Géodynamique actuelle du bassin versant de l'Oued Tensift (Maroc) : érosion mécanique et bilan des transports solides fluviaux. / Present geodynamic of the Tensift drainage basin (Morocco) : mechanical erosion and fluvial transports of sediments

Present geodynamic of the Tensift drainage basin (Morocco) : mechanical erosion and fluvial transports of sediments The aim of this study is to improve the knowledge on sediment mobilization in semi-arid climate through a global estimation of erosion-transport-sedimentation budget in the Tensift drainage basin (Morocco). The lithological material which could be mobilized by erosion of the main formations has been estimated using simple models to a specific value of 373 t.km-2.an-1. Sediment discharge to Atlantic ocean is calculated using river transport measurements to 200 t.km-2.an-1. This last flux represents 69% of the total sediments potentially eroded in the Tensift basin. The remaining 31% could be attributed to sedimentary deposits in the alluvial plain. The comparison between the fluvial transports of different floods during the same year exhibits the importance of the hydrological processes in the mechanical erosion control. Moreover, a survey of three hydrological years (dry. mean and humid) allows to follow the present interannual geodynamic evolution of this basin. The sediment budget shows that the climatic factor controls the fluvial transport for the different lithologies. Topography and lithology represent complementary factors, mainly in mountainous part of the drainage basin. Replaced in the whole Maghreb context, the mean values calculated for the Tensift river are in good agreement with those already obtained by various authors (Snoussi et al., 1989 et 1990 ; Probst et Amiotte Suchet, 1992) regarding moroccan. algerian and tunisian rivers. This study confirms the importance of the mechanical erosion and fluvial sediment transports in semi-arid climatic area such as the Tensift draining basin. The sediment yield measured in this region are for example much more higher (1 to 2 order of magnitude) than those observed in tropical Africa.