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Estimation of Rill Sediment Transport under Different Subsurface Hydrologic Conditions
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.
History
Degree Type
- Doctor of Philosophy
Department
- Agricultural and Biological Engineering
Campus location
- West Lafayette