Bed Material Discharge Prediction for Nonuniform Bed Sediments

An important element for simulating the evolution of a riverbed with nonuniform sediments is the reliable prediction of sediment discharge for each size fraction, which is essential for simulating hydraulic sorting and armoring of channel beds undergoing degradation or aggradation. Most of the existing sediment discharge relations either give total sediment discharge only or are unreliable for predicting size distribution of sediment discharge. This paper presents the development of a relation for predicting sediment discharge for each size fraction. The interaction between different size fractions in a nonuniform sediment bed is taken into account through the formulation of a weighting function for each fraction, consisting of two components: (1) Areal fraction of each grain size on bed; and (2) a “sheltering” factor reflecting reduction in transport of finer particles due to sheltering by the larger ones. The relation was applied to several data sets including both river and laboratory flows. Agreement between computed and observed values was good for both total sediment discharge and its size distribution. However, prediction was less accurate for flows with significant amounts of nonmoving coarser particles armoring the bed surface, indicating the need for further research for such flows. A simplified relation, based on median sediment size (D50) only, also is proposed. This relation is found to yield good agreement for predicting total sediment discharge and performs better than several existing sediment transport functions.