Stability state of a Mediterranean coastal environment threatened by erosion: the case of the Djerba Island (Gulf of Gabes, SE-Tunisia)

Abstract

Abstract The Djerba Island (Gulf of Gabes, SE-Tunisia) that is part of the South Rive of the Mediterranean is classified lately as an international heritage site. Despite this, it suffers from erosion. So, it's necessary to study and monitor its stability state (erosion/accretion) to predict future erosion. In the present work, the stability state was determined by studying the coastline evolution using photographs (1991) and satellite images treated by Remote Sensing and GIS tools. Extracted coastlines (schematized by polylines or extracted by the calculation of MNDWI index) have been superimposed. Their evolution (distances and rates) was calculated automatically using the Arc GIS Digital Shoreline Analysis System extension. Results show accelerated erosion over the last period 2016–2020 for all cells on the Island. The evolution over a period of 45 years, from 1975 to 2020, shows a coastline retreat along the entire Island (3 to 10 m/year), with the exception of the sandy spits of Ras Rmal and Borj Kastel (accretion due to the contributions of sediments by coastal drift currents). The sea level rise and the various hydrodynamic and anthropogenic forcings have led to this state instability and this generalized erosion.