Assessment of Equilibrium Scour by a Submerged Circular Turbulent Impinging Jet in Cohesive Soils

In this paper, results from a study where detailed measurements of the time development of the scour holes produced by a submerged circular vertical impinging jet in cohesive soils from long-term scour tests are presented. This type of jet is the one used in the ASTM standard Jet Erodiblity Test, which relies on measurements of the time development of scour for prediction of soil erodibility parameters. In the experiments, measurements of the entire scour hole were taken on a 2 mm grid using a computer-controlled laser displacement meter after scouring times of 5 min, 10 min, 15 min, 20 min, 30 min, 40 min, 50 min, and 1 h, 1.5 h, 2 h, 4 h, 8 h, 16 h, 24 h and then at an interval of 24 h thereafter until the scour hole was considered to have reached equilibrium based on the criterion used by Mazurek et al. (2001). For the tests, two types of manufactured pottery clays were used. The growth of the maximum depth of scour, average scour hole radii, centerline depth of scour, and scour hole volume were evaluated. Results for the equilibrium values are compared for a number of criteria typically used to decide when a scour hole has reached equilibrium, as well as models to predict equilibrium values. The maximum scour hole depth was appeared to be the appropriate characteristic length for defining equilibrium scour. Results also showed that the Blaisdell et al. (1981) method for predicting equilibrium scour, greatly overestimated equilibrium scour depths.