Embankment dam overtopping and collapse: an innovative approach to predict the breach outflow hydrograph

Dams play an essential role in water management. Many countries have adopted norms, laws and regulations in order to reduce the risk of dam failure. Preparing a dam safety emergency action plan starts with a study of dam-break flood waves in order to identify the zones at risk. The selected failure mechanism has a significant impact on the discharge at the dam and on the resulting water levels and velocities along the downstream valley. During an overtopping failure the breach formation is the result of fluid-structure interactions: for instance, water flowing over the crest of a dam induces shear stress and saturation in its surface layer which results in material erosion. This erosion, if not controlled, ultimately results in the formation of a breach, allowing the discharge to increase resulting in an accelerated rate of soil removal and growth of the breach. An accurate prediction of the failure discharge requires a breach development analysis based on the dynamic interactions between the various physical processes involved. In this paper, the ERODE numerical model is presented. The model allows for the assessment of breach formation and failure discharge during an overtopping event. Step by step calculations are performed, considering the hydraulics of the flow over the dam, the soil erosion and the potential slope instabilities in the breach during the entire process. Model validation has been performed by means of comparison between predicted processes and measured data obtained from real overtopping tests as performed on 6 m high embankments in Norway. Several results illustrate ERODE performance and its overall prediction sensitivity.