2016 – Tailings Dams: Failure Impact Assessment Framework

Matthew Ind, Kate Brand and Mark Ferrier

The framework for undertaking a dam breach analysis for water dams is reasonably well established with a depth of information and software available to guide practitioners on a consistent approach to undertaking failure impact assessments. In contrast, dam breach modelling for tailings dams is currently a developing field with a wide range of modelling approaches taken and an inconsistency in the quality of the failure impact assessments undertaken. Recent tailings dam failures at the Mt Polley Mine in British Columbia, Canada and the Fundäo and Santarém dams at the Samarco iron ore operation in Minas Gerais, Brazil have provided a sobering reminder of the hazards presented by tailings dams and the clean-up challenges that are significantly more complex than a similar failure of a water dam.

Current guidelines and approaches to dam breach modelling are often done assuming the run-out material from the breach is just water without due consideration of the impact from tailings loss. There is limited analysis undertaken on credible failure modes of tailings dams with an assumption that the embankment just “breaks” at some random point without appreciation of the failure mechanism. The misunderstanding of failure modes leads onto inconsistencies with application on whether a ‘sunny-day’ or extreme flood event modelling should be applied, with one or the other selected without explanation.

This paper outlines a framework that can be applied when undertaking a dam breach study for tailings dams to enable a consistent and credible assessment of potential failure impacts. The following tasks are discussed in detail in support of this framework:

  • Design Basis and Assumptions
  • Failure Modes and Effects Analysis
  • Credible Failure Modes during construction, operation and closure of a tailings facility
  • Estimation of dam beach parameters for credible failure modes (breach width, depth, tailings and water volume loss, time to breach, etc.)
  • Development of dam breach hydrograph (for water and tailings)
  • Hydraulic modelling of dam breach
  • Flood inundation mapping and tailings impact mapping
  • Estimation of population at risk and potential loss of life
  • Assigning a tailings dam failure consequence category
  • Emergency Preparedness Planning

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