Effect of the presence of a tailings dam beach on breach outflow and erosion during overtopping failure

Dam breach analyses for tailings dams currently rely heavily on relationships and methods derived for water retaining dams, despite significant differences in design, construction, dam materials, and geometry; particularly, the upstream face of the dam. Conventional tailings slurry deposition from the dam crest typically forms low angle upstream beaches (1–2 % inclination) within the impoundment. In this paper, we isolate the effect of tailings dam beach geometry at the time of overtopping on breach characteristics using physical and numerical modeling. Five 1 m high homogeneous fine sand dams with beach heights of 0.5 to 0.9 m and a beach slope of 5 % were brought to failure by v-notch overtopping. The laboratory data revealed that a threshold beach height existed above which the peak discharge was progressively limited by the geometry of the reservoir. Numerical simulations, performed in XBeach, captured this effect in the outflow hydrographs, with differences between physical and numerical model peak outflow generally within 25 %. Another key model parameter in tailings dam breach analysis is the volume of tailings solids lost through erosion during breach. Comparison of terrestrial laser scanning elevation profiles, cut through the centreline of the physical model, with XBeach simulations indicate XBeach can replicate the bulk characteristics of erosion when a tailings-style beach is present. These findings show that hazard analysis for overtopping failure in tailings dams should consider the effect of tailings dam beach geometry on the outflow hydrograph, and forms a growing case of evidence to support the use of XBeach for simulation of dam breach.