Le réaménagement du littoral Haut-Normand à Criel-sur-Mer (Seine Maritime)

Abstract

The lower Yeres valley was a small estuary on the coast of the Pays de Caux until the end of the last century, when it has been closed. The drastic reduction of the pebble flow along the coast, due mainly to jetty erection in several harbours and to overexploitation of pebbles is since fifteen years endangering the coastal equilibrium : expensive defense works such as groyne construction and repetitive fllling-up of the pebble beach are now necessary to maintain a retreating coast. A commitment to more definitive and satisfactory solutions happens to be necessary. In a first stage, a small scale sedimentological model was used to determine the best design for new defense works, such as optimized groynes or a dam along a retreated line suppressing the obliquity transport linked to predominant waves. But the kind of undistorted model used for monotoring pebble movements being unable to provide indications valuable enough on sandy and fine sediments transport, it was necessary to complete the survey, using borings in the alluvial plain, a geotechnical survey and laboratory tests in order to anticipate the behaviour of the sediments in case of a whole reopening of the former estuary. The mechanical resistance of all sediments filling the lower valley is very poor, due to saturation from the dominating chalk water body, impeding the building of dams and expanding their coast. Most sediments filling the valley are older than the last transgression and represent swamp material. Probable scenarii of the evolution of the estuary in case of a reopening are given, but the final decision on the option to be completed still depends on economical factors.