Etude du flux hydro-sédimentaire de la Soummam. Effets sur l’évolution de trait de côte et les infrastructures

Abstract

The hydro-sedimentary flow that characterizes the Soummam watershed is conditioned by morpho-structural and hydro-climatic frameworks and land use and cultural practices that allow fairly active water erosion in all its forms. The number and distribution of hydrometric stations that control the watershed is far from satisfactory (05 stations for an area of 9125 km2). Also, from 1985, the Soummam watershed the construction of four dams Lakhel (1985), Ain Zada (1986), Tilesdit (2004) and Tichy Haf (2007), which made the old network (1972/1985 series) of stations which are mostly obsolete. Thus, a second series of measures will be used by integrating the dams after they have been filled. The analysis and comparison of the two series allowed us to follow the evolution of the hydro-sedimentary flow before and after the hydraulic developments. The volume of solid input to the Sidi Aiche station (92% of the area of the Soummam watershed) is halved. The bathymetric data made it possible to verify the specific degradations obtained by the measurements of transport in suspension. However, these results are far from indicating the areas of contribution within the large Soummam watershed. The USLE model allows quantification and spatialization of erosion. It is a model developed from data on plots, but which has shown a certain flexibility which allows its application to watersheds. The average annual loss of soil, per unit of area, depends on several factors, namely: rain erosion, soil erodibility, plant cover, and topography and conservation practices. The annual intake obtained by this model does not reach 50% of the intake measured at the Sidi Aiche station. This contribution is only due to erosion in the water table. Thus, to improve the results and strengthen the applicability of the model to watersheds, we have added the contribution of two other forms of erosion: mass movements and gullying. Nonexistent forms on the experimental plots used by Wischmeier. The combination of data relating to each form and the modeling are carried out in a Geographic Information System (GIS) in order to obtain a map of the contribution areas in t / km2 / year. The model's land losses became equal to the sum of the contributions of the three forms of erosion and which is estimated at 90% of the measured annual contribution. We have successfully applied the same approach to sub-basins controlled by a hydrometric station. These encouraging results largely attenuate the contradictions resulting from the transition from the plot to the watershed. In addition, due to the hydraulic arrangements made in the Soummam watershed, the solid contribution to the mouth is halved. This situation is certainly at the origin of the increase in the speed of retreat of the coastline at Bejaia Bay.