International Symposium on Mitigative Measures against Snow ...

<strong>International</strong> <strong>Symposium</strong> on Mitigative Measures against Snow Avalanches
Egilsstaðir, Iceland, March 11–14, 2008
reduced and so the subsequent avalanches flow further and further. The big avalanches are
then able to reach the lower parts of the path. This scenario is documented very often by the
chronicle. Especially Heuberger reports about this situation during the event of 1951. In
addition the filling of the Ursprung is one of the criteria for the avalanche commission to act
(Nolf, 2008).
5. ENDANGERED VALUES
Six residential buildings are situated in the red zone, where substantial destruction of
buildings is expected. Seven non-enforced buildings are in the yellow zone, where damages
are supposed. The affected settlement has been built after 1950 and before the hazard zones
were determined. This means that currently 18 persons live in the red and 23 persons in the
yellow zone. Because of the weak structure of all houses, nobody in these two groups is safe
within their houses. So the avalanche commission has to evacuate the people on behalf of the
mayor of Innsbruck in case of avalanche hazard. This happens once in a decade on average
(Nolf, 2008). Furthermore, the avalanche endangers 450 m of roads and building sites of
9.000 m².
6. OPTIONS FOR MEASUREMENT
To reduce the risk for the settlement the Federal service of torrent and avalanche control
evaluated 3 security concepts:
6.1 Supporting structures in the starting zone
To reduce the avalanche mass in a considerable amount, the building of supporting structures
on 40 ha release area would be necessary (Fig. 1). Theses measures cause an expense of about
450.000 EUR/ha. An investment of about EUR 18.000.000 in total, is not justified by the
value endangered by the avalanche. The construction site is situated within the centre of the
ground water protection zone. These springs cover 80 % of the water supply of Innsbruck.
According to this we had to expect either a rejection by the authorities or the building costs
would become too high. The third heavy argument against this option is the influence on the
natural scenery north of Innsbruck by such a big expansion of the defences to stabilize the
snow cover. Environmentalists might have activated a public opinion against the project so
that there would be no political chance for acceptance.
6.2 Deflection dam above station km 2.6
The purpose of this deflection dam is to divert avalanches towards the western canyon side in
the upper quarter of the path (Fig. 1). The volume to be moved to build the embankment
would have an amount of 100.000 m³ – 150.000 m³, so that costs would come to EUR
2.200.000. The effect for the settlement would be completely insufficient. The dam would not
stabilize the main part of the catchments especially in the western part and it would cause
considerable damage on the forests on the right - hand side of the gorge without considerable
effect in regard of risk reduction in the settlement. In addition, a part of the excavation area
would take place within the ground water protection zone. The influence would have been
heavy during the construction phase but situated beneath the timber line these damages would
vanish after a decade.
62 A new avalanche braking system in the Mühlauer-Klamm