THE AVALANCHES OF GALTOR AND VALZUR IN FEB. 1999

Peter Holler and Horst Schaffhauser*

ABSTRACT: The avalanche of GaltOr (Feb.23 1999) was the most serious single accident inAustria since 1954. There were 31 fatalities, 6 buildings destroyed and 18 buildings damaged.Due to an area of high pressure over the Bay of Biscaya and an area of low pressure overScandinavia the Alps received moist air from the Atlantic and cold air from the Arctic. This situationleaded to intensive precipition in the Arlberg- AuBerfern- and Paznaun area in February 1999 andto many large avalanches.Calculations with the Samos Avalanche Model showed some interesting results. The pressure(which was calculated for those sites where most of the buildungs were destroyed) was in the orderof 100 kPa. About 26 % of the totally released snow mass (136 kilotons) was transferred to thepowder part of the avalanche.As a result of the avalanche the Austrian Government spent about 130 Mio ATS (about 10 Mio $)for different protection measures (supporting structures in the release zone, two catching dams....)in this area.The two accidents (GaltOr and Valzur) leaded to new design criteria for hazard maps inAustria. While the old decree determined 25 kPa for the Red Zone the modified guidelines haveextended it to 10 kPa (valid since JUly 1999).

KEYWORDS: Avalanche Disasters, Avalanche Zoning, Simulation of Avalanches

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Fig.1 Height of daily new snow in cm (rightaxis) at the observation site GaltOr in about1600 m from Jan 26. to Feb. 28, 1999 (Gabl.2000); the left axis shows the sum ofnew snow (in cm).

As shown in Fig. 2. the total depth of snow atthe observation site Madlein/lschgl (this is veryclose to GaltOr and Valzur) in 2280m increasedfrom about 80 cm (end of January) to 300 cm(Feb. 22nd); due to low air temperatures inFebruary 1999 the settlement of the snowpackwas relatively slow (Fig. 2).

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* Peter Holler and Horst Schaffhauser,Federal Forest Research Centre,Institute for Avalanche Research,Hofburg - Rennwg 1,A-6020 Innsbruck,tel:+43 512 573933, fax:+43 512 5739335250e-mail: Peter.Hoeller@uibk.ac.atHorst.Schaffhauser@uibk.ac.at

1. METEOROLOGICAL CONDITIONS

The synoptic situation in January and February1999 was characterized by an area of highpressure over the Bay of Biscaya and an areaof low pressure over Scandinavia. The resultingnorthwesterly oriented frontal zone did notcause very intensive daily snowfalls but leadedto three storm periods; the first one end ofJanuary (from 26th to 31 st) with about 100 cmof new snow, the second from 5th to 9th ofFebruary (appr. 130 cm of new snow) and athird period (from Feb. 17th to Feb. 24th) withmore than 200 cm. Especially the Arlberg-,AuBerfern- and Paznaun Area received plentyof snow. Fig 1. shows the height of new snowat the observation site GaltOr in about 1600 m(Jan 26. To Feb. 28, 1999); the total height ofprecipitation in February 1999 was 245 mm,which is about four times as much than theaverage (Gabl, 2000).

Snow depth (in em ) Madleinllschgl (2280 m)

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oJ....-<-+--+--+--+-f-----t-+_+__-+-+----+-+_+___+_m m mmmmmm mmmmm~ N N N N N N N N N N No 000 000 0 0 0 0 0m ~ ~ ~ m~. M ~ m~ ~ ~N 000 0 r r r r N N N

Temperature On 'C) MadieinJlschgl (2280 m)

·25.1-----------­m m m m m m m m m m m m m m m m mm m m m m m m m m m m m m m m m m~ ~ ~ N N N N N N N N N N N N N N000 0 0 0 0 0 ~ 000 ~ ~ 0 ~ 0ID ~ 0 ~ ~ ~ ~ mr ~ ~ ~ m r ~ ~ ~N N ~ 0 0 0 0 0 r r r r r N N N N

Fig. 2. Snow depth and air temperature atthe observation site Madlein/lschgl in about2280 m from Jan. 26 to Feb, 27,1999 (MAPData Centre, 1999)

2. THE AVALANCHES OF FEB. 23 ANDFEB. 24

On Feb. 23rd, 1999 (4pm) a dry-snow slabavalanche released on the south facing slopejust above the village of Galtor (which issituated in the Paznaun valley in the westernpart of the Tyrol). The starting zone of the twoavalanches ("WeiQ.e Riefe" and .AuQ.ere

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Wasserleiter") was near to the GrieQ.kogl inabout 2700 m (inclination 38 to 50°) andreached Galtor in about 1580 m above sealevel. There were 31 fatalities, 6 buildingsdestroyed and 18 buildings damaged (alsooutside the actual hazard zones). The damagewas estimated with about 72,5 millions ATS(appr. 5 Mio $). .The avalanche was running longer thanexpected (see Fig.3); the deposition area .reached into a part of the village (appr. 250 mfrom the Red Zone). A look to the avalanchestatistics (Holler, 1997) shows that it was thegreatest avalanche disaster in Austria sincethe 1950s.

Fig. 3. View from the starting zone of th~ .avalanche to GaltOr. The damage area IS In themiddle of the photograph.

One day later (Feb.24) there was a secondaccident only a few kilometersfrom GaItOr. The avalanche ("InnereRiefenbachlawine") starting zone was in2700 m and reached Valzur (appr. 3 kmnortheast of GaltOr) in about 1480 m.7 persons were killed, 7 buildings destroyedand 1 building was damaged. The damage wasestimated with about 56 MioATS. The mainflow was running straight on the alluvial cone(inclination 11 to 14°) and did not follow ~he

gully which was formed by the torrent (Fig. 4).The avalanche was totally different from theavalanches in GaltOr. While the avalanche pathin Galtor was on a open slope and thedeposition zone in the valley floor the path ofthe Riefenbachlawine was in a channel and therunout zone on a debris cone.

Fig. 4. Alluvial cone of Valzur and avalanchedeposit (Feb. 24, 1999).

The reasons for the great avalanches inFebruary 1999 can be found in the extremevalues of snow depth and fresh snow (threestorm periods) as well as in the lowtemperatures (almost no settlement of thesnow pack) during Feb.99

3. THE SAMOS MODEL, A SHORTDESCRIPTION

Generally the so called "catastrophicavalanche" is described asa dry dense flowavalanche with a high powder component. Dueto their completely different physical behaviourof both components a two dimensional granularflow model and a three-dimensional turbulentflow (Powder part) were generated undercoupling of a mass transfer model. On theexample of the GaltOr disaster 1999 it isreported about the application of the SAMOS(Snow Avalanche Modelling and Simulation,Sampl, Zwinger 1999).

4. RESULTS OF THE SAMOS-GALTORSIMULATION

The area of the release zone was determinedby the field observation of the Austrian FederalService for Torrent and Avalanche Control.Based on the before mentioned meteo-dataand own field studies directly in the startingzone (snow profile measurement) a total snowmass of 136 kts was calculated (Sampl,Zwinger, Schaffhauser 1999). In figure 5 it is

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represented the terrain model, the outlinedrelease areas and the maximum impactpressure over the whole sphere of action of thedense flow part with a scaling from 0 to 250kPa. In direction of the avalanche centre line

Fig. 5: Distribution of the max. pressure fromthe dense flow part of the avalanche

the dense flow part thrusted forward, crossedthe road and destroyed with a calculatedpressure round about 100 kPa the building infront of. The mapped damages are in good linewith the computed presentation. The same rulewill be applied to the deposition heightsbetween 0 to 5 meters. From figure 6 the

Fig. 6: Distribution of the max. pressure fromthe powder part of the avalanche (2.4 m abovethe ground)

Tab.1. New guidelines for avalanche zoning inAustria (BMLF, 1999)

houses were allowed to be rebuilt on the sameplace. Supporting structures in the startingzone are planned and will be done in the nextyears.In Valzur it was decided to have no buildingson the alluvial cone in future; the houses will berebuilt on a safe place, appro 300 m up thevalley.However, the two accidents leaded to newdesign criteria for hazard maps in Austria.While the old decree determined 25 kPa for theRed Zone (Holler, 1997) the modifiedguidelines (Tab.1) have extended it to 10 kPa(valid since July 1999).

influence of the powder snow impact is drawnin a scaling range betwen 0 to 25 kPa. Therecalculation of load limits of destroyed ordamaged roof frameworks corroborated alsotheese results and are consequently usefuldata for the verification of the SAMOS-model.

One month before the disaster in GaltOr theSAMOS-project-financed by the Ministry forAgriculture and Forestry in Austria - wassuccessfully brought to a close. In the case ofGaltor it is always again a tragic fact that suchdisaster open up rare and importantpossibilities for the research work to verify theresults and the usefulness of avalanchedynamics modelling. This mathematical modelis used to forecast the run-out distances ofavalanches, the flow height, the height ofdeposition, as well as the vertical distribution ofdensities, velocities and total pressures. Forthe last year the SAMOS model got animportant tool in the frame of the authoritieshazard zoning and to solve engineeringproblems.

5.CONSEQUENCES

RedYellow

Return period 150 y Return period up(design event) to 10 Yp> 10 kN/m2 p> 3 kN/m2

1 < P < 10 kN/m2 1 < P < 3kN/m2

As a result of the avalanche accidents theAustrian Government spent about 130 Mio ATS(about 10 Mio $) for different protectionmeasures (supporting structures in the releasezone, two catching dams....) in this area. InGaltOr two avalanche catching dams weredesigned which are already finished (Fig.7); the

Fig. 7. Avalanche catching dam in GaltOr

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6. REFERENCES

BMLF (Bundesministerium fOr Land- undForstwirtschaft), 1999. Erlaf1 betreffendRichtlinien fOr die Abgrenzung vonGefahrenzonen (Erlaf1 Nr. 52.240/10­VC6a/99).

BMLF (Bundesministerium fOr Land- undForstwirtschaft), 1989. Lawinen inOsterreich.

Gabl, K. 2000. Der Schnee im Februar 1999 imWesten Osterreichs aus meteorologischerund klimatologischer Sicht. Wildbach- undLawinenverbau 64 (141), 69-80.

Holler, P. 1997. Avalanche Control andAvalanche Research in Austria since WorldWar II. Avalanche Review 16 (1),6-7.

MAP Data Centre, 1999. Air temperature andsnow depth on observation site88706 (Madlein/lschgl), MAP Data Centre,ETH ZOrich. www.map.ethz.ch

Sampl, P; Zwing.er, T. 1999. A simulationmodel of dry snow avalanches. InXXVIIIIAHR-Congress, Aug. 22-27,Graz, Proceedings, p.287.

Sampl, P; Zwinger, T; Schaffhauser, H. 2000.Evaluation of Avalanche Defense Structureswith the Simulation Model SAMOS,Felsbau-Rock and Soil Engineering,1/2000, p.41- 46.

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