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

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

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2021
Publication Date: 9/9/2021
Citation: Wang, S., Flanagan, D.C., Engel, B., McIntosh, M.M. 2021. Impacts of subsurface hydrologic conditions on sediment selectivity and sediment transport in rills. Catena. 207. Article 105703. https://doi.org/10.1016/j.catena.2021.105703.
DOI: https://doi.org/10.1016/j.catena.2021.105703

Interpretive Summary: Erosion by flowing water detaches and transports soil particles away from their original locations in agricultural fields. Much sediment is moved by water flows in small concentrated channels, called rills. Most previous research has only used surface properties such as flow rate, slope gradient, and velocity to estimate the maximum amount of sediment that a given water flow can carry (value known as the sediment transport capacity). In this study, we examined the effects that subsurface hydrologic conditions can have on the amount of sediment that a flow can carry, as well as what can happen to the sizes of sediment moved. Hydrologic conditions used were free drainage, saturation, and seepage. We also varied the slope of the simulated rill channels and the water flow rates, and used an agricultural clay soil. Results showed that sediment transport capacity values increased greatly as subsurface hydrologic conditions changed from drainage to saturation and seepage, and differences widened as slope steepness increased. The subsurface hydrology also affected the sizes of particles leaving the rills. This research impacts scientists, engineers, faculty, students, and others involved in erosion science and prediction technology development. Findings from this study may help in development of improved equations and models to estimate sediment transport and soil losses.

Technical 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 from free drainage to 10 cm seepage head conditions with water discharge rates from 0.0003 to 0.0008 m**2/s 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. 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.