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Estimation of Rill Sediment Transport under Different Subsurface Hydrologic Conditions
thesisposted on 29.04.2021, 00:25 by Shuyuan Wang
The prediction of sediment transport capacity plays a pivotal role in soil erosion modeling. The commonly used definition of sediment transport capacity (Tc) is the equilibrium sediment transport rate for a given surface hydraulic condition. The lack of consideration of subsurface hydrologic conditions in this definition can influence the prediction accuracy. The overall goal of this study was to improve the estimation of the sediment transport process under different subsurface hydrologic conditions.
In this study, 300 experiments were conducted in a 3.0 m long flume including 216 runs using relatively uniform sands and 84 runs with a cropland Opal clay soil. First, twelve widely used sediment transport capacity equations were evaluated using data from 529 experiments from previous literature under no drainage or saturation conditions. A new sediment transport capacity equation was created based on the summary of parameters in the twelve equations including Yalin equation, simplified Yalin equation, Engelund and Hansen equation, Yang equation, Ali equation, Govers equation, Abrahams equation, Griffith University Erosion System Template (GUEST) equation, Guy equation, Abraham and Gao equation, Beasley and Huggins equation, and Zhang equation. Next, 107 experimental runs using relatively uniform sands (D50 = 0.46 mm) were conducted to compare the equilibrium sediment transport rate under detachment-limited and transport-limited conditions for each subsurface hydrologic condition. Additionally, on the basis of identification of a unique sediment transport capacity value for a given surface and subsurface hydrologic condition, the previously generated sediment transport capacity equation was modified using results from 216 experimental runs on sands through the adjustment of water discharge under different subsurface hydrologic conditions. Finally, the impacts of subsurface hydrologic conditions were estimated with consideration of sediment size distribution changes. The spatial and dynamic characteristics of sediment selectivity and sediment transport were investigated through 84 rill channel experiments with Opal clay soil.
The results indicated that the sediment transport capacity equations evaluated gave good performance for particles within a certain range of sizes, but none of the twelve equations gave satisfactory results for the overall dataset. The newly-generated sediment transport capacity equation provided good performance for the relatively wider range of hydraulic conditions with different particles including cohesionless sands, cropland soils, and loess soil. Similar sediment transport capacities were obtained under detachment-limited and transport-limited conditions, which indicated that there was only one equilibrium sediment transport rate for a given surface and subsurface hydrologic condition. The previously generated sediment transport capacity equation was based on close to no drainage datasets, and the predictions could be improved by the adjustment of water discharge with the impacts of infiltration or exfiltration. The accuracy was improved with the discrepancy ratio between predictions and observations (P.O.0.5-2.0) increasing from 73.0 to 90.5% under free drainage conditions and reaching 100% under saturation and seepage conditions. When subsurface conditions changed from free drainage to saturation, the critical shear stress decreased about 20% for the cohesionless sands and about 30% for the Opal clay soil. Correspondingly, sediment transport capacity increased significantly from free drainage to saturation. When subsurface conditions changed from saturation to 10 cm seepage head, the critical shear stress decreased slightly, and the sediment transport capacity increased slightly. The determination of sediment transport capacity includes the reach of both spatial and dynamic equilibrium conditions. The impacts of subsurface hydrologic conditions on the sediment selectivity process decreased as the slope increased.
The observations in this study indicated the importance of considering subsurface hydrologic conditions for soil erosion estimation. A great increase in sediment can be expected from free drainage to saturation or seepage even when no increase in surface water discharge occurs. The newly-generated sediment transport capacity equation with adjustment of water discharge under different subsurface hydrologic conditions gave more reliable predictions of sediment transport capacity.