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Avalanche prediction in Scotland: I. A survey of avalanche activity
Applied Geography (1984), 4, 91-108
Avalanche prediction in Scotland: I. A survey of avalanche activity R. G. W. Ward Department of Geology and Geography, City of London Polytechnic, Calcutta House Precinct, Old Castle Street, London El 7NT, England Abstract Avalanches are a common occurrence throughout the Scottish Highlands and have been responsible for several injuries and deaths amongst climbers and ramblers. The paper describes approximately 1000 avalanches which have been recorded over the last 200 years, the majority being observed in the Cairngorms between the winters of 1977-78 and 1979-80. Many different types of avalanche have been recorded, including slab avalanches and loose snow avalanches. Although the majority are comparatively small, a small proportion are extremely large. These may travel over a mile and involve a snow layer up to two or three metres thick and 200-300 m wide. Excluding small-scale sluffing from free faces, most avalanches release from slopes between 35” and 4.5”. Avalanche activity occurs during many different types of weather conditions, including thaws, cold snaps, storms and calm conditions.
Introduction In 1786 or thereabouts the Reverend McClaggan of Blair Atholl observed that some earth and trees had mysteriously removed themselves from one part of his parish to another during the course of one winter. The perceptive parson attributed this strange occurrence to the activity of snow avalanches, thereby providing the first historical record of these phenomena so far uncovered in Scotland. Some years later, on 1 January 1800, five people were killed in a mountain shelter in the forest of Gaick, and, despite an elaborate mythology that surrounds this incident (see for example Gordon 1949), it is highly probable that their deaths were the result of a very large avalanche issuing from the ravine above. In 1804 a dog was killed on Ben a Ghlo by an avalanche which spared his master, and in 1884 some deer were killed in Gaick forest. After 1890 avalanche records became more frequent due mainly to the proliferation of climbing clubs and the appearance of journals such as the Scottish Mountaineering Club Journal and The Cairngorm Club Journal. Early volumes carried accounts of accidents as well as opinions on Scottish snow, although interest in avalanches appeared to wane in the 1920s and 1930s. Surprisingly, no deaths, and only one minor accident, occurred before 1949, when a man was killed on Ben Nevis. Since that time deaths and injuries have increased noticeably and by 1980 over 30 people had been killed, and at least 80 injured in Scottish avalanches. Over 50 per cent of all accidents have occurred in the last ten years, and three people were killed in a recent accident on Ben Nevis in 1982. The reason for this increase is 0143~6228/84/020091-18
$03.00 0
1984 Butterworth
& Co (Publishers)
Ltd
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A survey of avalanche activity in Scotland
best explained by the rising number of climbers and walkers visiting the mountains in winter. In spite of more injuries and deaths, knowledge of Scottish avalanches and their causes has remained very limited. As early as 1890 mountaineers were already expressing their opinions about Scottish avalanches and during the last century three widely different views have been expressed (Ward 1981): 1. that Scottish avalanches (if they exist at all) are small and trivial (e.g. Mackenzie 1947); 2. that Scottish avalanches may be dangerous, but they are predictable since they occur in gullies during thaws, mainly as a result of cornice fracture (e.g. Raeburn 1920); 3. that Scottish avalanches are dangerous. They are predominantly of the windslab type and therefore most likely on lee slopes after a blizzard (e.g. Cliff 1978). The first of these views persisted in print (e.g. Firsoff 1949) almost until the 195Os, since when it has been confined to the non-climbing public. The second view is still strongly held by many mountain users, especially casual walkers, skiers and mountaineers. The third, and most recent view, is becoming the most popular amongst professional and regular climbers in Scotland. The major difficulty with the last two of these opinions is that they tend to pigeon-hole avalanches as phenomena caused in one particular way in a particular place and therefore essentially predictable. Furthermore they are unsupported in all cases by hard facts which list numbers, sizes, and descriptions of avalanches falling in the particular category. Thus it is fair to say that no-one really knows what types of avalanche occur, what magnitudes they have, what frequency they have, where they release or what meteorological conditions precede them. The objective of the work described in this paper is to present such information for the first time. In the second paper these data will form the basis of an analysis of Scottish avalanches which will be used to develop a predictive model. Sources of data A number of sources exist from which avalanche data may be obtained, including written or verbal accounts, published or unpublished material, personal observations by the author, and proxy data represented by scar tissue in trees and the geomorphological attributes of talus slopes. Published sources include local and national newspapers, an assortment of climbing books and journals and some deer-stalking books. So vast is the range of mountaineering literature that only a small portion has been examined so far. Table 1 lists those journals held in the Scottish Mountaineering Club’s private library which were examined. Though usually reliable, the information contained in these sources is sometimes questionable. For example, in one case a major avalanche fell in February from Dog Hillock in Glen Clova. It crossed the river Esk, was 150 m wide and ran 50 m beyond the river. Both the Scottish Mountaineering Club Journal and the Cairngorm Club Journal agree on all these points, but whereas the former states this was in 1946, the latter gives 1947. The most reliable source is the accident list published annually in the Scottish Mountaineering Club Journal, which is based on statistics collected by the Mountain Rescue Committee of Scotland. The lists vary in comprehensiveness and include only those avalanche accidents which involved a rescue team call-out. The
R. G. W. Ward Table 1. Published
journals
Journal * Cairngorm Club Journal Cambridge Mountaineering
Climbers’ Club Bulletin *Climbers Club Journal Edinburgh University Mountaineering Club Journal Etchachan Club Journal *The Fell and Rock Journal Glasgow University Mountaineering Club Journal Grampian Club Bulletin Grampian Club Journal (Journal of) Irish Mountaineering Club Irish Mountaineering Jacobites Journal *Ladies’ Scottish Mountaineering Club Journal Lairig Club Journal (Journal of) Lancashire Caving and Climbing Club Lomond Mountaineering Club Journal (Journal of) Manchester University Mountaineering Club Midland Association of Mountaineers (MAM) Moray Mountaineering Club Oxford Mountaineering Oxford and Cambridge Mountaineering *Rucksack Club Journal ‘Scottish Mountaineering Club Journal St Andrews University Mountaineering Club Journal *Yorkshire Ramblers’ Club Journal
* Denotes
a complete
examined
for avalanche Period
93
records examined
Vol. 1 (1) ff. (1893-1979) Various Years: (1925-26,1932, 1934, 1936, 1946,1948,1951,1953,1955-1965, 1967,1969, 1971,1976) Various years: (1924-32, 1964) Vol. 1 (1) ff. (1898-1979) Various years: (1946, 1950, 1952-53, 1967, 1970,1972) Various years: (1953-54,1956, 1960, 1962-63) Vol. l(1) ff. (1907-1979) One issue, 1970 One issue, 1977 (No. 24) One issue, 1937 Various years: (1950, 1952, 1954) Various years: (1959-61, 1963-65,1976, 1978) Two issues, 1975 and 1976 Nos. l-6 (1929, 1938,1952, 1959,1968, 1978) Two issues, 1975 and 1976 Various years: (1959-62) Vol. 2 (2) to Vol. 2 (5) One issue, 1956 Two issues, Session 1954-56 and Session 1964-65 Various years: (1933-38, 1947, 1951,1956, 1958,1960-H, 1965-66,1969-71) Two issues, 1935 and 1936 Various years: (1937,1947, 1950, 1953-56, 195&-60,1962-64,1966,1969,1972) Two issues, 1924 and 1929 Vol. l(1) ff. (1907-79) Vol. l(1) ff. (18961980) One issue, 1962 Vol. 1 (1) ff. (1899-1979)
set of journals
date of the accident report is usually correct, and never more than two days out. Occasionally, however, accident reports conflict. For example, conditions at an avalanche rescue on Beinn a Bhuird in 1964 were described as ‘cold and clear’ in the accident report whereas the police (less hardy perhaps) referred to them as ‘blizzard conditions’ (unpublished police record). This kind of difference may arise because one account relates to the time of the accident and the other to the time of the rescue. Unpublished sources include police records and mountain rescue statistics which
94
A survey
of avalanche
activity
in Scotland
go back to at least 1960. Two PhD theses from Edinburgh University on mountain geomorphology contain passing references to avalanches (King 1968; Shaw 1977), and two recent undergraduate theses from Lancaster University (Beattie 1976) and Strathclyde University (Sedgeworth 1980) deal with avalanches. The late Ben Beattie, author of the Lancaster thesis, kindly provided data for avalanches occurring in the Cairngorms in 1977’. The National Library of Scotland in Edinburgh houses records and manuscripts of the Scottish Mountaineering Club, which were examined in detail. Estate muniments held at the National Register of Archives may contain avalanche data but these have not been examined. New information was received in replies to 200 letters sent to all the Scottish climbing clubs and associates of the Scottish Mountaineering Club, and several skiers, walkers, climbers and stalkers agreed to talk about their experience of avalanches. Unpublished accounts are again not always reliable, but the major problem with these data is the current vogue for avalanches. Without observing fracture planes, many observers will confidently state that an avalanche is a ‘slab’, whereas 20 years previously they would probably have stated with equal confidence that it was a ‘cornice fall’. This type of data therefore needs to be used with caution. Whereas the sources described above cover all of Scotland, fieldwork was concentrated on the Cairngorms, and provided the majority of the avalanche data described in these papers. Observations were made chiefly between January and April of 1978, 1979 and 1980, and supplemented by additional observations in December and May of each winter season, chiefly by staff from Glenmore Lodge Outdoor Sports Centre. Observations depended on good weather conditions, permitting long forays into the mountains for perhaps two or three days. In addition, on 17 April and 6 May 1979, two aerial surveys were made from a light aircraft, during which the remains of over 500 avalanches were sighted. Direct observation of avalanches is often hampered by bad weather, as occurred, for example, during nearly all of March and much of February and April 1979. As a result, few data exist for a period in which avalanche activity was probably high. This is a perennial problem in all avalanche areas. As Fohn et al. (1977: 376) state: ‘Avalanche activity is most intensive during snowstorm conditions, i.e. at a time when . . . we cannot observe them’ and ‘Verification of avalanche activity in a whole area calls for a check of many slopes . a tremendous labour for any observation crew’. These problems are magnified when the ‘observation crew’ has only one member! Avalanche data were obtained from two other sources in the Cairngorms. In the Lairig Ghru avalanche activity had left clear geomorphological evidence where debris flow deposits had been eroded and transported by a large, recent avalanche some time before 24 February 1980. A more comprehensive record of avalanche activity was found in the scar tissues of trees growing alongside an avalanche track in Glen Feshie. Evidence for between 15 and 25 avalanches was obtained. Details of the method are given in Ward (1984). Results Of the 1000 avalanches which were recorded, approxiately 75 per cent fell in the Cairngorms, and 60 per cent of these were observed during the aerial surveys of 17 April and 6 May 1979. Where possible, the following information was obtained: 1. the location of the avalanche (as precise as possible); 2. the time of the avalanche (hour, day, month or year);
R. G. W. Ward 3. weather conditions preceding the avalanche and prevailing at the release; 4. the morphological type of avalanche (slab, sluff, wet or dry snow); 5. the slope angle; 6. the slope aspect; 7. the dimensions of the avalanche; 8. the trigger, if any, initiating avalanche release; 9. miscellaneous details.
time
95 of
Usually it was not possible to obtain all this information. As the majority of the avalanche data comes from the Cairngorms they will be described separately from the avalanches occurring in the rest of Scotland. Locations The location and number of avalanches recorded throughout the Cairngorms are shown in Fig. 1. Although this shows that avalanches are very widespread, certain locations appear to produce more avalanches than others, such as Coire an Lochain and Coire an-t Sneachda. Also some locations appear to favour certain types of avalanche, such as slab avalanches in Coire an Lochain or Coire Cas. In detail, the more important areas for avalanche activity are the northern carries of Cairngorm, the northern end of the Lairig Ghru, the slopes surrounding Loch Avon, and the Garbh Choire of Braeriach and Cairntoul. Creag Mhigheachaidh stands out on Fig. 1 due to the tree-ring research mentioned earlier, although three other avalanche records are shown from there. Coire Garbhlach, Glen Einich and Glen Derry are less important areas, and the Lairig an Laoigh between Beinn Meadhoin and Beinn a Chaorruin appears to be avalanche-free. Strath Nethy is also comparatively avalanche-free, with the exception of Coire Dheirg of Bynack More. On Coire Beannaidh the northwest spur of the Coire Ruadh arete produces several avalanches and in Coire cath nam Fion of Monadh Mor one track regularly produces avalanches. The east side of Coire an-t Sneachda and the Devil’s Point Burn area produce large avalanches, whilst the north-facing slope overlooking the Allt Mor by the junction of the Allt a Choire Chais regularly produces large cornice falls. Some slopes may appear to avalanche more often than others because they are more easily observed or because they gain something of a reputation which tends to be self-promoting. Even so, the locations mentioned above, some in quite remote parts of the mountains, do appear to be more avalanche-prone after due allowance has been made for bias in the frequency and ease of observation. The location of 252 avalanches in the rest of Scotland is shown in Fig. 2. For a record such as ‘eight avalanches fell from Ben Nevis’, eight avalanches are shown on the map, but for a record ‘some avalanches fell from Ben Nevis’, only one is added. In spite of the interpretive problems this system raises, some interesting patterns emerge. Ben Nevis stands out as the major avalanche area, with Glencoe, the Black Mount, Lochnagar and Creag Meagaidh as other important areas. By contrast, Skye, Mull, Arran and other mountainous islands off the west coast show no recorded avalanches. A number of avalanches occur outside the highland areas, in the Pentland and Ochil Hills. Certain factors explain this distribution. The chief avalanche areas are the most
96
A survey of avalanche activity in Scotland
R. G. W. Ward
..
/
..I
..
Figure 2. The location
of avalanches
recorded
in Scotland
outside
the Cairngorms.
97
98
A survey of avalanche activity in Scotland
popular winter climbing grounds, and therefore subject to more observations, whilst Skye, Mull and Arran are summer climbing areas, and therefore subject to fewer observations in winter. However, the major reason why few winter climbs are attempted in the western islands is that the hills rarely have adequate snow cover. Consequently, there is also a climatic reason why avalanches might be fewer in these areas. In summary, with the exception of the western islands, avalanches are widespread in Scotland. Avalanche
type
All the types of avalanches shown in Fig. 3 occur in Scotland. Table 2 shows that both sluffs and slabs are common in the Cairngorms, as are cornice falls. Wet snow avalanches appear to be more common than dry snow avalanches and open face avalanches are frequent. Surface avalanches are more frequent than full-depth avalanches, which appear to be relatively uncommon. Airborne powder avalanches have not been recorded, although some very far-travelled avalanches may have been airborne. Elsewhere in Scotland, 49 slab avalanches and 48 sluffs have been recorded, making a total of 97 avalanches for which the fracture type is known. Of all slabs, 10 were assumed by correspondents to be wet and 15 to be dry, and of all sluffs, 11 were thought to be wet and 11 dry. Whereas most slabs were open-faced, most sluffs were confined. Altogether roughly equal numbers of wet and dry snow avalanches have been recorded. Surface avalanches are much more common than full-depth avalanches (only three of the latter definitely known), and airborne MORPHOLOGICAL
FRACTURE
TYPE
FRACTURE
DEPTH
LIQUID
DEGREE
WATER
CHARACTERISTIC
CONTENT
OF CONFINEMENT
(
1
WEr
’
“““’
I
*
I
i
DRY
1
3
Fati’d
Figure 3. The morphological avalanche classification most commonly used in Scotland. (This was originally prepared for the Swiss Avalanche Service by De Quervain and Haefeli in 1955 and was adopted by the Mountain Leadership Training Boards of Great Britain and Northern Ireland, hence its popularity in Scotland.)
R. G. W. Ward Table 2. Avalanche
types in the Cairngorms
1977-78
1977-78 to 1979-80 by month. 1978-79
Avalanche type
D
F
M
A
Total
Cornices Slabs Sluffs
02211 2 6 9 0 1 22
6 12
5 3
6 28 38
00510 37 15 7 0 12 4
Wet Dry
0 0
15 0
1 0
18 28
7 0 1 17000
Wet slabs
00260 04500 00 0101 0 0 17 0
0
8 9 11 17
00017 17000 7 0 1 00000
Dry slabs Wet sluffs Dry sluffs
J
0 2 4 24
D
J
99
F
M
1
0
1979-80
A
Total
7 50”
6 23 73
0002 0161 0 0
59 8
0013 0020
4 2
8 8 58 0
0010 0020 0002 0000
1 2 2 0
50”
50”
D
J
F
12
M Total
9
2 8 21
U Indicates that several avalanches occurred all over the Northern carries. 50 is a large figure to emphasize the frequency of activity, not the actual number of avalanches
powder avalanches are rare, although they may have occurred on Ben Nevis in 1978 and on Loch Muick side (Watson 1975). Cornice falls are commonly reported. Mugnitude Histograms of avalanche runout distances are shown in Figs 4a and 4b. Although most Cairngorm avalanches are small, some are very large. The average height of corrie headwalls and talus slopes is around 300 m and several avalanches travel this distance. The largest avalanches are slab avalanches, and some may become airborne and travel huge distances as a result. For example, an avalanche was seen in Glen Geusachan from the air on 17 April 1979 which travelled about 2 km and part of the way up Beinn Bhrotain. Even more spectacular is an avalanche reported from Coire na Ciste in about 1959, which travelled at least 500 m beyond the position of the car park and restaurant facilities. The considerable distances achieved by these avalanches over uphill or gentle slopes strongly suggests airborne motion. Apart from these examples, very large slab avalanches have fallen from the headwalls of Coire Laoigh Mor and Coire Cas, the east slope of Coire an-t Sneachda, Coire an Lochain of Cairngorm and the Lairig Ghru. On 18 February 1980, the floor of the Lairig Ghru was at least 2 m deep in avalanche debris over a distance of 500 m. In March 1979 a very large avalanche fell from Cadha Mor at the head of Glen Einich and two climbers were killed by a large avalanche in Coire Bogha Cloiche on 26 December 1978. The crown fracture lines of events as large as these are typically between 300 and 500 m, and fracture depths may reach 3 or 4 m, although 0.5 to 1.0 m is a more common figure. The full-depth avalanche from the Great Slab of Coire an Lochain, which often releases in spring, is reputedly 5 m thick on occasions. Elsewhere, Fig. 4b shows the runouts obtained for 54 avalanches. The largest of these were recorded in Glencoe in 1946, on Ben Nevis in 1978, in Gaick in 1800 and Glen Clova (1946 or 1947). Calculated overland travel distances were over a kilometre.
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A survey of avalanche activity in Scotland
i
Figure 4. Avalanche Scotland (B).
runout
distances
recorded
in the Cairngorms
(A) and in the rest of
Widths are known for 23 avalanches, and are usually small with a median width of only about 50 m. It is likely that much larger avalanches than these figures suggest often release. Most fracture depths appear to be 1 m deep or less, although much greater thicknesses occur, with a maximum of 4 m recorded for a full-depth avalanche in the Ochils and a remarkable 8 m for a surface avalanche photographed by E. Langmuir on Lochnagar. This is, however, exceptional. Apart from the statistics presented above, there are also 26 references to ‘small’ avalanches, nine reports of ‘large’ avalanches and three records of ‘huge’ avalanches from the last 100 years. The conclusion must be that although the majority of Scottish avalanches are undoubtedly small, exceptions occur which produce very large events indeed, involving thousands of tonnes of snow and covering a very large area. This point has not been generally appreciated in Scotland. Annual pattern of avalanche activity
The timing of avalanches, by year, in the Cairngorms is shown until winter 1975-76 by the bars below the x-axis in Fig. 5. Avalanches from December 1977 to March 1980 are shown separately in Fig. 6. There is a considerable increase in avalanche sightings after 1960, and in fact the majority of the avalanches shown prior to 1960 in Fig. 5 are based on the tree-ring work. In recent years, during the winter 1976-77 Cilenmore Lodge staff are reported to have observed 150 avalanches, but data on only 74 has emerged (O’Donovan 1977). Between December 1977 and April 1978,
R. G. W. Ward
101
CAIRNGORMS
Figure 5. The number of avalanches recorded each year in the Cairngorms elsewhere in Scotland until 1980.
until 1976 and
over 100 avalanches were observed, and the flights of 17 April and 6 May 1979 boosted the winter of 1978-79 total to over 600. In winter 1979980, despite generally good viewing conditions, far fewer sightings were made, suggesting that in real terms avalanche activity was down on previous years. Yearly data from 252 avalanches which fell outside the Cairngorms is shown by the vertical bars above the x-axis in Fig. 5. This shows clearly the great increase in avalanche records in the last decade or so, as well as the paucity of records during most of the nineteenth century. For the last ten years, 100 avalanche records exist, with a peak in 1979 of 20 events. The reason for this increase has been suggested already and there is no reason to suppose that avalanches have suddenly become more common in Scotland.
Table 3 shows the percentage of avalanches seen each month for the periods 1893 to 1976,1977,1978 and 1980, excluding June and July. The figures for 1893 to 1976 do not include several ‘spring avalanches’ which may have occurred at any time between March and May. The table shows an interesting change. Prior to 1976, the spring months dominate, accounting for 50 per cent of all sightings, whilst in 1978 and 1980 particularly, February and March stand out. This pattern matches that shown in Table 4, for the rest of Scotland, using the periods 1790 to 1942, and 1790 to 1980. Between 1800 and 1942, about 44 per cent of all records relate to avalanches falling in April, with a further 36.5 per cent in March and May combined. January with 8.5 per cent and February with 4 per cent are, like December, almost avalanche-free. After 1943, this situation completely reversed,
102
A survey of avalanche activity in Scotland Table 3. The percentage of all avalanches seen in the Cairngorms during each month for the periods 1893-1976, 1977, 1978, 1979 and 1980
Month
1893-1976 (%I)
1977 (I%)
197x (1%)
1979 (%)
1980 (%)
2.3 0.0 2.3 22.7 6.8 15.9 29.5 20.5 0.0
0.0 0.0 0.0 91 0.0 3.0 84.8” 3.0 0.0
0.0 0.0 0.0 12.5 11.5 41.3 21.2 10.6 2.9
0.0 0.0 0.0 1.1 7.8 21.1 12.2 57@ O.Oh
0.0 0.0 0.0 0.0 3.0 51.5 424 0.0 3.0
September October November December January February March April May
(1Aerial survey, by Glenmore Lodge. March 1977 ’ Not including aerial surveys 17 April 1979 and 6 May 1979
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Figure 6. The frequency of avalanches each month between the winters of 1977-78 and 1979980.
M
D
79
from observations
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M
80
in the Cairngorms
R. G. W. Ward
103
Table 4. The percentage of avalanches seen in Scotland, excluding the Cairngorms, during each month for the
periods 1790 to 1942, and 1790 to 1980
Month October November December January February March April May June
179G1942 (%)
179C1980 (S)
0.0 1.4 4.2 8.5 4.2 19.7 43.7 16.9 1.4
0.5 0.5 5.1 16.2 23.6 16.7 27.3 9.7 0.5
with February and January together accounting for 50 per cent of all records and March, April and May together amounting to only 40 per cent. The probable explanation for this turn is a change in attitude amongst climbers that can be traced easily in climbing literature. Until about 1950, thaw avalanches in spring were thought to be the only avalanche type in Scotland, with few dissenters from this view. Increasingly, however, the ‘windslab avalanche’ of the snowier months, January and February, has been regarded as the chief danger, particularly after 1960. Another possibility is that it also reflects a change in winter climbing habits, with fewer climbers waiting for the milder spring months to come before venturing out. It may of course represent an actual change in the pattern of avalanching, but it is difficult to say why this should be. In any event, the following points should be noted: the ‘avalanche season’ can potentially last nine months, from September to May, but the main period of activity is December to April; peak months appear to be February, March and April, with December and January less important (Fig. 6). However, fewer observations could be made during these latter two months; Scottish avalanches are neither just spring thaw phenomena nor solely mid-winter slab phenomena, but they occur almost all year round. They must therefore include different types. As regards the time of day when avalanches fall, a common contention is that most avalanches fall in the afternoon ‘after the sun has got onto the slopes’. Of the 22 avalanches for which the time of release is known, 17 fell in the afternoon, one at noon, two in the morning and two overnight. Little significance should be attached to this as it could reflect the time at which most climbers (observers) reach the climbing area. In any case the effect of the afternoon sun will often be outweighed by other factors such as snowfall intensity, windspeed or ambient air temperature. Slope angle Slope angles were estimated
from OS 1:25000 topographic
maps, supplemented
by
104
A survey of avalanche
activity in Scotland
field measurements made with an abney level. Histograms of the percentage of avalanches in each slope class are shown in Fig. 7a for the Cairngorms, which does not include avalanches observed from the air. Free faces are excluded since they undergo almost continuous small-scale sluffing, whilst cornice falls account for all slopes up to 90”. Most avalanches release on slopes above 30”, although they have occurred on slopes as low as 26.5”. Where steep drifts form, the slope of the snow surface may bear little relation to the gradient of the underlying terrain. A slab avalanche at Coire na Ciste, for example, had a fracture surface inclined at 57”. although the ground slopes at less than 25”. Excluding free faces, slopes between 35” and 45” are most avalanche-prone. Slab avalanches cluster around 3S-39”, whereas sluffs tend to cluster around the steeper slope classes. Dry and wet slabs occupy similar slope class domains. but there is a slight suggestion that dry sluffs require steeper slopes for initiation than wet sluffs. Of those avalanches seen from the air, over 50 per cent released from slopes above 45” and about 40 per cent released from slopes of 4G4.5”. The remaining few released from slopes of 35-39”. Data from elsewhere in Scotland are shown in Fig. 7b, which shows that sluffs tend to fall on steeper slopes than slabs. Also slabs are infrequent on slopes below 30”, and sluffs absent, whilst a lower limit for all avalanching appears to be about 25”. Dry and wet avalanches appear to occupy similar slope class domains. A
Figure 7. The frequency of avalanches recorded Cairngorms (A) and elsewhere in Scotland (B).
on slopes
of different
gradient
in the
R. G. W. Ward
105
Aspect
Figures 8a and 8b show the aspects of all Cairngorm and other Scottish avalanches, and the dominance of the northeastern quadrant is apparent. This is probably because the main climbing slopes and steep headwalls have this orientation, for example Ben Nevis and Lochnagar outside the Cairngorms, and Coire an-t Sneachda and Coire an Lochain within the Cairngorm area. However, the preferred northeasterly orientation of Scottish carries is itself a reflection of meteorological factors, including the dominant direction of snow drift amongst other things. Avalanche
weather
Since this is analysed at length in the next paper, only a brief description is given here. The main point is that avalanches have been observed under a wide variety of weather conditions. For example, in the Cairngorms, an avalanche on 19 April 1908 from the Devil’s Point occurred in a blizzard, whereas an avalanche fell from the same place in 1931 after three consecutive sunny days. An avalanche in Coire an Lochain occurred on 30 March 1975 in cold clear weather, whilst an avalanche that struck the author on 4 February 1980 fell during heavy drifting. For 166 events occurring elsewhere in Scotland, weather data were obtained from nearby weather stations and eyewitness reports. A summary of the prevailing weather conditions is shown in Table 5, and again the most obvious point is that avalanches occur under a wide variety of conditions. Roughly equal numbers fell in sunny or snowy weather, Table 5. Weather conditions reported by witnesses on days with avalanches falling. Data not including the Cairngorms Comments on precipitation Snow/sleet/blizzard/drifting Rain/wet Mist Cloudy/dull (probably no precipitation) No precipitation
25 7 5 4 28
avalanche avalanche avalanche avalanche avalanche
days days days days days
14 avalanche 13 avalanche 12 avalanche
days days days
20.avalanche 4 avalanche
days days
Comments on temperature Cold/icy Thawing Warm/hot/strong
thaw
Comments on windspeed Driving/windy/drifting Calm Comments on wind direction Winds southeasterly Winds westerly
1 avalanche 1 avalanche
day day
4 avalanche 4 avalanche 4 avalanche
days days days
Combinations Sunny and hot Sunny and cold Windy and cold
106
A survey of avalanche activity in Scotland
GROUND
Figure 8. The orientation of avalanche elsewhere in Scotland (B).
activity
SURVEY
in the Cairngorms
(A) and
R. G. W. Ward
107
warm or cold weather and calm or windy weather. This again highlights the danger of ascribing avalanche activity to a single meteorological cause. As will be shown in the next paper, avalanches do follow certain weather patterns, but these are a little more complex than popular opinion in Scotland often allows. Triggers
The importance of a trigger, particularly human or animal as opposed to explosion or falling cornices, is difficult to prove. The mere fact that some avalanches initiate while people happen to be standing in the way hardly implies cause and effect, since sheer probability demands that this will happen occasionally, given a sufficient number of people walking or skiing on steep snow. Consequently, the distinction between triggered and spontaneous avalanches may sometimes be unhelpful. Bearing this in mind, the first recorded case of a ‘trigger’ in the Cairngorms was in 1952, when four climbers were avalanched, and since then a further 45 may have been triggered by human beings. Of these four were during 1976-77, followed by twelve in 1977-78, four in 1978-79 and three in 1979-80. In addition, deer may have started avalanches in 1978 and 1980. Excluding the effect of falling cornices, the majority of avalanches in the Cairngorms release spontaneously. Elsewhere in Scotland there are records of at least 110 events where triggering may have been a factor, most of these being climbing, walking and skiing accidents, and a smaller number where deer were involved. Cornice falls have started sluffs on the slopes beneath on 19 recorded occasions. Conclusions Although the data described above contain inevitable biases which reflect the difficulties of’observing equally under all conditions and in all locations, a number of conclusions may be drawn, chief amongst which are: 1. avalanches are a common occurrence in Scotland and may be a hazard in mountain recreation; 2. attempts to classify avalanches as being of a particular type related to only one or two kinds of weather conditions are misleading and potentially dangerous; 3. some avalanches in Scotland may be significantly larger than is commonly realized. Acknowledgements Many people helped to provide the data presented in this paper and I wish to express my gratitude to them all. David Sugden made many suggestions for improving the text and the technical staff of the Geography departments at Aberdeen University and City of London Polytechnic helped with equipment and cartography. The Natural Environment Research Council provided generous financial assistance. References Beattie, B. (1976) The densification of a seasonal snowpack in the Cairngorms with relation to avalanches. Unpublished BSc thesis, University of Lancaster. Cliff, P. (1978) Mountain navigation. Edinburgh: Diane Thompson.
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A survey of avalanche activity in Scotland
De
Quervain, M. R. (1965) On avalanche classification, a further contribution. In International Symposium on Scientific Aspects of Snow and Ice Avalanches, Davos. IASH Publication No. 69, pp. 196198. Firsoff, V. A. (1949) The Cairngorms on foot and ski. Edinburgh: W. and R. Chambers. Fohn, P., Good, W., Bois, P. and Obled, C. (1977) Evaluation and comparison of statistical and conventional methods of forecasting avalanche hazard. Journal of Glaciology 29 (81), 375-388. Gordon, S. (1949) Highways and Byways in the Central Highlands. London: Macmillan. King, R. B. (1968) Periglacial features in the Cairngorm Mountains. Unpublished PhD thesis, Edinburgh University. Mackenzie, W. M. (1947) The snow and ice climbs of Glencoe. Scottish Mountaineering Club Journal 23 (138), 365-379. McClaggan, D. (1786) An Account of the parish of Blair Atholl. Perth Museum Art Gallery. O’Donovan, R. (1977) Snow bas. Scottish Mountaineering Club Journal 31 (168), 133-137. Raeburn, H. (1920) Mountaineering art. London: T. Fisher Unwin. Sedgeworth, L. (1980) An introduction to avalanches with special reference to the Cairngorms. Unpublished BSc thesis, Strathclyde University. Shaw, R. (1977) Periglacial features in part of the South East Grampian Highlands of Scotland. Unpublished PhD thesis, Edinburgh University. Ward, R. G. W. (1981) Nearly two hundred years of avalanches in Scotland. Scottish Mountaineering Club Journal 32 (172), 140-145. Ward, R. G. W. (1984) An estimate of avalanche frequency in Glen Feshie, Scotland, using tree rings. In Palaeoenvironmental investigations: research design, methods and interpretation (N. R. J. Feiller, D. D. Gilbertson and N. G. A. Ralph, eds). Oxford: British Archaeological Reports. Watson, A. (1975) The Cairngorms. SMC district guide. Edinburgh: Scottish Mountaineering Trust. (Revised manuscript received 18 November 1983)
Avalanche prediction in Scotland: I. A survey of avalanche activity R. G. W. Ward Department of Geology and Geography, City of London Polytechnic, Calcutta House Precinct, Old Castle Street, London El 7NT, England Abstract Avalanches are a common occurrence throughout the Scottish Highlands and have been responsible for several injuries and deaths amongst climbers and ramblers. The paper describes approximately 1000 avalanches which have been recorded over the last 200 years, the majority being observed in the Cairngorms between the winters of 1977-78 and 1979-80. Many different types of avalanche have been recorded, including slab avalanches and loose snow avalanches. Although the majority are comparatively small, a small proportion are extremely large. These may travel over a mile and involve a snow layer up to two or three metres thick and 200-300 m wide. Excluding small-scale sluffing from free faces, most avalanches release from slopes between 35” and 4.5”. Avalanche activity occurs during many different types of weather conditions, including thaws, cold snaps, storms and calm conditions.
Introduction In 1786 or thereabouts the Reverend McClaggan of Blair Atholl observed that some earth and trees had mysteriously removed themselves from one part of his parish to another during the course of one winter. The perceptive parson attributed this strange occurrence to the activity of snow avalanches, thereby providing the first historical record of these phenomena so far uncovered in Scotland. Some years later, on 1 January 1800, five people were killed in a mountain shelter in the forest of Gaick, and, despite an elaborate mythology that surrounds this incident (see for example Gordon 1949), it is highly probable that their deaths were the result of a very large avalanche issuing from the ravine above. In 1804 a dog was killed on Ben a Ghlo by an avalanche which spared his master, and in 1884 some deer were killed in Gaick forest. After 1890 avalanche records became more frequent due mainly to the proliferation of climbing clubs and the appearance of journals such as the Scottish Mountaineering Club Journal and The Cairngorm Club Journal. Early volumes carried accounts of accidents as well as opinions on Scottish snow, although interest in avalanches appeared to wane in the 1920s and 1930s. Surprisingly, no deaths, and only one minor accident, occurred before 1949, when a man was killed on Ben Nevis. Since that time deaths and injuries have increased noticeably and by 1980 over 30 people had been killed, and at least 80 injured in Scottish avalanches. Over 50 per cent of all accidents have occurred in the last ten years, and three people were killed in a recent accident on Ben Nevis in 1982. The reason for this increase is 0143~6228/84/020091-18
$03.00 0
1984 Butterworth
& Co (Publishers)
Ltd
92
A survey of avalanche activity in Scotland
best explained by the rising number of climbers and walkers visiting the mountains in winter. In spite of more injuries and deaths, knowledge of Scottish avalanches and their causes has remained very limited. As early as 1890 mountaineers were already expressing their opinions about Scottish avalanches and during the last century three widely different views have been expressed (Ward 1981): 1. that Scottish avalanches (if they exist at all) are small and trivial (e.g. Mackenzie 1947); 2. that Scottish avalanches may be dangerous, but they are predictable since they occur in gullies during thaws, mainly as a result of cornice fracture (e.g. Raeburn 1920); 3. that Scottish avalanches are dangerous. They are predominantly of the windslab type and therefore most likely on lee slopes after a blizzard (e.g. Cliff 1978). The first of these views persisted in print (e.g. Firsoff 1949) almost until the 195Os, since when it has been confined to the non-climbing public. The second view is still strongly held by many mountain users, especially casual walkers, skiers and mountaineers. The third, and most recent view, is becoming the most popular amongst professional and regular climbers in Scotland. The major difficulty with the last two of these opinions is that they tend to pigeon-hole avalanches as phenomena caused in one particular way in a particular place and therefore essentially predictable. Furthermore they are unsupported in all cases by hard facts which list numbers, sizes, and descriptions of avalanches falling in the particular category. Thus it is fair to say that no-one really knows what types of avalanche occur, what magnitudes they have, what frequency they have, where they release or what meteorological conditions precede them. The objective of the work described in this paper is to present such information for the first time. In the second paper these data will form the basis of an analysis of Scottish avalanches which will be used to develop a predictive model. Sources of data A number of sources exist from which avalanche data may be obtained, including written or verbal accounts, published or unpublished material, personal observations by the author, and proxy data represented by scar tissue in trees and the geomorphological attributes of talus slopes. Published sources include local and national newspapers, an assortment of climbing books and journals and some deer-stalking books. So vast is the range of mountaineering literature that only a small portion has been examined so far. Table 1 lists those journals held in the Scottish Mountaineering Club’s private library which were examined. Though usually reliable, the information contained in these sources is sometimes questionable. For example, in one case a major avalanche fell in February from Dog Hillock in Glen Clova. It crossed the river Esk, was 150 m wide and ran 50 m beyond the river. Both the Scottish Mountaineering Club Journal and the Cairngorm Club Journal agree on all these points, but whereas the former states this was in 1946, the latter gives 1947. The most reliable source is the accident list published annually in the Scottish Mountaineering Club Journal, which is based on statistics collected by the Mountain Rescue Committee of Scotland. The lists vary in comprehensiveness and include only those avalanche accidents which involved a rescue team call-out. The
R. G. W. Ward Table 1. Published
journals
Journal * Cairngorm Club Journal Cambridge Mountaineering
Climbers’ Club Bulletin *Climbers Club Journal Edinburgh University Mountaineering Club Journal Etchachan Club Journal *The Fell and Rock Journal Glasgow University Mountaineering Club Journal Grampian Club Bulletin Grampian Club Journal (Journal of) Irish Mountaineering Club Irish Mountaineering Jacobites Journal *Ladies’ Scottish Mountaineering Club Journal Lairig Club Journal (Journal of) Lancashire Caving and Climbing Club Lomond Mountaineering Club Journal (Journal of) Manchester University Mountaineering Club Midland Association of Mountaineers (MAM) Moray Mountaineering Club Oxford Mountaineering Oxford and Cambridge Mountaineering *Rucksack Club Journal ‘Scottish Mountaineering Club Journal St Andrews University Mountaineering Club Journal *Yorkshire Ramblers’ Club Journal
* Denotes
a complete
examined
for avalanche Period
93
records examined
Vol. 1 (1) ff. (1893-1979) Various Years: (1925-26,1932, 1934, 1936, 1946,1948,1951,1953,1955-1965, 1967,1969, 1971,1976) Various years: (1924-32, 1964) Vol. 1 (1) ff. (1898-1979) Various years: (1946, 1950, 1952-53, 1967, 1970,1972) Various years: (1953-54,1956, 1960, 1962-63) Vol. l(1) ff. (1907-1979) One issue, 1970 One issue, 1977 (No. 24) One issue, 1937 Various years: (1950, 1952, 1954) Various years: (1959-61, 1963-65,1976, 1978) Two issues, 1975 and 1976 Nos. l-6 (1929, 1938,1952, 1959,1968, 1978) Two issues, 1975 and 1976 Various years: (1959-62) Vol. 2 (2) to Vol. 2 (5) One issue, 1956 Two issues, Session 1954-56 and Session 1964-65 Various years: (1933-38, 1947, 1951,1956, 1958,1960-H, 1965-66,1969-71) Two issues, 1935 and 1936 Various years: (1937,1947, 1950, 1953-56, 195&-60,1962-64,1966,1969,1972) Two issues, 1924 and 1929 Vol. l(1) ff. (1907-79) Vol. l(1) ff. (18961980) One issue, 1962 Vol. 1 (1) ff. (1899-1979)
set of journals
date of the accident report is usually correct, and never more than two days out. Occasionally, however, accident reports conflict. For example, conditions at an avalanche rescue on Beinn a Bhuird in 1964 were described as ‘cold and clear’ in the accident report whereas the police (less hardy perhaps) referred to them as ‘blizzard conditions’ (unpublished police record). This kind of difference may arise because one account relates to the time of the accident and the other to the time of the rescue. Unpublished sources include police records and mountain rescue statistics which
94
A survey
of avalanche
activity
in Scotland
go back to at least 1960. Two PhD theses from Edinburgh University on mountain geomorphology contain passing references to avalanches (King 1968; Shaw 1977), and two recent undergraduate theses from Lancaster University (Beattie 1976) and Strathclyde University (Sedgeworth 1980) deal with avalanches. The late Ben Beattie, author of the Lancaster thesis, kindly provided data for avalanches occurring in the Cairngorms in 1977’. The National Library of Scotland in Edinburgh houses records and manuscripts of the Scottish Mountaineering Club, which were examined in detail. Estate muniments held at the National Register of Archives may contain avalanche data but these have not been examined. New information was received in replies to 200 letters sent to all the Scottish climbing clubs and associates of the Scottish Mountaineering Club, and several skiers, walkers, climbers and stalkers agreed to talk about their experience of avalanches. Unpublished accounts are again not always reliable, but the major problem with these data is the current vogue for avalanches. Without observing fracture planes, many observers will confidently state that an avalanche is a ‘slab’, whereas 20 years previously they would probably have stated with equal confidence that it was a ‘cornice fall’. This type of data therefore needs to be used with caution. Whereas the sources described above cover all of Scotland, fieldwork was concentrated on the Cairngorms, and provided the majority of the avalanche data described in these papers. Observations were made chiefly between January and April of 1978, 1979 and 1980, and supplemented by additional observations in December and May of each winter season, chiefly by staff from Glenmore Lodge Outdoor Sports Centre. Observations depended on good weather conditions, permitting long forays into the mountains for perhaps two or three days. In addition, on 17 April and 6 May 1979, two aerial surveys were made from a light aircraft, during which the remains of over 500 avalanches were sighted. Direct observation of avalanches is often hampered by bad weather, as occurred, for example, during nearly all of March and much of February and April 1979. As a result, few data exist for a period in which avalanche activity was probably high. This is a perennial problem in all avalanche areas. As Fohn et al. (1977: 376) state: ‘Avalanche activity is most intensive during snowstorm conditions, i.e. at a time when . . . we cannot observe them’ and ‘Verification of avalanche activity in a whole area calls for a check of many slopes . a tremendous labour for any observation crew’. These problems are magnified when the ‘observation crew’ has only one member! Avalanche data were obtained from two other sources in the Cairngorms. In the Lairig Ghru avalanche activity had left clear geomorphological evidence where debris flow deposits had been eroded and transported by a large, recent avalanche some time before 24 February 1980. A more comprehensive record of avalanche activity was found in the scar tissues of trees growing alongside an avalanche track in Glen Feshie. Evidence for between 15 and 25 avalanches was obtained. Details of the method are given in Ward (1984). Results Of the 1000 avalanches which were recorded, approxiately 75 per cent fell in the Cairngorms, and 60 per cent of these were observed during the aerial surveys of 17 April and 6 May 1979. Where possible, the following information was obtained: 1. the location of the avalanche (as precise as possible); 2. the time of the avalanche (hour, day, month or year);
R. G. W. Ward 3. weather conditions preceding the avalanche and prevailing at the release; 4. the morphological type of avalanche (slab, sluff, wet or dry snow); 5. the slope angle; 6. the slope aspect; 7. the dimensions of the avalanche; 8. the trigger, if any, initiating avalanche release; 9. miscellaneous details.
time
95 of
Usually it was not possible to obtain all this information. As the majority of the avalanche data comes from the Cairngorms they will be described separately from the avalanches occurring in the rest of Scotland. Locations The location and number of avalanches recorded throughout the Cairngorms are shown in Fig. 1. Although this shows that avalanches are very widespread, certain locations appear to produce more avalanches than others, such as Coire an Lochain and Coire an-t Sneachda. Also some locations appear to favour certain types of avalanche, such as slab avalanches in Coire an Lochain or Coire Cas. In detail, the more important areas for avalanche activity are the northern carries of Cairngorm, the northern end of the Lairig Ghru, the slopes surrounding Loch Avon, and the Garbh Choire of Braeriach and Cairntoul. Creag Mhigheachaidh stands out on Fig. 1 due to the tree-ring research mentioned earlier, although three other avalanche records are shown from there. Coire Garbhlach, Glen Einich and Glen Derry are less important areas, and the Lairig an Laoigh between Beinn Meadhoin and Beinn a Chaorruin appears to be avalanche-free. Strath Nethy is also comparatively avalanche-free, with the exception of Coire Dheirg of Bynack More. On Coire Beannaidh the northwest spur of the Coire Ruadh arete produces several avalanches and in Coire cath nam Fion of Monadh Mor one track regularly produces avalanches. The east side of Coire an-t Sneachda and the Devil’s Point Burn area produce large avalanches, whilst the north-facing slope overlooking the Allt Mor by the junction of the Allt a Choire Chais regularly produces large cornice falls. Some slopes may appear to avalanche more often than others because they are more easily observed or because they gain something of a reputation which tends to be self-promoting. Even so, the locations mentioned above, some in quite remote parts of the mountains, do appear to be more avalanche-prone after due allowance has been made for bias in the frequency and ease of observation. The location of 252 avalanches in the rest of Scotland is shown in Fig. 2. For a record such as ‘eight avalanches fell from Ben Nevis’, eight avalanches are shown on the map, but for a record ‘some avalanches fell from Ben Nevis’, only one is added. In spite of the interpretive problems this system raises, some interesting patterns emerge. Ben Nevis stands out as the major avalanche area, with Glencoe, the Black Mount, Lochnagar and Creag Meagaidh as other important areas. By contrast, Skye, Mull, Arran and other mountainous islands off the west coast show no recorded avalanches. A number of avalanches occur outside the highland areas, in the Pentland and Ochil Hills. Certain factors explain this distribution. The chief avalanche areas are the most
96
A survey of avalanche activity in Scotland
R. G. W. Ward
..
/
..I
..
Figure 2. The location
of avalanches
recorded
in Scotland
outside
the Cairngorms.
97
98
A survey of avalanche activity in Scotland
popular winter climbing grounds, and therefore subject to more observations, whilst Skye, Mull and Arran are summer climbing areas, and therefore subject to fewer observations in winter. However, the major reason why few winter climbs are attempted in the western islands is that the hills rarely have adequate snow cover. Consequently, there is also a climatic reason why avalanches might be fewer in these areas. In summary, with the exception of the western islands, avalanches are widespread in Scotland. Avalanche
type
All the types of avalanches shown in Fig. 3 occur in Scotland. Table 2 shows that both sluffs and slabs are common in the Cairngorms, as are cornice falls. Wet snow avalanches appear to be more common than dry snow avalanches and open face avalanches are frequent. Surface avalanches are more frequent than full-depth avalanches, which appear to be relatively uncommon. Airborne powder avalanches have not been recorded, although some very far-travelled avalanches may have been airborne. Elsewhere in Scotland, 49 slab avalanches and 48 sluffs have been recorded, making a total of 97 avalanches for which the fracture type is known. Of all slabs, 10 were assumed by correspondents to be wet and 15 to be dry, and of all sluffs, 11 were thought to be wet and 11 dry. Whereas most slabs were open-faced, most sluffs were confined. Altogether roughly equal numbers of wet and dry snow avalanches have been recorded. Surface avalanches are much more common than full-depth avalanches (only three of the latter definitely known), and airborne MORPHOLOGICAL
FRACTURE
TYPE
FRACTURE
DEPTH
LIQUID
DEGREE
WATER
CHARACTERISTIC
CONTENT
OF CONFINEMENT
(
1
WEr
’
“““’
I
*
I
i
DRY
1
3
Fati’d
Figure 3. The morphological avalanche classification most commonly used in Scotland. (This was originally prepared for the Swiss Avalanche Service by De Quervain and Haefeli in 1955 and was adopted by the Mountain Leadership Training Boards of Great Britain and Northern Ireland, hence its popularity in Scotland.)
R. G. W. Ward Table 2. Avalanche
types in the Cairngorms
1977-78
1977-78 to 1979-80 by month. 1978-79
Avalanche type
D
F
M
A
Total
Cornices Slabs Sluffs
02211 2 6 9 0 1 22
6 12
5 3
6 28 38
00510 37 15 7 0 12 4
Wet Dry
0 0
15 0
1 0
18 28
7 0 1 17000
Wet slabs
00260 04500 00 0101 0 0 17 0
0
8 9 11 17
00017 17000 7 0 1 00000
Dry slabs Wet sluffs Dry sluffs
J
0 2 4 24
D
J
99
F
M
1
0
1979-80
A
Total
7 50”
6 23 73
0002 0161 0 0
59 8
0013 0020
4 2
8 8 58 0
0010 0020 0002 0000
1 2 2 0
50”
50”
D
J
F
12
M Total
9
2 8 21
U Indicates that several avalanches occurred all over the Northern carries. 50 is a large figure to emphasize the frequency of activity, not the actual number of avalanches
powder avalanches are rare, although they may have occurred on Ben Nevis in 1978 and on Loch Muick side (Watson 1975). Cornice falls are commonly reported. Mugnitude Histograms of avalanche runout distances are shown in Figs 4a and 4b. Although most Cairngorm avalanches are small, some are very large. The average height of corrie headwalls and talus slopes is around 300 m and several avalanches travel this distance. The largest avalanches are slab avalanches, and some may become airborne and travel huge distances as a result. For example, an avalanche was seen in Glen Geusachan from the air on 17 April 1979 which travelled about 2 km and part of the way up Beinn Bhrotain. Even more spectacular is an avalanche reported from Coire na Ciste in about 1959, which travelled at least 500 m beyond the position of the car park and restaurant facilities. The considerable distances achieved by these avalanches over uphill or gentle slopes strongly suggests airborne motion. Apart from these examples, very large slab avalanches have fallen from the headwalls of Coire Laoigh Mor and Coire Cas, the east slope of Coire an-t Sneachda, Coire an Lochain of Cairngorm and the Lairig Ghru. On 18 February 1980, the floor of the Lairig Ghru was at least 2 m deep in avalanche debris over a distance of 500 m. In March 1979 a very large avalanche fell from Cadha Mor at the head of Glen Einich and two climbers were killed by a large avalanche in Coire Bogha Cloiche on 26 December 1978. The crown fracture lines of events as large as these are typically between 300 and 500 m, and fracture depths may reach 3 or 4 m, although 0.5 to 1.0 m is a more common figure. The full-depth avalanche from the Great Slab of Coire an Lochain, which often releases in spring, is reputedly 5 m thick on occasions. Elsewhere, Fig. 4b shows the runouts obtained for 54 avalanches. The largest of these were recorded in Glencoe in 1946, on Ben Nevis in 1978, in Gaick in 1800 and Glen Clova (1946 or 1947). Calculated overland travel distances were over a kilometre.
100
A survey of avalanche activity in Scotland
i
Figure 4. Avalanche Scotland (B).
runout
distances
recorded
in the Cairngorms
(A) and in the rest of
Widths are known for 23 avalanches, and are usually small with a median width of only about 50 m. It is likely that much larger avalanches than these figures suggest often release. Most fracture depths appear to be 1 m deep or less, although much greater thicknesses occur, with a maximum of 4 m recorded for a full-depth avalanche in the Ochils and a remarkable 8 m for a surface avalanche photographed by E. Langmuir on Lochnagar. This is, however, exceptional. Apart from the statistics presented above, there are also 26 references to ‘small’ avalanches, nine reports of ‘large’ avalanches and three records of ‘huge’ avalanches from the last 100 years. The conclusion must be that although the majority of Scottish avalanches are undoubtedly small, exceptions occur which produce very large events indeed, involving thousands of tonnes of snow and covering a very large area. This point has not been generally appreciated in Scotland. Annual pattern of avalanche activity
The timing of avalanches, by year, in the Cairngorms is shown until winter 1975-76 by the bars below the x-axis in Fig. 5. Avalanches from December 1977 to March 1980 are shown separately in Fig. 6. There is a considerable increase in avalanche sightings after 1960, and in fact the majority of the avalanches shown prior to 1960 in Fig. 5 are based on the tree-ring work. In recent years, during the winter 1976-77 Cilenmore Lodge staff are reported to have observed 150 avalanches, but data on only 74 has emerged (O’Donovan 1977). Between December 1977 and April 1978,
R. G. W. Ward
101
CAIRNGORMS
Figure 5. The number of avalanches recorded each year in the Cairngorms elsewhere in Scotland until 1980.
until 1976 and
over 100 avalanches were observed, and the flights of 17 April and 6 May 1979 boosted the winter of 1978-79 total to over 600. In winter 1979980, despite generally good viewing conditions, far fewer sightings were made, suggesting that in real terms avalanche activity was down on previous years. Yearly data from 252 avalanches which fell outside the Cairngorms is shown by the vertical bars above the x-axis in Fig. 5. This shows clearly the great increase in avalanche records in the last decade or so, as well as the paucity of records during most of the nineteenth century. For the last ten years, 100 avalanche records exist, with a peak in 1979 of 20 events. The reason for this increase has been suggested already and there is no reason to suppose that avalanches have suddenly become more common in Scotland.
Table 3 shows the percentage of avalanches seen each month for the periods 1893 to 1976,1977,1978 and 1980, excluding June and July. The figures for 1893 to 1976 do not include several ‘spring avalanches’ which may have occurred at any time between March and May. The table shows an interesting change. Prior to 1976, the spring months dominate, accounting for 50 per cent of all sightings, whilst in 1978 and 1980 particularly, February and March stand out. This pattern matches that shown in Table 4, for the rest of Scotland, using the periods 1790 to 1942, and 1790 to 1980. Between 1800 and 1942, about 44 per cent of all records relate to avalanches falling in April, with a further 36.5 per cent in March and May combined. January with 8.5 per cent and February with 4 per cent are, like December, almost avalanche-free. After 1943, this situation completely reversed,
102
A survey of avalanche activity in Scotland Table 3. The percentage of all avalanches seen in the Cairngorms during each month for the periods 1893-1976, 1977, 1978, 1979 and 1980
Month
1893-1976 (%I)
1977 (I%)
197x (1%)
1979 (%)
1980 (%)
2.3 0.0 2.3 22.7 6.8 15.9 29.5 20.5 0.0
0.0 0.0 0.0 91 0.0 3.0 84.8” 3.0 0.0
0.0 0.0 0.0 12.5 11.5 41.3 21.2 10.6 2.9
0.0 0.0 0.0 1.1 7.8 21.1 12.2 57@ O.Oh
0.0 0.0 0.0 0.0 3.0 51.5 424 0.0 3.0
September October November December January February March April May
(1Aerial survey, by Glenmore Lodge. March 1977 ’ Not including aerial surveys 17 April 1979 and 6 May 1979
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D
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F
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Figure 6. The frequency of avalanches each month between the winters of 1977-78 and 1979980.
M
D
79
from observations
: : _. . . . .
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.: .:.
.: :.
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.
.
.
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r
M
.
A
*
M
80
in the Cairngorms
R. G. W. Ward
103
Table 4. The percentage of avalanches seen in Scotland, excluding the Cairngorms, during each month for the
periods 1790 to 1942, and 1790 to 1980
Month October November December January February March April May June
179G1942 (%)
179C1980 (S)
0.0 1.4 4.2 8.5 4.2 19.7 43.7 16.9 1.4
0.5 0.5 5.1 16.2 23.6 16.7 27.3 9.7 0.5
with February and January together accounting for 50 per cent of all records and March, April and May together amounting to only 40 per cent. The probable explanation for this turn is a change in attitude amongst climbers that can be traced easily in climbing literature. Until about 1950, thaw avalanches in spring were thought to be the only avalanche type in Scotland, with few dissenters from this view. Increasingly, however, the ‘windslab avalanche’ of the snowier months, January and February, has been regarded as the chief danger, particularly after 1960. Another possibility is that it also reflects a change in winter climbing habits, with fewer climbers waiting for the milder spring months to come before venturing out. It may of course represent an actual change in the pattern of avalanching, but it is difficult to say why this should be. In any event, the following points should be noted: the ‘avalanche season’ can potentially last nine months, from September to May, but the main period of activity is December to April; peak months appear to be February, March and April, with December and January less important (Fig. 6). However, fewer observations could be made during these latter two months; Scottish avalanches are neither just spring thaw phenomena nor solely mid-winter slab phenomena, but they occur almost all year round. They must therefore include different types. As regards the time of day when avalanches fall, a common contention is that most avalanches fall in the afternoon ‘after the sun has got onto the slopes’. Of the 22 avalanches for which the time of release is known, 17 fell in the afternoon, one at noon, two in the morning and two overnight. Little significance should be attached to this as it could reflect the time at which most climbers (observers) reach the climbing area. In any case the effect of the afternoon sun will often be outweighed by other factors such as snowfall intensity, windspeed or ambient air temperature. Slope angle Slope angles were estimated
from OS 1:25000 topographic
maps, supplemented
by
104
A survey of avalanche
activity in Scotland
field measurements made with an abney level. Histograms of the percentage of avalanches in each slope class are shown in Fig. 7a for the Cairngorms, which does not include avalanches observed from the air. Free faces are excluded since they undergo almost continuous small-scale sluffing, whilst cornice falls account for all slopes up to 90”. Most avalanches release on slopes above 30”, although they have occurred on slopes as low as 26.5”. Where steep drifts form, the slope of the snow surface may bear little relation to the gradient of the underlying terrain. A slab avalanche at Coire na Ciste, for example, had a fracture surface inclined at 57”. although the ground slopes at less than 25”. Excluding free faces, slopes between 35” and 45” are most avalanche-prone. Slab avalanches cluster around 3S-39”, whereas sluffs tend to cluster around the steeper slope classes. Dry and wet slabs occupy similar slope class domains. but there is a slight suggestion that dry sluffs require steeper slopes for initiation than wet sluffs. Of those avalanches seen from the air, over 50 per cent released from slopes above 45” and about 40 per cent released from slopes of 4G4.5”. The remaining few released from slopes of 35-39”. Data from elsewhere in Scotland are shown in Fig. 7b, which shows that sluffs tend to fall on steeper slopes than slabs. Also slabs are infrequent on slopes below 30”, and sluffs absent, whilst a lower limit for all avalanching appears to be about 25”. Dry and wet avalanches appear to occupy similar slope class domains. A
Figure 7. The frequency of avalanches recorded Cairngorms (A) and elsewhere in Scotland (B).
on slopes
of different
gradient
in the
R. G. W. Ward
105
Aspect
Figures 8a and 8b show the aspects of all Cairngorm and other Scottish avalanches, and the dominance of the northeastern quadrant is apparent. This is probably because the main climbing slopes and steep headwalls have this orientation, for example Ben Nevis and Lochnagar outside the Cairngorms, and Coire an-t Sneachda and Coire an Lochain within the Cairngorm area. However, the preferred northeasterly orientation of Scottish carries is itself a reflection of meteorological factors, including the dominant direction of snow drift amongst other things. Avalanche
weather
Since this is analysed at length in the next paper, only a brief description is given here. The main point is that avalanches have been observed under a wide variety of weather conditions. For example, in the Cairngorms, an avalanche on 19 April 1908 from the Devil’s Point occurred in a blizzard, whereas an avalanche fell from the same place in 1931 after three consecutive sunny days. An avalanche in Coire an Lochain occurred on 30 March 1975 in cold clear weather, whilst an avalanche that struck the author on 4 February 1980 fell during heavy drifting. For 166 events occurring elsewhere in Scotland, weather data were obtained from nearby weather stations and eyewitness reports. A summary of the prevailing weather conditions is shown in Table 5, and again the most obvious point is that avalanches occur under a wide variety of conditions. Roughly equal numbers fell in sunny or snowy weather, Table 5. Weather conditions reported by witnesses on days with avalanches falling. Data not including the Cairngorms Comments on precipitation Snow/sleet/blizzard/drifting Rain/wet Mist Cloudy/dull (probably no precipitation) No precipitation
25 7 5 4 28
avalanche avalanche avalanche avalanche avalanche
days days days days days
14 avalanche 13 avalanche 12 avalanche
days days days
20.avalanche 4 avalanche
days days
Comments on temperature Cold/icy Thawing Warm/hot/strong
thaw
Comments on windspeed Driving/windy/drifting Calm Comments on wind direction Winds southeasterly Winds westerly
1 avalanche 1 avalanche
day day
4 avalanche 4 avalanche 4 avalanche
days days days
Combinations Sunny and hot Sunny and cold Windy and cold
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A survey of avalanche activity in Scotland
GROUND
Figure 8. The orientation of avalanche elsewhere in Scotland (B).
activity
SURVEY
in the Cairngorms
(A) and
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107
warm or cold weather and calm or windy weather. This again highlights the danger of ascribing avalanche activity to a single meteorological cause. As will be shown in the next paper, avalanches do follow certain weather patterns, but these are a little more complex than popular opinion in Scotland often allows. Triggers
The importance of a trigger, particularly human or animal as opposed to explosion or falling cornices, is difficult to prove. The mere fact that some avalanches initiate while people happen to be standing in the way hardly implies cause and effect, since sheer probability demands that this will happen occasionally, given a sufficient number of people walking or skiing on steep snow. Consequently, the distinction between triggered and spontaneous avalanches may sometimes be unhelpful. Bearing this in mind, the first recorded case of a ‘trigger’ in the Cairngorms was in 1952, when four climbers were avalanched, and since then a further 45 may have been triggered by human beings. Of these four were during 1976-77, followed by twelve in 1977-78, four in 1978-79 and three in 1979-80. In addition, deer may have started avalanches in 1978 and 1980. Excluding the effect of falling cornices, the majority of avalanches in the Cairngorms release spontaneously. Elsewhere in Scotland there are records of at least 110 events where triggering may have been a factor, most of these being climbing, walking and skiing accidents, and a smaller number where deer were involved. Cornice falls have started sluffs on the slopes beneath on 19 recorded occasions. Conclusions Although the data described above contain inevitable biases which reflect the difficulties of’observing equally under all conditions and in all locations, a number of conclusions may be drawn, chief amongst which are: 1. avalanches are a common occurrence in Scotland and may be a hazard in mountain recreation; 2. attempts to classify avalanches as being of a particular type related to only one or two kinds of weather conditions are misleading and potentially dangerous; 3. some avalanches in Scotland may be significantly larger than is commonly realized. Acknowledgements Many people helped to provide the data presented in this paper and I wish to express my gratitude to them all. David Sugden made many suggestions for improving the text and the technical staff of the Geography departments at Aberdeen University and City of London Polytechnic helped with equipment and cartography. The Natural Environment Research Council provided generous financial assistance. References Beattie, B. (1976) The densification of a seasonal snowpack in the Cairngorms with relation to avalanches. Unpublished BSc thesis, University of Lancaster. Cliff, P. (1978) Mountain navigation. Edinburgh: Diane Thompson.
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Quervain, M. R. (1965) On avalanche classification, a further contribution. In International Symposium on Scientific Aspects of Snow and Ice Avalanches, Davos. IASH Publication No. 69, pp. 196198. Firsoff, V. A. (1949) The Cairngorms on foot and ski. Edinburgh: W. and R. Chambers. Fohn, P., Good, W., Bois, P. and Obled, C. (1977) Evaluation and comparison of statistical and conventional methods of forecasting avalanche hazard. Journal of Glaciology 29 (81), 375-388. Gordon, S. (1949) Highways and Byways in the Central Highlands. London: Macmillan. King, R. B. (1968) Periglacial features in the Cairngorm Mountains. Unpublished PhD thesis, Edinburgh University. Mackenzie, W. M. (1947) The snow and ice climbs of Glencoe. Scottish Mountaineering Club Journal 23 (138), 365-379. McClaggan, D. (1786) An Account of the parish of Blair Atholl. Perth Museum Art Gallery. O’Donovan, R. (1977) Snow bas. Scottish Mountaineering Club Journal 31 (168), 133-137. Raeburn, H. (1920) Mountaineering art. London: T. Fisher Unwin. Sedgeworth, L. (1980) An introduction to avalanches with special reference to the Cairngorms. Unpublished BSc thesis, Strathclyde University. Shaw, R. (1977) Periglacial features in part of the South East Grampian Highlands of Scotland. Unpublished PhD thesis, Edinburgh University. Ward, R. G. W. (1981) Nearly two hundred years of avalanches in Scotland. Scottish Mountaineering Club Journal 32 (172), 140-145. Ward, R. G. W. (1984) An estimate of avalanche frequency in Glen Feshie, Scotland, using tree rings. In Palaeoenvironmental investigations: research design, methods and interpretation (N. R. J. Feiller, D. D. Gilbertson and N. G. A. Ralph, eds). Oxford: British Archaeological Reports. Watson, A. (1975) The Cairngorms. SMC district guide. Edinburgh: Scottish Mountaineering Trust. (Revised manuscript received 18 November 1983)