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In late 2013, the Province of British Columbia, the Squamish Nation, and the District of Squamish collectively selected and appointed an independent Cheekye River and Fan Expert Review Panel (the Panel). The Panel was instructed to review all previous relevant documentation associated with geological, geomorphological, and geotechnical studies, re.search, reports, and publications associated with the Cheekye River and Fan, and to provide its opinion on possible future landslides, including debris flows and debris floods.
Specifically, the Panel was to provide its opinions on the:
- volume and frequency of future landslides;
- the character and volume of the ‘10,000-year’ landslide; and
- possible effects of climate change on future landslides.
This report summarizes the Panel’s opinions based on a thorough review of the referenced documents and careful consideration of the unknowns, uncertainties, and assumptions, which are discussed. In carrying out its review, the Panel identified other considerations that lie outside its Terms of Reference. As requested, these considerations are also included as part of this report. This Executive Summary summarizes the Panel’s opinions.
With respect to volume and frequency of future landslides, it is the Panel’s opinion that the magnitude-cumulative frequency (MCF) relationships developed by BGC Engineering Inc. (2008a) (redrawn as Figure 3 in this report) are the most reliable MCF relationships currently available for Cheekye Fan. Further, it is the Panel’s opinion that the part of the MCF curve representing smaller volume rainfall/surface water runoff-generated debris flows (solid line on Figure 3) is credible and could be a basis for considering debris flow mitigation strategies for this range of events. A spectrum of still smaller debris floods or stream floods, which are not shown on Figure 3, should also be considered in mitigation strategies.
With respect to the 10,000-year landslide, it is the Panel’s opinion that the upper (dotted) line on the right side of Figure 3 provides a prudent estimate of the volume of debris that could be transported to Cheekye Fan during a rock slide-generated debris flow. The volume of the 10,000-year debris flow is 5.5 million m3. This estimate is more conservative than that recommended by BGC (2008a), but is consistent with several other previous estimates, as reviewed in Section 4.2 of this report.
It is also the Panel’s opinion that the 10,000-year landslide, which is conceptually comparable to a ‘maximum credible earthquake’ or a ‘probable maximum flood’, is the appropriate extreme event for estimating the largest debris flow that could affect Cheekye Fan .With respect to possible effects of climate change on future landslides, it is the Panel’s opinion that climate change will increase the frequency of debris flows and debris floods of all sizes. This expectation would shift the lines on Figure 3 to the left, although by an unknown amount. This shift would have the effect of increasing the volumes of given-year events, including the 10,000-year event. However, it is not possible at present to quantify with certainty changes in the frequency of future debris flows due to climate change.
Consequently, possible climate change effects must be considered by selecting suitably conservative parameters during the design of any mitigation, and by selecting solutions that are flexible with respect to the magnitude of potential effects.
The Panel’s opinions with respect to the other considerations are as follows:
- Whether or not there is any future development on Cheekye Fan, existing development on the fan and on the west side of the Cheakamus/Cheekye river confluence including residential, public, and industrial buildings, BC Highway 99, the CN Rail line, roads and bridges, and other structures, should be protected from possible large volume (with relatively low probability of occurrence) and small volume (with relatively high probability of occurrence) debris flows, debris floods, and stream floods, and associated stream sediment movement.
- All forms of mitigation, singly or in combination, should be considered and carefully evaluated to protect existing development and to possibly allow some new development on Cheekye Fan and on the west side of the Cheakamus River opposite Cheekye Fan.
An up to date map of the reserves and areas concerned is below: