Geomorphology 199 (2013) Contents lists available at ScienceDirect. Geomorphology. journal homepage:

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1 Geomorphology 199 (13) 8 1 Contents lists available at ScienceDirect Geomorphology journal homepage: Storms, shoreface morphodynamics, sand supply, and the accretion and erosion of coastal dune barriers in the southern North Sea Edward J. Anthony Aix Marseille Univ, Institut Universitaire de France, CEREGE, UMR CNRS 733, Europôle Méditerranéen de l'arbois, B.P. 8, Aix en Provence, France article info abstract Article history: Received 9 November 11 Received in revised form 17 May 1 Accepted 6 June 1 Available online 16 June 1 Keywords: Storms Shoreface Tidal banks Sand supply Coastal dunes North Sea The coast of the southern North Sea is bound by dune barriers that have developed adjacent to a shallow storm- and tide-dominated shoreface comprising numerous shore-parallel to sub-shore-parallel tidal sand banks. The banks evolve under the joint control of tide-, wave- and wind-induced shore-parallel currents, which tend to stretch them, eventually leading to bank division, and to shoaling and breaking storm waves, which tend to drive them ashore. The banks, thus, modulate the delivery of storm wave energy to the coast, redirect currents alongshore and are the sand sources for the accretion of coastal dunes. Foredune accretion occurs where major sand banks have migrated shoreward over the last centuries to be finally driven ashore and weld under the impact of storm waves. Morphological changes in the bank field can impact on shoreline stability through dissipation or enhanced shoreward transmission of storm wave energy and effects on radiation stress, particularly when waves are breaking over the banks. Where banks are close to the shore, mitigation of offshore sediment transport, especially during storms, can occur because of gradients in radiation stress generated by the complex 3D bank structure. These macro-scale mechanisms involve embedded meso-scale interactions that revolve around the mobility of sand waves, mobility of beach bars and troughs and foredune mobility, and micro-scale processes of bedform mobility in the subaqueous and intertidal domains, and of swash and aeolian beach dune sand transport. These embedded interactions and the morphodynamic feedback loops illustrate the importance of synchroneity of sand transport from shoreface to dune on this coast. Large stretches of the foredunes show either signs of stability, or mild but chronic erosion. Furthermore, a demonstrated lack of a clear relationship occurs between storminess and coastal response over the second half of the th century. The present situation may be indicative of conditions of rather limited sand supply from offshore, notwithstanding the abundance of sand on the nearby shallow shoreface, except in areas where a nearshore storm-driven tidal sand bank has become shore-attached. Apart from the important influence of shoreface sand banks and of wave bank interactions, foredune accretion and erosion also depend on various context controls that include individual storm characteristics, wind speed and incidence relative to the shore, tidal stage during storms, and direct human intervention on the shore through foredune and beach management. The bewildering variability inherent in these intricately related parameters may also explain the poor relationship between storminess and barrier shoreline change and will still continue to render unpredictable the response of shores to individual storms. 1 Elsevier B.V. All rights reserved. 1. Introduction Storm effects on coastal sand barriers have received wide attention, in part because of increasing human occupation of storm-exposed coasts. Much of the literature has focussed on hurricane impacts on the North American coast (e.g., Morton, ; Stone et al., 4; Zhang et al., 5; Sallenger et al., 6; Wang et al., 6; Claudino- Sales et al., 8; Houser et al., 8), especially on the low-lying Atlantic seaboard where the transgressive history of much of the coast renders storm impacts pervasive (Forbes et al., 4). In Europe, efforts address: anthony@cerege.fr. have focussed on the various modes of coastal response to storms in a context of marked morphological variability, and have included work by Cooper et al. (4), Regnauld et al. (4), Ferreira (6), Mendoza and Jiménez (6), Ciavola et al. (7), Chaverot et al. (8), Sabatier et al. (9), Gervais et al. (1), Haerens et al. (1), and Suanez et al. (1). These efforts clearly show that the conceptual framework and capacity for accurate prediction of storminduced coastal change remain incomplete (Forbes et al., 4). The precise response of a coastal stretch to each storm event depends on numerous inter-related parameters. Whereas coastal erosion is a common but not exclusive result of storms, interactions between storms and the shoreline are complex. This complexity is inherent to the functional X/$ see front matter 1 Elsevier B.V. All rights reserved. doi:1.116/j.geomorph.1.6.7

2 E.J. Anthony / Geomorphology 199 (13) mechanism of storms, which includes various parameters such as air pressure, mean water level, wind speed, wind direction relative to the coast, and waves (e.g., Betts et al., 4; Backstrom et al., 8, 9; Haerens et al., 1). Coastal response to storms is also complex, because it is controlled by multiple factors related to incident storm characteristics and to coastal and shoreface morphology and sediment supply. These latter factors integrate a longer-term perspective on storm impacts on the coast. Of course, a commonly strong link exists between the beach and the shoreface in terms of sediment exchanges induced by storms, although Houser (9) has recently pointed out the insufficiencies in recognizing this synchroneity. Although coastal erosion is a common outcome of storms, storms may also rework sediment onto the beach, thus, generating accretion. For example, on the Atlantic coast of the US, northeast storms produce downwelling that results in offshore-directed sediment transport, whereas upwelling created by southwest storms results in onshore-directed sediment transport (Wright et al., 1994; Hill et al., 4). Stone et al. (4) also showed that barrier islands can conserve mass during catastrophic cyclones, and that less severe cyclones and tropical storms can promote rapid dune aggradation and can contribute sediment to the entire barrier system. Whereas the general link between storms and the response of sand barrier shorelines has been recognised in the literature, the role of shoreface modulation of storm impacts on the shore has received much less attention. Among the few studies having shown that the morphology of the shoreface can strongly influence coastal response to storms and sediment dispersal on the shoreface are those of Field and Roy (1984), Hequette et al. (1), and Backstrom et al. (8, 9). In this paper, the morphological response of a dune barrier coast in a storm-dominated macrotidal setting on the southern coast of the North Sea is examined with reference to storm impacts at long (order of decades) to short (order of years) timescales. The morphodynamic connections between storms and barrier shoreline response and the embedded macro-, meso- and micro-scale levels of interaction are reviewed. The essential role of storms in patterns of shoreline change comes out as being strongly influenced by the conditions prevailing on the inner shoreface.. The southern North Sea barrier and shoreface system.1. Barrier and shoreface morphology The North Sea coast of France from Cape Gris Nez to Belgium (Fig. 1) comprises two sand barriers, respectively, in Wissant Bay between Capes Gris Nez and Blanc Nez, and from Cape Blanc Nez along the southern North Sea as far as the Netherlands (Fig. 1). Each barrier consists of two to three generations of sub-shore-parallel dunes 1 to 6 m wide and with a maximum inland height of 5 m, impounding former tidal embayments. The barrier foredune is associated with beaches exhibiting multiple bar-trough (ridge-andrunnel) couplets that are widely exposed at low tide. The Wissant Bay barrier comprises variably eroding or accreting foredune sectors. The barrier stretching from Cape Blanc Nez to the Netherlands bounds the empoldered North Sea coastal plain, large parts of which lie below sea level, and face risks of flooding from storms. Large stretches of this latter barrier have been massively transformed or obliterated by urban and port development. The foredunes in both barrier sectors exhibit blowouts and deflation corridors. The gently sloping shallow shoreface, extending seaward of the beach bars and troughs, is characterised by an important field of prominent tidal sand banks and ridges (Fig. 1). The bank field off the coasts of France and Belgium, which forms the Flemish Banks complex, is particularly well developed as the narrow Dover Strait opens up on the epicontinental southern North Sea. These banks are several kilometres long and have heights of up to 1 m. The crests of sand banks closest to the shoreline lie at depths of less than 5 m below the mean low water line (Fig. 1). They practically impinge on the beach in places. These elongated sand bodies are commonly oriented WSW ENE, roughly parallel to sub-parallel to the coastline. Sediment distribution in the bank field is strongly related to the bathymetry, with fine to coarse sand in the interbank troughs, whereas Westhinder A-B Trapegeer Wissant Bay Line bank Cliffs Fig. 1. The dune barriers and shallow shoreface of the southern North Sea. Bathymetric contours are from Augris et al. (1995).

3 1 E.J. Anthony / Geomorphology 199 (13) 8 1 the shallower sectors of the banks and inner shoreface generally consist of fine to medium sand (Corbau et al., 1999; Van Lancker, 1999)... Winds, waves and tides The southern North Sea coast of France may be described as typical mixed storm-wave- and tide-dominated(anthony and Orford, ), subject to a complex pattern of time-varying influences of tides and storm waves, in addition to wind-forced flows. Winds mainly come from the southwest and northeast, but the strongest winds mostly originate from west to southwest (Fig. ). The overall aeolian dynamic regime of this North Sea coast operates within the framework of a relatively low frequency of strong (>1 m/s) onshore/offshore winds (1. 3.8%), and of a relatively balanced wind regime, in which the proportion of counteractive offshore winds in the significant wind category (>8 m/s) attains 41% and 34% in Calais and Dunkerque (Dunkirk), respectively. Overall, combined offshore and shore-parallel winds account for 71% and 57% of winds >8 m/s, respectively, in these two sites. The hydrodynamic context is that of a short-fetch, storm wave environment, characterised by marked short-term (order of days to weeks) fluctuations in wave height. The dominant waves are from southwest to west, originating from the English Channel, followed by waves from the northeast to north, generated in the North Sea. Long-term (Jan Dec. ) records of significant wave heights, obtained from waverider bouys, are shown (Fig. 3a) for two locations, Westhinder and A-B, respectively, about 35 km and 5 km offshore of the neigbouring coast of Belgium (see Fig. 1 for locations). Maximum significant wave heights are highest in autumn and winter but mean and minimum significant wave heights show smoother trends. Wave heights are lower at A-B closer to the shoreline because of refraction and shoaling over the sand banks. These differences are further highlighted by a recent record (1 March to 9 April 1) of mean significant wave heights at Westhinder and at Trapegeer (Fig. 3b), another station located close to the coast and nearer to the study area (Fig. 1). This graph also shows the short wave periods and the sharp short-term fluctuations in significant wave heights typical of this storm wave environment. Records from simultaneous deployments of pressure sensors on the inner shoreface and intertidal beach just east of Dunkerque, compared to Westhinder, further highlight the significant drop in wave heights caused by the sand bank field (Fig. 3c). In conjunction with the nearshore sand bank morphology, tidal modulation may result in a highly variable wave field close to the beach (Héquette et al., 9). Breaking waves are essentially from a north-northeast to northwest window, although the dominant deepwater directions are from north and west. The tidal regime in the region is semi-diurnal and macrotidal, the tidal range decreasing from 6.4 m in Calais to 5.6 m in Dunkerque during spring tides (Service Hydrographique et Océanographique de la Marine, 1968). Because of the large tidal range, tidal currents are strong on the shoreface (Fig. 4) and over the intertidal beach (Fig. 5). In calm weather, current directions are closely conditioned by the tide, with dominantly longshore east-northeast flood directions and westsouthwest ebb directions. Current reversals do not occur at high or low tide, but are typically retarded by up to h in Dunkerque and to 3 h in Calais because of the coexistence of stationary and progressive tidal waves. Because of the high velocity magnitude, tidal currents are only slightly modified in the shallow water column of the shoreface where the directions remain constant from the surface to the seabed, although speeds decrease bottomward. Current speeds generally diminish eastward from Calais towards Belgium from decreasing tidal amplitude in the same direction. Note, in Figs. 4 and 5, the influence of storm wind conditions on currents and waves. Strong winds enhance the strength of ebb or flood currents when blowing in the same direction, or limit, and even prevent tidal reversal when blowing in the opposite direction (Reichmüth and Anthony, 7; Héquette et al., 8a, 8b), but flow is more commonly flood-dominated. During conditions of significant wind stress (sustained wind speeds >1 m s 1 ), the peak current speeds can be two to three times higher than normal (tide-generated) peak spring tide speeds. Longshore currents can become particularly strong during storms because of direct wind stress and gradients in radiation stress that divert, alongshore, offshore mean currents generated by wave breaking and onshore mass transport as waves pass over or break over the sand banks. Storms may add up to 1 m of surge above high-tide swash excursion levels. 3. Storms, shoreface morphodynamics and the shoreline 3.1. Storms and shoreface morphodynamics: the big picture The abundant accumulation of fine to medium sand on the shoreface of the southern North Sea is considered as the product of large-scale wind- and tide-dominated hydrodynamic circulations in the eastern English Channel that have gradually sorted out, in the course of the Holocene, heterogeneous sea-bed sediments that accumulated under past changing sea-level and land drainage conditions (Anthony, ). The southern extremity of the North Sea is a net depocentre of sand circulating along coastal sediment pathways linking the eastern English Channel to the southern North Sea (Beck et al., 1991; Grochowski et al., 1993; Anthony,, ). In the course of the Holocene, this incoming sand has been reworked by the interplay of tidal currents and storm waves into the impressive jumble of tidal sand ridges and banks that have served as sources for coastal (foredune, estuarine and back-barrier) accretion. Sediment supply for the accumulation of the coastal dune barriers in the southern North Sea has depended essentially on storms driving sand from these shoreface banks. From high-resolution seismic profiles of these sand banks, Tessier et al. (1999) identified a pattern of bank mobility involving storm waves dominantly orthogonal to the crests and seaward flanks of the tidal banks, which tend to migrate Fig.. Wind roses for Calais and Dunkerque.

4 c Significant wave height (m) a Significant wave height (m) mean min J F M A M J J A S O N D b Significant wave height (m) : : : : : : :3 E.J. Anthony / Geomorphology 199 (13) : : : : : : : : : :3 Offshore (-7 m) Westhinder Nearshore (-1.9 m) site Nearshore (-1.4 m) site 1 Beach (+. m) site Westhinder Westhinder Trapegeer 4--3: : :3 A-B 4-6-: : : max Wave period 11 14/ 15/ 16/ 17/ 18/ 19/ / 1/ / 3/ 4/ 5/ 6/ 7/ 8/ Fig. 3. Wave conditions in the southern North Sea: (a) a 6-year (1977 1) record of maximum, mean and minimum significant wave heights (Hs) from two waverider bouy stations off the Belgian coast (see Fig. 1 for locations); (b) a typical recent (1 Mar. 9 Apr., 1) record of mean significant wave heights (Hs) from the Westhinder (in red) and Trapegeer (in blue) (see Fig. 1) bouys; the same record also shows typical wave periods at Westhinder (in black); (c) a comparaison of mean significant wave heights over a two-week period (14 Feb. 8 Feb., 7) at Westhinder and from inner shoreface and beach deployments near Dunkerque, showing the significant drop in wave heights due to dissipation on the shoreface. (a, b) from Administration Waterways and Marine Affairs of Flanders, and Flanders Marine Institute, Belgium ( midas/mvb.php), (c) from Héquette et al., 9. shoreward, whereas tidal currents, enhanced by wind stress during storms, tend to stretch the banks alongshore. The stretching process may eventually result, where the crest becomes very narrow, in bank division. Shoreward mobility may involve crest lowering as sediment is transferred through bedform (dune) migration from the exposed to the more sheltered landward flank of the bank (Houthuys et al., 1994). Sand moving towards the landward flanks may also be deposited in troughs dominated by tidal currents. These orthogonal directions of wave-versus tidal current influence on sand banks have been corroborated by Héquette et al. (8a) from measurements of bedload transport on the inner shoreface off Dunkerque (Fig. 6). Giardino et al. (1) have also shown, in a morphodynamic modelling study of sand banks, that the interaction between wave activity and tidal currents leads to a high increase in bottom shear stress, especially at the sand bank crests and, as a consequence, to an increase in sand tranport. The study showed that wave activity is also responsible for a change in direction of the net flux of sediments during the tidal cycle. The actual mechanisms of onshore bank migration, however, still remain elusive. The onshore sand transport occurring during storms is probably related to offshore currents being weaker across a shallow sand bank and/or to the alongshore diversion of these currents by radiation stress gradients generated by the 3D-bathymetry, such that waves become the dominant (onshore) transport mechanism. Where banks are very close to the shoreline, these diverted currents may also transport alongshore sediment eroded from the beach and foredune during storms, mitigating, in this way, sediment losses offshore. Long-term onshore bank migration under impinging storm waves appears to be modulated by: (1) shoreface morphology, with migration occurring where the seaward flanks of the banks are directly exposed to storm waves; and () tidal stage. These two points also highlight the important role of the bank field in modulating onshore wave energy transmission. Point 1 implies that the dissipation of storm wave energy over the most seaward banks may significantly limit the propensity for the migration of banks closer to the shore, whereas bank stretching, and eventual division, induced by longshore currents, become more prevalent. Point concerns the contrasting effects of neap and spring tides on bank migration and stretching. Neap tides and low waters at spring tides are generally

5 1 E.J. Anthony / Geomorphology 199 (13) 8 1 Fig. 4. A typical shoreface record of hydrodynamic and meteorological parameters recorded km offshore of Dunkerque (6/3 to 9/4, 4), showing from top to bottom: water depth, mean near bottom current direction, mean near-bottom current speed, significant wave height recorded in 5 m water depth (relative to Hydrographic Datum), and wind speed and direction measured in Dunkerque. The record shows the conjugate strengthening effect of spring tides and wind stress on tidal currents. Also note the clear relationship between wind speed and wave height. From Héquette et al. (8a). more favourable to bank migration as wave energy dissipation becomes concentrated on the bank field. These two points explain the strong gradients in wave energy from the deeper shoreface to the shoreline (Fig. 3), whereas high waters during spring tides favour more efficient shoreward transmission of wave energy (see relationship between tide-modulated water depth and wave height in Fig. 5). A final point to be considered is that the sand bank field may also be having a large-scale feedback effect on the intensity of incident storms. Notwithstanding the fair frequency of storms in the North Sea, storm intensity in the southern part of the North Sea appears to be lower than on the more exposed coasts of Belgium, Holland, Germany and Denmark, if one is to judge by the effects of historic storms on the countries

6 E.J. Anthony / Geomorphology 199 (13) Wind speed (m s -1 ) Offshore wave height (m) Water depth (m) Significant wave height (m) Mean overall current (m s-1) Mean overall current direction Mean longshore current (m s-1) Mean cross-shore current (m s-1) /1 16/1 18/1 /1 /1 4/1 6/1 8/ /1 16/1 18/1 /1 /1 4/1 6/1 8/ Speed Direction 14/1 16/1 18/1 /1 /1 4/1 6/1 8/1 nd Bar Bar 3 nd Idem for 4 to /1 16/1 18/1 /1 /1 4/1 6/1 8/ nd /1 16/1 18/1 /1 /1 4/1 6/1 8/ nd /1 16/1 18/1 /1 /1 4/1 6/1 8/ NE.5 nd SW /1 16/1 18/1 /1 /1 4/1 6/1 8/ Offshore.1 nd -.1 Onshore -. 14/1 16/1 18/1 /1 /1 4/1 6/1 8/ Wind direction Fig. 5. A typical record of hydrodynamic and meteorological parameters recorded at high tide over two intertidal bars on the beach in Wissant Bay (14/1 to 8/1, 4). Numbers in the water depth panel correspond to tides; nd: faulty data because of storm overturning and sand burial of sensors. From Sedrati and Anthony (7). Note, as in Fig. 4, the strong relationship between wind stress and currents and waves. bordering this sea (Lamb, 1991). Such regional differences are, however, still poorly known and await more complete investigation (Stone and Orford, 4). Lower intensity of storms in the southern North Sea may also be, in addition to a probable regional meteorological storm intensity gradient from the north to the south of this sea, an outcome of significant dissipation of storm wave energy by the impressive sand bank field concentrated in this southern area, as suggested by the marked cross-shelf wave energy gradients depicted in Fig. 3. This

7 14 E.J. Anthony / Geomorphology 199 (13) 8 1 Fig. 6. Time series of (a) computed shear velocity and (b) modeled sediment transport (bedload qb and total load qt) on the shoreface off Dunkerque (see also Fig. 4) based on the SEDTRANS96 model. The bedload transport directions are closely conditioned by the tide-induced longshore currents (7 ) and by onshore (36 ) wave-induced bottom stress. Adapted from Héquette et al. (8a). limiting condition is further exacerbated by the large tidal range in this area, compared to the more storm-wave dominated meso- to microtidal coasts bordering the rest of the North Sea. 3.. Sand supply from the shoreface and barrier accretion Based on the large-scale sand circulation patterns evoked in Section 3.1, Anthony (, ) suggested, for the storm and tidedominated coasts of the eastern English Channel and the southern North Sea, a sequential relationship of coastal (dune, estuarine and backbarrier) accretion involving shoreward attachment of tidal sand banks, followed by backshore sand flat accretion (and/or estuarine and backbarrier infill), embryo dune development and foredune development. Subsequent studies involving determination of shoreline changes at a secular scale (Aernouts and Héquette, 6; Chaverot et al., 8), bathymetric chart differencing at the same scale (Héquette and Aernouts, 1), monitoring of beach-dune budget changes (Anthony et al., 6, 7a, 9), monitoring of recent coastal progradation (Aubry et al., 9), and stratigraphic analysis of an isolated inland dune barrier (Anthony et al., 1), and of backbarrier deposits near Dunkerque (Mrani-Alaoui and Anthony, 11), confirm this relationship. In this setting, significant seaward shoreline translation is associated with the formation of a wide sand flat, followed, with a time lag of the order of years, by the growth of aeolian dunes. Such translation is essentially related to episodic wholesale onshore welding of a large tidal sand bank driven from the shoreface by repeated activity of storm waves. The welded sand bank forms extensive temporary (at scales of decades to centuries) intertidal backshore sand flats that serve as a basement and a sand source for the subsequent rapid accretion of aeolian dunes, essentially at the dry inner flanks of these flats (Anthony et al., 6, 7a). A fine recent example of this has been documented from the coast midway between Calais and Gravelines (Fig. 1), where, during the th century, localised accretion resulted in a shoreline sand-flat bulge up to 3 km-long and over 1 km-wide (Fig. 7), following the onshore welding of a large tidal sand bank (Garlan, 199). Bathymetric and shoreline changes in this area show an extensive sand flat that underwent progradation of more than 3 m between 1949 and (Héquette and Aernouts, 1). These authors calculated from the differences between bathymetric charts a welded bank volume that grew up to about m 3 in the course of the th century. Héquette and Aernouts (1) further showed, from SWAN wave propagation modeling based on sequential bathymetric changes in this site, the increasing effect of bank accretion on nearshore dissipation of wave energy. Significant sand supply to the backshore flat from a welded bank source is episodic, and results solely from major storms. These are capable of driving large amounts of sand onto the upper beach and to the backshore flat from the shore-attached sub-tidal sand bank source, as shown by correlative analysis of sequential digital elevation models of Fig. 7. A Formosat image (7/9/8) of the North Sea coast midway between Calais and Gravelines (Fig. 1), showing the eastern limit of a sand flat shore that significantly prograded in the course of the th century from the welding of a shoreface bank. The unvegetated part of the bank is subject to aeolian activity and foredune development, whereas the dark inner part consists of a mudflat that has evolved into a saltmarsh. The circular depressions in this saltmarsh are artefacts dug out by wildfowl hunters. In this transition zone, the present North Sea dune shoreline in the east has been isolated inland in the west by tidal flat progradation.

8 E.J. Anthony / Geomorphology 199 (13) the beach, the backshore flat and embryo dune front, and corresponding strong wind and storm episodes (Anthony et al., 6). Swash washing over the backshore sand flat rapidly percolates over this surface, enhancing its accretion. Wind action over the dry parts of the sand flat redistributes sand towards embryo dunes that develop over time into a longitudinal dune complex (Anthony et al., 7a). Apart from this dominant, but relatively rare, mode of large-scale bank-sourced accretion, sand supply to the dune barrier may also be assured via two other secondary modes: (1) successions of large sand waves up to 5 m-long and more than 5 m-high, detached from tidal banks and ridges close to the shore and driven ashore by storms (Beck et al., 1991; Anthony et al., 5), and () active storm-generated migration of small (.5.5 m) to medium (.5.5 m) D to 3D subaqueous dunes over beach longshore bars during tidal submergence (Sedrati and Anthony, 7) Barrier shoreline variability and erosion A remarkable feature of the southern North Sea coast is its marked alongshore and temporal variability in terms of accretion, erosion and stability (Anthony and Héquette, 5; Aernouts and Héquette, 6; Anthony et al., 6; Chaverot et al., 8). Fig. 8 shows detailed and highly variable patterns of shoreline changes determined by Chaverot et al. (8) for Wissant Bay and for part of the dune barrier shoreline west of Calais and covering the period The Wissant Bay dune barrier is in a particularly critical condition in terms of erosion. Nearly 8% of the 8 km-long shoreline in this bay is eroding and parts of transects 13 to 33 (Fig. 8a) show some of the highest rates of historical shoreline retreat in France. Parts of the central and western sectors of the bay retreated by up to 5 m between 1949 and, following an early period of stability and even progradation (Aernouts and Héquette, 6). This eroded part of the bay shows outcrops of peat on the beach that represent former backbarrier vegetation. In areas close to Cape Gris Nez, an upper beach frame of gravel has accumulated as the foredune has retreated (Anthony and Dolique, 1). This retreat constitutes a threat in the coming years, because of the likelihood of a storm breaching of the narrowing dune barrier. The profile of the beach in the eroding sectors of the bay is significantly lower than in the shorter accreting sector in the east. In terms of the overall shoreline dynamics, the western and central parts of the bay form an updrift erosional sector linked to a downdrift sand sink in the east, the latter characterised by significant foredune growth and active formation of embryo dunes (Anthony et al., 6). Reichmüth and Anthony () showed that the beach in the most strongly eroding central sector (Dune d'aval, Fig. 8a) of the barrier underwent a net volumetric loss of over 1% between 1996 and. Digital elevation models of this eroding sector highlight an upper beach subject to much stronger budget fluctuations than the dune front (Anthony et al., 6). The role of storms in barrier retreat in this sector has been demonstrated by joint monitoring of waves and currents, water levels and surges, and high-resolution beach profiling (Ruz and Meur-Ferec, 4; Sedrati and Anthony, 7, 8). Sedrati and Anthony (8) have shown that under strong wind conditions coinciding with spring tides, even the upper beach is subject to strong wind-, wave- and tide-induced longshore currents that lead to active eastward migration of subaqueous D and 3D dunes during tidal submergence. Such strong bedform development on the upper beach in Wissant is a source of significant day-to-day beach profile mobility. Because of chronic erosion, the lowered upper beach surface is subject to strong moisture control (Ruz and Meur-Ferec, 4), which, together with the complex surface topography imprinted by bedforms, tends to limit mobilisation of aeolian sand towards the foredune in this sector. Ruz and Meur-Ferec (4) further showed that aeolian transport and the evolution of the upper beach and dune front were strongly controlled by the magnitude and frequency of occurrence of high water levels, and that aeolian deposition on the upper beach above mean high water spring tide level in this eroding sector occurred only in summer, whereas upper beach and dune-scarp erosion were likely to occur the rest of the year whenever storms coincided with high (spring tide) water levels. These authors also indicated that a single storm could completely annul any mild gains because of summer aeolian accumulation. Sedrati and Anthony (8) showed that the annual mean shoreline retreat rate of over 4 m calculated for this central sector by Aernouts and Héquette (6) may be attained in just 4 h during storms associated with high surge levels (up to 1 m) and high spring tides. Marked alongshore variability is also shown by the dune barrier between Sangatte and Calais (Fig. 8b), east of Cape Blanc-Nez (Fig. 1). In this example, the relatively stable shoreline in Sangatte alternates alongshore with an erosional sector over a few transects (ca..5 km), and then with a sector of significant progradation (transects 33 to 41, Fig. 8b), associated with a very shallow accreted inner shoreface, succeeded alongshore to the northeast by a sector of variable retreat. Old maps of the Calais area suggest a welded sand bank in the area of strong progradation. Perhaps more significantly, Chaverot et al. (8) also highlighted alternating phases of erosion and accretion at several locations, indicating relatively sharp reversals from accretion to erosion and vice versa. East of Dunkerque (Fig. 1), the dune barrier was seriously damaged at the beginning of the th century by urban development and by the construction of the Atlantic wall and blockhouses of World War II. In the 198s, the 1 to m high foredune was affected by breaches and blowouts, and by erosional scarps cut during storms. The foredune is presently in a state of decadal-scale stability, attributed in part to human intervention (Ruz and Anthony, 8). Active rehabilitation carried out in the early 199s, based on a clear dune management scheme involving plantations, construction of fences, and protection of the foredune, has even resulted in mild accretion and incipient foredune development in places. This management scheme is still actively implemented. Mild dune scarping in winter in this area is often followed in spring and summer by limited formation of embryo dunes. Beach surveys at an experimental site east of Dunkerque reported by Reichmüth and Anthony (7, 8) highlight a relatively balanced sediment budget indicated by negligible profile volumetric variations, notwithstanding significant fluctuations in profile morphology. These surveys also bring out longshore variations in the characteristics of the beach profile that have been attributed to the effects of breakwaters and to offshore protection from waves by a nearshore bank, the Hills Bank, extending along the coast over a distance of about 9 km. The crest of the bank is commonly exposed at spring low tides, and forms a shoal at a distance of about 14 m from the beach near Dunkerque (Fig. 1). The bank is separated from the beach by a 1 to 15 m deep sub-shore-parallel channel that is constantly dredged for navigation. Stable sand budgets yielded by digital elevation models of the beach-dune interface (Anthony et al., 7b) tend to confirm the state of quasi-stability of this part of the foredune. Further east towards Belgium, the dune front shows more pronounced erosion, and various World War II blockhouses now lie on the beach. Looking at the entire southern North Sea coast of France between Cape Gris Nez and Belgium, large stretches show either signs of stability, or mild but chronic erosion of foredunes (Chaverot et al., 8), with the western end of Wissant Bay epitomizing extreme erosion. The gross stability of the beach-foredune system suggests conditions of rather limited sand supply from offshore, with exceptions in banksourced areas, such as near Calais. This situation embodies a clear paradox, given: (1) the abundance of fine to medium sand on the nearby shallow shoreface, and () the frequency of storms in the southern North Sea. The reasons for this require further studies, but one of these reasons may be the prevalence of equilibrium conditions between the bank field and the hydrodynamic forcing, other than in exceptional situations such as that of Calais.

9 16 E.J. Anthony / Geomorphology 199 (13) 8 1 a 5 53'N 5 53'3"N 5 54'N 1 36'E 5 5'3"N Dune du Châtelet Wissant Bay Dunes d'aval WISSANT Dune d'amont 5 5 m 1 37'E 1 38'E 1 39'E 1 4'E b Rate of shoreline change (m.yr -1 ) Transects 5 58'N Shoreline in Transect Blériot-Plage km.5 1 Sangatte 'E 6 Shoreline evolution (m) (reference year) Transects Fig. 8. Rates of shoreline change calculated for various time slices between 1949 and : (a) Wissant Bay, (b) between Sangatte and Calais. Note the strong fluctuations alongshore and over time. From Chaverot et al. (8).

10 E.J. Anthony / Geomorphology 199 (13) Discussion Given the apparently intricate relationship between the shoreface and the coastal sand barriers in the southern North Sea, the main question raised by the variability in barrier shoreline trends highlighted in the foregoing section, and posed in this paper, relates to the way this relationship is mediated by storms. This relationship revolves around two related elements: (1) interactions between storm waves and shoreface sand banks in this macrotidal setting, and () sand supply and dune accretion where banks are close to the shoreline, or sand removal and dune erosion where protection and sand-sourcing are not assured by a bank in proximity to the shoreline. The main regional controls are, thus, the 3D shoreface bathymetry and the storm and tidal setting. Embedded in these lower-order conditions are the mesoscale and micro-scale processes of sand delivery to, or removal from, the shoreline. These embedded relationships, which assume morphodynamic feedback loops, are summarised in Table 1. They provide a fine example of the synchroneity in sand supply from shoreface to dune highlighted by Aagaard et al. (4) and Houser (9). Macroscale interactions basically concern large-scale modulation of wave energy through dissipation, and the generation of strong longshore currents by gradients in radiation stress, wind stress and tides. Meso-scale interactions revolve around the mobility of sand waves, bar-trough beach mobility and foredune mobility. Micro-scale processes concern upperflow regime conditions and bedform mobility in the subaqueous domain (tidal banks, sand waves) and in the surf zone (beach bars and troughs), and, finally, swash-zone and aeolian beach-dune sand transport. These aspects are first discussed with reference to the contrasts in response of the various sectors of the barrier shoreline in the southern North Sea. The potential role of storm context is then evoked as a source of variability in shoreline response to storms. Following this, the morphodynamic regime prevailing in this area is briefly compared to that of more classical wave-dominated coasts. A situation of significant accretion is clearly expressed in parts of the barrier between Sangatte and Calais (transects 33 to 41, Fig. 8b), and especially between Calais and Gravelines (Fig. 7), where joint beach progradation and active foredune growth match the sustained secular sand sourcing by sand banks that have welded onshore under the influence of storms. Further east, the barrier in the Dunkerque sector shows overall relative equilibrium associated with a relatively stable foredune and intertidal bar-trough beach system. These conditions are typical of a site that has fluctuated over the past century between mild erosion/accretion and relative stability. Clearly, no generalised accretion of the foredune occurs in this eastern sector of the North Sea coast of France, as previous studies had already shown (Clabaut et al., ; Vasseur and Héquette, ). Although Reichmüth and Anthony (8) have attributed the relative stability of the Dunkerque shoreline sector to protection by the Hills Bank, Héquette et al. (9) did not identify a clear-cut effect of the bank on incident waves, the dissipation of which would appear to reflect the overall effect of the shoreface bank field (Fig. 3c), except during low tide stages, especially spring tides, when the Hills Bank may be particularly effective. Banks, as suggested in Section 3.1, may, however, mitigate offshore sediment transport, especially during storms, because of gradients in radiation stress generated by the complex 3D bank structure. The overall dense shoreface bank field off the Dunkerque site clearly moderates impinging storm waves and the potential for coastal erosion, but banks are probably not close enough to the shoreline in this sector to promote noteworthy coastal accretion, or such accretion is impeded by strong longshore currents in the troughs or channels between banks. Héquette et al. (9) have suggested that the relatively deep channel between the Hills Bank and the beach forms a barrier to onshore sediment transport, while conversely favouring longshore transport. This channel is dredged constantly to maintain adequate depths for high-tonnage ships. Table 1 Regional controls and spatial and temporal scales of morphodynamic interaction involved in the relationship between storms, the shoreface and the dune barrier shoreline in the southern North Sea. Regional controls Spatial and temporal scales of interaction Macro Meso Micro 1. Flat beds and D 3D dunes (subaqueous to intertidal). Beach-to-dune swash and aeolian transport (bar-trough fetch segmentation, back-beach swash and aeolian transport) 1. Inner shoreface sand wave mobility. Beach mobility (bars and troughs) 3. Foredune mobility (erosion/accretion) 1. Hydrodynamic modulation (wave energy dissipation, radiation stress gradients, longshore current generation, setup). Tidal bank characteristics and mobility (sand supply) 1. 3D shoreface bathymetry. Storm climate (barometric pressure, wind speed and direction, storm wave energy, wind stress) 3. Macrotidal setting (tidal stage, tidal currents)

11 18 E.J. Anthony / Geomorphology 199 (13) 8 1 The situation of the Wissant Bay barrier radically differs from that of the Dunkerque barrier sector. The reasons for the onset of erosion and the now chronic nature of this erosion in Wissant Bay, which was progradational up to the early th century, are still not clear. They do seem to involve, however, interactions between the Line Bank offshore, storms, longshore sand transport in the shoreface corridor of which the bank is a part, and probably the activity of current gyres (Anthony and Dolique, 1) related to the projecting headland of Cape Gris Nez (Fig. 9). The Line Bank underwent erosion during the th century, in part because of now-prohibited aggregate extraction on a massive scale. It is possible that the dynamics of the Line Bank itself are embedded in the process of larger-scale storm- and tidecontrolled sand migration (see Section 3.1) from the eastern English Channel towards the Dover Strait and the southern North Sea (Anthony, ). Aernouts and Héquette (6) showed that the shoreline retreat in the southwestern part of the bay has been matched by net sand loss by the Line Bank, with attendant bathymetric lowering. A SWAN wave propagation model simulation by these authors further showed that incident wave energy in this eroding sector had increased in relative to 1977 because of the lower bathymetry of the bank. As lowering of the Line Bank has occurred, dissipation of storm wave energy appears to have been largely transferred to the bar-trough beach and to the foredune (Sedrati and Anthony, 8). These have been rapidly retreating, releasing sand, which is then transported alongshore by strong storm-induced currents via actively migrating intertidal D 3D dunes that develop over the intertidal bars and in the troughs. Observations of erosion and accretion patterns along the bay sediment cell and sand transport calculations by Sedrati and Anthony (7) show that sand released from shoreline erosion in the western sector is not lost offshore, but is transported towards the accreting sink zone in the east (Figs. 8a, 9). This longshore transport is particularly strong during storms coinciding with high spring tides, most likely because of currents due to tides and wind stress, and because gradients in radiation stress, as waves pass or break over the Line Bank, result in the diversion of offshore flows alongshore, thus preventing sand loss offshore. One of the two trailing edges of the Line Bank is close to the shore in the accreting sector (Fig. 9), probably providing sand for the coastal dunes and sheltering the shore from the larger storm waves. Two clear aspects that come out from knowledge acquired on barrier dynamics in Wissant Bay are, thus, the close relationship between the historic lowering of the bathymetry of the Line Bank and foredune retreat (Aernouts and Héquette, 6), and the highly rhythmic nature of this foredune retreat which depends on the right combination of storm waves, spring tidal range and storm surge conditions (Sedrati and Anthony, 8). In this large tide-range setting, the impact of storms may only become significant when storm conditions coincide with large spring tides (e.g., Cooper et al., 4; Pirazzoli et al., 7; Ruz et al., 9). Whereas storm erosion is important in the recent dramatic evolution of this bay, however, the role of storms appears to have been exacerbated by changes in shoreface bathymetry and the influence on enhanced longshore currents and wave energy supply to the shoreline in this macrotidal setting, where high water at spring tides favours significant onshore wave impingement, while low tides at spring can strongly truncate impingement. The contrasting longshore patterns in foredune accretion and erosion exhibited by coastal sand barriers in the southern North Sea are, thus, to a large degree, strongly dependent on the interaction between storms and the shoreface morphology expressed by the jumble of tidal sand banks and ridges that characterise the southern North Sea. The response to high-energy events is strongly conditioned by the inner shoreface, the 3D structure of which, in turn, depends on the longer-term pattern of shoreface morphodynamics and sand redistribution embracing both the eastern English Channel and the southern North Sea. Bank location far offshore constitutes a limiting condition, which, together, with the relative inertia of large-scale shoreward bank movement, explains the commonality of foredune erosion in the southern North Sea. The important but localised foredune accretion in the Calais sector over the last century reflects the fortuitous location, at these sites, of sand banks that had migrated close enough to the shoreline over the last centuries to be finally driven ashore and weld under the impact of the regular storm regime that controls, together with tides and wind stress, the hydrodynamics and shoreface sediment transport in the southern North Sea. In contrast to these sites, morphological changes affecting the Line Bank are deemed to be responsible, at least in part, for the significant erosion of the western and central parts of Wissant Bay, the bank no longer acting in these sectors as a dissipator of storm waves nor as a supplier of sand. This erosion can only be balanced by fresh supplies of sand from offshore, a condition that is not likely to be met in the foreseeable future. The persistence of such erosion clearly suggests that a storm-cut and fairweather-fill regime, more typical of fully wave-dominated systems, does not operate on this storm- and tide-dominated coast because of the propensity for strong longshore sand transport, generally to the east, over the inner shoreface and intertidal zone during high tide. Whereas the foregoing considerations mainly concern embeddedscale process-response interactions, another important point concerns storm context variability. This includes variability in storm barometric pressure, wind speed and direction relative to the shoreline, setup and tidal stage during storms, all of which have a bearing on the outcome of storm shoreline interaction. The bewildering variability inherent in this context will still continue to render unpredictable the response of shores to many storms. One particular point concerns the relationship between storm wind direction relative to the southern North Sea shoreline, barometric pressure, and tidal stage. A favourable combination of strong onshore winds, storm waves, large spring tides, and low barometric pressure must be considered as a potential generator of barrier breaching that poses a permanent background threat to the southern North Sea coastal plain, large areas of which lie below present mean sea level. Ruz et al. (9) documented foredune changes near Dunkerque caused by a storm characterised by moderate but direct onshore winds blowing more than 48 h in March 7, associated with spring tides. Notwithstanding the moderate wind speeds, this storm resulted in major foredune retreat, following a decade marked by larger storms but with little impact on the coast. The results show that on this macrotidal coast, erosive events are not necessarily associated with strong winds, whereas wind direction and duration combined with a spring tide appear to hold one of the keys to medium-term foredune evolution. This example also illustrates a situation where a combination of high water levels because of the spring-tide influence and storm-related water setup in the coastal zone rendered the sand banks inefficient as wave dissipators, notwithstanding the moderate storm wave energy. The foregoing elements of analysis of the storm shoreline relationship in the southern North Sea may also provide a template for a better understanding of temporal trends in this relationship, such as those identified by Chaverot et al. (8). These authors have shown that the highly variable evolution of the shoreline of northern France during the second half of the th century, elements of which are depicted in Fig. 8, displayed no clear relationship with storminess in most cases. They showed, for instance, that from 1963 to 1977, the shoreline significantly advanced seaward at several sites whereas the period corresponded to that of maximum storm activity in this area. Conversely, shoreline retreat occurred at most sites during the 199s, despite an identified significant decrease in storm frequency. The storm shoreface shoreline interaction pattern identified for the southern North Sea coast is different from that of more classical wave-dominated coasts such as those documented by Hequette et al. (1) and Backstrom et al. (8, 9). It involves macro- to microscale morphodynamic feedback between storms and the numerous dissipative sand banks, and onshore sand transfer via dominantly mass bank migration. These sand bank movements most likely involve

12 E.J. Anthony / Geomorphology 199 (13) Fig. 9. Sketch of Wissant Bay, showing the barrier shoreline status, the Line Bank offshore and aspects of the hydrodynamic and sand circulations offshore and on the beach. Bathymetric contours are from Augris et al. (1995). Adapted from Anthony and Dolique (1). timescales of decades to centuries and include embedded smaller-scale sand movements involving sand waves in the subtidal zone close to the shoreline, beach longshore bar-trough formation and shoreward migration, and at increasingly smaller scales, bar bedform migration and both swash and aeolian transport in the intertidal zone (Table 1). In sediment-budget terms, this pattern also differs from that of the wave-dominated low-lying barrier island systems characterising the Atlantic and Gulf of Mexico seaboards, for instance. In many areas on these coasts, the shoreface, nearshore multiple bar complexes, and beaches evolve in a sand-limited context characterised by sand transfer landward into multi-decadal to century-scale storage in coastal dune, barrier, and flood-tidal delta sinks, in part via washovers during storms (e.g., Forbes et al., 4). 5. Conclusions 1. Sand barriers in the southern North Sea exhibit contrasting longshore and time-varying (multi-annual) patterns of foredune accretion and erosion that are, to a large degree, strongly dependent on the interaction between shoreface morphology, dominated by subaqueous tidal sand banks that characterise the shallow southern North Sea, and storms.. The rather irregular pattern of shoreline accretion, stability or erosion shown by barriers appears to be largely from: (i) longshore variations in the onshore supply of sand from the shore-parallel to sub-shore-parallel banks, and (ii) incident storm wave energy variations related to shoreface bathymetry. 3. Foredune accretion is associated with areas where major sand banks have migrated close to the shoreline over the last centuries to be finally driven ashore and weld under the impact of the storm regime that controls, together with tides and wind stress, the hydrodynamics and shoreface sediment transport in the southern North Sea. In contrast, morphological changes in nearshore banks, such as bank volume depletion through longshore stretching or aggregate extraction, can favour significant foredune erosion, such banks no longer acting as dissipators of storm waves. 4. These macro-scale interactions underline the synchroneity of sand transport from shoreface to dune, and include embedded mesoscale interactions relating to the mobility of sand waves, bartrough beach mobility and foredune mobility. These are, in turn, translated by micro-scale processes involved in 3D dune bedform mobility in the subaqueous domain (tidal banks, sand waves) and in the intertidal domain (beach bars and troughs), as well as in swash-zone and aeolian beach-dune sand transport. 5. In addition to the foregoing conditions, foredune accretion and erosion also depend on storm context variability that includes storm characteristics and wind incidence relative to the shoreline, and tidal stage during storms. These parameters introduce further complexity and unpredictability of barrier response to individual storms. A likely illustration of this may be the evidenced lack of a clear relationship between periods of storminess and coastal response. 6. Large stretches of the southern North Sea foredunes of France show either signs of stability, or mild but chronic erosion that may suggests conditions of rather limited sand supply from offshore, with exceptions in areas where a nearshore storm-driven tidal sand bank has become shore-attached. This paradoxical situation, given the abundance of sand on the nearby shallow shoreface, may result from large-scale shoreface equilibrium with the hydrodynamic context. Acknowledgements Special guest editors Nancy Jackson and Karl Nordstrom, two anonymous reviewers, and Jack Vitek, are thanked for their constructive review comments. Denis Marin and Patrick Pentsch prepared the illustrations. References Aagaard, T., Davidson-Arnott, R., Greenwood, B., Nielsen, J., 4. Sediment supply from shoreface to dunes: linking sediment transport measurements and longterm morphological evolution. Geomorphology 6, 5 4.