monitoring cornice dynamics and associated …

MONITORING CORNICE DYNAMICS AND ASSOCIATED AVALANCHE ACTIVITYWITH A TERRESTRIAL LASER SCANNER

Holt Hancock1*, Alexander Prokop1, Markus Eckerstorfer2, Christopher Borstad3, Jordy Hendrikx4

1 Department of Arctic Geology, The University Centre in Svalbard, Longyearbyen, Norway2 Earth Observation, Norut – Northern Research Institute, Tromsø, Norway

3Department of Arctic Geophysics, The University Centre in Svalbard, Longyearbyen, Norway4Snow and Avalanche Lab, Department of Earth Sciences, Montana State University, Bozeman, Montana, USA

ABSTRACT: Snow cornice accretion occurs seasonally on the edges of the plateaus above Long-yearbyen in central Svalbard, Norway. Cornice failures and associated cornice fall avalanches com-prise nearly 50% of observed avalanche activity near Longyearbyen and endanger human life and infrastructure annually. Despite the recognized hazard posed by cornices here and in other locations throughout the world, limited research on cornice dynamics exists and accurately forecasting cornice failure continues to be problematic. We monitored cornice dynamics on the slopes surrounding Long-yearbyen with terrestrial laser scanning (TLS) technology for two winter seasons (2016/2017 and 2017/2018). We observed and quantified changes to the cornice systems in detail not previously achieved. We investigated the evolution and failure of the lower cornice surfaces from scan positions underneath the cornices where accessibility has precluded previous research. Preliminary results indicate cornice accretion primarily coincides with winter storms. We observed full cornice failuresfrom the same location in both winter seasons following accretion events and downslope deformation of the main cornice mass. The results from this work provide insight into and refine our process un-derstanding of cornice dynamics and will help improve cornice fall avalanche forecasting and man-agement strategies in this and other locations where hazardous cornices exist.

KEYWORDS: TLS, Svalbard, cornice fall avalanche, ground-based LiDAR.

1. INTRODUCTION AND BACKGROUNDSnow cornices – defined as overhanging projec-tions of snow formed by wind deposition to the lee of a ridgeline or slope inflection – have long been recognized as a snow and avalanche haz-ard in irregular, mountainous terrain (Montagne et al., 1968; Seligman, 1936). Cornices pose an avalanche hazard when they fail, detach from their anchoring slope, and trigger slab ava-lanches on the slope below (e.g. Vogel et al., 2012). Despite the widespread use of both ex-plosives and structural defenses to mitigate the hazards associated with cornice failure (e.g. McCarty et al., 1986; Montagne et al., 1968), relatively little research exists on cornice dynam-ics. Furthermore, forecasting cornice failure con-tinues to be problematic in areas lacking struc-tural defenses or other active mitigation strate-gies. Early cornice research summarized by Vogel et al. (2012) focused on qualitative de-scriptions of cornice formation processes and resulting cornice structures (e.g. Montagne et al., 1968). Later studies more quantitatively in-vestigated mechanisms by which individual snow crystals adhere during cornice accretion (Latham and Montagne, 1970), the physical

snow characteristics at various structural loca-tions on individual cornices (Naruse et al., 1985), and monitoring cornice fall avalanches with time-lapse photography (Munroe, 2018).

Cornices play a key role in the avalanche regime in central Spitsbergen, the largest island in thehigh-Arctic Svalbard archipelago. Here, snow is transported by the wind across the broad, hori-zontal expanses of the region’s plateau moun-tains and accumulates annually as cornices on the edges of these plateaus. Avalanches related to cornice failure (hereafter: cornice fall ava-lanches) comprised the most observed ava-lanche type in recent regional studies (Ecker-storfer and Christiansen, 2011).

Within this context, Vogel et al. (2012) observed cornice processes on a mountain slope near Longyearbyen in central Spitsbergen, also the location of this study (Figure 1). Their results indicated cornice accretion occurs during or immediately after winter storm events when wind speeds were in excess of 10 ms-1, typically from a southeasterly direction perpendicular to the ridgeline. Smaller cornice failures were clustered in June near the end of the snow season and coincided with increasing air temperatures. However, less frequent failures in the earlier part of the snow season often involved the entire cornice mass and resulted in some of the largest cornice fall avalanches observed in the study.

* Corresponding author address: Holt Hancock, Department of Arctic Geology, The University Centre in Svalbard,Longyearbyen, Norway, N-9171; tel: +47 79 02 64 21email: [email protected]

Proceedings, International Snow Science Workshop, Innsbruck, Austria, 2018

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Figure 1: The shaded red area of the map in panel (a) indicates the study location. This same area is out-lined in red in the oblique photograph of Gruvefjellet’s western aspect in panel (b), taken from the TLS scan-ning location. The automated weather stations employed in this study are indicated in panel (a), and the black rectangle in panel (b) corresponds to the area highlighted in Figure 3c.

Terrestrial laser scanning (TLS) is an active remote sensing technology with documented applicationsincluding mapping snow depth and snow depth change (e.g. Deems et al., 2013) and observing avalanche activity to retrieve parameters for dynam-ic avalanche modeling (Prokop et al., 2015). De-spite its increasing use in snow avalanche research, to our knowledge TLS has not been previously em-ployed to specifically observe cornice processes.

We monitored cornice accretion, deformation, fail-ure, and associated avalanche activity on the west-ern aspect of Gruvefjellet (Figure 1) with TLS tech-nology over the past two winter seasons (2016/2017 and 2017/2018). Our primary objectives were to:

1) Illustrate cornice development, failure, and associated avalanche activity using the high spatial resolution snow surface data ac-quired via TLS, and;

2) Investigate the topographical and meteoro-logical controls on cornice dynamics and resulting avalanche activity.

The present study thus explicitly builds on the work of Vogel et al. (2012) with TLS data to refine our understanding of cornice processes in this and oth-er locations where cornices present an avalanche hazard.

2. METHODS AND DATA SOURCES

2.1 Terrestrial laser scanning (TLS) and post-processing

The primary dataset unique to this work consists of high spatial resolution snow surface data obtained using TLS. We scanned the western slope of Gruvefjellet 24 times over both seasons. We scanned with a Riegl® VZ-6000 laser scanner from

the same location across Longyeardalen from Gruvefjellet with good visibility of the main cornices throughout the 2016/2017 and 2017/2018 winter seasons (Figure 1). We emphasized scanning of pre- and post-event snow surfaces for major storm and avalanche events (Figure 2). We georeferenced and positioned scans using a DGPS and an iterative closest point algorithm in RiSCAN Pro, Riegl’s pro-prietary processing software. Equipment specifica-tions and initial data processing details are further described in Hancock et al. (2018).

Following initial georeferencing and resampling to a 0.10 m grid, we imported individual point clouds into CloudCompare (CloudCompare v2.10, 2018),where we extracted point cloud sections along manually defined axes using the polyline extraction tool. We then manually digitized lines connecting the points in the extracted sections in the ArcScene 3D Editing environment to construct the vertical cornice profile schematics displayed in Figure 3. Broader slope-scale snow distribution maps (e.g. Figure 4) were developed in ArcGIS by computing the difference in vertical distance between digital elevation models representing snow or bare ground surfaces.

2.2 Automated snow and weather dataWe obtained wind and air temperature data from the Gruvefjellet automated weather station (AWS) andprecipitation data from the Svalbard Airport AWS. The Gruvefjellet AWS is located less than 0.5 km east of the study site at 464 m – the approximate elevation of the cornices. The Airport AWS is situat-ed 5.5 km northwest (Figure 1) at 28 m. Further description of these sites is provided in Hancock et al. (2018).


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Figure 2: Meteorological summaries and timing of TLS observations during the 2016/2017 and 2017/2018cornice seasons. Daily average wind speed (scalar) and air temperature are from the Gruvefjellet AWS. Daily precipitation data (taken at 0600 UTC) is from the Svalbard Airport AWS. Vertical lines indicate the timing ofthe 24 TLS observations, with colored lines corresponding to the cross-sectional profiles in Figure 3 and grey lines representing other TLS observations.

3. RESULTSMore active weather patterns characterized the 2016/2017 winter season compared to the 2017/2018 season (Figure 2). We thus scanned much more frequently during the 2016/2017 winter season than during the 2017/2018 season, as long periods without winter storms resulted in relatively static snow surface conditions through much of the 2017/2018 winter. Additionally, the laser scanner was damaged and rendered unusable in late April 2018, preventing further scanning activity in the late snow season.

TLS observations from each winter season captured the development and full failure of the cornice indi-cated in Figures 1b and 3c, with resulting avalanche activity on the slope below (Figure 4). Here, we employ these TLS data to demonstrate a methodol-ogy for better clarifying and quantifying cornice pro-cesses while providing some valuable illustrative case studies as examples.

We documented full cornice failures that released small slab avalanches in both winter seasons, on 2017/04/10 and 2018/03/18, respectively. In both cases, almost the entire cornice released from an identical location in the cornice system. Cross-sectional profiles (Figure 3a and 3b) derived from TLS data illustrate the development and failure of this cornice through the course of both winters.

3.1 2016/2017Minimal early season snowfall limited cornice growth in November and December 2016 before a series of storms in late December and early January resulted in considerable cornice accretion by 2017/01/22. Over the next several weeks, the cor-nice’s leading edge rounded and developed the roll face observed on 2017/02/14. Maximum observed cornice extension was reached following a period of enhanced cornice accretion between 2017/02/14 and 2017/02/24. By 2017/03/21, this main cornice mass had again deformed downslope with a con-siderable overhanging roll face evident in the TLS data.


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Figure 3: Cross-sectional profile schematics of the cornice where we observed full cornice failures for select-ed scan dates during the (a) 2016/2017 and (b) 2017/2018 winter seasons. Dashed lines indicate interpola-tion where roll faces shadowed the roll cavity and scarp. Vertical profile schematics in (a) and (b) are from the location indicated in (c) and Figure 1.

Cornice failure occurred on 2017/04/10 following several days of increasing air temperatures and a light snowfall on 2017/04/08, with a cornice block detaching and triggering a small slab avalanche on the slope below. A clear cornice accretion meteoro-logical signal did not precede this event – wind speeds averaged below 7 ms-1 throughout the prior 72 hours and the Airport AWS recorded minimalprecipitation. However, TLS data from adjacent cornices that did not fail show some cornice accre-tion and horizontal extension of the main corniceoccurred prior to failure. Considerable downslope deformation of the main cornice masses also coin-cided with increasing temperatures in the weeks preceding failure. Minimal further changes were observed to the resulting failure face prior to melting in mid-June 2017.

3.2 2017/2018Considerable cornice accretion resulted from snow-falls in early December 2017, with maximum cornice extension for the 2017/2018 season observed on 2018/01/24. Roll face development and downslope cornice creep occurred over the next week (ob-served on 2018/01/31) and progressed slowly with only minimal additional accretion through 2018/03/02. Cornice failure on 2018/03/18 was pre-ceded by a winter storm; negative air temperatures combined with precipitation and mean wind speeds approaching 10 ms-1 from an easterly direction inthe 48 hours prior to the failure resulted in a period of sustained snow drift and wind loading on western aspects.

4. DISCUSSIONDifferentiating between accretion-induced mid-season failures and temperature-induced failures

clustered near the end of the snow season (e.g. Munroe 2018) is a necessary step in developing a strategy for forecasting cornice failure. Cornice ac-cretion seems to coincide with periods of snow drift when wind speeds are sufficient to mobilize snow particles on the surface. The challenging task of adequately characterizing snow available for transport is thus a critical component of forecasting cornice accretion and mid-season cornice failures, while observing downslope deformation and tem-peratures changes are necessary for attempting to predict temperature-induced failures. Furthermore,identical failure locations in both seasons suggest adegree of topographic control on cornice failure, also recognized by Vogel et al. (2012).

Slope-scale snow distribution patterns from TLS data also provide insight into avalanche activity on the western slope of Gruvefjellet following cornice failure (Figure 4). These data are illustrative of cor-nice fall avalanche dynamics in this location wherethe main cornice mass fails, removes snow from the bedrock face below as it descends, and then finally impacts the snow surface in start zones at the base of the bedrock face. The snow conditions in the starting zones at the base of bedrock face in combi-nation with the detached cornice mass help to de-termine the resulting avalanche size by entraining snow from an impact crater or triggering a larger slab avalanche. In both observed cornice failures, propagation on the slope below was limited to the immediate area surrounding the initial cornice im-pact, although more widespread propagation and larger avalanches have been observed previously (e.g. March 2009). In both observed events in our data, the detached cornice blocks remained largely intact throughout the avalanching process and ran further than the main avalanche mass, in line with previous observations.


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Figure 4: Overview of the change in snow height for the period 2018/03/02 – 2018/03/23, illustrating the ef-fects of multiple cornice fall avalanches on 2018/03/18 on Gruvefjellet’s western aspect indicated in Figure 1b. The inset highlights the slope immediately surrounding the cornice surfaces shown in Figure 3, the cor-nice impact crater, and the small slab avalanche release in the starting zone at the base of the bedrock face.

5. CONCLUSIONS AND FUTURE WORKIn this study we presented preliminary results from a two season TLS field campaign investigating cornice processes in high-Arctic Svalbard, Norway.Cross-sectional profiles extracted from TLS snow surface data illustrate changes to the cornices including accretion events, downslope deformation, and full cornice failures. Preliminary results indicate cornice accretion primarily coincides with winter storms and periods of snowdrift. Considerable downslope cornice deformation and a cornice accretion event preceded observed full cornice failures in both winters.

Continued research will entail attempting to link storm snow, mid-season cornice failures (as op-posed to late season, temperature-induced failures) to cornice accretion during snow storms. We will also work to display changes to the cornice systems in 3D. The conclusions from this work will hopefully improve our process understanding of – and thus ability to forecast for and manage hazards associat-ed with – cornice failures in this and other environ-ments.

REFERENCESCloudCompare (version 2.10) [GPL software]. 2018: Retrieved

from http://www.cloudcompare.org/

Deems, J.S., Painter, T.H. and Finnegan, D.C., 2013. Lidar measurement of snow depth: a review. Journal of Glaciology, 59(215): 467-479.

Eckerstorfer, M. and H. H. Christiansen, 2011: Topographical and meteorological control on snow avalanching in the Long-yearbyen area, central Svalbard 2006–2009. Geomorpholo-gy, 134(3): 186-196.

Hancock, H., A. Prokop, M. Eckerstorfer, and J. Hendrikx, 2018:Combining high spatial resolution snow mapping and mete-orological analyses to improve forecasting of destructive ava-lanches in Longyearbyen, Svalbard. Cold Regions Science and Technology, 154: 120-132.

Latham, J. and J. Montagne, 1970: The Possible Importance of Electrical Forces in the Development of Snow Cornices. Journal of Glaciology, 9(57): 375-384.

McCarty, D., R. Brown, and J. Montagne, 1986: Cornices: their growth, properties, and control, International Snow Science Workshop, Lake Tahoe, CA, USA, pp. 41-45.

Montagne, J., J. McPartland, A. Super, and H. Townes, 1968:The Nature and control of snow cornices on the Bridger Range, Southwestern Montana, Alta Avalanche Study Cen-ter, Miscellaneous Report No. 14.

Munroe, J.S., 2018. Monitoring snowbank processes and cornice fall avalanches with time-lapse photography. Cold Regions Science and Technology, 154: 32-41.

Naruse, R., H. Nishimura, and N. Maeno, 1985: Structural Char-acteristics of Snow Drifts and Cornices. Annals of Glaciology, 6: 287-288.

Prokop, A., Schön, P., Singer, F., Pulfer, G., Naaim, M., Thibert, E. and Soruco, A., 2015. Merging terrestrial laser scanning technology with photogrammetric and total station data for the determination of avalanche modeling parameters. Cold Regions Science and Technology, 110: 223-230.

Seligman, G., 1936: Snow structure and ski fields: being an account of snow and ice forms met with in nature and a study on avalanches & snowcraft. Macmillan, 555 pp.

Vogel, S., M. Eckerstorfer, and H. H. Christiansen, 2012: Cornice dynamics and meteorological control at Gruvefjellet, Central Svalbard. The Cryosphere, 6(1): 157-171.


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