Overland flow velocities measured using a high-resolution particle image velocimetry system

Abstract

Accurate measurements of the overland flow velocity along the flow depth are critical to understanding hydraulic and soil erosion processes on hillslopes; however, this topic has not been investigated in detail to date. This study used a high-resolution particle image velocimetry method to capture the flow fields of overland flow on a fixed smooth bed. Experiments were conducted under eight shallow flow conditions at flow depths ranging from 0.49 to 1.1 × 10−2 m. A fully developed open-channel flow condition was the control group. The Reynolds stress values indicated the presence of two regions, namely the inner and outer regions. The inner region was affected by viscous stress and Reynolds stress, whereas the outer region was affected only by Reynolds stress. The boundary between the two regions was variable, whereas the open-channel flow had a fixed boundary. Three subregions were observed in the inner region, and two subregions were detected in the outer region. The free surface was primarily affected by surface tension, which weakened as the flow depth H and the Weber number We increased. At We > 6, the surface tension effect was buffered, and no free surface region was observed in the velocity profile. Our results indicated that the division of the velocity region for overland flow differed from that of the open-channel flow, and the expressions of the subregions were also different in terms of the empirical constant A and integration constant C. Overland flow occurred when the dimensionless flow depth (H+=Hu∗/ν) was less than 550. This study provides an approach to improve the measurement accuracy and theoretical understanding of overland flow processes and hydrodynamic mechanisms on hillslopes.