Slope effects on flow resistance and soil erosion in tortuous rills

Purpose Even though rill erosion is strongly influenced by flow hydraulics, slope, and channel tortuosity, the influence of path tortuosity on rill flow resistance and soil erosion has been investigated little. In this study, experiments on tortuous rills were performed on a plot with a mean slope s(p) of 15% filled with clay soil to test a literature flow resistance equation previously proposed for tortuous rills and investigate if slope affects the potential link between rill tortuosity and soil loss. Methods The four pre-shaped rills, characterized by designed tortuosity values t(0) = 1, 1.08, 1.16, and 1.3, were subjected to a constant inflow discharge (0.3 L s(-1)), and the hydraulic variables were measured using the dye-tracing technique and 3D digital terrain models obtained by terrestrial surveys performed before and after the runs. Soil erosion volumes were quantified through DEMs (Digital Elevation Model) of difference. This analysis was also carried out for the experimental runs of a previous investigation with the same experimental setup and s(p) = 11%. Results The results confirmed that the literature flow resistance equation can be satisfactorily applied to estimate flow resistance in tortuous rills, and that tortuosity effects can be effectively captured by the a coefficient of the power velocity distribution regardless of the plot mean slope. The obtained results also revealed that lower slope (11%) unexpectedly produced higher flow velocities (except for the maximum tortuosity level) and soil losses, which can be mostly explained by the differences in channel bed roughness generated during the shaping phase. Conclusion Overall, this study highlights that slope influences rill hydraulics and erosion for tortuous rills. These insights advance the understanding of rill dynamics and support the refinement of predictive models for soil conservation.