Abstract
In November 2021, an atmospheric river event in southern British Columbia (BC) led to flooding, dike breaches, and an estimated $0.9 billion in damages in Abbotsford, BC. The flooding caused major agricultural losses, temporary closure of Highway 1, and environmental contamination of the Sumas Prairie area. This paper reviews the 2021 flooding of the Sumas Prairie as a case study in the differences in the current standard of practice for mining dams versus flood protection dikes in BC, and what elements of mining dam practices could be applied to the design and construction of flood protection dikes.
Unlike dike design criteria, mining dam design criteria and dam safety activities are based on the assessed consequence classification of the structure to manage the overall risk (i.e., consequence x likelihood). Mining dams (both tailings and water dams) are typically classified based on the Consequence Classification matrix presented by the Canadian Dam Association’s (CDA’s) Dam Safety Guidelines. Key criteria that are affected by the Consequence Classification include flood design criteria, seismic design criteria, frequency of dam status reports by the owner, and frequency of third-party dam safety reviews. The minimum flood design criteria ranges from a 100-year event for a Low Consequence structure to the Probable Maximum Flood for an Extreme Consequence structure.
Tailings dams require elevated minimum flood and seismic design criteria, regardless of Consequence Classification, when compared to water dams. Tailings dams also require a specific governance structure, including the Engineer of Record (typically a consultant), a qualified person (typically on the owner’s side), and an Independent Tailings Review Board.
Design guidelines and regulations for dikes in BC could benefit by learning from other industries, such as adopting the consequence-based design criteria and governance systems that are exemplified in the current BC water and tailings dam regulations and engineering guidelines, to meet evolving standards of practice and to avoid a recurrence of events like the November 2021 BC floods.