Marie-Hélène Ruz
email: ruz@univ-littoral.fr

Arnaud Héquette

Denis Marin

Development of large nebkhas along an accreting macrotidal coastline, Northern France

Keywords: nebkhas, incipient foredunes, accreting coastline

Very large isolated nebkhas, up to 4 m high and 14 m wide, have developed on the upper beach of a several hundred meters wide foreshore on the French coast of the Dover Strait. This macrotidal shoreline is characterized by abundant sediment supply from the shoreface related to the onshore welding of a nearshore sand bank. Continuing beach accretion provided the conditions for rapid seaward development of incipient foredunes. The nebkhas developed on the uppermost beach, slightly above the highest astronomical tide limit, forming hemispheric to oval-shaped mounds only reached by spring tides and/or storm surges. They do not form an incipient foredune zone, but mounds that grew vertically and remained in the form of isolated huge nebkhas disconnected from the incipient foredune zone by a swale. The nebkhas consist of scattered landforms that do not impede aeolian accretion landward in the incipient foredune zone where the maximum accretion rates were recorded. The distribution of such large coastal nebkhas is probably limited in beach environments since the descriptions of similar examples of aeolian coastal landforms are virtually absent in the scientific literature, suggesting that they are presumably restricted to low gradient macrotidal beaches associated with an excess of sand supply.

Highlights:

• We document the formation and evolution of huge isolated nebkhas on an upper beach.

• Such large nebkhas have not been reported yet in beach environments.

• The nebkhas do not limit the development of an incipient foredune zone landward.

• They are likely restricted to macrotidal foreshores with abundant sediment supply.

Introduction

Sandy beaches offer an exposed source of sediment and so most sandy beaches are fringed by some form of sand dune formed by sand transported by wind and deposited landward of the beach (Davidson-Arnott, 2010; [START_REF] Martinez | Coastal Dunes, Ecology and Conservation, Ecological Studies[END_REF]. Dunes of various size and morphology can be found depending on sediment supply, dominant wind velocity and direction, moisture and vegetation cover, but also on the geomorphology of the nearshore and intertidal beach [START_REF] Houser | Beach and Dune Interaction[END_REF]. Immediately landward of the active beach, incipient foredunes, which are also called embryo dunes by some authors (e.g., [START_REF] Davidson-Arnott | Introduction to Coastal Processes and Geomorphology[END_REF], commonly develop, consisting of low dunes forming by aeolian sand accumulation in areas covered by pioneer plant communities [START_REF] Hesp | Foredunes and blowouts: initiation, geomorphology and dynamics[END_REF][START_REF] Hesp | Surfzone-beach-dune interactions[END_REF]. Their formation is due to the presence of some roughness elements on the backshore that reduce wind flow velocity, resulting in sediment deposition. On beaches, the debris that accumulate at the drift line efficiently increase surface roughness, favouring the deposition of aeolian sand, but the formation of incipient dunes essentially takes place above the high tide limit where vegetation is the most common roughness element that initiates their formation. According to [START_REF] Hesp | Foredune formation in Southeast Australia[END_REF][START_REF] Hesp | A review of biological and geomorphological processes involved in the initiation and development of incipient foredunes[END_REF], their morphological development depends on plant species, density, height and cover, as well as wind speed, rates of sand transport, and beach progradation rates. In their early stage of development, incipient foredunes may form nebkhas, often with attendant shadow dunes [START_REF] Hesp | The formation of shadow dunes[END_REF]. Nebkhas (also called nabkhas, coppice dunes, dune mounds) form isolated hummocks, and are common features in many aeolian settings [START_REF] Melton | A tentative classification of sand dunes its application to dune history in the southern High Plains[END_REF][START_REF] Hesp | Dune coasts[END_REF]. They are sometimes considered as a distinct dune type [START_REF] Pye | Aeolian Sand and Sand Dunes[END_REF][START_REF] Nickling | The morphology and origin of nabkhas, region of Mopti, Mail, West Africa[END_REF][START_REF] Lancaster | Geomorphology of Desert Dunes[END_REF][START_REF] Hesp | Morphology, dynamics, ecology and fauna of Arctotheca populifolia and Gazania rigens nabkha dunes[END_REF]. They usually develop as a result of the deposition of wind-driven sediments around shrubs in arid and semi-arid regions [START_REF] Cooke | Desert Geomorphology[END_REF]. Nebkhas differ from shadow dunes that develop as a result of sediment deposition in the lee of an obstacle [START_REF] Bagnold | The Physics of Blown Sand and Desert Dunes[END_REF][START_REF] Hesp | The formation of shadow dunes[END_REF]. Nebkhas are also very common features along sandy coastlines [START_REF] Hesp | Dune coasts[END_REF]. They are found in arid and semi-arid coastal environments [START_REF] Khalaf | Sedimentological and morphological characteristics of some nabkha deposits in the northern coastal plain of Kuwait, Arabia[END_REF][START_REF] Langford | Nabkha (coppice dune) fields of south-central New Mexico[END_REF][START_REF] El-Bana | Role of host identity in effects of phytogenic mounds on plant assemblages and species richness on coastal arid dunes[END_REF][START_REF] Ardon | Dynamics of nebkhas superimposed on a parabolic dune and their effect on the dune dynamics[END_REF]Khalaf andAl-Awadhi, 2012: Hernandez-Cordero et al., 2015) as well as in temperate [START_REF] Hesp | Aeolian environments: coastal dunes[END_REF], cool temperate humid (Mountney and Russel, 2006) and cold environments [START_REF] Ruz | Sedimentary Coastal Zones from High to Low Latitudes: Similarities and Differences[END_REF]. They can develop at various locations, from the upper beach to low backbarrier areas [START_REF] Carter | Dune development in the aftermath of stream outlet closure: examples from Ireland and California[END_REF], although nebkhas can also form in interdune areas or in the slacks of active dunes [START_REF] Hesp | Morphology, dynamics, ecology and fauna of Arctotheca populifolia and Gazania rigens nabkha dunes[END_REF].

However, they usually develop on the backshore on swash-aligned drift material or on storm debris lines, through alongshore growth of pioneer plant seedlings and/or by rhizome growth, forming a quasi-continuous line of low (50-100 cm high) vegetated mounds [START_REF] Hesp | A review of biological and geomorphological processes involved in the initiation and development of incipient foredunes[END_REF].

The shape and size of coastal nebkhas is variable. Along the arid coastal plain of Kuwait for example, the nebkha height varies between 0.4 to 1.2 m on average [START_REF] Khalaf | Sedimentological and morphological characteristics of gypseous coastal nabkhas on Bubiyan Island, Kuwait, Arabian Gulf[END_REF]. Nebkhas developing in temperate environments are usually forming discrete hummocks that are typically 0.2 -0.5 m to 1 to 2 m in height [START_REF] Davidson-Arnott | Introduction to Coastal Processes and Geomorphology[END_REF][START_REF] Montreuil | Decadal and seasonal development of embryo dunes on an accreting macrotidal beach: North Lincolnshire, UK[END_REF]. In cool humid coastal environments, as along the southern coastline of Iceland, an extensive nebkha field developed, with individual nebkha up to 2 m high (Mountley and Russel, 2006). Small nebkhas, 0.5 to 1 m high are also found along arctic coastlines [START_REF] Ruz | Sedimentary Coastal Zones from High to Low Latitudes: Similarities and Differences[END_REF].

Incipient foredunes can be ephemeral features that can be partly or completely eroded during storm events, especially when they develop near the high tide limit. They may also withstand storms if they are high enough above high spring tide limit where they represent the initial stage of foredune development. Foredunes (incipient and established) are dependent aeolian landforms, linked to beach and nearshore processes and are sometimes considered as the only distinctive coastal dune forms [START_REF] Bauer | Coastal dune dynamics: problems and prospects[END_REF], where real dune formation can begin [START_REF] Arens | Patterns of sand transport on vegetated foredunes[END_REF]. According to [START_REF] Hesp | Foredunes and blowouts: initiation, geomorphology and dynamics[END_REF], all other factors being equal, large foredunes are more frequent on dissipative beaches, which are characterized by wide foreshore and maximum potential sediment supply. Macrotidal sandy beaches, with a wide wind-exposed foreshore at low tide, are also thought to represent optimal conditions for the development of coastal dunes [START_REF] King | Beaches and coasts[END_REF][START_REF] Carter | Coastal Environments -An Introduction to the Physical, Ecological and Cultural Systems of Coastlines[END_REF]. Well-developed coastal foredunes are thus common along low-gradient macrotidal beaches [START_REF] Battiau-Queney | Beach-dune systems in a macrotidal environment along the northern French coast[END_REF][START_REF] Anthony | 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[END_REF][START_REF] Anthony | Aeolian sand transport over complex intertidal bar-trough beach topography[END_REF][START_REF] Pye | Decadal-scale variation in dune erosion and accretion rates: An investigation of the significance of changing storm tide frequency and magnitude on the Sefton coast, UK[END_REF][START_REF] Montreuil | Decadal and seasonal development of embryo dunes on an accreting macrotidal beach: North Lincolnshire, UK[END_REF]. This is particularly the case on the northern coast of France where nebkha of substantial size have developed seaward of the foredune along a prograding macrotidal foreshore of the southern Dover Strait. In this paper, we report on the formation and evolution of isolated huge nebkhas that appear to be uncommon for seawardmost dunes and we try to determine the conditions leading to their development.

Study area

The French North Sea coastline is a 55 km long almost continuous sand beach barrier extending from the Cap Blanc Nez chalk cliff to the Belgium border, facing the eastern English Channel and the southern North Sea (Figure 1). The coast essentially consists of wide and gently sloping sandy beaches with multiple intertidal bars [START_REF] Anthony | Patterns of short-term intertidal bar mobility on a ridge and runnel beach, Merlimont, Northern of France[END_REF][START_REF] Reichmüth | Tidal influence on the intertidal bar morphology of two contrasting macrotidal beaches[END_REF] backed by coastal dunes that commonly exceed 10 m high and reach up to 25 m in places [START_REF] Battiau-Queney | Beach-dune systems in a macrotidal environment along the northern French coast[END_REF][START_REF] Ruz | Coastal dune evolution on a shoreline subject to strong human pressure : the Dunkirk area, northern France[END_REF]. The dunes merge with the French coastal plain that extends 10 to 20 km landward. The coastal plain is a low-lying reclaimed land with mean elevation of 2 m above mean sea level. Based on historical records (old topographic and hydrographic maps), this area experienced shoreline progradation since at least the 14 th century [START_REF] Briquet | Le Littoral du Nord de la France et son évolution morphologique[END_REF]. As the shoreline prograded seaward, dikes were erected in order to gain new farm lands. Nowadays, coastal dunes and dikes are the only protection against marine flooding. The study area consists of a 350 to 500 m wide beach at low tide, east of Calais (Figure 1), comprising a series of intertidal bars and an upper beach platform bounded landward by coastal dunes. The coastal dunes bound a salt marsh that has been largely modified by humans, with the excavation of numerous hunting ponds (Figure 1). Eastward, the intertidal zone forms a protruding and extensive sandflat. The beach and sandflat exhibit large variability in surface sediment size, depending on morphology, elevation and exposure to waves. The intertidal bars and troughs are composed of well-sorted fine to medium sand (mean grain size: 0.2 to 0.33 mm), while the upper beach sandflat mainly consists of fine sand (mean grain size: 0.23 mm).

The coastal dunes in the backshore are composed of fine to medium sand (mean grain size: 0.17 to 0.31 mm).

Offshore, tidal sand banks are widespread across the nearshore zone and the inner shelf where they form extensive linear sand bodies sub-parallel to the shoreline [START_REF] Augris | Le domaine marin du Nord-Pas-de-Calais -Nature, morphologie et mobilité des fonds[END_REF][START_REF] Beck | Sand bodies and sand transport paths at the English Channel-North Sea border: morphology, dynamics and radioactive tracing[END_REF]. Nearshore tidal sand banks have been migrating alongshore and/or onshore for centuries. Immediately east of Calais, a sand bank called "banc Braseux", parallel to the shoreline, merged to the shore during the 16 th century [START_REF] Briquet | Le Littoral du Nord de la France et son évolution morphologique[END_REF], which resulted in the formation of a convex-shaped sand platform protruding seaward (Figure 1). During the 19 th and 20 th centuries, the Ridens de la Rade, a shallow 13 km long and 1.4 km wide sand bank (Figure 1), migrated alongshore and onshore and became eventually attached to the shore [START_REF] Augris | Cartographie géologique des fonds marins côtiers Exemples le long du littoral français[END_REF]. Comparison of bathymetric charts revealed that this shore-attached sand bank grew up to about 100×10 6 m 3 during the 20 th century [START_REF] Héquette | The influence of nearshore sand bank dynamics on shoreline evolution in a macrotidal coastal environment, Calais, Northern France[END_REF], forming an extensive sub-tidal sand source for the development of the intertidal sandflat and coastal dunes in the backshore [START_REF] Anthony | Storms, shoreface morphodynamics, sand supply, and the accretion and erosion of coastal dune barriers in the southern North Sea[END_REF][START_REF] Anthony | 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[END_REF], therefore favouring shoreline progradation.

Except for episodic storm events, the coast is exposed to low-energy waves. The dominant winds are from west to southwest, with a secondary wind direction from north to northeast (Figure 1). Winds are usually moderate, with more than 45 % of winds having a mean velocity of less than 5 m/s; strongest winds (≥16 m/s) occur only 0.06 % of the time. Associated with this wind regime, waves predominantly come from the English Channel with a direction from southwest to west, followed by waves generated in the North Sea from the north to northeast.

Modal significant wave height is less than 1 m with wave periods typically ranging from 4 to 8 s, but maximum wave height may episodically exceed 5 m with periods of 9 to 10 s during major storms (http://candhis.cetmef.developpement-durable.gouv.fr/campagne/). Wave heights are significantly lower at the coast, due to significant shoaling and energy dissipation over the offshore sand banks, resulting in wave heights that hardly exceed 1 m in the intertidal zone even during storms [START_REF] Sedrati | Storm-generated morphological change and longshore sand transport in the intertidal zone of a multi-barred macrotidal beach[END_REF][START_REF] Héquette | Theoretical and observed breaking wave height on a barred macrotidal beach: implications for the estimation of breaker index on beaches with large tidal range[END_REF].

The tidal regime in the region is semi-diurnal and is characterized by a large tidal range that increases from the North Sea to the English Channel, the tidal amplitude reaching 6.5 m at Calais during spring tides. Due to this large tidal range and current funnelling through the Dover Strait, tidal currents are powerful along the northern coast of France, reaching near-surface velocities of 1.5 ms -1 during flood and 1.35 m s -1 during ebb in the narrow channels of the nearshore sand banks [START_REF] Augris | Le domaine marin du Nord-Pas-de-Calais -Nature, morphologie et mobilité des fonds[END_REF]. Tidal currents flow almost parallel to the shoreline, with flood currents directed to the east-northeast and ebb currents to the west-southwest.

Because the dominant waves come from the southwest and the tidal currents asymmetry is flood-dominated, net sediment transport on the shoreface and on the foreshore is directed to the east-northeast [START_REF] Héquette | Sediment transport under wave and current combined flows on a tide-dominated shoreface, northern coast of France[END_REF][START_REF] Cartier | Vertical distribution of longshore sediment transport on barred macrotidal beaches, Northern France[END_REF].

Methodology

The evolution of coastal dunes was based on the comparison of orthorectified vertical aerial photographs (years 1963, 1983, 2000 and 2012) and georeferenced and rectified aerial photographs (years 1938, 1949, 1957, 1961, 1965, 1969, 1970, 1972, 1975, 1977, 1980, 1982, 1986, 1988, 1989, 1992, 1993, 1997, 2005 and 2009), which were used for carrying out detailed geomorphological evolution of coastal dunes from stereo-pairs of aerial photographs. The error range of the orthorectified aerial photographs is ± 2 m.

High-resolution topographic data of the foreshore, upper beach and coastal dunes were obtained from airborne LiDAR surveys realised in 2011 (03/21/2011) and2014 (01/18/2014). The data were obtained with a Leica ALS60 LiDAR system that acquired topographic data with a density of 1.2 to 1.4 points/m 2 . The LiDAR operating system was combined with real time kinematic DGPS and inertial motion unit, ensuring a planimetric precision of less than 0.5 m.

The position accuracy of the LiDAR data points ranged from ± 0.10 to 0.17 m with a vertical accuracy less than ± 0.10 m as verified with several ground control points using a very high resolution differential GPS (Leica TPS Syst1200). However, vertical error can easily increase to ± 0.25 m or even more in areas covered by dense vegetation [START_REF] Saye | Beach-dune morphological relationships and erosion/accretion: An investigation at five sites in England and Wales using LIDAR data[END_REF]. The LiDAR topographic data were therefore filtered to remove vegetation and other objects above ground surface. Digital Terrain Models (DTM) were created from the filtered data using Golden Software Surfer™. The different DTMs were then used to calculate volume changes between each LiDAR survey. The DTMs were obtained using linear Delaunay triangulation interpolation, which resulted in a grid with a 1 m resolution. According to previous work on the spatial characterization and volume calculation of coastal dunes using LiDAR topographic data, a grid cell resolution of 1 m 2 provides reliable representation of topography and accurate volumetric measurements in coastal dunes [START_REF] Woolard | Spatial characterization, resolution, and volumetric change of coastal dunes using airborne LIDAR: Cape Hatteras, North Carolina[END_REF].

Observed and predicted hourly water levels at Calais were obtained from the SHOM (Service Hydrographique et Océanographique de la Marine). Water level data were analysed to determine the periods of time during which high water levels could reach the toe of the investigated coastal dunes. Tri-hourly and hourly wind data were obtained from Météo-France for the period 1994-1999 (tri-hourly data) and 2000-2014 (hourly data) to assess efficient winds for potential aeolian sand transport.

Results

Development of sandy islets and shoreline advance

On the 1938, 1949 and 1957 aerial photos, three sandy "islets", 190 m to 360 m long and 150 m to 210 m wide, detached from the main shore, are clearly visible on the upper beach platform (Figure 2). In 1938, these islets were poorly vegetated and were separated from each other by channels, and from the main shoreline by a wet upper beach 110 m wide, indicating tidal incursion around and past them at high tide. The main shoreline consisted of low vegetated coastal dunes, with a hummocky topography, and to landwards, reclaimed salt marshes backed against a dike erected in 1773 (Taaf dike). In 1957, the sandy islets were still disconnected, but appeared more vegetated and partially covered by low coastal dunes (Figure 2). According to [START_REF] Briquet | Le Littoral du Nord de la France et son évolution morphologique[END_REF], coastal dunes in this area were not continuous, but formed an archipelago of small sandy islands separated by narrow channels and spaced one hundred metres or more apart (Briquet, 1930, p. 298). In his description of the shoreline in ca. 1920-1925[START_REF] Briquet | Le Littoral du Nord de la France et son évolution morphologique[END_REF] called these coastal dunes, developed at the uppermost tide level, "insular dunes" as they formed isolated features, detached from the main foredune and surrounded by the sea during spring tides. It is likely that the three islets visible on the 1938 aerial photographs correspond to these "insular dunes" described by Briquet. In 1938, only three large sandy islets and four small ones were present along this section of the coastline. Between 1963 and 1993, these islets were progressively integrated to the main shore (Figure 3). In 1963, they were separated by channels, but by 1972, the two eastern ones were clustered together, and only a pathway to the beach remained (Figure 3). The density of vegetation cover, likely marram grass and shrubs, increased during this time period. Dune morphology was chaotic, but hemispheric mounds are discernible on the 1963 aerial photo, especially on the western islet. Behind, hunting ponds were dredged and connected to the sea by a channel which was artificially excavated in order to maintain seawater incursion in the ponds. By 1986, the connected sandy islets formed the new shoreline (Figure 3), resulting in a shoreline advance of 320 m since 1938. Incipient coastal dunes progressively developed on the upper beach and the "older" dunes were covered by shrubs. This was followed by the development of an established foredune visible on the 1993 aerial photo. The 1993 aerial photographs also shows that a new incipient foredune, with a hummocky topography, developed seaward of the established foredune, resulting in continuing shoreline progradation.

Formation of large isolated nebkhas

By 1986, a few discrete, scattered, bare, circular sandy hummocks, developed on the upper beach, 45 m to 50 m seaward of the incipient foredunes (Figure 3). By 1993, most of them had disappeared, but in 1997, a dozen of roughly circular sand patches, 13 to 40 m long and 10 to 28 m wide were visible (Figure 4). They were separated from the incipient foredune zone by a bare sand area 35 to 65 m wide. By 2000, a series of nascent nebkhas had developed. To the east, two lines of nebkhas developed. A first line of 5 nebkhas was close to the incipient foredune zone, while the seaward most nebkhas were located 30 to 50 m from the incipient foredune zone. In 2001 the nebkhas were partially vegetated and most of them were 1 to 1.5 m high, the highest ones reaching an elevation up to 2 m [START_REF] Tékin | Un type original de dunes embryonnaires sur la plage macrotidale du Fort-Vert (Pas de Calais, France)[END_REF]. By 2005, they formed well developed isolated nebkhas, still disconnected from the incipient foredune zone (Figure 4). On the aerial photo of 2005 (Figure 4), the wet sand surface between the nebkhas and the incipient foredune zone indicates they were episodically surrounded by water during high tides. of the main shoreline (seaward limit of the incipient foredune zone). Their seaward toe is slightly above the highest astronomical tide elevation (Figure 5). Table 1. Morphological parameters of the nebkhas. The location of each nebkha is shown on Figure 5. The main shoreline corresponds to the seaward limit of the incipient foredune zone. Most of them have a height of 2 to 3 m (Tabl. 1) and are located at the landward limit of a 20 m to 30 m wide, low gradient, upper beach (slope of 1.0 to 1.3% between the MHWS and the HAT level). The largest nebkha is 4 m high, 52.5 m long and 10 m wide. This particularly long nebkha results from the coalescence of two individual nebkhas. The variation in the size and the shape of the nebkhas over a short distance may be related to the time of initiation of each individual nebkha, but also to their coalescence during their subsequent development.

These scattered nebkhas, aligned along the uppermost tidal level on the very upper beach, form hemispheric to oval-shaped mounds only reached by spring tides and/or storm surges (Figure 6). They do not form an incipient foredune zone, but mounds that grew vertically and have remained in the form of isolated huge nebkhas. On the upper beach, some poorly developed nebkhas are still forming today, which are commonly observed in areas of extensive shell pavements (Figure 7). Such shell pavements, covering the beach, limit sediment transport [START_REF] Carter | Formation, maintenance and geomorphological significance of an aeolian shell pavement[END_REF] and it is also difficult for plants to establish in dense shell lags [START_REF] Davidson-Arnott | Seasonal patterns and controls on sediment supply to coastal foredunes, Long Point, Lake Erie[END_REF]. Surface shell lags also likely limit the development of pioneer species all along the wrack line. Wind driven fine sand preferentially accumulates around clumps of vegetation (Elymus farctus), resulting in the formation of circular low-elevated incipient nebkhas (Figure 7). Elymus farctus (sand couchgrass) is a perennial pioneer, salt-tolerant, strand species. It is a rhizomatous plant spreading outwards by means of long and wiry underground stems. By putting up vertical sympodial branches as a result of repeated tillering, Elymus farctus can keep pace with rapid though modest accumulation of sand [START_REF] Nicholson | A study of Agropyron junceum (Beauv.) in relation to the stabilisation of coastal sand and the development of sand dunes[END_REF]. Once the plant starts catching sand, its root system begins to develop a network of deep roots with side branches. To prevent being buried by the sand, the plant is continually growing. This plant, with culms 20-60 cm, can germinate (either by seedling or from rhizome fragments) in accumulations of organic tidal litter or directly on the beach and can survive seawater inundation. When Elymus farctus germinates, windblown sand preferentially accumulates around clumps of vegetation. A discrete, semicircular, convex dome is typically formed (Figure 7). Then, they progressively grow up and enlarge, forming nebkhas. Although most of the nebkhas are exclusively colonised by Elymus farctus, some well-developed ones have clumps of Ammophila arenaria on their crest. Located near the highest water level limit, they are episodically surrounded by seawater during wind-induced and/or barometric surges. When developing, the seaward most ones can be episodically eroded during storm events. Nebkhas are mainly parallel to southwesterly dominant winds and most of them have an elongated shape. However, depending on wind direction, nebkha morphology can change (Figure 8). Under strong dominant winds from the southwest, parallel to the beach, they have a tail parallel to the prevailing wind direction (240°) (Figure 8A). Under direct onshore winds, their stoss side can be eroded and sand preferentially accumulates on their landward side (Figure 8B). Most of the nebkhas have steep flanks, inclined up to 50° and can be steepened on their seaward side in response to episodic wave scarping.

Figure 8. Sand accumulation behind nebkhas under (A) longshore and (B) onshore winds.

The influence of hydro-meteorological conditions on the evolution of nebkhas and incipient foredune zone

The analysis of hourly uppermost water levels recorded in Calais harbor (Figure 9) reveals A second storm, Xaver storm, struck northern Europe in December 2013 [START_REF] Masselink | The extreme 2013/2014 winter storms: hydrodynamic forcing and coastal response along the southwest coast of England[END_REF]. Strong northwest winds, low atmospheric pressure, combined with a spring tide, generated a significant storm surge that has been responsible for erosion all along the coastline of the region. At Calais, on December 5 th and 6 th , southwestern to northwestern mean hourly winds above 8 m/s blew during more than 24 consecutive hours. The maximum instantaneous recorded wind speed was 24.4 m/s, while the maximum mean hourly recorded wind speed was 16 m/s. In this area, where wind speed rarely exceeds this value [START_REF] Chaverot | Changes in storminess and shoreline evolution along the northern coast of France during the second half of the 20 th century[END_REF], it has been shown that wind duration is a major factor inducing storm surges. Moderate winds, 8 to 10 m/s, blowing more than 24 hours and combined with a high tide, can induce significant erosion along this coastline [START_REF] Ruz | Identifying forcing conditions responsible for foredune erosion on the northern coast of France[END_REF]. In Calais harbour, on December 6 th 2013, the maximum recorded water level reached about 8.68 m above hydrographic datum [START_REF] Daubord | Caractérisation de 7 événements de tempête de l'automne-hiver 2013-2014 à partir des données disponibles au SHOM[END_REF] or 5.22 m relative to topographic datum (IGN69). This observed water level is greater than the numerical value of extreme high water levels calculated for a 100-year return period which is 5.04 m (IGN69).

The recent morphological evolution of the nebkhas was strongly influenced by this latter storm event. The LiDAR topographic data collected on January 18 th 2014 show that most of the nebkhas underwent erosion during the December 2013 storm event (Figure 10). During the storm surge, the maximum water levels reached the swale, isolating some nebkhas and eroding their landward slope. This happened only in areas where the swale depth was well under the storm surge level (Profile 4 and 5, Figure 10). The width of the seaward most nebkhas was reduced; however their height remained stable or increased due to vertical accretion. Two nebkhas were severely or completely eroded during this event (see Profile 1, Figure 10). The swale that formed between the nebkhas and the incipient foredune zone underwent both erosion and accumulation between 2011 and 2014 (Figure 11). Erosion mainly affected slightly lower areas located landward of the nebkhas (Figure 10), which were reached by the storm surge of December 2013. The incipient foredune zone was not reached by this storm surge and accumulation prevailed, with sand accretion resulting in up to 1.2 m of accumulation between 2011 and 2014 (Figure 10). Meanwhile, the established foredune remained fairly stable. A sediment budget calculation from a comparison of LiDAR data obtained in 2011 and 2014 along this portion of the coastline (Figure 11) was negative for the foreshore, with a mean erosion of -0.19 m 3 /m 2 . This negative budget can be related to the Xaver storm event, the 2014

LiDAR survey having been carried out only a few weeks later. However, on the upper beach, including the nebkhas, the sediment budget was positive, with a gain of +0.20 m 3 /m 2 . The maximum accumulation was recorded in the incipient foredune zone with a mean accumulation of 0.61 m 3 /m 2 . Even if huge nebkhas were present on the upper beach, they did not impede sediment transfer from the foreshore to the incipient foredune zone as well as in the swale where sediment accumulation locally occurred (Figure 11). 10) and correspond to the distance between the uppermost water levels during the windy events and the toe of the nebkhas (4.5 to 5 m IGN 69). Mean spring tidal range: 6.5 m; mean neap tidal range: 4 m. The grey lines correspond to potential favourable conditions for aeolian sand transport. from the southwest sector (the highest mean hourly recorded wind speed during this period of time was 18.5 m/s). Twelve significant wind events (efficient winds ≥ 12 m/s blowing during more than 6 consecutive hours) were recorded during this period (Tabl. 2). Seven of them occurred during neap tide or moderate tidal range, potentially favoring aeolian sand transport on the upper beach. During these events, the exposed upper beach was a hundred metres wide, ranging from 89 m to 264 m (based on tide gauge measurements), depending on the location of each nebkha. During such conditions, the maximum wind fetch at high tide, under parallel winds (SW) could exceed 2000 m, while under onshore winds (NNW) the fetch is restricted to less than 160 m. As stressed by [START_REF] Anthony | Aeolian sand transport over complex intertidal bar-trough beach topography[END_REF], along macrotidal beaches, strong oblique onshore winds blowing on the dry upper beach during neap tides likely represent favorable conditions for incipient foredune accretion. Isolated nebkhas were not an obstacle for aeolian sand transport as they are scattered over the upper beach zone. Furthermore the bare sandy swale between them and the incipient foredune zone can also be a sand source for the incipient foredune.

Discussion

The nebkhas described in the present study are the largest we are aware of developed in the foreshore/intertidal region of a beach. The development of huge, spatially isolated nebkhas disconnected from the main foredune and situated this far out on the foreshore/intertidal region is probably uncommon as, to our knowledge, it has not been specifically reported in the scientific literature. Very large nebkhas have been described in desert environments where they can reach greater heights [START_REF] Cooke | Desert Geomorphology[END_REF][START_REF] Langford | Nabkha (coppice dune) fields of south-central New Mexico[END_REF][START_REF] Lang | Nebkha (coppice dune) formation and significance to environmental change reconstruction in arid and semi-arid areas[END_REF]). In China for example, nebkha-like features (Tamarix cones) up to 15 m high have been reported in the Taklimakan Desert [START_REF] Qong | Formation and internal structure of Tamarix cones in the Taklimakan Desert[END_REF]. Large sand mounds that accumulate around clumps of trees are called mega-nebkhas [START_REF] Khalaf | Sedimentological and morphological characteristics of some nabkha deposits in the northern coastal plain of Kuwait, Arabia[END_REF]. Such mega-nebkhas, reaching 10 m high and up to 1 km long, have notably been reported by [START_REF] Cooper | Coastal sand dunes of Oregon and Washington[END_REF] on the edges of Oregon coastal dunes.

The large nebkhas described in this study, developed at the seaward edge of an upper beach platform close to the highest astronomical tide limit, are scattered and grew up in situ, reaching an elevation up to 4 m. According to [START_REF] Ranwell | Ecology of Salt Marshes and Sand Dunes[END_REF], an irregular zone of scattered nebkhas and shadow dunes can develop if only a few seeds are transported to the backshore by swash or wind, or if only a few plants survive. In the study area, the development of scattered nebkhas is likely linked to the presence of shell lags that are commonly observed on the upper foreshore and backshore. In the presence of such shell lags, restricting sediment supply from the foreshore and the backshore, the wind is unsaturated, which reduces the sediment trapping capacity of the vegetation [START_REF] Houser | Controls on coastal dune morphology, shoreline erosion and barrier island response to extreme storms[END_REF][START_REF] Hesp | Aeolian environments: coastal dunes[END_REF]. In this case, the establishment of pioneer plants is restricted [START_REF] Davidson-Arnott | Seasonal patterns and controls on sediment supply to coastal foredunes, Long Point, Lake Erie[END_REF] and, in the study area, Elymus farctus is the only pioneer species colonising scattered nebkhas. In addition, because the observed nebkhas are isolated from the incipient foredune zone and are surrounded by bare sandy areas, they are exposed to winds from various directions that may supply or remove sediment from one part of the nebkha (Figure 8) as exposure and shelter vary depending on wind direction as underlined by [START_REF] Hesp | Foredunes and blowouts: initiation, geomorphology and dynamics[END_REF].

It is well known that beach width and sediment supply, which are closely tied to surfzonebeach type, are important parameters controlling foredune development, particularly where sediment supply is not an important limiting factor (e.g., [START_REF] Short | Wave, beach and dune interactions in South Eastern Australia[END_REF][START_REF] Psuty | Sediment budget and dune/beach interaction[END_REF][START_REF] Davidson-Arnott | Measurement and prediction of long-term sediment supply to coastal foredunes[END_REF]. The development of nebkhas on the backshore east of Calais seems to be associated with a wide foreshore and upper beach in a context of abundant sediment supply along a macrotidal coastline. The formation of large nebkhas just above the uppermost tide level is probably favoured by the macrotidal regime characterizing this coastal area. Not only can the wide foreshore exposed at low tide serve as a sediment source for onshore aeolian transport, but the frequency of potentially erosive high water levels is extremely low.

Another key factor in foredune development is the supply of sediment from the shoreface to the beach (e.g., [START_REF] Anthony | Storms, shoreface morphodynamics, sand supply, and the accretion and erosion of coastal dune barriers in the southern North Sea[END_REF][START_REF] Houser | Beach and Dune Interaction[END_REF], which is commonly controlled by the onshore migration of nearshore and/or intertidal bars (e.g., [START_REF] Aagaard | Sediment supply from shoreface to dunes: linking sediment transport measurements and long-term morphological evolution[END_REF][START_REF] Houser | Synchronization of transport and supply in beach-dune interaction[END_REF]. In this area, the welding of nearshore sand banks, which started since at least the 16 th century resulted in the development of a broad sandflat (1500 m at low tide) in the early 20 th century [START_REF] Briquet | Le Littoral du Nord de la France et son évolution morphologique[END_REF]. The local increase in sand supply to the beach resulted in beach progradation, which created space in the backshore for coastal dune development. As stressed by [START_REF] Anthony | Storms, shoreface morphodynamics, sand supply, and the accretion and erosion of coastal dune barriers in the southern North Sea[END_REF], in this area, the seaward advance of the shoreline was related to the development of this extensive sandflat, followed by the development of coastal dunes with a time lag on the order of years.

Historical evidence reveals that beach progradation resulted in successive episodes of dune formation. By the mid-19 th to the beginning of the 20 th century, sandy islets 150 m to 210 m wide, covered by small coastal dunes, formed on the upper beach, disconnected from the main shoreline [START_REF] Briquet | Le Littoral du Nord de la France et son évolution morphologique[END_REF]. These sandy islets were likely related to the presence of the extensive sandflat over which aeolian sand transport was active. Such large and low-elevated sandy islets disconnected from the main foredune have been reported in north Lincolnshire along the eastern coast of UK on a dissipative macrotidal beach [START_REF] Montreuil | Aeolian Dune Development and Evolution on a Macro-tidal Coast with a Complex Wind Regime[END_REF][START_REF] Montreuil | Decadal and seasonal development of embryo dunes on an accreting macrotidal beach: North Lincolnshire, UK[END_REF]. Even after 10 years of development, they were still expanding but had not merged with the established foredunes [START_REF] Montreuil | Decadal and seasonal development of embryo dunes on an accreting macrotidal beach: North Lincolnshire, UK[END_REF]. East of Calais, however, the sandy islets observed on the aerial photography of 1938 (Figure 2) progressively merged to the main shoreline, resulting in a shoreline advance up to 320 m seaward.

Following this phase of sandy islets coalescence and significant shoreline advance, continuing beach accretion resulted in the development of incipient foredunes, and nebkhas formed at the uppermost tide level, disconnected from the incipient foredune zone by a swale.

Whereas, according to [START_REF] Briquet | Le Littoral du Nord de la France et son évolution morphologique[END_REF], the formation of foredunes all along this coastline during the 19 th century and the early 20 th century resulted from the coalescence of large "insular dunes", only isolated nebkhas developed during the late 20 th century. This is presumably linked to the progressive eastward migration of the extensive sandflat [START_REF] Héquette | The influence of nearshore sand bank dynamics on shoreline evolution in a macrotidal coastal environment, Calais, Northern France[END_REF] that resulted in the reduction in beach width along this stretch of the coastline.

Nebkhas started to develop by the mid 90's and progressively grew up, reaching 1 to 2 m high in 2001 [START_REF] Tékin | Un type original de dunes embryonnaires sur la plage macrotidale du Fort-Vert (Pas de Calais, France)[END_REF] and up to 3.5 m to 4 m in 2011. Since 2011, very little change in their elevation was recorded (Figure 11). While the nebkhas formed, a shore-parallel partially vegetated foredune zone developed at the toe of the established foredune (Figure 4). In a situation with shore-parallel incipient and established foredune ridges, there are usually two sedimentation peaks, one in the incipient zone and the other one in the established foredune, with lower sedimentation in the bare zone separating the two [START_REF] Arens | Patterns of sand transport on vegetated foredunes[END_REF][START_REF] Keijsers | Vegetation and sedimentation on coastal foredunes[END_REF]. In the study area, maximum accumulation was recorded in the incipient foredune zone, with vertical accretion up to 1.2 m between 2011 and 2014, while no major changes were observed in the established foredune (Figure 10). Although large nebkhas were present on the upper beach, they only trapped a small amount of wind-blown sand that was transferred from the foreshore to the incipient foredune zone. The swale between the nebkhas and the incipient foredune zone underwent both slight erosion and accumulation. Swales are generally low to limited aeolian deposition zones and become deeper as seaward incipient foredunes become higher [START_REF] Olson | Lake Michigan dune development 2. Plants as agents and tools in geomorphology[END_REF][START_REF] Hesp | Morphodynamics of incipient foredunes in N.S.W., Australia[END_REF][START_REF] Hesp | Foredunes and blowouts: initiation, geomorphology and dynamics[END_REF], but here, the swale remained fairly stable.

The analysis of aerial photographs and LiDAR topographic data revealed that the incipient foredune zone developed seaward at the expanse of the swale (Figure 4 and Figure 11). This evolution suggests that the nebkhas may progressively merge with the prograding incipient foredunes, being eventually included in a new incipient foredune zone. The merging of the nebkhas would therefore occur due to maximum sand accumulation in the incipient foredune zone and not as a result of sediment accumulation in the nebkha zone. This would contrast with what is generally observed along foredunes that develop seaward through the development of the seawardmost embryo dunes [START_REF] Pye | Aeolian Sand and Sand Dunes[END_REF][START_REF] Hesp | Dune coasts[END_REF]. Here the scattered nebkhas would not interrupt sand transfer from the foreshore to the incipient foredune zone located landward.

Our observations suggest that the most developed nebkhas have reached an equilibrium stage as their height, length and width remained fairly stable since 2011, although some of them were eroded by the major storm surge of December 2013. According to Mountley and Russel (2006), nebkhas have a characteristic shape and tend to adopt a similar morphology in any bioclimatic setting, suggesting they reach equilibrium once they have developed beyond their embryonic state. According to [START_REF] Hesp | Aeolian environments: coastal dunes[END_REF], as nebkhas build they usually evolve through an evolutionary cycle of development from small to large forms. Cycles of nebkha build-up and erosion, processes controlling their size and density, and vegetation behaviour were described by several authors [START_REF] Suslov | Physical Geography of Asiatic Russia[END_REF][START_REF] Gile | Coppice dunes and the Rotura soil[END_REF][START_REF] Mahmoudi | Les Nebkhas de Lut, Iran[END_REF][START_REF] Gibbens | Soil movement in mesquite dunelands and former grasslands of southern New Mexico from 1933 to 1980[END_REF][START_REF] Hesp | Morphology, dynamics, and internal stratification of some established foredunes in southeast Australia[END_REF][START_REF] Hesp | A review of biological and geomorphological processes involved in the initiation and development of incipient foredunes[END_REF][START_REF] Khalaf | Sedimentological and morphological characteristics of some nabkha deposits in the northern coastal plain of Kuwait, Arabia[END_REF]. Three stages of development were proposed by Tengberg and Chen (1998): (1) sand deposition and nebkha development, (2) steady state, and

(3) nebkha erosion due to the disappearance of the vegetation and of the mound of sand. Nebkhas can develop and eventually attain equilibrium on the upper beach were they are only very episodically reached by storm surges. A major storm event associated with an extreme water level, such as the one of December 2013, can nevertheless partially or strongly erode some of the fully-developed nebkhas (Figure 12 C1 and C2) or even completely wash away smaller nebkhas (Figure 10, profile 1). However, in this context of high sediment supply, nebkhas may eventually merge with the incipient foredune zone depending on the magnitude and frequency of future storm events.

Conclusions

(i) Large nebkhas, up to 4 m high and 14 m wide, have developed on the upper beach east of Calais, forming aeolian landforms parallel to the dominant southwest winds near the uppermost tide level and disconnected from the main foredune.

(ii) These large nebkhas formed in a coastal area characterized by abundant sediment supply from the shoreface due to the onshore migration of a nearshore sand bank that resulted in the formation of a wide sandy foreshore which favoured active incipient foredune development and significant seaward shoreline progradation.

(iii) They do not form an incipient foredune zone, but mounds that grew vertically and remained in the form of isolated huge nebkhas disconnected from the incipient foredune zone by a swale.

(iv) The nebkhas consist of scattered landforms that did not prevent aeolian accretion landward in the incipient foredune zone where the maximum accretion rates were recorded. (vii) Such uncommon nebkhas developed on an upper beach are presumably restricted to low gradient macrotidal beaches associated with an excess of sand supply.

Figure 1 .

 1 Figure 1. Location map of the study area. The inset on the aerial photograph shows the location of the investigated coastal dunes. Wind rose diagram based on three-hourly mean wind speed and direction from the Météo-France station in Calais.

Figure 2 .

 2 Figure 2. 1938 and 1957 aerial photographs showing the evolution of sandy islets on the upper beach east of Calais. The white circle corresponds to the location of a Second World War blockhouse.

Figure 3 .

 3 Figure 3. Series of aerial photographs from 1963 to 1993 showing the progressive merging of the sandy islets that resulted in shoreline advance. The white circle corresponds to the location of the Second World War blockhouse shown on figure 2.

Figure 4 .

 4 Figure 4. Series of aerial photographs from 1997 to 2012 showing the development of large nebkhas on the upper beach and seaward expansion of the incipient foredune zone. Note the inland position of the Second World War blockhouse shown on figures 2 and 3 (white circle), evidencing significant seaward shoreline displacement since the 1950s.

Figure 5 .

 5 Figure 5. Vertical aerial photograph (upper left), ground photograph (upper right), and Digital Terrain Model of the study area based on airborne LiDAR topographic data (bottom map) showing large nebkhas on the upper beach. The red circle corresponds to the location of the Second World War blockhouse shown on figures 2, 3 and 4.

Figure 6 .

 6 Figure 6. Typical cross-shore profile of the beach and coastal dunes east of Calais (based on LiDAR topographic data acquired in January 2014) and photograph showing the presence of large nebkha on the upper beach.

Figure 7 .

 7 Figure 7. Low dune surrounded by shell pavements on the upper beach east of Calais.

  that between 1993 and 2012, uppermost water levels reaching the nebkhas occurred only 15 times, less than once a year on average. The very low occurrence of extreme water levels between 1996 and 2000 may have also favored nebkhas and incipient foredune zone development. In 2013, however, 9 very high water levels were recorded, during two major storm events. A first storm, the Godehart storm, from 3 to 6 November 2013, was characterized by strong southwest winds, with instantaneous wind speed exceeding 21 m/s. A surge of 0.63 m at high tide resulted in water levels up to 8.08 m above HD (4.54 m IGN 69) on November 3, 2013.

Figure 9 .

 9 Figure 9. Annual frequency of (A) efficient winds for Aeolian transport (see text for explanation) and winds with velocities ≥ 8 m/s (data: Météo-France), and (B) of water levels above highest astronomical tide (HAT) measured at Calais tide gauge (data: Service Hydrographique et Océanographique de la Marine).

Figure 10 .

 10 Figure 10. LiDAR-derived Digital Terrain Model showing the morphology of the study area in March 2011 and the location of topographic profiles showing changes in elevation on the upper beach and incipient foredune zone between March 2011 and January 2014.

Figure 11 .

 11 Figure 11. Net changes in surface elevation over the study area between March 2011 and January 2014 based on LiDAR topographic data. The location of the nebkhas in 2011 is also indicated.

Figure 12 .

 12 Figure 12. Examples of stages of nebkha development east of Calais: (A) initial development of scattered low dunes developing on shell lag close to highest astronomical tide level; (B) fully-developed nebkha having reached equilibrium state; (C) partially-eroded nebkhas.

  (v) Due to the seaward expansion of the incipient foredune zone, the nebkhas could be progressively amalgamated in the prograding foredune zone if they are not eroded during storms associated with high water levels. (vi) The distribution of such large coastal nebkhas is probably very limited since the descriptions of similar examples of aeolian coastal landforms are virtually absent in the scientific literature.

Table 2 .

 2 Analysis of strong wind events (≥ 12 m/s during more than 6 consecutive hours) between March 2011 and January 2014 based on hourly wind data recorded at Calais meteorological station. Tidal range and uppermost water level reached during each event recorded at Calais harbour tide gauge do not include wave-induced set-up. The upper beach widths were extracted from LiDAR topographic profiles (Figure

Acknowledgements

This study was partly funded by the CNRS (Centre National de la Recherche Scientifique)

by the Ministère de l'Education Nationale et de l'Enseignement Supérieur through operating research funds to the Laboratoire d'Océanologie et de Géosciences. The LiDAR data were acquired thanks to the CLAREC project. Thanks are due to P. Bretel and F. Levoy for the acquisition and processing of the raw LiDAR data. Vertical aerial photographs were obtained from the IGN (Institut National de l'Information Géographique et Forestière). Water level data were obtained from the French Naval Hydrographic and Oceanographic Service. Wind data were provided by Météo-France though a research agreement with the Université du Littoral Côte d'Opale. Thanks are due to V. Sipka, A. Crapoulet and A. Cartier for their help during the field surveys. The authors would also like to thank Patrick Hesp for his comments and suggestions on an earlier draft of the manuscript. The constructive comments from two anonymous reviewers are gratefully acknowledged.


Adrien Crapoulet

Arnaud Héquette
email: arnaud.hequette@univ-littoral.fr

Franck Levoy

Patrice Bretel

Using LiDAR topographic data for identifying coastal areas of northern France vulnerable to sea-level rise

Keywords: Coastal hazards, marine flooding, climate change

). However, our study also shows that if coastal retreat proceeds during the next decades at the same or higher rates as today, several coastal dune systems will be entirely eroded in the next future, which would result in extensive storm-induced marine flooding in several coastal areas.

INTRODUCTION

Although demographic projections for the 21 st century only show a slight increase of population living along European coasts, about 50 million people in Europe are living in low lying coastal areas exposed to coastal hazards [START_REF] Neumann | Future coastal population growth and exposure to sea-level rise and coastal flooding -A global assessment[END_REF]. This is particularly the case in the densely populated coastal areas bordering the North Sea, such as in Belgium, the Netherlands, and Denmark, where reclaimed lands below mean sea level are subject to marine flooding. The northern coast of France is part of this extensive set of low-elevated shorelines exposed to coastal risks that are likely to worsen in the next decades with predicted sea level rise associated with climate change (IPCC, 2013).

It is well established that global sea-level has been rising since the late 19 th century and that the rate of sea-level rise increased during the last decades [START_REF] Church | Sea-level rise from the late 19 th to the early 21 st century[END_REF]. However, tide-gauge data as well as satellite altimetry clearly show that the rate of rise displays strong regional variations due to climate-related processes, such as steric effects and water mass redistribution, and/or to vertical land movement caused by natural or anthropogenic geological processes (Nichols and Cazenave, 2010). Recent observations of mean sea-level records from tide gauge sites along the North Sea and English Channel coastlines revealed that the rate of (local) sea-level rise was particularly high in the North Sea, exceeding 4.0 mm yr -1 between 1993 and 2009 [START_REF] Wahl | Observed mean sea level changes around the North Sea coastline from 1800 to present[END_REF] which is significantly higher than mean rate of global sea-level rise of 3.2 mm yr -1 measured by altimeter satellites during the same period [START_REF] Church | Sea-level rise from the late 19 th to the early 21 st century[END_REF].

In this paper, we present the results of a study carried out along the coast of northern France, using a series of LiDAR topographic data collected between 2008 and 2014 which were used for analyzing shoreline change and for mapping areas at risk of erosion and flooding. The possible impacts of future sealevel rise by 2050 are also assessed based on a moderate scenario of greenhouse gas emissions from the IPCC (2013) Fifth Assessment Report.

STUDY AREA

The coast of northern France is located in the eastern English Channel and southern North Sea (Figure 1). A major portion of this coast consists of wide dissipative barred beaches (300-600 m) and coastal dunes protecting low-lying backshore areas (mostly reclaimed lands) from marine flooding, despite a considerable expansion of urban and port areas during the 20 th century (Figure 1). Although the shoreline was stable or even prograded seaward in places during the last decades, several coastal areas underwent severe erosion during the same period [START_REF] Chaverot | Changes in storminess and shoreline evolution along the northern coast of France during the second half of the 20th century[END_REF], while flooding sporadically occurred locally during major storms [START_REF] Maspataud | Potential impacts of extreme storm surges on a low-lying densely populated coastline: the case of Dunkirk area, Northern France[END_REF].

The tidal regime in the region is semidiurnal and macrotidal, the tidal range increasing from the North Sea to the English Channel, with a mean spring tidal range of more than 5.4 m at Dunkirk to approximately 8.5 m at Berck-sur-Mer (Figure 1). Due to the large tidal amplitude, tidal currents are strong along the northern coast of France with a flood-dominated asymmetry responsible for a net regional sediment transport to the north in the coastal zone of the English Channel and to the east-northeast along the North Sea coast [START_REF] Héquette | Sediment transport under wave and current combined flows on a tidedominated shoreface, northern coast of France[END_REF]. The dominant winds in the eastern English Channel and southern North Sea are from west-southwest and from north to northeast. In response to these winds, the offshore wave regime is characterized by short-fetch waves from southwest to west, originating from the English Channel, followed by waves from the northeast to north, generated in the North Sea. Most waves have periods and significant heights of less than 5 s and 1.5 m, respectively, but may episodically exceed heights of 4 m during major storms [START_REF] Ruz | Identifying forcing conditions responsible for foredune erosion on the northern coast of France[END_REF]. Wave heights are much lower at the coast, however, due to significant wave refraction and shoaling over the tidal sand banks of the eastern Channel and southern North Sea [START_REF] Héquette | Effects of nearshore sand bank and associated channel on beach hydrodynamics: implications for beach and shoreline evolution[END_REF].

METHODS

The potential impacts of sea-level rise on the coast of northern France were assessed based on high-resolution topographic data of the intertidal zones and coastal dunes, which were obtained from airborne LiDAR surveys carried out between 2008 and 2014. A complete topographic coverage of the coast of northern France from the Authie estuary to the Belgium border (Figure 1) was obtained in May 2008, March 2011 and December 2013/January 2014. Partial LiDAR surveys were also conducted in November 2012 in specific areas.

The 2008 survey was carried out with an Optech ALTM 1020 LiDAR having a planimetric position accuracy of ± 0.25 m and a vertical accuracy of ± 0.1 m over bare surface areas. The other surveys were conducted using a Leica ALS60 LiDAR system that acquired topographic data with a planimetric accuracy ranging from ± 0.1 to 0.17 m with a vertical accuracy < ± 0.10 m as verified by several ground control points using a very high resolution differential GPS (Leica TPS Syst1200). The vertical error range can easily increases to ± 0.25 m or more in areas covered by dense vegetation however [START_REF] Saye | Beach-dune morphological relationships and erosion/accretion: An investigation at five sites in England and Wales using LIDAR data[END_REF].

A density of data points of 1.2 to 1.4 points/m 2 was obtained during the different LiDAR survey. LiDAR topographic data were filtered to remove vegetation, buildings and other objects. Filtered data were then used to create contour Digital Terrain Models (DTM) using Golden Software Surfer™. The DTMs were obtained by linear interpolation using a Delaunay triangulation resulting in a grid with a 1 m resolution, a grid cell resolution of 1 m 2 appearing to provide reliable representation of topography and accurate volumetric measurements in coastal dunes using LiDAR data [START_REF] Woolard | Spatial characterization, resolution, and volumetric change of coastal dunes using airborne LIDAR: Cape Hatteras, North Carolina[END_REF].

Shoreline evolution between 2008 and 2014 was calculated based on different sets of LiDAR topographic data mentioned above. Shoreline position was determined from each DTM as the upper beach/coastal dune limit based on a change in slope gradient on the upper beach calculated as:

‖𝑔 ⃗‖ = √( 𝜕𝑧 𝜕𝑥 ) 2 + ( 𝜕𝑧 𝜕𝑦 ) 2 (1)
Where is the slope gradient, z the elevation, x and y the coordinates of each point of the calculation of the grid. The value of the gradient is 0 for a horizontal surface and tends to infinity for a slope approaching the vertical. The transition between the upper beach and the coastal dunes corresponds to a sharp increase in the value of the slope gradient that is then selected for mapping shoreline position. To validate this semiautomatic shoreline extraction method, shoreline position detected from LiDAR data was compared at several sites with the position of the dune toe simultaneously determined in the field using a high-resolution differential GPS. The comparison of these two shoreline indicators usually showed small position differences, especially along erosive shoreline stretches, with a standard deviation of approximately 1.5 m. The margin of error associated with the delimitation of the shoreline was thus estimated at ± 1.5 m based on this comparison. Shoreline change measurements were carried out on cross-shore transects spaced every 50 m along the shore for each period between the different LiDAR surveys. When comparing shoreline position between two dates, the error margin can be considered to be ± 3 m

To assess the risk of coastal erosion and marine flooding, critical high water levels were determined based on storm surge observations from Boulogne-sur-Mer, Calais and Dunkirk tide gauge stations (Figure 1) and tidal predictions computed by the French Hydrographic Service (SHOM). A joint probability method was used for determining the 100-year return period high water level for each tide gauge station based on the probability distributions of extreme water levels (observed positive surges and predicted high tide levels) [START_REF] Shom | Statistiques des niveaux marins extrêmes des côtes de France (Manche et Atlantique)[END_REF].

In order to evaluate the maximum elevation reached by waves during extreme high water events, wave run-up was estimated according to [START_REF] Cariolet | Runup estimations on a macrotidal sandy beach[END_REF] Where Rmax is the maximum wave run-up elevation, Ho is the significant offshore wave height and ξo is the Iribarren parameter (tanβ/(Ho/Lo) 1/2 ), Lo is the deep water wavelength and tanβ is the beach slope. The beach slope was measured between the dune toe and the mean high water level, which closely corresponds to the active section of the beach defined by [START_REF] Cariolet | Runup estimations on a macrotidal sandy beach[END_REF] to be used in Equation (2). Run-up was computed for two storm wave conditions based on wave statistics from the Vergoyer wave buoy off Boulogne-sur-Mer (Figure 1), using Ho = 3.5 m and T = 9 s for moderate storm conditions and Ho = 6 m and T = 10 for extreme storm conditions.

Sea level rise by 2050 was estimated based on the RCP6.0 scenario from the last IPCC report (IPCC, 2013). Under this scenario, the increase will be 7.3 mm yr -1 by 2050, or about + 29 cm (relative to 2010). This value was added to the high water levels and run-up elevations previously calculated as explained above for estimating water levels in 2050.

Coastal hazard mapping was conducted using the latest LiDAR topography data set (December 2013 and January 2014), based on the method of [START_REF] Sallenger | Storm impact scale for barrier islands[END_REF] which allows to distinguish coastal dune erosion (collision regime) when the maximum run-up elevation reaches an altitude between the dune toe and the dune crest and marine flooding (overwash and inundation regimes) when run-up reaches or exceeds the elevation of the dune crest. Overwash and inundation were not discriminated from one another because both processes can be responsible for marine flooding since overwash may lead to dune breaching and eventually to flooding. Foreshore erosion (i.e., Sallenger's swash regime) was not considered in the present study, because the beaches of the region do not necessarily undergo erosion during storms as intertidal bars prove to be remarkably stable even during high wave-energy events [START_REF] Anthony | Morphodynamics of intertidal bars on a megatidal beach, Merlimont, Northern France[END_REF].

RESULTS

Mapping of high water levels over the LiDAR-generated DTMs showed that most of the region's coastline is subject to erosion under present sea level conditions, even during moderate storm events. Coastal erosion is already significant at several sites along the coast of northern France such as in Wissant Bay where the shoreline locally retreated by more than 250 m during the second half of the-20 th century [START_REF] Chaverot | Changes in storminess and shoreline evolution along the northern coast of France during the second half of the 20th century[END_REF]. Our LiDAR surveys revealed that the coast of Wissant Bay continued to erode during recent years, the shoreline having retreated landward by more than 20 m in several places between May 2008 and January 2014, with a maximum retreat of about 40 m in the western part of the bay (Figure 2), corresponding to annual retreat rates on the order of 3.5 to 7.0 m yr -1 . The shoreline was stable or slightly prograded seaward in some areas however, notably east of the village of Wissant, which is likely due to the redistribution of eroded material along the coast.

Most of the coastline retreat took place between November 2012 and January 2014 with shoreline retreat in excess of 7 m along extensive stretches of the coast and up to nearly 20 m at several locations (Figure 2). These high values of coastal retreat can be explained by the occurrence of extreme water levels (return period > 100 years) associated with a series of severe storms during the fall and early winter of 2013 [START_REF] Daubord | Caractérisation de 7 évènements de tempête de l'automne-hiver 2013-2014 à partir des données disponibles au SHOM[END_REF]. These storms were responsible for significant coastal erosion at several other sites along the coast of northern France like along the northern shore of the Authie estuary where shoreline retreat ranged from approximately 15 to 30 m between March 2011 and December 2013, reaching about 70 m along a low-elevated stretch of coastal dunes named "Bois de sapins" (Figure 3).

The determination of coastal areas subject to marine flooding based on LiDAR-generated DTMs and storm-induced high water levels (including maximum run-up elevation) showed that only localized and spatially restricted, low-elevated, coastal areas would be at risk of inundation along the coast of northern France during a 100-year return period event. One of these areas is the "Bois de sapins" mentioned above, on the northern shore of the Authie estuary, where a breach has formed through the dune crest that hardly exceeds 7 m above French vertical datum (IGN69) (Figure 4). Even under present sea level, our data show that marine flooding could occur in this area under high-energy storm wave conditions associated with a 100-year return period water level, which would result in partial inundation of the lowlying reclaimed land located behind the coastal dunes (Figure 3). With a 29 cm higher sea level in 2050, as projected with the RCP6.0 scenario (IPCC, 2013), this area would obviously be more easily and more completely flooded as an inundation regime would last during a longer period of time at or near high tide (Figure 4A) and would affect a longer stretch of coast. Similar storm impact regimes would prevail in the western part of Wissant Bay where overwash and limited inundation could occur locally under present sea level conditions during an extreme storm event associated with a 100-year return period, while more extensive inundation would take place in 2050 with a higher sea level (Figure 4B). However, our results suggest that although a higher proportion of coastal dunes would undergo erosion with a higher sea level in 2050, marine flooding would still be essentially restricted to the shores of the Authie estuary and Wissant Bay.

DISCUSSION AND CONCLUSION

This study shows that the dune coast of northern France is subject to widespread shoreline erosion during high-energy wave events associated with a 100-year return period water level, even under present sea level. Severe coastal dune erosion is already taking place at a number of sites along the northern coast France, notably in Wissant Bay and the Authie estuary where the shoreline retreated by tens of meters in places in response to the storms that hit western Europe during the fall and winter of 2013-2014 [START_REF] Castelle | Impact of the winter 2013-2014 series of severe Western Europe storms on a double-barred sandy coast: Beach and dune erosion and megacusp embayments[END_REF]. With a higher sea level in 2050, coastal dune erosion would not only be more widespread, but also more severe since the dune front should generally experience a collision regime up to a greater height. Moreover, a rising sea-level will also result in a higher frequency of extreme water levels [START_REF] Haigh | Rising sea levels in the English Channel 1900 to 2100[END_REF], which will also result in intensified coastal erosion.

Our results show that the risk of marine flooding is currently limited to a few sites. This work also suggests that although an increase of 29 cm in mean sea level by 2050 would induce more extensive flooding at sites already vulnerable to this hazard, flooding would still be essentially limited to the same sites. Such interpretation only based on the mapping of storm-generated high water levels over LiDAR-generated DTMs could be misleading, however, because coastal dunes are already eroding at high rates in places and coastal erosion will likely increase during the next decades with rising sea level [START_REF] Nicholls | Sea-level rise and its impact on coastal zones[END_REF]. On the northern shore of the Authie estuary, for example, the low-elevated polders are only protected from marine flooding by a narrow (20 m) dune barrier that is affected by considerable erosion. If coastal erosion continues to proceed at the same or higher rates, coastal flooding will certainly take place well before 2050 at this location. The same situation occurs at a number of sites along the northern coast of France where coastal flooding is not an immediate threat, but where coastal dunes protecting low-elevated coastal lowlands are presently undergoing erosion, which could lead to flooding hazards in the next future.

_________________________________________________________________________________________________

  Journal of Coastal Research, Special Issue No. 75, 2016

Figure 1 .

 1 Figure 1. Location of the study area.

  equation based on runup measurements on a macrotidal beach: _________________________________________________________________________________________________ Journal of Coastal Research, Special Issue No. 75, 2016 Rmax = 0.67 ξo Ho(2)

Figure 2 .

 2 Figure 2. Shoreline changes along the coast of Wissant Bay between May 2008 and January 2014. The shoreline positions are extracted from LiDAR Digital Terrain Models (see text for explanation) and are relative to the May 2008 shoreline.

Figure 4 .

 4 Figure 4. Cross-shore topographic profiles of the littoral zone showing maximum run-up elevation during extreme storm wave conditions (Ho = 6 m, T = 10 s) associated with a 100-year return period water level with present sea level (H100) and with predicted sea level rise in 2050 (H1002050); (A) "Bois de sapin" on the northern shore of the Authie estuary, (B) western shore of Wisant Bay.

† Laboratoire d'Océanologie et de Géosciences, UMR CNRS 8187 Université du Littoral Côte d'Opale Wimereux, France www.cerf-jcr.org www.JCRonline.org ‡ Laboratoire M2C, UMR CNRS 6143 Université de Caen Basse Normandie Caen, France

ACKNOWLEDGMENTS

Financial support for this work was provided by the Fondation de France through a Ph.D. scholarship to A. Crapoulet. LiDAR data were obtained through the CLAREC project funded by Regional Councils of France and by the French Centre National de la Recherche Scientifique. Additional funding was provided by DREAL of Nord-Pas de Calais.