Assessing shoreline dynamics in southwestern Madagascar, using DSAS tools

The phenomenon of shoreline erosion may cause significant ecological and economic losses, including the degradation of biodiversity and the deterioration of infrastructure. The majority of studies on shoreline dynamics have focused on the description of its temporal evolution. This paper aims to identify the principal factors influencing shoreline dynamics, with a focus on the coasts of southwestern Madagascar, which encompass extensive mangrove areas. In order to observe the processes of coastal retreat (erosion) and advance (accretion), satellite images from different dates (1990/2000/2010/2020) were analysed using Digital Shoreline Analysis System (DSAS) tools. These processes were also associated with a set variables that have the potential to act as drivers of shoreline dynamics. These included coastal land use (presence/absence of mangroves), distance to the nearest river mouth, Bouguer anomaly, which serves as a proxy for tectonic subsidence risk, and foreshore slope. The variables were derived from three distinct sources: the 2020 vegetation map of the study site, the digital topographic map of Madagascar, and the Bouguer gravity anomaly map of Madagascar. Ultimately, a co-inertia analysis was employed to determine the relative importance of the four variables in shoreline dynamics. The shoreline of the study site is predominantly subject to erosion, which appears to be worsening over time. The occurrence of shoreline erosion was found to be significantly linked to proximity to river mouths and high Bouguer anomaly. The presence of mangroves along the shoreline of the study site does not appear to have impeded the process of shoreline erosion.