Impacts of sediment load and size on rill detachment under low flow discharges

Abstract

Rigorous validation of the first-order detachment-transport coupling concept using different sediment sizes is nonexistent in the literature. We hypothesized that particle sizes in a sediment-laden flow can affect soil detachment. The objectives of this study were to investigate the relationship between detachment rates by rill flows and hydraulic parameters under low flow discharges, and to quantify the responses of soil detachment rates to sediment loads and median sizes. A concentrated flow flume test was conducted under three low flow discharges (0.00033, 0.00067 and 0.00100 m2·s−1) and three sediment concentrations (50, 100 and 200 kg·m−3) with five median sizes (0.233, 0.204, 0.169, 0.115 and 0.078 mm). The results showed that the relationship between soil detachment capacity and shear stress fits the Water Erosion Prediction Project (WEPP) model well (R2 = 0.89). Stream power, compared with shear stress, is a better hydraulic parameter in simulating soil detachment rates under different sediment concentrations. The soil detachment rates by sediment-laden rill flows were negatively related to both sediment loads and sediment median sizes, validating the first-order sediment feedback relationships used in the WEPP model. The overall responses of soil detachment rates to sediment loads and sediment median sizes followed multivariate linear relationships, further demonstrating the negative feedbacks of sediment load and size on soil detachment by rill flow.