Overtopping of the core in rockfill dams — Internal erosion

During extreme flood events, there is a risk for internal overtopping of the core in rockfill dams and resulting erosion on the top surface of the core or of the sand filter above it. If the geometric filter criterion is not satisfied in the interface between two soil layers subjected to through-flow, both a theoretical investigation and observations from experiments at prototype-scale clearly indicate that erosion starts at the downstream side of the lower, finer stratum and propagates rapidly upstream. The time-scale of the process is markedly sensitive to variation in the ratio of the grain sizes in the layers and the void ratio of the upper stratum; however does not depend significantly on the height of overtopping. If only a small fraction of the erodible grains is retained at the interface by mechanical interaction (filtration), erosion ceases rather quickly and the degradation of the interface is limited.