Impacts of subsurface hydrologic conditions on rill sediment transport capacity

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

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/24/2020
Publication Date: 9/29/2020
Citation: Wang, S., Flanagan, D.C., Engel, B.A., Zhou, N. 2020. Impacts of subsurface hydrologic conditions on rill sediment transport capacity. Journal of Hydrology. 591. Article 125582. https://doi.org/10.1016/j.jhydrol.2020.125582.
DOI: https://doi.org/10.1016/j.jhydrol.2020.125582

Interpretive Summary: Soil erosion by water is a serious problem that affects soil resources around the world. Loss of soil can negatively affect soil productivity, crop growth, and off-site water bodies. Detached soil is also called sediment, and it can be carried by water flows away from where it was initially detached from the soil surface, and may deposit somewhere lower downslope. In this experiment we studied how the water conditions within the soil affected the amount of sediment that surface flow was able to carry. The soil conditions included free drainage (where water could freely exit the bottom of the soil layers), saturation (where the water pores were filled with water to the soil surface), and seepage (where the water table was above the soil surface, and water was slowly seeping from the soil onto the soil surface). For the sand that we used here, we found that under the saturation and seepage conditions, the surface flow was able to carry more sediment compared to the drainage condition. This was because the saturation/seepage conditions decreased the soil strength and there was a slight increase in flow from the seeping water. These results impact scientists, students, and others involved in soil erosion mechanics and prediction research, to better understand how subsurface hydrology can affect the transport of sediment. They may also ultimately help to improve modeling of soil erosion processes, to better account for soil subsurface hydrologic conditions. More research is underway to examine these condition effects on other soils.

Technical Abstract: Sediment transport capacity is the equilibrium sediment transport value under steady-state flow and channel conditions. 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 non-erodible beds or erodible beds without drainage. However, infiltration under drainage conditions and exfiltration under seepage conditions cannot be ignored in the real world, and their impacts on soil strength and water discharge may affect transport capacity. The objective of this study was to estimate the effects of subsurface hydrologic conditions on rill sediment transport capacity, and the observations are discussed under both detachment-limited and transport-limited conditions. A series of experiments were carried out with a relatively uniform sand using three water discharges on a 4.56% slope flume with four rills from 0.5 to 3.0 m long under four subsurface hydrologic conditions varying from free drainage to 10 cm seepage head. The determination of equilibrium sediment transport capacity was based on the spatial change of sediment transport as slope lengths increased and the elevation change of the erodible surface in rills. 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 sand, observed critical shear stress decreased around 20% from the drainage to the saturation condition, and decreased slightly from the saturation to the seepage condition. Measured transport capacity increased 13% to 15% from drainage to seepage conditions for the studied water discharges, which was the result of decreased soil strength and increased water discharge. The differences in transport capacities between drainage and saturation increased as water discharge increased, but the differences between saturation and seepage were relatively stable for all studied water discharges.