A Cautionary Note for Rock Avalanche Field Investigation – Recent Sequential and Overlapping Landslides in British Columbia

Canadian Geotechnical Journal A cautionary note for rock avalanche field investigation – recent sequential and overlapping landslides in British Columbia Journal: Canadian Geotechnical Journal Manuscript ID cgj-2019-0751.R2 Manuscript Type: Note Date Submitted by the 06-Mar-2020 Author: Complete List of Authors: Geertsema, Marten; British Columbia Ministry of Forests Lands and Natural Resource Operations; University of Northern British Columbia Bevington, Alexandre; British Columbia Ministry of Forests Lands and Natural ResourceDraft Operations; University of Northern British Columbia Keyword: landslides, rock avalanche, field investigation, glacier, British Columbia Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? : https://mc06.manuscriptcentral.com/cgj-pubs Page 1 of 11 Canadian Geotechnical Journal 1 A cautionary note for rock avalanche field investigation – recent sequential 2 and overlapping landslides in British Columbia 3 Marten Geertsema1,2, Alexandre Bevington1,2 4 5 1 British Columbia Ministry of Forests, Lands, Natural Resource Operations and Rural Development, Prince George, V2L 1R5, 6 Canada; 7 2 University of Northern British Columbia, Prince George, V2N 4Z9, Canada; 8 9 Abstract 10 11 Large rock avalanches on glaciers are an annual occurrence in the mountains of western North America. 12 Following an event, landslide investigators may strive to quickly arrive on site to assess the deposit. 13 Satellite remote sensing imagery demonstrates that caution is warranted for on-site field assessments. 14 We combine Landsat, Sentinel-1(radar), Sentinel-2 and Planet imagery to reconstruct the events of four 15 recent overlapping rock avalanche deposits in British Columbia. In our examples substantial rock 16 avalanches are closely followed (days - months) and buried by much larger landslides. We suggest that 17 landslide investigators exercise caution when assessing fresh rock avalanche deposits in the field. 18 Keywords: landslides, rock avalanche, fieldDraft investigation, glacier, British Columbia 19 Résumé 20 Les grandes avalanches de rocheuses sur les glaciers sont une occurrence annuelle dans les montagnes 21 de l'ouest de l'Amérique du Nord. À la suite d'un événement, les enquêteurs peuvent s'efforcer d'arriver 22 rapidement sur place pour évaluer le glissement. L'imagerie par télédétection par satellite démontre 23 qu'une prudence est justifiée. Nous combinons les images Landsat, Sentinel-1(radar), Sentinel-2 et 24 Planet pour reconstituer les événements de quatre récentes de double avalanches rocheuses en 25 Colombie-Britannique. Dans nos exemples, les avalanches de rocheuses initiales sont suivies de près 26 (jours - mois) et ensevelies par des glissements de terrain beaucoup plus importants. Nous suggérons 27 aux enquêteurs de glissement de terrain de faire preuve de prudence lorsqu'ils évaluent les avalanches 28 de roches fraîches sur le terrain. 29 30 31 Introduction 32 33 Large landslides in western North America appear to be on the increase (Evans and Clague 1994; 34 Geertsema et al. 2006; Huggel et al 2011; Cloutier et al. 2017; Coe et al. 2018; Hibert et al. 2019) 35 especially in recently deglaciated terrain (Holm et al. 2004; Deline et al. 2015). Increases in landslide 1 https://mc06.manuscriptcentral.com/cgj-pubs Canadian Geotechnical Journal Page 2 of 11 36 occurrence may call for increased field assessments. In many instances rapid response mapping and 37 evaluation of landslides may include field investigation (Lerouil et al. 2006; Marconi et al. 2014; Collins 38 and Jibson 2015). In many cases it is desirable to examine and describe fresh rock avalanche deposits, 39 especially where the deposit is subject to rapid change due to the potential melting of a snowy and icy 40 matrix, as is the case for many ice-rock avalanches on glaciers (e.g. Dufresne et al. 2019). 41 42 Accounts of multiple surges of debris flows are well reported (e.g. Davies 1990; Kean et al. 2013), even 43 those triggered by rock slope failure (Walter et al. 2019). For sensitive clay landslides in Quebec, Locat 44 et al. (1984) and Locat and Leroueil (1997), showed with multi-temporal airphotos, that what appeared 45 to be single landslide morphologies, were actually the deposits of separate events. Sequential rock 46 avalanches from the same source area have also been observed, with examples provided by Eberhardt 47 et al. (2004) at Randa, Switzerland (two events between 18 April and 9 May 1991), and, through seismic 48 detection and remote sensing methods, by Ekstrom and Stark (2013) at Siachen Glacier, Pakistan (7 49 events between 6-12 September 2010). There are also examples of much longer timespans between 50 landslides originating in the same source areas.Draft A recent example comes from two landslides associated 51 with glacial thinning at Taan Fiord Alaska - separated by some two decades (Dufresne et al. 2018). 52 53 Here we provide a short account of four recent examples of sequential rock avalanches (Figure 1) that 54 originated from steep, ice-capped rock walls (Figure 2) in British Columbia. In all these examples the 55 second landslide deposit was longer, and the deposits covered more area than the first. In three of the 56 cases, landslide deposits more than doubled in length, and increased up to almost an order of 57 magnitude in area. While much landslide research centers on risks to and fatalities of local populations 58 (Guzzetti 2000; Nadim et al. 2006; Petley 2012; Kirschbaum et al. 2015; Blais-Stevens et al. 2018), we 59 intend this to be a cautionary note to rapid-response landslide investigators themselves. 60 61 Methods 62 63 In this study, we identified rock avalanches on glaciers discovered through satellite image browsing, 64 helicopter flights, and media reports (Mitchell et al. 2019; Friele et al. 2020). We reviewed satellite 65 archives to refine the timing and geometry of the events, taking advantage of the rapidly growing 66 archive of medium to high resolution multispectral optical and synthetic aperture radar (SAR) data in 67 recent years (Bevington et al. 2018), using Landsat, ASTER, Sentinel-1, Sentinel-2, and RapidEye and 2 https://mc06.manuscriptcentral.com/cgj-pubs Page 3 of 11 Canadian Geotechnical Journal 68 PlanetScope Dove images (Planet Team 2017). We mapped elevations and travel angles using a 69 provincial digital elevation model (TRIM). 70 71 72 Draft 73 74 Figure 1. Overview map of four recent sequential rock avalanches (red dots) on glaciers in British Columbia (QG - 75 Quanstrom Glacier, NC – North Canoe, KK - Klinaklini, JP - Joffre Peak). 76 77 78 Results 79 3 https://mc06.manuscriptcentral.com/cgj-pubs Canadian Geotechnical Journal Page 4 of 11 80 The four rock avalanche sites (Figure 1; Table 1) all occurred at high elevations (1750 – 2600 m asl), 81 initiating from steep rock slopes (ranging from 55-70o). Bedrock geology varies between the sites - from 82 diorite and orthogneiss for Joffre and Klinaklini, to metasedimentary rock for North Canoe and 83 Quanstrom Glacier (https://maps.gov.bc.ca/ess/hm/imap4m/). We suspect all events were likely a 84 combination of rock and ice/avalanches as confirmed for the Joffre Peak events by Friele et al. (2020) 85 which show glacier ice in main scarps (Figure 2) as well as in the deposits. The other sites also showed 86 ice-covered cliffs on satellite imagery. 87 88 The Mount Quanstrom and North Canoe Glacier rock avalanches were the first to occur in our small 89 dataset. Both happened in 2016 in east-central British Columbia with the smaller event followed 90 approximately a month later (Table 1) by a much larger event (Table 2; Figure 3). Respective deposit 91 area increased by 4.8 and 5.1 times between the first and second events. Similarly, runout length 92 increased by factors of 2.4 and 4.6 for Mt. Quanstrom and North Canoe’s first and second landslides. We 93 measured travel angles by measuring the slopeDraft angle between the crown and tip along the length of the 94 central path of the landslide as described by Cruden and Geertsema (2008). At Mt. Quanstrom’s first 95 slide had a travel angle of 19.5o but the second larger landslide had a travel angle of 14.2o. The first 96 landslide at North Canoe had a travel angle of 17.9o but the second, larger landslide had a lower angle of 97 15.9o, respectively - reductions of 5.3o and 2.0o. The North Canoe events are the only two to come from 98 completely different sources, but still have overlapping deposits (Figure 3). 99 The Klinaklini Glacier rock avalanches occurred in 2017 and 2018, approximately one year apart (Table 100 1). Here the deposit area increased by a factor of 7.9, while the runout length more than doubled with 101 an increase of 2.6 times between the first and second events (Table 2; Figure 3). Travel angles fell from 102 an initial 31.8o to 20.0o for the second event – a reduction of 11.8o over an increased travel distance of 103 1.15 km 104 The largest landslides in our dataset occurred at Joffre Peak in southeastern BC (Figures 1-3) in May 105 2019. These rock/ice avalanches occurred only three days apart as confirmed by imagery (Table 1) and 106 seismic analysis (Friele et al. 2020). This case also differs because the Joffre rock avalanches travelled 107 beyond glaciers into forested terrain, and the second event triggered a debris flood, which extended the 108 reach of the landslides (Friele at al. 2020). Here landslide area increased only modestly by 1.1 times, but 4 https://mc06.manuscriptcentral.com/cgj-pubs Page 5 of 11 Canadian Geotechnical Journal 109 the length, due to the debris flood, increased by a factor of 1.5, reducing the travel angle from 16.7o to 110 12.4o (Friele et al.

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