RESISTANCE TO OVERLAND-FLOW ON DESERT PAVEMENT AND ITS IMPLICATIONS FOR SEDIMENT TRANSPORT MODELING

Seventy-three overland flow experiments were performed on eight runoff plots located on a gentle piedmont hillslope in Walnut Gulch Experimental Watershed, southern Arizona. These plots were sited on gravel-covered desert pavement surfaces where overland flow occurs during natural rainstorms. Multiple regression analyses indicate that the Darcy-Weisbach friction factor ff can be well predicted by the Reynolds number and gravel concentration and size, with gravel concentration being by far the single best predictor. Grain resistance ff, calculated using Savat's algorithm, is found to be typically less than 10% of ff. Given that sediment transport capacity is controlled by grain shear stress tau' rather than total shear stress tau and that the ratio of tau' to tau is the same as that of ff' to ff, sediment transport modeling on the gravel-covered hillslope under study must be based on tau' rather than tau to avoid gross overestimation of transport capacity. This conclusion is relevant to all gravel-covered desert hillslopes and probably also to most humid hillslopes and agricultural lands where plant stems, litter, and tilled surfaces replace gravel as sources of form resistance and ensure that grain resistance remains relatively small.