RETROSPECTIVE FEEDBACKON THE AVALANCHES OF … · 2011. 11. 8. · RETROSPECTIVE FEEDBACKON THE AVALANCHES OF FEBRUARY 1999 IN THE EUROPEAN ALPS Richard M.Lambert * CISM, University

RETROSPECTIVE FEEDBACK ON THE AVALANCHES OF FEBRUARY 1999IN THE EUROPEAN ALPS

Richard M.Lambert *CISM, University of Savoy, Chambery, France.

ABSTRACT:The winter of 1999 was particularly snowy and prone to avalanches on the western sideof the alpine arc and several European countries were hit by devastating drypowder snowavalanches.The author has examined the 4 main sites in question (Montroc, Evolime, Morgex, Galtiir)and carried out a synthesis of his research (real catchment area,orientation of slopes,differenttrajectories,historical precedence... ).He makes several proposals, especially as regards cartography,in order to anticipate "extreme" situations and thus to refine risk zoning.

KEYWORDS: catastrophe, major avalanche, catchmen area, risk cartography.

1. INTRODUCTION

Winter 1999 , especially the month ofFebruary, was particularly snowy anddevastating avalanches occurred on the Alpinearc in Europe. Among them, one in Montroc(France), then in Evolene (Switzerland),Morgex (Italy) and Galtur (Austria) wereusually and surprisingly wide and have lefttheir mark on people's minds.

As an expert, I could analyse theavalanche of Montroc 24 hours after thecatastrophe and I noted the characteristics onthe spot. When working again on this site insummer and autumn 1999 , what I noted "afterthe facts' allowed me to confirm hypotheses ,to understand the facts observed in Februaryeven better: It seemed important to me toproceed in the same expert way for three othersites.

The "a posteriori" field analysishinged on the refine topography of the sites,the traces left by the avalanche, the realtrajectories.

*Richard M. Lambert, Laboratory ofGeography, CISM, University of Savoy, 73360Le Bourget du Lac, France, tel. 00-33-4-50-02­19-25 Fax: 00-33-4-79-75-87-77; email: ReneeFenestraz @univ-savoieJr

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A retrospective feedback has beencarried out to try to draw conclusions for thefuture as regards prevention.

A quick study of the 4 sites will befollowed by a synthesis about the numeroussimilarities between the 4 catastrophes.

To enable the reader to comparebetter, the 4 sites'plans are presentedtoghether in appendix 1.

2. THE AVALANCHE OF MONTROC (Valleyof Chamonix-Mont-Blanc-France)

This avalanche flowed down on 9february 1999 , at about 2.40 p.m. , hitting 20chalets and striking 12 victims.

It started from a cirque at analtitude of 2 400 m, real conch of about O,~km2, with slopes of 40° or more, it entirelydeveloped on a North West oriented side. Theflowing zone divides into 2 parts :

the higher part shows constant slopes of30°, then softer slopes, almost a shelf atabout 1 900 m (the flowing partly wentdown more to the right, but a large part ofthe flows of snow went beyond the shelfand rushed straight down toward thehouses) ;the lower part : after the rock step, a slopebreaking dominates steep slopes (400

- 450

or more) where the avalanche sped up,taking in more snow and air.

At the bottom of the valley, at 1 400m, the avalanche rushed across the river andended its flow on the field facing the slope thusgoing up 15 to 20 m.

The damaged chalets werethere.The soft slab turned into a dry powdersnow avalanche followed by a very destructivedense phase.

The key to understand theavalanche of February 1999 in its major shapeis the passing of the shelf by the snow flowsgone from the top.

No more details can be given fornow: a judicial information has been opened.Beforehand expert reports and the mediamention several historical avalanches ,especially in 1908 and 1843.

The general outline of the site bothuphill and downhill would make convincingprotection works almost impossible.

3. THE AVALANCHE OF EVOLENE(Brequet-La Tour-Valais-Switzer1and)

The avalanche flowed down on 21february 1999 at about 8.20 p.m.

It left from 2 900-3 000 m with abreak which spreaded over 4 km's width (> 3km2) (that is to say the whole side) ; it partiallywent down to the bottom of the valley, down to1 400 m in tongue-shaped flows whichfollowed pronounced talwegs.The snow waspowdery in altitude and down to 1 800 m but alot wetter downhill.

The pictures of the snow depositwhich went down to the lowest point and of theslightly devastated vegetation above the arrivalzone confirm it. If the avalanche haddeveloped over the 1 600 m entirely coveredwith dry cold snow (just like in Montroc andGaltur), there would have been a very powerfulaerosol and the avalanche would have beeneven more destructive.

On the whole of the very largedeparture zone, the slopes situated immediatlywest the Pointe du Prelet are the very oneswhich originated the catastrophe over Brequet-

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La Tour at 1 400 m, with several victims inchalets near the road at the bottom of thevalley.

The 0,2 to 0,3 kill2 departure zoneis steep (over 35°) and oriented south-west. Ascrutiny of the topography shows how the crestwhich closes those western slopes allowed thewest-north-west prevailing winds to over1oadthis windy area which is the real accumUlationzone of the avalanche of La Tour.

Avalanche maps did mention thisrisk with an extension slightly inferior to whatwas noted on 21 february 1999.

The protection works whichare being planned or carried out back up thisanalysis and are done bearing in mind theknowledge of the contrasted topography of thisslope; they also include a good managementof real risks in keeping with the rare frequencyofthe major phenomenon and of the availablebudgets.

4. THE AVALANCHE OF MORGEX(Dailley-Lavancher-Aosta Valley-Italy)

"An avalanche in the vineyards·The avalanche flowed down on 23

February 1999 at about 6.30 a.m.

It started from a wide deep cirque.dominated by the Tete de Uconi at 2930 m, tostop 4,2 km futher against the mountainopposite, going up more than 60 m on thisslope.The lowest point reached is only at 940m of altitude, which gives a total descent ofnear1y 2 000 m !

Down on its way, it hit a hamlet on the right bank, Dailley, crossed the mainroad of the Val d'Aoste and the river. It was apowerful powder snow avalanche with a densephase ; the aerosol and the airblast reachedthe future highway of the Mont-Blanc outside atunnel.

Then against the potential feed .basin which was actually sollicited is very wide(> 2,5 km2) with one side oriented south eastand the other south-west. The slopes arerather steep (sections at 35° or more), therunout zone is a wide "dejection cone· where

vines were planted . The whole site iscompletely oriented south. In those southernparts of the Alps, the probability of veryimportant accumulations of snow whichremains powdery on very steep south-orientedsides, is very rare. The particular weatherconditions of February 1999 allowed the majoravalanche to trigger off and to spread very lowdown the slope and widely to the sides.

Nevertheless, this avalanche in itsmajor shape, was historically known foranother hamlet was on the left bank of itstrajectory: Lavancher, which means"avalancheBin old French (a language spokenin this Italian area ! )

This hamlet is like a boundarymark, a witness of history. The toponymy is anactual reminder of big avalanches whichfrequency of return is very weak.

Why was the old hamlet of Dailley ,situated on the right bank, hit ?

The study of the traces in thevegetation and of the topography (in details) ofthe slopes and talwegs, allow us to suggest adouble explanation :

either the avalanche, around 1 600 m,suddenly widened (fan-shaped tracesosberved on each side over the forest).Either the south-west oriented side belowLa Tete de Drumianaz dragged by thewhole of the faults of the cirque at the topalmost got straight down over Dailley. Ifound the positive traces of an intensivedense phase in this very flowing axis uphillfrom Dailley.

Those 2 hypotheses are not contradictory.

Protection projects are beingstudied: they.are to take the various levels ofvulnerability into account: 2 little villages, onenational road, one international highway.

5. THE AVALANCHE OF GALTOR(paznaun Valley-Austria)

The avalanche struck the very heartof the Village, a ski resort, and made morethan 30 victims on 23 february 1999 at about4 p.m. .

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Once more, it was a hugeavalanche of dry powder snow with aerosol,together with a most destructive intensivedense phase.

Seen the altitudes of the departurezone (Grieskopf at more than 2 700 m), of therunout zone (village at 1 580 m) and the"continentalB position of this high valley with aharsh climate, the snow was cold and dry allthe way down.

The successive and intensive snowfalls over several days, together with west­north-west winds, created the criticalconditions for the triggering off since they arefavorable to accumulations on the slopesdominating the village.

The departure zone is large (morethan 0,3 km2) and very steep (over 45°). Theflowing zone includes one steep slope (30°).The slopes get softer at the bottom of thevalley.

Usually, on this south-orientedsteep-sloped side, the snow layer never getstoo thick and most of all, it transforms morequickly.

Thus the avalanches which usuallyflowed down this side spreaded over the .slopes down to the river, maximum, thereforenot threatening the village itself.

But by the end of February 1999,the huge amounts of snow and the quality ofthat snow which had remained cold and dry,allowed a wider longitudinal extension of theavalanche than if it had been a heavy snowone which took an ·unusual" distance to stop.

Unfortunately, the village is right inthe axis of this zone prone to avalanches.

A few gathered elements are saidto indicate that formerly (XII-XIII century), theabbey of Galtur owned fields on which nothinghad ever been built for centuries andcorrespond to the most seriously hit sector atthe center of the present village . A path usedto bypass those fields. But I could notpositively establish whether there was a link

with the risk of a major avalanche or with otherfactors.

Since it is a matter of safety for awhole resort, consequent works of activeprotection in the departure zone (supportingstructures) and of passive protection down theway (dikes) are being carried out or alreadycompleted.

6. SYNTHESIS

This overall vision of the 4 sitesenables us to draw several conclusions:

These avalanches have all hit inhabited areas,sometimes long settled. Each one of them hadalready occurred in the past, with rathersimilar extensions, but they had been"forgotten" and their extents surprisedeverybody. They have three common points:

6.1 The "exceptional" weather conditions bythe end of January and in February 1999originated the 4 catastrophes examined. Thesuccessive disturbances from north-west,together with strong winds, corresponding tovery active cold fronts heaped up a thick snowlayer (over 2,5 m in 4-5 days) and brought bothdry and cold snow qualities, which favoredpowder snow avalanches of great extent;those avalanches could spread far down thevalleys, especially if the ground configurationallowed it, which is particular1y true as regardsMontroc and Galtur.

In Morgex, the extent and altitudeof the departure zone, the steepness and thelength of the track favored a very powerfulpowder snow avalanche.

In Evolime, the snow was wetterover the loWer third of the way. Thus only a fewtongue-shaped dense snow flows reached thevalley.

Therefore, it ought to be admittedthat the falls of dry, cold, intense snow whichlast are always possible, even if they are seenonly a few times a century or twice over threehundred years.

6.2 The potential feed basins which wereactually activated over their whole surface in

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February 1999 were very important and/or verypronounced, sometimes misleading sincesouth-oriented. They generated (and accountfor) strong cubages which went very lowdownhill. They must not be underestimated.

6.3 The so-called "unusual" or "unpredictable"trajectories, as well as the surprisinglongitudinal developments which go 50, 100 or500 meters beyond the classical limits to stop,milst be carefully analysed.

Actually, they can be much betterunderstood if we check the generating feedbasins and if they can be explained by the waybig powder snow avalanches develop ; thoseavalanches are rare since they depend uponthe convergence of weather conditions whichare not recurrent, hence their being all themore dangerous as they are "forgettable".

These avalanches are known toacquire high speed, to spread very quiCkly,have particular physical flowing characteristicsand tend to flow down in straight line or fromsteep slopes. over distances longer than anyother type of avalanche. This was the case forthe 4 avalanches studied.

7. PERSPECTIVES AND CONCLUSIONS

7.1 Historical precedence of the events

One chance for the mountainousvalleys of old Europe compared to those ofAmerica is a long ancestral memory ofcatastrophes, for these mountains have beeninhabited for 4 centuries or more, and archivesare kept. The resort to history is important :the possibility to "trace" an avalanche ought tobe preserved and "exceptional" phenomenashould be inquired over 200-300 years.

7.2 An interactive and prospective cartographY

As we have seen, there is a clearconnexion between :

the avalanche type (hence the type ofsnow in motion)the frequency (= probability of return)the trajectory (ies) followed by theavalanches and extreme reach ofavalanches.

It seems necessary that those 3criteria appear together in order to show or topredict the consequences of a powder snowflow as regards weak frequency and·unexpected" trajectories.

What is more, the real feed basinsmust be identified and mapped, together withthe prevailing winds which overload them.

On avalanche maps, this option torelate those 3 criteria often reveals majorphenomena and may influence hazard zoningpolicies.

These proposals may enable toanticipate "extreme"situations, to limitprospective tragedies even if the climate is stillup to several big snowstorms.

8. REFERENCES

Barbolini, M., 2000. The catastrophic 1999alpine winter: analysis of the "Lavancher".event,Morgex,ltaly. Proceedings ISSW 2000.Big Sky.Montana.Briindl,M., Wiesinger,T., Wilhelm,C.,Amman,W., 2000. The avalanche winter 1999in Switzerland. An ovelView. ProceedingsISSW 2000.Big Sky.Montana.Heumader,J., 2000. The catastrophicavalanche disasters of Galtuer and Valzur onthe 23 and 24 of february 1999 in thepaznaun Valley (Tyrol).lnterpraevent2000.1,397-409.Lambert, M., 1992. The representation ofForeseeable Natural Risks: a contribution of anew cartography of avalanches. Theoreticaland Applied Climatology. 45,83-88.Rapin, F. and Ancey, C., 2000. Conditions oftwo major avalanches at Chamonix.France.Proceedings ISSW 2000.Montana,Schaffhauser, H., Holler,P., 2000. The 1999avalanche disasters at Galtur and Valzur .Proceedings ISSW 2000.Big Sky.Montana.

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