Physical Modelling of Tailings Dam Breach

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Authors
Walsh, Andrea
Keyword
dam breach , tailings , mining , outflow hydrograph , physical modelling , overtopping
Abstract
Tailings dams are constructed in stages, exhibit sediment deposits located in the reservoir upstream of the dam crest (beaches) generated from slurry deposition, and are commonly designed with shallower upstream slopes in contrast with conventional water storage dams. Current breach models for the failure mode of overtopping, used to inform flood routing and emergency preparedness plans, have been developed for water storage dams and have not been calibrated to the tailings dam case. This thesis presents the results of eleven 1m high homogeneous fine sand dams brought to failure by overtopping to investigate the influence of dam geometries varying from water storage dams to tailings dams on the outflow hydrograph. Laboratory observations indicate that the breach crest geometry can be described as a semi-ellipse, and that the size and shape of the breach crest controls instantaneous discharge during breach. This geometric relationship, coupled with an observed linear breach crest growth rate and a discharge equation were shown to permit an estimation of discharge from the onset of breach to peak discharge. Peak discharge was demonstrated to occur when the combination of breach crest length and depth of water flowing over the crest were at their maximum, given a finite reservoir volume. The reservoir storage function was shown to limit breach discharge when the reservoir elevation fell below the sum of the dam height below the breach crest location and maximum potential depth of water flowing over the breach crest. These findings enabled calculation of the maximum discharge rate and time to reach peak discharge, based on reservoir and dam upstream geometry. The presence of a shallowly sloping beach in the reservoir impacted peak discharge only in the case where the beach was intersected by the breach crest while the discharge rate was on the rising limb of the outflow hydrograph. A reduction in peak discharge was observed in tests where the beach height was greater than this threshold, including scenarios that resulted in an approximately 50% reduction in peak discharge rate.
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