The Equivalent Sand Particle Diameter Approach to Rationally Estimate the Erosion Behavior of Fine-Grained Riverbed Soils

The evaluation of erosion is often conducted by dedicated computer software, such as Hydrologic Engineering Center’s River Analysis System (HEC-RAS), which considers several input parameters such as the flow conditions, river geometry, and soil properties. It considers the soil condition primarily based on the mean particle size (${\mathrm{D}}_{50}$). Research showed that grain size mainly controls the erosion characteristics of coarse particles, while additional factors such as the particle shape, size, and interparticle microelectrical forces control the erosion characteristics of fine particles, resulting in higher erosion resistance as the particle size becomes smaller. The borderline between these different behaviors is approximately at the mean particle size of 0.2 mm. Therefore, it needs a careful approach in evaluating the erosion depth of fine riverbed materials based on HEC-RAS and similar software. This study conducted erosion tests on four different riverbed soils in Nebraska using the Mini-JET. Then, it correlated the mean grain size to the critical shear stress and developed the logic of the “equivalent sand particle diameter method for fine particles” to rationally convert the particle diameter of fine particles to that of coarse ones with the same critical shear stress. HEC-RAS was then used with the equivalent sand particle diameter concept. It found that this method predicted the erosion depth for fine soils rationally well, opening the possibility of easy assessment for the erosion of fine-grained riverbed soils.