Goneri Le Cozannet, Thomas Bulteau, Bruno Castelle, Roshanka Ranasinghe, Guy Wöppelmann, Jeremy Rohmer, Nicolas Bernon, Déborah Idier, Jessie Louisor, David Salas-y-Mélia

Sandy shorelines are constantly evolving, threatening frequently human assets such as buildings or transport infrastructures. In these environments, sea level rise will exacerbate coastal erosion to an amount which remains uncertain. Sandy shoreline change projections inherits the uncertainties of future mean sea level changes, of vertical ground motions, and of other natural and anthropogenic processes affecting shoreline change variability and trends. Furthermore, the erosive impact of sea level rise itself can be quantified using two primary different coastal impact models. Here, we show that this latter source of uncertainty, which has been little quantified so far, can account for 20 to 40% of the variance of shoreline projections by 2100 and beyond. This is demonstrated at four contrasted sandy beaches relatively unaffected by human interventions in south-western France. Over these four sites, a variance-based global sensitivity analysis of shoreline projections uncertainties can only be achieved owing to previous observations of beach profile and shoreline changes. This means that sustained coastal observations and efforts to develop sea level rise impact models are needed to understand and eventually reduce uncertainties of shoreline change projections, in order to ultimately support coastal land use planning and adaptation.