Impacts of subsurface hydrologic conditions on sediment selectivity and sediment transport in rills

Abstract

Sediment size distribution and sediment load are important factors for the estimation of sediment transport processes. Subsurface hydrologic conditions including drainage, saturation, and seepage can affect the sediment selectivity process and sediment transport, given that downward infiltration under drainage and upward exfiltration under seepage can add one more force on surface particles compared to saturation conditions. The aim of this study was to investigate characteristics of sediment selectivity and transport rates under different subsurface hydrologic conditions in small laboratory flume channels. Series of experiments were conducted with an Opal clay soil under four subsurface hydrologic states ranging from free drainage to 10 cm seepage head conditions with water discharge rates from 0.0003 to 0.0008 m2 s−1 at three slope gradients from 4.56% to 12.17%. Results showed that the fraction of coarse-sized particles increased from saturation to 5 cm and 10 cm seepage head conditions. Compared to saturation conditions, the greater proportion of coarse particles leaving in flow in the free drainage experiments may have been caused by infiltration enhancing the deposition processes and regenerating a coarser surface when reaching the dynamic equilibrium. As slope gradients increased, the subsurface hydrologic impacts decreased on the sediment selectivity process. Both the sediment size distribution and the sediment transport rate changed dynamically and spatially before reaching an equilibrium condition. The determination of sediment transport capacity should consider both dynamic and spatial equilibrium conditions. For the Opal soil, the sediment transport capacity values greatly increased from free drainage to saturation conditions, and the differences between these two subsurface hydrologic conditions increased for greater water discharge rates and slope gradients. From saturation to 5 cm and 10 cm seepage head conditions, the sediment transport capacity values increased slightly, and the differences were stable for the slope gradients and water discharge rates studied.