There are three main categories for consequences of failure that should be taken into account in an impact assessment when assigning a class to a dam:
- Population at risk and loss of life
- Environmental and cultural values
- Infrastructure and economics
The class should be determined by the highest potential consequences, whether it be loss of life or environmental, cultural or economic losses.
Since many mining dams in Canada are located far from populated areas, the potential for loss of life is often not as prevalent as it would be for conventional dams. However, it is possible that people may be in the area downstream of the dam temporarily because of seasonal cottages, roads and highways, rail corridors and recreational activities. Mining dam failures could also threaten mine employees working downstream, for example, in an open-pit mine. In this instance, training mine staff on evacuation procedures and the potential for reducing possible loss of life can be considered.
Environmental losses are often the most significant aspect of a mining dam failure. Specific studies may be required to predict the degree of environmental loss. This could include damage to the downstream environment, but in some cases, mining dams have supported ponds and wetlands that are important or critical fish and terrestrial habitat, which can be impacted or destroyed in the event of a dam failure.
The economic losses to a mining company can be substantial and may be far greater than the financial burden directly associated with a failure. Mining dam failures can result in lost production, have negative impacts on the company’s market capitalization and limit the company’s ability to engage in other mining projects.
Dam breach and inundation studies are necessary to assess the consequences of potential mining dam failure. However, there are several challenges associated with dam breach for tailings dams, because the science of predicting tailings dam breaches and flows is relatively new. Current techniques for predicting tailings flow slide inundation are limited, and the lethality of tailings dam failures can be quite different than for conventional dam breach flooding. Limitations in accurately modelling the effects of a tailings flow slide need to be considered when assessing the consequences.
In 2020, the Canadian Dam Association (CDA) developed a technical bulletin and guidelines for tailings dam breach analysis. These documents provide the steps for tailings dam breach analysis and inundation studies for proper downstream impact assessment. The population at risk and potential loss of life are determined using CDA standard approaches based on the number of temporary or permanent residents in the dam-breach inundation zone.
Because mining dams contain contaminated fluids and solids, the environmental consequences of a “normal” failure (sunny-day failure) could be worse than a flood induced failure, even though the extent of distribution of the contaminated fluids and solids may be less. Since the conditions associated with each mining dam can vary, experts specialized in the fields of ecosystems, land, water quality, fisheries and cultural values should be called upon.
Classification criteria and terminology, such as “significant loss” and “critical habitat”, should be defined and agreed upon early in the design or safety review process, with input from specialists and regulatory authorities. Because it is difficult to predict the environmental and ecosystem effects from accidental releases, it is often necessary to be on the conservative side when applying dam classifications.
Economic losses should be considered for third parties beyond the limits of the mining lease on which the mining dam is situated. In many cases, the failure of a mining dam has no effect on third parties, if the failure and runout is wholly contained on the mine property. Nevertheless, the financial consequences to a mine owner can be much greater than the financial effects on third parties.
BBA’s team is experienced in this field and can advise you on the best approach to adopt. Contact us to discuss your projects!