Storm influences on a tidal sandbank's surface (Middelkerke Bank, southern North Sea)

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Marine Geology 121 (1994) 23-41

Storm influences on a tidal sandbank's surface (Middelkerke Bank, southern North Sea) Rik Houthuys a, Alain Trentesaux b, Peter De Wolf c Eurosense Belfotop N. V., Nervierslaan 54, B-1780 Wemmel, Belgium b Laboratoire de Sddimentologie et Gdodynamique, U.KA. 719 CNRS, Universit~ de Lille 1, F-59655 Villeneuve d'Ascq, Ckdex, France c Coastal Harbours Service, Vrijhavenstraat 3, B-8400 Oostende, Belgium

Received 1 February 1994; revision accepted 22 June 1994

Abstract The results are presented of two types of "pre-storm" and "post-storm" surveys, carried out on the Middelkerke Bank (southern North Sea). The first type of survey was planned to detect detailed morphological changes in two selected areas of the bank totalling 3 km2; for the survey in these shallow waters to be completed in a fast and accurate way, use was made of a special hovercraft-based hydrographic system. The other type of survey concerned grab samplings covering the sandbank in order to study spatial variations in surficial grain size. The pre-storm and post-storm morphological surveys showed that small and medium dunes disappear under stormy conditions. A 5 m westerly displacement of most of the large and very large dunes has been interpreted as the result of bedform migration under normal (fair-weather), locally dominant ebb flow. The lowering of the large dunes' crests by up to 1.2 m is considered an effect of the storms. The accretion of the lower part of the northwestern flank is attributed to the deposition of sand, derived from shallow parts of the sandbank during storms. The grain-size surveys showed that especially the central and northern parts of the sandbank were affected by grain-size changes, the northwestern flank showing a clear coarsening and the landward southeastern flank a fining with respect to the fair-weather situation. It is put forward that the waves approaching from the north cause an extra winnowing on the exposed flank of the bank. On the southeastern flank of the bank, which is better protected against the wave action, fine particles are then deposited.

1. Introduction The morphology and regional setting of the Flemish Banks are described by Lanckneus et al. (1994-his volume). M a n y papers and publications agree on the fact that sandbanks like the Flemish Banks are heavily determined by their setting in a pronounced tidal environment. The Flemish Banks are widely considered as mainly maintained (if not shaped) by tidally induced forces (Stride, 1982). The spring tidal range in the area amounts to nearly 5 m, while peak near-surface ebb and flood 0025-3227/94/$7.00 © 1994 Elsevier Science B.V. All fights reserved SSDI 0025-3227(94)00078-Y

velocities are of the order of 1 m s - 1. The influence of wave-induced forces is relatively unknown. Due to the bank's shallowness, a strong influence of wave action is, however, expected. Water motion and related forces on sediment due to wind waves are extremely complex to describe. In this field, observations from controlled experiments together with time-averaged wave and current measurements are employed in an attempt to make predictions or model the impact on sediment transport and bed morphology. In wave-related sedimentology especially, there is a sore need for reliable field

24

R. Houthuys et al./Marine Geology 121 (1994) 23-41

measurements. Wave action can be directly observed by either remote sensing techniques or sensors mounted on a fixed, bottom-attached frame, pile or platform. The present paper presents the results of an indirect approach, in which the effect of storms on the morphology and surficial sediment grain size was assessed through "prestorm" and "post-storm" surveys. The work was part of the EC co-sponsored MAST ("Marine Science and Technology") project RESECUSED ("Relationship between Sea Floor Currents and Sediment Mobility in the southern North Sea"). The detailed hovercraft bathymetric surveys were supported by the Ministry of the Flemish Community, Waterways Infrastructure and Marine Affairs Administration, Coastal Harbours Service (Oostende, Belgium).

2. Storm influence on the Middelkerke Bank's morphology

2.1. Setup of the measurements In order to assess the influence of a storm period on the detailed sandbank morphology, it was planned to perform a pre-storm and a post-storm bathymetric survey on part of the project area. In view of the possibly small morphological response, the utmost precision in positioning and depth measurements was required for the bathymetric surveys in very shallow water environments. These requirements were entirely met by using the hovercraft-based hydrographic BEASAC ® system1 . This system, designed and operated by the private company Eurosense (Wemmel, Belgium), has been regularly used since 1985 for the production of official nautical charts of very dynamic harbour access channels and navigation lanes (Maes, 1985; Kerckaert et al., 1989) (Fig. 1). The advantages of employing a hovercraft platform for echo sounding are obvious: fast operational speed, ability to measure in very shallow water areas, high manoeuvrability and stability. The problems that had to be overcome during the ~BEASAC = Belfotop Eurosense Acoustic Cushion platform.

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development of the system mainly relate to the high rate of data acquisition, which is inherent to the high operational speed, the improvement and integration of positioning data, and the design of a system to compensate for the movements of the craft (De Putter et al., 1992). The BEASAC ® III hovercraft, used in the RESECUSED project, combined and integrated the use of Trisponder (Del Norte-Euless, Texas, USA) and Toran (Sercel-Nantes, France) for positioning to eliminate the weaknesses of both systems used separately. The circular Trisponder system is dependent on line of sight conditions. The hyperbolic Toran system has a lane ambiguity problem and lane slips may occur. When using both the Trisponder and the Toran 'Lines of Position' (LOP's), it is possible to make a position estimate, also in cases where not all data of both systems are available, or even if some data are corrupted. Depths were measured using a Deso 20 (Krupp Atlas Elektronik GmbH) echo sounder. The 'raw' depth data had to be corrected for heave, roll, pitch, air cushion height or draught of the transducer, and tidal influences. As the echosounder transducer is rigidly attached to the craft, the movements of the platform produce errors on the measured depth values. A good knowledge of the craft's movements allows one to compensate for these errors. Special sensors measure the hovercraft's relative movements above the sea surface. The use of six independent ultrasonic sensors and a vertical gyro provides information redundancy and reliability. The advantage of using a hovercraft-based bathymetric system for morphological studies in highly dynamic and shallow environments is evident. Also the availability of a survey vessel at the required moment is important, especially in the present study on the impact of a storm period. Eurosense guaranteed this essential service. The soundings were to take place along closely spaced survey tracks in two small test areas of 1 x 1.5 km 2 each. The test areas were selected after an introductory morphological study. The areas are shaded in Fig. 2. The main arguments for choosing these particular areas were the following: - both areas represent elevations that rank among the shallowest of Middelkerke Bank. It was

R. Houthuys et al./Marine Geology 121 (1994) 23-41

25

Fig. 1. Aerial view of the BEASAC® III hovercraft during bathymetric survey. expected that storm influences on bed morphology would be more intense in shallow areas; both areas are sufficiently large to show a differentiation with respect to the degree of seaward exposure (steeper N W flank, level top zone, gentle SE flank); and - Area 1 shows some large, widely spaced compound hydraulic dunes, whereas Area 2 displays numerous and more closely spaced compound dunes 2 . The survey tracks were planned with spacings of 25 m. The lines are oriented N W - S E , parallel to the 1.5 km long sides of the test areas and perpendicular to the Middelkerke Bank's long axis. A basic problem in this setup is the weather forecast and storm prediction. Ideally, a survey should be carried out immediately before a storm. It was decided, however, not to risk waiting too long lest conditions at the last moment would prove too rough to actually carry out the prestorm survey. Therefore, the pre-storm survey was carried out after several weeks of quiet, fair weather, the underlying assumption being that the bed morphology adapts completely to the fairweather tidal flows when no important surface waves, and wind or wave-driven currents are pre-

2Bedform terminologyaccording to Ashley (1990).

sent. The post-storm survey was to take place immediately after a storm or a stormy period, as soon as conditions allowed. As such, the following bathymetric surveys have been carried out: (1) A first survey of the two selected representative areas on 2 September 1991. This date was preceded by several weeks of quiet, fair weather. In fact, during all of the months of July and August 1991, no peak wind speed velocities in excess of 20 m s-1 were recorded in the nearby coastal town of Oostende; (2) A second survey on 27 November 1991, after successive storms had crossed the southern North Sea in mid-October and especially in the first half of November. The survey was carried out on the first day that conditions allowed.

2.2. Data processing and visualization o f results

In the cross-profiles (Figs. 3 and 4), use has been made of all of the originally stored data, typically 1500 points per 1.5km long line. By doing this, also some of the smaller bedforms show on the cross-profiles. In the other visualizations and calculations, use has been made o f digital terrain models.

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R. Houthuys et al./Marine Geology 121 (1994) 23-41

A digital terrain model (DTM) is a numerical description of the terrain elevation, such as is recorded at a given moment in time. It is essentially an enumeration of height (or depth) values, geographically arranged in a rectangular or square grid. The unit distance between two adjacent data points determines the resolution of the terrain coverage. The unit grid distance should neither be too small nor too large. If the unit distance is very small compared to the mean spacing of the original data, nonexistent terrain elements might be introduced as a result of the interpolation procedure used. Often, such elements show directional relationship with the orientation of the grid. If the unit distance is excessively large, obvious loss of information will occur. Once a digital terrain model has been established, computer techniques allow the construction of contour line maps, pseudo 3-D views, volume calculations, and so on. A DTM proves to be an essential aid in quantitative morphological monitoring. As such, in the present study, 4 DTMs have been established, i.e. a DTM has been made for each of the pre- and post-storm surveys of Area 1 and Area 2. Special algorithms exist to calculate a DTM out of a set of height values. They work well when data are evenly distributed over the terrain and when no sharp breaks in the topography exist. Neither of these conditions is fulfilled for the Middelkerke Bank data sets. The first attempts to make visualizations based on a normally calculated DTM were a failure. Echo sounder surveys are commonly carried out along parallel lines. In planning the survey, the lines are oriented so that they are perpendicular to the major slopes. The result of this survey technique is that data are concentrated on the survey lines while in between the lines there is no information. Like most tidal current ridges, the Middelkerke Bank shows a superposition of large bedforms that strike at oblique angles with the large-scale bank axis (Lanckneus et al., 1994-this volume). As such, crest points of large bedforms appear at different distances in the adjacent survey lines. A person who is somewhat familiar to underwater morphology is capable of making the mor-

29

phological link between adjacent data lines. Manual drawing of depth contours is based on this skill. As such, new information in the form of depth contour lines can be added to the original data set; this process is based on the hydrographer's experience and knowledge of the bed morphology. For the final calculation of the Middelkerke Bank test area DTMs, use was made of some additional depth contours to extend the original data set. As to the second condition, sharp breaks in the Middelkerke Bank topography are formed by the large dunes' crestlines. Common DTM calculation procedures tend to smooth sharp crestlines. However, in the procedure used for the present study, the position and height of the large dunes' crestlines such as derived by manual interpolation from the original data set, have been imposed to persist throughout the calculation. Using the pre- and post-storm DTM of each area, a height difference DTM was then established for Area 1 and Area 2. A height difference DTM contains in each of its grid points the height difference between the corresponding grid points of the two initial DTMs. Negative height differences correspond to a decrease in height (erosion) between the pre-storm and the post-storm surveys. From the height difference DTM isoline maps or coloured height difference maps can be established. A black and white reproduction of one such height difference map is shown in Fig. 5. 2.3. Morphological description of the test areas at the pre-storm survey

The morphological situation at the pre-storm survey, performed on 2 September 1991, can be considered to represent the sandbank's morphology in dynamic equilibrium with predominant tidal flow forces. Weather and wave conditions had, indeed, been mild during the preceding months. The steepest overall slopes are found at Middelkerke Bank's NW flank, which is exposed to the sea and the flood currents. Slopes steeper than 3° (5.2%) are rare. The SE flank has an average overall slope of no more than 0.5 ° (1%). Small dunes (often called "megaripples") occur with a height of less than 20 cm on the lower part

R. Houthuys et aL/Marine Geology 121 (1994) 23-41

30

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of the steeper slopes, whereas their height may reach to 50 cm on the upper part of the N W flank where slopes are milder.

Large compound dunes ("megaripple-covered sandwaves") with heights of 1 to 3 m occur in the top area and on the mild SE flank. They are less

R~Houthuyset al./Marine Geology121 (1994)23-41 abundant in the shallower Area 1, which contains the shallowest point of the Middelkerke Bank (-4.3 m). The large dunes' steeper slopes mostly face the West. They strike across the sandbank at oblique angles, may bifurcate, and are often laterally traceable over several hundreds of metres. The large dunes' spacing varies from some tens to over 100 m. With the exception of their crest area, which is often clearly marked, the large dunes' slopes are relatively mild.

2.4. Description of morphological changes The morphological changes in Area 1 and Area 2 are described by comparing the results of the pre-storm survey, performed on 2 September 1991, to those of the post-storm survey, carried out on 27 November 1991. The description is based on quantified, processed data like differential height maps (of which Fig. 5 is an example) and volume differences. Compared to the sandbank scale, morphological changes are modest. Changes are however significant and consistent of the following points, which apply equally to Area 1 and Area 2: the shape of the NW flank is smoother in the post-storm survey; small dunes can no longer be recognized; this change especially holds true for the upper parts of the NW flank (Figs. 3 and 4); - the large compound dunes have a smoother profile at the post-storm survey. Slopes are often reduced. Sharp "peaks" no longer occur at the top of the large dunes; the large compound dunes have undergone a height reduction. The amount of the lowering is 0.3-1.2 m in the sandbank crest area, 0.2-0.8 m in the upper reaches of the SE flank, and 0.0-0.5 m in the lower parts of the SE flank. Thus the height reduction rate decreases with depth, but not in a significant way. Remarkably enough there is no difference in large-dune height reduction between Area 1 and Area 2, although the first area is distinctly shallower; the large compound dunes' crestlines shifted 5 m (locally up to 8 m) to the west; only in the northwesternmost part of the sandbank top area, in a very limited area, some translations to the east occurred. In the amount of the overall west-

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31

ward 5 m shift, there is no difference between the lower and the upper parts of the SE flank. The height differences between survey 2 and survey 1 of Area 1 are presented in the "Middelkerke Bank, Area 1" map (reproduced in black and white in Fig. 5). The largest area corresponds to height differences between + 0.25 m and -0.25 m. Due to the inaccuracy of absolute depth measurements, as explained above, such height differences are considered to be insignificant. The significant height changes are restricted to two distinct zones. One of these is the NW flank, where an areal increase in height of no more than 25 to 30 cm occurs. The accretion is especially clear at the base of the steepest slope segment. The other zone is the large dunes area. Here narrow bands of erosion of more than 50 cm are found at the 2 September 1991 position of the large dunes' crestlines. Accretion occurred systematically at the west side of the survey 1 crestlines and often exceeds 50 cm. A similar pattern is found in Area 2, where a greater number of large dunes is present. The volume differences have been calculated in both areas separately for the NW flank, the bank's top zone, the upper SE flank, and the lower SE flank. The volume differences are of the order of some thousands of cubic metres. When the extent of the areas is taken into account, it appears that no significant volume differences occurred. The overall change in mean height, for both areas, is a small increase of 2.8 cm. This figure is insignificant compared to the absolute depth measurement accuracy, which is of the order of 30 cm. As in the case of the height difference maps, some differentiation was found in the volume differences according to the sandbank's morphology. In Area 1 as well as in Area 2, the NW flank which represents less than 1/3 of the area, accreted with over 30,000 m 3 whereas the top and SE flank show a total increase of less than 10,000 m 3.

2.5. Interpretation of results and discussion It should first be noted that the morphological changes, described above, reflect the resultant situation of different processes that took place within the 86 days' period from 2 September to 27 November 1991. In this section, a description

32

R. Houthuys et al./Marine Geology 121 (1994) 23-41

is first given of the meteorological and hydrodynamic conditions that occurred during the 86 days' period. This description is then used to propose an interpretation for the observed morphological changes. The meteorological and hydrodynamic data were retrieved from the Hydro Meteo System (HMS), owned and operated by the Coastal Harbours Service, Oostende. They included: - wind velocity and direction, observed at the measuring pile "MOW5". This permanent station is situated at an offshore location near Zeebrugge harbour, at about 10 km from the shore and 30 km from Middelkerke Bank; - water level, recorded at the Nieuwpoort harbour tide gauge, situated at about 17 km from Middelkerke Bank; - significant wave height and wave period, measured by a waverider buoy near Westhinder Bank. This offshore station is situated at 32 km from the shore and about 22 km from Middelkerke Bank. The sensors were selected to best represent conditions at Middelkerke Bank. Unfortunately, no permanent sensor was present in the study area itself. Also, no flow current data were available at any significant location for the 86 days' period under consideration. The readings of wind speed and wind direction were averaged per calendar day. Also, daily peak wind speed values were determined. The data are represented in Fig. 6. In the period from 2 September to 15 October 1991, relatively mild wind speeds prevailed. From 16 to 20 October 1991, a first short stormy period occurred. Peak wind speeds slightly exceeded 40 m s-X. Daily mean velocities were in the order of 18 to 20 m s - 1 , which classifies the wind speed as gale to strong gale in the Beaufort scale (Open University Oceanography Course Team, 1991). Winds blew from the northwest. After 9 mild days, a series of depressions crossed the southern North Sea from 1 November to 25 November 1991. Daily average wind speeds often reached 12 to 16 m s -1 while peak speeds were of the order o f 35 m s -1. Mean wind directions first veered from SW to W and NW, and afterwards backed to SW. Wind speed in the Beaufort scale was near-gale to gale.

Though the second stormy period did not have the force of the first, the wind action continued for a significantly longer time. Observed high and low water levels were obtained from the Nieuwpoort tide readings. They represented clearly the fortnightly spring-neap cycle. The tidal range is 5 m at spring tide while the neap tidal range is less than 3 m. Survey 1 as well as survey 2 were performed in the stage of falling tidal range after springs. There was a clear influence of the 16 to 20 October storm on water level. Low waters were as much as 1 m above normal, probably due to wind setup. In the second, longer stormy period, there was an influence on tides in particular from 1 November to 12 November 1991. The wave action is described in Fig. 7 using the parameters of significant wave height (Hs) and wave period (T). For the calculation of Hs, only the one-third highest waves are taken into account. The wave period is determined as the mean duration of all waves within a measuring cycle of 15 minutes. Out of the set of H~ and T values, daily average and maximum values have been calculated and represented in Fig. 7. The transmission of data from the Westhinder wavebuoy unfortunately failed from 12 November 1991 onwards. In both stormy intervals, daily mean Hs values of between 2.0 and 2.5 m occurred. Maximum Hs values almost reached 4 m. It is seen that daily mean wave periods increased from about 4 s to about 5 s during the storms. It is thought that the conditions between 15 October and 25 November 1991 are representative of a normal southern North Sea autumn storm period. Conditions were certainly not extreme. One of the most severe storms to cross the southern North Sea these last few years occurred during the night of 28 February to 1 March 1990. The extreme value of H~ recorded at Westhinder Bank was then 5.1 m, while mean wind speeds of up to 33 m s-1 (violent storm to hurricane) were observed at the MOW5 pile (source: Coastal Harbours Service, Oostende). The morphological changes described in the previous section are interpreted here on three scales: that of the small to medium bedforms (dune wavelength range 1-10 m), that of the large to very large bedforms (dune wavelength range

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R. Houthuys et al./Marine Geology 121 (1994) 23-41

25-200 m), and finally the scale of Middelkerke Bank seen as a "giant" bedform.

(a) Small to medium bedforms The excellent BEASAC ® hydrographic recordings allow us to recognize small to medium dunes down to dune heights below 15 cm and lengths smaller than 10 m. The detailed cross-profiles of Figs. 3 and 4 show that, (i) after the stormy period, the number of small to medium dunes was drastically reduced, and (ii) in the limited number of small to medium dunes present at the poststorm survey, dune heights remained below 25 cm approximately, whereas in the first survey dune heights over 50 cm were observed. Within Area 1 and Area 2, the conclusions hold true for all major morphological parts of Middelkerke Bank: the NW flank, the top area and the SE flank. The few small dunes observed at the post-storm survey without any exception occurred near the crests of large dunes; this is particularly clear in profile G - H in Fig. 4. Small to medium dunes ("megaripples") are considered as a bedform constructed under relatively strong steady flow conditions (Guy et al., 1966, in Reineck and Singh, 1973; Allen, 1968; Harms et al., 1982). They are extremely frequent in tidal environments with large supplies of sandsized clastic sediments (Belderson et al., 1972, 1982). It was demonstrated in the tidal channel environment that small and medium dunes develop and migrate under the peak flow stages of the tidal cycle (Boersma and Terwindt, 1981; Kohsiek and Terwindt, 1981; De Mowbray and Visser, 1984). The observation of numerous small to medium dunes at the pre-storm survey corresponds to the fact that these bedforms are built and migrate under normal tidal flow conditions. The present study clearly shows that these bedforms had disappeared at the end of the observed stormy conditions. The strong oscillatory currents associated with the surface waves probably prevented the development of dunes or wiped out the existing dunes. Possibly the sediment near the dunes' crests is taken away and partly deposited in the troughs. It can be expected that fair-weather tidal flows will reconstruct the small dunes. The presence at the post-storm survey of some small dunes near

35

the large dunes' crests is in agreement with the observation that, under normal flow conditions, shear stresses at the large dunes' crests are higher than in their troughs (Terwindt, 1971; Houthuys, 1990). Under survey l's fair-weather conditions, small bedforms are higher near the large dunes' crests than in the large dunes' troughs.

(b) Large to very large dunes Two main morphological changes characterize the large to very large dunes between survey 1 and survey 2: (i) a shift in the crestline's position mostly 5 m to the west, and (ii) a lowering of the bedform height by up to 1.2 m; the degree of lowering is greater in the sandbank's shallower parts than on the lower SE flank. The 5 m displacement is interpreted as the result of bedform migration under (locally dominant) ebb flow conditions. This movement is in agreement with the bedform's asymmetry. Similar conclusions were found for large, asymmetrical dunes in the Channel (Langhorne, 1982; Bern6 et al., 1988); however, large, nearly symmetric dunes in the southern North Sea off Holland appear not to migrate (Terwindt, 1971; Smith, 1988). The large dunes' migration under normal tidal conditions could predominantly have occurred in the fair-weather parts of the 86-day period. Stages with increased sand transport when tidal currents were modified by storm wave oscillations may have occurred; however, the present study does not allow to differentiate such processes. The lowering of the large dunes' crests is considered an effect of the storms. This hypothesis is supported by the relation between the amount of lowering and the water depth. Water motions due to surface waves are indeed more intense in shallow water. There was no significant difference in large dunes' lowering between Area 1 and Area 2, although Area 1 is shallower than Area 2. It is therefore suggested that exposure to wave attack is at least as significant a factor as depth. The observation of large dunes' lowering confirms earlier observations by Terwindt (1971) and Langhorne (1982).

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R. Houthuys et al./Marine Geology 121 (1994) 23 41

( c) Sandbank flanks and top Two main morphological observations relate to a scale exceeding that of the dunes: (i) accretion of the NW flank, and (ii) no net volume difference on the sandbank's top and SE flank. The amount of accretion of the NW flank falls close to the detection limit (see detailed crossprofiles of Figs. 3 and 4). Yet, as the phenomenon has been systematically observed in both test areas and as the accretion in some patches exceeded the measurement inaccuracy (differential height maps), it is believed to reflect a real morphological change. Based on an analysis of undisturbed bottom samples, accretion of the bank's surrounding channels and its NW flank as a result of storms was more or less expected for tidal sandbanks in the same region, while erosion of the NW flank was seen as the morphological response to fair-weather tidal flow conditions (Houthuys, 1990). In the same line of reasoning, the present accretion of the NW flank could be attributed to the deposition of sand, derived from shallow parts of the sandbank during storms. The observed storms in the present study were generated by westerly winds. The larger waves must have migrated in easterly, shoreward directions. The location of an accretion on the seaward NW flank is therefore somewhat surprising. Also somewhat surprising is the "zero" sand volume balance. The observed zero balance might either be due to small net amounts of displaced sediment or to the result of a partial period of bank accretion by fair-weather processes and a partial period of net sediment loss during storm. Though such a view of "dynamic equilibrium" is supported by longer-term observations (Van Cauwenberghe, 1971; De Moor, 1985, 1986), the elements of the present study are insufficient to verify the supposition.

3. Storm influence on the Middeikerke Bank's surficial sediments

3.1. Background and data Two surface sediment surveys of Middelkerke Bank were made, the first one on 16 May 1990

and the second on 24 April 1991. Each time 85 samples have been taken along five longitudinal lines spaced 350 m apart, with a spacing of 750 m for the 17 samples along the lines. The results of the first survey are presented by Trentesaux et al. (1994-this volume). Basically the sediment distribution follows the morphology of the study area. The coarsest sediments are located in the shallowest sectors, near the crest of the bank, whereas finer sands are located in the deepest parts i.e. in the two adjacent channels. This distribution follows the general pattern observed on the majority of the Flemish banks (Lanckneus, 1989). However, the sediment distribution corresponding to the second survey was less "systematic" than the first one. Several factors can explain these differences: positioning inaccuracy, sampling method, hydrodynamics. For both surveys, the same sampling method (Van Veen grab), grain-size analysis technique and granulometric parameters calculation (Folk and Ward, 1957) have been used. The accuracy of the positioning systems which have been used (DECCA and SYLEDIS) (Van Cauwenberghe and Denduyver, 1993), is far better than the grid size. Thus, hydrodynamic factors should be invoked to explain the grain-size differences. Although hydrodynamic forces due to the governing tidal currents in the Middelkerke Bank area can move all grain sizes present (Van Rijn, 1989; Stolk, 1993), it is assumed that tidal current forces only are not the major factor to explain the observed changes in the areal granulometric pattern. The completion of a single sampling survey indeed takes a significant part of a 13-hour tidal cycle, so that possible grain-size variations due to the variation of the tidal flow will occur within each of the sampling surveys and cannot be studied by comparing the results of two successive surveys. Significant differences between both surveys nevertheless occurred. It was thought, therefore, that in a first stage the non-tidal forces should be examined. Wave-induced water motions have been reported to significantly influence near-bed water velocities observed in the area (Stolk, 1993). In order to verify whether waves could have played a role, the meteorological conditions recorded at Dunkerque harbour, located 40 km southwest of

K Houthuyset al./Marine Geology121 (1994) 23-41 the study area, of the 15 day period preceding each survey have been compared.

conditions on the southern North Sea. Most winds came from north or northwest with velocities often higher than 5 m s -1 and up to 21 m s -1 (Fig. 8b). These wind conditions were rougher: force 3-5 (up to a strong breeze) on the Beaufort scale.

3.2. Comparison of meteorological conditions The 15 days preceding the first survey (16 May 1990) were characterized by anticyclonic conditions with moderate winds: the mean speed was less than 10 m s -1 and the winds blew from the north-northeast and the southwest (Fig. 8a), especially the 11 days before the survey. These wind conditions of force 2-3 (light to gentle breeze) on the ten steps Beaufort scale were associated with a low sea state. The 15 days preceding the second campaign (24 April 1991) were marked by low pressure

3.3. Grain-size differences and discussion The granulometric differences between the two surveys are expressed in Fig. 9. They have been calculated in ~b units to facilitate a quantitative comparison. The difference values range from - 1 . 5 6 to 1.17 ~b. They correspond to variations from - 5 6 7 to +463 #in which points to a high diversity of sediment types (from very fine to very

N Wind velocity per wind sector

Period : 1-16 May 1990. Values in m.s-1, circle : 20 m.s-l. W-

-E

~-q Averagevalues Peak values

N

Wind velocity per wind sector

Period : 8-24 April 1991. Values in m.s-t, circle : 20 m.sq. W-

37

-E

[-""] Averagevalues Peak values

Fig. 8. Comparison of wind velocityper wind sector.

1L Houthuys et al./Marine Geology 121 (1994) 23-41

38

Differences of the mean grain-size in April 1991 relative to May 1990. Depths bellow MLLWS in metres. el, u n i t s . . : samples

Coa

r

°20

-°' "0''

:!!+.

-o

,o--

\ N 51°16 '

~o ,q.

g~ 5000 m Fig. 9. Mean grain-size differences between April 1991 and May 1990.

coarse sand) and important changes in sediment type. Three different situations are encountered: (1) The mean grain-size values observed in May 1990 and April 1991 present differences lower than 0.1 ~b.This is especially the case for the fine-grained sediments located in the southwestern corner of the study area, as well as for a few other locations. Because of the uncertainty in the measurement of the mean, these granulometric variations should be interpreted as no change. (2) The mean grain-size values decrease significantly from the first to the second survey (more than 0.1 ~). This corresponds to a coarsening of the sediments between 1990 and 1991. These changes concern principally the sediments located on the steeper northwestern flank of the bank, on the northern part of the bank and in some more

or less isolated other localities. These differences can be explained by a winnowing of the finest particles. Nevertheless, it should be noted that, apart from the northwest flank, the most obvious differences concern dune field areas, which suggests there could be grain-size differences determined by the sample location on either the lee side or the stoss side of the dunes. (3) The mean grain-size values increase significantly (more than 0.1 ~). This corresponds to locations where the average grain size diminished from the first to the second sampling survey. The sediments affected by this difference are essentially located on the southeastern part of the bank. This suggests abundant fine particles have been deposited in this area before the second survey. It is thought that the observed differences in

IL Houthuys et al./Marine Geology 121 (1994) 23--41

grain size can be attributed to the different hydrodynamic conditions that characterized the periods immediately preceding both sampling surveys. After a sufficient time of fair-weather conditions, the surficial grain-size distribution reaches an equilibrium state which is clearly tied to the bathymetry. This equilibrium corresponds to the tidal currents' action. Opposed to this, stormy periods cause an important "disequilibrium". That wave action has an important influence on sediment transport in the relatively shallow southern North Sea is a well-established fact. Initiation of motion under waves-only conditions of typical Middelkerke Bank sand (median grain-size range of 250-400/~m) at typical wave periods (4-6 s) would occur at near-bed orbital velocities of 0.15-0.25 m s -1 (literature review in Van Rijn, 1989). Such velocities are already attained in 10 m water depth by 0.25-0.75 m high waves and in 20 m water depth by 1.5-2.5 m high waves. Such waves are very common in the southern North Sea. It should be noted, however, that the number of direct field observations to support the thresholds cited are still very limited. When the waves approach from the north, an extra winnowing is supposed to affect the exposed flank of the bank where sediments became coarser and the mean grain-size decreased. On the southeastern flank of the bank, which is better protected against the wave action, fine particles were then deposited. The degree of possible cross-bank transport in this process is not revealed by the present observations.

4.

Summary

and

conclusions

In this paper the results are presented of two types of "pre-storm" and "post-storm" surveys, carried out on the Middelkerke Bank (southern North Sea). The first type of survey was planned to detect detailed morphological changes in two selected areas of the bank totalling 3 km2; for the survey in these shallow waters to be completed in a fast and accurate way, use was made of a special hovercraft-based hydrographic system. The other type of survey concerned grab sampling of the

39

sandbank in order to study spatial variations in surficial grain size. The hovercraft surveys have been performed on 2 September 1991 and 27 November 1991, respectively. The quantified comparison of both surveys made use of digital terrain models (DTM) which have been established in a special way: use was made of some additional depth contours to extend the original data set, and also, the position and height of the large dunes' crestlines such as derived by manual interpolation from the original data set, were imposed to persist throughout the DTM calculation. The main morphological changes between the pre-storm and post-storm surveys of both selected areas were: a more or less evenly distributed increase in height on the NW flank, which locally exceeded 25 crn; disappearance of the small dunes on the NW flank; most of the large hydraulic dunes' crestlines moved 5 m westward; - the large dunes' height lowered up to 1.2 m; - the amount of their lowering decreased with increasing depth along the SE flank; neither in Area 1 nor in Area 2 was there a significant net change in bank volume between the two surveys. - there was no important difference in morphological response between Area 1 and Area 2. These morphological changes reflect the resultant situation of different processes that took place within the 86 days' period from 2 September to 27 November 1991. Within this period, the conditions between 15 October and 25 November 1991 were representative of a normal southern North Sea autumn storm period, but conditions were certainly not extreme. The observed morphological changes have been interpreted on three scale levels. It is shown in the present study that small and medium dunes disappear under stormy conditions. Probably, the strong oscillatory currents associated with the surface waves prevented the development of dunes or wiped out the existing dunes. All large dunes present in the surveyed areas have been affected by this process in more or less the same way,

40

R. Houthuys et al./Marine Geology 121 (1994) 23-41

regardless of their depth below mean low low water springs, which ranged from 5 to 15 m. The westerly displacement of most of the large and very large dunes has been interpreted as the result of bedform migration under normal (fair-weather), locally dominant ebb flow. The lowering of the large dunes' crests by up to 1.2 m is considered an effect of the storms. The accretion of the NW flank is attributed to the deposition of sand, derived from shallow parts of the sandbank during storms. As the larger waves must have migrated in easterly, shoreward directions, the location of accretion at the seaward NW flank is somewhat surprising. Most of the surficial grain-size variations observed between two surveys have also been ascribed to the influence of waves. The distribution of sediments, which is clearly related to the sandbank bathymetry after a sufficient time of fairweather conditions, showed a much less regular pattern at the second survey, after some rough weather crossed the southern North Sea. Especially the central and northern parts of the sandbank were affected by grain-size changes, the NW flank showing a clear coarsening and the landward SE flank a fining with respect to the fair-weather situation. It is put forward that the waves approaching from the north cause an extra winnowing on the exposed flank of the bank. On the southeastern flank of the bank, which is better protected against the wave action, fine particles are then deposited. From the present observations, no statements can be made about the degree of possible cross-bank transport. The present study clearly shows the outstanding quality of BEASAC~s bathymetric soundings for use in morphological process studies and the importance of carrying out grain-size surveys in relation with acquisition of hydro-meteorological data. Future research in the same interesting and well-documented area should strive to further single out the action of the different morphological and sedimentary agents. A few surveys in a period of fair weather could more clearly and quantifiably demonstrate the morphological action of the tidal currents only. Even variations in function of the spring-tide cycle could be studied.

Acknowledgements The work presented in this paper was part of the EC co-sponsored MAST project 0025-C RESECUSED. The Ministry of the Flemish Community, Waterways Infrastructure and Marine Affairs Administration, Coastal Harbours Service (Oostende, Belgium), supported the detailed hydrographic hovercraft surveys and supplied the hydro-meteorological data used for the interpretation of the morphological results. The crews of the R.V. Navicula and Belgica helped for the onboard work. A. Trentesaux performed this work thanks to a fellowship provided by both IFREMER and Nord-Pas de Calais Council. Dr. P. Castaing and dr. A.H. Stride are gratefully acknowledged for their fruitful comments and suggestions.

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