Effects of detachment capacity on soil erodibility to concentrated flow with flume experiments

Abstract. Concentrated flows in rills or ephemeral gullies are major sediment sources and the primary channel to convey runoff and sediments from hillslopes. To understand and model concentrated flow erosion appropriately, a clear insight into the soil susceptibility against water erosion process is of vital importance. Soil erodibility and critical shear stress are two important indices to measure soil susceptibility to concentrated flows in rill or ephemeral gully erosion. The objective of this study was to quantify soil erodibility and critical shear stress to concentrated flow under controlled flume experiments. The experiment was conducted at sixteen combined hydraulic conditions with four slope gradients (8.74 %, 17.62 %, 26.78 %, and 36.38 %) and four flow rates (32, 64, 128 and 256 L min-1). The experimental soil materials were packed into the flume of 0.5 m wide, 0.6 m deep and 12 m long, to a thickness of 0.5 m at a bulk density of 1300 kg m-3. Regulated water flow was introduced to soil surface at the upper end of flumes. The detachment capacity was calculated with a linear relationship with detachment rate and sediment load from the spatial distribution data of sediment concentrations simultaneously taken along the gullies at one meter intervals. Soil erodibility and critical shear stress were then regressively estimated with linear relationship between soil detachment capacity and flow shear stress under different flow regimes. Detachment capacities calculated with different slope lengths were found to have significant contributions to the relative errors for determination of soil erodibility and critical shear stress. The results confirmed that the slope lengths of experiment flume have significant effects on detachment capacity determination, which would affect calculation efficiency of soil erodibility and critical shear stress. The correlative coefficient between soil detachment capacity from different slope lengths and those estimated with all distributed experimental data reduces from 0.88 to 0.46 with slope lengths increasing from 1 m to 12 m. Relative errors were larger as slope lengths for determination of detachment capacities were longer. Short slope length was suggested to use for calculating soil erodibility and critical shear stress.