Prediction of rill sediment transport capacity under different subsurface hydrologic conditions

Authors: WANG, SHUYUAN - Purdue University; Flanagan, Dennis; ENGEL, BERNARD - Purdue University; ZHOU, NA - Heibei University Of Engineering

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2021
Publication Date: 5/4/2021
Citation: Wang, S., Flanagan, D.C., Engel, B., Zhou, N. 2021. Prediction of rill sediment transport capacity under different subsurface hydrologic conditions. Journal of Hydrology. 598. Article 126410. https://doi.org/10.1016/j.jhydrol.2021.126410.
DOI: https://doi.org/10.1016/j.jhydrol.2021.126410

Interpretive Summary: Soil loss by water flow detachment and transport is a major problem throughout the world. In equations and computer simulation models that estimate soil erosion, there is often a value known as the sediment transport capacity that is the maximum amount of soil particles that a given flow of water can carry, and this acts to put an upward limit on the soil loss that can occur from a given rainfall and runoff event. Up till now, ways to estimate this sediment transport capacity have usually relied only upon surface hydrologic parameters such as flow rate, flow velocity, and slope gradient. However, in this research we studied how subsurface hydrology factors such as water infiltration into a draining soil or water exfiltration from a seeping saturated soil might affect the transport capacity. We found that these subsurface hydrology conditions can affect the amount of sand sediment a flow can carry, and in particular when soils are in a saturated seepage state the transport capacity will be greater than when they are in a drainage condition. We also improved a previously developed prediction equation with data from these experiments. This research impacts scientists, university faculty, students and others involved in soil erosion by water studies and development of prediction technologies to estimate soil loss.

Technical Abstract: When estimating rill sediment transport capacity, 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 exfiltration force under seepage conditions and the downward infiltration force under drainage conditions provide opposite effects on sediment detachment, transport, and deposition. 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 length 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 their differences increased for greater water discharge rate and slope gradient. The small increases of 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.O.0.5-2.0) increasing from 73.0 to 90.5% under the free drainage condition and reaching 100% under the saturation and seepage conditions.