Investigating the Early Stages of a Dune Restoration in Front of a Dyke on a Microtidal Urban Beach Affected by Chronical Erosion

Robin, N.; Raynal, O.; Palvadeau, E.; René, C.; Balouin, Y., and Valentini, N., 2024. Investigating the early stages of a dune restoration in front of a dyke on a microtidal urban beach affected by chronical erosion. 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. 130-134. Charlotte (North Carolina), ISSN 0749-0208. Coastal dunes are globally recognized as natural features that can be important adaptation approaches for climate change along urban and natural coastal system. The aim of the study is to investigate the effects of sand trapping fences installed in front of a grey dyke along a 2300-meter stretch of coastline in St. Marie-LaMer (Southeastern France) in November 2021. This area has been experiencing severe chronic erosion for several decades. The methodology involves analyzing bi-annual topographic Lidar surveys conducted between May 2020 and September 2023, along with an examination of hydro-meteorological conditions. The research indicates several sediment balances within the traps between October 2021 and September 2023, with volumes in stability -0.05 m3/m2 (-14 m3) at the more eroding zone and in exceeding between 0.2 and 0.35 m3/m2 (1200 m3) elsewhere. The erosive trend preceding October 2021 has ceased, with each DEM demonstrating stability or gain. Throughout the study, the beach's sediment budget remains negative (ranging between -0.12 and -0.27 m3/m2, -6100 m3) except in SA2. Thus, despite adverse conditions for aeolian sediment transport and deposition, such as narrow and steep beach, predominant offshore wind, coarse grain size, limited sediment supply, slow vegetal colonization, along with persistent erosion of the lower beach, initial stages of upper beach restoration are encouraging. Moreover, this soft dune management approach is expected to mitigate the impacts of major storm events. This was observed in 2021 and 2022 where two significant storms (Hs>5m), had minimal impact on the upper beach compared to past incidents where similar conditions led to deterioration of the incipient dune and the dyke. This study provides valuable insights into the implementation of such systems in environments prone to chronic erosion and unfavorable conditions for aeolian sediment transport