Effect of gravel content on the detachment of colluvial deposits in Benggang

Accurately calculating detachment capacity is the most fundamental issue when establishing a soil erosion process model. Colluvial deposits of Benggang are typical soil-gravel mixtures, whereas the understanding of the soil detachment of colluvial deposits is limited. This work investigated the effects of the gravel contents on the soil detachment capacity of colluvial deposits and its hydrodynamic mechanism. The colluvial sample was collected in Anxi County, Fujian Province, Southeast China, and a small-sample scouring test was used. The slope steepness ranged from 18% to 84%, unit discharge ranged from 0.56×10 −3 to 2.22×10 −3 m 2 s −1 , and gravel content ranged from 0% to 70%. The results indicated that the gravel content is the primary factor that influences the detachment capacity, followed by the discharge and then the slope. The detachment capacity trend with the gravel content varied over different slopes and discharges. Stream power represents the best hydrodynamic parameter for modelling the detachment capacity of colluvial deposits and can be used to establish a fitting equation for the colluvium together with the mean weight diameter (MWD) (Nash-Sutcliffe efficiency (NSE)=0.96). As the gravel content increased, the soil erodibility parameters increased several folds, in some cases more than 10 folds, mainly because the soil shear strength decreased gradually. Meanwhile, as the gravel content increased, the gravel specific surface area increased, the obstruction of gravel to runoff increased, and the energy needed for runoff to overcome gravel obstruction increased, leading to 2–3 folds higher critical shear stress of runoff for soil-gravel mixtures compared with pure soil. In summary, gravel can influence the detachment capacity by changing the soil properties, and the gravel content also affects the relationship between soil detachment capacity and the hydrodynamic parameters. These findings deepen the understanding of the influence of gravel on soil erosion and provide a basis for establishing a soil erosion process model in colluvial deposits.