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Despite its widespread appearance, the scour of cohesive materials has received relatively less attention than the granular counterpart. In particular, the different modes of erosion of cohesive soils, its long-term temporal evolution, and the effect of the impinging angle and jet regime on scour features still represent open questions. To shed light on these aspects, a series of long-term experiments were conducted involving a circular jet impinging on a clay bed at angles of 45°, 60°, 75°, and 90°. Experimental evidence allowed us to provide unprecedented insights into the peculiar evolution of the scour process, as well as empirical formulas to estimate the main geometrical features of the scour hole. Scour evolution is distinguished between continuous or discontinuous. These two modes differ for scour evolution kinetics and scour profile similitude. Distinct erosion mechanisms are documented, including the formation of a peculiar ring scour shape. The transition between strongly and weakly deflected jet regimes is investigated and, for the first time, related to the soil friction angle. Overall, the analysis of physical evidence allowed us to reach the following main conclusions: (1) the inclination of the jet and the tailwater depth affect the scour process and its evolution and (2) the long-term scour behavior should not be disregarded to provide trustable predicting tools.
Palermo et al. (Sun,) studied this question.