Surface sediment dynamics along the Tunisian coast at Skhira (Gulf of Gabès, south-eastern Tunisia)

Abstract

An investigation was conducted in the summer of 2012 at 24 study stations along the Tunisian coast near Skhira (south-eastern Mediterranean Sea) through high resolution of analyses, grain size and mineral composition of surface sediment. An Acoustic Doppler Current Profiler showed Skhira's main coastal current to follow a direction 60°E, with its main axis south-west/north-east parallel to the coastline and its minor axis east-west. Current speeds were approximately 13 cm s−1 and 4.5 cm s−1 for the major and minor axes, respectively. Parallel to the coast, currents were generally mild to moderate, but often exceeded 20 cm s−1. An Argonaut meter recorded the dominant current direction as north/north-east with a speed not exceeding 18 cm s−1. Orthogonal Empirical Function analysis of tidal currents showed that the major axis velocity dispersion was north-east/south-west, the water mass flow parallel to the shoreline being almost unidirectional, driving littoral drift parallel to the coast. Spatial distribution of particle size, along with speed and current direction analysis, furnish an overview of the Skhira area's sediment dynamics and transport. Average sand grain size shows that the bottom consists of fine and especially of medium sands near the coast, with muddy sands offshore. The fine fraction percentage (<63 microns) as opposed to the coarse fraction (>63 microns) is higher at the two offshore study stations, 20 m deep. Fine particles are discharged into the sea by rip currents.Sediment dynamics along the Skhira coast are complex, being subject to the combined effect of swell and tide. Sediments are permanently re-suspended and in constant movement, especially during storms.