HYDRAULICS OF INTERRILL OVERLAND-FLOW ON STONE-COVERED DESERT SURFACES

The hydraulics of interrill overland flow on stone-covered desert surf aces depend on the resistance to flow, which may be partitioned into g rain 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 R eynolds number, Froude number, flow depth, and the size, shape, spacin g, 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 l aboratory and field studies alike, f-Re relations have been found to b e convex-upward and negatively sloping, and these shapes have been exp lained in terms of the progressive inundation of the roughness element s. Where laboratory- or field-based models have been developed for pre dicting f, they invariably contain percent stone cover. The prominence of this variable reflects the strong influence of stone size and spac ing on flow resistance. A laboratory study shows that where the Froude number F > 0.50, wave resistance increases with stone cover and domin ates 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 ab out 5% and 95% of f, respectively. These findings have important impli cations for sediment transport modeling because percent grain resistan ce is equal to percent grain shear stress, and it has recently been su ggested that in overland flow, as in river flow, sediment transport ca pacity is determined by grain shear stress rather than total shear str ess. A laboratory study, however, demonstrates that this is not the ca se. Sediment transport capacity is in fact greater than predicted by g rain 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.