The effect of gravel content on the velocity of hyperconcentrated rill flow under steep slope conditions

Abstract

Flow velocity serves as a key hydraulic parameter in examining rill erosion on slopes. Analysing the characteristics of flow velocity changes and their influencing factors is critical for understanding the rill erosion process and hydrodynamic mechanisms. However, the understanding of the mechanism of flow velocity changes in gravel-laden hyperconcentrated flows on steep slopes is still limited. In this study, different soil–gravel mixtures (with gravel mass contents ranging from 0% to 70% and gravel clast median diameters ranging from 0.08 to 2.95 mm) were used in indoor runoff scouring tests to examine the influence and mechanisms of gravel content on flow velocity under varying unit flow discharges (1.11–4.44 × 10⁻³ m² s⁻¹) and slopes (18–84%). These results indicate that a relatively high gravel content in the sediment significantly impedes the flow velocity. The structural equation model indicates that gravel content primarily reduces flow velocity by positively influencing settling velocity (path coefficient = 0.990, p < 0.001), which in turn exerts a suppressive effect on flow velocity (path coefficient between settling velocity and flow velocity = −0.295, p < 0.001). The contributions of flow discharge, slope and gravel content to flow velocity are 79.7%, 15.6% and 0.5%, respectively. The flow velocity prediction equation established on the basis of these three factors is not only highly accurate (NSE = 0.918) but also has easily obtainable parameters, making it the preferred equation for predicting the rill flow velocity. This study explored the mechanism of flow velocity changes in hyperconcentrated flows on steep slopes, enhancing the understanding of the relationship between sediment particles and the flow velocity, which aids in elucidating the erosion process in soil-gravel mixtures.