Hydrodynamic characteristics of runoff and sediment yield on slopes in typical mountainous areas under incremental and continuous rainfall conditions

The complex underlying surface conditions in mountainous areas lead to the complexity of the hydrological response process in small mountainous watershed. As the basic unit of hydrological response in small mountainous areas, it is of great significance to study the hydrodynamic characteristics of runoff under different underlying surface conditions on mountainous slope. In this paper, combined with the underlying surface characteristics of the slope in a typical mountain watershed, Guanshan River watershed, multiple simulated rainfall experiments were conducted in the laboratory, the runoff and erosion hydrodynamic characteristics of the soil slope under different slope gradients, soil thickness distribution and soil base permeability were studied. The results showed that the Reynolds number, shear stress and runoff power increased with the increase of rainfall intensity, while the Darcy-Weisbach resistance coefficient decreased, conversely. The increase of rain intensity initiates significantly increase of the mean values of Reynolds number and runoff power (P <0.05), the mean values of Reynolds number and runoff power at rain intensity of 60mm h-1, 90mm h-1 and 120mm h-1 increased by 130%, 276%, 366% and 171%, 328% , 435% , compared with the mean values of 30mm h-1; respectively. Slope gradients has significant positive effects on mean shear stress and mean runoff power(P <0.05), the mean shear stress for the 15 and 25 slope gradients treatments increased 135% and 187% compared with the 5 slope gradients treatment; mean runoff power increased 224% and 357%, respectively. Soil layer thickness distribution has significant effects on mean Darcy-Weisbach resistance coefficient, mean Reynolds number and mean runoff power under permeable soil base. The soil base permeability has a significant effect on the mean Reynolds number and the mean shear stress when the thickness of the soil layer is thin at the top. The hydrodynamic characteristics were mainly affected by rainfall intensity. The variance contributions to the Reynolds number reached 80.5 %. The dominant factors affecting shear stress and runoff power were rainfall intensity and slope gradients, the sum of both variance contributions to shear stress reached 67.64%, to runoff power reached 80.58%, respectively. Under a single rainfall intensity condition, slope and soil thickness are the main factors affecting the hydrodynamic parameters, followed by the interactions between soil thickness and slope gradients, the bottom permeability and soil thickness also have a certain influence on the hydrodynamic parameters. Therefore, the influence of soil bottom permeability, soil thickness and their interaction on hydrodynamic characteristics should be considered, as well as the rainfall intensity and slope gradients. The results can provide some theoretical support for the mechanism of runoff and sediment yields on complex slope surface, and also some parameter basis for improving the distributed simulation of hydrological process in small mountainous watershed.