Influence of Soil Grain Roughness and Roughness Type on Flow Resistance in Rectangular Rill Channels

The knowledge of rill flow resistance is required to improve the modelling of upland erosion processes in different hydraulic conditions. Using controlled laboratory conditions, experimental runs in flumes are useful to investigate a specific effect, such as the boundary roughness, on flow resistance. In this paper, the effects of the soil roughness and roughness types on flow resistance are investigated using laboratory measurements. Flow resistance measurements, which were performed for rectangular fixed-bed flumes with different bed and wall arrangements (smooth, covered by a clay-loam, a sandy-loam, a clay soil), were jointly used with literature measurements obtained for rectangular flumes covered by sand or grass. In agreement with previous studies, the analysis showed that, for open-channel rill flows on a fixed bed, the influence of different soil grain roughness on the Darcy–Weisbach friction factor is restrained (≈ ±1%). On the other hand, the results highlighted that the effect of a different roughness type (i.e., sand or grass) on flow resistance is appreciable. Ultimately, laboratory measurements of flow resistance in a rectangular cross-section flume are characterised by limited influence of soil grain roughness, and characteristic values of the roughness parameters in the flow resistance equation can be estimated for a non-specific ‘soil roughness’ condition.