Profile evolution and active layer measurements on a macrotidal composite gravel beach, Somme Estuary, France

Morphological change, active layer variations and nearshore hydrodynamics were measured over a complete neap-spring-neap tidal cycle on a mixed sediment sand-and-gravel spit at Cayeux-sur-Mer, northern France. During the study period the tidal level reached 7.33 m above Hydrographic Datum (HD) at the lowest neap tide and 9.59 m at the spring tide. Incident waves approached from the southwest at angles of 15-40°; to the beach. During the survey period significant wave heights < 1 m occurred 49.7%, <2 m 96.4% and >2 m 3.6% of the time. The significant wave period was 4-9 secs. Three key phases of morphological change were identified, mainly associated with maximum tidal levels: (i) berm accretion during calm wave conditions (tidal range moved from neap to spring tide); (ii) berm retreat and erosion in relation to sediment supply and wave conditions (tidal range still moving towards spring tide); (iii) berm reconstruction and seaward progradation followed by stability (through the spring tide and approaching the neap tide). Cusps occurred twice indicating the reflective nature of the beach. The method used to measure active layer depth made it possible to discern the contributions of both erosion and accretion to profile changes. Maximum activation layer depths were associated with the zone of wave breaking and berm movement and empirical formulae linking the active layer depth to the wave height have been calculated.