Combined tracing of salt and heat to characterizevelocity profile of sheet flow

The velocity of sheet flow on slopes is one of the key hydrodynamic parameters, and the velocity profile of sheet flow is important for understanding the soil erosion mechanism. The measurements of the slope sheet flow velocity were conducted under various underlying surfaces roughness through the tracing methods with the salt tracer and heat tracer, respectively, to investigate the influence of the underlying surface on the vertical velocity distribution of the sheet flow. The velocities of sheet flow were measured at three slope lengths (2, 3, and 4 m) under three slope gradients (5°, 10°, and 20°) and three flow rates (2, 5, and 8 L/min) on the slopes with various underlying surface roughness of polymethyl methacrylate, 80-mesh sandpaper (0.16 mm), and 24-mesh sandpaper (0.53 mm), respectively. The measurements were conducted in the water flumes with fixed bed, and the internal size of the flume was 5 m long, 0.2 m wide, and 0.2 m deep. The centroid velocity was adopted as the sheet flow velocity, and hydraulic parameters such as water depth, Reynolds number, Froude number, and Darcy-Weisbach resistance coefficient were also calculated to provide the background information of the experimental water flows. The results showed that the roughness of the underlying surface and the water depth both have significant effects on the velocity profile of the sheet flow (P<0.05). On the smooth underlying surface of polymethyl methacrylate, the boundary layer was fully developed to achieve a turbulent flow. The combined tracing method by using the salt and thermal tracers can be used to characterize the velocity profile of sheet flows when the water depth was 2-4 times of the bed roughness height and the flow was laminar; however, when the water depth were within or well beyond the roughness height, or when the flow was turbulent under the high velocity conditions, the combined tracing method failed to characterize the vertical velocity distribution of the sheet flow. A significant linear correlation was observed between the flow velocities measured by the two tracers, respectively, under the three experimental underlying surfaces (polymethyl methacrylate, 80-mesh sandpaper, and 24-mesh sandpaper), with the slopes of fitted lines of 1.015, 1.094, and 1.078, and the coefficients of determination R² of 0.892, 0.824, and 0.760, respectively. As the roughness of underlying surface increased, the velocity difference between the two tracers increased as the results of the degree of turbulence of the sheet flow increased, which in turn affected deference in the water flow velocity measured by salt and thermal tracers, respectively. The results can provide a reference for further understanding of the dynamic processes of sheet flow on the slopes.