Slope threshold in rill flow resistance

The applicability of a theoretical rill flow resistance equation, based on the integration of a power velocity distribution, was tested using measurements carried out in mobile and fixed bed rills, shaped on plots having different slopes (9, 14, 15, 18, 22, 24, 25 and 26%) and soil textures (clay fractions ranging from 32.7% to 73% and silt of 19.9–30.9%), and flume measurements available in the literature. The Darcy–Weisbach friction factor resulted dependent on the slope, Froude number, Reynolds number and clay and silt percentages, used as variables representative of soil transportability and detachability, respectively. This theoretical approach was applied to two different databases distinguished by the slope threshold of 18%. The results showed that for both steep (≥18%) and gentle (<18%) slopes, the flow resistance increases with slope and is affected by the soil particle detachability and transportability. The analysis also demonstrated that the effect of sediment transport on flow resistance is different for gentle and steep slopes and the detected difference is due to the circumstance that for gentle slopes the flow, characterized by a low transport capacity, can transport less sediment than the flow moving on a steep slope