Transport Capacity of Overland Flow for Sediment Mixtures

Sediment transport capacity of overland flows is crucial to understanding and modeling the erosion and deposition processes. The transport capacity for sediment mixtures is particularly complex because of the need for an algorithm to allocate the total available shear stress or stream power to individual size classes. The transport capacity for sediment mixtures may not even be unique, depending on whether the flow is eroding or depositing sediments unless all sediments are of a single size class. Using data from a set of flume experiments involving steep slopes and high sediment concentrations, a formula was proposed for the sediment transport capacity limited by available stream power and available accommodation space for sediments. Sediments used for the experiment were further sieved into five distinct size classes. The sediment transport capacity was measured for 25 flow-slope combinations to validate a new formula for individual size classes, and to propose and test a method to compute the sediment transport capacity for sediment mixtures from that for individual size classes. An independent set of experimental data was used to validate the new formula for individual size classes. The results showed that the formulation for sediment transport capacity was consistent for all size classes, and the sediment transport capacity was not as sensitive to the settling velocity as expected. With the effect of settling velocity moderated, the predicted and observed sediment concentrations at the transport limit showed excellent agreement with the Nash-Sutcliffe coefficient of efficiency (Ec) of 0.96 for individual size classes. The paper also shows that the transport capacity for sediment mixtures can be computed as an average of the sediment transport capacity for individual size classes weighted by the size distribution and moderated settling velocity. The sediment transport capacity can also be predicted using the average settling velocity for sediment mixtures for this set of experiments. An independent validation test showed that the predicted and observed sediment concentration agreed well for the experimental data with the coefficient of determination R2=0.85, Ec=0.71, and a bias of approximately 25%.