Influence of Gravel Coverage on Hydraulic Characteristics and Sediment Transport Capacity of Runoff on Steep Slopes

Gravel coverage on slopes influences overland flow and soil erosion. However, the effect of different gravel sizes on the soil erosion process remains underexplored. In this study, a runoff scour test was performed to examine the effects of gravel coverage on the hydrodynamic characteristics of slope runoff and sediment transport capacity (Tc). The slope gradient varied from 18% to 84%, the unit flow discharge ranged from 0.27 × 10−3 to 1.11 × 10−3 m2 s−1, and gravel coverage was adjusted from 0% to 90%. The results reveal that water depth, shear stress, and stream power increased with gravel coverage. However, once coverage exceeded 20%, flow velocity and unit stream power decreased and stabilized. As gravel coverage increased, the hydraulic regimes transitioned from laminar to turbulent flow and shifted from supercritical to subcritical. Consequently, Tc first increased and then decreased with the increase in gravel coverage, reaching a peak at 20% coverage (1.66 kg m−1 s−1). Moreover, the degree of coverage indirectly influenced Tc through grain shear stress. The new equations, based on the Box–Lucas function, incorporated slope, grain shear stress, and flow velocity, thereby effectively simulating Tc for runoff on gravel-covered slopes (R2 = 0.94, NSE = 0.94). These findings provide a basis for modeling soil erosion on gravel-covered slopes.