Water Stewardship Information Sources
| Citation | BGC Engineering. 2015. Catiline Creek Debris Flow Hazard and Risk Assessment. Prepared for Squamish Lillooet Regional District. |
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| Organization | Squamish-Lillooet Regional District |
| URL | http://www.slrd.bc.ca/sites/default/files/reports/20150122%20Catiline%20Creek%20Hzd%26RiskAssmt_ecopy.pdf |
| Abstract/Description or Keywords | BGC Engineering Inc. (BGC) was retained by the Squamish-Lillooet Regional District (SLRD) as agent for Emergency Management BC (EMBC) to assess debris-flow hazards and risks on Catiline Creek on the north side of Lillooet Lake. The primary objectives of this assessment were to: Assess geohazard safety risk for residential development located at the outlet of the Catiline Creek drainage Develop conceptual debris-flow risk reduction options and costs. BGC assessed risk for four debris flow scenarios representing a range in debris-flow return periods from 5 - 30 to 3000 - 10,000 years. Debris flows were numerically simulated for each scenario at volumes ranging from 6000 m3 for the smallest event to 300,000 m3 for the largest event. The risk assessment involved estimating the probability that debris flows will impact residential dwellings and cause loss of life. It considered the existing channel configuration and conservatively assumed that no evacuation is possible during the event. This assessment used two different metrics to estimate safety risk: individual risk and group risk. Individual risk evaluates the chance that a specific individual (the person judged to be most at risk) will be affected by the hazard. Group risk, also known as societal risk, evaluates the chance that any people present in the area will be affected by the hazard. Results were compared to quantitative risk tolerance or risk acceptance criteria to help guide the development of options to reduce risk to tolerable levels. Such criteria have not been defined for British Columbia by formal legislation. For this study, estimated risks were compared with individual risk tolerance criteria formally adopted by the District of North Vancouver, British Columbia (DNV 2009), and with group risk tolerance criteria formally adopted in Hong Kong (GEO 1998) and previously applied by DNV. The DNV criteria for individual landslide risk tolerance are as follows (DNV 2009): Maximum 1/10,000 (10-4) risk of fatality per year for existing developments Maximum 1/100,000 (10-5) risk of fatality per year for new developments. In summary, BGC’s best-estimate of individual risk exceeded 1:10,000 risk of fatality per year for 76 of the 114 occupied, residential-classed lots within the study area. Of these, 18 lots exceeded 1:1,000 annual risk of fatality, more than one order of magnitude above the DNV individual risk tolerance threshold. Estimated group safety risk also fell entirely into the “Unacceptable” range when compared to the above risk tolerance standards. Table E-1 summarizes mitigation options and estimated costs. Each option was developed for 100,000 m3 and 300,000 m3 design volumes, which correspond to approximately 1000 – year and 10,000 year return period events, respectively. The estimated cost of mitigation for the smaller design volume is about half that of the larger option, primarily due to lower earthworks requirements. The larger design volume is intended to reduce risk to tolerable levels according to DNV standards (e.g. tolerable residual risk). Preliminary analyses suggest that mitigation of the smaller design volume may reduce individual safety risk but not group risk to tolerable levels according to DNV criteria. This is subject to confirmation during detailed mitigation design. While risks other than safety were not quantified in this assessment, the mitigation options listed in Table E-1 would also reduce risk for a broad spectrum of other elements on Catiline fan including roads, utilities, and water and power transmission. |
| Information Type | report |
| Regional Watershed | Howe Sound & Sunshine Coast |
| Sub-watershed if known | Catiline Creek |
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| Project status | complete |
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