Identifying Forcing Conditions Responsible for Foredune Erosion on the Northern Coast of France

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Journal of Coastal Research SI 56 356-360 ICS2009 (Proceedings) Portugal ISSN 0749-0258 Identifying Forcing Conditions Responsible for Foredune Erosion on the Northern Coast of France M-H. Ruz, A.

and AASPATAUD, A., 2009. Identifying forcing conditions responsible for foredune erosion on the northern coast of France.

1 Journal of Coastal Research SI ICS2009 (Proceedings) Portugal ISSN Identifying Forcing Conditions Responsible for Foredune Erosion on the Northern Coast of France M-H. Ruz, A. Héquette and A. Maspataud Laboratoire d'océanologie et de Géosciences (UMR CNRS 8187) Université du Littoral, Côte d Opale, Wimereux, France ruz@univ-littoral.fr ABSTRACT RUZ, M-H., HEQUETTE, A. and AASPATAUD, A., Identifying forcing conditions responsible for foredune erosion on the northern coast of France. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), Lisbon, Portugal, ISSN Along the southwestern coast of the North Sea a large proportion of the Flemish coastal plain consists of densely populated reclaimed land, much of which lying below mean sea level. This sandy coast is exposed to fetchlimited, relatively low-energy waves punctuated by storm activity and experiences tidal range of 5.6 m at spring tides. A number of recent studies suggested medium term (10 years) gross stability of the beach-dune system. This stability was related to moderate wind regime and efficient dune management practices. In March 2007, a storm event resulted in major foredune retreat. This episodic erosive event was induced by moderate direct onshore winds blowing during more than 48 hours associated with a spring tide. Our results show that along this macrotidal coast, erosive events are not necessarily associated with strong winds. Wind direction and duration combined with a spring tide appear to hold the key to understanding the relative importance of processes controlling medium term foredune evolution. ADITIONAL INDEX WORDS: coastal dunes, coastal erosion, southern North Sea INTRODUCTION Along sandy beaches, foredunes are usually defined as shoreparallel dune ridges formed on the backshore by aeolian sand deposition within vegetation (HESP, 2002). The importance of foredunes is well recognised (CARTER, 1988; ARENS et al., 2001). They can delay coastal retreat and protect low-lying backshore areas against marine invasion as they act as a buffer to extreme waves and wind (PSUTY, 1988; PYE, 1991; SHERMAN and BAUER, 1993). This is particularly the case along the southwestern coast of the North Sea where a large proportion of the Flemish coastal plain consists of densely populated reclaimed land, much of which lying below mean sea level. The extreme northern coast of France (Fig. 1) is a macrotidal coast (tidal range of 5.6 m during mean spring tides) characterised by a 300 to 600 m-wide beach/surfzone consisting of parallel bars and troughs (MASSELINK and ANTHONY, 2001; REICHMÜTH and ANTHONY, 2002). This is a moderate mixed energy coast, influenced by tides and waves. Mean significant wave height at the coast is generally below 1 m and mean wave period is in the order of 5 to 6 s. Strong tidal currents are canalized by shore parallel sand banks, with a predominance of the flood. From Dunkirk to the Belgium border (Fig. 1), inland parabolic dunes fronted by a foredune ridge form a 7 km long well developed coastal dune system, 5 to 25 m high and 700 to 1100 m wide (CLABAUT et al., 2000). The established foredune is 50 m to 150 m wide and 10 m to 15 m high. This coastline is dominantly exposed to shore-parallel moderate winds from a south to southwesterly window (Fig. 1). Northerly onshore winds, the most efficient in terms of potential dune accretion, are less frequent, but they occur in winter and can induce storm surges responsible for upper beach/dune erosion (VASSEUR and HÉQUETTE, 2000; RUZ and ANTHONY, 2008). This macrotidal moderate energy coast presently functions under conditions of rather limited sand supply from the shoreface in spite of the abundant stocks of sand locked up in the shoreface tidal ridges and banks (ANTHONY and HÉQUETTE, 2007). A number of recent studies suggested medium term (10 years) gross stability of the beach-foredune system (RUZ et al., 2005; ANTHONY et al., 2006; 2007), mild dune scarping in winter being Figure 1. Location map and wind conditions at the Dunkirk meteorological station. W and T refer to the Westhinder and Trapegeer buoys respectively. 356

2 Forcing Conditions of Foredune Erosion usually followed by limited sand accumulation at the dune toe in spring and summer. In March 2007, however, the foredune underwent dramatic erosion in response to a single storm event. The aim of this contribution is a first attempt to define forcing conditions responsible for foredune evolution along this coast. METHODS In the central part of the beach, between Dunkirk and the Belgium border (Fig. 1), a topographic profile perpendicular to the foredune and the beach was monitored using a high resolution laser electronic station along a representative coastal sector that was characterized by relative stability prior to the March 2007 storm. Along most of the monitored site the established foredune is 6 to 10 m high with a steep stoss slope partly vegetated. The junction between the dune toe and the foreshore is a narrow (< 20 m wide) upper beach not reached by the mean highest tides. This site was monitored prior and after the March 2007 erosive event. Forcing conditions during this storm event were analysed using meteorological and hydrographic data. Hourly mean wind speed and direction were obtained from Dunkirk meteorological weather station located 7 km from the study area and from a Belgium buoy (Westhinder) located 36 km offshore (Fig. 1). Predicted and observed hourly tidal levels at Dunkirk were obtained from the SHOM (Service Hydrographique et Océanographique de la Marine). In this study water levels as well as beach-dune profile elevations refer to metres above Hydrographic Datum (HD, the French Hydrographic Datum corresponds approximately to the lowest astronomical tide level). Significant wave heights (H s ) and wave periods were recorded at the offshore Westhinder buoy and at the nearshore (3 m water depth) Trapegeer buoy located 11 km northeast of the study area (Fig. 1). In order to define potentially erosive event occurrence over longer time periods, observed hourly water levels recorded at Dunkirk from 1956 to 2006 were also analysed. RESULTS Recent foredune evolution Shoreline evolution in this area during much of the 20 th century was dominated by retreat (CLABAUT et al., 2000), related to both human pressures and natural erosional processes. Coastal dunes in this sector have been massively transformed by urban and port development. Coastal dunes were also badly damaged during World War II (RUZ et al., 2005). From 1971 to 1994 the mean retreat rates were on the order of m/year. The foredune was affected by breaches and blowouts, mainly due to human disturbance and by erosional scarping during storms. In the mid 1990s, measures to prevent degradation of the dunes and reduce the threat of marine erosion were implemented by the Departmental Authority of the North (Conseil Général du Nord) in charge of the management of these coastal dunes. Wooden and brushwood fences were erected in order to encourage sand accumulation in the most sensitive areas. In order to promote the recovery of natural habitats, these rehabilitation measures have involved, since 1994, manual collection of detritus and debris accumulating at the high tide lines. Such measures were very successful and have contributed to coastal dune rehabilitation and foredune stabilisation (RUZ et al., 2005; ANTHONY et al., 2007). The foredune seaward slope remained however relatively steep and partly vegetated. At the dune toe, episodic wave attack was responsible for basal undercutting. On the other hand, the refilling of dune blowouts and the development of a vegetation cover suggested a relatively balanced sand budget (CLABAUT et al., Figure 2. Frequency of winds 16 m/s in Dunkirk for the period From CHAVEROT et al.(2008). 2000). Along most of this coast the foredune ridge was described in a state of meso-scale (decadal) stability (RUZ et al., 2005). This relative stability was attributed in part to human intervention and decrease in storminess. The analysis of strong winds ( 16 m/s) frequency from three-hourly wind data recorded at Dunkirk (Fig. 2) showed that the period was characterised by a high frequency of strong winds (CLABAUT et al., 2000) while the last two decades were periods of decreasing storminess (CHAVEROT et al., 2008). Foredune response to a major erosive event A major erosive event occurred in March Between March 17 th and March 22 nd, the hourly mean wind speed recorded at Dunkirk varied from 7 to 13 m/s with a mean wind speed of 10.5 m/s and maximum mean hourly wind velocity of 15.3 m/s recorded on the 20 th (Fig. 3). From March 17 th 18:00 to the 22 nd 6:00 the wind blew at 10 m/s and more during 73 hours over this 110 hours period. Offshore, at the Westhinder station, the wind speed was higher, with a mean wind speed of 14.5 m/s and maximum mean hourly wind speed of 23 m/s recorded on the 18 th (Fig. 3). Offshore, wind speed remained above 12 m/s during 88 hours. At Dunkirk, dominant wind direction was west to southwest on March 17 th and 18 th, then veered to the northwest on the 19 th and maintained to the north on the 20 th, 21 st and 22 nd. The same wind directions were recorded offshore (Fig. 3). In response to these moderate but constant winds, significant wave heights (H s ) recorded offshore and in the shallow subtidal zone increased significantly. On March 18 th, high velocity (>20 m/s offshore) southwestern winds induced a rapid increase in offshore wave heights that exceeded 3 m (Fig. 3). At the coast, this response was less obvious, with maximum wave heights reaching only 1.5 m. Between March 20 th 9:00 and 21 st, 12:00, with direct onshore winds up to 15 m/s at Dunkirk and up to about 19 m/s offshore, H s of 4 m were recorded offshore and H s above 3 m were recorded near the coast. It is noticeable that at the coast wave height is modulated by the vertical tidal fluctuations as maximum wave heights were recorded at high tide. Wave periods recorded offshore and nearshore reached maximum values of 6.7 and 6.4 s respectively. Direct and persistent strong northern winds probably induced wind and wave set up at the coast. This stormy event was combined with a spring tide. At Dunkirk, predicted tidal range increased from 5.36 m on March 18 th to 5.48 on the 19 th, reaching a maximum of 6.10 m on March 21 st and then decreasing to 5.9 m on the 22 nd. With increasing tidal range, observed water level at Dunkirk reached a maximum of 6.8 m above HD at 0:00 on March 19 th, a level well above the highest predicted astronomical tides 357

3 Ruz et al. Figure 3. Wind, waves and tide conditions during the March 2007 storm event. (6.48 m). This observed level was 1.06 m above the predicted high tide of the day. This surge, also observed at low tide (Fig. 3), was not exceptional in this area where surges of 1 m have a return period of 0.1 year (TOMASIN and PIRAZZOLI, 2008). On March 20 th and 21 st, the spring tide was combined with a surge of 0.5 m and water level was above 6.7 m HD. These high water levels were the result of the conjunction of spring tides, strong northerly winds and high waves at the coast. High water levels reached an elevation above the dune toe on several occasions between March 18 th and March 21 st (Fig. 3). Combined with waves they were responsible for significant foredune erosion along the coast. In response to this event, the upper beach was flattened and lowered and the foredune front retreated about 4 m (Fig. 4). In response to intense wave action associated with extra surge levels, the foredune was uniformly cut into a steep scarp (Fig. 4) along 5 km of shoreline. In the western part of the study area, sand fences erected in 2004 at the dune toe were vanished by storm waves (RUZ and ANTHONY, 2008). The analysis of high water levels recorded at Dunkirk for the period reveals a low occurrence of water levels above the dune toe (Fig. 5). From 1996 to 2006, a period characterized by foredune stabilization, such high water levels only occurred 4 times and over the complete period, potentially erosive high water levels occurred 27 times, with a high frequency in early 80 s (Fig. 5). DISCUSSION AND CONCLUSION For individual dune systems, coastal sediment budgets, aeolian sand transport and destructive marine events are key factors to understand the relative importance of controlling processes. Under erosive conditions with a high influx of wave energy and a dissipative nearshore, the dunes often erode (KROON et al., 2007). Along the macrotidal coast of northern France, the patterns of short-term to decadal-scale morphological evolution appear determined by episodic erosive events. After almost 10 years of relative meso-scale stability, the foredune retreated by about 3 to 5 m in response to a single event. As outlined by SHERMAN and BAUER (1993), meso-scale variations of foredune response may incorporate the annual to decadal sequences of morphological attenuation and recovery. In this area, foredune growth observed over the last decade reflected the absence of major storm events combined with large spring tides during which the upper beach and foredune can be exposed to surge conditions. This evolution must, therefore, be viewed within a favorable context of sand supply combined with the absence of significant erosive storm event. 358

4 Forcing Conditions of Foredune Erosion near the coast (Trapegeer station, Fig. 3) where waves can have major impacts on coastal dunes. These results show that along this coast a major erosive event can occur when moderate to strong direct onshore winds, blowing during more than 2 days, induce a positive surge combined with a high water level associated with a mean spring tide. Along this fetch-limited macrotidal coast, strong winds are therefore not necessarily a criterion for explaining foredune evolution. Coastal dune evolution appears to be primarily dependent on high water level frequency. The analysis of past uppermost water levels reaching the dune toe gives an overview of potential erosive events (Fig. 5). It is obvious that periods of high frequency of strong winds (Fig. 2) do not correspond with periods of very high water levels (Fig. 5). Therefore, coastal dune erosion is not necessarily a response to periods of increasing frequency of strong winds as previously assumed (CLABAUT et al., 2000; CHAVEROT et al., 2005). Along this coast it seems that the greatest morphological impacts at the shoreline result from locally generated short period waves associated with coastal proximal storms and spring tides. Figure 4. Pre- and post-storm upper beach and foredune profile. The photograph shows the dune scarp after the March 2007 storm. Our study suggests that major erosive events along this macrotidal coast are not necessarily related to strongest winds. Tidal elevation, wind duration and direction appear to be major factors controlling coastal dune erosion. Strong winds occurring at low tide have no impact along these beaches as well as offshore blowing winds. As also noted by COOPER et al. (2004), when tidal range is relatively large, the probability of wave set-up during high wave conditions causing water levels exceeding normal high tide levels is reduced. Our analyses of offshore and nearshore wave heights during the March 2007 storm event show that higher wave heights were not generated by the strongest offshore winds (>20 m/s), but by more moderate winds (about m/s) persistently blowing onshore during more than 48 hours (Fig. 3). Wind direction and duration appear therefore to be major forcing parameters responsible for an increase in wave heights, especially Figure 5. Frequency of high water levels observed at Dunkirk from 1956 to m corresponds to water levels above highest astronomical tides. 6.8 m corresponds to water levels above the dune toe. LITERATURE CITED ARENS, S.M., JUNGERIUS, P.D., and VAN DER MEULEN, F., Coastal dunes. In: WARREN A. and. FRENCH J.R (eds.), Habitat Conservation: Managing the Physical environment. John Wiley & Sons Ltd., pp ANTHONY, E.J., and HÉQUETTE, A., The grain size characterisation of coastal sand from the Somme estuary to Belgium: sediment sorting and mixing in a tide and stormdominated setting. Sedimentary Geology, 202, 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, ANTHONY, E.J., VANHÉE, S., and RUZ, M-H., An assessment of the impact of experimental brushwood fences on foredune sand accumulation based on digital elevation models. Ecological Engineering, 31, CARTER, R.W.G., Coastal Environments; An Introduction to the Physical, Ecological and Cultural Systems of Coastlines. London, Academic Press, 617 pp. CHAVEROT, S., HÉQUETTE, A. and COHEN, O., Evolution of climatic forcings and potentially eroding events on the coast of Northen France. Proceedings of the 5 th International Conference on Coastal Dynamics (Barcelona, Spain), 11 p. 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), CLABAUT, P., CHAMLEY, H. and MARTEEL, H., Evolution récente des dunes littorals à l est de Dunkerque (Nord de la France). Géomorphologie, 2, COOPER J.A.G., JACKSON D.W.T., NAVAS F., MCKENNA J. and MALVAREZ G., Identifying storm impacts on an embayed, high-energy coastline: examples from western Ireland. Marine Geology, 10, HESP, P., Foredunes and blowouts: initiation, geomorphology and dynamics. Geomorphology, 48, KROON, A., QUARTEL, S. and AAGAARD; T., Impact of a major storm on sediment exchange between the dunes, beach and nearshore. Proceedings of the International Conference Coastal Sediments '07 (New Orleans, USA), pp

5 Ruz et al. MASSELINK, G., and ANTHONY, E.J., Location and height of intertidal bars on macrotidal ridge and runnel beaches. Earth Surface Processes and Landforms, 26, PSUTY, N.P., Sediment budget and beach/dune interaction. Journal of Coastal Research Special issue No. 3, pp.1-4. PYE, K., Beach deflation and backshore dune formation following erosion under storm surge conditions: an example from Northwest England. Acta Mechanica, 2, 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, RUZ, M-H., ANTHONY, E.J., and FAUCON, L., Coastal dune evolution on a shoreline subject to strong human pressure: the Dunkerque area, northern France. Dunes & Estuaries 2005, Proceedings of the International Conference on Nature Restoration Practices in European Coastal Habitats, (Koksijde, Belgium), pp RUZ, M-H., and ANTHONY, E.J., Sand trapping by brushwood fences on a beach-foredune contact: the primacy of the local sediment budget. Zeitschrift für Geomorphologie, Suppl. Bd. 52 (3), SHERMAN, D.J. and BAUER, B.O., Dynamics of beach-dune systems. Progress in Physical Geography, 17, TOMASIN, A. and PIRAZZOLI, P.A., Extreme sea levels in the English Channel: calibration of the Joint Probabilities Method. Journal of Coastal Research, 24, VASSEUR B. and HEQUETTE A., Storm surges and erosion of coastal dunes between 1957 and 1988 near Dunkerque (France), southern North Sea. In: PYE, K. and ALLEN, J. R. L. (eds.), Coastal and Estuarine Environments: sedimentology, geomorphology and geoarcheology, Geological Society of London, Vol. 175 pp ACKNOWLEDGEMENTS This study was partly funded by the French Agence Nationale pour la Recherche (ANR-06-VMC-009) 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. 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 and to Vincent Sipka for technical assistance in the field. 360

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