Morphosedimentary evolution of Mamelles and Ouakam beaches (Dakar, Senegal)

Résumé

The coasts are among the most dynamic environments on the surface on the earth. The most fundamental impact of waves with respect to the coastal zone, is that they introduce energy which is responsible for sediment erosion and transport. Thus, the evolution of the coastal morphology result from the distribution of this energy through time and space. The purpose of this article is to study the morphosedimentary evolution of Mamelles and Ouakam beaches in response to the swells and waves action through a regular monitoring of beach profiles and sedimentological analysis (vertical movements and granulometric variations) between August 2013 and August 2014.  Data obtained show that each of beaches have a relatively low width with an average of 27.53 m in Ouakam and 32.98 m in Mamelles. The slopes are respectively 7.82% and 9.37%.  The widths, slopes and shapes of the beach are more variables in Mamelles beach. The annual balance of vertical movements is negative in Mamelles and positive in Ouakam. The accretion are more marked during the dry season whereas the rainy season recorded a predominant erosion. Sediments show fine to medium size in Mamelles and medium size in Ouakam. There are very well to moderalety classified in Mamelles and moderately well classified in Ouakam. They are homogeneous in Ouakam and heterogeneous in Mamelles. All these differences could indicate a lack of linkage between these two beaches or different hydrodynamic conditions. Indeed, the beach of Mamelles, unlike that of Ouakam presents a form of bay which causes waves refraction. There are also some beach rocks playing a role of natural breakwater against the waves. All these conditions reduce the waves energy.