst suspension feeding, than those without an enhanced diet who deposit fed. Thus an increase of material in suspension will favour suspension feeding by Macoma balthica and indirectly reduce its vulnerability to lethal and sub-lethal siphon browsing by fish and decapods. Macoma balthica is therefore assessed as 'not sensitive' with the potential for growth and reproduction to be enhanced by the increased food supply.

Increase in water flow rate Macoma balthica thrives in low energy environments such as estuaries (Tebble, 1976) where the substratum has a high proportion of fine sediment. Increased water flow rate will change the sediment characteristics in which the species lives, primarily by re-suspending and preventing deposition of finer particles (Hiscock, 1983). This would result in erosion of the preferred habitat, which may cause mortality of some portion of the population of Macoma balthica. Green (1968) recorded that towards the mouth of an estuary where sediments became coarser and cleaner, Macoma balthica was replaced by another tellin species, Tellina tenuis. Sensitivity is therefore recorded as intermediate. Recoverability is recorded as high. Newell (1965) (cited in Green, 1968) noted that Macoma balthica populations in the Thames Estuary, UK, were denser where the grade of deposit was finer, possibly due to greater food availability.

Decrease in temperature The species occurs in the Gulfs of Finland and Bothnia where the sea freezes for several months of the year (Green, 1968). It must therefore tolerate lower temperatures. Furthermore, Macoma balthica was apparently unaffected by the severe winter of 1962/3 which decimated populations of many other bivalve species (Crisp, 1964), and De Wilde (1975) noted that Macoma balthica kept at 0°C maintained a high level of feeding activity.

Increase in wave exposure Fine sediments would be eroded (Hiscock, 1983) resulting in the likely reduction of the habitat of Macoma balthica. Strong wave action may cause damage or withdrawal of the siphons, resulting in loss of feeding opportunities and compromised growth. Furthermore, individuals may be dislodged by scouring from sand and gravel mobilized by increased wave action. For example, Ratcliffe et al. (1981) reported

II Biogeomorphology Small activities with large effects? that juvenile Macoma balthica are susceptible to displacement by water currents due to their small mass and inability to bury deeply.

Changes in oxygenation Brafield and Newell (1961) frequently observed that in conditions of oxygen deficiency (e.g. less than 1 mg O2/l) Macoma balthica moved upwards to fully expose itself on the surface of the sand. Specimens lay on their side with the foot and siphons retracted but with valves gaping slightly allowing the mantle edge to be brought into full contact with the more oxygenated surface water lying between sand ripples. In addition, Macoma balthica was observed under laboratory conditions to extend its siphons upwards out of the sand in to the overlying water when water was slowly deoxygenated with a stream of nitrogen. The lower the oxygen concentration became the further the siphons extended. This behaviour, an initial increase in activity stimulated by oxygen deficiency, is of interest because the activity of lamellibranches is generally inhibited by oxygen deficient conditions (Brafield and Newell, 1961).

III Biogeomorphology Small activities with large effects?

Appendix 2: Grain size distribution Molenplaat Silt

384600.00 75.00 70.00 384400.00 65.00 60.00 55.00 384200.00 50.00 45.00 384000.00 40.00 Par y 35.00 383800.00 30.00 25.00 383600.00 20.00 15.00 383400.00 10.00 5.00 383200.00 March 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 53: Silt percentage Molenplaat March

384600.00 75.00 70.00 384400.00 65.00 60.00 384200.00 55.00 50.00

384000.00 45.00 40.00 Par y 35.00 383800.00 30.00 25.00 383600.00 20.00 15.00 383400.00 10.00 5.00 383200.00 June 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 54: Silt percentage Molenplaat June

IV Biogeomorphology Small activities with large effects?

384600.00 75.00 70.00 384400.00 65.00 60.00 384200.00 55.00 50.00 384000.00 45.00

Par y 40.00

383800.00 35.00 30.00 25.00 383600.00 20.00 15.00 383400.00 10.00 5.00 383200.00 September 0.00 54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 55: Silt percentage Molenplaat September

384600.00 75.00 70.00 384400.00 65.00 60.00 384200.00 55.00 50.00 384000.00 45.00

Par y 40.00

383800.00 35.00 30.00 25.00 383600.00 20.00 15.00 383400.00 10.00 5.00 383200.00 December 0.00 54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 56: Silt percentage Molenplaat December

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Medium sand

384600.00 80.00 75.00 70.00 384400.00 65.00 60.00 384200.00 55.00 50.00 384000.00 45.00

Par y 40.00 383800.00 35.00 30.00 25.00 383600.00 20.00 15.00 383400.00 10.00 5.00 383200.00 March 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par y Figure 57: Medium sand percentage Molenplaat March

384600.00 80.00 75.00 384400.00 70.00 65.00 384200.00 60.00 55.00 50.00 384000.00 45.00 Par y 40.00 383800.00 35.00 30.00 383600.00 25.00 20.00 15.00 383400.00 10.00 5.00 383200.00 June 0.00 54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 58: Medium sand percentage Molenplaat June

VI Biogeomorphology Small activities with large effects?

384600.00 80.00 75.00 384400.00 70.00 65.00 60.00 384200.00 55.00 50.00 384000.00 45.00

Par y 40.00 383800.00 35.00 30.00

383600.00 25.00 20.00 15.00 383400.00 10.00 5.00 383200.00 September 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 59: Medium sand percentage Molenplaat September

384600.00 80.00 75.00 384400.00 70.00 65.00

384200.00 60.00 55.00 50.00 384000.00 45.00

Par y 40.00 383800.00 35.00 30.00 383600.00 25.00 20.00 15.00 383400.00 10.00 5.00 383200.00 December 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 60: Medium sand percentage Molenplaat December

VII Biogeomorphology Small activities with large effects?

Appendix 3: Distribution Chlorophyll a Molenplaat The values of Chlorophyll a are given in µgg-1 sediment

384600.00 36.00 34.00 384400.00 32.00 30.00 28.00 384200.00 26.00 24.00 22.00 384000.00 20.00

Par y 18.00 383800.00 16.00 14.00 12.00 383600.00 10.00 8.00 383400.00 6.00 4.00 2.00 383200.00 June 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 61: Distribution of Chlorophyll a at the Molenplaat, June

384600.00 36.00 34.00 32.00 384400.00 30.00 28.00 384200.00 26.00 24.00 22.00 384000.00 20.00

Par y 18.00 383800.00 16.00 14.00 12.00 383600.00 10.00 8.00 383400.00 6.00 4.00 2.00 383200.00 September 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 62: Distribution of Chlorophyll a at the Molenplaat, September

VIII Biogeomorphology Small activities with large effects?

384600.00 36.00 34.00 32.00 384400.00 30.00 28.00 384200.00 26.00 24.00 22.00 384000.00 20.00

Par y 18.00 383800.00 16.00 14.00 12.00 383600.00 10.00 8.00 6.00 383400.00 4.00 2.00 383200.00 December 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 63: Distribution of Chlorophyll a at the Molenplaat, December

IX Biogeomorphology Small activities with large effects?

Appendix 4: Distribution macrozoobenthos density at the Molenplaat

384600.00 95000.00 90000.00 85000.00 384400.00 80000.00 75000.00 384200.00 70000.00 65000.00 60000.00 384000.00 55000.00 50000.00 Par y 45000.00 383800.00 40000.00 35000.00 30000.00 383600.00 25000.00 20000.00 383400.00 15000.00 10000.00 5000.00 383200.00 March 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 64: Distribution of the macrozoobenthos density at the molenplaat, March

384600.00 95000.00 90000.00 85000.00 384400.00 80000.00 75000.00 384200.00 70000.00 65000.00 60000.00 384000.00 55000.00 50000.00 Par y 45000.00 383800.00 40000.00 35000.00 30000.00 383600.00 25000.00 20000.00 383400.00 15000.00 10000.00 5000.00 383200.00 June 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 65: Distribution of the macrozoobenthos density at the Molenplaat, June

X Biogeomorphology Small activities with large effects?

384600.00 95000.00 90000.00 85000.00 384400.00 80000.00 75000.00 384200.00 70000.00 65000.00 60000.00 384000.00 55000.00

Par y 50000.00 45000.00 383800.00 40000.00 35000.00 383600.00 30000.00 25000.00 20000.00 383400.00 15000.00 10000.00 September 5000.00 383200.00 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 66: Distribution of the macrozoobenthos density at the Molenplaat, September

384600.00 95000.00 90000.00 85000.00 384400.00 80000.00 75000.00 70000.00 384200.00 65000.00 60000.00 384000.00 55000.00 50000.00 Par y 45000.00 383800.00 40000.00 35000.00 30000.00 383600.00 25000.00 20000.00 15000.00 383400.00 10000.00 5000.00 383200.00 December 0.00

54200.00 54600.00 55000.00 55400.00 55800.00 56200.00 Par x Figure 67: Distribution of the macrozoobenthos distribution at the Molenplaat, December

XI Biogeomorphology Small activities with large effects?

Appendix 5: Determination α and b

Depth-averaged velocity -1 The dept-averaged flow velocity u (ms ) is expressed in the bed shear stress τb (Nm-2): § u ·2 C 2 τ = gρ¨ ¸ → u 2 = τ b © C ¹ b gρ

This gives for u:

1 C = τ 2 ⋅ u b (Eq. 44) gρ where C is the Chezy coefficient (m1/2s-1), ρ the density of water (kgm-3) and g the gravity acceleration (ms-2).

Bedload transport formulas The bed-slope coefficient is neglected in the bedload transport formula of van Rijn (1993). When the bed-slope effects are neglected the bedload transport (m2s-1) can be expressed as:

S = αub (Eq. 45)

Substitution of(Eq. 44) in (Eq. 45) gives:

1 b = α()τ 2 S b c (Eq. 46) where c is a constant (m3/2kg-1/2).

A second bedload transport formula of van Rijn (1984) uses two dimensionless parameters, a dimensionless bed shear stress parameter (T) and a dimensionless 2 -1 grain size diameter (D*). The bedload transport (m s ) is expressed as:

= θ ∆ 3 Sv.Rijn b g d50 (Eq. 47) 2.1 θ = T with: b 0.053 0.3 for T<3 and D*

Where ∆ is the relative density (1.65) and d50 the grain size diameter (m). The dimensionless bed shear stress parameter (T) is given by:

τ −τ T = b cr τ (Eq. 48) cr -2 Where τcr is the critical bed shear stress (Nm ). The dimensionless grain size diameter (D*) is given by:

XII Biogeomorphology Small activities with large effects?

1  ∆g  3 D = d   (Eq. 49) * 50  v2  where v is the viscosity (m2s-1).

Substitution of (Eq. 48) and (Eq. 49) in (Eq. 47) gives:  τ 2.1 S = K * b −1 (Eq. 50) v.Rijn τ   cr  = 1 ∆ 3 = with K 0.053 0.3 g d50 constant . D*

The determination It is supposed that both bedload transport formulas must give the same bedload transport. The same is supposed for the differentiation of both bedload transport formulas. This gives: = S Sv.Rijn (Eq. 51) 1 = 1 S Sv.Rijn (Eq. 52)

The differentiation of the first bedload transport formula (Eq. 45) is given by: 1 − 1 = 1 α τ 2 b 1 S 2 b( b c) (Eq. 53)

The differentiation of the second bedload transport formula (Eq. 50) is given by: 1.1  τ  1 S1 = 2.1* K b −1 * (Eq. 54) v.Rijn τ  τ  cr  cr

Solution of (Eq. 51) and (Eq. 52): 2.1 1  τ  α(τ 2c)b = K * b −1 (Eq. 55) b τ   cr  1.1 1  τ  1 1 αb(τ 2c)b = 2.1* K b −1 * (Eq. 56) 2 b τ  τ  cr  cr

A multiplication of (Eq. 56) with τb gives: 1.1 1  τ  τ 1 αb(τ 2c)b = 2.1* K b −1 * b (Eq. 57) 2 b τ  τ  cr  cr

Now both equations (Eq. 55) and (Eq. 57) can be subtracted. This gives an expression for α.

XIII Biogeomorphology Small activities with large effects?

τ −τ 1.1  b cr  τ + τ K  (11 b 10 cr )  τ  (Eq. 58) α = 0.2 cr ()τ bτ − + bc cr ( 2 b)

The expression for α (Eq. 58) is substituted in equation (Eq. 55) and gives an expression for b. τ b = 4.2 b τ − τ (Eq. 59) b cr

Now the expression for b (Eq. 59) is substituted in (Eq. 58). This gives a final expression for α in terms of the bed shear stress and the critical bed shear stress. 0.1 § τ · ¨ b ¸ §τ −τ · 1 ¨ −4.2 ¸ 2 b cr 2 τ −τ ()τ −τ ¨ ¸ ()τ © b cr ¹ K b cr ¨ ¸ b c τ (Eq. 60) α = © cr ¹ τ 2 cr

 τ   b  1  −4.2  2 τ −τ α = α ()τ  b cr  The expression used in paragraph 6.3.2 is scaled with ' / b c .

XIV