Erodibility and Scour by a Vertical Submerged Circular Turbulent Impinging Jet in Cohesive Soils

Abstract

The erodibility of cohesive soils is commonly represented by the critical shear stress, which is the applied shear stress for which erosion is initiated, and the erodibility coefficient, which is a measure of the rate of erosion once the critical shear stress threshold is exceeded. These erodibility parameters are typically considered properties of the soil and can be measured using a number of laboratory or field testing devices. In this study, a submerged, turbulent jet with a circular nozzle, impinging vertically on a soil surface, was used for testing 18 natural and 10 manufactured cohesive samples. There are a number of different methods available for analyzing the test data and the first objective of this study was to compare the resulting erodibility parameters calculated using multiple methods. The second objective was to use the calculated erodibility parameters to develop a relationship that can be used for estimating the ultimate dimensions of a scour hole formed using a vertical jet.

Four data analysis methods (Hanson and Cook (2004), Visual, Equilibrium, and Thomas (Personal Communication, 2010))were used in determining the critical shear stress for the tested samples. While it was expected that the results from these four methods should provide similar results, this was not found to be the case. Differences in the calculated values were largely attributed to the disparity between theoretical estimates for the equilibrium centre line scour depths and the equilibrium scour depths observed from testing. Results were also found to be impacted by the duration of test data used in the analysis. The relative ranking of the critical shear stress values for the soils tested are very close to being the same between analysis methods.

Two data analysis methods (Hanson and Cook (2004) and Thomas (Personal Communication, 2010)) were used in determining the erodibility coefficient for the tested samples and were found to have a significant discrepancy in results. Each analysis method is a theoretical model of how the scour hole depth grows with time and both are based on a form of the excess shear stress equation. This equation is generally assumed to be linear; however, results from this study support the inclusion of an exponential term. This term was found to vary between 0.5 and 2.0 for Hanson and Cook’s (2004) analysis method and between 0.6 and 6.1 for Thomas’ Method. Adopting nonlinear time development of scour equations affects how the erodibility coefficient can be compared between samples since its units depend on the value of the excess shear stress exponent. It is recommended that a nondimensional form of the erodibility coefficient be considered when using a nonlinear expression of the excess shear stress equation for data analysis.

A number of scour analysis methods from the literature were highlighted in this study and Mazurek’s (2001) approach was chosen. When using the critical shear stress from Hanson and Cook’s (2004) method and the equilibrium method, reasonable relationships were developed for estimating the centre line scour depth and scour hole radius at the equilibrium state. As expected, data from the natural samples showed more variability than the manufactured samples.