Cécile Picouet

Empirical and conceptual modelling of the suspended sediment dynamics in a large tropical African river: the Upper Niger river basin

A 7-year sediment transport monitoring on the Upper Niger rivers was used to study the relationship between suspended sediment concentration and river discharge. During annual floods, these relationships show positive hysteresis. This paper presents the results of two models that estimate the time evolution of suspended sediment concentration using water discharge data only. The first model is based on a statistical approach using two relationships, one for the rising stage period of the flood and one for the recession period of the annual flood; the second model is a lumped conceptual one; it supposes that the sediment flux observed in the river comes from two different sources of sediment and that these two sources may be regarded as two different reservoirs. The erosion of the first reservoir represents hillslope erosion observed during the runoff season. Sediment supply from this ‘reservoir’ is limited in time because depletion occurs during the runoff season. The second reservoir is unlimited in time and quantity and its erosion represents contributions coming from bank erosion and mobilisation of deposits in the channel network. Both of the models are compared with a simple rating curve based model. The model results show that the conceptual model has the highest efficiency to reproduce from weekly discharge only the time evolution of weekly suspended sediment concentrations, the time evolution of weekly sediment fluxes, and the global annual sediment yields.