Abstract
Coral reefs, iconic environments of global biodiversity, face the challenges of climate change, rising sea levels, and increasing coastal development. Despite their ecological and economic significance, beaches adjacent to these reefs, especially fringing back-reef beaches, have received relatively little attention. This manuscript presents doctoral research aimed at enhancing our understanding of the hydro-morphodynamics of these beaches across various time scales, utilizing a Video Monitoring System (VMS) coupled with a pressure sensor. The study site, Hermitage Beach, situated in a microtidal zone on the western coast of La Réunion in the Indian Ocean, is frequently exposed to southern swells and cyclonic events. Since 2014, this beach has been monitored by Differential Global Positioning System (DGPS) and drone as part of the Dynalit National Observation System (SNO Dynalit). Findings spanning from 1950 to the present highlight a significantly eroded beach, attributed to various anthropogenic factors. The results of this thesis illustrate the relevance and constraints of utilizing such instrumentation (video camera and pressure sensor) for investigating back-reef beaches. Seasonal fluctuations in the waterline position, with a retreat of 1.4 m in winter and an advance of 0.9 m in summer, are observed. On average, the reef attenuates 97% of incoming swells and functions as a low-pass filter. Additionally, an analysis of runup on the foreshore reveals that low-frequency waves contribute 50% to this phenomenon, with water level setup contributing up to 40%. This research underscores the critical role of long waves (infragravity (IG) and Very Low Frequency (VLF)) in both submersion and erosion processes. It sets the stage for further exploration in this environmental context, particularly in light of climate change and rising sea levels.