Impact of Surface Roughness Measurements on the Erosion Function of Soils

The erosion function apparatus (EFA) is a laboratory device that can determine the relationship between the erosion rate and the hydraulic shear stress of a soil specimen tested; this relationship also estimates the critical shear stress required to initiate soil erosion. The computed hydraulic shear stresses from EFA testing are a function of the mass density of water, the mean water flow velocity, and a friction factor relating to the Reynolds number and the relative roughness of the soil sample after erosion. Since the water and flow geometry characteristics are well defined, the primary variable in computing shear stress is the soil surface roughness. There are a variety of methods available to measure the roughness; however, the location, accuracy, and resolution of the surface measurements can affect the final roughness value used in computing the corresponding hydraulic shear stress. This paper presents the results of different soil roughness measurement methods on five field samples after EFA testing and the impact those measurements have on the computed hydraulic and critical shear stresses. Findings suggest that measuring points across the entire sample (including points close to the perimeter) and taking readings every 0.3 points/cm² using a laser texture scanner or similar will result in a reasonable approximation of the erosion function of soils.