Le système plage-dune de Merlimont-Berck et son évolution depuis la seconde guerre mondiale - HAL-SHS - Sciences de l'Homme et de la Société Accéder directement au contenu
Article Dans Une Revue Dynamiques Environnementales - Journal international des géosciences et de l’environnement Année : 2014

The beach-dune system of Merlimont-Berck and its evolution since the Second World War

Le système plage-dune de Merlimont-Berck et son évolution depuis la seconde guerre mondiale

Résumé

The Merlimont-Berck dune-field which extends over more than 1000 ha is the best example of "Picardy dunes". Seven main units are observed from the sea eastwards: the intertidal zone, the upper beach, an outer dune system, an interdune sandy plain, the inner dunes and a low flat wetland limited eastwards by a marine palaeo-cliff. First the paper analyses the origin and development of the dunes before the 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 and constantly moving inland. In most places the foredune was absent. In the second part the paper analyses the evolution of the outer dunes after the second World War. In 1947, just after the war and the mine clearance operations, the outer dunes have not changed significantly in comparison with 1935. South of Merlimont-Plage, huge bare-surfaced sand tongues stretch northeastwards from the beach and the foredune is 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 accurately the photos. The "Bunker archeology" refers to the utilization of the German bunkers and other works of the "Atlantic Wall" to reconstitute their architecture, military purpose and exact original position. It is based on a careful analysis of their remnants in the field, completed by the study of military and civil archives. It may give very interesting and precise data on the shoreline change and foredune development since 1944. In this paper a bunker complex located south of Merlimont-Plage, in front of the National Biologic Reserve, has been chosen to show the great interest of these benchmarks. The results concern two main points: the shoreline change and the development of a foredune in the 20 years following the war. The successive IGN aerial photographs allow to follow up the bunker complex in relationship with the changing morphology. In October 1947, all the bunkers are still in their original position and badly seen on the photo. The access road is partly covered with sand but still recognizable. The foredune is low and very discontinuous north of the bunkers and completely absent to the south. 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 the beach. The sand deposit was certainly massive since no bomb-holes are seen in this area, which was strongly bombed by the British in 1943 and 1944. In April 1955, the whole outer dune system has been remodeled, more or less flattened and squared off with windbreaks oriented NNW-SSE. Wide areas are bare-surfaced. The bunkers are better seen than in 1947. Two gun tanks lie on the upper beach. The foredune is totally absent. In August 1961, the main casemate has moved down but it is still leaning against the dune slope. A narrow sand bench has formed in front of the previous shoreline. On the top of the photo (to the south) there is a dense grid of windbreaks associated with marram planting. Along the beach the foredune is not well formed yet (it is present on the 1963 photo). Inland numerous small sand mounds are being colonized by vegetation (marram?). In 1968, the bunkers position has not changed significantly. The bare-surfaced sand bench along the upper beach is well developed. An incipient foredune is forming but it is discontinuous with numerous transverse troughs. Inland the effect of human intervention is even more spectacular than in 1961 with wide areas which are equipped with windbreaks and planted with marram. In May 1974, the coastal strip has changed: the sand bench has disappeared and all the bunkers lies on the beach. This is related to several storm events which happened just before. To the north a small incipient foredune has formed in front of a higher dune covered with 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 the same 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 and the 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 blowout is changing into a parabolic dune ("C" on the photo). Other blowouts have formed inland: the natural wind dynamics is working to create more 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 sandy cliff. Once again this erosion feature must be connected with successive storms in the 1990’s. The present shoreline (in 2014) has advanced since 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 does not reflect the long term evolution: the retreat took place mainly between 1974 and 2000, then the shoreline has advanced, so that the present 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 future long term evolution. Especially in a macrotidal environnement the effects of storms depend not only of their occurrence frequency but also of 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, but well formed in 1963 combined with incipient dunes on the upper beach. Well established in 1971, the foredune height has increased significantly in the following years. A layer of compost mud which was deposited in 1969 north of Berck and visible in the sand cliff of the foredune was 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 developed naturally 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 third part of the paper analyses the short-term shoreline change and emphasizes the resilience of the beach-dune system thanks to sediment exchanges between the beach and foredune. All along the beach of Merlimont (and in many other places of the Opale Coast) the contact between upper beach and foredune face is constantly changing at a very rapid rate (from one tide-cycle to another and throughout the year). The German bunkers are good benchmarks to observe and evaluate the change. It is due to massive volumes of sand moving from dune to beach during the storms combined with high tide, or from beach to dune when the prevailing winds blow over a wide sandy 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 be removed by storm waves without threatening the beach equilibrium. These natural exchanges of sediment are the key to the good state of the beach. They work very well all along the North-South coast between the Canche and Somme estuaries thanks to the prevailing sea winds, abundant sand supply and wide sandy surfaces at low tide. The fourth part of the paper questions how a beach-dune system might work without any foredune. Two cases have to be considered: either the absence of foredune is natural, or it has been destroyed. The first case existed before the 2nd World War south of Merlimont. During storms the sea could enter the dune-field through wide troughs and deposit marine sediment which was later reworked and moved inland by the wind. The beach-dune system was less resilient. Nevertheless the shoreline was not retreating considerably because the amount of sand coming from the sea could compensate the inland export of blown sand, thanks to the voluminous sediment supply present in nearshore sand bars inherited from the last Glacial Period, when the sea-level was 120 m lower and the Channel received the load of several large rivers (Rhine and Seine were the most important). The second case exists where a seawall has been constructed and the dune replaced by a built area. In front of the seawall of Merlimont the beach is narrow or absent 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 in stock 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 gets lower after each storm. Any sustainable solution has to take account of both marine and wind dynamics. So in May 2014 a system of beach dewatering (ECOPLAGE®) was installed with 2 sets of 6 drains and a pump station. The purpose is to increase the accumulation of sand on the 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 partie de 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’article reconstitue 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 peu changé depuis 1935 : grandes dunes mouvantes s’allongeant vers l’ENE et directement alimentées depuis la plage. L’avant-dune n’est nette qu’à partir des années 1960. Les photos aériennes soulignent l’ampleur des interventions humaines sur tout le domaine dunaire au sud de Merlimont (quadrillage de brise-vent et plantation d’oyat). Le trait de côte a reculé assez nettement à partir des années 1970 jusqu’aux années 1990 avant de ré-avancer. La morphologie des dunes externes (avant-dune bien formée et système couloirs/ pourrières), si caractéristique du paysage actuel, ne date que du début des années 1980. Le suivi topographique complété par celui des bunkers allemands de 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 une remarquable résilience de ce système côtier macrotidal, grâce au rôle de stockage de l’avant-dune, à une dynamique éolienne très active et à l’abondance de sable dans les petits-fonds. A contrario, toute absence ou destruction de l’avant-dune est une menace pour le budget sédimentaire de la plage, comme c’est le cas le long de la digue de Merlimont.
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halshs-01722408 , version 1 (03-03-2018)

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  • HAL Id : halshs-01722408 , version 1

Citer

Yvonne Battiau-Queney. Le système plage-dune de Merlimont-Berck et son évolution depuis la seconde guerre mondiale. Dynamiques Environnementales - Journal international des géosciences et de l’environnement, 2014, Coastal dunes management strategies and practices. Perspectives and case studies, 33, pp.92-106. ⟨halshs-01722408⟩
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