Soil Erosion by Concentrated Flow: Shear Stress and Bulk Density

Abstract

Soil detachment by concentrated flow is often predicted using hydraulic shear stress (t) as a predictor variable. The nature of this relationship is uncertain, however, and may depend on many factors that influence soil resistance to detachment. Research was conducted to better understand the relationship between soil detachment from concentrated flow and t and the influence of soil bulk density (rb) and erosion duration on detachment. Overland flow was simulated using a 4.9-m-long hydraulic flume. Soil cores from a Mexico silt loam soil (fine, montmorillonitic, mesic Udollic Ochraqualf), 102-mm in diameter, were prepared at 1.2 and 1.4 Mg m3 bulk density and exposed to overland flow at shear stresses ranging from 1.1 to 38.2 Pa. Soil detachment increased with test duration and t for both rb soils. Within a t range of 1.1 to 15.0 Pa, soil detachment was observed to be 5.5 times greater for the 1.2 versus the 1.4 Mg m3 rb samples with a mean detachment of 11.1 and 2.1 kg m2, respectively (P = 0.01). Soil erodibility (Kr), defined as the detachment rate per unit increase in t, was 1.10 g m2 s1 Pa1 ( 103 s m1) for the 1.2 Mg m3 rb and 0.23 g m2 s1 Pa1 for the 1.4 Mg m3 rb, samples indicating that the 1.4 Mg m3 rb samples were 4.7 times as resistant to detachment compared to samples packed at 1.2 Mg m3 rb soil. Critical shear stress, defined as the shear stress where detachment is not different from zero in a graph of detachment rate versus t, was 1.3 Pa for the 1.2 Mg m3 rb and 4.0 Pa for the 1.4 Mg m3 rb samples. Vane shear strength difference between the two bulk densities was highly significant with mean strength of 3.0 kPa for the 1.2 Mg m3 rb and 14.4 kPa for the 1.4 Mg m3 rb soil.