Resistance to overland flow due to bed‐load transport on plane mobile beds

During bed‐load transport by overland ow, momentum is transferred from the ow to the bed via grain collisions, resulting in a decrease in ow velocity and an increase in ow resistance, herein termed bed‐load transport resistance. In overland ow on mobile plane beds, total ow resistance f consists of grain resistance f g and bed‐load transport resistance f bt . In order to identify and evaluate the relative importance of the factors controlling f bt , 38 ume experiments were performed on slopes of 2·7 and 5·5° using sediment with median diameters of 0·74 and 1·16 mm. All ows were supercritical and turbulent. This study is an extension of a recent study by Gao and Abrahams ( Earth Surface Processes and Landforms 2004, vol. 29, pp. 423–435). These authors found that f bt is controlled by three factors: sediment concentration C , dimensionless sediment diameter D * , and relative submergence h/D , where h is ow depth, D is median sediment diameter. However, a new dimensional analysis identies two additional factors: Froude number F and slope S . Multiple regression analyses reveal (1) that these ve factors together explain 97 per cent of the variance of f bt , and (2) that S controls f bt entirely through C . The variable C is therefore redundant, and a new functional equation relating f bt to D * , h/D , S and F is developed. This equation may be used to predict f bt . An advantage of this equation is that it may be used to predict f bt without measuring bed‐load transport rate. Copyright © 2004 John Wiley & Sons, Ltd.