Numerical Simulation of Submerged Jet Test and Associated Soil Erosion

Estimates of bank erosion typically require field measurements to determine the soil erodibility since soil characteristics are highly variable between sites, especially for cohesive soils. In situ methods are often preferred due to the difficulties of collecting an undisturbed sample. The submerged jet test device is an in situ method of determining the critical shear stress and soil erodibility of cohesive soils. A constant velocity jet, applied perpendicular to the soil surface, creates a scour hole which is measured at discrete time intervals. While the results of these tests are able to provide values of critical shear stress and soil erodibility, the results are often highly variable and do not consider certain aspects of scour phenomena found in cohesive soils. Jet test measurements taken on the lower Roanoke River showed that the results varied for samples from similar sites and bulk failures of large areas of soil were common on the clay banks. Cohesive sediments exhibit variable erosion patterns and the effect of these scour hole shapes on the applied shear stress has not been thoroughly explored. Computational Fluid Dynamics (CFD) can be used to determine the effect of scour hole shape changes on the applied shear stress. This study uses the CFD program FLUENT to calculate shear stresses within the jet test device with differing scour hole geometries. The geometries reflect scour holes ranging from narrow to wide including potential effects of bulk failures. The modeling results are used to improve the estimation of critical shear stress and soil erodibility for cohesive soils using the jet test device.