Multi-Scale Analysis of Erosion and Sediment Transfer Processes on Grand-Lahou Beach

Abstract

This study was carried out to report on the hydro-sediment dynamics of Grand-Lahou beach at different time scales over the last twenty-five years. It is based on two complementary methods: the method of sediment reworking based on continuous altimetric monitoring of posts during a tidal cycle and in situ tracing using fluorescent-coloured sand, and the method of monitoring changes to the beach based on topographical profiles at different time intervals (seasonal, annual, multi-year, ten-year). The sediment balances were established by determining the areas eroded and/or greased using the trapezoidal or triangular surface calculation method to precisely quantify the sediment movements in the profile and the associated dynamics. The results obtained show that the dynamic swash zone varies according to the phases of the tide. On the ebb tide, this zone is reduced to the low foreshore and a few points on the mid-foreshore, depending on the tide-swell couple. When the tide is flowing, the zone widens, taking into account the high foreshore. The morphology of the submerged zone changes constantly as the tide passes over it. The low offshore shows a trend towards stability with weak erosion episodes. The mid-foreshore shows a predominance of fattening, with the highest movements of instantaneous reworking (RrI) on the foreshore. The maximum amplitude of sediment movement of 25.5 cm was obtained at mid foreshore. The morpho-sedimentary monitoring of the beach reveals that the most important retreats, estimated at 25 m.an-1 were observed during exceptional events including storms. Erosion in the area around the mouth of the Bandama River is the main threat to the people living along the river, given the damage that has been caused.