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
Development, assessment and effect of a new avalanche braking
system in the Mühlauer-Klamm, Innsbruck, Austria
Manfred Pittracher
Austrian Service for Torrent and Avalanche Control, Liebeneggstraße 11, A-6020 Innsbruck, AUSTRIA
e-mail: manfred.pittracher (at) die-wildbach.at
ABSTRACT
North of Innsbruck the Mühlauer-Klamm–avalanche endangers Mühlau, a part of Innsbruck
the capital of Tyrol. The starting zone spreads over an area of 2.6 km² and leads to a gorgestyled
avalanche track. The average inclination of the track is 15°. The run out close to the
lower end of the gorge is densely settled. Six buildings are in the red zone and 7 buildings in
the yellow zone of the hazard-map of Innsbruck. This situation was not accepted by the
authorities. So, a special protection-system was developed, which consists of 2 braking –
structures to enhance the retarding effort of the gorge.
The environmental conditions of the catchment and the selection-procedure of the protection
measures with regard to economical, ecological and technical conditions, are described.
Finally, the influence of the avalanche braking – system on land-use planning is presented.
Since this protection-measure is unique, no experience about its effects exists. Therefore, in
cooperation between the federal service of avalanche and torrent control (WLV) and the
federal forest research centre (BFW), a study was carried out to verify the expected effect of
the system. A numerical investigation was conducted with SAMOS and ELBA avalanche
simulation models. In addition, model tests in a water-tank will be performed.
The decision procedure of the project and results of the numerical analysis are presented.
1. INTRODUCTION
The population have increased considerably in the Alps since 1950 especially in Innsbruck.
The number of inhabitants has increased from 95.000 in the year 1950 to 150.000 in 2007
(Grieser, 2007). The city of Innsbruck covers an area of 105 km², but only 35 % or 36 km² of
this area is suitable for permanent settlement. Innsbruck have a population density of 3.830
persons per km². More than half of the available area for settlement is already planned for
construction. In this situation the price range of housing space lies between EUR 3.000 and
EUR 4.000 per m². Because of the continuous increasing population these prices are expected
to grow further.
During the last 50 years, the settlements have expanded into areas which are endangered by
natural hazards like the Mühlauer-Klamm-avalanche. This avalanche endangers 41 people
(Nolf, 2007) and therefore causes the biggest risk in Innsbruck. The resettlement of these
persons to safe parts of the city is not a political option. No politician could stand the public
storm caused by a drastic measure like that. So in the past, Innsbruck decided to evacuate the
affected persons in hazardous periods. These evacuations are also extremely unpopular and
have not always been accepted by some of the inhabitants. Therefore the municipal
58 A new avalanche braking system in the Mühlauer-Klamm