Response of roll wave to suspended load and hydraulics of overland flow on steep slope

Abstract

A roll wave is frequently observed in overland flow and it can accelerate the soil erosion on slopes. However, the feedback effect of eroded sediment on roll wave has not been studied. The aim of this study was to investigate the response of roll wave to sediment concentration and overland flow hydraulics on steep slope. The experiment was carried out in a hydraulic flume. The unit flow rate varied from 1.0 to 3.0×10−3m3s−1, and sediment concentration from 0 to 400kgm−3. The sediment transport was dominated by suspension. Slope gradient was 9°. As the sediment concentration increased, the critical slope length for roll wave formation increased, implying that the suspended sediment in flow could inhibit the formation of a roll wave. The roll wave in overland flow is a short water long wave. The roll wave length increased with the increasing sediment concentration, while the wave frequency and velocity decreased. The decreased wave velocity meant a decrease in flow erosion potential caused by a roll wave. Roll wave frequency and velocity significantly increased with Reynolds number, Froude number and mean flow velocity, and decreased with the hydraulic resistance, while there were no notable relationships between roll wave length and overland flow hydraulics. Both roll wave frequency and velocity had the strongest dependency on Froude number and could be estimated by the linear equations between them. When the sediment concentration was larger than 300kgm−3, all the roll waves in overland flow disappeared due to the high sediment supply. The results indicated that the suspended sediment can ease the acceleration influence of a roll wave on soil erosion and should be considered in the soil erosion models.