Seasonal evolution on reef fringed pocket beaches in a context of relative sea level rise.

In tropical environment, studies concerning pocket beaches dynamics are scare compared to open sandy beaches. Consequently, seasonal morphological adjustment of pocket beaches is still poorly understood and seasonal evolution rates are poorly quantified in these specific environments. Mayotte, a coral reef-fringed island in the Mozambique channel, western Indian Ocean, is characterised by a highly indent shoreline, submitted to important seasonal weather and sea variation conditions (monsoon and trade wind). Since 2018, Mayotte has undergone a seismic crisis characterized by clustered epicentres located offshore between 10 and 50 km east of the Main Island (Grande-Terre). This sismo-volcanic activity caused rapid subsidence and eastward movement of up to 20cm, leading to a relative sea level rise, the hydro-geomorphological consequences of which are the subject of investigations.

Beach profiles, sedimentary analysis and hydrodynamic surveys were conducted over four beaches located on the different facades of the island. The analyse of those survey data revealed that the alternation between monsoon and trade winds lead to a drift in the sediment transport, inducing seasonal beach rotations. During trade winds (austral winter), beaches show a tendency to erosion in their south part and accretion in their north part, this dynamic reverse during the wet season (austral summer).

This study provides the first elements of understanding the reef fringed pocket beaches morphodynamics at a seasonal scale, subsequently allowing an attempt to discriminate interannual variations and the possible impacts of subsidence as well as morphogenic thresholds.