Sedimentary dynamics of the South Lagoon of Tunis (Tunisia, Mediterranean Sea).

Abstract

The South lagoon of Tunis, a shallow Mediterranean lagoon, had undergone an important restoration management (during the period 1998-2001), leading to structural and functional changes of this coastal ecosystem. In this work, a comprehensive study of sedimentary dynamic based on hydrodynamic data, granulometric analysis, mineralogy and impact evaluation is presented. Restoration project, especially the establishment of two groups of inlets gates of the channel of Rades and Tunis (recalibrated during the restoration management), imposed an east-west water flow direction, created from high tide submersion, with an average flow exchanged with the sea about 80 m3s-1. This current control local sediment sorting. The grain sizes analysis of superficial sediment shows that the lagoon bottom characterized by fine sediments (<63 µm, 50-90%). The sediment dynamic is controlled by the lagoon water currents inducing an east-west grain-size sorting. The extreme eastern side of the lagoon, close to the inlet gate, is lined by medium sand, moderately sorted and transported by saltation. The central and western sides of the lagoon are covered by fine sand, poorly sorted, and deposed in relatively calm hydrodynamic conditions. Mineralogical results reveal the following association: quartz (13 to 69%), biogenic calcite, (15 to 81%), aragonite (0 to 8%), pyrite (0 to 1,75%) and accessory magnetite, smectite, illite and kaolinite. The lagoon seems to be a protected zone as a result of restoration project that forms a physical barrier for sedimentary materials amount from Gulf of Tunis.