Effect of gravel coverage on the hydrodynamic characteristics of overland flow on the Loess Plateau in China

In the Loess Plateau of China, the presence of gravel mulching is widespread, and investigating the dynamic changes in hydrodynamic properties induced by gravel coverage is crucial for optimizing soil and water conservation in this region. In this study, the effect of gravel coverage on the hydrodynamic parameters characterizing the overland flow was investigated through indoor artificially simulated scouring experiments, considering ten different gravel coverages, four slope gradients, and nine flow discharges. The results demonstrated significant differences in hydrodynamic parameters under various experimental conditions (P < 0.01). The flow depth exhibited a linear increase with increasing gravel coverage. Compared to flow velocity observed on non-gravel-covered slope, the reduction percentage of flow velocity ranged from 13.62 % to 72.4 % on gravel-covered slopes. Furthermore, a prediction model was developed to quantify the impact of gravel coverage on the mean flow velocity of overland flow. The model achieved a high R2 of 0.858 and a low RME of 13.61 %. The experiments revealed that the overland flow was distributed in the “virtual laminar”-subcritical and transitional-supercritical zones, with mutual restrictions between the influence of gravel coverage and slope gradient on the flow regime. Finally, it was observed that the Darcy–Weisbach resistance coefficient increased with an increase in gravel coverage. These findings provide a solid theoretical basis for optimizing soil erosion prediction models based on the hydrodynamic characteristics of overland flow, and offer guidance for the rational allocation of soil and water conservation measures for gravel-covered slopes.