Hazard and risk from large landslides from Mount Meager volcano, British Columbia, Canada

Abstract

During the past 8000 years, large volcanic debris flows from Mount Meager, a Quaternary volcano in southwest British Columbia, have reached several tens of kilometres downstream in Lillooet River valley, with flow velocities of many metres per second and flow depths of several metres. These debris flows inundated areas that have become settled in the past 100 years and are now experiencing rapid urban growth. Notably, Pemberton, 65 km from Mount Meager, has doubled in size in the past five years. Approval of subdivision and building permits in Pemberton and adjacent areas requires assessment and mitigation of flood hazards, but large, rare debris flows from Mount Meager are not considered in the permitting process. Unlike floods, some volcanic debris flows occur without warning. We quantify the risk to residents in Lillooet River valley from non-eruption triggered volcanic debris flows based on Holocene landslide activity at Mount Meager. The calculated risk exceeds, by orders of magnitude, risk tolerance thresholds developed in Hong Kong, Australia, England, and in one jurisdiction in Canada. This finding poses a challenge for local governments responsible for public safety.

Keywords:

• Quaternary volcano
• Mount Meager
• landslide hazard and risk
• quantitative risk assessment
• debris flow
• volcanism

Acknowledgements

We thank Stephen Black, John Vanloon, Niel Vanloon, Bruce Miller, Jack Ronayne, Bob Menzel, and John Beks for allowing us to drill on their properties and for clearing snow for the drill rigs. Drilling was carried out by Sonic Drilling Ltd. and Geotech Drilling Services Ltd. Many of the radiocarbon ages were provided by Roger McNeely (Geological Survey of Canada Radiocarbon Laboratory). The paper has benefited from careful review by Christophe Bonnard and an anonymous reviewer.