Turbulence Intensities of Shallow Rain-Impacted Flow over Rough Bed

A thorough understanding of the effect of raindrop impacts on the characteristics of a shallow flow is very helpful to properly predict and control water erosion. The turbulence properties of a shallow rain-impacted flow over a rough bed were studied using a two-dimensional fiber-optic laser Doppler velocimeter and an oscillating type raindrop generator. It was found that the value of momentum correction factor BETA for a rough boundary was much smaller than that for a smooth boundary at a given value of Reynolds number because of the vertical mixing effect of the boundary roughness. For the condition without rainfall, the vertical distribution of the longitudinal turbulence intensity became more uniform as the boundary roughness increased. For the "fully rough" condition with rainfall and constant roughness size, the effect of roughness on the distributions of turbulence intensities was small. Similar to the condition with smooth boundary, the rainfall-generated turbulence was greatest near the water surface and decreased downward at an exponential rate for the flow over rough bed. Two types of statistical correlations were developed to predict the distributions of both the longitudinal and the vertical turbulence intensities depending on whether the flow is completely penetrated by the raindrops.