The Scottish Parliament and Scottish Parliament Infor mation C entre l ogos .

SPICe Briefing

Avalanches in Scotland 8 December 2010

10/91

Alasdair Reid

This short briefing provides some basic information about avalanches in Scotland. Following a period of persistently cold and snowy weather, a number of news sources are reporting avalanches and the potential danger of unstable snow conditions, even in areas where they have not commonly occurred in the past.

Figure 1: Unstable cornices and avalanche debris in Coire an Lochan, Aonach Mor.

Photo: SAIS

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CONTENTS

INTRODUCTION .......................................................................................................................................................... 3

WHAT IS AN AVALANCHE, AND HOW DOES IT OCCUR? ..................................................................................... 4

RESEARCH, FORECASTING, TRAINING AND RESCUE ......................................................................................... 6

ROOF SNOW LOADING .............................................................................................................................................. 7

SOURCES .................................................................................................................................................................... 8

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INTRODUCTION

Avalanches can happen wherever there is a build up of snow lying on ground of sufficient angle, and they occur in Scotland’s highlands every year, as well as in the English Lakes, the Cheviots, the Pennines and Wales. The Scottish Avalanche Information Service (SAIS) provides forecasts between December and April.

The number of people accessing the mountains each winter is hard to determine, however in 2009-10 the SAIS website had over 420,000 hits, and a survey of 300 users showed that they had had 9000 hill days that year. The number of hill-goers is thought to be rising significantly each year. Whilst climbers, hill walkers and ski-mountaineers are expected to be self reliant and personally responsible in winter conditions, recent significant snowfall in Scotland’s cities and lower lying rural areas has increased the risk of snow slides from roofs and the possibility of avalanche activity in areas where they are unlikely to have been previously recorded. This means those with no knowledge or experience of snow structure and stability may come into danger (SAIS 2010a).

In late February 2009, four avalanches from the slopes of Beinn Odhar closed the West Highland Line between Tyndrum and Bridge of Orchy, and the risk of further slides prevented it from being cleared for over a week. When snow blowing equipment was finally brought in, 15,000m3 of avalanche debris had to be removed before the line could be reopened. Avalanches also affected sections of the A9, and the SAIS noted that “if future winters were the same then avalanches should be considered a serious threat to the country's transport network” (BBC 2009a & 2009b).

Figure 2: One of four avalanches blocking the West Highland Line in February 2009.

Photo: Network Rail

Recent media coverage of dramatic snow fall from an office roof in Edinburgh (BBC 2010a), coupled with an avalanche warning being issued jointly by Edinburgh City Council, SAIS and the Pentland Hills Ranger Service for all hills in the Edinburgh area (including Pentlands, Lammermuirs, Moorfoots, Lomond Hills and Bathgate Hills) (ukclimbing.com 2010) alongside reports of a school-girl being crushed by snow and ice which fell from the roof of a farmhouse in Berwickshire (BBC 2010b) mean that the dangers of an unstable snow-pack to people and infrastructure have become increasingly of interest.

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WHAT IS AN AVALANCHE, AND HOW DOES IT OCCUR?

An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes part or all of the snow pack to release.

Snow can be deposited in successive layers as the winter progresses. These layers may have dissimilar physical properties and an avalanche occurs when one layer slides on another, known as a surface avalanche, or the whole snow cover slides on the ground, known as a full depth avalanche. Avalanches may be of loose snow, starting at a single point; or a slab avalanche which occurs when an area of more cohesive snow separates from the surrounding snow and slides out. Typically, occurring in mountainous terrain, an avalanche can mix air and water with the descending snow, and powerful ones are capable of moving rocks, trees and other material (Langmuir 1995).

In Scotland, avalanche risk is classified on a scale of low to very high, as follows (SAIS 2010b):

There are a number of significant factors that are taken into account when determining avalanche risk. These are (Barton and Wright 1985, and Langmuir 1995):

Weather: The evolution of the snowpack is entirely dependent on weather conditions; these are therefore critical in determining where and when avalanches are likely to happen. Scotland’s maritime climate means that certain meteorological occurrences play a significant role in determining avalanche risk, not least the effects of wind, rain, hail, and a cycle of rapid freezing and thawing on the snow.

Even a light breeze can contribute to a rapid accumulation of snow on sheltered downwind slopes (windslab), whilst wind pressure at a favourable angle can stabilise other slopes. Windslabs are particularly fragile and brittle; they are heavily loaded and poorly bonded to their adjacent layers, and often form in Scotland.

Snowstorms and rainstorms are also important contributors to avalanche danger. Heavy snowfall will cause instability in the existing snow pack, both because of the additional weight and because the new snow has insufficient time to bond to underlying snow layers. Rain has a similar effect. In the short-term, rain causes instability because, like a heavy snowfall, it imposes an additional load on the snow pack; and, once rainwater seeps down through the snow, it acts as a lubricant, reducing the natural friction between snow layers that holds the snow pack together. Most avalanches happen during or soon after a storm.

Snowpack: Observation of where and how the snow is lying, including evidence of recent avalanche activity, the main snow accumulation zones, and fresh loading by new and drifting snow are important in assessing the risk of avalanche. Risk of avalanche can be further

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assessed by digging a pit to examine the layers of snow, and performing a shear test to assess how well bonded they are (Forest Service National Avalanche Centre 2009a and 2009b).

Cornices are overhanging edges of snow on a ridge or at the top of a cliff. They form by wind blowing snow over the edge onto the leeward side. Cornices often collapse, and are therefore extremely dangerous to walk on or under. When they break, they often trigger larger avalanches that permeate several snow layers. Cornices are particularly vulnerable to collapse during snow storms or heavy drifting and between 24 - 48hours afterwards, as well as during heavy thaw or sudden temperature rises.

Terrain: Most large slab avalanches occur on slopes between 25 and 45 degrees. This range includes the average angle of the wall at the back of many corries. Smooth ground such as rock slab and grass is also pre-disposed to full-depth avalanches. Rough ground such as large boulders will tend to anchor base layers in position, making avalanches less likely. Once these boulders are covered, however, surface avalanche activity is unhindered. Convex slopes are generally more hazardous than uniform or concave slopes. The point of maximum convexity is a frequent site of tension fractures, with the release of slab avalanches.

The hill ranges of the Southern Uplands, the Pentlands, the Ochils, and the Campsies, have many steep grassy slopes. When it snows, a poorly bonded layer is likely to form with the underlying ground, and human triggered or natural avalanches may occur. Figure 3 shows various types of avalanches:

Avalanche debris including the remains of trees, March

2010; Creag Meagaidh (Photo: SAIS).

Full depth avalanche from the great slab Coire an

Lochain, Cairngorms, April 2000 (Photo: Angus

Robson).

Full depth avalanche from steep grassy slope,

December 2010; Wood Hill, Ochils (Photo: Bob Easson)

Massive snow boulder, debris from an avalanche visible

from A93 south of Glenshee Ski Centre. Estimated

mass of 80 tonnes, April 2010 (Photo: SAIS)

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RESEARCH, FORECASTING, TRAINING AND RESCUE

The SAIS, established in 1993, provides daily forecasts at five key climbing areas1, covering 30% of the highlands, between mid-December and April each year. In early and late season, it also provides weekend reports, and in 2009-10 it received £121,700 of Scottish Government funding through sportscotland (Scottish Government 2009).

The SAIS, in their Report for Winter 2009-10 (2010c) show that they have also worked with a number of non-mountaineering agencies and groups; including SEPA to help assess the amount of water being retained by snow to enable flood planning; the Met Office to provide snow samples from which the environmental risk of volcanic ash could be evaluated; and Network Rail to assess the risk of further avalanches to the West Highland Line. In this instance, explosive charges were used to control the slope. This is thought to be the first time explosives have been used to safeguard infrastructure from avalanche threat in the UK.

In 2001, the Snow and Avalanche Foundation of Scotland was founded. This is an expert avalanche and snow science advisory group, whose role is to improve and formalise the current knowledge and research available as well as provide information to the Scottish Avalanche Information Service (Scottish Government 2001).

The Mountain Weather Information Service (MWIS) produces forecasts for eight UK mountain areas. The MWIS receives £42,300 annual government funding through sportscotland, and covers Scotland's five main upland areas (Scottish Government 2009).

The Mountaineering Council of Scotland (MCofS) was founded in 1970, and represents hill walkers, climbers and ski-tourers who live in Scotland or who enjoy Scotland’s mountains. It provides advice on winter safety and in 2010-11 received £155,000 of sportscotland funding (MCofS 2009).

A number of specific courses for avalanche awareness and winter mountaineering skills are available in Scotland. Glenmore Lodge near Aviemore runs subsidised courses, and is sportscotland’s National Outdoor Training Centre; Plas Y Brenin, sportengland’s equivalent (based near Ballachulish during winter months) does the same. Other relevant organisations include the Association of Mountain Instructors, and the British Mountain Guides.

Twenty-eight teams make up the Mountain Rescue Committee of Scotland (MRC of S), ranging from Galloway to Assynt. These mountain rescue teams are primarily made up of civilian volunteers who are on call 24 hours a day throughout the year to locate and rescue those in need. There are also some police and RAF units, e.g. Grampian Police, Tayside Police and at RAF Leuchars and Kinloss. One of the key functions of the Committee is to collate and analyse mountain accident information in Scotland. It produces an annual report listing each and every incident (by Region) in Scotland, which is published in the Scottish Mountaineering Club Journal. The MRC of S has charitable status and is funded entirely through voluntary contributions. Currently, the Order of St. John is funding vehicles for all teams.

1 Creag Meagaidh, Glen Coe, Lochaber, Northern Cairngorms and Southern Cairngorms

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ROOF SNOW LOADING

A quantity of snow lying, poorly bonded to a slope, at risk of sliding off onto pavements or streets is technically considered as an avalanche. Not unlike avalanches in the mountains, the following factors have an impact (Bartlett Quimby 2007):

angle of roof slope

roof material

thermal characteristics of the structure (insulation)

exposure to wind

On 7 December, Scottish Borders Council (2010) published the following advice:

If possible use alternative entrances if there is a danger of snow sliding from roofs or icicles or gutters falling.

Roofs should have an element of snow loading built into their design. However vigilance is required and where roofs show signs of sagging or stress from excessive snow, the building should either be evacuated or the snow cleared off the roof.

Temporary structures and agricultural sheds are usually most at risk – […] the agricultural and business community [should] inspect roofs on a regular basis. […] no one should go onto roofs without carrying out a full risk assessment and without using the correct equipment.

The Scottish Government (Scottish Government 2010) states:

“The climatic conditions in Scotland including temperature, snow, wind, driving rain and flooding and the impact of climate change should be carefully considered in the assessment of loading (actions) and in the structural design of buildings.”

Currently, the loadings to which a building will be subjected should be calculated in accordance with Structural Eurocode 1: Actions on Structures, which takes snow loading into account (Scottish Government 2010).

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SOURCES

Bartlett Quimby, T. (2007). A Beginner’s Guide to ASCE 7-05. Available at: http://www.bgstructuralengineering.com/BGASCE7/BGASCE7008/index.htm [Accessed 6 December 2010]. Barton, B and Wright, B. (1985). A Chance in A Million? Scottish Avalanches. Glasgow: Scottish Mountaineering Trust.

BBC. (2009a). Image Reveals Avalanched Railway. Available at: http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/8547394.stm [Accessed 6 December 2010]. BBC. (2009b) Avalanches Threat to Transport. Available at: http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/8572961.stm [Accessed 6 December 2010]. BBC. (2010a) Lucky escape from snow ‘avalanche’. Available at: http://www.bbc.co.uk/news/uk-scotland-11922246 [Accessed 6 December 2010]. BBC. (2010b) Girl ‘stable’ after being hit by ice on Borders farm. Available at: http://www.bbc.co.uk/news/uk-scotland-south-scotland-11909544 [Accessed 6 December 2010]. Forest Service National Avalanche Centre. (2009a). Learn how to: Dig a Pit. Available at: http://www.fsavalanche.org/Default.aspx?ContentId=20&LinkId=25&ParentLinkId=3 [Accessed 7 December 2010]. Forest Service National Avalanche Centre. (2009b). Learn how to identify a potential weak layer: Perform A Shovel Shear. Available at: http://www.fsavalanche.org/Default.aspx?ContentId=21&LinkId=26&ParentLinkId=3 [Accessed 7 December 2010]. Langmuir, E. (1995). Mountaincraft and Leadership. Edinburgh: Scottish Sports Council & Mountain Leader Training Board.

Mountaineering Council of Scotland. (2009). Annual Report and Accounts. Available at: http://www.mcofs.org.uk/assets/pdfs/annual%20report%202009-2010_final_v%2012%20august%202010.pdf [Accessed 7 December 2010]. Scottish Avalanche Information Service. (2010a). Personal Communication. [Unpublished]. Scottish Avalanche Information Service. (2010b). Snow and Avalanche Report – Northern Cairngorms. Available at: http://www.sais.gov.uk/page_northern%20cairngorms.asp [Accessed 7 December 2010]. Scottish Avalanche Information Service. (2010c). Sportscotland Avalanche Information Service Report for Winter 2009/10. Available at: http://www.sais.gov.uk/pdf/SAIS_report_2009_10.pdf [Accessed 6 December 2010]. Scottish Borders Council. (2010). Weather Updates. Available at: http://www.scotborders.gov.uk/council/departmentsandservices/chiefexecutives/corporatecommunications/34270.html [Accessed 7 December 2010].

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Scottish Government. (2001). Avalanche research group inaugurated. Available at: http://www.scotland.gov.uk/News/Releases/2001/10/397 [Accessed 7 December 2010]. Scottish Government. (2009). Safety on the hills. Available at: http://www.scotland.gov.uk/News/Releases/2009/12/07145657 [Accessed 7 December 2010]. Scottish Government. (2010). Technical Handbooks 2010 Domestic. Available at: http://www.scotland.gov.uk/Topics/Built-Environment/Building/Building-standards/publications/pubtech/thb2010dom [Accessed 7 December 2010]. ukclimbing.com. (2010) Avalanche Report – Edinburgh Area. Available at: http://www.ukclimbing.com/news/item.php?id=59262 [Accessed 7 December 2010].

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Scottish Parliament Information Centre (SPICe) Briefings are compiled for the benefit of the Members of the Parliament and their personal staff. Authors are available to discuss the contents of these papers with MSPs and their staff who should contact Alasdair Reid on extension 85375 or email alasdair.reid@scottish.parliament.uk. Members of the public or external organisations may comment on this briefing by emailing us at spice@scottish.parliament.uk. However, researchers are unable to enter into personal discussion in relation to SPICe Briefing Papers. If you have any general questions about the work of the Parliament you can email the Parliament’s Public Information Service at sp.info@scottish.parliament.uk.

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