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AVALANCHES: SAFETY CONCEPTS FOR TEMPORARY MEASURES AND APPLICATION OF ARTIFICIAL RELEASE
Lukas Stoffel1, Stefan Margreth1
The extraordinary winter 1999 clearly demonstrated the importance of temporary avalanche protection measures. Many accidents were prevented with the evacuation of endangered set- tlements, road closures or artificial release of avalanches. The event analysis showed defi- ciencies in the safety concepts and decision-making process of many communal and road ad- ministration security services. Safety concepts have therefore been improved and standardized in the last years. The obtained experiences are summarized in the following abstract.
SAFETY CONCEPT
Safety concepts regulate the application of temporary measures and should contain the fol- lowing points:
x the boundary of the affected area / overview, basic documents
x the procedure to assess the prevailing snow and avalanche situation (decision-making process)
x the procedure for closures, evacuations and artificial release of avalanches.
A map of the avalanche paths should be one of the basic documents. The damage potential must be evaluated. Hazard zone maps are helpful to determine endangered objects, but ob- served damages also give valuable information. The probability that an avalanche will reach a certain location can be derived from the avalanche history and avalanche dynamic calcula- tions. In addition, the post event analysis allows to estimate critical new snow amounts..
The assessment of the prevailing snow and avalanche situation is based on snow-, weather- and wind-data from the region. Visual observations such as the influence of wind and local avalanche activity are very important. It is recommendable to record the data in a prepared form. The national avalanche forecast and the weather forecast including the forecast for new snow, wind and temperature must also be considered. The result of the decision-making proc- ess is to determine if an area is endangered or not. The decision-making process of a security service has to involve the terrain factors of the avalanches paths, the avalanche activity during the current winter, the efficiency of existing permanent measures and the current weather and snow data. For release areas the terrain factors include for example elevation, exposition, in- clination, topography and surface area.
For road closures fixed barriers at defined locations are recommended. Closure / evacuation maps are helpful for villages (Fig. 1). Such maps prevent time consuming discussions during a critical situation (e.g. as to whether a house is in a safe area or not) and determine the loca- tion for a closure in advance. An evacuation concept can be included. For the application of artificial release at least the release areas, the methods, the necessary closure measures (e.g.
road closures, curfew), the detonation points (Fig. 2) and general specifications for the inter- ventions have to be determined.
1 WSL Swiss Federal Institute for Snow and Avalanche Research SLF, Flüelastrasse 11, CH-7260 Davos Dorf, Switzerland (phone: +41-(0)81-4170126; fax: +41-(0)-81-4170110; e-mail: stoffell@slf.ch / margreth@slf.ch
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Fig. 1: Closure map plan A Dorfbachlawine Davos Fig. 2: Detonation points, Breitzug, Davos Glaris (X=Road closure, B=end of blue zone)
APPLICATION OF ARTIFICIAL RELEASE
For the application of artificial release the key factors “terrain, damage potential, method, closure measure and snow data / check of the result” must be evaluated (Tab. 1). For the deci- sion to use artificial release at a certain location also the cost-benefit ratio and the overall situation (e.g. along a road section) has to be taken into account. The remotely controlled ava- lanche release systems e.g. Gazex, avalanche tower / avalanche guard allow the regular use of explosives in almost any weather conditions. With regular use smaller avalanches can occur, which is one goal of artificial release.
Tab. 1: Key factors for the application of artificial release of avalanches
Factors Favourable Unfavourable
Release area:
Parts of the area (single evaluation) Total area
Inclinationt 35° (bigger area) N-exposition
Small area (i.e. few detonation points)
For big areas: separated parts
Inclination < 32°
S-exposition (i.e. early interven- tions)
Big area (i.e. lot of detonation points)
Track Confined (e.g. gully) Steep, unconfined
Runout zone Flat (< 10°) Inclination > ~ 20°
Terrain
Secondary release
of avalanches Steep slopes on both sides (prim./sec. area)
Permanent protection measures in the secondary release area
Flat ridge between primary and secondary area
Primary avalanche
path No or small damage potential for small avalanches (T=10 y) Small damage potential for return period of 100 years Permanent protection measures (dam, reinforced houses)
Damage potential for avalanches with return period T=10 years High damage potential for return period of 100 years
Possible forest damage Damage potential
Secondary area Small damage potential High damage potential Method for artifi-
cial release Possible use without visibility
Good detonation effect At the detonation point > 35°
Enough detonation points
No use of method without visibility Small detonation effect
At the detonation point < 32°
Not enough detonation points
Closure measures Small effort Big effort (persons to evacuate)
Snow/wind data Automatic station nearby No data near the release zone Visibility Observation possible, e.g. from
the valley No observation because of terrain Available snow
dataCheck of the result
Support e.g. Geophone
Keywords: Natural hazard, temporary measures, artificial release.
