Hydraulic Conditions of Incipient Rill by Raindrop-Induced Overland Flow on Steep Slopes of Sandy Soil

Abstract

Hydraulic characteristics of rill incipience on steep slopes with sandy soils are different from those of gentle slopes in the mountainous watersheds. In other to better understand the processes of the incision and expansion of rill, rainfall simulations were conducted in a laboratory soil box with an inflow device installed at the top of the steep slope. The location and shape of the rill incision were tested with respect to different slopes (20, 24, and 28°), rainfall intensity (80 and 100 mm/h), and segment distances with various inflow rates (0.0, 8.8, 17.5, and 26.3 ml/s). The result showed that rills with heatcut were mostly incised on the 2.4 m segments and progressively moved upslope on the 3.2 m segments. The steps of rill evolution with increase of inflow are identified as four stages; (1) sheet flow with impact of raindrops, (2) overland flow with flowpaths, (3) microrill with headcut incision, and (4) rill enhanced by headcut expansion. It appears that incision and headcut of rill is prevalent when the Froude number (Fr) changes from subcritical flow to supercritical flow and the maximum velocity of rill reaches over 0.065 m/s. Unit stream power using the maximum velocity of rill was the best parameter for estimating rill erosion as rill evolution greatly depends on maximum velocity of the local flow enhanced by rill incision and expansion As a result, it was found that critical conditions for rill incipient of sandy soils on steep slope are observed greater than those of loamy soils or gentle slopes.

Applicability Comparison of GIS-Based RUSLE and SEMMA for Risk Assessment of Soil Erosion in Wildfire Watersheds

“…However, they must be simulated so that the erosion rate increases on the hillslope of the cell unit due to rill development with an increase in surface runoff. In post-fire mountainous hillslopes, the incision and expansion of rills are not frequently observed due to obstruction by residues and the stems and roots of plants [74,84]. The FAV of the RUSLE greatly overestimated compared to the TV, as shown in Figure 13a.…”