Vertical Hydraulic Gradient and Run-On Water and Sediment Effects on Erosion Processes and Sediment Regimes

During rainfall events, runoff water carries sediment downslope. It has been hypothesized that the detachment potential of the runoff water would decrease as the sediment content is increased until a sediment transport capacity, Tc, is reached and the excessive amount of sediment beyond Tc would then be deposited. Translating this concept to a hill slope setting implies that water and sediment from up-slope areas would affect erosion processes on a downslope segment. However, there are few experimental data to support this hypothesis or the run-on water and sediment transport capacity effect. A dual-box system, consisting of a 1.8-m-long sediment feeder box and a 5-m-long test box, was used to quantify run-on water and sediment effects. The test box has holes at the bottom for controlling the vertical hydraulic gradient. Experimental variables were rainfall intensity, slope steepness, near-surface hydraulic gradient, and run-on sediment concentration. Results demonstrated that under free soil drainage conditions, a decrease in run-on sediment content caused a corresponding increase in downslope sediment detachment, resulting in a relatively constant sediment delivery from the test box. Under artesian seepage conditions, sediment deliveries were three to six times greater than those under drainage conditions and decreased as the erosion event progressed, possibly because of a reduction in soil erodibility. Sediment detachment by the run-on water increased as either slope or rainfall intensity was increased or when the surface was changed from a drainage to a seepage condition. These results demonstrated that the sediment delivery reached a dynamic equilibrium and the dual-box system can be used to study erosion processes similar to those occurring on a hill slope. A process-based erosion model needs to account for surface hydraulic conditions and the associated erosion processes and their interactions.