Abstract
Predicting the morphodynamic response to sea-level rise is one of the major challenges of this century. The rapid rise in relative sea level, of around twenty centimetres in Mayotte between May 2018 and July 2020, following an underwater volcanic eruption, has created an exceptional opportunity to assess, in situ, the impacts of this rise on beach/coral reef systems. The aim of this study was to take stock of post-subsidence knowledge and assess any short-term changes linked to this rapid rise in sea level. Hydrodynamic, sedimentary and topographical measurements showed a phenomenon of seasonal beach rotation, typical of pocket beaches, influenced by the seasonal variability of marine weather conditions and by the presence of fringing coral reefs. Comparison of data before and after subsidence did not reveal any significant morphological changes that could be directly linked to the subsidence. Similarly, no significant variation in biogeomorphological units was observed between 2016 and 2020 from the satellite images. However, the kinematics of the coastline has revealed a recent acceleration in shoreline retreat, although this acceleration cannot be attributed exclusively to subsidence. Ongoing monitoring and in-depth analysis will be required to understand the response of the beach/reef system and its geomorphological and biological compartments in the coming years. Therefore, this study represents an important first contribution, after subsidence, to the understanding the impacts of sea-level rise on the coasts of Mayotte.