Effects of Raindrop Impact on the Resistance Characteristics of Sheet Flow

Abstract Raindrops impact (RDI) can disturb the flow structure of sheet runoff and thus change the flow resistance. In this study, multiple experimentally simulated rainfalls were applied on a flat and smooth flume with variable slopes. To identify the RDI effect, one half of the data was collected with the flume covered by a gauze screen near runoff surface. Comparisons of results indicated that RDI increased the flow resistance, reduced flow velocity and increased flow depth. Darcy-Weisbach resistance and Manning coefficient decreased in the slope range 2-12%, and kept almost constant in higher slope range. Corresponding approximately to 12% slope, Reynolds number, Re = 800 appears to be a critical value separating the laminar and turbulent flow regime under rainfall conditions. Empirical equations relating Darcy-Weisbach resistance coefficient and Manning coefficient to Reynolds number and rainfall intensity were obtained. In addition, empirical equations of the incremental resistance, due to RDI were established for laminar flows. This study revealed some aspects of RDI in the rainfall-runoff hydrologic processes.