Canadian Geotechnical Journal

A cautionary note for rock avalanche field investigation – recent sequential and overlapping landslides in

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

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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 ; 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 . 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

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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

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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

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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 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

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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. 2020) – a reduction of 4.3o over an increased travel distance of 1.8 km

111 Table 1: Landslide timing windows Event Date (dd-mm-yy) and Imagery Source (abbrev.)

Latitude, Elevation (m asl) Pre 1st event Post 1st event Pre 2nd event Post 2nd event Longitude

Mt Quanstrom 52.95 ~2400 18-08-16 (RE) 24-08-16 (PS3) 13-09-16 (S2) 27-09-16 (RE) -120.18 13-09-16 (RE) 07-10-16 (S1D)

North Canoe Glacier 52.76 ~1750 24-11-16 (S1) 01-12-16 (S1) 25-12-16 (S1) 30-12-16 (S1) -119.67

Klinaklini Glacier 51.60 ~2500 26-07-17(AST) 03-08-17 (S1) 14-08-18 (PS3) 15-08-18 (PS3) -125.93 14-08-18 (S1D) 26-08-18 (S1D)

Joffre Creek 50.36 ~2600 12-05-19 (PS3) 13-05-19(PS3) 15-05-2019 (L8) 16-05-2019 (S1) -122.41

112 Abbreviations: S1 = Sentinel-1, S2 = Sentinel-2, PS3 = PlanetScope (3 Band), RE = RapidEye, AST = Aster 113 Draft

114 115 Figure 2. experienced two overlapping rock avalanches occurring three days apart in May 2019 116 (Mitchell et al. 2019; Friele et al. 2020). The earlier event is represented by the dashed yellow line in the frame on 117 the right. Note the cliff-top glacier at the crown of the landslides. See Figure 1 for location. Photos by Tom Millard, 118 Province of British Columbia. 119 120 121

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122 Table 2: Landslide metrics Landslide Metrics

1st event 1st event 1st event 2nd event 2nd event 2nd event Travel angle (o) Travel distance Area (ha) Travel angle (o) Travel distance Area (ha) (km) (km)

Mt Quanstrom 19.5 0.73 11.5 14.2 1.78 55

North Canoe Glacier 17.9 0.72 17.1 15.9 3.28 87

Klinaklini Glacier 31.8 0.73 8.2 20.0 1.88 65

Joffre Creek 16.7 4.0 1330 12.4 5.8 1430

123 124

Draft

125 126 127 Figure 3. Planform outlines of four sequential and overlapping rock avalanches in British Columbia drawn to scale 128 and oriented north. Both the first and second events, the year of the events, the time between the first and

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129 second events, and the movement directions are shown. See Figure 1 for locations. Note that the two North 130 Canoe events, despite having different source areas, still overlap. 131 132 Discussion/Conclusion

133 These rock avalanches initiated on steep rock walls (Figure 2) above glaciers that have experienced 134 thinning (Bolch et al. 2010; Menounos et al. 2019) and are likely to have mountain permafrost (Gruber 135 2012a,b; Hasler and Geertsema 2013). Based on field investigation (Joffre) and /or high resolution 136 satellite imagery all the landslides crowns were capped with ice from hanging glaciers.Perhaps these 137 high elevation environments undergoing cryospheric change are especially prone to landslides (Deline et 138 al. 2015; Cloutier et al 2017; Coe et al. 2018; Hock et al 2019). 139 140 The second landslides at each of the four locations were up to 4.6 times longer, covered up to 7.9 times 141 more area, and experienced reductions of 2.0o – 11.8o in travel angles (Table 2, Figure 3) than the first, 142 large precursory landslides. Time lags betweenDraft the first and second landslides ranged from as long as 143 one year to as short as three days. The travel angles fit within the ranges of rock slides on glaciers 144 reported for the Canadian Cordillera by Geertsema and Cruden (2008, 2014). The largest of the 145 landslides at Joffre Peak (Mitchell et al. 2019; Friele et al. 2020) travelled beyond the glacier limits into a 146 forested valley. Though similar in size, the second rock avalanche transformed into a channelized debris 147 flood and thus extended the reach of the combined landslide event (Friele et al. 2020). The other three 148 rock avalanches did not undergo flow transformation and remained on glaciers. 149 150 Our dataset is very small, but these four groups of rock avalanches could represent a larger population, 151 because our work is based on a cursory, rather than an exhaustive survey. In addition, underreporting 152 can arise from limitations of detectability including low image resolution, or cloud and snow cover. High 153 mountain rock avalanches are often difficult to detect from optical imagery alone as they may be quickly 154 obscured by seasonal snow. In some cases they can also remain snow covered for years. Nonetheless, 155 our ability to detect these events has increased with the wide availability of free satellite image archives 156 (e.g. Bevington et al. 2018; Zhu et al. 2019). We suggest that a subset of other previously reported rock 157 avalanches may also be a product of multiple events. 158 159 While we were able to discern larger subsequent movements because of the contrast with landslide 160 debris on snow, it would be more difficult to detect a smaller second failure emplaced on an existing

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161 deposit. An example of a large rock avalanche followed by a second smaller one (2 hours later) occurred 162 at Lamplugh Glacier in Alaska (Dufresne et al. 2019). These two events were identified by seismic traces 163 and then corroborated through a careful morphological and sedimentological investigation of the 164 deposit. 165 166 Even though this dataset of overlapping rock avalanches is too small to be statistically reliable, our 167 findings do show that a second overlapping event may occur, and therefore caution is warranted during 168 field investigations. 169 170 We recommend three things: 171 172  That caution is exercised when conducting a rapid-response ground investigation of rock 173 avalanches on glaciers and that such cautions be expressed in professional guidelines (Couture 174 et al. 2013; APEGBC 2010). This may include monitoring or assessments of unstable source areas 175 prior to field investigation of the depositsDraft and limiting the duration of visits on site. 176  That further seismic inversion and remote sensing be used to inventory multiple landslides with 177 the same source areas or overlapping deposits. 178  That further research is conducted into why large rock avalanches are sometimes preceded by 179 smaller scale rock avalanches. This includes investigations of the role of glaciers in steep 180 mountain environments, the role of permafrost degradation, and other factors. 181

182 Acknowledgements

183 We thank PlanetLabs for access to imagery through academic licences. We thank three anonymous 184 reviewers for suggestions that improved our manuscript.

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