Effects Of Nearshore Sand Bank And Associated Channel On Beach Hydrodynamics: Implications For Beach And Shoreline Evolution

Transcription

1 Journal of Coastal Research SI ICS2009 (Proceedings) Portugal ISSN Effects Of Nearshore Sand Bank And Associated Channel On Beach Hydrodynamics: Implications For Beach And Shoreline Evolution A. Héquette, M.H. Ruz, A. Maspataud and V. Sipka Laboratoire d'océanologie et de Géosciences (UMR CNRS 8187), Université du Littoral Côte d Opale, Wimereux, France arnaud.hequette@univ-littoral.fr ABSTRACT HÉQUETTE, A., RUZ, M.H., MASPATAUD, A. and SIPKA, V., Effects of nearshore sand bank and associated channel on beach hydordynamics: implications for beach and shoreline evolution. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), Lisbon, Portugal, ISSN Tidal banks are common in the southern North Sea where they form linear shore-parallel or slightly oblique sand bodies. A 13 day field experiment was conducted in February 2007 on a macrotidal barred sandy beach of northern France, on the southwestern shore of the North Sea, in order to assess the effects of a shallow nearshore sand bank on beach/nearshore hydrodynamics. Two Acoustic Doppler Current Profilers (ADCP) were moored in the surf zone and two electromagnetic current meters were deployed on the middle beach. The instruments were deployed along two shore-perpendicular transects, the first being located onshore of a sand bank extending along an area characterized by shoreline retreat, while the other was positioned in a prograding area of the coastline, about 2 km eastward of the bank edge. Results obtained in the intertidal zone during moderate wind events, showed that significant wave height was similar or even slightly higher behind the bank compared to the other experimental site, indicating that the bank did not significantly enhance wave energy dissipation. Strong flood tidal currents, up to 0.7 m/s, were recorded at both sites during the experiment, but current speed was generally higher behind the bank, presumably because tidal flows are constrained in the channel located between the sand bank and the beach. These results suggest that sand banks do not necessarily protect the coast from the action of incoming waves and may locally favor downcurrent sediment deposition at the coast due to increased sediment transport in nearshore channel. ADITIONAL INDEX WORDS: Tidal sand bank, macrotidal coast, North Sea INTRODUCTION The shoreface and inner shelf of the southern North Sea is characterized by the presence of numerous tidal banks forming linear shore-parallel or slightly oblique sand bodies (LANCKNEUS et al., 1994; TESSIER et al., 1999) Although a large body of literature has been dedicated to the dynamics of tidal sand banks, notably in the North Sea (e.g., DYER and HUNTLEY, 1999; BERNÉ et al., 1994; DELEU et al., 2004; HORRILLO-CARABALLO and REEVE, 2008), only a few studies have been conducted on the influence of nearshore banks on shoreline behaviour (e.g., SHAW et al., 2008) even if it is generally considered that these nearshore sand bodies may have important effects on coastal hydrodynamics and shoreline evolution (MCDONALD and O CONNOR, 1996: CORBAU et al., 1999). In this paper, we present the results of a field experiment conducted on the coast of northern France, on the southwestern shore of the North Sea, in order to assess the effects of a shallow nearshore sand bank on beach and nearshore hydrodynamics. The study was carried out on a barred sandy beach (ridge-and-runnel) east of Dunkirk, such intertidal bartrough morphology being typical of the macrotidal coast of northern France (REICHMÜTH and ANTHONY, 2002). The beach is backed by coastal dunes that are eroding in places (Fig. 1B) in response to high water levels associated with stormy conditions, although aeolian deposition and dune development also occurs at some locations (Fig. 1C). Seaward, a shallow sand bank (Hills Bank) extends along the coast over a distance of about 9 km. The crest of the bank may be exposed at spring low tides, forming a shoal at a distance of about 1400 m from the beach in the study area (Fig. 1). The bank is separated from the beach by a 10 to 15 m deep channel (Fig. 1C), sub-parallel to the coastline, that is sometimes dredged for navigation. Due to macrotidal conditions (mean spring tidal range at Dunkirk: 5.6 m), tidal currents are strong with peak near-surface velocities reaching 1.5 m.s -1 during spring tides in narrow interbank channels. Tidal currents are alternating in the coastal zone, flowing almost parallel to the coastline. Flood currents are oriented towards the east-northeast and ebb currents towards the west-southwest. Measurements in various sectors of the coastal zone show that the speeds of flood currents exceed those of the ebb, resulting in a flood-dominated asymmetry responsible for a net regional sediment transport to the east-northeast (HÉQUETTE et al., 2008). The speed of tidal currents decreases onshore, however, whilst the action of wave oscillatory flows become dominant in the shallower water depths of the upper shoreface and intertidal zone (AUGRIS et al., 1990). Winds mainly come from the southwest and northeast, but the strongest winds mostly originate from west to southwest. Associated with this wind regime is a fetch-limited environment dominated by short period waves from southwest to west originating from the English Channel, followed by waves from the northeast to north generated in the North Sea. Offshore modal significant wave heights are less than 1.5 m, but 59

2 Effects of nearshore sand bank on beach hydrodynamics Figure 1. Location of the study area and instruments deployed during the field experiment (14 to 27 February 2007). A) Nearshore bathymetry profile at sites 1 and 2; B) Photograph of eroding coastal dune at site 1; C) Photograph of incipient dune development on the upper beach at site 2. may episodically exceed heights of 4 m during storms (DELFT HYDRAULIC, 2004). Wave heights are much lower at the coast due to significant refraction and shoaling over the shallow banks and low gradient shoreface of the southern North Sea. METHODS Hydrodynamic measurements were carried out in the coastal zone east of Dunkirk (Fig. 1) during a two-week field experiment in February Two Acoustic Doppler Current Profilers (ADCP) were moored in the nearshore zone in approximately 1.45 and 1.9 m below Hydrographic Datum (HD) and two electromagnetic wave and current meters (Valeport Midas current meters) were deployed on the middle beach at an elevation of about 2.2 m above HD (Fig. 1). All instruments were programmed to measure wave parameters at a frequency of 2 Hz for 512 consecutive seconds (8 minutes 32 seconds burst record duration), every 15 minutes. Spectral analyses of the raw data yielded values of significant wave height (H s ), period and direction. Mean current speed and direction were also recorded by all instruments. The electromagnetic current meters recorded velocity components at 15 cm above the bed during 1 minute every 15 minutes, providing values of mean near-bottom flow speed and direction. The ADCP data collected at a frequency of 2 Hz were used for computing time-averaged residual current velocity and direction over 5 minutes at 1 m above the bed. The instruments were deployed along two shore-perpendicular transects, the first being located onshore of the Hills Bank in an area characterized by shoreline retreat (Site 1, Fig. 1B), while the other was positioned in a prograding area of the coastline (Site 2, Fig. 1C), about 2 km eastward of the bank edge. Because the ADCPs were not deployed at the same water depth, the data obtained from these instruments can not be easily compared as wave height and tidal current velocity decrease with water depth (AUGRIS et al., 1990). The similar elevations of the instruments located on the beach, however, allow comparisons of hydrodynamic data obtained on a coastal stretch sheltered by the bank (Site 1) with those collected in an area that is not directly protected by the bank (Site 2). Unfortunately, one of the current meter deployed on the beach failed during several days and the period of common measurements with the other instrument was limited to three days. Beach and nearshore hydrodynamic measurements were complemented by wave data collected at an offshore wave buoy (Westhinder) in 27 m water depth, approximately 36 km seaward of the Belgian coast (Fig. 1), providing significant wave height every 15 minutes. Hourly wind data (speed and direction) were obtained from the Meteo-France meteorological station of Dunkirk. RESULTS Several wind events occurred during the experiment. Winds essentially came from southwest to northwest with speeds generally ranging from 9 to 10 m s -1. Associated with these wind events, offshore wave heights in excess of 1.5 m were measured on several occasions, reaching about 2.2 m on 26 February (Fig. 2) in response to northwesterly winds that exceeded 11 m s -1. Wave measurements at the shallow nearshore ADCP stations showed a considerable decrease in wave height from the offshore to nearshore stations (Fig. 2), due to refraction and wave energy dissipation across the shelf and shoreface. It is noteworthy that wave heights were quite similar at both nearshore sites, showing that the Hills Bank was not responsible for significant wave energy dissipation during these events. Wave height was nevertheless slightly lower at site 1 (behind the bank) on some occasions (on 21, 24 and 25 February, for example), but because the instrument at this site was in shallower water depth (-1.45 m HD) than the one located in the nearshore zone at site 2 (-1.9 m HD), such decrease in wave height is not necessarily due to the presence of the bank as wave heights are expected to be lower in shallower water depths. 60

3 Héquette et al. Figure 2 Time-series of significant wave height (H s ) measured offshore (Westhinder buoy), in the nearshore zone and on the beach east of Dunkirk between 14 and 27 February 2007 (see Fig. 1 for location of the instruments). Water depths and elevation are relative to Hydrographic Datum. Measurements of wave heights on the beach also showed an important reduction in wave height from the nearshore stations to the middle beach (Fig. 2), revealing very significant energy dissipation across the intertidal zone. Comparison of significant wave heights recorded on the beach at both sites during conditions of moderate wave agitation (offshore H s ranging from about 0.9 to 1.8 m, Fig. 2) indicates, however, that wave height was generally higher at site 1 (Fig. 3A), showing again that the proximity of the bank did not result in more energy dissipation of the incoming waves. Lower wave heights at site 2 may probably be explained by the low gradient, highly dissipative, slopes (tan ) of the nearshore/shoreface (0.0045) and beach (0.012), whereas the somewhat steeper coastal profile at site 1 (nearshore slope: 0.01; beach slope: 0.017) (Fig. 1A) would result in less wave energy loss. Another difference in hydrodynamics between the two middle beach stations concerns the speeds of the mean currents. As Figure 3B shows, the middle beach is strongly dominated by eastwardflowing flood currents that can be temporarily reinforced by shore-parallel winds, like on the afternoon of 24 February when winds with speeds of 8 to 9 m s -1 were blowing from the southwest. Our measurements reveal that mean current speeds were higher at site 1, compared to site 2, whether the mean flow is essentially tidally-induced (i.e., during moderate wind conditions) or intensified by shore-parallel winds. These data suggest that a longshore gradient in mean current velocity develops on the beach during flood, which may be related to the canalization of the tidal flows between the bank and the coast. The action of ebb currents is limited to very short time intervals on this part of the beach (Fig. 3B), which therefore appears to be mainly affected by floodcurrents that can potentially induce eastward-directed sediment transport. The two instruments in the nearshore zone were always submerged and consequently recorded complete tidal cycles of flood and ebb currents. As one would expect, the speeds of the tidal current currents were higher than those measured in the intertidal zone and flood currents were still dominant, with maximum flood current velocities reaching about 0.7 m s -1 during several tides whilst ebb current velocities never exceeded 0.4 m s -1. DISCUSSION Several authors have suggested that nearshore tidal banks may play a significant role on shoreline evolution, notably along the coast of northern France. According to ANTHONY et al. (2006) and CHAVEROT et al. (2008), the formation of an extensive sand flat near Calais, about 30 km west of Dunkirk, would be related to the onshore migration of a prominent nearshore sand bank. Shoreline progradation of more than 150 m during the last decades (CHAVEROT et al., 2008) is probably related to onshore sediment transfer from this bank that is almost completely welded to the beach nowadays. The exact mechanisms that could be responsible for this shoreward movement of sand are not known however, as modeling of shoreface sediment transport in that area suggests longshore rather than onshore transport (HÉQUETTE et al., 2008). In their study of a massive headland-associated nearshore sand bank off the coast of prince Edward Island, Canada, SHAW et al (2008) demonstrate how this bank may have been responsible for sediment deposition and infilling of embayments on the nearby coast. Using detailed multibeam bathymetry data and threedimensional current modeling, they show that sediment is not necessarily supplied from the bank to the coast, but that the bank causes significant disturbance of the coastal hydrodynamics that control sediment transport. In contrast, very high coastal erosion rates have been observed onshore of an elongated headlandassociated sand bank that extends across Wissant Bay, on the French coast of the Dover Straight, which may be partly due to variations in incident wave propagation across the bay caused by changes in bank morphology during the 20 th century (AERNOUTS and HÉQUETTE, 2006). Similar reasoning involving disturbing effects of nearshore tidal banks on coastal hydrodynamics has been applied by several authors to the sand banks of the southern North Sea. Based on wave propagation modeling, CORBAU et al. (1999), suggested that the nearshore sand banks near Dunkirk induce complex deformations of wave propagation that generate alternating patterns of energy concentration and dissipation along the coast. According to these authors, the Hills Bank, significantly protect the coast from wave attack by dissipating the energy of incoming 61

4 Effects of nearshore sand bank on beach hydrodynamics on the upper beach and coastal dunes. During high water level conditions, however, which may be associated with positive surges and/or high astronomical tides, the dissipation of wave energy is lower due to greater water depths over the bank, resulting in less protection for the upper beach and coastal dunes. This is especially the case for short period waves, which are representative of the fetch-limited North Sea, that undergo refraction and shoaling in shallow water depths (compared to long period swells that characterize open seas). An additional factor that can explain why the coastal dunes are not efficiently protected by the bank is the presence of a relatively deep channel between the bank and the beach, probably allowing waves to reform. Wave energy dissipation actually appears very significant across the gently sloping nearshore/shoreface at site 2. Our data suggest that the proximity to the coast of the Hills Bank and associated channel constitutes a major barrier limiting onshore sediment transport, which conversely favors alongshore transport. The channel between the bank and the beach represents a relatively deep trough characterized by high-velocity tidal flows that are likely responsible for longshore sediment transport. The coarse nature of the sediments on the channel floor (CORBAU et al., 1999) attests the strength of tidal currents that winnow the finer-grained sediments. Most of the sand that is driven landward by onshore-directed wave oscillatory flows across the bank is therefore probably transported alongshore in the channel, in response to flood-dominated tidal currents, and can hardly be transported onshore to the beach. Eastward of the bank, however, sand can more likely be transported onshore by waves over the gently sloping shoreface (Fig. 1A), potentially resulting in a higher sediment supply to the beach. As shown in several studies, gently sloping dissipative shoreface profiles increase onshoredirected transport and are commonly associated with foredune accretion (AAGAARD et al., 2004; COOPER and NAVAS, 2004).The observed eastward decrease in mean current velocity on the beach, which should result in downcurrent deposition, is another factor that may contribute to the accretion observed in the eastern sector near the Belgium border (site 2). Figure 3. A) Comparison between significant wave heights measured on the middle beach at site 1 and site 2 from 22 to 25 February 2007; B) Time-series of mean current velocity on the middle beach at sites 1 and 2. waves. They attribute coastal erosion in this sector to the combined action of tidal currents and shore-parallel winds from WSW, but they had no wave or current measurements for supporting this hypothesis. In their study of beach morphology east of Dunkirk, REICHMÜTH and ANTHONY (2002) also considered that the presence of the Hills Bank controls the exposure of the beach to wave energy, which would represent a major factor explaining the variability of the observed intertidal bar morphology. Although we acknowledge that waves can undergo significant refraction and shoaling over the shallow sand banks of the southern North Sea, resulting in energy dissipation, our measurements show that the Hills Bank does not significantly reduce wave energy, at least during moderate energy conditions like the ones encountered during this study. Wave energy dissipation is probably high at low water levels, especially when waves break over the crest of the bank. During such conditions, the bank can certainly protect the beach and may play a role on the morphodynamics of the intertidal zone, notably on the morphological behaviour of the intertidal bars as suggested by REICHMÜTH and ANTHONY (2002), but this would have no effects CONCLUSION Nearshore sand banks may have different effects on coastal hydrodynamics, including the redistribution of wave energy along the coast through wave refraction and disturbance of current patterns, depending on the depth, orientation and distance of the bank to the coast. Our wave measurements on the beach at two different sites along the shore show that the presence of a bank in the nearshore zone does not necessarily result in more wave energy dissipation at the coast, even with offshore wave heights exceeding 2 m. This may be explained by the presence of a relatively deep channel between the bank and the beach in which waves can reform and by a steeper nearshore slope compared to the second experimental site located eastward of the bank. Our current measurements also suggest that the occurrence of a channel close to the coast results in tidal flow canalization, which favours longshore sediment dispersal rather than cross-shore transport. This could limit onshore sediment transport to the beach by shoreward-directed wave oscillatory flows and may explain the apparent sediment deficit and coastal dune erosion observed behind the bank. Conversely, nearshore sediment can probably be more easily driven onshore by waves on the low gradient nearshore slope eastward of the bank where a decrease in longshore current velocity also contributes to sand deposition, thus favouring shoreline progradation. 62

5 Héquette et al. LITERATURE CITED AAGAARD, T., DAVIDSON-ARNOTT, R., GREENWOOD, B. and NIELSEN, J., Sediment supply from shoreface to dunes: linking sediment transport measurements and long-term morphological evolution. Geomorphology, 60, AERNOUTS, D. and HÉQUETTE, A., L évolution du rivage et des petits fonds en Baie de Wissant pendant le XX è siècle, Pas-de-Calais, France. Géomorphologie : relief, processus et environnement, 1, ANTHONY, E.J., VANHÉE, S. and RUZ, M.H., Short-term beach-dune sand budgets on the North Sea coast of France: sand supply from shoreface to dunes and the role of wind and fetch. Geomorphology, 81, AUGRIS, C., CLABAUT, P. and VICAIRE, O., Le domaine marin du Nord-Pas-de-Calais Nature, morphologie et mobilité des fonds. Edition IFREMER Région Nord-Pas-de- Calais., 93 p. BERNÉ, S., TRENTESAUX, A., MISSIAN, T. and DE BATIST, M., Architecture and long term evolution of a tidal sand bank : the Middelkerke Bank (Southern North Sea). Marine Geology, 121, CHAVEROT, S., HÉQUETTE, A. and COHEN, O., Changes in storminess and shoreline evolution along the northern coast of France during the second half of the 20th century. Zeitschrift für Geomorphologie, Suppl. Bd. 52 (3), pp COOPER, J.A.G. and NAVAS, F., Natural bathymetric change as a control on century-scale shoreline behavior. Geology, 32, CORBAU, C., TEISSIER, B. and CHAMLEY, H., Seasonal evolution of shoreface and beach system morphology in a macrotidal environment, Dunkerque area, Northern France. Journal of Coastal Research, 15, DELEU, S., VAN LANCKER, V., VAN DEN EYNDE, D. and MOERKERKE, G., Morphodynamic evolution of the kink of an offshore tidal sandbank: the Westhinder Bank (Southern North Sea). Continental Shelf Research, 24, DELFT HYDRAULICS, Dunkerque Port Development Wave Climate Study. Unpublished report for the Port Autonome de Dunkerque, Dunkirk, France. DYER, K.R. and HUNTLEY, D.A., The origin, classification and modelling of sand banks and ridges. Continental Shelf Research, 19, HÉQUETTE, A., HEMDANE, Y. and ANTHONY, E.J., Sediment transport under wave and current combined flows on a tidedominated shoreface, northern coast of France. Marine Geology, 249, HORRILLO-CARABALLO, J.M. AND REEVE, D.E., Morphodynamic behaviour of a nearshore sand bank system: The Great Yarmouth sandbanks, U.K. Marine Geology, 254, LANCKNEUS, J., DE MOOR, G. and STOLK, A., Environmental setting, morphology and volumetric evolution of the Middelkerke Bank (Southern North Sea). Marine Geology, 121, MACDONALD, N.J. and O CONNOR, B.A., Changes in wave impact on the Flemish coast due to increased mean sea level. Journal of Marine Systems, 7, REICHMÜTH, B. and ANTHONY, E.J., The variability of ridge and runnel beach morphology: Examples from Northern France. Journal of Coastal Research, Special Issue No. 36, pp SHAW, J., DUFFY, G., TAYLOR, R.B., CHASSÉ, J. and FROBEL, D., Role of submarine bank in the long-term evolution of the northeast coast of Prince Edward Island, Canada. Journal of Coastal Research, 24, TESSIER, B., CORBAU, C., CHAMLEY, H., and AUFFRET, J-P., Internal structure of shoreface banks revealed by highresolution seismic reflection in a macrotidal environment (Dunkerque Area, Northern, France). Journal of Coastal Research, 15, ACKNOWLEDGEMENTS This study was partly funded by the French Agence Nationale pour la Recherche (ANR) through the project VULSACO (VULnerability of SAndy COast systems to climatic and anthropic changes) and by European funds (FEDER) through the INTERREG IIIA project Beaches At Risk. Bathymetry data were provided by the French Service Hydrographique et Océanographique de la Marine (SHOM). The authors would like to thank Mr. Hans Pope of the Belgian Agency for Maritimes Services and Coast Division COAST for providing the offshore wave data. Thanks are also due to Denis Marin for drafting of the figures. 63