This unprecedented Integrated Flood Hazard Management Plan systematically considers and responds to the interplay of physical, economic, social, and environmental risks.
In 2014 the District of Squamish undertook a ground-breaking, multi-year study to assess its flood hazards and recommend mitigation. the resulting plan sets out an array of mitigation choices to balance the need for flood protection with impacts to stakeholders, the community, and the environment.
The District faces an unparalleled range of flood-related hazards: river, urban, and coastal floods, dam breaches, debris flows, dike failures, and tsunamis. Rapid growth leaves the community with billions of dollars of assets within its floodplains. Floodplain risk management is a local responsibility for BC communities, and the District must manage its risks with resources available to a community of 19,500.
In 2011, BC’s local governments were directed to plan for 1m of sea level rise by Year 2100 and 2m by 2200. Subsequent development proposals triggered the need for a formal plan to manage sea level rise. The ‘obvious’ solution of a sea dike was not so simple: if the river dikes breached upstream, a sea dike would trap water in Squamish’s historic downtown and turn it into a giant bathtub.
Assessing Squamish’s flood hazards required an integrated, systems-based approach. Hazard assessments considered dependencies between different sources of flooding and mitigation options. Consequence assessments considered community priorities for different economic, social, and environmental outcomes. Quantitative risk assessments brought hazard, probability and consequence together to show how different areas are affected. Mitigation strategies balanced flood protection, community growth, and environmental objectives.
Implementation looked beyond existing conditions to focus on what can be expected in the coming decades. The project produced several engineering innovations, including western Canada’s most detailed floodplain-scale hydraulic model, a first-principles approach for establishing sea dike design criteria, and inundation probability maps that weight the effects of dike breach along a linear river floodplain.
The IFHMP took an unprecedented comprehensive approach to incorporating future development and adopted European methods to highlight potential challenges for floodplain evacuation. As well, new GIS tools extrapolated results from a small number of dike breach models to ensure that planning maps capture the possibility of a dike breach at any location along a 20 km dike.
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Kerr Wood Leidal Associates (KWL)
District of Squamish,