Previously undiscovered landslide deposits in Harrison Lake, British Columbia, Canada
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K. E. Hughes I M. Geertsema I E. Kwoll I M. N. Koppes I N. J. Roberts I J. J. Clague I S. Rohland
Previously undiscovered landslide deposits in Harrison Lake, British Columbia, Canada
Abstract A bathymetric survey of Harrison Lake in southwest British Columbia revealed deposits of three large landslides on the lake floor. The blocky and flow-like surface morphology of the deposits suggests rapid emplacement from subaerial sources. The multibeam survey, together with a subbottom acoustic survey, allowed us to estimate deposit volumes of 2.4 Mm3, 1.3 Mm3, and 0.2 Mm3 for the Mount Douglas, Mount Breakenridge, and Silver Mountain landslides, respectively. The large volumes and inferred rapid emplacement of the Mount Douglas and Mount Breakenridge landslides suggest they were tsunamigenic. Because people live along the shoreline of Harrison Lake, our discovery and characterization of these landslide deposits and their tsunamigenerating potential form an important foundation for further landslide-tsunami hazard analysis in the region. Keywords Landslide . Multibeam bathymetry . Subottom profile . Tsunami . Harrison Lake . British Columbia Introduction Fjord coastlines, notably those in Canada, Alaska, Greenland, Norway, Chile, and New Zealand, are vulnerable to landslidegenerated waves (Roberts et al. 2014; Gauthier et al. 2018; Higman et al. 2018). Additionally, there is a growing awareness of the hazard posed by landslides into lakes bordered by steep, potentially unstable slopes in mountains worldwide (e.g., Roberts et al. 2014; Haeberli et al. 2017). Landslide-generated waves can be much larger than earthquake-triggered tsunamis, at least near their sources (Tappin et al. 2001; Graziani et al. 2006; Omira et al. 2019) and are common worldwide (Roberts et al. 2014). Historic events exemplify their potential magnitude. The 1934 Tafjord (Norway) rockslide generated a displacement wave that reached up to 63 m above sea level (asl), and the 1936 Lake Loen (Norway) rockslide produced a wave with a 74 m run-up (Grimstad 2005; Oppikofer et al. 2009). The 1963 Vajont rockslide (Italy) occurred in a reservoir and generated a displacement wave, which overtopped the reservoir damn by 245 m, causing an estimated 2000 casualties (Müller 1964). The 1958 Lituya Bay (Alaska) rockslide produced a wave that reached up to 517 m above sea level (Miller 1960), and the 2015 Taan Fiord (Alaska) rockslide generated a 193-m-high wave at the head of the inlet (Dufresne et al. 2018; Haeussler et al. 2018; Higman et al. 2018). In 2007, a 3 Mm3 rockslide entered Chehalis Lake, 80 km east of Vancouver, British Columbia, generating a displacement wave that reached up to 38 m above the lakeshore and ran down the 8-km length of the lake (Brideau et al. 2012; Roberts et al. 2013). The Chehalis lake event, in part, motivated our research on the exposure risk of large subaerial landslides in British Columbian lakes. Given the apparent increase in large landslides in British Columbia (BC) in recent years, due in part to loss of glacier ice and permafro
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