Evaluating the Erodibility of Cohesive Riverbanks with the Jet Erosion Test

Erosion rates for natural cohesive soils depend on both the complex inter-particle forces within the soil and the applied shear stress by the moving fluid.  Due to the difficulty in describing these parameters theoretically, empirical methods are typically used to determine soil erodibility.  To analyze erosion of cohesive riverbanks on the lower Roanoke River in North Carolina, jet erosion tests were performed at field sites.  The jet erosion test scours the soil by applying a constant headwater jet and the measured scour depth and time period provide empirical estimates of soil erodibility parameters.  The erodibility parameters assume an excess shear stress model with a linear relationship between erosion rate and applied shear stress.  The advantages of the test are (1) it is performed at the site, negating the need for sample removal, (2) it can be performed on steep slopes such as those typical of many riverbanks, and (3) the results are easy to interpret with a spreadsheet program.  The general testing procedure and analysis is reviewed and the implications of the underlying model discussed.  The results from the Roanoke River are presented and erosion rates are compared with soil properties determined from other in situ and laboratory tests.  Integration of jet erosion test results with numerical modeling of river flows is also discussed.