Prediction Of Rill Sediment Transport Capacity Under Different Subsurface Hydrologic Conditions

When estimating rill sediment transport capacity for overland flow conditions, almost all widely used models focus on the impacts of surface hydraulic conditions and particle characteristics. However, the subsurface hydrologic impacts on the soil erosion process cannot be ignored. The upward force by exfiltration and the downward force by infiltration give opposite direction forces on the particles, which further influence sediment transport under seepage and drainage conditions. The aim of this research was to investigate the quantitative impacts of subsurface hydrologic conditions on sediment transport capacity in rills and improve the estimation of one existing transport capacity model by considering subsurface hydrologic effects. In this study, 216 rill flow experiments were completed in a 3.0 m long flume which contained four rill channels with slope lengths of 0.5, 1.0, 2.0, and 3.0 m. The experiments were conducted on close to uniform sands with four water discharge rates on 4.56%, 8.75%, and 12.17% slopes under four subsurface hydrologic conditions. The sediment transport rates from 192 of the 216 runs were estimated to have reached the sediment transport capacities for the corresponding condition based on the changes of sediment transport rates along the slope length and the surface elevation changes of the bottom region in the rills. The results indicated that rill sediment transport capacity increased significantly from free drainage to saturation conditions, and these differences increased with greater water discharge rates and greater slope gradients. Small increases in rill sediment transport capacity were observed from saturation to the 10 cm seepage condition with relatively stable differences. The selected sediment transport capacity equation provided good predictions under saturation and seepage conditions. The adjustment of water discharge rates improved the sediment transport capacity predictions with the discrepancy ratio between predictions and observations (P.O0.5-2.0) increasing from 73.0 to 90.5% under the free drainage condition and reaching 100% under the saturation and seepage conditions.