A theoretically-based overland flow resistance law for upland grassland habitats

Abstract

Sediments conveyed from interrill areas to rills are transported by a thin flow (overland flow) and the knowledge of overland flow resistance is useful to evaluate overland flow velocity and sediment transport capacity.The aim of this paper was to verify the applicability of a theoretical overland flow resistance law, based on a power-velocity distribution, using field measurements for four upland grassland types used in different management strategies (Hay Meadows, Low-density Grazing, Rushes and Rank Grassland). Particular attention was deserved to the effects of vegetation growth cycles on the surface roughness and the corresponding reduction of overland flow velocity.The relationship between the parameter Γ of the power velocity profile, the flow Froude number and the Reynolds number, obtained by dimensional analysis, was calibrated using the available data. A specific calibration for each upland grassland type and taking into account the temporal phases of the vegetation growth was also carried out.The theoretical approach, checked by the available field measurements, allowed to establish that a) the proposed theoretical flow resistance law, which considers also the effect of the flow Reynolds number, allows an accurate estimate of the Darcy–Weisbach friction factor, b) the theoretical overland flow resistance law considers the temporal variability of vegetation characteristics, c) this flow resistance law can be specifically calibrated for taking into account for each upland grassland type the effect of the vegetation temporal variability and d) a low effect of the seasonal variation of roughness in grassland environments is detected.The interrill soil erosion processes are strongly related to hillslope hydraulics and the proposed overland flow resistance law is useful to model soil erosion processes.