Environmental setting, morphology and volumetric evolution of the Middelkerke Bank (southern North Sea)

MARINE GEOLOGY IN11RNAlI(WJ¢ JOUIINaL OF MAP.#~ aEO~k~k~THY AND GEOf*HYS~S

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Marine Geology 121 (1994) 1-21

Environmental setting, morphology and volumetric evolution of the Middelkerke Bank (southern North Sea) Jean Lanckneus a, Guy De Moor a, Ad Stolk b "Research Unit Marine and Coastal Geomorphology, University of Gent, Krijgslaan 281, $8, 9000 Gent, Belgium b Institute for Marine and Atmospheric Research Utrecht (IMA U) Department of Physical Geography, Utrecht University, P.O. Box 80115, 3508 TC Utrecht, The Netherlands Received 1 February 1994; revision accepted 20 June 1994

Abstract

The Middelkerke Bank, a tidal sandbank located in the southern North Sea on the Belgian continental platform has been the object of a multidisciplinary and international research project. The morphological and hydrodynamical setting of the study area is presented. A complete side-scan sonar coverage of the bank allowed a detailed analysis of the morphology of the bank and of its superimposed bedforms. Large dunes cover the flanks and summit of the Middelkerke Bank. They have a constant orientation, a height ranging from 0.5 to 5 m, a wavelength from 75 to 150 m and in most cases an asymmetrical profile. The slopes of both lee and stoss flanks are very low. Small and medium dune fields occur on the entire bank and in the adjacent swales. They are 2-D and present a wavelength ranging from 1 to 15 m. Flood and ebb oriented small dunes cover respectively the western and eastern flank of the bank. Bathymetric surveying of the Middelkerke Bank was carried out to assess the importance of volumetric variations of the bank mass caused by seasonal factors. Results show that changes in the bank volume can be quite important. The bank's volume decreases in periods of heavy weather after which the process of sand uppiling acting during long periods of fair weather conditions causes the bank to restore itself. Volumetric variations of the total bank volume in a winter-summer period can range along certain sections of the bank from - 2 1 % to +26%. Despite these large seasonal volumetric variations, the bank shows a long-term dynamic equilibrium as proven by the obtained evolution trend.

I. Introduction

The Middelkerke Bank, a tidal sandbank located on the Belgian continental platform, has been the object of a multidisciplinary research project, carried out by several European institutions. The project was partly financed by the European Commission in the framework of its M A S T I p r o g r a m m e and was known under its acronym RESECUSED ("RElationship between SEa floor CUrrents and SEDiment mobility in the Southern N o r t h Sea") (De M o o r et al., 1993). The project was intended as a field study of the 0025-3227/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0025-3227 (94) 00075-V

present-day interaction between water movement, sediment transport and bedform mobility (or stability) in a macrotidal offshore shelf sea environment. It especially aimed to perform a detailed monitoring of the present-day behaviour of a sandbank under changing hydrodynamic conditions, combined with the analysis of the impact of sediment and bedform displacements involved in the short-term evolution of the sandbank. Moreover shallow subsurface small-scale and deeper internal macrostuctures were detected and interpreted in relation with the recent-, the medium- and the long-term development of

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J, Lanckneus et al./Marine Geology 121 (1994) 1 21

the sandbank and with its genesis. In that prospect it strove to produce models of sedimentary structures in actual bedforms helpful for interpretation of sedimentary structures in fossil deposits and for their genetic diagnosis. The final aim was to obtain a better insight into the genesis, the evolution and the maintenance of a sandbank, in the mobilisation and mobility of the non-cohesive material, in the involvement of moving bedforms in its present-day behaviour and in the sediment exchanges with the surrounding sea floor, by analysing the sustaining processes in the field and using information on their morphological expression and their fundamental actions. Moreover the project was intended to produce qualitative and quantitative data on phenomena, on processes and on environmental conditions usable for model calibration and validation and for prediction of bedform behaviour (De Moor, 1993, p. 6). This paper gives an introduction to the morphological setting and the hydrodynamical environment of the Middelkerke Bank. It presents results on the morphology, the hydrodynamics, the superimposed bedforms and the volumetric evolution of the Middelkerke Bank. Other results of the RESECUSEDproject are presented in three accompanying papers in this volume of Marine Geology (Bern6 et al., 1994-this volume; Houthuys et al., 1994-this volume; Trentesaux et al., 1994-this volume). They deal with (1) the internal structures of the bank, (2) the assessment of the effect of a storm period on the morphology of the bank, and (3) the sedimentology of the surficial deposits.

2. General morphological setting of the Middelkerke Bank The Middelkerke Bank is one of the Flemish Banks (Fig. I). These parallel sandbanks or tidal current ridges (Off, 1963) are situated off the French-Belgian North Sea coast and stretch in a SW-NE direction, slightly oblique to the sandy macrotidal coastline. These banks are separated by swales that dip to the northeast and generally do not reach 30 m below low water level at spring

tide. Their morphology has been studied by Van Veen (1936), Off (1963), Houbolt (1968), Van Cauwenberghe (1971), Caston (1972), Bastin (1974), Kenyon et al. (1981), De Moor (1985), Ceuleneer and Lauwaert (1987), De Moor and Lanckneus (1988, 1989), Vlaeminck et al. (1989) and Lanckneus et al. (1989). Each of these banks is about 20-30 km long, 10-20 m high and 1-2 km wide. Their dimensions and general size decrease to the east. They all present an asymmetric cross-section with the steep flank towards the northwest. In some parts of the banks the crest zone rises up to less than 4 m below low water level at spring tide. Flanks and summit of the banks are covered with various types of bedforms especially at their northern edges, where large dunes reach heights up to 8 m, and where the energy of waves and currents is higher. Upslope on the flanks large and small dunes gradually become more parallel to the crest line of the banks. A detailed analysis of the bank morphology and of the bedfi ~m patterns is given by De Moor (1985).

3. Morphology of the Middelkerke Bank The Middelkerke Bank has a length of 12 km, a mean width of 1.5 km and a height above the sea floor varying from 8 m in the northeast up to 15 m in the southwest (Fig. 2). The bank's axis runs obliquely to the coast line and has a SW-NE orientation except in its landwards end where it is slightly curved towards the south. The bank shows a pronounced asymmetric cross-section with a steep side (1.7°-3 °) to the northwest and a more gentle slope to the southeast (0.5°). The coast is at a distance of 11 km (Middelkerke) to 14km (Oostende). The low water depth varies from 4 m in the southwest to 20 m in the northeast. The southern edge of the bank is relatively wide while the northern extremity tends to be rather narrow. The two swales adjacent to the Middelkerke Bank (Fig. 3) are known as the Negenvaam, to the northwest, and the Uitdiep to the southeast. The Negenvaam swale is 2 to 3 km wide and 12 to 20 m deep. The Uitdiep swale has a width of 1 to

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Fig. 1. Location of the MiddelkerkeBank on the Belgiancontinental platform. The location of the referencepole (RP no. 7) on the beach is given too. 3 km and a depth of 12 to 20 m. Both swales become narrower and shallower landwards where they merge into a coast-parallel swale system. Cross-sections through the northern, central and southern part of the bank (Fig. 4) illustrate that superimposed bedforms occur on the flanks and summit o f the entire bank and that their geometric characteristics and distribution vary substantially along the bank's axis. The northern part of the bank is characterised by large to very large dunes which occur on the bank summit. On both flanks much smaller bedforms can be seen. The orientation of their steep side suggests a possible migration towards the bank top. The southern part of the bank has a flatter morphology and only two large dunes with a height of more or less 1 m are

present on the profile rG20. The pronounced asymmetry of the bank can be well appreciated on a bathymetric profile through the central part of the bank (reference track rG23). In this area the bank is split up into two parts by a deeper section reaching - 1 3 m. On both parts bedforms with a height up to 1 m can be found.

4. Surficial currents 4.1. Surficial water movement in the Southern North Sea

Knowledge of hydrodynamical characteristics is essential to understand the morphological and

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J. Lanckneus et al./Marine Geology 121 (1994) 1-21

Fig. 2. 3-D view of the morphology of the Middelkerke Bank. View from the northwest, grid unit size 200 x 200 m (from Houthuys, 1993).

sedimentological phenomena occurring in a shelf sea. Several techniques have been used to measure surficial currents and residual currents (movements of water masses over a period larger to that of a tide) in the Southern N o r t h Sea. The tidal currents on the Belgian continental platform are measured by the Belgian Service of Coastal Harbours with the help of current meters. The results are published in a current atlas (Van Cauwenberghe, 1992) providing the direction and velocity of the superficial currents at different tidal phases and in a restricted number of reference stations. Information on waves and surface currents can be deduced from high frequency radar echoes backscattered from the sea surface. An experiment with a ocean surface current radar (Palmer, 1990) has been carried out on the Belgian coast in the framework of the M A S T II CSTAB Project (O'Connor, 1993). The system was deployed in November 1992 for a period of 2 months during which surface current speeds and directions were measured in an area including the Middelkerke Bank, the Kwintebank and the Oostende Bank. The Management Unit of the Mathematical Model North Sea ( M U M M ) has been developing

for m a n y years mathematical models of the surficial water circulation in the Southern N o r t h Sea ( M U M M , 1987). Several authors such as Salomon and Breton (1991 ) developed a 2-D mathematical model of the residual current field for an average tide in the Channel. Sequential thermal infrared imagery (Jegou and Salomon, 1991), together with information of a 2-D numerical model of the English Channel, was found to be an effective tool for the detection of long-term (time scale of a few weeks) residual water movements. The A V H R R images of the N O A A satellites reveal a residual water movement across the Strait of Dover in a northeastern direction. Visser (1989) applied salinity measurements to detect the residual currents in the Southern Bight of the N o r t h Sea. His data revealed an eight year average residual velocity of 3.0 cm s - 1, in a northnortheastward direction, 70 km offshore from the Dutch coast. In the Dutch coastal zone, between 0 and 30 k m offshore, the residual current seems to increase, in a north-northeastward direction, from almost zero near the Belgian coast to about 6 cm s-1 near the isle of Texel.

J. Lanckneus et al./Marine Geology 121 (1994) 1-21

Fig. 3. Bathymetry of the Middelkerke Bank and adjacent swales. The thick lines across the bank are the reference tracks sailed for volumetric evaluation. The position of the current measurements (Stolk, 1993) are given too.

4.2. Tidal currents on the Belgian eJntinental platform

Tidal currents play an essential role in the presence and morphology of sandbanks on continental shelves (Stride, 1982). A brief account is given here of the bidiumal tides and tidal flows in the Flemish Banks area. This description is based on tidal observations (Van Cauwenberghe, 1977, 1985) and near-surface flow data from the Flow Atlas (Van Cauwenberghe, 1992). The account is valid for the spring tide situation. The neap tide image is almost similar, but flow velocities only amount to 50-70% of the spring tide velocities.

5

Neap tidal range is only 65% of the spring tidal range (the latter being 4.85 m at Nieuwpoort). During low water off Nieuwpoort, the flow is parallel to the coastline and directed from northeast to southwest (heading 250°). Peak surface ebb flow velocities of 0.85 to 1.0 m s-1 are reached between 0.75 and 1.5 h after low water and have a heading of 245 °. In the following hours, the flow velocity gradually decreases and turns south. Slack waters occur between 2.5 and 2.0 h before high water and are characterised by low velocities (0.35 to 0.45 m s -1) heading southeast, towards the coast. In the following hour the velocities increase and swiftly turn east. Between 1 and 2 h after high water, the flow direction is again parallel to the coast, but now heading northeast (60°). The peak flood velocities of 0.80 to 1.0 m s -1 are attained around high water. Peak currents occur first inshore and later offshore. The currents are again minimal at about 4 h after high water: 0.1 to 0.2 m s-1 with a northwest heading. Afterwards, the velocities increase and flow turns to the southwest. The rotation of the tidal current vectors is counterclockwise (Houthuys, 1993). North of the Flemish Banks area the tides are symmetric, i.e. the peak ebb currents are approximately equal to the peak flood currents. However, near the Flemish Banks and more inshore, there is a flood dominance. The flood dominance increases shoreward: the ratio of peak ebb to peak flood velocities decreases from 0.9 in the Flemish Banks area to 0.7 and 0.5 in the swales under the coast. As shown below, local flow conditions close to the sandbanks may differ from the overall, regional picture described here.

5. Tidal movement and bottom currents in the Nliddelkerke Bank area

From a sedimentological point of view, the nearbottom currents play a much more important role for instantaneous and for residual sediment transport than the surficial movement of water masses. Numerous authors [Caston and Stride (1970), Caston (1972), McCave (1979), Caston (1981), Kenyon et al. (1981), McCave and Langhorne

6

J. Lanckneus et al./Marine Geology 121 (1994) 1-21

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Fig. 4. Cross-profilesfor the northern (reference line rH03.0), central (reference line rG23.0) and southern (reference line rG20.0) parts of the Middelkerke Bank recorded on 24 February 1993. (1982), Stride (1982), Venn and D'Olier (1983), Howarth and Huthnance (1984), De M o o r (1985) and De M o o r and Lanckneus (1988)], point out that in the inter-bank swales, the near-bottom

currents are parallel to the channels' long axis, and that they are gradually deflected towards the sandbanks' crests when approaching these crests. Moreover, a local flood dominance of the near-

J. Lanckneus et al./Marine Geology 121 (1994) 1-21

bottom currents is observed on the northwestern flanks of the sandbanks, whereas the ebb currents are dominant on the opposite slopes (De Moor, 1985, 1986). Methods have been developed to study specifically the near-bottom currents and the resulting residual sediment transport directions. On the Middelkerke Bank, the near-bottom currents were studied by Stolk (1993). Tidal cycle current measurements in 7 different locations were carried out with propeller-type Elmar current meters. The currents were measured at 6 different depths from 0.5 m below the sea surface to 0.8 m above the sea bed during a tidal cycle of 13 hours. The discrete monitoring of wave and tidal movement, current characteristics and suspendend sediment concentration was carried out with the help of bottom located measuring frames [Van de Meene (1994), Stolk (1993)]. The frames were equipped with electro-magnetic flow meters, optical suspension meters and a pressure sensor. The deployment of the measuring frames in April/May 1991 covered a spring/neap tidal cycle during which fair weather conditions prevailed so that the water level variations merely showed the tidal amplitudes without wave influence. The water levels on a tidal scale were calculated each hour as the mean of the waterlevels during a burst of 34 minutes. Fig. 5 shows the tidal curve for a spring tidal cycle and a neap tidal cycle on the northwest flank of the Middelkerke Bank near station G1 (Fig. 3). The tidal amplitudes at spring and neap tide are 4.5 and 2.7 m, respectively. The curves show a slight asymmetry with low water at 6 h before high water and 6.5 h after high water. The spring tide curve shows another asymmetry as well. From low to high water there is first a slow rise (2 m in 3.5 h) and then a fast rise (2 m in 1.5 h). The drop in water level from high to low water is more uniform. This asymmetry is much less pronounced at neap tide. The spring tidal range shown here is relatively low. The tidal amplitude during other spring tides on the Middelkerke Bank can be some decimeters higher. The shape of the tidal curve is very similar to that of Oostende (Van Cauwenberghe, 1977). The tidal current ellipses measured in the four stations on the Middelkerke Bank (Fig. 6) pointed

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Fig. 5. Tidal curves for spring tide and neap tide recorded on the Middelkerke Bank on 29 April and 7 May 1991, respectively.

out that differences in speed and direction exist over the bank. Measurements of wind speed and density profiles during deployment showed that wind- or density-driven currents contributed only minimally to the current velocities and could therefore be neglected. The currents are given at two levels in the vertical profile: slightly above the sea bed (+ 1.3 m) and below the sea surface ( - 4 to - 5 m). The ellipses are elongated in the direction of the flood current to the northeast and in the direction of the ebb current to the southwest. The tidal current vectors rotate counterclockwise. The change in tidal current direction from swale to bank is shown by the current ellipses for the Negenvaam (station G5) and for the flanks of the bank (stations G1 and G2). The main axis of the current ellipse is oriented at a clockwise angle of 10° to the axis of the bank-swale system in the Negenvaam swale. On the flanks of the bank the direction changes in a clockwise sense to an angle of 28 ° in relation to the bank axis. This happens on both the steep northwestern flank and the gentle southeastern flank of the bank. These facts

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J. Lanckneus et al./Marine Geology 121 (1994) 1 21

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Fig. 6. Current ellipses recorded in stations G1 (on 4 March 1992; 3 days from spring tide), G2 (on 5 March 1992; 2 days from spring tide), G5 (on 22 April 1992, 5 days from spring tide) and G6 on 22 July 1992, 3 days from spring tide). See Fig. 3 for location of stations.

fit the theory of Huthnance (1982) which suggests such changes in angle between the peak flood and ebb currents and the morphological axis of the bank-swale system. There are also variations in the tidal current characteristics within a single morphological unit, e.g. the northwestern flank of the bank. The current speed and direction in the southern part of the bank (station G6) differ from those at the

northern part (G1) (Fig. 3). The maximum current speed near the surface is 1.0 m s-1 at G1 and 0.6 m s -1 at G6. Near the bottom the speeds are 0.7 and 0.4 m s-1, respectively. This difference is too large to be explained by tidal phase differences which could interfere as measurements were not carried out simultaneously. The orientation of the current ellips at G6 differs significantly from that at G1. At G6 the angle between the ellips and the

J. Lanckneus et al./Marine Geology 121 (1994) 1-21

bank axis is only 15°. It is closer to the value measured in the swale than to the one recorded on the northern part of the bank. The water depth at the two stations is nearly the same. Therefore, the difference in current characteristics at stations G1 and G6 points to a real diversity of the hydrodynamics along the Middelkerke Bank. This difference can be partly due to the value of the bank slope which is gentler in the south (0.8%) than in the north (3%). This, however, cannot be the only cause because the current ellipses at G1 and G2 correspond to stations with different slope values (3 and 1.6%, respectively) but they have nevertheless a similar shape. The theory of Huthnance (1982) is based on the case of a linear sandbank. The northern part of the Middelkerke Bank meets this condition. The southern part, however, is less linear as it forms a plateau with the western end of the Oostende Bank (Fig. 3).

9

direction in a narrow coastal section and a southwestward direction more offshore. The asymmetry of large and small dunes, based on side-scan sonar mapping, has been used to study maintenance processes on the Kwintebank, situated west to the Middelkerke Bank (De Moor, 1985, 1986). Finally, recent developments in sedimentology raised the possibility of the use of variations in the areal pattern of grain-size parameters for the determination of residual sediment transport paths (McLaren, 1981; McLaren and Bowles, 1985). A slightly altered version of this method (Gao and Collins, 1990) has been used to study residual sediment transport directions on the Kwintebank (Lanckneus et al., 1992) and on the Gootebank (Lanckneus et al., 1993). Results of these studies show that although a complete similarity between the residual transport directions derived from the sediment trend analysis and the bedform analysis is lacking, the sediment trend analysis was able to distinguish flood from ebb dominated areas.

6. Residual bedload transport 7. Morphology of superimposed bedforms Radioactive tracer experiments have been introduced to measure the dispersion of a substance on the sea floor. Beck et al. (1991) recently carried out eight radioactive tracing experiments at different sites in the English Channel. The results of the radioactive tracing experiments, together with the data obtained from current meters and from the geometry of sand bodies, made it possible to propose a model about residual bottom-current directions at the English Channel-North Sea border. The general pattern of the residual sediment displacement in that area shows an offshore domain with residual sand transport to the southwest and a narrow southern coastal domain with residual sediment transport from the English Channel towards the North Sea. Bedforms and more specifically their asymmetry, can be used too to determine the residual direction of the sand transport. Stride et al. (in Johnson and Baldwin, 1986) analysed the dominant sand transport paths and their relationship with areas of large dunes on the northwest European Shelf. They distinguish two directions of residual sand movement off the Belgian coast: a northeastward

7.1. Data acquisition and processing The reconnaissance of the Middelkerke Bank was carried out on the 16th of May 1990 (Lanckneus et al., 1991) according to a procedure established during previous surveys on the Flemish Banks (De Moor, 1985; De Moor and Lanckneus, 1988). Side-scan sonar and bathymetric registrations were recorded along profiles across the bank. Track lines were sailed with an interval of 120 m in order to obtain complete sonar coverage for a sonograph mosaic. A Deso echosounder and a Klein two-channel analogue side-scan sonar, coupled with a 500 kHz high resolution transducer, were used for the bathymetric and sonograph recordings. A ship speed of 4 knots was maintained during all survey operations. Navigation and positioning during all operations were performed by the Syledis system whose accuracy (variation of measured values in a single location) calculated according severe statistical standards (Van Cauwenberghe and Denduyver, 1993) is better than 10 m in the area of the Middelkerke Bank.

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J. Lanekneus et al./Marine Geology 121 (1994) 1-21

During the data processing, a distinction was made between the large (height between 0.5 and 3 m) to very large dunes and the small (height smaller than 0.25 m) to medium dunes according to Ashley (1990).

7.2. Geometry of large to very large dunes Fig. 7 shows the position of the crest lines of the large dunes, deduced from the sonograph mosaic. Large to very large dunes occur on both flanks and on the summit of the bank and are completely absent in the two adjacent swales. The strike of the crest lines is quite uniform and varies between N - S and N15°W on the whole bank except on the southern end where it bends towards the west. In this part several large dunes have a strike of N30°W. The crest lines of most large dunes are nearly straight to slightly sinuous. They display a good lateral continuity and are traceable in most cases from one flank of the bank to the other over a distance up to 3 km. Shorter large dunes with a crest length of 200-300 m frequently occur between the longer ones. Branching of crest lines was observed occasionally. The height of the large to very large dunes ranges from 0.5 m to some exceptional values of 4 to 5 m, the most common height varying between 1.5 and 2 m. The height of an individual dune can vary considerably along its crest and variations were observed between 1 and 4 m. The wavelength of the dunes is the distance measured between two adjacent crest lines. The large dunes on the Middelkerke Bank have a rather constant wavelength ranging in most cases from 75 to 150 m. The angle of the steep slopes of the dunes varies between 1° and 10°, the most common value being 2°-3 °. Stoss slopes are generally in the range of 1°-7 ° . Present-day large dunes are known to present slopes with small angles, but values of 10 ° are commonly found (McCave 1985). Bern6 et al. (1989) even mention values of 30°-35 ° which were measured in several very large dune fields of the continental shelf around France. The slope values of the large dunes on the Middelkerke Bank can thus be considered as quite low. Most large dunes show a distinctly asymmetric

cross-sectional profile. Many authors (e.g. Caston, 1972) describe asymmetric large dunes migrating on both bank flanks of linear sand banks towards the bank's crest. In this case, the asymmetry of the large dunes is opposite on both bank flanks pointing to a convergence of sand streams towards the crest line of the bank (De Moor, 1985). Symmetric large dunes can occur in the transition area between the dunes of opposite asymmetry (Bernr, 1991 ). The detailed side-scan sonar recordings allowed an accurate analysis of the asymmetry of the dunes along the whole bank. The results are presented in Fig. 7. All large dunes of both bank ends have their steep slopes dipping toward the northeast. The central area of the bank is characterised by large dunes with steep slopes facing towards the crest of the bank on the two bank flanks. Between the central area and the two bank ends large dunes with steep slope dipping towards the southwest are found. This asymmetry pattern is much more complex than the model in which the large dunes have their steep slope dipping in opposite directions on both bank flanks pointing to a convergence of sand streams towards the bank's crest. The evolution in time of the asymmetry of large dunes on the Middelkerke Bank was studied with the help of time-series of bathymetric and side-scan sonar recordings (Lanckneus and De Moor, 1993). This analysis pointed out that situations exist in which nearly all large dunes on the bank can be ebb asymmetric (lee side dipping to the southwest). This was recorded for instance in May 1990. An opposite image was obtained with the recordings of September 1992 which revealed a majority of flood asymmetric large dunes (lee side dipping to the northeast). Between these two extremes, all intermediate situations, as the one presented on Fig. 5, can occur. Swell and wind from a dominant direction and acting during a prolonged period are able to increase or decrease the strength of either flood and ebb current and to induce a particular asymmetry to the large dunes (Lanckneus and De Moor, 1994). In the case of the recordings of May 1990, swell and wind from the north to northeast were dominant in that period. Similarly, a dominant southwest to west wind was blowing during the survey period of September 1992.

J. Lanckneus et al.IMarine Geology 121 (1994) 1-21 I 2*42'E

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12

J. Lanekneus et al./Marine Geology 121 (1994) 1-2l

7.3. Geometry of small to medium dunes All small dune fields were mapped and the asymmetry of the structures was deduced from the sonographs (Fig. 8). Small dunes occur on the whole bank and in a large part of the adjacent swales. They often cover both flanks of the large dunes. They are 2-D and present a wavelength ranging in most cases from 1 to 15 m. The strike of the small dunes ranges between N10 ° and N30°W. This means that they are usually oriented at an oblique angle to the large dunes with a counterclockwise angle offset of approximately 20 °. Current measurements carried out on the Middelkerke Bank attribute a direction of N60°-N65°E to the peak flood current (Stolk, 1993). Taking into consideration the strike of both small and large dunes, it is clear that it is the small dunes that are perpendicular to the peak currents. Situations in which the small dunes are a product of the peak currents and, therefore, have an orientation perpendicular to the peak currents, are commonly found. Dalrymple (1984) explains the oblique orientation of the large dunes relative to the peak currents as a result of different migration rates of certain portions of the crest line. Because of their limited lateral extent, the small dunes would remain more or less nearly perpendicular to the local peak currents. The steep slopes of the small dunes are dipping in opposite directions on both sides of the bank. In the western swale and western section of the bank, they dip to the northeast while in the eastern swale and eastern section of the bank they dip towards the southwest. The division between the ebb and flood oriented small dune fields is sharp and a line can be drawn running from northeast to southwest, more or less parallel with the bank axis, which delimits the small dune fields with opposite steep slopes (Fig. 8). This line often coincides with crests of large dunes.

8. Volumetric evolution trend

8.1. Introduction Caston (1972) suggested that a tidal sandbank is subject to growth both vertically (limited by the

depth of water) and laterally (parallel to the direction of the tidal currents). Information about the directions along which sand is moving residually can be deduced from the geometric characteristics of bedforms (orientation and steep slope direction) which can be recorded with the help of side-scan sonar. The sonographs, however, only provide the location and direction of the residual sand transport paths and not the net effect of the sand supply on the volume of the bank. Volumetric measurements of the Middelkerke Bank and of a beach section opposite to the bank were carried out to assess the occurrence and the importance of the volume variations caused by occasional ~torms) or seasonal factors.

8.2. Acquisition and processing of volumetric data Volumetric monitoring of a sandbank Volumetric monitoring of sandbanks (De Moor, 1984, 1985, 1986; De Moor et al., 1989) has been carried out by bathymetric echosounding along a number of fixed tracks crossing the Middelkerke Bank (Fig. 3) and sailed three to eight times a year from 1990 onwards. After corrections for tidal movement and variations in ship's speed and heading a net depth profile is plotted in relation to a zero datum defined as the local mean lowest low water spring (MLLWS) level. The tidal reduction is carried out with the help of a tidal reduction method (Van Cauwenberghe, 1977) based on tidal measurements in coastal stations. Different profiles sailed along the same track can then be compared. A numerical processing of the depth data is performed comprising the calculation of unit volumes along that track. These unit volumes which have a width of 1 m are d~,qimited by the bank profile and by reference levels situated at intervals of 2.5 m beneath the zero reference level. Two important unit volumes are the top slice volume, defined by the highest reference plane intersecting the bank and the total bank volume defined by the lowest reference plane intersecting both bank flanks (Fig. 9). All calculated values can be expressed in absolute (m 3 m -1) and in relative values (%). These relative values are obtained by normalising the absolute values in relation to a reference unit volume. After having sailed several times along a fixed track, a time

J. Lanckneus et aL/Marine Geology 121 (1994) 1-21 I 2042'E

I 2"44'E

q~

13

u 2046'E

f

N

57020'N

/

/2 /

/ / /

/ /

/

/

I

/

)

,

8°N

)

)

fJ

/

,f

/ / f

/

/

J

/

)

f

~\ /

lJ

\,f Limits of survey area ~

/

Small dune fields Area with flood oriented small dunes Area with ebb oriented small dunes Boundary line between flood and ebb oriented small dune fields

qb

0

,/

1

2

I

I

3km I

I

Fig. 8. Location and asymmetry of the small dunes on the Middelkerke Bank (situation May 1990).

Z Lanckneus et aL/Marine Geology 121 (1994) 1-21

14

zero datum

.-~7.5NW

(MLLW~

SE

2.5

5.0

~

51o.o

~

12.5

E VOLUME

15.0

----

17.5 20.0

500

1000

1500

2000

2500

I

I

I

]

I

distance (m)

3000

I

zero datum(MLLW5

NW

SE

2.5 5.0 7.5 ~'~-10.0 TOTAL BANK VOLUME

~12.5 15.0

/

17.5 20.( - - -

--

500

1000

1500

2000

2500

I

I

l

1

I

distance (m)

300(

I

Fig. 9. Definition of the top slice volume and of the total bank volume.

series of unit volumes is obtained on which a regression analysis is applied. This analysis indicates the present volumetric trend. Bathymetric recordings were carried out with the help of a Deso XX echosounder coupled to a TSS heave compensator. Recordings prior to the second half of 1992 were, however, performed without heave compensation. Navigation and positioning during all operations were performed by the Syledis system. Volumetric monitoring of a beach Volumetric measurements of a beach section opposite to the Middelkerke Bank were carried out at a reference pole situated at 7 km east of the

French border near Koksijde (Fig. 1). Data are available for the period 1990-1993. At this station, the beach comprises an important high beach and a low foredune backed by an inactive sea wall; the near shore is very slightly sloping. Volume calculations of beaches is based on beach profiling with a 3 m step along reference profiles oriented transversal to the coast line (De Moor, 1979). The topographic measurements are carried out from the dune front convexity to the spring tide low water line. An objective description of the local beach evolution cannot be fulfilled without the use of a numerical parameter for the beach condition (De Moor, 1992). If monitoring is performed with an

J. Lanckneus et aL/Marine Geology 121 (1994) 1-21

adequate frequency, the resulting time-series will allow, as it is the case for the volumes of the sandbanks, the definition of a volumetric trend. The basic numerical parameter which is calculated is the absolute unit volume (AUV in m 3 m -a) along a profiling section (De Moor, 1992). This parameter corresponds to the volume defined by the vertical cross section along the rectilinear transverse beach profile delimited by a vertical at the begin and another at the end of the profile and by its intersection with a horizontal plane situated at a fixed depth below a local elevation datum, and further by an equal cross section at 1 m parallel to the former. Such volumetric monitoring and the resulting time-series form an excellent mean to obtain a synthetic and easy-reference image of the beach evolution.

8.3. Results Volumetric results of the Middelkerke Bank The Middelkerke Bank is a sandbank which is not subject to aggregate exploitation, contrary to

15

the adjacent Kwintebank where sand has been exploited pro rata of 0.5 x 106 m 3 per year between 1979 and 1988. From 1989 on this quantity doubled and extraction in 1991 reached the figure of 1.4 x 106 m 3 (Bestuur Mijnwezen, 1993). The volumetric time-series of the top slice of the Middelkerke Bank shown in Fig. 10 is based on recordings carried out along the reference track rH00. Fig. 10 illustrates distinctly the existence of seasonal variations in the recorded volumes. Possible interference with an error in tidal reduction is controlled by matching the time-series for different adjacent tracks (Fig. 11). If variations in volume are the effect of such error, the displayed trends should be different for each series as each bathymetric profile during a single survey was recorded in different phases of the tidal cycle. The general trends present on each time-series are, however, the same for all tracks which suggests that the recorded volumetric variations correspond basically to real changes in volume.

Middelkerke Bank Reference track rH00 T o p slice v o l u m e a b o v e - 1 0 m 1000 900 A

800

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700 600

O

500

._o

400

¢3. O

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1

200 100 0 1/01/90

i

1/01/91

1/01/92

31/12/92

i

31/12/93

Date of recording

Top slice volume - -

Regression line

Fig. 10. Time-series of the top slice volume across the Middelkerke Bank based on recordings carried out from February 1990 till May 1993 along the reference track rH00 (see Fig. 3 for location of track).

J. Lanckneus et aL/Marine Geology 121 (1994) 1-21

16

Middelkerke Bank

w

Top slice volumes 1200

03

1000

1

z 800

~

~-

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;

~

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:::::::::::::::::

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1/01/90

1/01/91

1/01/92

31/12•92

31/12/93

Date of recording •

rG19

~

rG20

'

rG22

~

rH00

"~

rH02

]

J

Fig. 11. Superimposed time-series of the top slice volume across the Middelkerke Bank, based on recordings carried out from February 1990 till May 1993 along the reference track rG19, rG20, rG22, rH00 and rH02. A question mark represents a volume which could not be calculated.

Examination of the trends show that minima are reached in a period corresponding to the first three months of each year and maxima are obtained in the last three to four months of each year. This suggests that winter storms occurring in December to February of each year could be responsible for an important downslope dispersion of sand resulting in a top volume decrease. The better weather conditions during summer make an uninterrupted upslope movement of sand under the influence of near-bed currents possible causing an uppiling of sand resulting in a top volume increase. In June 1991 and June 1992, however, nearly all volumes as observed on the time-series decreased for a short period. This shows that erosion not merely occurs in winter. Periods of rough weather indeed can be found both in winter and summer. These data suggest that the volume of the bank is in equilibrium with the natural hydrodynamic conditions. Cyclic changes in volume occur

through the year reflecting changing dominant weather conditions. Severe storm periods are able to decrease the small bank top volume by 70% (Table 1) after which fair weather allow the reconstruction of the bank to its original volume. The available data cover only a rather short period of three years which is, however, large enough to observe the impact of seasonal phenomena on the bank's volume. A longer time-series of observations will be necessary to define the longer-term volumetric trend of the bank. Volumetric results of the beach The presented volumogram (Fig. 12) provides the AUV time-series for the total beach (beach unit situated between the dune foot and the low water line) and for the high beach (the beach unit between the dune and the mean spring high water mark) recorded in reference station no. 7. Periods of residual erosion and volume increase are seasonal events. The winters of 1989, 1990,

J. Lanckneus et aLIMarine Geology 121 (1994) 1-21

17

Beach Station Reference Pole n ° 7 (Koksijde)

~

: :::, :~ !

8300 ,,~

8200

.=

8100 8000

o

~

ul

w ~

(n ..:~:.

rr w

if"

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25600

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%

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'~

25100

7700

25000

7600

24900

7500

24800

7400 1/01/89

.~

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7900 e 7800

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i

25300

~ S

i

24700

1/01/90

1/01/91 Date

1/01/92 of recording

I--.--o-.-- High Beach

•

31/12/92

31/12/93

Total Beach

Fig. 12. Time-series of the Absolute Unit Volume for the total beach and the high beach along reference pole no. 7 (Koksijde) based on recordings carried out from January 1989 till March 1993.

1991 and 1992 are clearly visible on the volumetric figures of high beach and total beach. The winter of 1991 had less impact on the high beach volume than the others. In station no. 7, the winter of 1990, although characterized by very severe storms and dramatic impacts on other sections along the coast (De Moor, 1991), had only little impact on the total beach volume but caused a relatively strong erosion on the high beach. The summers from 1989 till 1992 were characterized by seasonal sediment accumulatL,n. The volumetric evolution of the Middelkerke Bank and the beach at reference pole 7 shows a similarity in seasonal variations. The relative value variations in that station for 1992 indicate volumetric losses of 3% for the total beach and of 6% for the high beach between summer and winter. The periodic summer restorations may partly be due to artificial sand supply on the high beach.

9. Conclusions A multidisciplinary and international research project was carried out on the Middelkerke Bank, a tidal sandbank on the Belgian continental platform. The principal objective of the project was to perform a detailed analysis of the behaviour of sediments and bedforms of and on a sandbank in a macrotidal offshore environment over short-, medium- and long-terms, and to study the interactions between water movement, sediment transport and bedform mobility. The tidal current ellipses are orientated clockwise to the axis o f the bank-swale system with an angle of 10° in the swales and 25 ° on the flanks of the bank. In the southern part of the bank this angle is only 15 ° . This difference is attributed to the less linear morphology of the bank in that area. Large dunes cover the flanks and summit of the Middelkerke Bank. They have a constant orienta-

18

J. Lanckneus et aL/Marine Geology 121 (1994) 1-21

tion, a height ranging from 0.5 to 5 m and a wavelength from 75 to 150 m. The slopes of both steep and stoss flanks are very slight. Most of the large dunes have an asymmetrical cross-profile. Small and medium dune fields occur on the whole bank and in the adjacent swales. They are 2-D and present a wavelength ranging from 1 to 15 m. They are oriented at an oblique angle to the large dunes with a counterclockwise angle offset of approximately 20 ° which make them perpendicular to the peak tidal currents. Flood and ebb oriented small dunes cover respectively the western and eastern flank of the bank. The boundary line between these 2 groups of features with opposite asymmetry follows more of less the bank's axis and coincides often with the large dune's crest lines. The importance of volumetric variations of the Middelkerke Bank due to seasonal factors was assessed by sequential bathymetric surveying along reference track. Results obtained show that natural changes in the top slice volume of the bank can reach up to 170% (Table 1). The bank's volume

decreases in periods of heavy weather after which the process of sand uppiling acting during long periods of fair weather conditions causes the bank to restore itself. I f the volume of the total bank is taken into consideration, volumetric changes can still range between - 2 1 % and + 2 6 % for one winter-summer period. These figures have to be considered as extreme values and observed volumetric variations are usually much less pronounced. Fig. 13 gives an overview o f the volumetric variations calculated for the total bank volume along reference track rG19 and based on 15 successive recordings in a time span of 2 years; percentages express the degree of volumetric variation in respect to a mean value. The beach opposite to the Middelkerke Bank shows a volumetric evolution characterized by seasonal variations. The relatively large distance, the intermediate topography and the simultaneity in the behaviour do not sustain the idea of a direct sediment exchange but rather that of local storages and remobilizations.

Table 1 Volumetric variations of the Middelkerke Bank along the reference track rG19 expressed in absolute and relative values. All presented variations have to be considered as maximum and are normally much smaller Cause of volumetric variations

Unit volume

Volumes expressed in absolute values (m3/m)

Volumes expressed in relative values (%)

Seasonal processes

Top slice volume

Mean: 554 Winter '92:164 Summer '92:1100

Mean: 100 Winter '92:30 Summer '92:199

Total bank volume

Mean: 5576 Winter '92:4418 Summer '92:7021

Mean: 100 Winter '92:79 Summer '92:126

N 642-

i.n

~

-

,

b~

b~

i.n

b~

i.n

+

+

+

+

+

+

tn

i.n

tn

b~

b~

b~

V o l u m e v a r i a l i o n in %

Fig. 13. Histogram representing the variations of the total bank volume of the Middelkerke Bank along reference track rG19 based on recordings carded out from May 1990 to June 1993. The percentages were calculated with respect to a mean value.

J. Lanckneus et al./Marine Geology 121 (1994) 1-21

The periodic volumetric variations lack behind the climatological seasons. The lack is more important on the Middelkerke Bank than on the opposite beach station.

Acknowledgements The research on the Middelkerke Bank was supported by the EEC in the framework of its MAST I programme (Project RES~CUSED,0025-C). We greatly appreciated the help and interest of its representative, Mr. C. Fragakis. We are grateful for the logistic support provided by the Belgian National Fund for Scientific Research, the Mathematical Model North Sea of the Belgian Ministry of Public Health, the Service of Coastal Harbours of the Flemish Government, the Geological Survey of the Netherlands and the Netherlands Department of Public Works and Water Management. The authors wish to thank the entire crew of the research vessels Belgica and Mitra and of the hydrographic vessel Ter Streep, for their constant help and co-operation during the monitoring surveys. The authors express their gratitude as well to Dr. Stride and Dr. Castaing for critically reading and improving the manuscript.

References Ashley, G.M., 1990. Classification of large-scale subaqueous bedforms: a new look at an old problem. J. Sediment. Petrol., 60: 160-172. Bastin, A., 1974. Regionale sedimentologie en morfologie van de Zuidelijke Noordzee en van bet Schelde Estuarium. Ph.D. Thesis, Kath. Univ. Leuven, Belgium, Fac. Sci., 91 pp. Beck, C., Clabaut, Ph., Dewez, S., Vicaire, O., Chamley, H., Augris, C., Hoslin, R. and Caillot, A., 1991. Sand bodies and sand transport paths at the English Channel-North Sea border: morphology, hydrodynamics and radioactive tracing. In: H. Chamley (Editor), Proc. Symp. Environment of Epicontinental Seas. Oeeanol. Acta, 11: 111-121. Bern6, S., 1991. Architecture et Dynamique des Dunes Tidales. Universit6 des Sciences et Techniques de Lille FlandresArtois. Ph.D. Thesis, 295 pp. (Unpubl.) Bern6, S., Allen, G., Auffret, J.P., Chamley, H., Durand, G. and Weber, O., 1989. Essai de synth6se sur les dunes hydrauliques g6antes tidales actuelles. Bull. Soc. Geol. Fr., 6: 1145-1160.

19

Bern6, S., Trentesaux, A., Stolk, A., Missiaen, T. and De Baptiste, M., 1994. Architecture and long term evolution of a tidal sandbank: the Middelkerke Bank (southern North Sea). Mar. Geol., 121. Bestuur Mijnwezen (Editor), 1993. Effecten op bet marien leefmilieu van de zand- en grindwinningen op bet belgisch continentaal plat. Ann. Mijnen Belgie, 2. Caston, G.F., 1981. Potential gain and loss of sand by some sand banks in the Southern Bight of the North Sea. Mar. Geol., 41: 239-250. Caston, V.N., 1972. Linear sand banks in the southern North Sea. Sedimentology, 18: 63-78. Caston, V.N.D. and Stride, A.H., 1970. Tidal sand movement between some linear sand banks in the North Sea off northeast Norfolk. Mar. Geol., 9: 3842. Ceuleneer, G. and Lauwaert, B., 1987. Les s6diments superficiels de la zone des Vlaamse Banken. Report and maps published by MUMM (Management Unit of the Mathematical Model of the North Sea and the Sebeldt Estuary), Gulledelle 100, 1200 Brussels. Dalrymple, R.W., 1984. Morphology and internal structure of sandwaves in the Bay of Ftmdy. Sedimentology, 31: 365-382. De Moor, G., 1979. Recent beach evolution along the Belgian North Sea coast. Bull. Soc. Belg. G6ol., 1979, 88: 143-157. De Moor, G., 1984. Morfodynamiek en sedimentdynamiek rond de Kwintebank. Brusscl, Ministerie Economische Zaken, Vol. I, II, III, IV, V; 219, 71, 65, 39, 36 pp. Dc Moor, G., 1985. Shelf bank morphology off the Belgian coast. Recent methodological and scientificdevelopments. In: M. van Molle (Editor), Recent Trends in Physical Geography. Liber Amicorum L. Peeters Brussels,VUB Stud. Ser., 20: 47-90. De Moor, G., 1986. Geomorfologisch onderzock op bet Bclglsch kontinentaal plat. Bull. Belg. Vcr. Aardr. Stud. (BEVAS-SOBEG), 55: 133-174. De Moor, G., 1991. The february 1990 storms and their impact on the beach evolution along the Belgian coast. In: M. dc Dapper (Editor), De Aardrijkskundc Spec. Vol., Gent, V.L.A., 1991, 37 pp. De Moor, G., 1992. A quantitative evaluation of erosive and accrctional sections along the Belgian coast in the period 1978-90. Bull. Belg. Ver. Aardr. Stud. (BEVAS-SOBEG), 2: 413-424. De Moor, G. and Lanckneus, J., 1988. Acoustic tclcdctcction of sea-bottom structures in the Southern Bight. Bull. Soc. Belg. G6ol., 97: 199-210. Dc Moor, G. and Lanckncus, J., 1989. Stabilit6 et apports s6dimentaires sur les Banes dc Flandre. Proc. Syrup. Gdologie et Am6nagcrncnt du Territoirc. (Lille, April 26-27, 1989.) Ann. Soc. Geol. Nord, 109: 129-139. Dc Moor, G., Lanckneus, J., Bern6, S., Chamley, H., De Batist,M., Houthuys, R., Stolk, A., Tcrwindt, J.,Trentesaux, A. and Vincent, C., 1993. Relationship between sea floor currents and sediment mobility in the southern North Sea. In: K.G. Barthel, M. Bohie-Carbonell, C. Fragakis and M. Weydert (Editors), Proc. MAST Days Syrup. (March 15-17, 1993.) CEC, Brussels, pp. 193-207.

20

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