Charroux, S.; Jeanson, M.; Morisseau, S., and Pennober, G., Seasonal morphodynamics of fringing reef pocket beaches and responses to rapid vertical tectonic movements.
Determining the morphological response of coastal environments to a general rise in sea level constitutes one of this century’s greatest challenges. In 2018–19, Mayotte, a coral reef–fringed island in the Indian Ocean, underwent a volcanic crisis that resulted in island subsidence of up to 0.2 m. This seismo-volcanic crisis made the island an ideal workshop site to observe the response of pocket beaches to sea-level variations. Morphodynamic analyses were carried out on three beaches located on different sides of the island on a biannual basis between 2019 and 2022. Analysis of these survey data showed that the alternation between monsoon and trade winds led to a seasonal rotation of the beaches. Furthermore, the central profiles did not appear to be affected by seasonal variations. As result, they were used to characterize subsidence-related impacts. When the pre- and postsubsidence profiles were compared, morphological changes were not noticeable. However, the rise in the sea level caused by subsidence led to an increase in the frequency of spring flooding at high tide in certain areas of low-lying beaches and roads on the densely populated NE coast of the archipelago, the area most affected by this tectonic movement.