Slope Effect of Soil Detachment Capacity on Steep Colluvial Deposits in Southern China

Abstract

Soil erosion is a major environmental issue affecting agricultural productivity and land conservation, and soil detachment capacity is a key parameter governing the erosion process. However, the mechanisms of soil detachment differ significantly across slope gradients, and the underlying causes remain to be further explored. In this study, scour experiments were conducted using small soil samples (0.1 m in diameter, 0.05 m in height) under eight slope gradients (9%, 18%, 27%, 36%, 47%, 58%, 70%, and 84%) and five unit discharges (0.56 × 10⁻³, 1.11 × 10⁻³, 1.67 × 10⁻³, 2.22 × 10⁻³, and 3.33 × 10⁻³ m² s⁻¹) to simulate rill flow detachment processes on colluvial deposits. The objectives were to systematically elucidate the influence of slope gradient on soil detachment capacity and to quantify the difference in slope contribution rates between gentle to moderate slopes (9%–36%) and steep slopes (47%–84%). The results show that soil detachment capacity increases with the slope gradient according to a power function, but the rate of increase in detachment capacity decreases as the slope steepens. Additionally, the contribution of slope gradient to soil detachment capacity is lower for steep slopes (47%–84%) (27.2%) than for gentle to moderate slopes (9%–36%) (43.2%). Furthermore, the representation of soil detachment capacity using basic indicators (flow velocity), mechanical indicators (flow shear stress), and energy indicators (stream power) exhibits slope-dependent effects. The threshold slope for rill erodibility parameters (k) and critical hydraulic variables (v_c, τ_c, ω_c) derived from linear fitting was approximately 47%. Similarly, the threshold slope for the coefficients (a) derived from the power-law fitting between soil detachment capacity and hydrodynamic variables was also 47%, whereas the exponents (b) showed comparatively limited changes, with the stream power exponent (b₃) remaining relatively stable across the tested slope range. Therefore, the slope-dependent effects on soil detachment capacity suggest that results from gentle to moderate slopes cannot be directly applied to steep slopes. The slope-dependent effects on soil detachment capacity confirm that results from gentle to moderate slopes cannot be directly applied to steep slopes. This study quantitatively reveals the variation rule that the contribution rate of slope to soil detachment decreases from 43.2% to 27.2%, clarifies the internal physical mechanism of the 47% slope as a hydrodynamic transition zone, and improves the theoretical understanding of soil detachment processes in steep colluvial deposits. It should be noted that this study is based on colluvial deposits derived from granite weathering crust in southern China (30% gravel, 2% clay), and the results are primarily applicable to steep slopes with similar coarse-grained, low-cohesion soils. Extrapolation to other soil types (e.g., loess, purple soil, cinnamon soil) or colluvial deposits at different erosion stages should be undertaken with caution and site-specific validation.