Study on Overland Flow Resistance Under Simulated Rainfall

The knowledge of the friction factor of flows moving over rough hillslopes is required for modelling soil erosion and transport processes. The available literature studies regarding the rainfall effect on overland flow resistance agree with the conclusion that the friction factor increases with rainfall intensity for the laminar flow regime. In this paper, a focus on the Darcy-Weisbach friction factor f, for which a theoretically based approach is available, was developed, with particular attention to the effects of rainfall intensity on a laminar overland flow. Several data available in the literature were used for calibrating and testing a theoretically based approach. The available dataset, which was restricted to the laminar flows (Reynolds number values less than 2000), was divided by distinguishing overland flows on a smooth bed from those in which the effects of roughness (fixed bed) and sediment transport (mobile bed) are considered. The results for the laminar flow regime on a smooth bed confirmed the increase of the friction factor with rainfall intensity. For the mobile rough bed condition, the highest friction factors were detected as overland flow resistance is due to both grain resistance and sediment transport. Finally, the main topics for future research are highlighted.