Battiau-Queney, Yvonne, auly, teddy, Laboratoire d’Océanologie et de Géosciences (LOG) - UMR 8187 (LOG), Centre National de la Recherche Scientifique (CNRS)-Université du Littoral Côte d'Opale (ULCO)-Université de Lille-Institut national des sciences de l'Univers (INSU - CNRS), Université du Littoral Côte d'Opale (ULCO), EUCC-France, and Institut national des sciences de l'Univers (INSU - CNRS)-Université du Littoral Côte d'Opale (ULCO)-Université de Lille-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD [France-Nord])
The Merlimont-Berck dune-field which extends over more than 1000 ha is the best example of "Picardy dunes". Seven main units areobserved from the sea eastwards: the intertidal zone, the upper beach, an outer dune system, an interdune sandy plain, the inner dunesand a low flat wetland limited eastwards by a marine palaeo-cliff. First the paper analyses the origin and development of the dunes beforethe 2nd World War thanks to old maps and photos. In the first 30 years of the 20th century the outer dunes were mainly bare-surfaced andconstantly moving inland. In most places the foredune was absent. In the second part the paper analyses the evolution of the outer dunesafter the second World War. In 1947, just after the war and the mine clearance operations, the outer dunes have not changed significantlyin comparison with 1935. South of Merlimont-Plage, huge bare-surfaced sand tongues stretch northeastwards from the beach and the foreduneis absent except close to Berck. The methods to follow up the changing dune morphology and shoreline use aerial photos (IGN archives),bunker-archeology and field monitoring. A stereoscopic analysis of the photos was often necessary to clearly identify the shoreline,incipient dunes and other dune landforms. Moreover a good geomorphological knowledge of the field is required to interpret accuratelythe photos. The "Bunker archeology" refers to the utilization of the German bunkers and other works of the "Atlantic Wall" to reconstitute theirarchitecture, military purpose and exact original position. It is based on a careful analysis of their remnants in the field, completed by thestudy of military and civil archives. It may give very interesting and precise data on the shoreline change and foredune development since1944. In this paper a bunker complex located south of Merlimont-Plage, in front of the National Biologic Reserve, has been chosen to showthe great interest of these benchmarks. The results concern two main points: the shoreline change and the development of a foredune inthe 20 years following the war. The successive IGN aerial photographs allow to follow up the bunker complex in relationship with the changingmorphology. In October 1947, all the bunkers are still in their original position and badly seen on the photo. The access road is partlycovered with sand but still recognizable. The foredune is low and very discontinuous north of the bunkers and completely absent to thesouth. A parabolic dune is forming in the north (on the left side of the photo). There are wide areas of bare sand directly connected with thebeach. The sand deposit was certainly massive since no bomb-holes are seen in this area, which was strongly bombed by the British in 1943and 1944. In April 1955, the whole outer dune system has been remodeled, more or less flattened and squared off with windbreaks orientedNNW-SSE. Wide areas are bare-surfaced. The bunkers are better seen than in 1947. Two gun tanks lie on the upper beach. The foredune istotally absent. In August 1961, the main casemate has moved down but it is still leaning against the dune slope. A narrow sand bench hasformed in front of the previous shoreline. On the top of the photo (to the south) there is a dense grid of windbreaks associated with marramplanting. Along the beach the foredune is not well formed yet (it is present on the 1963 photo). Inland numerous small sand mounds arebeing colonized by vegetation (marram?). In 1968, the bunkers position has not changed significantly. The bare-surfaced sand bench alongthe upper beach is well developed. An incipient foredune is forming but it is discontinuous with numerous transverse troughs. Inland the effectof human intervention is even more spectacular than in 1961 with wide areas which are equipped with windbreaks and planted withmarram. In May 1974, the coastal strip has changed: the sand bench has disappeared and all the bunkers lies on the beach. This is relatedto several storm events which happened just before. To the north a small incipient foredune has formed in front of a higher dune coveredwith marram. In August 1983, the main casemate lies a few meters in front of the dune. It is noticeable that the bunker position is exactly thesame than today, 30 years later, the sand bench has disappeared in that area (but present further north) where a cliff cuts the dune face.The shoreline has distinctly retreated since 1968. The appearance of the outer dunes has changed also: the white dune has extended andthe grid of marram planting is well seen. A system of SSE-NNW troughs and bare sand tongues ("P" on the photo) has formed. A large blowoutis changing into a parabolic dune ("C" on the photo). Other blowouts have formed inland: the natural wind dynamics is working to createmore natural landforms. But a new bike trail through the dunes proves that the battle for nature conservation is not completely won. In 2000,the situation has just slightly changed. The system of troughs/sand tongues is very well developed. The foredune has been cut into a sandycliff. Once again this erosion feature must be connected with successive storms in the 1990’s. The present shoreline (in 2014) has advancedsince 2000. Finally the shoreline retreat between 1944 and 2014 is about 35 to 40 m, i.e. 0.50 to 0.57 m per year on average. But this rate doesnot reflect the long term evolution: the retreat took place mainly between 1974 and 2000, then the shoreline has advanced, so that thepresent situation is the same than in 1983. So it would be incorrect to use an average rate measured in a short period to anticipate the futurelong term evolution. Especially in a macrotidal environnement the effects of storms depend not only of their occurrence frequency but alsoof their simultaneity with spring tide. The analysis of aerial photos clearly shows that the origin of the foredune south of Merlimont has been strongly helped by human actions (windbreaks and marram planting) in the years following the war. The foredune was absent in 1944, butwell formed in 1963 combined with incipient dunes on the upper beach. Well established in 1971, the foredune height has increased significantlyin the following years. A layer of compost mud which was deposited in 1969 north of Berck and visible in the sand cliff of the foredunewas covered by 3 to 4 m of sand in 1994. So the foredune of the south beach of Merlimont was initiated by man but then it developednaturally thanks to the deposit of blown sand from the beach. Presently it is 15 to 25 m high and continues to trap the beach-sand. The thirdpart of the paper analyses the short-term shoreline change and emphasizes the resilience of the beach-dune system thanks to sedimentexchanges between the beach and foredune. All along the beach of Merlimont (and in many other places of the Opale Coast) thecontact between upper beach and foredune face is constantly changing at a very rapid rate (from one tide-cycle to another andthroughout the year). The German bunkers are good benchmarks to observe and evaluate the change. It is due to massive volumes of sandmoving from dune to beach during the storms combined with high tide, or from beach to dune when the prevailing winds blow over a widesandy beach at low tide. The wind dynamics is the most important factor to explain the resilience of the beach-dune system of this coast.The blown sand piles up on the upper beach and at the foot of the dune. It will stay there and form a stock of sediment which might beremoved by storm waves without threatening the beach equilibrium. These natural exchanges of sediment are the key to the good state ofthe beach. They work very well all along the North-South coast between the Canche and Somme estuaries thanks to the prevailing seawinds, abundant sand supply and wide sandy surfaces at low tide. The fourth part of the paper questions how a beach-dune system mightwork without any foredune. Two cases have to be considered: either the absence of foredune is natural, or it has been destroyed. The firstcase existed before the 2nd World War south of Merlimont. During storms the sea could enter the dune-field through wide troughs and depositmarine sediment which was later reworked and moved inland by the wind. The beach-dune system was less resilient. Nevertheless theshoreline was not retreating considerably because the amount of sand coming from the sea could compensate the inland export of blownsand, thanks to the voluminous sediment supply present in nearshore sand bars inherited from the last Glacial Period, when the sea-level was120 m lower and the Channel received the load of several large rivers (Rhine and Seine were the most important). The second case existswhere a seawall has been constructed and the dune replaced by a built area. In front of the seawall of Merlimont the beach is narrow orabsent at high tide and the waves tend to sap the foot of the dike. The wind dynamics is still active but the blown sand is no more kept instock but spread over the sea front where it tends to bury streets and buildings. It is lost for the sediment budget of the beach which getslower after each storm. Any sustainable solution has to take account of both marine and wind dynamics. So in May 2014 a system of beachdewatering (ECOPLAGE®) was installed with 2 sets of 6 drains and a pump station. The purpose is to increase the accumulation of sand onthe upper beach in the swash zone. Then wooden fences will be set up to prevent the beach sand to be blown out., Les dunes de Merlimont-Berck, d’un seul tenant sur plus 1000 ha, sont l’illustration la plus achevée des dunes « picardes ». La 1ère partiede l’article décrit leur origine et leur évolution jusqu’à la deuxième guerre mondiale, grâce aux cartes anciennes et aux archives photographiques.Les dunes externes étaient très mobiles et peu végétalisées et l’avant-dune absente ou discontinue. La 2e partie de l’articlereconstitue la mobilité du trait de côte et la naissance et l’évolution de l’avant-dune après la 2e guerre mondiale, à partir de la « bunker-archéologie» des ouvrages du Mur de l’Atlantique et de l’analyse des photos aériennes de l’IGN. En 1947, les dunes externes avaient peuchangé depuis 1935 : grandes dunes mouvantes s’allongeant vers l’ENE et directement alimentées depuis la plage. L’avant-dune n’estnette qu’à partir des années 1960. Les photos aériennes soulignent l’ampleur des interventions humaines sur tout le domaine dunaire au sudde Merlimont (quadrillage de brise-vent et plantation d’oyat). Le trait de côte a reculé assez nettement à partir des années 1970 jusqu’auxannées 1990 avant de ré-avancer. La morphologie des dunes externes (avant-dune bien formée et système couloirs/ pourrières), si caractéristiquedu paysage actuel, ne date que du début des années 1980. Le suivi topographique complété par celui des bunkers allemandsde 1993 à nos jours montre la mobilité extrême du trait de côte sur le court terme. L’alternance de recul et d’avancée fait apparaître uneremarquable résilience de ce système côtier macrotidal, grâce au rôle de stockage de l’avant-dune, à une dynamique éolienne très activeet à l’abondance de sable dans les petits-fonds. A contrario, toute absence ou destruction de l’avant-dune est une menace pour lebudget sédimentaire de la plage, comme c’est le cas le long de la digue de Merlimont.