Subsurface Hydrologic Impacts on Rill Sediment Transport Capacity

Abstract

The sediment detachment-transport coupling concept is widely used in soil erosion prediction models including WEPP (Water Erosion Prediction Project), EUROSEM (European Soil Erosion Model), LISEM (Limburg Soil Erosion Model) and KINEROS2 (Kinematic Wave Overland Flow, Channel, Routing and Erosion Model). Sediment transport capacity is one of the key values in this concept, which impacts the prediction of sediment detachment, transport, and deposition. Therefore, it is essential to obtain more reliable transport capacity predictions for better soil erosion modeling based on the sediment detachment-transport coupling concept. Sediment transport capacity is the equilibrium sediment transport rate under steady-state conditions. When measuring transport capacities in the lab, most previous studies considered surface hydrologic impacts alone. However, the impacts of subsurface hydrology on sediment transport cannot be ignored in the real world. The downward infiltration under drainage conditions and the upward exfiltration under seepage conditions introduce opposite forces on sediment particles, and influence soil strength and water discharge which may affect transport capacity. A series of experiments were carried out with a relatively uniform sand and an Opal clay soil using four water discharges at three slope gradients. The flume contained four rills with slope lengths of 0.5, 1.0, 2.0 and 3.0 m (Figure 1). Four different subsurface hydrologic conditions were studied including free drainage, saturation, 5 cm seepage head and 10 cm seepage head. By introducing sediment at the top of the flume at different rates, sediment transport capacities were measured under both detachment-limited and transport-limited conditions. The determination of transport capacity was based on the spatial changes of sediment transport as slope length increased and the elevation changes of the erodible bed surface. The results indicated that there was only one equilibrium sediment transport value for a given surface and subsurface hydrologic condition, given similar observations were obtained under detachment-limited and transport-limited conditions. The critical shear stress decreased 20% from the drainage to the saturation condition, and decreased slightly from saturation to the seepage condition. Measured transport capacities increased from drainage to saturation conditions (Figure 2), and increased slightly from saturation to seepage conditions, which was the result of decreased soil strength and increased water discharge. The impacts of subsurface hydrology on transport capacity increased as water discharge and slope steepness increased. Dramatic increases of transport capacities were obtained for the greatest water discharge and steepest slope in this study when subsurface hydrologic condition changed from free drainage to saturation. The differences in transport capacities between saturation and seepage conditions were relatively stable for all considered water discharges and slopes. This study improves the estimation of transport capacity by introducing the subsurface hydrologic impacts, and the sediment transport capacity predictions were examined with various water discharges, slopes, and subsurface hydrologic conditions.