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Fine sediment deposits in shelf seas
JOURNAL
OF
MARINE SYSTEMS ELSEVIER
Journal
of Marine
Systems 7 (1996) 119-131
Fine sediment
deposits in shelf seas
J. Dronkers
*,
A.G. Miltenburg
Institute for Marine and Atmospheric Research Utrecht (ZMXlJ), Princetonplein 5, 3584 CC Utrecht, The Netherlands Received
9 September
1994; accepted
7 March
1995
Abstract From field observations it appears that the top layer of a shelf bottom in general exhibits an intricate geographical pattern of sediment formations. Sediments of different composition are confined in distinct regions.
This contradicts the idea that current and wave forces stir up bottom sediment and disperse it in a random way over the shelf; the dispersal process is counteracted by sorting mechanisms. In this paper the bottom patterns of fine cohesive sediments are considered. A specific sorting mechanism is studied which may explain the patchy structure of fine sediment deposits. It is shown that fine sediments can be trapped in bottom deposits which contain a fine sediment fraction high enough to prevent pore water motion in the shelf bed. This mechanism opposes sediment dispersal away from existing deposits. It may also explain the formation or the preservation of mud patches, even in regions where the bottom shear stress is relatively high.
1. Introduction Coastal systems in general exhibit intricate structures, both in topography and in sediment composition. On geological time scales coastal systems are of an ephemeral, temporary nature (Postma, 1980). They evolve under the influence of hydrodynamic forcing often without reaching complete equilibrium. The evolution of coastal structures due to Holocene sea level rise is slowing down, but disturbance due to human interventions is becoming more important. The morphodynamics of coastal structures has received much attention in recent years (de Vriend et al., 1993). Less attention is given to the sedimentary structure of the shelf bottom; field evidence of sedimentary structures in the coastal zone is sel-
* Corresponding
Author.
0924-7963/96/$15.00
dom considered in studies of coastal dynamics. This alternate viewpoint is the subject of the present study. Geological maps of the top layer of the shelf bottom in general display complex geographical patterns (Shepard, 1932). Distinct formations of different types of sediment on different spatial scales can be clearly identified in spite of the dispersive action of currents and waves. Many of the observed sedimentary structures are probably related to relict deposits formed during past geological ages. The fact that such structures are preserved implies a quasi dynamical equilibrium with present hydrodynamic conditions. The basic assumption of this study is that the sedimentary patterns are indeed sensitive to present hydrodynamic conditions (Swift et al., 1971). A possible mechanism underlying the formation and preservation of the observed sedimentary structures on the coastal shelf is discussed.
0 1996 Elsevier Science B.V. All rights reserved
SSDI 0924-7963(95)00036-4
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Fig. 1. Schematic
representation
of deposition
This study focuses mainly on deposits of fine cohesive sediment. In Section 2 some current ideas on shelf sedimentation are reviewed. In Section 3 a qualitative comparison is made with deposition patterns observed on different coastal shelves. The most striking characteristic of the observed patterns which is not well explained by current theory is the patchy nature of many fine sediment deposits and the apparent lack of dispersal of fine sediments away from original deposits. In Section 4 a mechanism is described which may explain these features in a qualitative way. This mechanism is based on an assumption put forward by Clarke and Swift (1984) concerning the interaction between bed structure and sediment deposition and resuspension. The basic
Fig. 2. Bathymctry and general Atlantic Bight shelf (2d).
circulation
pattern
in the Bering
of Marine Systems 7 (1996) 119-131
and erosion
processes
on the continental
shelf.
idea is that under certain conditions less resuspension may occur from a bottom with higher fine sediment content than from a bottom with lower fine sediment content. It is shown that alternating currents then may generate patchiness. Furthermore this mechanism may contribute to the stability of sediment deposits formed during past geological ages.
2. Sedimentation
processes
on the shelf
In this section some current ideas on shelf sedimentation theory are reviewed (McCave, 1972; Drake, 1976 and Dyer, 1986). The pathways of fine sediment and the major mechanisms re-
shelf (2a), Yellow
Sea shelf (2b),
North Sea
shelf (2~) and the
J. Dronkers, A.G. Miltenburg/Journal
2a
m
depth < 30 m
Ezl
depth < 75 m
-
circulation
121
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2b
4 2d
circulation
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sponsible for deposition and erosion on the coastal shelf are schematically represented in Fig. 1 for a characteristic coastal zone. The most important sources of fine sediment are rivers, coastal and seabed erosion and primary production. Deposition areas are sheltered coastal basins and the deeper outer shelf. Near the coastal zone, river outflow plays an important role; fine sediments are trapped in a coastal fringe due to density induced cross-shore circulation. Recent studies (van der Giessen et al., 1990) emphasise the strength of this circulation. Residual alongshore transport takes place by tidal asymmetry and by the residual alongshore currents. Fine sediments can escape towards the shelf during periods of strong onshore winds causing downwelling. Cusps in the coastline may also force the current to separate from the coast causing an offshore transport of sediment. Variable wind-driven currents may further disperse fine suspended sediment over the shelf. The action of waves and tide prevents sedimentation in the shallow parts of the shelf. On the deeper shelf, where the influence of waves and tide is less pronounced, fine sediment will settle. The location of deposition areas is influenced by several factors, in particular the distribution of storm surge currents, flow circulation patterns and convergence zones of bottom currents. The sediment can originate from the coastal zone, but also from the deeper ocean by upwelling. The general picture that can be inferred from this paradigm is, (1) deposits of fine sediment occur in sheltered basins along the coast, (2) the shallow shelf is sandy and (3) the deep shelf is covered with fine sediments. The geographical structure of sediment deposits may reflect the general near bottom circulation pattern. However, specific geographical features may modify this general picture, for instance: the location and abundance of sediment supply; the exis-
Fig. 3. Deposits (3d).
of fine sediment
in the Bering
shelf (3a), Yellow
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tence of seabed depressions; topographical structures and related current patterns; the prevailing wind directions and the influence of ocean waves.
3. Fine sediments distribution on four shelves The above sedimentation picture is compared with field evidence from four continental shelves: the Bering Sea, the Yellow Sea, the North Sea and the Atlantic Coast of the USA. The first three are semi-enclosed seas, having a comparable surface area and a relatively gentle bottom slope. The Atlantic Coast of the USA is a narrow shelf. The observed fine sediment patterns are briefly discussed, indicating qualitative agreements and differences with theoretical expectations. 3.1. The Bering Sea The bathymetry is shown in Fig. 2a. Information on hydrodynamic and sedimentary characteristics is given by Sharma (1979) and Nelson (1982). The bering sea has a high sediment input of the Yukon river, approximately 4 * lo* ton per year. The Bering shelf is exposed to ocean waves which may strongly affect the bottom of the inner and middle parts of the shelf. Tidal currents are not very strong except in some near coastal regions; ice-cover in winter opposes the generation of strong wind waves and wind driven currents. The fine sediment distribution is shown in Fig. 3a. The middle shelf is sandy. With increasing depth the medium grain size becomes smaller and most of the deep shelf is covered with fine sediments. These observations are consistent with the theoretical picture. Most of the shallow shelf is sandy too. It is remarkable that in the coastal waters around the mouth of the Yukon river wave and current action cannot prevent the Yukon sedi-
Sea shelf (3b), North
Sea shelf (3~) and the Atlantic
Bight shelf
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3a
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3b
3d
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ment from settling and forming a muddy bed. Isolated deposits of fine sediment also occur at greater distances in the near coastal zone. 3.2. The Yellow Sea The bathymetry of the Yellow Sea is shown in Fig. 2b. Data on hydrodynamic and sedimentary conditions in the Yellow Sea is derived from the studies of Larsen et al. (19851, Graber et al. (1989) and Lee and Chough (1989). Rivers around the Yellow Sea basin provide a very high input of fine sediments, in particular the Huanghe (Yel-
low) River (11. lo8 ton/yr) and the Changjiang (5 * lo8 ton/yr). The Yellow Sea is a meso-tidal sea (tidal range 2-4 m) with tidal currents being strongest in the middle part of the shelf and in some coastal regions. The influence of ocean waves is relatively small. Major storms have winds from the north, causing highest wave action in the central and southern part of the shelf. As a result, higher bottom stresses occur in the central part of the shelf than in the shallow northern part and the deep southern part. In the southern part of the shelf ocean currents and upwelling may play a role. From the distribution of bottom sediments shown in Fig. 3b, the following conclusions can be drawn. Fine sediment is deposited mostly in the shallow northern part of the shelf. This is consistent with the hydrodynamic conditions described above. The fine sediments originate from riverine input in present but also past ages. It is remarkable that no fine sediment deposits are found in the deeper parts of the shelf near the Korean coast. A possible explanation for their absence is the occurrence of large scale three-dimensional circulation cells (Hu, 19841, which may also explain the existence of a large mud patch in the southern part on the shelf. 3.3. The North Sea
Fig. 4. Bottom surveys in the highly turbulent Dutch coastal zone reveal isolated patches with an increased content of fine sediment. Stronger sedimentation occurs along the less turbulent northern edge of the Southern Bight. In this region the sediment deposits also have a patchy structure.
Sedimentation patterns in the North Sea have been extensively described by many authors (see, for instance, Pratje, 1949; Eisma, 1981; Larsonneur et al., 1982 and Eisma, 1987). The bottom topography is shown in Fig. 2c. The surface area and the average slope are comparable to the Bering and Yellow Sea shelves. During past ice ages, local depressions in the sea bed were formed which presently act as deposition sites. Tidal currents and wave action are most important in the Southern Bight of the North Sea and in the Channel. Storm surge currents can be strong in the same region, and in particular around the northern boundary of the Southern Bight and around the boundaries of the Irish Sea. The riverine sediment input to the North Sea shelf is small. The major sediment source is coastal and seabed erosion. The most important sedimentation areas in the North Sea correspond to depres-
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sions and trenches in the shelf bottom, as can been seen by comparing Figs. 2c and 3c. Other sedimentation areas in the central part of the North Sea seem to be related to the convergence of bottom currents in tidal mixing fronts (Simpson et al., 1978). There is no explanation for the patchy structure of the sediment deposits. This is true also for the sediment patches occurring along the Dutch coastal zone, see Fig. 4.
and deposition properties. The model describes a mechanism which counteracts dispersion of fine sediment away from deposition areas and provides an explanation for the patchy nature of the sediment distribution on the coastal shelf. The starting point is the continuity equation for the transport of sediment
3.4. The Atlantic coast of the USA
where M is sum of the sediment mass in the bottom and in suspension per unit area and S denotes the sediment flux integrated over the watercolumn, i.e. the sediment transport rate. If the sediment is not uniform this equation holds for each sediment class separately. From here on we will consider only fine cohesive sediments. For the vertical distribution of these fine sediments a two layer description is adopted, one layer representing the suspended state and the other layer representing the immobile sediment bed. It is assumed that the two layers can effectively be described by quantities such as an effective depth and an average concentration. Such a description does not apply to the situation where a fluid mud layer is present above the bed. To deal with this situation in our model we ignore the horizontal motion of the fluid mud layer and suppose that it is part of the immobile bottom. With this simplification the sediment mass is represented by
This continental shelf is quite distinct from the previous three shelf regions, see Fig. 2d. It is a narrow shelf with only a small sediment input from rivers. The tide is weak but the impact of ocean waves on the shelf bottom is strong. As shown in Fig. 3d no large deposits of fine sediment occur on the shelf with the exception of a large patch south of Cape Cod (Hathaway, 1972). Fine sediments are mainly deposited on the shelf slope of the Atlantic Bight. However, in the near coastal zone several small sized patches have been observed, in spite of strong wave action (Freeland et al., 1979). Summarising the findings for the different shelf seas the following picture emerges. The observed sedimentation patterns are in general agreement with the assumption that fine sediments mainly deposit in regions of low wave and current activity. However, deposits are not uniform and fine sediment deposits are also found in regions where wave and current activity are important. In some cases hydrodynamic structures such as fronts and gyres may explain the non-uniformity of the sediment distribution. In other cases such explanations are not plausible. Often the size of sediment patches is smaller than the scale of topographical or hydrodynamic structures. In the following further attention will be given to the formation of mud patches which are not directly related to topography or to gradients in current and wave properties. 4. Dynamics of shelf sedimentation In this section a simplified model for sediment transport is formulated based on assumed erosion
g+wo,
M=
/
cdz = HJ,
+ H,C,,
(2)
where H, and C, are representative suspension layer depth and suspension concentration respectively. H, and C, are the representative depth and concentration of the bottom layer. We will only consider the top layer of the bottom which exchanges sediment with the water column. The sediment flux is represented by
s=e’,c, where Q, is the transport suspension layer,
(3) flux of water in the
Q, = WJ, and U, is the representative velocity.
suspension
layer
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The sedimentary structures under consideration evolve on a time scale T, which is much longer than most of the fluctuations in the current distribution on the coastal shelf. This time scale T can be in the order of several years or even several centuries. For that reason we are more interested in the long term averaged values of suspended sediment concentrations and transports than in the instantaneous values. For the time interval T the transport flux Q, is decomposed as follows, e’,=
(4)
Here: (e’,) is the time averaged flux of water in the suspension layer, (@ > is the transport flux of all non residual water motions in the time interval T, due to turbulence, waves, tides, wind and other non residual forcings. The same decomposition is used for the suspended sediment concentration: C,=(C,)+C’,
(5)
Here: (C,) is the time averaged sediment concentration in the suspension layer, and C’ represents fluctuations of the suspended sediment concentration with respect to the mean. The following decomposition of the sediment flux results; = +
(6)
The first transport term corresponds to residual flow. The second transport term represents the residual sediment transport due to fluctuating water motions. This term would vanish if (1) dispersion processes are absent and if (2) no deposition and resuspension takes place. The contributions of (1) and (2) may be distinguished, although they are not independent. Contribution (2) represents transport due to asymmetry existing in the fluctuating water motions (for example, ebb-flood asymmetry of tidal current strength and variation or onshore-offshore asymmetry of wave orbital velocities) together with settling and resuspension time lag effects. Contribution (1) represents the effect of mixing processes which take place on the scale of the fluctuating water motions (for example, by generating shear).
of Marine Systems 7 (1996) 119-131
The residual flow term and the deposition-resuspension induced transport represent advective transport of suspended sediment and can be written to a first approximation as:
c&f.c,, where &r is an effective transport capacity and C, the average suspended sedimenf concentration. If the spatial structure of Qeff contains convergence and divergence zones, then this term may generate a patchy sedimentary structure. Some of the sediment structures found on the coastal shelves can probably be explained in this way. In the following this mechanism will not be considered, however. This study focuses on fine sediment deposits occurring in shelf regions where no structures in the current field are expected to occur at the scale of the deposits. By analogy with dissolved substances the residual transport due to mixing processes will be represented by a gradient type dispersive transport, involving a dispersion coefficient D. It should be noted, however, that the magnitude of the dispersion coefficient may be quite different. The spatial distribution of dissolved substances and suspended sediment is not similar, the latter in general being more patchy. The physical processes responsible for dispersion therefore involve different temporal and spatial scales. Otherwise the same type of mechanisms plays a role, in particular the combination of eddy diffusion and shear in the current field. The dispersive transport of suspended sediment will be written: @~C’>
= -D.H;tk,.
As a further approximation the concentration in suspension C, will be replaced by the equilibrium concentration C,,. The equilibrium concentration C,, is a quantity which in principle can be determined experimentally. It depends on the composition and structure of the bottom and on the hydrodynamic forces acting on the bottom. A large number of bottom characteristics can influence Ceq, for instance, the degree of consolidation and the presence of organic matter and biota. For simplicity it is assumed that the most important bottom characteristic is the fine sedi-
.I. Dronkers, A.G. Miltenburg/Journal
merit concentration C,. Also for simplicity it is assumed that the forces acting on the bottom are represented by a unique hydrodynamic parameter ut,, such as, for instance the bottom drag velocity. In most field situations the hydrodynamic forces on the bottom fluctuate on a shorter timescale than the deposition and resuspension time lags so that the equilibrium suspended concentration is not reached. This implies that it is not the equilibrium suspended concentration which appears in the time averaged transport formula, but an average of many equilibrium concentrations. This quantity will not be computed, but it will be assumed that the dependency on C, and z+, will be similar as for C,,. Considering that C,, = C, = C&C,, us), the gradient can be rewritten and the following expression for the averaged transport is obtained,
(7) The second and third term are dispersion terms. The second term corresponds to dispersion from regions with high ub towards regions > 0. This with low ub, provided that K,/au, condition means that the suspended concentration should increase when the forces acting on the bottom increase. Normally this will be the case; dispersion of fine sediment away from regions with high bottom stresses is consistent with classical sedimentation theory. The third term will be considered in more detail now. If aC,/iK, > 0, there is dispersion away from regions where the fine sediment content in the bottom is high and towards regions with low fine sediment bottom concentrations. This is what may be expected intuitively. However, if
ac,/ac,
(8)
then dispersion is from regions with low fine sediment content towards regions with high fine sediment concentration in the bottom. Note that the actual dispersion acts on the suspended concentration and always corresponds to transport from high to low suspended concentrations.
of Marine Systems 7 (1995) 119-131
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Clarke and Swift (1984) have shown that C, is not necessarily a monotonously increasing function of C,. This has important consequences for the residual transport of suspended sediment and for the evolution of sedimentary structures on the coastal shelf. One of the consequences is patch formation and patch preservation, as will be discussed in the next section.
5. Discussion As shown before, the residual transport of fine suspended sediment depends in a crucial way on the fine sediment concentration C, of the bottom. This relationship will now be discussed in a qualitative way. Existing field measurements can hardly be used to quantify this relationship. One of the difficulties is that apart from C, and ub many other characteristics play a role, as indicated earlier. Different measurements can only be compared if all these characteristics are similar, but this is in general not the case. Therefore we will rely on two different conceptual models for sediment settling and resuspension. In the first model (Clarke and Swift, 1984) the bottom is assumed to form a sand matrix with interstitial holes which are more or less filled with fine sediment particles (see also Dyer, 1986). Two regimes exist for different concentrations of fine sediment. The first regime corresponds to a low concentration of fine sediment, such that the bottom is permeable and pore water motion can take place. Small particles are then easily extracted from the sand matrix and a high percentage is brought in suspension. The second regime corresponds to a high concentration of fine sediment such that the holes will be filled, so halting pore water motion. Thus, in this regime, not only the cohesive forces have to be overcome, but particles must also be extracted from the seabed. With respect to the first regime a smaller percentage of the fine sediment will be brought in suspension even though the concentration in the seabed is higher. From this model, a relation between C, and ub can be derived which is tentatively drawn in Fig. 5a. In this figure the equilibrium concentration
J. Dronkers, A.G. Miltenburg/Journal
a)
I
I
I
I
I
I
I
I
I
I
of Marine Systems 7 (1996) 119-131
C, medum
C, high
Fig. 5. (a) Qualitative variation of suspended concentration C as a function of u,, for three different values of bottom concentration C,. Higher hydrodynamic forcing of the bottom leads to increasing suspension of fine sediment. At low bottom concentration the hydrodynamic forcing is more efficient due to the permeability of the bottom. The suspension efficiency decreases with increasing bottom concentration especially if the hydrodynamic forcing is small. At high bottom concentrations a minimum hydrodynamic see for instance, Postma, 1967). (b) Qualitative forcing is required for suspending fine sediments (the so-called “yield-stress”, variation of C as a function of C, for three different values of ub. This figure is completely equivalent with Fig. 5a.
versus the bottom forcing parameter ui, is presented for low, medium and high bottom concentrations C,. Some experimental support is presented in Clarke et al. (1982). Fig. 5b shows that the suspended concentration C is not an increasing function of the bottom concentration C, under all conditions. The decrease of suspended concentration with C, occurs when the permeable regime changes into the impermeable regime. Therefore a range of intermediate bottom compositions may exist which release less fine sediment into suspension than bottoms with higher or lower fine sediment content. The suspended equilibrium concentration will therefore be lower in regions where the fine sediment content of the bottom has intermediate values than in the surroundings where the fine sediment content of the bottom is lower. Due to the gradient nature of the dispersive sediment transport, fine sediment is transported towards regions where the bottom has such an intermediate composition. This leads to the formation of mud patches with relatively high content of fine sediment embedded within regions of lower content of fine sediment, see Fig. 6. Advective transport of fine sediment also affects the mud patch. It G assumed that the effective transport capacity Qeff is uniform in the shelf region under consideration. In that case sedimen-
tation occurs in zones where the equilibrium suspended concentration decreases and erosion occurs in zones where the equilibrium suspended concentration increases. Fig. 7 shows that by this process mud patches are brought towards an equilibrium condition. Fine sediments are redistributed in such a way that the equilibrium suspended concentrations inside and outside the patch become equal. Dispersion of fine sediment away from the patch is counteracted. An alternative model for dispersion of cohesive fine sediment into mud patches is based on energy dissipation arguments. When wave and current action is exerted on a sandy bottom most of the energy is transformed into shear stress and
ywer t__________ ______?trh_____ “,,,
&CA+
acs/acb
cs
Fig. 6. Formation of a mud patch. Suspended dispersed towards a region where the bottom impermeable.
fine sediment is has become just
J. Dronkers, A.G. Miltenburg/Journal
cb .
T
ac,/ acb
Fig. 7. Different stages of mud patch evolution under the influence of a spatially uniform, unidirectional effective transport capacity. The patch is enhanced until1 the equilibrium suspended concentration is continuous across the patch. At the rear a negative patch is formed which travels downstream and which will finally disappear as a result of dispersion.
turbulence. Floes of fine sediment which settle in such regions, have a high probability to be broken up and resuspended (van Leussen, 1993). When wave and current action is exerted on a muddy bottom part of the energy is transferred to the bottom where it is used for fluidizing the upper bed layer. A fluid mud layer is formed which moves with the current. The shear stresses at the interface with the water column are relatively small and turbulence is strongly damped. Floes of fine sediment settling to a muddy bottom will therefore not be broken up and are easily incorporated in the fluid mud layer. This mechanism of mud patch formation is illustrated in Fig. 8. The two models described above relate to different physical processes, although they are not mutually exclusive. The first model deals with mechanisms playing a role in the erosion of fine sediments from the bed. The second model addresses bed properties which relate to the settling of fine sediment. Therefore the two models can
of Marine Systems 7 (1995) 119-131
129
be viewed as being complementary. The two models only describe the variation of the suspended equilibrium concentration C,, as a function of C, and ub in a qualitative way. The mechanism of mud patch formation and mud patch preservation described in this study has a very qualitative and even speculative character. There is no firm evidence for decreasing suspended equilibrium concentrations when the concentration of fines in the bottom increases. This should be observed when exposing shelf bottoms with different sediment compositions to the same hydrodynamic forcing ui,. In reality the forcing z+, is highly variable during the process of patch formation. This is not taken into account in the very simplified model presented in this study. The model may explain some of the characteristic patchy features of shelf sedimentation, but is not suitable in its present form for direct comparison with patchy structures observed in the field. The model does not include the influence of extreme events on the dynamics of mud patch formation. It may be expected that such events, where large amounts of fine sediment are brought into suspension, play an important role. Smaller mud patches may entirely disappear and the suspended fine sediments may disperse over a large region. Settling will generally not result in a homogeneous bed cover of fine sediment; the areas with the highest concentration may act as poles for new mud patch formation. Field observations confirm that the mud patches in the Dutch coastal
Fig. 8. Formation of a mud patch by trapping of floes in a fluid mud layer. Wave and current energy is dissipated in the fluid mud layer and turbulence is damped. Settling floes are not broken up and incorporated in the mud patch.
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zone are not permanent structures, but that their presence is related to the occurrence of storms (Eisma, 1981). Large mud patches seem to be rather stable. Under average conditions they seem to attract fine sediments, possibly by the mechanisms described in this study. Therefore they form distinct structures on the continental shelf, which do not necessarily coincide with geographical patterns in topography and current structure. One may expect that their growth is controlled by extreme events which resuspend and disperse part of the sediment accumulated during average conditions. More detailed field work on mud patches is required to confirm these ideas. Numerical modelling of shelf sedimentation requires a quantitative description of deposition and erosion processes. In present models (Pohlmann and Puls, 1994) the interaction of these processes with the sediment composition of the bed is not taken into account. For a proper representation of mud patch formation these interaction mechanisms should be considered. This requires further study of the processes of erosion and deposition as a function of bed structure and composition. Laboratory studies in flumes are easiest to perform and may provide an experimental basis for the theoretical considerations developed in this study. Field studies in selected sites are essential, however, to understand the full range of processes, physical, chemical and biological, which contribute to the formation of fine sediment deposits.
References Clarke, T.L., Lesht, B., Young, R.A., Swift, D.J.P. and Freeland, G.L., 1982. Sediment resuspension by surface-wave action: an examination of possible mechanisms. Mar. Geol., 49: 4-59. Clarke, T.L. and Swift, D.J.P., 1984. The formation of mud patches by non-linear diffusion. Cont. Shelf Res., 3: l-7. Drake, D.E., 1976. Suspended sediment transport and mud deposition on continental shelves. In: D.J. Stanley and D.J.P. Swift (Editors), Marine Sediment Transport and Environmental Management. Wiley, New York, pp. 127158. Dyer, K.R., 1986. Coastal and Estuarine Sediment Dynamics. Wiley, Chichester, 342 pp.
of Marine Systems 7 (1996) 119-131 of suspended matter Eisma, D., 1981. S upply and deposition in the North Sea. Spec. Publ. Int. Assoc. Sedimental., 5: 415-428. Eisma, D., 1987. The North Sea: An overview. Philos. Trans. R. Sot. London B, 316: 461-485. Freeland, G.L., Swift, D.J.P. and Young, R.A., 1979. Mud deposits near the New York dumpsites: Origin and behaviour. In: H.D. Palmer and M.G. Gross (Editors), Ocean Dumping and Marine Pollution. Dowden, Hutchinson and Ross, Stroudsburg, pp. 73-95. van der Giessen, A., de Ruijter, W.P.M. and Borst, J.C., 1990. Three-dimensional current structure in the Dutch coastal zone. Neth. J. Sea Res., 25: 45-55. Graber, H.C., Beardsley, R.C. and Grant, W.D., 1989. Stormgenerated surface waves and sediment resuspension in the East China and Yellow Seas. J. Phys. Oceanogr., 19: 1039-1059. Hathaway, J.C., 1972. Regional clay mineral facies in estuaries and continental margin of the United States East coast. In: B.W. Nelson (Editor), Environmental Framework of Coastal Plain Estuaries. Geol. Sot. Am. Mem., 133: 293316. Hu, Dunxin, 1984. Upwelling and sedimentation dynamics: I. The role of upwelling in sedimentation in the Huanghai Sea and East China Sea A description of general features. Chin. J. Oceanol. Limnol., 2: 12-19. Larsen, L.H., Cannon, G.A. and Choi, B.H., 1985. East China tide currents. Cont. Shelf Res., 4: 77-103. Larsonneur, C., Bouysse, P. and Auffret, J.P., 1982. The superficial sediments of the English Channel and its western approaches. Sedimentology, 29: 851-864. Lee, H.J. and Chough, S.K., 1989. Sediment distribution, dispersal and budget in the Yellow Sea. Mar. Geol., 87: 195-205. van Leussen, W., 1993. Estuarine Macroflocs and their Role in Fine-grained Sediment Transport. Ph.D. Diss., Univ. Utrecht, 488 pp. McCave, I.N., 1972. Transport and escape of fine-grained sediment from shelf areas. In: D.J.P. Swift, D.B. Duane and O.H. Pilkey (Editors), Shelf Sediment Transport: Process and Pattern. Dowden, Hutchinson and Ross, Stroudsburg, pp. 225-248. Nelson, C.H., 1982. Late Pleistocene-Holocene transgressive sedimentation in deltaic and non-deltaic areas of the Northeastern Bering epicontinental shelf. Geol. Mijnbouw, 61: 5-18. Pohlmann, T. and Puls, W., 1994. Currents and transports in water. In: J. Siindermann (Editor), Circulation and Contaminant Fluxes in the North Sea. Springer, Berlin, pp. 345-402. Postma, H., 1980. Sediment transport and sedimentation. In: E. Olausson and I. Cato (Editors), Chemistry and Biogeochemistry of Estuaries. Wiley, Chichester, pp. 153-186. Postma, H., 1967. Sediment transport and sedimentation in the marine environment. In: G.H. Lauff (Editor), Estuaries. Am. Assoc. Adv. Sci., Washington, pp. 158-179.
.I. Dronkers, A.G. Miltenburg/Journal Pratje, O., 1949. Die bodenbedeckung der nordeuropaischen Meere. Handb. Seefisch. Nordeurop., 1: 3. Sharma, G.D., 1979. The Alaskan Shelf. Springer, New York, 498 pp. Shepard, F.P., 1932. Sediments on continental shelves. Bull. Geol. Sot. Am., 43: 345-360. Simpson, J.H., Allen, C.M. and Morris, N.C.G., 1978. Fronts on the continental shelf. J. Geophys. Res., C83: 4607-4614.
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Swift, D.J.P., Stanley, D.J. and Curray, J.R., 1971. Relict sediments on continental shelves: a reconsideration. J. Geol., 79: 322-346. de Vriend, H.J., Capobianco, M., Chesher, T., de Swart, H.E., Latteux, B. and Stive, M.J.F., 1993. Approaches to long term modelling of coastal morphology: a review. In: H.J. de Vriend (Editor), Coastal Morphodynamics: Processes and Modelling. Coastal Eng., 21: 225-269.
OF
MARINE SYSTEMS ELSEVIER
Journal
of Marine
Systems 7 (1996) 119-131
Fine sediment
deposits in shelf seas
J. Dronkers
*,
A.G. Miltenburg
Institute for Marine and Atmospheric Research Utrecht (ZMXlJ), Princetonplein 5, 3584 CC Utrecht, The Netherlands Received
9 September
1994; accepted
7 March
1995
Abstract From field observations it appears that the top layer of a shelf bottom in general exhibits an intricate geographical pattern of sediment formations. Sediments of different composition are confined in distinct regions.
This contradicts the idea that current and wave forces stir up bottom sediment and disperse it in a random way over the shelf; the dispersal process is counteracted by sorting mechanisms. In this paper the bottom patterns of fine cohesive sediments are considered. A specific sorting mechanism is studied which may explain the patchy structure of fine sediment deposits. It is shown that fine sediments can be trapped in bottom deposits which contain a fine sediment fraction high enough to prevent pore water motion in the shelf bed. This mechanism opposes sediment dispersal away from existing deposits. It may also explain the formation or the preservation of mud patches, even in regions where the bottom shear stress is relatively high.
1. Introduction Coastal systems in general exhibit intricate structures, both in topography and in sediment composition. On geological time scales coastal systems are of an ephemeral, temporary nature (Postma, 1980). They evolve under the influence of hydrodynamic forcing often without reaching complete equilibrium. The evolution of coastal structures due to Holocene sea level rise is slowing down, but disturbance due to human interventions is becoming more important. The morphodynamics of coastal structures has received much attention in recent years (de Vriend et al., 1993). Less attention is given to the sedimentary structure of the shelf bottom; field evidence of sedimentary structures in the coastal zone is sel-
* Corresponding
Author.
0924-7963/96/$15.00
dom considered in studies of coastal dynamics. This alternate viewpoint is the subject of the present study. Geological maps of the top layer of the shelf bottom in general display complex geographical patterns (Shepard, 1932). Distinct formations of different types of sediment on different spatial scales can be clearly identified in spite of the dispersive action of currents and waves. Many of the observed sedimentary structures are probably related to relict deposits formed during past geological ages. The fact that such structures are preserved implies a quasi dynamical equilibrium with present hydrodynamic conditions. The basic assumption of this study is that the sedimentary patterns are indeed sensitive to present hydrodynamic conditions (Swift et al., 1971). A possible mechanism underlying the formation and preservation of the observed sedimentary structures on the coastal shelf is discussed.
0 1996 Elsevier Science B.V. All rights reserved
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Fig. 1. Schematic
representation
of deposition
This study focuses mainly on deposits of fine cohesive sediment. In Section 2 some current ideas on shelf sedimentation are reviewed. In Section 3 a qualitative comparison is made with deposition patterns observed on different coastal shelves. The most striking characteristic of the observed patterns which is not well explained by current theory is the patchy nature of many fine sediment deposits and the apparent lack of dispersal of fine sediments away from original deposits. In Section 4 a mechanism is described which may explain these features in a qualitative way. This mechanism is based on an assumption put forward by Clarke and Swift (1984) concerning the interaction between bed structure and sediment deposition and resuspension. The basic
Fig. 2. Bathymctry and general Atlantic Bight shelf (2d).
circulation
pattern
in the Bering
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and erosion
processes
on the continental
shelf.
idea is that under certain conditions less resuspension may occur from a bottom with higher fine sediment content than from a bottom with lower fine sediment content. It is shown that alternating currents then may generate patchiness. Furthermore this mechanism may contribute to the stability of sediment deposits formed during past geological ages.
2. Sedimentation
processes
on the shelf
In this section some current ideas on shelf sedimentation theory are reviewed (McCave, 1972; Drake, 1976 and Dyer, 1986). The pathways of fine sediment and the major mechanisms re-
shelf (2a), Yellow
Sea shelf (2b),
North Sea
shelf (2~) and the
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2a
m
depth < 30 m
Ezl
depth < 75 m
-
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2b
4 2d
circulation
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sponsible for deposition and erosion on the coastal shelf are schematically represented in Fig. 1 for a characteristic coastal zone. The most important sources of fine sediment are rivers, coastal and seabed erosion and primary production. Deposition areas are sheltered coastal basins and the deeper outer shelf. Near the coastal zone, river outflow plays an important role; fine sediments are trapped in a coastal fringe due to density induced cross-shore circulation. Recent studies (van der Giessen et al., 1990) emphasise the strength of this circulation. Residual alongshore transport takes place by tidal asymmetry and by the residual alongshore currents. Fine sediments can escape towards the shelf during periods of strong onshore winds causing downwelling. Cusps in the coastline may also force the current to separate from the coast causing an offshore transport of sediment. Variable wind-driven currents may further disperse fine suspended sediment over the shelf. The action of waves and tide prevents sedimentation in the shallow parts of the shelf. On the deeper shelf, where the influence of waves and tide is less pronounced, fine sediment will settle. The location of deposition areas is influenced by several factors, in particular the distribution of storm surge currents, flow circulation patterns and convergence zones of bottom currents. The sediment can originate from the coastal zone, but also from the deeper ocean by upwelling. The general picture that can be inferred from this paradigm is, (1) deposits of fine sediment occur in sheltered basins along the coast, (2) the shallow shelf is sandy and (3) the deep shelf is covered with fine sediments. The geographical structure of sediment deposits may reflect the general near bottom circulation pattern. However, specific geographical features may modify this general picture, for instance: the location and abundance of sediment supply; the exis-
Fig. 3. Deposits (3d).
of fine sediment
in the Bering
shelf (3a), Yellow
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tence of seabed depressions; topographical structures and related current patterns; the prevailing wind directions and the influence of ocean waves.
3. Fine sediments distribution on four shelves The above sedimentation picture is compared with field evidence from four continental shelves: the Bering Sea, the Yellow Sea, the North Sea and the Atlantic Coast of the USA. The first three are semi-enclosed seas, having a comparable surface area and a relatively gentle bottom slope. The Atlantic Coast of the USA is a narrow shelf. The observed fine sediment patterns are briefly discussed, indicating qualitative agreements and differences with theoretical expectations. 3.1. The Bering Sea The bathymetry is shown in Fig. 2a. Information on hydrodynamic and sedimentary characteristics is given by Sharma (1979) and Nelson (1982). The bering sea has a high sediment input of the Yukon river, approximately 4 * lo* ton per year. The Bering shelf is exposed to ocean waves which may strongly affect the bottom of the inner and middle parts of the shelf. Tidal currents are not very strong except in some near coastal regions; ice-cover in winter opposes the generation of strong wind waves and wind driven currents. The fine sediment distribution is shown in Fig. 3a. The middle shelf is sandy. With increasing depth the medium grain size becomes smaller and most of the deep shelf is covered with fine sediments. These observations are consistent with the theoretical picture. Most of the shallow shelf is sandy too. It is remarkable that in the coastal waters around the mouth of the Yukon river wave and current action cannot prevent the Yukon sedi-
Sea shelf (3b), North
Sea shelf (3~) and the Atlantic
Bight shelf
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3a
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ment from settling and forming a muddy bed. Isolated deposits of fine sediment also occur at greater distances in the near coastal zone. 3.2. The Yellow Sea The bathymetry of the Yellow Sea is shown in Fig. 2b. Data on hydrodynamic and sedimentary conditions in the Yellow Sea is derived from the studies of Larsen et al. (19851, Graber et al. (1989) and Lee and Chough (1989). Rivers around the Yellow Sea basin provide a very high input of fine sediments, in particular the Huanghe (Yel-
low) River (11. lo8 ton/yr) and the Changjiang (5 * lo8 ton/yr). The Yellow Sea is a meso-tidal sea (tidal range 2-4 m) with tidal currents being strongest in the middle part of the shelf and in some coastal regions. The influence of ocean waves is relatively small. Major storms have winds from the north, causing highest wave action in the central and southern part of the shelf. As a result, higher bottom stresses occur in the central part of the shelf than in the shallow northern part and the deep southern part. In the southern part of the shelf ocean currents and upwelling may play a role. From the distribution of bottom sediments shown in Fig. 3b, the following conclusions can be drawn. Fine sediment is deposited mostly in the shallow northern part of the shelf. This is consistent with the hydrodynamic conditions described above. The fine sediments originate from riverine input in present but also past ages. It is remarkable that no fine sediment deposits are found in the deeper parts of the shelf near the Korean coast. A possible explanation for their absence is the occurrence of large scale three-dimensional circulation cells (Hu, 19841, which may also explain the existence of a large mud patch in the southern part on the shelf. 3.3. The North Sea
Fig. 4. Bottom surveys in the highly turbulent Dutch coastal zone reveal isolated patches with an increased content of fine sediment. Stronger sedimentation occurs along the less turbulent northern edge of the Southern Bight. In this region the sediment deposits also have a patchy structure.
Sedimentation patterns in the North Sea have been extensively described by many authors (see, for instance, Pratje, 1949; Eisma, 1981; Larsonneur et al., 1982 and Eisma, 1987). The bottom topography is shown in Fig. 2c. The surface area and the average slope are comparable to the Bering and Yellow Sea shelves. During past ice ages, local depressions in the sea bed were formed which presently act as deposition sites. Tidal currents and wave action are most important in the Southern Bight of the North Sea and in the Channel. Storm surge currents can be strong in the same region, and in particular around the northern boundary of the Southern Bight and around the boundaries of the Irish Sea. The riverine sediment input to the North Sea shelf is small. The major sediment source is coastal and seabed erosion. The most important sedimentation areas in the North Sea correspond to depres-
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sions and trenches in the shelf bottom, as can been seen by comparing Figs. 2c and 3c. Other sedimentation areas in the central part of the North Sea seem to be related to the convergence of bottom currents in tidal mixing fronts (Simpson et al., 1978). There is no explanation for the patchy structure of the sediment deposits. This is true also for the sediment patches occurring along the Dutch coastal zone, see Fig. 4.
and deposition properties. The model describes a mechanism which counteracts dispersion of fine sediment away from deposition areas and provides an explanation for the patchy nature of the sediment distribution on the coastal shelf. The starting point is the continuity equation for the transport of sediment
3.4. The Atlantic coast of the USA
where M is sum of the sediment mass in the bottom and in suspension per unit area and S denotes the sediment flux integrated over the watercolumn, i.e. the sediment transport rate. If the sediment is not uniform this equation holds for each sediment class separately. From here on we will consider only fine cohesive sediments. For the vertical distribution of these fine sediments a two layer description is adopted, one layer representing the suspended state and the other layer representing the immobile sediment bed. It is assumed that the two layers can effectively be described by quantities such as an effective depth and an average concentration. Such a description does not apply to the situation where a fluid mud layer is present above the bed. To deal with this situation in our model we ignore the horizontal motion of the fluid mud layer and suppose that it is part of the immobile bottom. With this simplification the sediment mass is represented by
This continental shelf is quite distinct from the previous three shelf regions, see Fig. 2d. It is a narrow shelf with only a small sediment input from rivers. The tide is weak but the impact of ocean waves on the shelf bottom is strong. As shown in Fig. 3d no large deposits of fine sediment occur on the shelf with the exception of a large patch south of Cape Cod (Hathaway, 1972). Fine sediments are mainly deposited on the shelf slope of the Atlantic Bight. However, in the near coastal zone several small sized patches have been observed, in spite of strong wave action (Freeland et al., 1979). Summarising the findings for the different shelf seas the following picture emerges. The observed sedimentation patterns are in general agreement with the assumption that fine sediments mainly deposit in regions of low wave and current activity. However, deposits are not uniform and fine sediment deposits are also found in regions where wave and current activity are important. In some cases hydrodynamic structures such as fronts and gyres may explain the non-uniformity of the sediment distribution. In other cases such explanations are not plausible. Often the size of sediment patches is smaller than the scale of topographical or hydrodynamic structures. In the following further attention will be given to the formation of mud patches which are not directly related to topography or to gradients in current and wave properties. 4. Dynamics of shelf sedimentation In this section a simplified model for sediment transport is formulated based on assumed erosion
g+wo,
M=
/
cdz = HJ,
+ H,C,,
(2)
where H, and C, are representative suspension layer depth and suspension concentration respectively. H, and C, are the representative depth and concentration of the bottom layer. We will only consider the top layer of the bottom which exchanges sediment with the water column. The sediment flux is represented by
s=e’,c, where Q, is the transport suspension layer,
(3) flux of water in the
Q, = WJ, and U, is the representative velocity.
suspension
layer
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The sedimentary structures under consideration evolve on a time scale T, which is much longer than most of the fluctuations in the current distribution on the coastal shelf. This time scale T can be in the order of several years or even several centuries. For that reason we are more interested in the long term averaged values of suspended sediment concentrations and transports than in the instantaneous values. For the time interval T the transport flux Q, is decomposed as follows, e’,=
(4)
Here: (e’,) is the time averaged flux of water in the suspension layer, (@ > is the transport flux of all non residual water motions in the time interval T, due to turbulence, waves, tides, wind and other non residual forcings. The same decomposition is used for the suspended sediment concentration: C,=(C,)+C’,
(5)
Here: (C,) is the time averaged sediment concentration in the suspension layer, and C’ represents fluctuations of the suspended sediment concentration with respect to the mean. The following decomposition of the sediment flux results;
(6)
The first transport term corresponds to residual flow. The second transport term represents the residual sediment transport due to fluctuating water motions. This term would vanish if (1) dispersion processes are absent and if (2) no deposition and resuspension takes place. The contributions of (1) and (2) may be distinguished, although they are not independent. Contribution (2) represents transport due to asymmetry existing in the fluctuating water motions (for example, ebb-flood asymmetry of tidal current strength and variation or onshore-offshore asymmetry of wave orbital velocities) together with settling and resuspension time lag effects. Contribution (1) represents the effect of mixing processes which take place on the scale of the fluctuating water motions (for example, by generating shear).
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The residual flow term and the deposition-resuspension induced transport represent advective transport of suspended sediment and can be written to a first approximation as:
c&f.c,, where &r is an effective transport capacity and C, the average suspended sedimenf concentration. If the spatial structure of Qeff contains convergence and divergence zones, then this term may generate a patchy sedimentary structure. Some of the sediment structures found on the coastal shelves can probably be explained in this way. In the following this mechanism will not be considered, however. This study focuses on fine sediment deposits occurring in shelf regions where no structures in the current field are expected to occur at the scale of the deposits. By analogy with dissolved substances the residual transport due to mixing processes will be represented by a gradient type dispersive transport, involving a dispersion coefficient D. It should be noted, however, that the magnitude of the dispersion coefficient may be quite different. The spatial distribution of dissolved substances and suspended sediment is not similar, the latter in general being more patchy. The physical processes responsible for dispersion therefore involve different temporal and spatial scales. Otherwise the same type of mechanisms plays a role, in particular the combination of eddy diffusion and shear in the current field. The dispersive transport of suspended sediment will be written: @~C’>
= -D.H;tk,.
As a further approximation the concentration in suspension C, will be replaced by the equilibrium concentration C,,. The equilibrium concentration C,, is a quantity which in principle can be determined experimentally. It depends on the composition and structure of the bottom and on the hydrodynamic forces acting on the bottom. A large number of bottom characteristics can influence Ceq, for instance, the degree of consolidation and the presence of organic matter and biota. For simplicity it is assumed that the most important bottom characteristic is the fine sedi-
.I. Dronkers, A.G. Miltenburg/Journal
merit concentration C,. Also for simplicity it is assumed that the forces acting on the bottom are represented by a unique hydrodynamic parameter ut,, such as, for instance the bottom drag velocity. In most field situations the hydrodynamic forces on the bottom fluctuate on a shorter timescale than the deposition and resuspension time lags so that the equilibrium suspended concentration is not reached. This implies that it is not the equilibrium suspended concentration which appears in the time averaged transport formula, but an average of many equilibrium concentrations. This quantity will not be computed, but it will be assumed that the dependency on C, and z+, will be similar as for C,,. Considering that C,, = C, = C&C,, us), the gradient can be rewritten and the following expression for the averaged transport is obtained,
(7) The second and third term are dispersion terms. The second term corresponds to dispersion from regions with high ub towards regions > 0. This with low ub, provided that K,/au, condition means that the suspended concentration should increase when the forces acting on the bottom increase. Normally this will be the case; dispersion of fine sediment away from regions with high bottom stresses is consistent with classical sedimentation theory. The third term will be considered in more detail now. If aC,/iK, > 0, there is dispersion away from regions where the fine sediment content in the bottom is high and towards regions with low fine sediment bottom concentrations. This is what may be expected intuitively. However, if
ac,/ac,
(8)
then dispersion is from regions with low fine sediment content towards regions with high fine sediment concentration in the bottom. Note that the actual dispersion acts on the suspended concentration and always corresponds to transport from high to low suspended concentrations.
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Clarke and Swift (1984) have shown that C, is not necessarily a monotonously increasing function of C,. This has important consequences for the residual transport of suspended sediment and for the evolution of sedimentary structures on the coastal shelf. One of the consequences is patch formation and patch preservation, as will be discussed in the next section.
5. Discussion As shown before, the residual transport of fine suspended sediment depends in a crucial way on the fine sediment concentration C, of the bottom. This relationship will now be discussed in a qualitative way. Existing field measurements can hardly be used to quantify this relationship. One of the difficulties is that apart from C, and ub many other characteristics play a role, as indicated earlier. Different measurements can only be compared if all these characteristics are similar, but this is in general not the case. Therefore we will rely on two different conceptual models for sediment settling and resuspension. In the first model (Clarke and Swift, 1984) the bottom is assumed to form a sand matrix with interstitial holes which are more or less filled with fine sediment particles (see also Dyer, 1986). Two regimes exist for different concentrations of fine sediment. The first regime corresponds to a low concentration of fine sediment, such that the bottom is permeable and pore water motion can take place. Small particles are then easily extracted from the sand matrix and a high percentage is brought in suspension. The second regime corresponds to a high concentration of fine sediment such that the holes will be filled, so halting pore water motion. Thus, in this regime, not only the cohesive forces have to be overcome, but particles must also be extracted from the seabed. With respect to the first regime a smaller percentage of the fine sediment will be brought in suspension even though the concentration in the seabed is higher. From this model, a relation between C, and ub can be derived which is tentatively drawn in Fig. 5a. In this figure the equilibrium concentration
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a)
I
I
I
I
I
I
I
I
I
I
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C, medum
C, high
Fig. 5. (a) Qualitative variation of suspended concentration C as a function of u,, for three different values of bottom concentration C,. Higher hydrodynamic forcing of the bottom leads to increasing suspension of fine sediment. At low bottom concentration the hydrodynamic forcing is more efficient due to the permeability of the bottom. The suspension efficiency decreases with increasing bottom concentration especially if the hydrodynamic forcing is small. At high bottom concentrations a minimum hydrodynamic see for instance, Postma, 1967). (b) Qualitative forcing is required for suspending fine sediments (the so-called “yield-stress”, variation of C as a function of C, for three different values of ub. This figure is completely equivalent with Fig. 5a.
versus the bottom forcing parameter ui, is presented for low, medium and high bottom concentrations C,. Some experimental support is presented in Clarke et al. (1982). Fig. 5b shows that the suspended concentration C is not an increasing function of the bottom concentration C, under all conditions. The decrease of suspended concentration with C, occurs when the permeable regime changes into the impermeable regime. Therefore a range of intermediate bottom compositions may exist which release less fine sediment into suspension than bottoms with higher or lower fine sediment content. The suspended equilibrium concentration will therefore be lower in regions where the fine sediment content of the bottom has intermediate values than in the surroundings where the fine sediment content of the bottom is lower. Due to the gradient nature of the dispersive sediment transport, fine sediment is transported towards regions where the bottom has such an intermediate composition. This leads to the formation of mud patches with relatively high content of fine sediment embedded within regions of lower content of fine sediment, see Fig. 6. Advective transport of fine sediment also affects the mud patch. It G assumed that the effective transport capacity Qeff is uniform in the shelf region under consideration. In that case sedimen-
tation occurs in zones where the equilibrium suspended concentration decreases and erosion occurs in zones where the equilibrium suspended concentration increases. Fig. 7 shows that by this process mud patches are brought towards an equilibrium condition. Fine sediments are redistributed in such a way that the equilibrium suspended concentrations inside and outside the patch become equal. Dispersion of fine sediment away from the patch is counteracted. An alternative model for dispersion of cohesive fine sediment into mud patches is based on energy dissipation arguments. When wave and current action is exerted on a sandy bottom most of the energy is transformed into shear stress and
ywer t__________ ______?trh_____ “,,,
&CA+
acs/acb
cs
Fig. 6. Formation of a mud patch. Suspended dispersed towards a region where the bottom impermeable.
fine sediment is has become just
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cb .
T
ac,/ acb
Fig. 7. Different stages of mud patch evolution under the influence of a spatially uniform, unidirectional effective transport capacity. The patch is enhanced until1 the equilibrium suspended concentration is continuous across the patch. At the rear a negative patch is formed which travels downstream and which will finally disappear as a result of dispersion.
turbulence. Floes of fine sediment which settle in such regions, have a high probability to be broken up and resuspended (van Leussen, 1993). When wave and current action is exerted on a muddy bottom part of the energy is transferred to the bottom where it is used for fluidizing the upper bed layer. A fluid mud layer is formed which moves with the current. The shear stresses at the interface with the water column are relatively small and turbulence is strongly damped. Floes of fine sediment settling to a muddy bottom will therefore not be broken up and are easily incorporated in the fluid mud layer. This mechanism of mud patch formation is illustrated in Fig. 8. The two models described above relate to different physical processes, although they are not mutually exclusive. The first model deals with mechanisms playing a role in the erosion of fine sediments from the bed. The second model addresses bed properties which relate to the settling of fine sediment. Therefore the two models can
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be viewed as being complementary. The two models only describe the variation of the suspended equilibrium concentration C,, as a function of C, and ub in a qualitative way. The mechanism of mud patch formation and mud patch preservation described in this study has a very qualitative and even speculative character. There is no firm evidence for decreasing suspended equilibrium concentrations when the concentration of fines in the bottom increases. This should be observed when exposing shelf bottoms with different sediment compositions to the same hydrodynamic forcing ui,. In reality the forcing z+, is highly variable during the process of patch formation. This is not taken into account in the very simplified model presented in this study. The model may explain some of the characteristic patchy features of shelf sedimentation, but is not suitable in its present form for direct comparison with patchy structures observed in the field. The model does not include the influence of extreme events on the dynamics of mud patch formation. It may be expected that such events, where large amounts of fine sediment are brought into suspension, play an important role. Smaller mud patches may entirely disappear and the suspended fine sediments may disperse over a large region. Settling will generally not result in a homogeneous bed cover of fine sediment; the areas with the highest concentration may act as poles for new mud patch formation. Field observations confirm that the mud patches in the Dutch coastal
Fig. 8. Formation of a mud patch by trapping of floes in a fluid mud layer. Wave and current energy is dissipated in the fluid mud layer and turbulence is damped. Settling floes are not broken up and incorporated in the mud patch.
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zone are not permanent structures, but that their presence is related to the occurrence of storms (Eisma, 1981). Large mud patches seem to be rather stable. Under average conditions they seem to attract fine sediments, possibly by the mechanisms described in this study. Therefore they form distinct structures on the continental shelf, which do not necessarily coincide with geographical patterns in topography and current structure. One may expect that their growth is controlled by extreme events which resuspend and disperse part of the sediment accumulated during average conditions. More detailed field work on mud patches is required to confirm these ideas. Numerical modelling of shelf sedimentation requires a quantitative description of deposition and erosion processes. In present models (Pohlmann and Puls, 1994) the interaction of these processes with the sediment composition of the bed is not taken into account. For a proper representation of mud patch formation these interaction mechanisms should be considered. This requires further study of the processes of erosion and deposition as a function of bed structure and composition. Laboratory studies in flumes are easiest to perform and may provide an experimental basis for the theoretical considerations developed in this study. Field studies in selected sites are essential, however, to understand the full range of processes, physical, chemical and biological, which contribute to the formation of fine sediment deposits.
References Clarke, T.L., Lesht, B., Young, R.A., Swift, D.J.P. and Freeland, G.L., 1982. Sediment resuspension by surface-wave action: an examination of possible mechanisms. Mar. Geol., 49: 4-59. Clarke, T.L. and Swift, D.J.P., 1984. The formation of mud patches by non-linear diffusion. Cont. Shelf Res., 3: l-7. Drake, D.E., 1976. Suspended sediment transport and mud deposition on continental shelves. In: D.J. Stanley and D.J.P. Swift (Editors), Marine Sediment Transport and Environmental Management. Wiley, New York, pp. 127158. Dyer, K.R., 1986. Coastal and Estuarine Sediment Dynamics. Wiley, Chichester, 342 pp.
of Marine Systems 7 (1996) 119-131 of suspended matter Eisma, D., 1981. S upply and deposition in the North Sea. Spec. Publ. Int. Assoc. Sedimental., 5: 415-428. Eisma, D., 1987. The North Sea: An overview. Philos. Trans. R. Sot. London B, 316: 461-485. Freeland, G.L., Swift, D.J.P. and Young, R.A., 1979. Mud deposits near the New York dumpsites: Origin and behaviour. In: H.D. Palmer and M.G. Gross (Editors), Ocean Dumping and Marine Pollution. Dowden, Hutchinson and Ross, Stroudsburg, pp. 73-95. van der Giessen, A., de Ruijter, W.P.M. and Borst, J.C., 1990. Three-dimensional current structure in the Dutch coastal zone. Neth. J. Sea Res., 25: 45-55. Graber, H.C., Beardsley, R.C. and Grant, W.D., 1989. Stormgenerated surface waves and sediment resuspension in the East China and Yellow Seas. J. Phys. Oceanogr., 19: 1039-1059. Hathaway, J.C., 1972. Regional clay mineral facies in estuaries and continental margin of the United States East coast. In: B.W. Nelson (Editor), Environmental Framework of Coastal Plain Estuaries. Geol. Sot. Am. Mem., 133: 293316. Hu, Dunxin, 1984. Upwelling and sedimentation dynamics: I. The role of upwelling in sedimentation in the Huanghai Sea and East China Sea A description of general features. Chin. J. Oceanol. Limnol., 2: 12-19. Larsen, L.H., Cannon, G.A. and Choi, B.H., 1985. East China tide currents. Cont. Shelf Res., 4: 77-103. Larsonneur, C., Bouysse, P. and Auffret, J.P., 1982. The superficial sediments of the English Channel and its western approaches. Sedimentology, 29: 851-864. Lee, H.J. and Chough, S.K., 1989. Sediment distribution, dispersal and budget in the Yellow Sea. Mar. Geol., 87: 195-205. van Leussen, W., 1993. Estuarine Macroflocs and their Role in Fine-grained Sediment Transport. Ph.D. Diss., Univ. Utrecht, 488 pp. McCave, I.N., 1972. Transport and escape of fine-grained sediment from shelf areas. In: D.J.P. Swift, D.B. Duane and O.H. Pilkey (Editors), Shelf Sediment Transport: Process and Pattern. Dowden, Hutchinson and Ross, Stroudsburg, pp. 225-248. Nelson, C.H., 1982. Late Pleistocene-Holocene transgressive sedimentation in deltaic and non-deltaic areas of the Northeastern Bering epicontinental shelf. Geol. Mijnbouw, 61: 5-18. Pohlmann, T. and Puls, W., 1994. Currents and transports in water. In: J. Siindermann (Editor), Circulation and Contaminant Fluxes in the North Sea. Springer, Berlin, pp. 345-402. Postma, H., 1980. Sediment transport and sedimentation. In: E. Olausson and I. Cato (Editors), Chemistry and Biogeochemistry of Estuaries. Wiley, Chichester, pp. 153-186. Postma, H., 1967. Sediment transport and sedimentation in the marine environment. In: G.H. Lauff (Editor), Estuaries. Am. Assoc. Adv. Sci., Washington, pp. 158-179.
.I. Dronkers, A.G. Miltenburg/Journal Pratje, O., 1949. Die bodenbedeckung der nordeuropaischen Meere. Handb. Seefisch. Nordeurop., 1: 3. Sharma, G.D., 1979. The Alaskan Shelf. Springer, New York, 498 pp. Shepard, F.P., 1932. Sediments on continental shelves. Bull. Geol. Sot. Am., 43: 345-360. Simpson, J.H., Allen, C.M. and Morris, N.C.G., 1978. Fronts on the continental shelf. J. Geophys. Res., C83: 4607-4614.
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Swift, D.J.P., Stanley, D.J. and Curray, J.R., 1971. Relict sediments on continental shelves: a reconsideration. J. Geol., 79: 322-346. de Vriend, H.J., Capobianco, M., Chesher, T., de Swart, H.E., Latteux, B. and Stive, M.J.F., 1993. Approaches to long term modelling of coastal morphology: a review. In: H.J. de Vriend (Editor), Coastal Morphodynamics: Processes and Modelling. Coastal Eng., 21: 225-269.