Overland flow resistance: A review

Shallow water flows over rough natural hillslopes contribute to interrill erosion and floods. The friction factor, that describes the hydraulic resistance, is particularly important for modeling soil erosion and transport processes. The present review focuses on the Darcy-Weisbach friction factor f for both large and small-scale roughness conditions and addresses the effects of rainfall intensity, vegetation cover, and sediment transport on overland flow resistance. All the studies on rainfall effect agree regarding the increase of the friction factor with rainfall intensity for the laminar flow regime and their independence for flows characterized by Reynolds number higher than a threshold varying between 800 and 2000. The analysis of the literature allows for concluding that f always increases with vegetation cover. Moreover, f (or its component due to sediment transport) increases with sediment concentration, slope, and Reynolds number, while it decreases with increasing values of Froude number and dimensionless sediment diameter. Finally, the focus areas for future research are highlighted.