Thetis Lake Dams – Breach Analysis and Inundation Mapping

Project Highlights

Thetis Lake is a popular recreation destination near Victoria, BC and lies within Thetis Lake Regional Park, managed by Capital Regional District (CRD).  Dams on Thetis Lake were originally constructed between 1887 and 1927 for water supply purposes, first for Esquimalt and then for the City of Victoria until Sooke Lake was made the primary water supply for the region.  The dams are now used to maintain levels in the Thetis Lake for aesthetic and recreation purposes, and to store water to augment low summer flows to Craigflower Creek for fish habitat protection.

Residential development downstream of the dams over the past 100 years has increased the potential consequences if the dams were ever to fail.  KWL was retained by CRD to complete a dam breach analysis and inundation mapping study to:

• determine the magnitude of peak flows resulting from a hypothetical breach of any of the three dams; and
• map the limits of downstream inundation that could occur as a result of breaches.

Both one-dimensional MIKE 11 and two-dimensional MIKE 21 models were used to perform the dam breach analysis for the three Thetis Lake dams.  Inundation maps were prepared and used to assess the risk classification of the dams, which is used to establish flood and earthquake design criteria, and for emergency planning purposes.