The spillway dilemma for mine tailings closures.

A hydrologic closure design concept is presented to minimise the risk of water discharges from closed mine tailings facilities. The concept takes into account that the dam requires enough freeboard to prevent overtopping and discharge to the environment, but dam regulations require a spillway to prevent the dam from overtopping and provide a means for discharge. The concept was applied to the tailings facility at the Red Dog Pb-Zn mine in Alaska, USA, where the tailings main dam is a 192-foot high rock fill with a geomembrane liner system for seepage control which extends down to relatively competent bedrock. Current plans are for the dam to be raised by an additional 16 feet before closure. The applicability was assessed of the US Nuclear Regulatory Commission guidelines to develop acceptable designs for the closure of tailings facilities that have a calculated adequate surcharge capacity for environmental protection and an emergency spillway for dam safety. The guidelines provide a basis to estimate an acceptable surcharge capacity of a probable maximum flood series for storage of surface runoff. To be conservative and for redundancy with respect to dam safety, an emergency spillway is provided to pass the probable maximum flood. Depending on the annual water balance, the surcharge capacity may be used by surface runoff and minor amounts of sediment that accumulates in the impoundment from year to year. The emergency spillway crest is then set at this level or above the surcharge capacity, whichever is higher. The method was employed for the preliminary design of the tailings main dam at its ultimate closure configuration.
A hydrologic closure design concept is presented to minimise the risk of water discharges from closed mine tailings facilities. The concept takes into account that the dam requires enough freeboard to prevent overtopping and discharge to the environment, but dam regulations require a spillway to prevent the dam from overtopping and provide a means for discharge. The concept was applied to the tailings facility at the Red Dog Pb-Zn mine in Alaska, USA, where the tailings main dam is a 192-foot high rock fill with a geomembrane liner system for seepage control which extends down to relatively competent bedrock. Current plans are for the dam to be raised by an additional 16 feet before closure. The applicability was assessed of the US Nuclear Regulatory Commission guidelines to develop acceptable designs for the closure of tailings facilities that have a calculated adequate surcharge capacity for environmental protection and an emergency spillway for dam safety. The guidelines provide a basis to estimate an acceptable surcharge capacity of a probable maximum flood series for storage of surface runoff. To be conservative and for redundancy with respect to dam safety, an emergency spillway is provided to pass the probable maximum flood. Depending on the annual water balance, the surcharge capacity may be used by surface runoff and minor amounts of sediment that accumulates in the impoundment from year to year. The emergency spillway crest is then set at this level or above the surcharge capacity, whichever is higher. The method was employed for the preliminary design of the tailings main dam at its ultimate closure configuration.