Citation | Kerr Wood Leidal. 2015. District of Squamish Integrated Flood Hazard Management Plan, Final DRAFT Background Report. Prepared for District of Squamish. |
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Organization | District of Squamish |
URL | http://squamish.ca/assets/IFHMP/20150224-FINAL%20DRAFT_BackgroundReport%20-%20web.pdf |
Abstract/Description or Keywords | The District of Squamish (District) is set in a beautiful but hazardous natural environment that includes: • flood hazards from the Squamish, Mamquam, Cheakamus, Cheekeye, and Stawamus Rivers; • debris flow hazards from the Cheekeye River and smaller local creeks; and • coastal flood and tsunami hazards from Howe Sound. The District lies within traditional territories claimed by the Squamish Nation. Ten Squamish Nation reserves located throughout the shared floodplain create an inseparable common interest in flood protection. The District also lies within traditional territories claimed by the Tsleil-Waututh Nation. In 1994, the District of Squamish completed a Flood Hazard Management Plan (FHMP) to help achieve an appropriate balance between flood protection and community development. Twenty years after its release, key parts of the 1994 FHMP have become obsolete as a result of community growth, improved understanding of flood hazards, and the emergence of new tools for flood hazard management. Integrated Flood Hazard Management Plan In February 2014, the District retained a multi-disciplinary consulting team led by Kerr Wood Leidal Associates Ltd. (KWL) to prepare a new Integrated Flood Hazard Management Plan (IFHMP). The IFHMP project will be supported by a multi-stage public consultation process and a technical working group that includes regulators, key major industry representatives, and the Squamish Nation. This Background Report is the first major deliverable of the IFHMP, which is scheduled for completion in 2016. The Background Report provides an overview of: • specific hazards that will be addressed in the IFHMP and the areas at risk from each; • the current state and context of the District’s flood protection program, including policy instruments as well as the local portfolio of structural flood protection works; • regional, provincial, national and international guiding principles for the IFHMP process; • pre-requisite technical updates including river modelling, coastal flood analysis, and a high-level geohazards assessment; and • preliminary conclusions and recommendations that arose during the compilation of background materials. River Hazards Most river floods occur in the fall and early winter when large and intense multi-day storms create high flows on the local rivers, and when precipitation falling as rain throughout the watershed can bring additional runoff contribution from alpine snowmelt. Sediment aggradation gradually or periodically increases the flood risk in some areas by filling in the river channels. Erosion is a separate but related hazard where riparian development has encroached into the flood corridor. Areas at risk of river flooding include Paradise Valley (Cheakamus River), the low-lying corridor that follows Highway 99 from Brackendale to Downtown Squamish (Squamish and Mamquam River), and from Valleycliffe to Stawamus I.R. No. 24 (Stawamus River). The value of infrastructure vulnerable to the Squamish River alone exceeds $2.4 billion. The majority of community services and commercial areas are at risk of flooding, as are most of the local Squamish Nation reserves. Coastal Hazards Coastal water levels at Squamish are a function of tide, storm surge, local effects, and wind waves. Coastal floods typically occur when external storm surges combine with the highest tides of the year during the winter storm season. The IFHMP has adopted the Province’s climate change guidelines, which recommend an allowance of 1 m of Sea Level Rise (SLR) by year 2100. The coastal flood assessment has identified other key gaps in the current understanding of local tsunami hazards, subsidence, datum adjustments, wind set-up, and wave generation. Debris Flow Hazards Debris flows involve very large peak discharges of substantially-sized material moving at relatively high velocities. The Cheekeye River is one of the most studied debris flow hazards in BC; the corresponding hazard area includes the entire Cheekeye Fan from Cheakamus I.R. No. 11 to Brackendale. Other small creeks around the District are potentially subject to debris flows, and previous studies have found the Stawamus River may be subject to a transitional process referred to as a debris flood. Policy In 2004, responsibility for development in flood hazard areas was shifted from the provincial government to local municipalities. The District’s 2009 Official Community Plan (OCP) provides strategic policies for flood hazard management planning and protection but does not specify Development Permit Areas for flood and erosion hazards. The District does not have a floodplain bylaw. Hazard assessments are currently mandated under rezoning, subdivision and building permit processes through provincial statutes such as the Land Title Act and the Community Charter. The District’s complex hazards are frequently beyond the scope of these site-specific reviews. This Background Report includes a review of flood hazard management policy measures adopted by Canadian jurisdictions ranging from nearby local municipalities to the federal government. Many international jurisdictions face even higher levels of flood risk and have naturally developed more sophisticated flood hazard management policies. International examples reviewed for this report include case studies from Europe and the United States. Structural Protection In addition to implementing policy and planning measures, the District maintains a portfolio of structural flood protection works to protect the Squamish community. The District portfolio primarily consists of dikes, riprap erosion protection revetments, and ancillary structures regulated under the Dike Maintenance Act. The District’s structural flood protection works are complemented by a number of unregulated First Nation, privately-owned, “orphaned”, and de facto dikes and training berms on many of the local creeks and rivers. The most significant element of the District portfolio is the integrated ±20 km long Squamish River and Mamquam River dike system constructed by the province in the early 1980s. Other structural flood protection works are located throughout the Paradise Valley (Cheakamus River), along the Cheekeye River upstream of Highway 99, and adjacent to the Valleycliffe neighbourhood (Stawamus River). Updated hydraulic modelling, including an allowance for flow increases as a result of climate change, confirms that some structures currently do not provide the intended level of flood protection. Coastal flood protection is currently provided by a variety of low, non-standard works around downtown. The District’s only regulated sea dike extends from the foot of Cleveland Avenue around to the west end of Winnipeg Street. Conflicts with development have created challenges for future dike raising. Flood Protection Gap Analysis The Background Report closes by contrasting current and future flood hazards against the District’s flood hazard management program, drawing conclusions about where the existing program may be unable to deliver the desired level(s) of protection. These conclusions take the shape of a gap analysis focussed on both policy and structural program elements. Key policy gaps are identified in the categories of risk management and analysis, regulation, and public education. The most notable gaps include planning for climate change, particularly SLR, and the need for a floodplain bylaw and/or flood hazard development permit areas. Key structural flood protection gaps are identified in the categories of design standards, jurisdiction and access, inspection, reporting and compliance, and environment and community. The most notable gaps include coastal defences, access challenges, particularly the lack of a continuous Statutory Right-of-Way, and outstanding maintenance issues. The subject matter for this report was discussed by the IFHMP Technical Working Group at a meeting on June 16, 2014. The draft report was presented to District Mayor and Council on August 19, 2014. |
Information Type | report |
Regional Watershed | Howe Sound & Sunshine Coast |
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Project status | complete |
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