STUDY OF THE 1999 AVALANCHES IN THE OBERGOMS VALLEY, , WITH RESPECT TO AVALANCHE HAZARD MAPPING

Urs Gruber and Perry Bartelt * Swiss Federal Institut of Snow and Avalanche Research

ABSTRACT: The Obergoms valley is a highly visited tourist region in south-western Switzerland. It was dramatically struck by avalanches during the Winter of 1999. Avalanches occurred nearly in all known avalanche tracks. Fortunately, avalanche hazard maps and a historical avalanche database existed. The ood weather conditions after the most extreme avalanche events allowed a good documentation. §UbSequently, the intensity of the avalanches, the existence of avalanche hazard maps and the good information about the actual events make the Obergoms Valley an ideal area to appraise avalanche hazard maps. The comparison of the avalanche events to the existing hazard maps reveals that more than 80% of the avalanche events are in good agreement to the existing hazard zones. However, in three villages, Reckingen, Blitzingen and Geschinen, avalanches clearly exceeded the hazard limits. The main reason for these failures were (1) the underestimation of the runout of powder snow avalanches, (2) the omission of multiple avalanche events in a single avalanche track and (3) the under-dimensioning of deflecting dams. Therefore, the study of the Obergoms Valley led to the conclusion, that research should be focused primarily on powder snow avalanches, starting conditions and the inclusion of entrainment in snow avalanche models.

KEYWORDS: Avalanche Hazard Mapping, Avalanche Dynamics, Deflecting Dams, Deposition Depths.

1. INTRODUCTION withstand impact pressures of up to 3 tm'2. The yellow zone delineates areas where either The Obergoms Valley, Canton Wallis, is avalanches occur very rarely or where the impact one of the most endangered populated areas in of a powder snow avalanche is expected. Until Switzerland. Catastrophic avalanche events now the delineation of this yellow zone was not reaching the valley settlements were reported in mandatory. The fourth zone (white) is considered 1720 (84 persons killed), in 1827 (51), in 1888 (0), to be free of avalanche hazard. Since 1981, in the in 1951 (0), in 1970 (30) and in 1984 (2) (Lauber, Obergorils valley a local avalanche security 1999). The population is well aware of the service exists that is responsible for the local avalanche hazard. Historical avalanche events avalanche warning and provides the local have been systematically recorded and for the authorities with suggestions for the closing of settled areas hazard maps were drawn up roads and the evacuation of people. according to the recommendations of avalanche In 1999 the avalanches struck the valley. hazard mapping in Switzerland (BFF and SLF, Es~ecially in the time period between February 1984) and the Swiss calculation guidelines (Salm 201 and 25 th avalanches of extraordinary et aI., 1990). Four different zones, distinguished dimensions led to the death of one person, from each other by the impact force and the damaged or destroyed 47 buildings and destroyed avalanche frequency, are used to define the nearly 15'OOOm3 of wood. Almost 7000 people ­ hazard degree of avalanches. In the area of the more than the half of them were tourists - were ~ighest hazard (red) it is forbidden to build houses; stuck in the region for about 10 days due to the In the zone of moderate hazard (blue) houses are closed roads and railway line. allowed to be built, but must be reinforced to A few days after the avalanche events the weather conditions were good enough to take aerial stereo-photographs in the scale of 1:10'000 • Corresponding author address: Urs Gruber, Swiss that allowed a detailed mapping of all the ~:derallnstitute of Snow and Avalanche Research, avalanches in Obergoms. In some tracks the luelastr. 11, 7260 Dort, Switzerland; tel: deposition volumes of the avalanches were ++41-81-4170-262; fax++41-81-4170-262; email: photogram metrically measured. [email protected]

495 The intensity of the avalanches, the wet snow at lower elevation had an important existence of avalanche hazard maps and the influence on the runout distance of many photographs of the actual avalanches make the avalanches. Obergoms Valley an ideal study area for the existing avalanche hazard mapping procedures in Switzerland. 3. AVALANCHE ACTIVITY In the first section of this paper the main characteristics of the valley and the meteorological In the first period (January 2S th to January th conditions are described. Afterwards, the 1999 30 ) 17 avalanche events occurred. Several avalanche activity is put into relation to the avalanches reached and covered the road and historical events. Then, the usefulness and railway. On February 9th only three large accuracy of the existing avalanche hazard maps avalanches were observed in the north-eastern are discussed and illustrated with several part of the valley. In the third period, 56 avalanche examples. events were recorded· on almost all known avalanche tracks. Figure 1 provides an overview 2. SITE DESCRIPTION AND of the avalanche events in February. Several METEOROLOGICAL CONDITIONS 1999 avalanches occurred on the same track.

The Obergoms Valley is located on the north-eastern part of the Canton Wallis, near to the central Alps of Switzerland at the origin of the Rhone River. The bottom of the Valley is at an altitude of about 1300m.a.s.1. and its width varies between 500 and 1000m. Figure 1 shows an overview of the site. Lateral to the South­ West/North-East directed main valley, many small, mostly narrowed side valleys exist, through which the avalanches flow into the main valley. The potential avalanche release areas are located up to altitudes of 3000m.a.s.1. in the north-western part and up to 2700m.a.s.1. in the south-eastern FebnJaty 1999 avalanche perimeters part. Several small villages are situated near the .- Railway N ~e entrances of these side valleys. The villages are 1,_ 0 1______. 2 3 4km -T ! connected by one road and a railway line. -_._...._- - .__.- --_.__..------_ .. During the Winter 1999 there were three large snowfall periods within a time period of less Figure 1: Overview af the February 1999 than one month (Wilhelm et aI., 2000). In the avalanches in the Obergams Valley. Source: Obergoms valley the snow depths were measured (Lauber, 1999). at five different locations. Between January 2ih th A comparison between and 29 , the new snow amount at the bottom of the valley was. SO to 130cm accompanied by avalanches and the historically strong winds. The beginning of February was free avalanches revealed, that especially avalanches of precipitation and cold. Between February 5th coming from the south-eastern side of the valley and 10th another SO to 130cm snow fell. After a often exceeded the known runout distances and short sunny, cold period, snowfall restarted on damaged large forested areas. Many of these h avalanches were in unsettled areas where no February 1i • Together with stormy north-westerly winds between 190cm and 2S0cm of new snow avalanche hazard map was drawn up. Two th was deposited by February 25 • Between avalanches in the community of Reckingen that February 20th and February 22nd the temperatures were previously not known as avalanche trackS rose remarkably. In this period the new show was damaged even very old forest. The average slope very wet and sometimes it rained in the bottom of in the release area of these avalanches was with 0 the valley. Thus the existing snow cover was 31 0 and 33 , respectively. These values are settled and moistened. For example in the village extremly low. of a new snow amount of 109cm was measured in this period but the increase of the snow depth was only 21cm (Lauber, 2000). The

496 EVALUATION OF THE EXISTING avalanche in the south-west end of Reckingen 4. AVALANCHE HAZARD MAP stopped within the boundaries of the hazard map. In 1970 this avalanche caused the death of 30 The avalanches exceeded the frontal persons. Afterwards, deflecting dams had been I'mits of the existing avalanche hazard maps in six built. In 1984, this avalanche occurred again and lases. At a few additional locations, the covered approximately the area of the existing Cvalanches exceeded the lateral limits of the avalanche hazard map. In 1999 the avalanche ~azard maps. More than 80% of the avalanches reached only the main road, i.e. it stopped within remained within the areas marked as endangered . the deflecting dams and filled. them with large by the maps. The most important failures were at deposits. Fortunately no secondary avalanche was Reckingen, ~~schinen a~d Blitzinge~, since released in the same track, otherwise the dams residential bUl.ldlngs were hit or ~early hit. In. the would have been overflowed. following we diSCUSS these cases In more detail. In Figure 3 the avalanche hazard map of Figure 2 shows the avalanche hazard map Geschinen is shown. The avalanche in the north­ of Reckingen with an overlay of the 1999 eastern part of the map fits perfectly with the avalanches. In 1999 two avalanches occurred in avalanche hazard map. However, the avalanche the track in the north-east end of the village. One near the village Geschinen exceeds the mapped avalanche was a dense flowing avalanche that area at the front as well as at the side. remained within the limit of the existing avalanche hazard map.

Yellow Hazard Zone _ AvalancheEvents

I~$~$~N Powder Avalanche

,i{%~ '00 " 'OO.~ ~.

~~ Red Hazard Zone Yellow Hazard Zone ~ E z:z Blue Hazard Zone _ Avalanches1999 s Figure 3: A valanche hazard map of Geschinen with an overlay of the 1999 avalanches. Figure 2: Avalanche hazard map of Reckingen The reasons for the failure of the avalanche with an overlay of the 1999 avalanches. hazard map in Geschinen was twofold. Firstly, the extreme amount of snow fall led to fracture depths The powder snow avalanche, however, exceeded . much higher than those specified for the ~e limit of the existing map and endangered the avalanche dynamics calculations for this particular Village of Reckingen. Not only in Reckingen but region of Switzerland (Salm et aI., 1990). almost in the whole Obergoms valley, yellow Secondly, the release area was very large. In hazard Zones were not delineated. This was due Figure 4 the perimeter of the whole avalanche is to the fact that no powder snow avalanche model shown. The avalanche started in a large basin and as available for practical applications up to 1999. 7 flowed through a gully before entering the main n three other cases powder snow avalanches valley. Were the main reason for the failures of existing The total size of the observed release area (i.e. ~~~anche hazard maps. In addition to the failure the area with a slope> 30°) was more than 60ha. tr eke avalanche hazard map in the north-eastern a of Reckingen, Figure 2 shows that the

497 Geschinen was overflowed due to the preceding avalanche deposits.

4-6m l. 500 0 500 ~

Figure 5: Avalanche deposits at Geschinen. Three Figure 4: Perimeter of the Geschinen avalanche. avalanches occurred within 36 hours. The dotted line was the first avalanche, the thick line the A large part of the fracture area is exposed to the second. The third avalanche was split into two East and is directly located below the crest of the arms. The numbers indicate the snow depths mountain. Therefore, large fracture depths caused measured by photogrammetry. Picture: © Swiss by blowing snow from the western side of the crest Air Force. led to large fracture depths. nd On February 22 a first powder snow The photogrammetrically measured volume of the avalanche released and crossed the river Rhone, deposits of all three avalanches was 1'400'OOOm3 stopping far past the hazard map limits. 3 with an measurement error of ± 40'OOOm . The Fortunately, this avalanche did little damage since avalanches clearly entrained snow along the no houses were located in this area. In Figure 5 avalanche track. The models used in Switzerland the extent of this avalanche is marked with a th to calculate dense snow avalanches do not take dotted line. On the afternoon of February 23 , a into account this process and therefore, the second avalanche occurred that was a purely deposits were generally underestimated by the dense snow avalanche. It stopped directly at the models. entrance of the main valley. In Figure 5 the frontal Another example of the failure of limits of this avalanche are marked with a thick avalanche hazard maps occurred in the village of line. Snow depth measurements that were Blitzingen. A small avalanche filled up the performed by the method of aerial deflector dam that was designed to protect the photogrammetry revealed that the sum of the village. A second much larger avalanche deposits of these two avalanches at the entrance overflowed the dam without any visible deviation into the main valley was as high as 18m. by the dam and flowed towards the center of the On the same day, a third avalanche - also village (Figure 6). Fortunately, the avalanche a dense flowing avalanche - occurred and was stopped before hitting the first house. splitted in two directions due to the large deposits at the entrance of the valley. The western arm of this avalanche exceeded the existing hazard map laterally and at the front. Numerical back­ calculations of this avalanche were only possible by using very low friction values. Therefore,. we believe that the previous avalanche smoothed the gully and provided a good gliding surface. A small deflector dam that should protect the village of

498 Thanks to the avalanche recordings of 1827 the people in Blitzingen had been evacuated in 1999 before the avalanche came down. Since there are still many houses in the blue zone, the inhabitants of Biel and Selkingen did not feel save anymore. They feared that in future an even larger avalanche may occur than the one of 1999 and 1827. In order to be able to stay at their villages they agreed to build a deflector dam. Based on the bad experience in Blitzingen and Geschinen with deflecting dams, the new dam dimensions were' much higher. The example of Biel/Selkingen shows the importance of avalanche hazard maps as basis for Figure 6: Overflow of the deviation dam that the local authorities for making decisions to should protect the village of Blitzingen. Picture: evacuate endangered areas. In the whole Lauber, 1999. Obergom~ Valley, up to the 20 th February, only persons In the red zones were evacuated. On Another large avalanche flowed between the two February 21 almost all houses in the red zones small villages of Biel and Selkingen. This example and also some blue zones were evacuated. After demonstrate the importance of historical the events in Geschinen, Reckingen and avalanche records. The last catastrophic Blitzingen, that exceeded the blue zones avalanche that occurred on this track - 172 years remarkably, the local authorities decided to ago - in 1827 killed many persons. The outline of evacuate additionally houses situated in the white this historic avalanche was stored in the archives. zones. In this situation it was often unclear which When the avalanche hazard maps were prepared areas remain completely save. A "worst case" in the 1970s, the outline of this historic avalanche boundary in the avalanche hazard map would had been chosen as the limiting case for the avalanche map. However, before 1970 they had have helped to find houses in which the evacuated already built houses within the red and blue persons could stay. In one case an avalanche hazard zones. In 1999 the avalanche nearly stopped just in front of a house where many extended to the line of the 1827 avalanche and persons stayed that had been evacuated. In destroyed some of the houses in the red zone. In summary in the Obergoms Valley over 1300 Figure 7 t.he existing avalanche hazard map with persons were evacuated out of 290 houses. the overlaId 1999 avalanche event is shown. 5. CONCLUSIONS

More than 80% of the avalanches occurred within the boundaries of the existing aval~nche hazard maps. The hazard maps proVided a good basis for the evacuation of the inhabitants of endangered areas and were primarely responsible for the fact, that "only" one person was killed during this avalanche period. Comparable historic avalanche periods killed often many more people. However, three main deficiencies were clearly identifiable and the avalanche hazard L ','. . mapping procedure must be improved according 50 0 50100150 m! to them. (1) The runout distances of powder snow =fr~re 7: Avalanche hazard map of Biel/Selkingen avalanches were underestimated. To overcome an overlay of the 1999 avalanche. this failure, research must be focused on the development of a mixed dense and powder snow model that can be used by practioners. In future the yellow hazard zones must be better

499 delineated. Work in this direction has already report of the local avalanche security started (Bartelt and Kern, 2000). service). (2) The hazard caused by the occurrence Salm, B., Burkard, A. and Gubler, H., 1990. of multiple avalanche events on the same track Berechnung von Fliesslawinen. Eine within a short time period has to be taken into Anleitung fOr den Praktiker mit Beispielen. account more carefully, - especially in avalanche Eidg. Inst. fUr Schnee- und tracks with a large potential release area. Multiple Lawinenforschung. Mitt. 47. Davos. avalanche events change the lateral spreading in Wilhelm, C., BrOndl, M., Wiesinger, T., and the deposition area and also may lower the friction Ammann, W., 2000. The avalanche Winter along the track. Therefore, the secondary 1999 in Switzerland - An overview. avalanches may have longer runout distances Proceedings of the International Snow than primary avalanches. Science Workshop (ISSW), Big Sky, (3) The existing design of deflecting dams Montana. was strongly questioned in two cases in the Obergoms valley. Both dams did not protect the villages at all. The main reason was also that the influence of multiple avalanche events were not considered, but also that the deposition depths in general were underestimated due to the lack of entrainment along the track. These three definiencies were also observed in other parts of Switzerland (Gruber and Margreth, 2000). The Winter 1999 showed that the avalanches hazard maps in the Obergoms Valley were not perfect but they helped protect houses and to evacuate persons. However, when the first avalanches exceeded the existing avalanche hazard maps, it became uncertain which places were unconditionally safe. Since not many safe places were available in this valley, it would had been helpful to have an additional "worst case" boundary in the avalanche hazard map. This would help find houses in which a lot of (evacuated) persons could safely stay.

6. REFERENCES

Bartelt, P. and Kern, M., 2000. A mixed flowing/powder snow avalanche model. Proceedings of the International Snow Science Workshop ISSW, Big Sky, Montana. BFF and SLF, 1984. Bundesamt fOr Forstwesen (BFF) and Eidg. Institut fOr Schnee- und Lawinenforschung (SLF): Richtlinien zur BerOcksichtigung der Lawinengefahr bei raumwirksamen Tatigkeiten. , Eidg. Drucksachen- und Materialzentrale. Gruber, U. and Margreth, St., 2000: Winter 1999: a valuable test of the avalanche hazard mapping procedure in Switzerland. Annals of Glaciology, 32. Lauber, G., 1999. 17. Winterbericht 1998/99. Lawinenwarndienst der Region Obergoms, CH- 3981 Geschinen. (1999

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