Recent shoreline mobility and geomorphologic evolution of macrotidal sandy beaches in the north of France

Marine Geology 194 (2003) 31^45 www.elsevier.com/locate/margeo

Recent shoreline mobility and geomorphologic evolution of macrotidal sandy beaches in the north of France Yvonne Battiau-Queney  , Jean Francois Billet, Sylvain Chaverot, Philippe Lanoy-Ratel Geomorphology and Environmental Management, EA 3339, University of Lille 1-USTL, 59655 Villeneuve d’Ascq Cedex, France Received 17 February 2001; accepted 21 October 2002

Abstract This paper examines shoreline mobility and sediment budget conditions on a sandy macrotidal coast with a well developed foredune. Aerial photographs and ‘bunker archaeology’ covering the last 50 years were analysed in order to determine the meso-scale evolution of Le Touquet beach, in northern France. Both methods show that shoreline retreat has been negligible or very slow and has been accompanied by an elevation of the foredune. DGPS cross-shore profiles show a balanced sediment budget over the last 7 years. Only a slight change of the beach face profile (0.2^ 0.5 m rise or fall) was observed while the foredune exhibited a sharper change. The French coast of the eastern English Channel differs from that of the French Atlantic coast which is currently being strongly eroded. The balanced sediment budget of the former is attributed to sand inputs from nearshore banks, which compensate exports due to longshore drift and to foredune development. 8 2002 Elsevier Science B.V. All rights reserved. Keywords: northern France; shoreline mobility; macrotidal beach; beach morphodynamics; beach pro¢les; foredune

1. Introduction In France, beach erosion is considered a generalised phenomenon which has worsened in recent times especially along the Atlantic coast (Lorin and Migniot, 1984; Prat and Salomon, 1997; Pasko¡, 1998). Shoreline retreat and sand depletion are seen as a major economic threat for the many seaside resorts. In the north of France, sand shortage and repeated sea-wall destruction are spectacular in a * Corresponding author. Fax: +33-3-20-33-60-75. E-mail address: [email protected] (Y. Battiau-Queney).

few places like Wissant (Fig. 1) (Clabaut and Chamley, 1986; Battiau-Queney et al., 2001). A widely held opinion claims that it is an ineluctable phenomenon, which concerns the whole coastline of the north of France. This seems to be con¢rmed by data obtained from the comparison of aerial photographs between 1947 and 1977 (Clique and Lepetit, 1986). The resulting map of coastal mobility has been frequently reproduced (see for example ENR, 1999) and used by coastal planners and conservationists to deduce average rates of shoreline retreat or progradation. It has appeared clear, however, from ¢eld experience over the last 10 years that the rates of change proposed by this map are no more than

0025-3227 / 02 / $ ^ see front matter 8 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0025-3227(02)00697-7

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a rough estimate, which does not take into account the whole beach^dune system. Moreover the range of error when comparing aerial photographs depends on the accuracy of landmarks, which are not frequent in coastal dunes. Although repeated topographic surveys have been carried out by the Services Maritimes (a French National Agency) since 1996, these have to be used cautiously from a geomorphologic point of view. The transverse pro¢les are established between the low tide level of the day of the survey and the foot of the dune. They do not refer to the French Ordnance Datum (IGN 1969) and cannot be compared with con¢dence from one year to the next. In fact very little work has been done previously on the meso-scale sediment budget conditions of this coast. The widely believed idea that the sandy northern French coasts are inexorably retreating at a rate ranging from 0.4 to s 2 m/ year must be called into question and compared with ¢eld data. It is the reason why a research programme between the Somme estuary and the Belgian border has been undertaken at the University of Lille (Battiau-Queney et al., 2001). The paper focuses on an area which is assumed to be typical of most beaches in the north of France. Pro¢ling of the beach and dune complex highlights little change for the last 50 or 60 years.

2. The study area Besides tide-dominated estuaries and rocky cli¡s, wide macrotidal beaches and dunes characterise the 160-km-long French coast between the Somme estuary and the Belgian border (Fig. 1). Spring tide range decreases from over 10 m in the Somme estuary to 5 m east of Dunkerque. Dominantly westerly wind waves generate in a fetchlimited environment due to the narrow eastern English Channel. The signi¢cant wave height does not exceed 1 m. The highest wave height might reach 7.4 m at Boulogne and 6.8 m at Dunkerque (Clique and Lepetit, 1986). The main morphological features of this coast were ¢rst described by Briquet (1930). Recent research has focussed on sand-dune morphodynamics and

short-term coastline evolution (Battiau-Queney et al., 1995; Fauchois, 1998; Sipka, 1998; Corbau et al., 1999; Anthony and Dobroniak, 2000; Clabaut et al., 2000; Chaverot, 2000), as well as the o¡shore and nearshore hydrodynamic circulation and sedimentation patterns (Larsonneur et al., 1982; Despeyroux and Chamley, 1986; Dewez et al., 1989; Salomon and Breton, 1993; Augris et al., 1995; Levoy et al., 1998; Tessier et al., 1999; Anthony, 2000). The coast between the Authie and Canche estuaries in the Picardy Holocene coastal plain (Fig. 2) is characterised by the strong development of coastal tourism that has resulted in the growth of several important seaside resorts over the last century. The beach in this area is therefore a major economic and environmental resource. As a result, it is important to know whether or not the beach is retreating, especially in the present context of rising sea level (Cazenave, 1999; Pasko¡, 2001; Simon, 2001). The study area is a 16-km-long stretch of macrotidal beach backed by dunes. It lies between the Authie and the Canche estuaries and includes the resorts of Le Touquet, Stella-Plage, Merlimont and Berck-Plage (Fig. 1). The mean spring tidal range reaches 7.9 m. At low spring tide the beach is 750^900 m wide, and exhibits ridge and runnel topography (Sipka, 1998). The nearby dunes are organised into two ¢elds separated by a 500^2000m-wide sandy plain: an outer 200^1000-m-wide dune ¢eld with a 10^20-m-high foredune and a 700^1000-m-wide inner one (Battiau-Queney et al., 1995; Chaverot, 2000). The environmental conditions are very similar from Berck-Plage to Le Touquet. However, the beach and foredune management strategy di¡ers from one site to the other. Le Touquet is a prosperous seaside resort and wide parts of the dune ¢elds have been built up. The commune of Merlimont Beach has been partly integrated into a protected area managed by the O⁄ce National des Fore“ts (ONF). This protected area includes 800 ha of dunes set aside as a Nature Biological Reserve since 1985. In this commune, the choice of management is one of ‘laissez-faire’ (i.e. let natural processes work, with as little human intervention as possible). For this reason, it is a particu-

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N 1 0

10km

Cape Blanc Nez Cape Gris Nez

3

2

4

5 6

7

Canche Bay

8 9

dune field

10

sandy coast and foredune rocky cliff

Authie Bay

seawall or artificial shoreline

depth -10m Belgian border town

Somme Bay Fig. 1. Main coastal characteristics of the study area. 1: Dunkerque; 2: Calais; 3: Sangatte; 4: Wissant; 5: Boulogne; 6: Hardelot; 7: Le Touquet; 8: Stella-Plage; 9: Merlimont; 10: Berck-Plage.

larly interesting site for surveying the ‘natural’ coastline change and dune dynamics. From a general point of view, the study area forms a unique sediment cell with two ¢xed limits: the Authie and Canche estuaries. The cell hy-

drodynamics is dominated by a northward longshore current resulting from the prevailing southwesterly winds. The southern updrift beach of Berck-Plage supplies sand to the northern downdrift beach and spit of Le Touquet. Merli-

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Pointe de Lornel

f g

a b c d e

h i

ETAPLES LE TOUQUET 33 36

HARDELOT Ca n

che

R.

Canche estuary L.T.

32

STELLAPLAGE

M.

Authie B. estuary

47 31

MERLIMONT Biological reserve

61

N

Somme estuary

27 38 24

0

5

Palaeo-cliff Chalk plateau Sea-cliff

52

BERCK PLAGE

ABBEVILLE 10km

33

0

1

2

3km

N 42

th i e R Au

Fig. 2. The Picardy coastal plain between the Canche and Authie Estuaries. (a) Altitude in metres (NGF = Nivellement Ge¤ne¤ral de la France). (b) Shoreline. (c) 5 m (NGF) contour line. (d) Lowest sea level. (e) Dune ¢eld. (f) Chalk scarp and post-glacial palaeo-cli¡. (g) Pool and freshwater marsh (‘marais de Balancon’). (h) Tidal £at. (i) Built area. The map shows the double system of dune ¢elds and the limit of the Biological Reserve of Merlimont.

mont and Stella-Plage beaches are in a transit zone. The presence of northerly residual tidal currents has been proved on a sand bank lying o¡shore at a few kilometres from Merlimont Beach. They contribute to the longshore sand transport: the volume has been estimated at between 35 and 150 m3 /m/year (Dewez et al., 1989).

Besides the longshore sand transport, there are also transverse sand transport patterns, although these are less well known. The geomorphologic changes of the upper beach and dune foot after storm conditions prove that sand is exported by backwash processes. The quantity of nearshore sand entering the sediment cell has never been

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Fig. 3. Merlimont upper beach and foredune (30 July 2001). Along the foredune foot a microcli¡ (formed a few weeks before when a storm event coincided with a spring tide) is still recognisable although partly buried by blown sand transported from the beach by the predominant southwest wind. It de¢nes the shoreline. In the foreground, embryo dunes are associated with Cakile maritima. The view is southward.

considered previously except from a long-term perspective (Anthony, 2000).

3. De¢nition of the shoreline In this study we de¢ne the shoreline as the ‘foredune foot’, which coincides with the upper level of the highest spring tide. The ‘foredune foot’ is a feature which changes in time, according to prevailing tidal and wave conditions. Following a storm event coupled with spring tide, the dune foot is a microcli¡ generally less than 1 m high. However, sand will slide from the top of the dune and blown sand from the beach tends to accumulate rapidly on the windward (seaward) face of the dune. Both processes contribute to the rapid burial of the microcli¡ (Fig. 3). If the coastline is easily de¢ned when the foredune front is a cli¡, it is, in contrast, harder to identify its exact position in other cases. In the

¢eld the coastline is de¢ned as the more or less sharp break of slope from the gentle upper beach to the steep dune front. When embryo dunes form (Fig. 3), they are considered to belong to the beach, below the coastline : most of them are seasonal forms that will disappear during the next storm events. In contrast, on a photograph it is not easy to distinguish the break of slope and embryo dunes. The interpretation has to be checked in the ¢eld but this is possible only for the most recent photographs. The ‘foredune foot’, which is the reference line in any study of coastline migration, is therefore not so much a lineation but, rather, a narrow, 2^3-m-wide zone (cf. Guille¤n et al., 1999). This is a potential source of error when evaluating the shoreline migration.

4. Methods The methods di¡er according to the time scale

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LE TOUQUET 225

P-810

dyke 25m 20

10

0

-20

-10

Erosion

Progradation

shoreline in 1998 metric point

P-820

1 450

675

P-830

1

-0,17m / y (55-98) 900

P-831

1125

1350

1575

P-840

2 0,28 m / y (55-98) 1800

2025

2250

fig.5 2475

3 4 2700

3

3150

0,24 m / y (55-98) 2925

-20

-10

0

10

3375

5

P-850

4

STELLA

3600 3690

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250

0

1968 1998

1955 (reference year) 1987 1979 maximum range of error (+ / - 6m)

GOULET

dyke

Y. Battiau-Queney et al. / Marine Geology 194 (2003) 31^45 36

N

2

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of the research: (1) on the time scale of the last 60 years, the changing position of the shoreline is determined from two sources of information: a series of aerial photographs and the position of the ‘Atlantic Wall’ military bunkers, which were built in 1942^1944; (2) on the time scale of the last 8 years, topographic and geomorphologic data were directly obtained in the ¢eld. 4.1. Analysis of aerial photographs The vertical aerial photographs were taken by the IGN (Institut Ge¤ographique National) from 1947 to 1998. The scale is 1:25 000 or 1:30 000. Apart from those taken in 1947, all were suitable for stereoscopic examination. The photographs were digitised and processed with Imagine0 8.3 of ERDAS0 , a software package created by the Rhodanian Laboratory of the University of Lyon 2. This process produces orthophotographs which can be superimposed on the basis of a few wellreferenced landmarks on the IGN 1:25 000 topographic map and observed on the photographs (Chaverot, 2000). The most critical point is the precise location (Lambert 1 x,y,z references) of these landmarks (buildings, bunkers, crossing paths) : the number, spatial distribution and precision with which the landmarks can be identi¢ed determine the quality of the analysis. In this study at least 20 landmarks were identi¢ed on each photograph. Close to the shoreline, the best landmarks are the German bunkers, but they are few and unevenly distributed. The ¢rst stage was to rectify the 1998 photographs and relate these to the Lambert 1 topographic map control with the 1:25 000 IGN sheet of Montreuil-Berck. Thereafter, older images were processed, using the 1998 recti¢ed image as a model. An average range of error of T 3 m is usually accepted during this process (Dolan et al., 1980; Chaverot, 2000). The comparison and superimposition of successive recti¢ed images has allowed not only a dynamic vision of dune development through time,

37

but also a study of the shoreline migration. In the present study, the shoreline, de¢ned as the ‘foredune foot’, has been identi¢ed stereoscopically by the change from the steep dune face to the gentle beach slope. It often coincides with the limits of dune vegetation, except when embryo dunes have formed. Taking into account all sources of error (identi¢cation of the coastline, geo-references of landmarks, computer processes), the ¢gure of 12 m as the maximum margin of error seems appropriate when comparing successive aerial photographs (Chaverot, 2000). 4.2. Bunker archaeology Shoreline and foredune change was also assessed on the basis of the position of German bunkers, given the fact that, as major landmarks, their positions were well-established (LanoyRatel, 1998). The method is based on the knowledge that each bunker had a distinctive design and site, perfectly adapted to its speci¢c purpose, and consequently, its natural environment. Five bunkers are present along Le Touquet/StellaPlage Beach (Fig. 4). All of them are still in their original position, with a quite horizontal concrete base. For example, number 3 in Fig. 4 was a casemate (type 612 bunker) built in a £anking position for an assault gun. Fig. 5 shows quite clearly the rectangular ¢ring embrasure, protected on the right by a concrete £anking wall. This type of bunker is especially useful in the identi¢cation of the original shoreline position ; when it was built, the whole beach lying northward (on the left side of the photograph) had to be seen from the ¢re axis of the embrasure and the gun entrance opened on the right side of the photograph. The bunker was located very near the foot of the dune. 4.3. Cross-shore pro¢les The coastline mobility on a short time scale (1^ 8 years) depends on both climatic (especially the

Fig. 4. Shoreline change (1955^1998) along Le Touquet-Stella Beach from aerial photo-interpretation. The schematic map on the right gives the exact position of the reference metric points. The stars (Nos. 1^5) correspond to the bunkers.

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Fig. 5. A German bunker (No. 3 of Fig. 4) (December 1998). The photograph was taken from the beach at low tide. The ¢ring embrasure (on the left) is now half concealed by the foredune. Initially, it was in a position that allowed surveillance of the whole beach area to the north.

occurrence of storm events) and human parameters (i.e. all the e¡ects of coastal activities), and is an important criterion for coastal managers. To determine coastline mobility, the beach pro¢le and beach^dune exchanges have to be surveyed directly in the ¢eld. As indicated earlier, in 1996 the ‘Services Maritimes’ started a programme of beach pro¢ling along the northern French coast. From the Canche estuary to Stella-Plage, six transverse west^east pro¢les are surveyed once a year at least, generally in October. They were also surveyed in February 2000, after the major storm event of December 1999. Each pro¢le starts at the dune foot and goes down to the lowest possible low tide level on the beach. Data refer to the hydrographic datum reference (and not to the IGN). Normally, each pro¢le refers to the same point (the dune foot of the ¢rst survey) when it is repeated. In fact, this is not precise, because the surveyor has only a short time to work between low and high tide, and has to sight all the points of the pro¢le from the same station.

Along the beach of the Merlimont Biological Reserve, our research team has been engaged in a survey programme, with the help of the ONF. In October 1993, two cross-shore pro¢les were established from the lower beach to the inner dunes by professional surveyors (accuracy T 1 cm). Data were referenced to the French vertical Ordnance Datum, Lambert 1 IGN 1969. Using DGPS, the survey was repeated in March 1994 and August 2000 along exactly the same pro¢les (accuracy T 1 cm). The ¢rst and second surveys assessed the situation before and after a series of severe winter storms, with a view to con¢rming the widely assumed seasonal cycle of beach pro¢le change. The third may usefully be compared with the ¢rst as both were established before the onset of the winter wave conditions ; it is therefore possible to accurately evaluate shoreline migration and beach accretion or depletion over a period of 7 years. This is a short period indeed but our data will serve any future scienti¢c study. We expect to repeat the survey along the same pro¢les every third year.

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have subsequently been covered by sand and dune growth.

5. Results 5.1. Change on the time scale of the last 60 years 5.1.1. Data from aerial photographs When considering the shoreline migration from 1955 to 1998, from Le Touquet to Stella-Plage, four sectors are recognised along the 3500-mlong coastline (Chaverot, 2000) (Fig. 4): (1) Between metric points 475 and 1125, the trend is slightly negative (30.17 m/year); the results are within the margin of error, except from 1968 to 1979, a period of more pronounced shoreline retreat. (2) Between metric points 1125 and 2475, a seaward migration of the shoreline has been persistent, with an average of 0.28 m/year. Indentations of the curve correspond to the foredune blowouts. (3) Between metric points 2475 and 3465, a similar trend has been observed, the average seaward migration being 0.24 m/year. Sectors 2 and 3 are separated by a narrow zone, rich in blowouts, where the trend lies within the range of error. (4) Between metric points 3465 and 3690, the shoreline is not easy to identify on the photographs. Although, clearly, it has retreated, the mean rate is di⁄cult to ascertain. 5.1.2. Data from the German bunkers The example of bunker 3 in Fig. 4 (seen also in Fig. 5) proves a remarkable stability of the shoreline and a considerable rise of the foredune. Today the ¢re axis is half concealed and the bunker entrance totally hidden by sand, revealing a considerable morphological change of the foredune since the bunker was built. Moreover, the foredune crest has risen: to be adequately camou£aged, the upper concrete slab of the bunker had to be exactly at the same level as the foredune crest. The present crest is about 4 m above the upper concrete slab. On the back slope of the dune (not visible in the photograph), the apparent absence of small concrete shelters which are usually associated with this type of bunker is abnormal. In fact, many were constructed, as shown on contemporary maps; the fact that they cannot now be identi¢ed is presumably because they

5.1.3. Interpretation Here the bunker archaeology method entirely con¢rms the results from aerial photographs: in front of metric point 2655 (site of bunker 3), the 1998 coastline lies 6 5 m seaward from the 1955 coastline. Similar observations on the other bunkers of this area (Nos. 1, 2, 4 and 5 of Fig. 4) ¢t the photo-interpretation well. Between metric points 1125 and 2025, where the photographs suggest a slight advance of the coastline, bunkers are presently absent along the dune foot. If present in 1944 they might have been buried by sand, a hypothesis that has still to be con¢rmed in the ¢eld. Clearly, on this part of the Picardy coast, the shoreline has not, or just slightly ( 6 0.28 m/ year) migrated since 1944. Moreover, the present foredune must be higher and wider than that of 60 years ago. It acts as a sand sink in the coastal sediment cell. 5.2. Change on a short time scale (1993^2000) Despite their poor quality, the 1996^2000 pro¢les of the Services Maritimes along Le Touquet Beach have been analysed (Chaverot, 2000). They give interesting information about the changing seasonal topography of the beach, which con¢rms previous observations. After a storm event (for example in December 1999), the runnels are less deep and the beach £atter. The pro¢les also show a slight accretion of the middle and upper beach. However, this is just a trend that must be interpreted with care, given the short time of the survey. The results obtained from Merlimont Beach are more precise. 5.2.1. October 1993 to March 1994 (e¡ect of winter storm events) On pro¢le AB, at 600 m south of the Merlimont urban zone, we observe (Figs. 6a and 7a): ^ no signi¢cant change of the lowest part of beach; ^ a slight elevation (0.40^0.50 m) of the lower middle beach beneath mean sea level (MSL); at MSL, the new pro¢le is 1 m higher;

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Fig. 6. Cross-shore pro¢les AB, at 600 m south of Merlimont village. Altitudes and distances are given in metres. Altitudes refer to the French Ordnance Datum NGF 1969. The position of point A refers to the Lambert 1 kilometric grid.

B : X=545699,47 - Y=306407,12 - Z=20,47 October 1993 cross-shore profile March 1994 cross-shore profile

20

August 2000 cross-shore profile

15

a

m

10

1

5 0 100

200

300

2 (x 2)

1 (x 2)

0

m

G : X=545539,31 - Y=304953,29 - Z=16,53 20

m

15 2

b

10 5 0

400

300

200

100

J.F. Billet, 2001

0

m

Fig. 7. Shoreline and foredune change between 1993 and 2000 at Merlimont, along pro¢les AB (above) and FG (below). The position of points B and G refers to the Lambert 1 kilometric grid.

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Fig. 8. Cross-shore pro¢les FG passing near the southern border of the Merlimont Nature Reserve. Altitudes and distances are given in metres. Altitudes refer to the French Ordnance Datum NGF 1969. The position of point F refers to the Lambert 1 kilometric grid.

^ above MSL, the upper beach pro¢le has been slightly lowered: 0.30^0.50 m on a ridge and 0.70 m in a runnel; ^ in March, a 0.60-m vertical cli¡ formed at the dune foot which is at +6.70 m, about 2 m above the highest ‘normal’ sea level. This corresponds to storm wave conditions; ^ no change of the front slope of the dune; ^ the ridge and runnel topography was more pronounced in March: three longshore runnels were prominent. They are 0.20^0.30 m deep, 6 to 25 m wide. On pro¢le FG, near the southern border of the Reserve, we observe (Figs. 7b and 8a): ^ no signi¢cant change on the lowest part of the beach; ^ a slight elevation (around 0.30 m) of the pro¢le just beneath MSL. The two runnels noted in October 1993 had completely disappeared by March 1994. ^ on the upper beach pro¢le, a wide runnel

prominent in October had been ¢lled up (0.40^ 0.95 m) in March. Closer than 50 m to the dune foot, the pro¢le was slightly depressed (0.20^0.45 m) and a 0.30-m-high cli¡ had formed; ^three narrow, 0.20^0.30-m-deep runnels appeared on the March pro¢le, above MSL. 5.2.2. October 1993 to August 2000 We observe on pro¢le AB (Figs. 6b and 7a): ^ a slight depression (0.20^0.60 m) of the lowest part of the beach, except on a ridge which was prominent in 2000, but which did not exist in 1993; ^ seaward progradation of the bank located just above MSL ; ^ a generally slight depression (0.80^1 m) of the upper beach (above MSL) except a small area where it has undergone elevation (from 0.12 to 0.40 m); runnels were very wide (around 80 m) and shallow (c. 0.30 m) in August 2000; ^ no retreat of the dune foot and a complete

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stability of the front slope of the dune, which bears a dense cover of Ammophila arenaria; ^ the dune summit was 0.50 m higher in 2000. A strong change was observed on the lee side of the foredune, with lateral spreading of sand rather than direct nourishment from the beach. On pro¢le FG (Fig. 7b and 8b), we note: ^ no signi¢cant change of the lowest 220 m of beach; ^ 0.20^0.50 m depression of the low middle beach; ^ 0.30^0.50 m depression of the upper beach; ^ the nearest runnel from the dune was located at the same place in 2000 and 1993 but was shallower in 2000; ^ the 1993 pro¢le crossed an isolated patch of the foredune ; by 2000, the same place was occupied by a de£ation trough; ^ important elevation of the foredune (from 1.85 to 2.40 m), implying considerable direct nourishment from the beach. 5.2.3. Interpretation Both pro¢les (AB and FG) show a similar slight change after winter wave conditions: overall, the change is negligible on the lowest parts of the beach. There is a slight elevation of the lower part of the middle beach and a depression of the beach above MSL. In both cases, a small cli¡ has formed at the foot of the foredune. In winter, most runnels disappear ; however, in March, a few runnels are deeper and narrower than in October. Merlimont Beach has undergone only limited change during the last 7 years. The dune foot has not retreated. On pro¢le FG, a de£ation trough has widened and modi¢ed the pro¢le, but, overall, the coastline has not retreated. Meanwhile, the beach pro¢le above MSL has been generally slightly depressed, by 0.20^0.50 m. It must be emphasised that these ¢gures hardly di¡er from the winter changes observed between October 1993 and March 1994. They cannot be considered signi¢cant in the long term. Our data con¢rm the 50-year trend observed at Le Touquet: the Picardy coast from Merlimont to Le Touquet shows a remarkable stability and is clearly evolving only very slowly.

Bunker archaeology shows that the foredune has risen in the past 60 years. The Merlimont Beach pro¢les con¢rm that the foredune is constantly rising and receives a considerable quantity of beach sand. This is a highly signi¢cant result of our study. Part of this sand is transported through the blowouts and forms transgressive sand tongues at the rear of the foredune (Fig. 9) (Fauchois, 1998; Battiau-Queney et al., 2001). Despite this export of sand, beach pro¢les show a negligible or just a very slight depletion. It means that sand supply to the beach is su⁄cient to compensate wind de£ation from the beach. The sediment transport by the longshore current is assumed to result in an equilibrium state, import equalling export. Therefore another source of sand must exist.

6. Discussion The results acquired in this area are similar to those observed east of Dunkerque at Bray-Dune, along the southern North Sea (Corbau et al., 1999; Brulez, 2002; Lanoy-Ratel, work in progress). From May 1992 to December 1994, bathymetric and topographic surveys showed a balanced sediment budget. The geomorphological evolution from 1995 to 2002 was marked by a slight elevation of the foredune and closure of blowout troughs. German bunkers con¢rm a noteworthy stability of the local coastline over the last 60 years. So, the macrotidal beaches and associated foredunes of the north of France show a recent evolution, which invalidates the widespread opinion of a long-term erosional trend. The short period which has been considered in this study does not seem uncommon in terms of climate: it was marked by a series of winter storms, especially in February 1953, November 1984, February 1990, and during the winters of 1993^94 and 1994^95 and again in December 1999. Each time, the geomorphological consequences were rapidly healed up along the coast, except in the Wissant Bay. Clearly, for 50 years at least, the study area has experienced the same trend, implying a balanced sediment budget for the beach and

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Fig. 9. Sand tongues on the lee side of the Merlimont foredune. They move inland in the direction of the predominant wind. The view is southward (the sea on the right). Each tongue is nourished with beach-sand through a blowout trough. In the foreground: a sandy £at with a dense cover of shrubs (mainly Hippophae rhamnoides).

a slight or negligible coastline change. It means that besides the longshore transport, sand export to the coastal dunes has been compensated by sand import. The problem is to ¢nd the available source. It cannot be the river load, which is negligible in this region. On the other hand, the shoreface may act as a major sedimentary source for the beach^dune system (Corbau et al., 1999). In the Dunkerque area, nearshore sand banks presently move coastward at an average rate of 1^5 m/year (Corbau et al., 1993). They supply the beach with sand, although this phenomenon has been locally slowed down by dredging the access of Dunkerque Harbour. We assume that the same process is of major importance in the Merlimont area. A series of sand banks and ribbons are present in the nearshore zone between 310 and 330 m (Dewez et al., 1989) and could substantially participate in the coastal sediment budget. The banks are several metres high, 1^6 km wide and 30^75 km long. They are parallel to the shoreline. According to Dewez et al. (1989), they appeared between 9000

and 7000 years BP. The sand properties (grain size, sorting, shape, mineral composition) of the submarine banks and beaches suggest a common provenance. All along the Picardy coast, the beach sand is remarkably uniform with an average size of 200^220 Wm and a sorting index of around 1.10^1.15 (Fauchois, 1998; Sipka, 1998). In this region, the beach morphodynamics exhibits a highly site-speci¢c character: the tidal regime in the eastern English Channel leads to the concentration of nearshore sand banks along the French coast. So a great volume of sand is available, which contributes to the beach morphodynamics. By contrast, sand shortage on the lower shoreface could explain the long-term shoreline retreat along the French Atlantic dominantly sandy coast (from Brittany to Biarritz). It has been estimated to be from 0.5 to s 2 m/year between the Gironde estuary and Biarritz (Lorin and Migniot, 1984) and from 4 to s 9 m/year along the Charente coast (Prat and Salomon, 1997). In both cases, the spring tidal range reaches 5 m. The marine hydrodynamics of this coast di¡ers consider-

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Y. Battiau-Queney et al. / Marine Geology 194 (2003) 31^45

ably from the Channel environment: swell generates in the Biscay Bay, and may give very high waves (up to 13 m during a storm event) in the surf zone. Sediment inputs from the Loire River and the Gironde are estimated around 3.5^3.9 million metric tons per year, but they are mainly silt and clay rather than sand and gravel. South of the Gironde estuary, sand supply has been globally insu⁄cient to compensate sea erosion and wind de£ation over the beach, since at least the middle of the 19th century. Signi¢cantly, the German bunkers presently lie on the lower beach or in the subtidal area and the front of the coastal dune is generally an eroded cli¡. In contrast, the strong negative sedimentary budget, which is presently observed along the Charente coast, is a recent phenomenon: from the 15th century to the middle of the 20th century, the shoreline was migrating seaward and foredunes were regularly prograding, despite the high-energy environment. Change occurred rather abruptly 30 years ago, when the shoreline began to retreat (Prat, 2001). A similar evolution characterises most Brittany beaches. According to Pasko¡ (1998), this erosion trend marks the depletion of sand reserves, which have been accumulated on the shoreface in response to the post-glacial marine transgression. The Dinan Bay, on the west coast of Brittany, could be the exception proving the rule (Hallegouet and Hena¡, 1995). The local shoreline has migrated seaward (up to 150 m since 1775) and dunes have formed behind the beach. This unusual evolution is explained by the presence of abundant biogenic calcareous sand on the nearshore sea bed, and a coastal con¢guration favouring sand storage in the bay. Similarly but on a much larger scale, the French coast of the eastern English Channel and southern North Sea is atypical in the present context of ‘beach crisis’.

coastline retreat has been negligible or very slow, with simultaneous elevation of the foredune. In the present macrotidal environment of the beach at Merlimont, a balanced sediment budget is shown by cross-shore pro¢ling over the last 7 years. The amplitude of change observed on the beach over 7 years hardly di¡ers from the winter change between October 1993 and March 1994. It never exceeds 1 m and generally ranges between 30 and 60 cm. Similar conditions of negligible meso-scale change appear to prevail along much of the northern French coast, contrary to the widely believed idea that this coast is eroding. This is an infrequent situation around northwest Europe coastlines. We presume that the voluminous nearshore sand banks lying along the French coast of the eastern English Channel and southern North Sea contribute signi¢cantly to the beach sediment budget.

Acknowledgements The authors gratefully acknowledge ONF for allowing ¢eld survey in the Merlimont Biological Reserve. They thank Fredy Groux who surveyed Merlimont pro¢les and Jacqueline Domont (University of Lille) for her help in drawing the ¢gures. The authors also thank Edward Anthony, Colin V. Murray-Wallace and two anonymous reviewers for their constructive comments, which helped to improve the paper. The main author (Y.B.-Q.) is indebted to Peter Walsh who signi¢cantly improved the ¢rst English text. This paper is a contribution to IGCP Project 437, ‘Coastal environmental change during sea-level highstands: a global synthesis with implications for management of future coastal change’.

References 7. Conclusion Two di¡erent methods of investigation, aerial photograph analysis and ‘bunker archaeology’, lead to the same conclusion for the last 60 years : along the beaches of Le Touquet and Stella, the

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