Estimating flow resistance in steep slope rills

Recent research recognized that the slope of 18% can be used to distinguish between the ‘gentle slope’ case and that of ‘steep slope’ for the detected differences in hydraulic variables (flow depth, velocity, Reynolds number, Froude number) and those representatives of sediment transport (flow transport capacity, actual sediment load). In this paper, using previous measurements carried out in mobile bed rills and flume experiments characterized by steep slopes (i.e., slope greater than or equal to 18%), a theoretical rill flow resistance equation to estimate the Darcy‐Weisbach friction factor is tested. The main aim is to deduce a relationship between the velocity profile parameter Γ, the channel slope, the Reynolds number, the Froude number and the textural classes using a data base characterized by a wide range of hydraulic conditions, plot or flume slope (18%–84%) and textural classes (clay ranging from 3% to 71%). The obtained relationship is also tested using 47 experimental runs carried out in the present investigation with mobile bed rills incised in a 18%—sloping plot with a clay loam soil and literature data. The analysis demonstrated that: (1) the soil texture affects the estimate of the Γ parameter and the theoretical flow resistance law (Equation 25), (2) the proposed Equation (25) fits well the independent measurements of the testing data base, (3) the estimate of the Darcy‐Weisbach friction factor is affected by the soil particle detachability and transportability and (4) the Darcy‐Weisbach friction factor is linearly related to the rill slope. According to previous investigations, the hydraulic mechanisms of rill erosion for steep slopes are different from those for gentle slopes. For steep slopes the high sediment transport capacity is not a limiting factor for the actual sediment transport, which depends only by the detachability and transportability of the soil particles. A relationship to estimate the Darcy‐Weisbach friction factor f on steep slope rills, accounting for soil particle detachability and transportability, is positively tested.