The mud deposits and the high turbidity in the Belgian–Dutch coastal zone, southern bight of the...

Continental Shelf Research ] (]]]]) ]]]–]]]

The mud deposits and the high turbidity in the Belgian–Dutchcoastal zone, southern bight of the North Sea

Michael Fettweis*, Dries Van den Eynde

Management Unit of the North Sea Mathematical Models (MUMM), Royal Belgian Institute of Natural Science, Gulledelle 100,

B-1200 Brussels, Belgium

Received 22 August 2002; accepted 27 January 2003

Abstract

The suspended sediment processes and the mudfields found in the Belgian/Dutch coastal area (Southern North Sea)

are discussed by presenting an integrated data-modelling approach of the suspended sediment transport along the

Belgian–Dutch coast, using a fine-grid coupled 2D hydrodynamic and sediment transport model and existing field and

literature data. These mudfields and turbidity maxima are situated in a well-mixed, highly energetic hydrodynamic

environment. In the past the occurrence of this high turbidity zone (more than a few hundreds mg/l of suspended

matter) was ascribed to a closed hydrodynamic system (gyre) in front of the coast. This study shows that the SPM input

through the Strait of Dover, the shallowness of the considered area, the decreasing magnitude of the residual transport

vectors from the French/Belgian border towards Zeebrugge and the specific hydrodynamic features are the main

processes responsible for the presence of the turbidity maximum. The origin and the formation of these mud deposits in

front of the coast are explained by the neap-spring tidal cycles and the presence of SPM sources (import of SPM

through the Strait of Dover and through erosion of clay layers).

r 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Mud; SPM; Turbidity maximum; Sediment transport modelling; Belgium; North Sea

1. Introduction

The bottom sediments in the southern Bight ofthe North Sea (see Fig. 1) consist mainly of fine tomedium sand. Between Oostende and the Wester-schelde estuary (geographically the mouth of theestuary is situated at Vlissingen, although geo-morphologically the area between Zeebrugge andWalcheren is part of the mouth) the mud depositsand high concentrations of suspended particulate

matter (SPM) occur (values of a few hundred mg/lare common). The marine environment in theconsidered area is shallow and well mixed withhighly energetic hydrodynamic conditions.The presence of mudfields and high turbidity in

such an energetic environment has been the subjectof various studies. McManus and Prandle (1997)showed that only the Dover Strait, the northernboundary (56�N), The Wash and the Suffolk coastare statistically significant sources of SPM in theSouthern North Sea. The smectite content in thefine-grained surface sediment in the Belgian coast-al zone is high, which points to the Cretaceous

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*Corresponding author.

E-mail address: [email protected] (M. Fettweis).

0278-4343/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0278-4343(03)00027-X

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Fig. 1. Belgian–Dutch coastal area. On the map are indicated the numerical model boundaries, the dumping sites (B/1, B/2, B/6 and

B/9), the navigation channels and the 5, 10, 20 and the 30m depth lines (MSL). The co-ordinates are in longitude and latitude.

M. Fettweis, D. Van den Eynde / Continental Shelf Research ] (]]]]) ]]]–]]]2

formations in the Dover Strait as a source area(Irion and Z .ollmer, 1999). Goss!e (1977) and laterVan Alphen (1990) and Salden and Mulder (1996)mention the erosion of the Flemish Banks as asource of SPM in the Belgian/Dutch coastal area.Other authors (Bastin, 1974; Gullentops et al.,1976; Baeteman, 1999) discuss the occurrence ofexposed Tertiary clay and Holocene mud and peatlayers. These sediments are locally a source ofSPM, although quantities are not provided. Otherstudies used a hydrodynamic trap consisting ofgyres, divergences or convergence’s in the residualcurrent or transport pattern to explain theaccumulation of cohesive sediments (Nihoul,1975; Gullentops et al., 1976; Van Alphen, 1990).The abundance of fine-grained sediments and

the shallowness of the area are the cause of theintensive dredging activities in the navigationchannels and the coastal harbours. In the lastdecades the depth of the navigation channelstowards Zeebrugge and the Westerschelde hasbeen gradually increased; it varies between 13.8and 15.9m below mean low low water spring tidelevel (MLLWS) depending on the location. Thetotal amount of fine sediments could thus haveincreased if one assumes that the sediment budgetin the high turbidity area consists of an almostclosed system with very low exchange of matterwith neighbouring areas (Malherbe, 1991). It isremarkable that theories of closed or nearly closedsystems in front of the Belgian/Dutch coastalwaters remain common till today. Present compu-ter infrastructure allows to reduce the grid size of anumerical model sufficiently to simulate the localhydrodynamics in detail. Results of such a fine-grid 2D model are presented below. These resultsdo not show any gyre like structure in the residualcirculation, which could explain the trapping ofsediments or identify a closed sediment system.The short introduction shows that the origin of

the mud and the SPM in the Belgian/Dutch coastalareas remains controversial. Our study thereforemakes use of sediment data and of numericalmodel results in order to re-examine two topicsboth responsible for the accumulation of mud onthe bottom and the formation of a high turbidityzone: (1) the possible origins of the mud in themudfields and of the suspended mud occurring in

the Belgian/Dutch coastal waters (input throughthe Strait of Dover; erosion of Tertiary clay andHolocene peat and mud layers) and (2) thehydrodynamic processes which are responsiblefor the sedimentation/erosion processes and theincrease of turbidity.

2. The 2D modelling system

The model system consists of a coupled 2Dhydrodynamic model (MU-BCZ), a wave model(MU-WAVE) and a 2D sediment transport model(MU-STM). The year 1999 has been chosen as thesimulation period. A 2D horizontal model isappropriate for the Belgian coastal zone, becausethe water column is not stratified through theentire year (de Ruijter et al., 1987). The high tidalamplitude (mean spring amplitude at Oostende is4.6m, mean neap amplitude is 3.0m), the strongtidal currents (>1.2m s�1) and the low fresh waterdischarge of the Schelde (yearly average is100m3 s�1, peak values are 20m3 s�1 duringsummer and 600m3 s�1 during winter) result inwell mixed water. The fresh water run-off of theSchelde can induce horizontal density gradients inthe area between Zeebrugge and the mouth of theestuary. The evolution of salinity during a tidalcycle has been measured regularly in the vicinity ofZeebrugge. The maximum salinity difference dur-ing a tide is usually small but can reach values upto 4.0 (Fettweis, 2001).The meteorological data for the hydrodynamic

and wave models were provided by the meteor-ological forecasts from the United KingdomMeteorological Office, Bracknell. The wind vec-tors and the atmospheric pressure are given on agrid of 1.25� by 1.25� and with a time increment of6 h.

2.1. Hydrodynamic model (MU-BCZ)

The two-dimensional hydrodynamic modelMU-BCZ calculates the depth-integrated currentvelocity and the water elevation resulting from thetides and the meteorological forcing (wind andatmospheric pressure). The model solves theclassical shallow-water wave equations in a system

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M. Fettweis, D. Van den Eynde / Continental Shelf Research ] (]]]]) ]]]–]]] 3

of Cartesian co-ordinates. Horizontal diffusion isincluded in the model, a constant diffusioncoefficient of 20m2 s�1 is used. The bottom stressis computed using a quadratic friction law, whilethe wind friction coefficient is calculated followingSmith and Banke (1975). The equations are solvedusing a fully explicit finite difference method on astaggered Arakawa-C grid.The model covers the Belgian coastal area and

the Flemish Banks (Fig. 1), with a grid resolutionof 2500 � 4000 (7750m� 750m). Along the openboundaries, the MU-BCZ model is coupled with a2D hydrodynamic model, which covers the NorthSea and the English Channel (MU-STORM). Foursemi-diurnal tidal components ðM2;S2;N2;K2Þ andfour diurnal tidal components ðO1;K1;P1;Q1Þ areused to force the tidal elevation on the openboundaries of the Continental Shelf Model. Atthe outflow of the Westerschelde, the MU-BCZmodel is coupled with a one-dimensional model forthe estuary and its main tributaries.The hydrodynamic model has been validated

(Van den Eynde, 1997) using OSCR currentmeasurements, which were collected by ProudmanOceanographic Laboratory at the MiddelkerkeBank (CSTAB Group, 1996). The root-mean-square-error (RMSE) on the major axis and onthe minor axis of theM2 tidal ellipses are restrictedto 0.057 and 0.034m s�1, respectively. The corre-sponding scatter indices (SI), which are defined asthe RMSE divided by the mean of the observa-tions, are 9% and 20%, respectively. The RMSEon the residuals (Z0-component) was 0.042m s

�1.Also tidal elevation and current measurements,executed by MUMM over a period of 8 years andin different stations on the Belgian ContinentalShelf (BCS), were used to validate the modelresults. This gave a RMSE on the M2 tidalelevation of 0.09m and a RMSE on the majoraxis of the M2 tidal ellipses of 0.13m s

�1.The residual water transport ~UU res(m s

�1) wascalculated for the year 1999 (see Fig. 2) using thefollowing expression:

~UU res ¼Pn

i¼1 hi~uuciPni¼1 hi

; ð1Þ

where ~uuci is the velocity vector (m s�1) at time i; hi

the total water depth (m) at time i and n the

number of current vectors, used to calculate theresidual water transport.

2.2. Wave model (MU-WAVE)

The wave environment in the Belgian coastalarea is calculated with the MU-WAVE model(Van den Eynde, 1992). The core of the model isformed by the second generation HYPAS wavemodel (G .unther and Rosenthal, 1985). This hybridmodel combines the independent calculation of

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1998-2001

0

5

10

15

20

25N

NNE

NE

ENE

E

ESE

SE

SSE

S

SSW

SW

WSW

W

WNW

NW

NNW

>8 Bf

7 Bf

6 Bf

5 Bf

4 Bf

<3 Bf

1998-2001

0

5

10

15

20

25N

NNE

NE

ENE

E

ESE

SE

SSE

S

SSW

SW

WSW

W

WNW

NW

NNW

>1.5m

1-1.5m

0.75-1m

0.5-0.75m

<0.5m

(a)

(b)

Fig. 2. (a) Frequency and wind speed (sector: indicates from

where the winds are blowing) and (b) significant wave height

(direction indicates from where the waves are coming)

occurring in the coastal zone of Belgium. Arithmetic mean of

monitoring data (01/01/1998–31/08/2001, Ministry of the

Flemish Community, Waterways and Marine Affairs Admin-

istration, Coastal Waterways Division. The coast is in WSW–

ENE direction.


swell energy for each frequency and directionthrough a ray technique, with a parametrical windsea model, using the JONSWAP parameters andthe mean wind sea direction as prognosticvariables. For the current application, the modelis implemented on two nested stereographicalgrids. The North Sea grid has a resolution of50 km� 50 km, whereas for the Southern Bight aresolution of 5 km� 5 km is implemented.The MU-WAVE model has been tested exten-

sively and is used in operational mode for theprediction of the waves at the BCS. Results of thefirst validation were presented in Van den Eynde(1992). The RMSE of the significant wave heighton the BCS was 0.35m, which gives a SI of 31.8%.During later validations of the MU-WAVE model,the results were compared with ERS-1 data, withbuoy data and with results of the third generationwave model WAM (Ovidio et al., 1994; Bidlotet al., 1995). For the entire North Sea, a SI of 38%was found for the significant wave height results,while in the Southern North Sea, the SI was 27%.Taking into account the uncertainty in the windfield forecasts, let to the conclusion that theseresults are very satisfactory.The wind sector, wind velocity, sector of the

high frequency waves (2–5 s) and significant waveheight for the period 01/01/1998–31/08/2001 arepresented in Fig. 2. The most frequent wind sectoris from the SW. The strongest wind speeds areoccurring during winter. The dominant wavedirection from where the waves are coming is theW–WSW. Nevertheless, the highest waves arecoming from the NW–NNW, they occur mostfrequently during winter.

2.3. Mud transport model (MU-STM)

Transport of mud is determined by the settlingof mud particles under the influence of gravity andby erosion and sedimentation due to the localcurrent velocity. In the southern North Sea thesediment composition is not uniform: it variesfrom sand to mud (with different grain sizes,mineralogical compositions, organic contents,etc.). The bed structure, density and consolidationof the cohesive sediments are in general poorlyknown. The MU-STM model, which is presented

here, should be used as an instrument to betterunderstand some of the aspects of mud transport.The model solves the 2D depth-averaged advec-

tion–diffusion equation, which describes the trans-port of the material in suspension through themodel grid:

qðhCÞqt

þqhuC

qxþ

qhvC

qy

¼qqx

hK1qC

qx

� �þ

qqx

hK2qC

qy

� �þ E � D; ð2Þ

where C is the depth-averaged SPM concentration(kgm�3), h is the water depth (m), u and v are thedepth-averaged velocity components which in-cludes Stokes drift (m s�1), K1 and K2 are thediffusion coefficients (m2 s�1), E and D the erosionand deposition rates (kgm�2 s�1), which arecalculated using the formulations of Ariathurai–Partheniades (Ariathurai, 1974) and Krone (1962),respectively, see the appendix for a detaileddescription. The local bottom stress is calculatedunder the combined effect of currents and waves(Eq. (A.1) in the appendix). The model uses thesemi-Lagrangian Second Moment Method (Eganand Mahoney, 1972; de Kok, 1994) for theadvection of the material in suspension. In thismethod all material in each grid cell is representedby one rectangular mass, with sides parallel to themodel grid, characterised by its zero-order mo-ment (total mass), first-order moments (masscentre) and second-order moments (extent). Thediffusion of suspended matter is based on the workof Johnson et al. (1988) and calculates the enlarge-ment of the rectangular mass assuming a Fickiandiffusion. K1 and K2 have a value of 10m

2 s�1.Research on the erosive characteristics of non-

homogenous sediment compositions (sand/mudmixtures) has shown that the sand content hasan important effect on the erosion threshold, therate of erosion and the bed consolidation (Torfset al., 1996; Williamson and Torfs, 1996). Aconcept of active layers overlaying an unremova-ble parent bed that can be used for varyingfractions of sediment sizes, has been presented byLou and Ridd (1997). In the present model, onlycohesive sediments (mud, grain sizeo62.5 mm) areconsidered and a different concept of active layersand parent bed is used. The formation of a

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cohesive sediment layer is the result of settling,deposition and consolidation of particles. Theseprocesses change the erosion sensitivity of thesediments and are implemented in the active layersof the model. The consolidation model is based onthe work of Hayter (1986), Le Normant (1995) andWilliamson and Torfs (1996), where the depositedmud will consolidate and increase its bulk densityin function of its age. The bulk density determinesthe critical shear stress of erosion. The parent bedis used as initial condition for the model simula-tions and is thus removable. The concept of parentbed which is applied here is based on theassumptions that (1) the sediments are consoli-dated, (2) the sand and mud are well mixed, (3) thesediments can only be eroded when the overlayingactive layers are empty and (4) the erosion of mudfrom the parent bed is proportional to the mudcontent in the parent bed. This latter is imple-mented by multiplying the erosion constantM (seeEq. (A.3) in the appendix) by the mud content.The mud content in the parent bed is based onfield data and has been calculated in each cell ofthe numerical grid by a distance-weighted inter-polation (Fig. 3).

The values used in the model for the criticalshear stress for erosion tce vary between 0.5(freshly deposited mud) and 0.8 Pa (after 48 h).The shear strength of the parent bed was set to2.0 Pa, which is a compromise between the erosionshear strength of Holocene mud layers found inthe coastal zone near Zeebrugge, of the mainlysandy sediments outside the coastal area (Fig. 3)and of the results of calibration simulations basedon seafloor erosion data (see Section 3.1). The bulkdensity of the Holocene mud is about 1500–1600 kgm�3, the one of the fresh mud from thenavigation channels about 1400 kgm�3 (Francken,2002). The critical shear stress for erosion of theHolocene mud is thus about 1.4–1.6 Pa usingEq. (A.4) (see the appendix). Williamson andTorfs (1996) have measured in situ values for tce

of 1.9 Pa for sandy, consolidated beds. For 100%mud the erosion constant M was set to0.12� 10�3 kgm�2 s�1. In sediments with lowermud content, which can only occur in the parentbed, the erosion rate is multiplied by the mudfraction. This value is in the same range asreported by Williamson and Torfs (1996), wheremeasurements have indicated that the erosion rate

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Fig. 3. Mud content (% weight) in the surface sediments. The mud content is calculated from a distance weighted average of the data

lying in a diameter of 3.75 km (i.e. 5 times the grid distance). The vectors indicate the residual water transport pattern of 1999 (tide and

wind forcing) calculated with the MU-BCZ model.


for pure mud beds (0.05–0.1� 10�3 kgm�2 s�1)was an order of magnitude higher than the20% and 40% sand beds (0.005–0.03� 10�3

kgm�2 s�1). In the model a constant fall velocityof 0.001m s�1 has been used. This relative highvalue implicitly accounts for flocculation pro-cesses. Typical values for the critical shear stressfor deposition tcd which have been obtained in alaboratory environment vary between 0.05 and0.2 Pa (Berlamont et al., 1993). Here a value of0.5 Pa has been taken. A high value of tcd favoursthe deposition of mud. The sediment parametervalues used in the model are presented in Table 1.The SPM concentration condition along the

open boundaries of the mud transport model hasbeen constructed using measurement data (Eismaand Kalf, 1987; Goss!e, 1977; Groenendijk, 1992;Gullentops et al., 1976; Salden, 1998). Thesuspended sediment concentration varies withseason. The seasonal influence is calculated byusing the measurements of suspended sedimentconcentration of the MUMM Monitoring Pro-gram (1976–1996). The seasonal-averaged valueover the period shows that the SPM concentrationis 1.2 times higher in winter and 0.8 times lower insummer than in spring or autumn (Fig. 4).

2.4. Sensitivity analysis

Gerritsen et al. (2000) mention three funda-mental uncertainties in modelling SPM fluxes: (1)insufficient information on sediment sources, (2)relatively simple erosion and deposition formulae(especially with respect to flocculation effects) and(3) insufficient information on bed surface sedi-

ments available for erosion or resuspension. Theparameters used in the model presented here andwhich affect sediment behaviour have been men-tioned above. Their values have been chosen basedon a commonly accepted range found in literature.This parameterisation is site specific and remainsthus fairly arbitrary. Direct comparison withmeasurements is difficult because of their limitedavailability in time and space. A sensitivityanalysis allows evaluating the model results. Theresults of the sensitivity simulations will bepresented in the following chapter where sourcesof mud (Section 3.1), mud deposition (Section 3.2)and hydrodynamic effects (Section 3.3) are dis-cussed. Evaluation of the mud sources has beencarried out by changing the SPM concentrationboundary condition (variation of the Dover Straitinput) and by varying the erosion strength of thesediments in the parent bed (seafloor erosion).Discussion of the result of different fall velocitiesand critical shear stresses of deposition togetherwith the hydrodynamic condition gives someinsight into the origin of the mud deposits. Theinfluence of meteorological forcing and of waveshas also been investigated.

3. The high turbidity and the mudfields

The Belgian–Dutch coastal area is shallow(depth between 5 and 35m) and major navigationchannels connect the open sea to the harbour ofZeebrugge and the Westerschelde (Fig. 1). Themean tidal range at Zeebrugge is 4.3m (2.8m) atspring (neap) tide; the maximum current velocityat the B/1 dumping site is more than 1m s�1

(0.6m s�1) at spring (neap) tide. As alreadymentioned the winds are dominantly from thesouthwest and the highest waves occur duringnorthwestern winds. The distribution of suspendedmatter is influenced by a range of interrelatedprocesses such as meteorological and hydrody-namic conditions, turbulence, re-suspension anddeposition, aggregation and flocculation, salinity,temperature, biological activity and, of course,the availability of fine-grained sediments. Theimportance of each process is difficult toquantify, partly because of a lack of observa-

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Table 1

Parameter values used for the reference simulation

Fall velocity, w 1mms�1

Critical shear stress for deposition,

tcd

0.5 Pa

Erosion constant, M 0.12� 10�3 kgm�2 s�1

Critical shear stress for erosion, tcd :

fresh deposited

0.5 Pa


after 48 h

0.8 Pa


parent bed

2.0 Pa


tions and partly because of its complex natureand the coupling existing between them. There-fore, we will focus on the most important ones:the hydrodynamic conditions, sedimentation–ero-sion processes and the sources of mud. Theinfluence of the dredging/dumping operations isalso evaluated.

3.1. Sources of mud

The high SPM concentration in the coastalzone is a secondary phenomenon. The primaryprocesses are those responsible for the presenceof the mud in the area. Two major sources ofmud and SPM can be identified: the erosion

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(a)

(b)

Fig. 4. SPM (mg l�1), arithmetic mean of minimum 10 samples per point of MUMM’s monitoring data (1977–1996). (a) December–

February, (b) March–May, (c) June–August, (d) September–November. Also indicated is the 10m depth contour.


of exposed Tertiary or Holocene clay layersand the import of SPM from the Strait of Dover.Input of SPM could also occur throughwashing out the fine grains from the sandmatrix; however, no quantitative data on thissubject have been found.

3.1.1. Along-shore SPM transport

The mud on the Belgian Continental Shelfpartly owes its origin to the import of SPM fromthe Strait of Dover. The high smectite content inthe fine-grained surface sediment in the Belgiancoastal zone, which points to the Cretaceous

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(c)

(d)

Fig. 4 (continued).


formations in the Dover Strait as a source areaconfirms this assumption (Irion and Z .ollmer,1999). An extensive scientific literature on theresidual SPM transport through the Strait ofDover exists. Some of these articles are citedbelow. They underline that the import of sedi-ments actually is the major source of fine-grainedsediments (mud) in the Southern Bight of theNorth Sea and thus in the Belgian/Dutch coastalarea. Therefore, part of this literature is reviewedin order to quantify the flux of SPM at the French/Belgian border.The values in literature of the residual SPM

transport through the Strait of Dover varybetween [2.5–57.8]� 106 t yr�1. These variationsare due to differences in measurement techniquesas well as to naturally occurring fluctuations.Eisma and Kalf (1979) calculated a SPM transportof [11.5–15.0]� 106 t yr�1. Later, values of [22–30]� 106 t yr�1 (Eisma and Irion, 1988), of17� 106 t yr�1 (Van Alphen, 1990) and of19.2� 106 t yr�1 (Lafite et al., 1993) from which5–22% along the French coast at Cap Griz Nez(i.e. [0.9–4.2]� 106 t yr�1) where mentioned. Dur-ing the MAST-FLUXMANCHE II project (Vele-grakis et al., 1997), the SPM transport wasestimated as [21.672.1]� 106 t yr�1. McManusand Prandle (1997) used numerical models andmeasurement data to obtain a yearly averagedvalue of 44.4� 106 t yr�1 ([21.8–57.8]� 106 t yr�1).Measurements in the English Channel (at theWight-Contentin section) gave a net flux of [2–71]� 106 t yr�1 SPM from the western to theeastern part of the Channel (Velegrakis et al.,1999). Sediment accumulation measurements inthe Kattegat and the Skagerrak resulted in a fourtimes higher accumulation rate as previouslyaccepted (de Haas, 1997). This could implicate aSPM transport through the Dover Strait of at least46� 106 t yr�1 and confirm the value of McManusand Prandle (1997). Residual current (Prandle,1993; Prandle et al., 1993) and residual SPMtransport data (Eisma and Kalf, 1979; VanAlphen, 1990) suggest that about half of theSPM flux through the Strait of Dover is deviatedtowards the south-eastern coastal zone of theNorth Sea. Based on the above-mentioned values,the SPM transport along the French–Belgian–

Dutch coastal zone is thus between [1.3–28.9]� 106 t yr�1. The most recent value is of22.2� 106 t yr�1 (seasonal variation: [11.9–28.9]� 106 t yr�1) and is based on the data ofMcManus and Prandle (1997). The Westerscheldeestuary is not considered as source of mud (Bastin,1974); quantities between 0.6� 106 t yr�1 (VanAlphen, 1990) and [0.170.2]� 106 t yr�1 (vanMaldegem and Vroon, 1995) are estimated toenter the estuary.The yearly residual SPM transport calculated by

the MU-STM model and entering the modelthrough the western boundary is equal to15.4� 106 t yr�1 (Table 4). This result is for asimulation with wind and wave forcing and withan initial distribution of mud on the bed corre-sponding to values presented in Fig. 2. If no mudis initially assumed to be present in the parent bed,a value for the yearly residual SPM transportthrough the western boundary of 15.6� 106 t yr�1

is obtained. During a period of 1 year 12.8� 106 tof mud or 73% of what is entering, is leaving themodel domain through the northern boundarywhen no mud is initially present in the parent bed.With mud in the parent bed the amount increasesto 14.5� 106 t yr�1. This slightly higher value is aconsequence of the northeastward residual watertransport (Fig. 3).The residual SPM transport obtained with the

numerical model fits the values found in literature(see above). The influence of variation in DoverStrait input has been simulated by changing theSPM concentration boundary condition (Table 4).When the inflow of SPM is set to zero (along thesea boundaries) then the input of mud from theWesterschelde has increased by almost a factor 4whereas erosion of the parent bed remains thesame. Seventy-three percent of the mud enteringthe model leaves the domain in the northeasterndirection while 20% ends up deposited in themodel area. For a simulation with realisticboundary conditions of SPM concentrationand without meteorological forcing only 56%of the mud leaves the domain towards theNetherlands, while 35% is deposited. Whenincreasing the SPM concentration boundarycondition by a factor of 7.5, relatively moremud (84%) is leaving the domain along the

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northern boundary and relatively less (12%) isdeposited in the model area.These results show that a major part of the

inflow of mud coming from the Dover Strait isactually leaving the domain towards the north andthat this flux is regulated by tides and windconditions. The numerical model results fit theassumption that the origin of the matter in theturbidity maximum in front of the coast is mainlydue to the SPM transport through the Strait ofDover, and partially due to resuspension anderosion of the sea floor. Although measurementdata have been used to set up the boundarycondition, a lower value for the residual SPMtransport is obtained from the numerical modelresults than from literature data. The lower valueis, however, realistic and could be due to anunderestimated suspended sediment boundarycondition or most probably to an overestimatedresidual mud transport along the French/Belgiancoast.

3.1.2. Seafloor erosion

Part of the mud in the considered area has itsorigin in the recent geological history of thesouthern North Sea. From about 3000 years BPthe coastline started to retreat and the mud, clayand peat layers, which have been deposited duringthe Holocene together with the Tertiary substra-tum were systematically eroded (Baeteman, 1999).Bastin (1974) confirms that clay erosion from thegeological substratum is occurring and that it isthus also a source of mud and SPM. Thegeological substratum of the Belgian ContinentalShelf consists mainly of clay: Ypresian clay in thewestern part, Bartonian and Rupelian clay in theeastern part. The top of the Tertiary substratumforms a more or less smooth transgression horizonand is situated between 25m in the east and 40mat the French/Belgian border below MSL. In thearea formed by the mouth of the Westerscheldeand the eastern part of the Belgian ContinentalShelf the Tertiary substratum is locally upwellingup to 10m below MSL and can thus be eroded inthe deeper channels. During RV Belgica cam-paigns in 2000 and 2001 compact clay and claypebbles in a sand matrix were found along theslopes of the navigation channel connecting the

open sea to the Westerschelde. ConsolidatedHolocene mud layers are exposed west of theharbour of Zeebrugge. The clay and consolidatedmud in these samples is significantly different fromthe soft, less compacted and recent mud samplessituated in the access channels to the harbour ofZeebrugge. By comparing successive depth chartsBastin (1974) estimated the erosion to be of theorder of [0–2.4]� 106 t yr�1. In the model simula-tion erosion of the parent bed results in a yearlyinput of 3.2� 106 t (tce ¼ 2 Pa) into the model area(Table 4). The wave forcing is responsible for 10%and the wind forcing for 4% of the erosion, 86% isthus caused by the tidal currents. When theerosion threshold of the parent bed layer isdecreased to 1.5 Pa then the yearly erosionincreases by a factor 3 to reach 10.6� 106 t. Thedifference between field-based estimation andmodel data, and the high sensitivity of the modelto the erosion threshold underline the importanceof an accurate knowledge of the ‘erodibility’ andthe sediment composition of the bed when settingup a sediment transport model.

3.2. Mud deposition and dredging/dumping

activities

The maintenance of the navigation channels andthe harbours (Zeebrugge and Oostende) is thecause for the high amount of sediments dredgedyearly (on average 107 t dry matter). Calculatingthe ratio between the amount of matter, which isyearly dredged and dumped, and the amount ofmatter which is naturally entering the area can givean indication of the importance of the humanactivities. If this ratio is high, dredging anddumping are important processes in the coastalzone. Otherwise dumping is less important com-pared to the natural sediment input. Applied to theBelgian coastal zone and using the literature anddumping data of Table 2, the ratio is equal to0.37–0.43. It has been calculated using a value forthe residual SPM transport through the Strait ofDover of 22.2� 106 t yr�1 and for the erosion ofthe sea floor of 1.2� 106 t yr�1. The ratio suggeststhat the deposition, dredging and dumping of fine-grained sediments are important processes in thesediment budget of the Belgian coastal zone. The

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quantities of dredged and dumped matter are ofthe same order of magnitude as the naturalsediment supply through the Strait of Dover andthrough seafloor erosion. The dumping activities,however, can be only to a small part responsiblefor the occurrence of the high turbidity found inthe area. Maintenance dredging and dumping isnot a source of mud. It only displaces matter overa relative short distance.

3.3. Hydrodynamic effects

Van Veen (1936) explained—without beinghimself in favour of the theory—the origin of themud outside the turbidity maximum by theoccurrence of a meeting area of residual currents.Gullentops et al. (1976) attributed the increasedconcentration of suspended sediments to a largegyre in the residual transport pattern and thepumping effect of the Schelde estuary. Nihoul(1975) was able to simulate this gyre using a 2Dhydrodynamic numerical model.

In the results of the fine grid MU-BCZ model,which includes tidal and meteorological forcing,such a large gyre in the residual water transportpattern in front of the coast could not be identified(cf. Fig. 3). Small gyres in the residual watertransport occur to the west and the east of theouter harbour of Zeebrugge. They are local andrelated to the presence of the harbour dams. Fig. 5shows the tidal current ellipses and the maximumbottom shear stresses during a spring and a neaptide in 1999. The highest maximum bottom shearstresses are situated in the areas with elongatedcurrent ellipses: in the western part of the grid andbetween Zeebrugge and the Westerschelde. In thecentre part of the grid and in the Vlakte van deRaan the current ellipses are semi-circular and themaximum bottom shear stresses are lower. TheVlakte van de Raan is a shallow area, whichmainly consists of sand. The results of thesimulations with the MU-STM model indicatethat it is part of the high turbidity area (Fig. 7,compare with Fig. 4). The very low mud contenton the bed in this area (Fig. 3 and Fig. 8) can beexplained by the fact that during most of the tidethe bottom stress is higher than the critical bottomstress for erosion of mud (Fig. 6). The sameapplies to the western part of the model grid (theSW–NE elongated areas correspond with theorientation of the Flemish banks) and the mouthof the Westerschelde (Figs. 6–8). The bed in theseareas also consists of sand (in the deeper channelsclay is exposed) and the mud stays in suspension.The results of the hydrodynamic model MU-BCZshow that in the coastal areas the current ellipsesare elongated and that the bottom stress is duringhalf of a tidal cycle lower than the critical erosionstress value. These are thus sedimentation areas.The mud which is deposited during a neap tide cansufficiently consolidate to resist (partly) to the highbottom stresses occurring during a spring tide.During spring tide the deposits are partly resus-pended. This also explains, besides the fact thatduring spring tide more mud is imported in thearea through the W-boundary, the higher SPMconcentration during spring than neap tide(Fig. 7). The mud inflow from the Dover Straitand thus the outflow towards the Netherlands isalso determined by the deposition parameters. If

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Table 2

Amount of dry matter dredged in 1998 (1999) in 106 t from

navigation channels and harbours (95% is from Zeebrugge) and

dumped at the different dumping sites

Sand+Mud Sand Mud

Dredged from navigation channels

B/1 3.78 (8.02) 0.94 (4.42) 2.84 (3.60)

B/2 0.72 (0.08) 0.19 (0.02) 0.53 (0.06)

B/6 0.00 (0.00) 0.00 (0.00) 0.00 (0.00)

B/9 0.00 (0.45) 0.00 (0.08) 0.00 (0.37)

Dredged from harbours

B/1 1.39 (2.91) 0.01 (0.03) 1.38 (2.88)

B/2 0.18 (0.04) 0.01 (0.00) 0.17 (0.04)

B/6 3.00 (2.96) 0.03 (0.03) 2.97 (2.93)

B/9 0.62 (0.17) 0.01 (0.01) 0.61 (0.16)

Sum from

navigation

channels

4.50 (8.55) 1.13 (4.52) 3.37 (4.03)

Sum from

harbours

5.18 (6.07) 0.05 (0.07) 5.13 (6.01)

Total 9.68 (14.62) 1.18 (4.58) 8.50 (10.04)

See Fig. 1 for a site map (data are from the Ministry of the

Flemish Community, Waterways and Marine Affairs Admin-

istration, Coastal Waterways Division).


parameter values more unfavourable to deposition(low critical shear stress of deposition: 0.2 Pa, lowfall velocity: 0.0001m s�1) are chosen then theinflow from the Dover Strait reduces by 17% and33% and the outflow towards the Netherlands by10–17% (Table 4), in comparison to the referencesimulation without meteorological and wave for-

cing. The sedimentation areas reduce, the SPMconcentration increases and the Westerscheldeestuary acts as a sink for mud.In situ data sets of suspended sediment con-

centration have been used to validate the generaltrends observed in the simulated suspended sedi-ment concentration distribution. The data from

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(a)

(b)

Fig. 5. Tidal current ellipses and maximum bottom stress (Pa) during (a) spring tide of 19/03/1999 and (b) neap tide of 26/03/1999,

calculated with the MU-BCZ model. Also indicated is the 10m depth contour.


MUMM’s monitoring programme at 30 stationsfor a 20 year period (1977–1996) are shown as acontour plot of the mean suspended sedimentconcentration at 3m below surface (Fig. 4). Themaximum averaged SPM concentration is situatednear Zeebrugge (>100mg l�1). Tidal variation ofSPM concentration is important as is shown in the

SPM concentration derived from OBS data ofabout 10 through tide measurements carried outbetween 1999 and 2001 in the coastal zone nearZeebrugge (Fettweis, 2001). Some of the results ofthese measurements are summarised in Table 3.Values of up to 7600mg l�1 have been measuredat 3m above the bottom. Fig. 9 shows the vertical

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(a)

(b)

Fig. 6. Percentage of time during which the bottom stress exceeds the critical shear stress for erosion of soft mud (tce ¼ 0:5Pa) duringthe (a) spring and (b) neap tides of 1999.


averaged current (compare with the model resultsin Fig. 5) and the SPM transport vectors for thesethrough tide measurements.Based on the hydrodynamic numerical results

one can conclude that the decreasing residualwater transport vectors between Oostende and

Zeebrugge, the shallowness of the area and thedifference in magnitude between neap and springtidal currents and their effect on the erosion andtransport capacity are responsible for the presenceof the high turbidity area. In Fig. 10 are presentedthe time evolution of the tidal averaged suspended

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(a)

(b)

Fig. 7. Tide-averaged SPM concentration (mg l�1) for the reference simulation where the SPM concentration boundary condition is

based on MUMM’s monitoring data (see Fig. 4) and where mud is initially present in the parent bed. (a) Spring tide of 21/03/1999 and

(b) neap tide of 26/03/1999. Also indicated is the 10m depth contour.


and deposited matter together with the tidalamplitude. Suspension and deposition clearly varywith tidal amplitude: high SPM values during hightidal amplitude (>3.5m) and low values duringlow tidal amplitude (o2.5m). During the monthlyvery low neap tidal amplitudes the SPM decreases

to almost zero. The mud deposits show anopposite variation. Also visible is the seasonalvariation of the SPM due to the seasonal varyingSPM concentration boundary condition.The prevailing wind direction being from the

SW (see Fig. 2), the input of mud through the

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(a)

(b)

Fig. 8. Mud deposition (in kgm�2 from 01/01/1999 on) in the active layers of the model for the reference simulation where the SPM

concentration boundary condition is based on MUMM’s monitoring data (see Fig. 4) and where mud is initially present in the parent

bed. (a) Spring tide of 21/03/1999 and (b) neap tide of 26/03/1999. When a bulk density of the mud of 1300 kgm�3 is assumed then

100kgm�2 correspond to a mud layer of 78 cm. Also indicated is the 10m depth contour.


western boundary of the model (Dover Straitinflow) decreases by 33% to 10.8� 106 t yr�1

without wind forcing (Table 4). The outflowthrough the northern boundary decreases evenby 45% to 8.0� 106 t yr�1 without wind forcing,which is caused by an increase in mud depositiondue to the lower bottom shear stresses in absenceof wind and wave forcing.

3.4. Sediment balance

The fluxes of mud simulated for the year 1999and those available from literature are given inTable 4 (see observation and reference simulation‘Waves’). The data show that inflow from theStrait of Dover is important and that outflowtowards the northeast (N-boundary) is only a littleless than the inflow. Mud is also brought into thearea by erosion of exposed mud and clay layers;the quantities are smaller but the uncertainties arehigh. Reliable data on mud accumulation is notavailable in literature. Mud deposition is mainlydescribed through the amounts dredged in theharbours and navigation channels. These data,however, only give information on human inducedsediment transport. In the model a deposition ofmud of the order of 4� 106 t is simulated, fromwhich 725% in the navigation channel (harboursare not included in the model grid). 75% (i.e.3� 106 t) of the total deposition represents thus theyearly mud accumulation on the Belgian Con-tinental Shelf.

4. Conclusions

The Belgian/Dutch coastal area is characterisedby high SPM concentrations. The formation ofthis high turbidity zone has been investigated usinga coupled 2D hydrodynamic, wave and sedimenttransport model forced with real meteorologicaldata. The main conclusions of this study are:

(1) The processes responsible for the high turbid-ity zone formation are the currents and theimport of SPM through the Strait of Dover.Mainly because of the decreasing magnitudeof residual transport and the shallowness ofthe area, the SPM is concentrated in theBelgian–Dutch coastal waters and forms aturbidity maximum in front of Zeebrugge.The occurrence of a high turbidity zone canthus best be compared to a kind of congestionin the sediment transport, an open sedimentsystem thus. Our finding is in opposition tothose of previous studies where the turbiditymaximum was thought to be caused by ahydrodynamic trap consisting of a residualeddy (a closed system) in which the sedimentwas captured.

(2) The erosion of clay and Holocene mud layersis partly responsible for the increase of SPMconcentration in the considered area. How-ever, the Strait of Dover remains the majorsource of SPM.

(3) Mud is continuously deposited and re-sus-pended. Significant variations occur during

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Table 3

Maximal and minimal suspended particulate matter concentration during through tide measurements (see Fig. 9 for location of the

stations)

Station Date Tidal coef. Wind (Bf) SPM max (mg l�1) SPM min (mg l�1)

99/07 8–9/03/1999 0.87 1–4 W 204 20

99/17 13/07/1999 1.17 1–6 S–E 20 11

00/14 29/05/2000 0.83 2–7 N–NE 370 64

00/19 10/07/2000 0.78 4–8 NE–SE 20 9

00/26 23–24/10/2000 0.91 5–8 NE 236 46

01/01 25–26/01/2001 0.99 3–6 NNE 340 49

01/06a 7–8/03/2001 0.99 1–4 N–NE 509 46

01/06b 8–9/03/2001 1.08 3–4 SW 592 65

The data have been measured with an OBS sensor. The instrument was situated at about 3m from the bottom and the signal was

calibrated with filtered water samples. Also indicated are the tidal coefficient (1=mean tide, 0.77=neap tide and 1.19=spring tide) and

the wind velocity and direction.


tidal cycles and during neap-spring cycles.Also seasons and meteorological condi-tions have an influence on the mudbehaviour. Because of the many processesinvolved, it remains still difficult to compare

quantitatively model output to measurementdata.

(4) The results of sensitivity analysis have shownthat a major part of the inflow of mud fromthe Dover Strait is leaving the domain

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(a)

(b)

Fig. 9. Through tide measurements during 1999–2001 in the high turbidity zone using the RV Belgica. Presented are every hour the (a)

vertically averaged current vectors and (b) SPM transport vectors. Also indicated is the 10m depth contour.


towards the north and that tides and windsregulate this flux. The difference betweenfield-based estimation and model results isalso depending on the erosion and deposition

parameters. Their values have been chosenbased on a commonly accepted range found inliterature. The accurate knowledge of the‘erodibility’, the sediment composition of the

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Table 4

Sensitivity simulations and mass balance.

W-bound N-bound W-Schelde Erosion Deposit. Susp.

Observations +22.2(1) �22.0(2) �0.170.2(3) +0–2.4(4) �10.00(5)

Waves +15.4 �14.5 +1.1 +3.2 �4.0 1.2

Waves, no mud +15.6 �12.8 +1.9 0 �3.7 1.0

Meteo +15.9 �14.5 +1.1 +2.9 �4.1 1.2

No waves +10.8 �8.0 +0.7 +2.8 �5.0 1.3

No waves, tce, +10.7 �12.8 �0.4 +10.6 �6.7 1.4

No waves, boundary 0 0.0 �4.1 +2.8 +2.8 �1.1 0.4

No waves, boundary 7.5 +123.2 �106.7 +0.7 +2.8 �15.6 4.4

No waves, fall velocity +7.3 �6.6 �1.1 +2.8 �0.6 1.8

No waves, tcd ; +9.0 �7.2 �0.1 +2.8 -2.8 1.7

Indicated are the fluxes of mud (106 t/year) entering (+) or leaving (�) the model domain through the W-boundary, through the N-boundary, through the Westerschelde estuary, from erosion of the parent bed and by deposition in the active layers. The sum of these

fluxes equals the amount of mud in suspension (Susp.). The observations represent the fluxes found in literature: (1) McManus and

Prandle (1997); (2) Fettweis and Van den Eynde (1999); (3)van Maldegem and Vroon (1995); (4)Bastin (1974); (5)is the amount of mud

dredged in 1999 (Table 2), these values can give an indication of the deposition of mud. The sediment parameters are as defined in

Section 2.3 (Table 1) except when indicated. ‘Waves’: simulation with meteorological and wave forcing; ‘Meteo’: simulation with

meteorological but without wave forcing; ‘No mud’: initially no mud in parent bed. In ‘tce’ the critical shear stress of bottom sediments

in the parent layer has been reduced to 1.5 Pa. In ‘Boundary’ the SPM concentration boundary condition has been changed; ‘0’: SPM

concentration=0mg/l (no inflow of matter), ‘7.5’=7.5 times higher boundary condition along W-boundary. In ‘Fall velocity’ the fall

velocity is reduced to 0.1mms�1 and in ‘tcd ’ the critical shear stress for deposition is reduced to 0.2 Pa.

0

1e+06

2e+06

3e+06

4e+06

5e+06

6e+06

7e+06

0 30 60 90 120 150 180 210 240 270 300 330 360-1

0

1

2

3

4

5

6

Mas

s (t

on)

Tid

al a

mpl

itude

(m

)

Time (days)

Suspended MatterDeposited Matter

Sum(Susp+Depo)Tidal amplitude

Fig. 10. SPM, deposited matter and tidal amplitude in function of time for the reference simulation (wave and wind forcing).


bed and the fall velocity of the particles insuspension is still important when setting upcohesive sediment transport models, thesedata must be obtained through field measure-ments.

(5) The difference in magnitude between springand neap tidal currents is partly responsiblefor the fact that the mud deposits arepermanent. During neap tide the mud has ahigher probability to build up a structure, toconsolidate and to increase its erosion resis-tance. More mud is thus found on the bottomand the SPM concentration is relatively low.During spring tide the opposite happens andpart of these deposits are again resuspended.

(6) Comparison between the natural input ofsuspended sediment and the quantitiesdredged and dumped at sea shows that animportant part of the SPM is involved in thedredging/dumping cycle. The deposition ofmud in the dredging areas, can be seen as atemporarily storage of mud. The effect ofdredging and dumping on the sediment balanceis of minor importance, because dredging anddumping occurs almost continuously andbecause the major dumping places are situatedin the turbidity maximum zone.

Acknowledgements

This work is part of the SEBAB project,financed by the Coastal Waterways Division ofthe Waterways and Marine Affairs Administra-tion, Ministry of the Flemish Community (AWZ).We wish to thank S. Wartel and R. Schallierfor the discussions. K. Ruddick (MUMM) pro-vided the long-term averaged SPM concentrationdata and G. Dumon (AWZ) the wave andwind data. The measurements have been takenwith the RV Belgica; these data have been treatedand analysed at the measurement service ofMUMM in Oostende.

Appendix A. Erosion and deposition

The erosion and deposition rates E and D

(Eqs. (A.3) and (A.7)) depend on the local bottom

stress tb ([Pa), which is calculated under thecombined effect of currents and waves usingBijker’s formulae of 1966 (Koutitas, 1988):

tb ¼ tc 1þ 0:5cffiffiffiffiffi2g

p ffiffiffiffiffifw

p ub

uc

!224

35; ðA:1Þ

where tc is the bottom stress due to currents (Pa), gthe acceleration due to gravity (m s�2), ub theorbital wave velocity (m s�1), uc the depth-aver-aged current velocity (m s�1), fw the wave frictionfactor and c; originally, an empirical constant. Thefollowing relation has been used in order tocalculate c (Van den Eynde and Ozer, 1993):

c ¼ 77:5þ 14:18 minuc

ub

; 0:67

� �: ðA:2Þ

Erosion of mud is defined as the sum of theprocesses responsible for the resuspension andtransport of particles attached to the bed. Mehtaet al. (1989) distinguish in general between threetypes of erosion: surface erosion, where mudparticles are successively brought in suspension;re-entrainment of a stationary high density mudsuspension, with quick resuspension of mud overthe water column, and mass erosion, where largeparts of the bottom are brought in suspension. Thelatter is a typical phenomenon of tidal areas withhigh current velocities. The formulation of Ar-iathurai–Partheniades (Ariathurai, 1974), whichuses a linear relation between the excess shearstress and the rate of erosion E is used in themodel:

E ¼ Mpðtb � tceÞ=tce; tb > tce; ðA:3Þ

where M is the erosion constant (kgm�2 s�1), p

the weight fraction of mud in the bed and tce thecritical shear stress for erosion (Pa). In theformula, the rate of erosion is independent ofthe sediment concentration in the eroding current.The critical shear stress for erosion tce of estuarineor coastal mud varies between 0.1 and 2 Pa(Berlamont et al., 1993) and is calculated as afunction of the bulk density rb (kgm

�3) of thesediment (Williamson and Torfs, 1996):

tce ¼ 0:015ðrb � 1000Þ0:73: ðA:4Þ

This relationship was derived for artificial mixedsediment beds. The bulk density is a function of

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the salinity S (a value of 32, representing theaveraged salinity in the area was used) and the drydensity rd (kgm

�3) (Odd, 1988):

rb ¼ 1000þ 0:7 S þ 0:62rd : ðA:5Þ

The dry density is equal to the sediment concen-tration in the bed. In case of pure mud it iscalculated as a function of time (Le Normant,1995). The dry density of mud/sand mixtures iscalculated as (Mulder, 1995):

rd ¼ rd;sand � 1750 p þ 650 p2; ðA:6Þ

where rd;sand is the dry density of sand(1550 kgm�3).The equation of Krone (1962) is used for

calculating deposition. In the formula, the prob-ability of mud particles to be attached to thebottom varies linearly between 0 and 1 if thebottom stress reduces from a critical value to zero.The deposition rate D then is written as

D ¼ wCðtcd � tbÞ=tcd tbotcd ðA:7Þ

with w the fall velocity (m s�1) and tcd the criticalshear stress for deposition (Pa). This equationessentially represents the rate of deposition in caseof low suspended sediment concentration. Aggre-gation is very often indirectly considered as afunction of concentration (Mehta, 1986). Morecomplicated laws for deposition exist, but their usein tidal conditions does not significantly improvethe results when compared to the Krone formula(Teisson, 1994).

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