Morphologic changes and sedimentary budgets along a Mediterranean coastline with a sand spit: case of the littoral fringe Sidi Ali El Mekki–Gammarth (NE Tunisia)

Abstract

Morphological changes and both surface and volume budgets along the Mediterranean coastline Sidi Ali El Mekki–Gammarth which is located at the western fringe of the Gulf of Tunis (NE Tunisia) are studied, over the 1887–2010 period, by ancient minute of bathymetry and aerial photographs treated using digital photogrammetric methods and GIS tools. The accretion areas are located upstream the fishing port of Ghar El Melh which interrupts alongshore sediment transport (0.22–0.66 ± 0.21 m/year, i.e., 21,309 ± 1,113 m²/year, i.e., 106,546 ± 5,565 m³/year) and around the recent mouth of the Medjerda River because of fluvial deposits and accumulation of sediment transported by drift currents (2.35–3.35 ± 0.21 m/year, i.e., 4,956 ± 1,050 m²/year, i.e., 24,780 ± 5,250 m³/year). The erosion areas are situated downstream the fishing port of Ghar El Melh (−4.32 to 0.9 ± 0.21 m/year, i.e., −4,822 ± 1,232 m²/year, i.e., −24,114 ± 6,163 m³/year), around the ancient mouth of the Medjerda River which was abandoned and deprived from fluvial sediment supply (−15.44 to −4.55 ± 0.21 m/year, i.e., −4,499 ± 264 m²/year, i.e., −22,495 ± 1,323 m³/year). The eroded sediments are displaced to the south giving rise to the sand spit of Kalâat El Andalous. Coastline retreated also along the Raoued–Gammarth sector which lost sediments by SE–NW drift currents (−0.75 to −0.23 ± 0.21 m/year, i.e., −3,765 ± 1,113 m²/year, i.e., −18,828 ± 5,565 m³/year). The alongshore distribution of accretion/erosion patterns lead to divide the studied coastline into three littoral cells limited by natural boundaries: (a) Cap Sidi Ali El Mekki—the ancient mouth of the Medjerda River, (b) from the ancient to the recent mouth of the Medjerda river, and (c) the recent mouth of the Medjerda River–Cap Gammarth.