ABSTRACT: Overview of the 2010 Mount Meager landslide, British Columbia, Canada.

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Abstract

The 2010 Mount Meager landslide is the most recent and most damaging of frequent landslides from the Mount Meager Volcanic Complex, British Columbia (BC). We documented the event based on: multiple field visits from August 6th until mid-autumn 2010; interpretation of pre-event air photos and post-event GeoEye imagery; comparison of a digital elevation model derived from post-event GeoEye images and a pre-event, 25-m resolution Province of BC digital elevation model; analysis of seismograph records from British Columbia and Washington State; and two-dimensional numerical simulation using DAN/W. In the early morning of 6 August 2010, the secondary peak of Mount Meager failed catastrophically. Approximately 45 Mm3 of Pleistocenerhyodacitic volcanic rock slid away from the steep flank of the volcano and impacted and liquefied an apron of colluvium on the footslope below. Fragmentation and liquefaction were facilitated by and the high water content both within the rock slope and the flanking colluvial apron. Within a short distance, the landslide transformed into a highly mobile debris flow that traveled 7.8 km down the valley of Capricorn Creek at an average velocity of 60 m/s, banking high on outer bends along its path. At the confluence of Capricorn and Meager creeks, the leading edge of the flow impacted and ran 235 m up the south side of the Meager Valley. Some debris continued 3.7 km up the valley of Meager Creek. Much of the debris was deposited at the mouth of Capricorn Creek, where it blocked Meager Creek for 19 hours; the remainder was carried down Meager Creek to the Lillooet Valley, where it temporarily blocked Lillooet River. The total height (H) and length (L) of travel are 2078 m and 12.6 km, respectively, giving H/L = 0.165 and a fahrböschung of 9.4°. The impact of the landslide was equivalent to an earthquake of local magnitude 2.6. The landslide significantly altered the planforms of Meager Creek and Lillooet River. The 2010 event is the third major landslide in the Capricorn Creek watershed since 1998 and the fifth large mass flow in the Meager Creek watershed since 1930. The anticipated breach of the Meager landslide dam prompted evacuation of the town of Pemberton on the night of 6 August. The dam, however, breached slowly enough that flooding was averted. Fluvial reworking of the landslide deposit, however, will lead to a pulse of bedload down Lillooet River, causing some channel aggradation that will decrease dyke freeboard and increase the likelihood of flooding over the next several years. Many tens of landslides have occurred in basins draining the Mount Meager Volcanic Complex over the past century; three of these events, each larger than 1 Mm3, have occurred in the Capricorn Creek catchment since 1998. The 2010 event is comparable in size to the 1965 Hope Slide, which is considered to be the largest historic landslide in British Columbia.

Bibliography of Canadian Geomorphology