The Avalanche Catastrophe of Galtür.pdf


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The Avalanche Catastrophe of Galtür.pdf

  1. 1. The Avalanche Catastrophe of Galtür, 23 February 1999, from the vantage point of meteorological and avalanche research Rudi MAIR Avalanche Warning Service of Tyrol, Innsbruck, Austria. E-mail: ABSTRACT: On 23 February 1999, an enormous powder snow avalanche destroyed several districts of the Tyrolean village of Galtür, causing 31 fatalities. The next day another powder snow avalanche cost an additional 7 lives in the neighbouring village of Valzur. The major cause of this catastrophe was a combination of unusual weather conditions and extremely stable snow layering. This stable snow layering hindered and/or prevented earlier potential avalanches, thereby leading to the accumulation of enormous masses of snow in the avalanche starting zones and subsequently, to the avalanche catastrophes. Keywords: avalanche catastrophe, extreme snowfall, avalanche danger, avalanche situation, Galtür. 1. INTRODUCTION The community of Galtür (altitude 1583 meters) is located in the innermost Paznaun Valley, in the furthermost southwestern corner of the state of Tyrol. The “Paznauntal” extends over a length of about 40 kilometers, from the Bielerhöhe ( 2,036 m) on the border to the Austrian state of Vorarlberg in the northeast, to the opening of the valley (at about 950 meters altitude) below Wiesberg Castle. Paznaun has dense forests in its early sector, filled with gorges and steep gradients; it then becomes quite narrow in the middle sector with heavily wooded, steep slopes; in the innermost sector of the upper part of the valley it has a relatively wide valley floor and a fertile alluvial delta. Due to its very exposed site, Galtür has been subject to avalanches since time immemorial. 2. SYNOPTIC METEOROLOGY AND GENERAL WEATHER SITUATION IN WINTER 1998/1999 A rather tense avalanche situation prevailed far and wide in Tyrol in the winter of 1998/1999. In February 1999, conditions led to extremely high avalanche activity which culminated in the avalanche catastrophes of Galtür and Valzur. A stable northwestern barrier effect, with clouds being lodged against the mountain flanks, is established when the distribution of air pressure systems brings about a low pressure system northeast of the British Isles and an Atlantic high pressure system southwest of it. This general weather situation led to strong northwesterly air currents which in their turn brought moist, mild air masses from the central Atlantic up against moist, cold air masses from the northernmost Arctic regions. The resulting front, when combined with leverage effects along the mountain ranges, leads to heavy precipitation throughout the Alps. This circulatory pattern occurs regularly in the air currents circling over the northern Atlantic, since an equilibrium is thereby established between the masses of cold air coming from the Arctic and the mild air masses coming from sub-tropical latitudes. Ordinarily, such a northwesterly air current lasts only a few days, after which other circulatory currents take over. In winter 1998/1999, however, a similar air current repeated itself three times in close succession, with only a week between each bout. 3. LOCAL WEATHER AND SNOW LAYERING IN GALTÜR IN WINTER 1998/1999 The winter of 1998/99 can be sub-divided into three phases: • In early winter, from October until December, unusually mild temperatures prevailed, accompanied by insufficient amounts of precipitation. Greater amounts of snowfall did not occur until the end of November and the early part of December. • The months of high winter, January and February, were very cold at high altitudes. Intense precipitation subsequently led to severe avalanches. • During the late winter months of March and April, quite mild temperatures prevailed overall. Precipitation was average. In mid-April in particular, several thrusts of cold air increased the avalanche danger again. During January and February 1999, a northwesterly air current dominated three different times (27-31 Jan. 1999; 5-12 Feb. 1999; 17-25 Feb. 1999) at intervals about one week apart, which brought about heavy precipitation in Tyrol. The snowfall was accompanied by storm-strength northwesterly winds which led to deep snowdrift accumulations forming widespread. Between the periods of precipitation, continental cold air streamed towards the Alps. The low temperatures hindered the snowpack from settling and stabilising. A further aggravating factor was that during the final period of precipitation, 20 - 21 February, a current of warm air was released over the Alps. The snowfall level ascended to 1,800 meters, and ongoing rainfall made the snowpack 1
  2. 2. thoroughly wet, thus increasing its weight enormously. The 210 cm overall snow depth at the end of February was the highest snow depth ever measured in Galtür. 4. GENERAL DISTRIBUTION OF EXTREME VALUES (GEV) OF PRECIPITATION AT THE GALTÜR MEASUREMENT STATION In meteorology, the analysis of extreme values - which occur rarely but which are linked to immense financial damage - are of importance in assessing risks. Ordinarily, an extreme event is the maximum in a random sample measured over a certain specified length of time. The extreme events defined in such a way are then depicted in the general distribution of extreme values (GEV). Rank 1 2 3 4 5 Date [cm] Date [cm] Date [cm] Date [cm] Date [cm] 1 day 23.04.1928 110 04.02.1992 80 19.01.1910 79 18.01.1974 74 10.01.1995 70 2 days 17.01.1974 115 22.04.1928 110 03.02.1992 105 18.01.1910 104 10.01.1995 100 3 days 16.01.1974 145 03.02.1992 125 10.01.1995 125 19.01.1951 115 18.01.1910 112 4 days 16.01.1974 148 09.01.1995 145 18.01.1951 129 01.02.1935 125 02.02.1992 125 5 days 15.01.1974 148 08.01.1995 145 01.02.1935 145 18.02.1999 140 17.01.1951 129 10 days 16.02.1999 245 12.12.1962 189 18.12.1991 162 31.01.1961 159 12.01.1951 156 15 days 10.02.1999 250 08.12.1962 213 26.01.1999 209 10.02.1937 197 29.12.1994 194 Month Feb.1999 375 Feb.1970 266 Jan 1995 263 Feb.1937 242 Feb.1935 240 Year 1998 952 1994 826 1999 794 1991 779 1974 777 Table 1: Ranking various periods of extended snowfall (Mair, 2009) This table shows that the greatest amount of snowfall ever measured on one sole day in Galtür, i.e. over a period of 24 hours, was 110 cm on 23 April 1928. In winter 1998/1999, on the other hand, the greatest amount of snowfall measured on one day was “merely” 50 cm. However, the longer the measured period, the more the winter of 1998/1999 thrusts itself into the foreground: thus, the 140 cm of snowfall in a 5-day period from 18 Feb. 1999 rank fourth in the 100 years of measurement statistics; and both the 245 cm during a 10-day period (16.2.1999) and the 250 cm during a 15-day period (10.2.1999) rank first. It was not the amount of snowfall during a short period which was remarkable during the winter of 1998/1999, but much more, the long-lasting, persistently heavy precipitation. Return interval 5 10 20 30 50 100 150 200 300 [Years] 1 day 45 53 61 65 71 78 83 86 90 [cm] 2 days 64 76 87 93 102 113 119 123 130 [cm] 3 days 77 91 105 113 123 137 145 150 158 [cm] 4 days 85 101 117 125 137 151 160 166 175 [cm] 5 days 93 110 126 136 148 164 173 180 189 [cm] 10 days 118 140 161 173 188 209 221 229 241 [cm] 15 days 137 162 187 201 218 242 256 266 279 [cm] Month 144 166 188 201 217 239 252 261 274 [cm] Year 518 617 713 768 836 929 983 1021 1075 [cm] Table 2: Calculated values of various return intervals of snow depths in cm (Mair, 2009) The calculated values of various return intervals for snow depths in Galtür underscore how the winter of 1998/1999 turns out to be more extraordinary, the longer the measured periods of precipitation are. The graphic evaluation of the return interval times with the General Distribution of Extreme Values (GEV) shows, for example, that the 10-day sum total of 245 cm of new fallen snow in 1999 ranks ahead of all other years; the annuality is greater than 200 years. Fig. 1: 10-day maximum new fallen snow (left) and overall snow depth (right) in Galtür in February 1999 (Mair, 2009) 2
  3. 3. To draw to a close these considerations of extreme statistics, a final glance at the overall snow depth in Galtür in February 1999 is called for: a snow depth of 210 cm ranks first in the one hundred years of measurement, the theoretical return interval time is calculated at more than 200 years. To summarize, it can be established that the daily precipitation during the winter of 1999 in Galtür does not appear to be extraordinary, since that pattern is repeated every few years. However, the longer the period of measurement is, the more extraordinary the events of winter 1999 prove to be. The return interval time both for the 10-day total snowfall as well as for the overall snow depth in Galtür in February 1999 is far beyond 200 years! 5. AVALANCHE CATASTROPHES The massive avalanche in Galtür on 23 February 1999 was largely the result of very intense and unusually long lasting periods of snowfall in winter 1998/1999 as well as the prevailing weather conditions, in particular the winds and air temperatures. One immediate consequence of the meteorological conditions was the relatively well consolidated snowpack base, which prevented any earlier avalanches. As regards the triggering mechanism, the large-scale avalanches were, at a percentage of probability bordering on certainty, naturally triggered. That means that the avalanches were unleashed due to the accrued load of ever-increasing amounts of fresh fallen snow and snowdrift, which ultimately caused the trigger-sensitive transition layer between the old snowpack and fresh fallen snow to be overburdened beyond its capacity. During the afternoon of 23 February 1999 at about 4:05 pm, beneath the ridge linking Grieskogel (2641 m) and Grieskopf (2754 m), on the so-called “Sonnenberg”, a naturally triggered, large-scale, dry slab avalanche was triggered. The depth of the fractures were approximately 2.5 to 3.5 meters, unusually deep; the greatest fracture depths were observed in steep, rocky places with a weak snowpack fundament. Fig. 2: Overview map of Galtür (source: Orthofoto) with Sonnberg and the avalanche contours (left); overall view of the “Sonnberg” from the south with the fracture region between Grieskopf and Grieskogel, together with the sub-division into “Weisse Riefe” (“White Flute”) and outer and inner “Wasserleiter” (“Waterline”) avalanches (right) (Mair, 2009) 3
  4. 4. The snow masses which were unleashed initially plunged down towards a slope ledge whose altitude varies between 2450 and 2550 meters. The avalanche flowed over this ledge at high speed, below which the gradient of the mountainside significantly increases. Over a distance of 500 meters, maximum gradients of about 40° were measured, the average gradient was about 36°. The avalanche which plummeted towards the valley probably swept along additional large snow masses with it, since at the time it was triggered the snow in that area was more than 4 meters deep. Due to the topography of the terrain, it can be assumed that the avalanche was about 800 meters wide down to a point in the terrain at 2350 meters altitude. Beneath that spot, the avalanche, because of the increasing steepness of the slope and the ever-increasing velocity of the snow masses, developed into a powder avalanche. In the process of this, the fine-grained, dry and loosely packed snow is whipped and tumbled about in such a way that a mixture of snow and air is formed, the definition of a so-called “powder avalanche”, which then divided into two fall tracks. The left sector pitched through the steep, craggy gully of the so-called “Weissen Riefe”. The right sector of the avalanche, which was larger, probably plummeted over the approximately 45° steep, rocky slope still as a flat avalanche, to begin with. In this phase, its speed escalated further and even more snow masses were stirred up. Below about 2000 m, this sector of the avalanche was divided yet again by a broad, scarcely prominent ridge above the so-called “Egge” into two further sectors. The left part of this sector, the so-called “outer waterline avalanche”, plummeted through a shallow bowl about 50 to 100 meters wide. At an altitude of 2200 to 2000 m the flat fall-line of the terrain then becomes steeper again, which led to the disgorged snow masses being stirred up into eddies in the air which are conducive to the formation of a powder avalanche. The fall line of this bowl points straight in the direction of the village district of “Winkl” in Galtür. The powder avalanche was hardly able to spread out at this stage of its descent, since on its left the snow masses of the Weisse Riefe avalanche were plummeting towards the valley and on its right, the broad ridge “Egge” prevented its being able to disperse laterally. Due to these circumstances, the snow masses of this particular branch of the avalanche were concentrated still further, which ultimately explains the extraordinarily large breadth and destructive power of the outer waterline avalanche in Galtür’s district of Winkl. In addition, the rough convexities and concavities of the natural terrain had already been filled up and smoothed over by previous avalanches, which contributed to this avalanche attaining a far higher velocity and longer avalanche path. The avalanche catastrophe of Galtür on 23 February 1999 was by and large, both in the hallmarks of its particular track and from the point of view of the damage caused, a powder avalanche. The flow part of the avalanche, insofar as all available evidence is taken into consideration, must have been quite small. All in all, eleven houses were completely buried by the avalanche, some of which had been standing for more than 400 years; 17 further houses were partially destroyed or damaged. The largest damages in Galtür’s district of Winkl were apparently caused by the so-called “saltation layer”, that is, the lowermost and densest part of the powder avalanche. Since the mixture of snow and air is far more dense in the saltation layer than in the layer lying on top of it, the so-called “suspension layer”, the pressures and resulting potential for destruction are correspondingly higher. The suspension layer itself is essentially a stirred-and-eddied snow cloud which bursts, yanks and swirls its way through the air, thus the name “powder avalanche”. Fig. 3: Utterly destroyed house ‘Litzner’ (left) and destroyed apartment house in Winkl district (right) (Mair, 2009) According to calculations, the maximum avalanche pressure at the Litzner house was 55.6 kPa; and at the Walter house, 33.6 kPa. If the deposited snow masses of the Outer Waterline and the White Riefe avalanches are cubed, in accordance with a number of different calculation models, it comes to approximately 145,000 m3 to 160,000 m3, which corresponds to a snow mass of 80,000 tons. 6. INTEGRAL RISK MANAGEMENT For avalanche protection, a sustained programme to minimize risk requires, above all else, a comprehensive understanding of the associated and interconnected factors of snow layering, the formation and the dynamics of avalanches. Only such a thorough understanding can lead to appropriate precautionary measures being taken which are thereby necessitated. As regards an integral risk management, the following themes must be addressed above all others: 4
  5. 5. Meteorology and Snow layering The challenges for a modern avalanche warning service are continually rising. This makes the necessity of ever improving meteorological and nivological (snowflake physics) data as a basis for evaluation indispensable. Whereas Tyrol has an outstanding network of automated meteorological measurement stations in operation, there is still a lack of truly effective models for evaluating snowpack stability. Organisational Measures Above all else this includes the entire sector of timely avalanche alerts, in other words, the early warning system of avalanche forecasts all the way to road closures and evacuations. The Avalanche Warning Service of Tyrol, both with regard to personnel as well as technical facilities, has to be adapted to the changing safety and informational needs of society. Both the expansion of the existing network of automated weather stations and the development of new analytical and forecasting models are essential. Mapmaking In this area, the compilation and/or adaptation of danger-zone maps is required. Such maps are often lacking. If responsibly constructed danger-zone maps are compiled and taken into due consideration, most of the damages to buildings can be prevented or, at very least, minimized. It is also imperative to inspect and overhaul existing danger-zone maps in ongoing fashion and adapt them to new avalanche knowledge and calculation models. Technical Measures This comprises all construction measures, including fracture prevention walls, avalanche breakers, diversion and retention walls as well as avalanche protection tunnels. In this area as well, the conclusions of modern avalanche simulation have to be reflected in the dimensions of the construction, that is to say, existing structures must be updated and adapted to the latest discoveries and insights with regard to avalanches. It goes without saying, all technical measures must take into consideration both settled residential areas and tourist facilities in equal measure. Integral Risk Management The most important element in minimizing avalanche danger is doubtless a concerted programme of avalanche protection combining all available knowledge and actions which complement each other in maximizing protection. In the final analysis, the damages which resulted from the avalanche winter of 1999 were far less the consequence of lacking or inadequate protective measures than they were of intensified land utilization. For that reason, it is utterly essential to analyse and depict changing avalanche risks with state-of-the-art methods; and to devote our fullest attention to the potential damage avalanches can cause. Ideally, this should lead to a strategic risk-minimizing plan which can (and should) draw into the calculation a variety of differentiated safety levels which are required, that is, it should distinguish between tourist-frequented, traffic-utilised and other places based on the extent to which they are used. At the end of the day, this is not only a matter of safety, but an issue affecting the entire body politic and economy. This is even more the case when you realize that from a scientific point of view, neither flawless construction measures nor perfect avalanche forecasts, all of which are meant to utterly exclude the perils of avalanches, are possible.... 7. REFERENCES Mair, Rudi (2009). Meteorologische Analyse und Extremwertstatistik der Lawinenkatastrophe von Galtür 1999. Wildbach- und Lawinenverbau, Nr. 162, p. 68-92 Mair, Rudi (2009). Untersuchungen zur Lawine von Galtür 1999. Blickpunkt der Forschung, Band 1, p. 6-63, Lahnbrück-Verlag, ISBN 978-3-9812777-0-8 5