Production and transport of suspended sediment transport in the Upper-Tafna river basin (North West Algeria)

Knowledge of suspended sediment loads carried by rivers provides insight into the underlying erosion mechanisms. In Maghreb rivers there are few data available on suspended sediment transport (PROBST and SUCHET, 1992). In this context knowledge of suspended sediment loads in the Oued Sebdou River (Mediterranean Algeria), which is a main stream in the Upper-Tafna basin, is of interest. The purpose of this study was to determine periods of active erosion and estimate the contributions of slope erosion and channel erosion in this basin. Situated in northwest Algeria, the Upper-Tafna basin covers an area of 256 km super(2). Rising in Ouled Ouriach, the upper reaches of the river flow through Jurassic soils at altitudes up to 1400 m. These tributary streams join on the Sebdou plain (900 m), which is composed of Plio-quaternary alluviums. The soil is composed of calcareous-marnes, calcareous and Jurassic dolomites (BENEST, 1972; BENEST et al., 1999) until the Beni-Bahdel dam. The study zone is characterized by a semi-arid climate. Analysis of the rainfall data at the Beni-Bahdel station (X=34 degree 42'33"; Y=01 degree 29'48"; Z=660 m) from 1939-1940 to 1997-1998 demonstrates that the rain supply has decreased greatly since 1975, with more than an 18% decrease in rainfall amounts. Hydrological data and methodology The study used instantaneous water discharge and suspended sediment load values (1257 values) measured and supplied by the National Agency of Hydrologic Resources [ANRH]. Five annual water years have been investigated during the period from September 1988 to August 1993. The discharge values (Q sub(L), in m super(3)/s) were determined from the rating curve for the measured heights of the water at the gauging station. The suspended load (g/l) was determined on water samples taken from the stream channel banks. The number of samples taken was adapted to the hydrological regime (i.e. every other day or during flood periods) and samples were collected as frequently as increments of 10 cm. The suspended sediments were carried from the slopes into the river by surface runoff or by stream channel erosion caused by flowing water. In order to express the contribution of these two mechanisms, we applied hydrograph separation methods used by ETCHANCHU (1988), ETCHANCHU and PROBST (1986), KATTAN et al. (1987) and PROBST and BAZERBACH (1986). Balance sheet of solid and liquid contributions The annual liquid contributions from 1988 to 1993 were evaluated to be 30.9