The SLRD received grant funds from the 2020 Union of BC Municipalities (UBCM) Community Emergency Preparedness Fund (CEPF) for the purpose of flood hazard mapping and risk assessment for the SLRD Electoral Area D community of Paradise Valley / Tunnel Station. Ebbwater Consulting Inc. has now completed this study, which includes mapping and risk assessments of both hydrological and geomorphological hazards.
This project completes the assessment of the Cheakamus River floodplain from Hwy 99 at the Daisy Lake Dam in the north, to the District of Squamish municipal boundary in the south of the study area. In 2016, the District of Squamish (DOS) completed a comprehensive floodplain assessment which included the Cheakamus River floodplain area within its boundaries.
The Ebbwater study will inform both emergency management plans and development criteria in an area that is subject to recurrent flooding and an increase in development. This information, in combination with the DOS study already completed, will give a complete flood risk profile for the Cheakamus River system south of the BC Hydro Daisy Lake Dam. That complete picture will benefit the SLRD and the DOS in moving forward with applications for future flood mitigation funding where available. This study was periodically vetted by a Project Steering Committee consisting of representatives from the SLRD; Squamish Nation; DOS; the Ministry of Forests, Lands, Natural Resource Operations and Rural Development (MFLNRORD); and Emergency Management BC (EMBC).
This study also provides updated Flood Construction Level mapping, produced in accordance with provincial guidelines.
The study finds that:
- The estimated proportion of people potentially exposed to the Very Low magnitude flood (i.e., 450 m3/s at Cheakamus River near Brackendale WSC gauge) is relatively small at 6%; [MF1] [MF2] however, this number jumps to 37% and 40% for the Low (i.e., 800 m3/s at WSC gauge) and Moderate (i.e., 1200 m3/s at WSC gauge, which is also ~0.5% AEP or “200 year”) flood magnitudes, respectively. When fluvial geohazards are considered with the Moderate flood magnitude, the number of affected people increases by 14%.
- In terms of economic consequences, the building values exposed to flood ranges from $79M to $82M. When fluvial geohazards are considered (which includes damages to land values) with the Moderate flood magnitude, the consequences increase to $98M (i.e., a 20% increase).
- In terms of critical infrastructure, eight facilities (primarily electrical power stations) are exposed to the Very Low flood magnitude and one additional facility is exposed to the Moderate flood magnitude or higher.
- A negligible length of roads is exposed to the Low flood magnitude; however, approximately 1 km is exposed to the Moderate flood magnitude or higher. When fluvial geohazards are considered with the Moderate flood magnitude, the length of exposed roads increases by approximately 200 m. Out of the four river crossings in the project area, only the pedestrian crossing is flooded, and this occurs for the High flood magnitude or higher.
- The risk from the Very Low flood magnitude (i.e., almost certain, small event) is generally higher than the risk for the Moderate and High flood magnitudes (i.e., more rare, larger events). This means that, cumulatively over time, the damages from small events can potentially add up to be larger than those from larger events that governments typically focus on for flood mitigation.
- Geohazards are interlinked with flood hazards, and damages stemming from them can be substantial.
- Although it is difficult to project future flows under climate change, the flood flows that have been experienced in the past are getting more frequent and larger with time. Climate change conditions will also exacerbate the frequency and magnitude of geohazards.
Key recommendations from the study:
- Work with the Province to improve the climate monitoring network in the region. Establishing long-term high-elevation stations would contribute to understanding changes in precipitation processes (e.g., raising of the snow line) that are responsible for flash flooding.
- Work with the Province to improve the hydrometric monitoring network in the region. This data would improve the accuracy of hydrology studies, and understanding changes in the flow regime including peak flow trends.
- Strengthen partnerships with NGOs and industry to facilitate collaborative research and the integration of new science into policy and practice.
- Conduct follow-up field assessment to better characterize the geohazards, based on the overview analysis and mapping results
- Work with the relevant health authority to obtain and map septic systems within the project area, if the data is available.
- Work with others to support a motion at the UBCM to require that BC Assessment property information and rental agreements disclose current and projected flood risk levels.
- Work together with local NGOs, First Nations, and the District of Squamish, to develop and implement public awareness initiatives.
- Implement an initiative to show important recent and historical water surface elevations in prominent places (e.g., on bridge piers or utility poles) to keep flood hazards at top of mind for residents.
For more information:
Flood Hazard Mapping and Risk Assessment - Upper Paradise Valley Presentation