Grain-size and morphodynamical state of the bay-of-Mahdia shoreface (Tunisia). Contribution to the assessment of coastal sensitivity

Abstract. Improving our knowledge about coastlines worldwide can definitely contribute to understanding how sand coastlines with offshore bar systems operate. The study undertaken on the coastal sedimentary unit on the Bay of Mahdia where moderately active tectonic activity prevails fits into that general approach. This preliminary work presents the morphology and sedimentology of a 15 km long coastline with a sandy beach and an irregular dune ridge bordering the offshore between cape Africa and cape Dimas along the Bay of Mahdia. Morphology displays a sandy sedimentary prism composed of 3 compartments spreading from south to north. At both ends, the prism is poor in sediments along the cape promontory. In the central part, it is more regular for about 7 to 8 km, forming a 1.4 % slope leading, at -10 to -12 m, to a slightly sloping plateau with relief lines highlighting the general contour of the bay. This sedimentary slope is abruptly limited to the north by a relief alignment, located in the extension of the Moknine fault. In the central part, the prism is labeled by the existence of two offshore bars of which the innermost shows instability under the hydrodynamic regime forcing. A bathymetric sounding carried out in August 2006 registered a segmentation of the inner bar, not seen on two sets of previous comparable aerial photos. This segmentation seems to be influenced by the incidence of storms, common during the summer season. A simulated wave refraction diagram of the N to E quadrant swells shows the good relationship of the wave orthogonal deviation with the distribution of the directions of the bar sections observed. The disruption and reorganization of inner bar sections under the influence of average energy storms bear witness to the symptomatic instability of a volume of sand to be displaced that can only be modest in size. Textural analysis and the nature of sediments confirm this diagnosis. The response of sediments to the selection processes shows that grain size assemblage depends on a mixture of sedimentary types in limited numbers, characteristic of the reduction in the number of sedimentary sources. The most important type uses the material from the beach and dune as a reservoir, which is in turn under the influence of reduction processes having required erosion abatement measures. Coarser populations, mainly bioclastic, become incorporated into the fine fraction of the sand whose distribution they modify. The change is however discreet, influencing primarily the asymmetry and leading to bimodality only in rare instances. In the case of Mahdia, the sedimentary sources are therefore exclusively local, rare and in small quantities