Along the coast of the Hauts de France, various mussel farms can be found, including those between the bays of Somme and Canche. These farms are formed by rows of piles which create a porous environment and a loss of flow energy (tide and waves), liable to generate sand deposits and thus constitute protection for the dune lines at the top of the beach.
The farm located between the towns of Berck and Merlimont was in greater detail. It is 1 km long and 100 m wide. Each row consists of two lines separated by 3.5 m. On a line, the spacing is 1 m. Measurements of waves and changes in beach profiles were carried out and supplemented by numerical modelling.
The follow-up measures made it possible to analyse the effects of mussel beds on beach profiles. Analysis of over 20 years of biannual topographic measurements of beach profiles shows seasonal and annual variability, with sediment exchanges between the top and bottom of the intertidal profile, but no significant trend in total sand volume change. In particular, the area of the beach behind the bouchots shows the same long-term behaviour (stability) as the surrounding areas (stability in the north, and slight accretion in the south), both in terms of the total volume of sand and in terms of the position of the coastline.
Due to its limited width, the presence of the farm has no significant impact on the propagation of the swell. However, the digital model shows an effect on tidal currents. These run parallel to the beach and, over the course of a kilometre, lose intensity as they pass the different rows. The presence of the farm could therefore temporarily generate sedimentary deposits during the summer. These deposits are remobilized during the winter. To constitute a defence of the dune lines on the profile, it would nevertheless be necessary to increase the width of the farm and increase the density of the piles.