Hydraulics of interrill overland flow on stone-covered desert surfaces

Abstract

The hydraulics of interrill overland flow on stone-covered desert surfaces depend on the resistance to flow, which may be partitioned into grain resistance, form resistance, wave resistance, and rain resistance. Efforts to model large-scale roughness in open channels suggest that resistance to overland flow on such surfaces is a function of grain Reynolds number, Froude number, flow depth, and the size, shape, spacing, and pattern of the roughness elements. The effect of flow depth on flow resistance f is often hidden in the flow Reynolds number Re. In laboratory and field studies alike, f-Re relations have been found to be convex-upward and negatively sloping, and these shapes have been explained in terms of the progressive inundation of the roughness elements. Where laboratory- or field-based models have been developed for predicting f, they invariably contain percent stone cover. The prominence of this variable reflects the strong influence of stone size and spacing on flow resistance. A laboratory study shows that where the Froude number F > 0.50, wave resistance increases with stone cover and dominates resistance to flow on all surfaces with stone covers greater than 10%. A field study indicates that where F < 0.50 and wave resistance is inconsequential, grain and form resistance typically account for about 5% and 95% of f, respectively. These findings have important implications for sediment transport modeling because percent grain resistance is equal to percent grain shear stress, and it has recently been suggested that in overland flow, as in river flow, sediment transport capacity is determined by grain shear stress rather than total shear stress. A laboratory study, however, demonstrates that this is not the case. Sediment transport capacity is in fact greater than predicted by grain shear stress because energy dissipated in the wakes of roughness elements in overland flow is transformed into turbulence sufficiently close to the bed to affect sediment transport.