Pluriannual beach-dune evolutions at regional scale: Erosion and recovery sequences analysis along the aquitaine coast based on airborne LiDAR data

Abstract : With apparent uniformity, the 230 km of the Aquitanian sandy coast presents many differences in geomorphological characteristics and evolutions at the event, seasonal, annual and pluriannual time scales. This contribution highlights work based on extensive airborne LiDAR coverage campaigns realized in 2011, 2014, 2016 and 2017. Geomorphological evolutions are studied at the scale of the sedimentary cells and subcells of the Aquitanian sandy coast, specifically through the automatic detection of dune erosion scarps and incipient foredunes, planimetric indicators analysis, and the sediment budget of the beach-dune interface. Analyses were performed for two periods: 2011–2014 (including the erosional impact of the 2013–2014 winter) and 2014–2017, presenting continuous and extensive beach recovery at the beach-dune interface. Dune erosion scarps and incipient foredune characteristics are analyzed. A classification based on the alongshore length of the erosion scarp shows great diversity of erosional dynamics, which are directly linked to the geomorphologic characteristics of the beach sediment availability and the nearshore bar system. This diversity includes homogeneously large erosional bands above a 1 km alongshore length, periodic Mega cusp embayments from 200 to 800 m, and a local erosion mark less than 200 m long. Analysis for the following period shows a prompt and massive recovery at the upper beach and dune foot proxies on the majority of the coast. However, the recovery process as well as erosion is strongly marked by a North–South gradient. The dune front was massively impacted in Gironde, with a retreat exceeding 25 m that was poorly prograded since 2014, whereas in Landes a progradation of the front dune is observed compared to 2011. During the first period (including the 2013–2014 winter), more than 15 500 × 103 m3 of sediment was eroded from the beach-dune interface, which represents on average a loss of 66 m3 per linear meter. The gains during the following period (2014–2017) are also massive, above 13 500 × 103 m3, with an average gain of 57 m3 per linear meter. Given the budget between the two periods, 86.5% of the sand eroded in 2014 had moved back in 2017 at the beach-dune interface but 90 % of the sand stocks are localised between the upper beach and the dune foot. The progressive recovery tendency at the beach-dune interface is massive but fragile because the sediments are potentially removable by storm marine processes in case of energetic events. At the pluriannaual, decadal and pluridecadal scales, the different sections of the coast follow various trends concerning erosion as well as recovery processes resulting in a contrasted long term coastline evolution from the north to the south of the coast.