Comparison of outflow hydrographs following dam breach arising from overtopping, wide-width overtopping, and geotechnical seepage failure mechanisms.

Dam breach is a large-scale, highly unsteady, and complex water-sediment flow. Dam breach events span scenarios involving natural dams (created following valley blocking landslides) to scenarios involving anthropogenic structures such as water-retaining dams and dams designed specifically to store mine waste (e.g. tailings). Management of the risk posed by a potential breach of a dam structure requires a careful analysis of the consequences of failure. These analyses, often called dam breach studies, aim to improve safety and risk management through the prediction of travel time, flow velocity, and spatial extent of the hazard (e.g. map of depth of inundation) in the event of a breach. These factors in turn define the consequence classification of a dam and guide the development of emergency preparedness plans. The key boundary condition required for flood routing numerical simulations often conducted for dam breach studies is the outflow hydrograph which describes the relationship between outflow of the retained volume with time. In this study we explore the effect of failure mechanism on the characteristics of the outflow hydrograph of otherwise identical physical model dams. Physical model dams of 1 m in height were constructed of fine sand near the midspan of the 36 m long, 2.1 m wide, and 1.2 m high large landslide flume facility at Queen’s University Coastal Engineering Laboratory. Three failure mechanisms are explored; a) notch overtopping, initiated by incising a v-notch into the dam crest and allowing the impounded reservoir to intersect this local low point; b) wide-width overtopping, where a retaining wall placed along the crest of the dam allows for an additional reservoir capacity above the height of the crest that when rapidly removed a full width sheet of water cascades over the dam; and c) geotechnical seepage failure, where closing the toe drain allows a seepage face to develop that causes a failure in the downstream face of the dam. The reservoir surface elevation during breach was monitored with a series of five Akamina AWP-24 wave capacitance height gauges distributed centerline in the upstream reservoir. The evolution of breach shape is captured every 3 s using five Canon EOS Rebel T5 Digital Single-lens Reflex (DSLR) that capture a plan view area of the entire 2.1 m width of the dam and a combined upstream and downstream length of 4.3 m. To further capture the evolution of failure a Blickfeld LiDAR sensor was positioned oblique to the downstream slope to capture a point cloud scan ever 1.4 s. These data sets are then used to compare the physical characteristics of each breach process and the resulting implications for the observed outflow hydrographs for each failure mechanism.