Field Investigation of Spatio-Temporal Change of Sediment Grain Size and Beach Slope on Wave-Dominated Sandy Beaches

Castelle, B.; Labarthe, C.; Marieu, V.; Bujan, S., and Garlan, T., 2024. Field investigation of spatio-temporal change of sediment grain size and beach slope on wave-dominated sandy beaches. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 432-437. Charlotte (North Carolina), ISSN 0749-0208. Beach slope is a critical parameter which is usually assumed constant in time and to depend on a single time-invariant median grain size. In the present work, we conducted a three-year intensive in situ bi-yearly monitoring of the 3D distribution (surface and depth across a 50-cm profile) of sediment grain size and beach topography at five sandy beaches in France. These five beaches are representative of the diversity of the metropolitan France beaches in terms of wave climate, tide range, and anthropogenic pressure. Results show that, generally, average sediment grain size and beach slope are finer and more gently sloping at the end of the winter and at the end of the summer, respectively. Results also show a large spatial and temporal variability of both sediment grain size (horizontally and vertically) and of the sandy beach morphology. Sediment grain size shows a consistent vertical gradient, with finer sand closer to the surface. The spatial and temporal patterns of beach slope and beach sediment variability are found to be linked with beach state (dissipative, intermediate, reflective) and anthropogenic pressure at the engineered beaches. Our results also suggest that tide range and modal wave conditions must be considered to improve empirical models linking beach slope and sediment grain size.