Subsurface hydrology effects on rill sediment transport capacity

Authors: WANG, SHUYUAN - Purdue University; Flanagan, Dennis; ENGEL, BERNARD - Purdue University

Submitted to: ASABE Annual International Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 3/20/2020
Publication Date: 7/12/2020
Citation: Wang, S., Flanagan, D.C., Engel, B. 2020. Subsurface hydrology effects on rill sediment transport capacity [abstract]. American Society of Agricultural and Biological Engineers. Paper No. 2000725.

Interpretive Summary:

Technical Abstract: Sediment transport capacity is a key factor in the sediment detachment-transport coupling concept which is widely used in soil erosion 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). In this concept, the detachment rate can be calculated based on the deficit of transport capacity, and net deposition occurs when sediment load exceeds transport capacity. Therefore, reliable prediction of transport capacity is essential for the prediction of sediment detachment, transport and deposition in soil erosion modeling. The definition of transport capacity is under steady-state condition. Given the difficulties in controlling hydrologic parameters close to steady-state in the field, most previous studies measured transport capacity in the laboratory on either nonerodible beds or erodible beds without drainage. However, infiltration under drainage conditions and exfiltration under seepage condition cannot be ignored in the real world, and their impacts on soil strength and water discharge may affect transport capacity, even though inflow hydrologic parameters do not change. Moreover, different methods have been used to obtain transport capacity in experimental setups including measurements under detachment-limited conditions and transport-limited conditions. However, limited studies exist to compare measurements using these two methods, which may challenge the uniqueness of sediment transport capacity for a given surface and subsurface hydrologic condition. A better understanding of transport capacity is necessary with the consideration of subsurface hydrologic impacts and the comparison of measurements under detachment-limited and transport-limited conditions. A series of experiments were carried out with a relatively uniform sand using three water discharges on a flume with four rills under four subsurface hydrologic conditions varying from free drainage to 10 cm seepage head. The flume contains four rills with slope lengths of 0.5, 1.0, 2.0 and 3.0 m, and the determination of equilibrium sediment transport capacity was based on the spatial change of sediment transport and the elevation change of the erodible surface. By introducing sediment at the top of the flume with different rates, transport capacity was measured under both detachment-limited and transport-limited conditions. Our results from 107 runs 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. For the studied sands, observed critical shear stress decreased around 20% from drainage to saturation condition, and decreased slightly from saturation to seepage condition. Measured transport capacity increased from drainage to seepage conditions, which was the result of decreased soil strength and increased water discharge. The differences of transport capacity between drainage and saturation increased as water discharge increased, but the differences between saturation and seepage were relatively stable for all studied water discharges. This study indicates the uniqueness of transport capacity for a given surface and subsurface hydrologic condition and improves the estimation of transport capacity which is based on surface hydrologic condition by adding consideration of subsurface hydrologic impacts.