Impact of Vertical Hydraulic Gradient on Rill Erodibility and Critical Shear Stress

Abstract

Field observations and subsequent support from controlled lab experiments have shown the linkage between vertical hydraulic gradients and initiation of rills or incised channels. Nevertheless, current erosion prediction models do not include the hydraulic gradient effect in the erodibility parameters. The objectives of this study were to: (i) quantify the impact of a subsurface hydraulic gradient on rill erodibility and critical shear stress; and (ii) determine the intrinsic critical shear stress of the soil, which is the critical shear stress of the soil without the influence of the vertical hydraulic gradient. Two series of concentrated flow experiments were conducted on a silt loam soil subjected to hydraulic gradients varying from −2 to 2 m m−1 The first series measured soil loss at a given shear stress, while the second was a direct measurement of the critical shear stress by calculating the shear stress at the point of incipient motion of particles. The intrinsic critical shear stress was determined using a fluidized bed approach. We found that the average erodibility under an upward or positive hydraulic gradient was 5.64 times larger than that under a downward hydraulic gradient. The critical shear stress decreased from 1.0 to 0.2 Pa as the hydraulic gradient was increased from −2 to 2 m m−1 Using the fluidized bed approach, we obtained an inherent critical shear stress value of 1.61 Pa. We concluded that the presence of positive pore pressure near the soil surface was a major factor in reducing soil cohesion. We also concluded that with some improvements, the fluidized bed concept can be applied to determine the critical shear stress independent of the hydraulic gradient effect.