Shear stress partitioning and sediment transport by overland flow

Grain shear stress has been used in many earlier studies to estimate the se diment transport capacity of interrill overland flow. However, this procedu re may be flawed because turbulent eddies generated by large-scale roughnes s elements contribute to sediment transport in this type of flow, whereas t hey do not in deeper (river) flow. In this study a procedure is developed f or estimating the proportion of total shear stress contributing to sediment transport in overland flow. Application of this procedure to 1506 flume ex periments representing a wide range of discharge, slope and surface roughne ss conditions reveals that the proportion of total shear stress contributin g to sediment transport decreases from 1.0 to 0.83 as roughness concentrati on increases from 0 to 0.37. Concomitantly, the grain shear stress expresse d as a proportion of total shear stress decreases from 1.0 to 0.14. Clearly , the proportion of total shear stress involved in sediment transport is hi gher than is indicated by grain shear stress. Consequently, use of grain sh ear stress to predict the sediment transport capacity of overland flow on r ough surfaces will result in significant underestimation of the transport c apacity.