Interaction between bed roughness and flow hydraulics in eroding rills

Govers [1992] showed that the flow velocity in rills eroding loose, nonlaye red materials could be predicted from knowledge of discharge only (without significant slope or soil effect). The objective of this paper is to invest igate to what extent the observed slope independence of flow velocity in er oding rills can be explained by the interaction between rill bed roughness and flow hydraulics. In a laboratory study, two situations were compared: ( 1) rills which can freely erode a uniform soil layer and (2) rills with a f ixed bed geometry. During the experiments, rill discharge and flow velocity were recorded. After each experiment, a detailed topographic survey of the rill bed was carried out using a laser scanner. From these data the main h ydraulic variables (mean values of flow depth, wetted perimeter, and hydrau lic radius) were estimated. The experiments confirmed the slope independenc e of rill flow velocities on mobile beds. When the bed is fixed, the flow v elocity in rills is clearly slope-dependent. The slope independence of flow velocity on mobile beds is due to a feedback between rill bed morphology a nd flow conditions. The roughness amplitude (which was assessed from the st andard deviation of corrected height values) and the frequency of macroroug hness elements (which was assessed by counting the number of peaks per unit length in a previously simplified longitudinal profiles) both increase and tend to counteract the effect of the increase in slope gradient on rill fl ow velocity. The final rill flow velocity appears to be characterized by a constant average Froude number.