A flood risk assessment for the City of Chilliwack on the Fraser River, British Columbia, Canada

ABSTRACT

Flood hazard management in Canada is predicated on an event of arbitrary return period – the ‘design flood’, which on the Fraser River is the flood of record, with an estimated return period of 500 years. Recent studies have demonstrated that the existing dikes will not contain design flood levels, in a location where the potential consequences of flooding have increased exponentially during the past century. This suggests that a hazard-based approach is questionable for effective flood management. A pilot flood risk assessment was completed for the City of Chilliwack, 80 km east of Vancouver. We assessed direct and indirect economic losses for three hypothetical dike breach scenarios for flood return periods from 100 to 1000 years. We estimate that total losses would exceed CND $1 billion (€720 million) for the flood scenarios examined, not including intangibles such as emotional and cultural losses, environmental contamination, or changes in the quality and abundance of aquatic and terrestrial habitat. The study concludes with recommendations that would lead to a risk-based decision-making framework for flood mitigation design and emergency preparedness.

Keywords:

• Risk assessment
• flood hazard
• consequence assessment
• economic analysis

Acknowledgements

Tara Friesen and David Blain of the City of Chilliwack provided much helpful advice throughout the study.

Funding

Terasen Gas, Kinder Morgan Pipeline, Ministry of Environment, Ministry of Transportation, the City of Chilliwack, Fraser Basin Council, and DIAND are acknowledged for providing funding for this study.