Réf. Noetzli & al. 2003 - P

Référence bibliographique complète
8th INTERNATIONAL CONFERENCE ON PERMAFROST (2003, Zürich). Mountain permafrost and recent Alpine rock-fall events: a GIS-based approach to determine critical factors. NOETZLI J., HOELZE M., HAEBERLI W., Zürich, 2003, 5p.

Abstract : Glacier retreat and permafrost changes, as related to climate change, are supposed to affect stability conditions of steep rock walls in cold mountain ranges. Several rock-fall events, which have occurred inthe European Alps during the 20th century, are possibly related to warm permafrost. This study undertakes a systematic parameterization of rock-fall events in order to increase information about thermal and topographic conditions under which rock instabilities develop in areas of mountain permafrost. Thermal conditions of historically documented starting zones are parameterized by applying either empirical rules or GIS-based spatial models; slope is derived from DTMs. Despite the relatively small number of events documented so far (around 20), the first results presented clearly indicate that the factor ‘permafrost’ must be considered in connection with rock-falls from high mountain slopes.

Mots-clés
Rock falls, permafrost, GIS-based spatial models, rock surface reconstructed temperatures.

Organismes / Contact
Glaciology and Geomorphodynamics Group, Department of Geography, University of Zurich
jnoetzli@geo.unizh.ch ; haeberli@geo.unizh.ch

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
  Permafrost Mass movements Rock falls 

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Swiss, Italian and German Alps       2100-4200 m asl 1717-2002

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
Observations
 
Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
The PERMAKART model calculated temperatures range from -6.3 to 5.9°C. 3 sites show warm temperatures, several degrees above the freezing point. The other events show neither very warm nor very cold temperatures: 80% of the events have a parameterized temperature between -4 and +4°C and 50% between -2 and +2°C.

The PERMAMAP results show that only 2 events (on 20 events) are completely outside permafrost area. 2 others are located at the permafrost limit. All other events are situated within or close to permafrost areas. Some starting zones are located in warm permafrost areas, but events from cold permafrost areas also occurred. In view of the melting process that might trigger a landslide, maximum radiation values seem to be important as they indicate maximum energy input. Parameterized temperatures range from -9.7 to 3.3°C. 4 sites have shown positive temperatures, the other events vary between -6.4 and 1.8°C. About 84% of the events have a surface temperature between -6 and +2°C and 47% between -4 and 2°C.

Temperatures calculated with the second approach are lower than those calculated with the first. The difference ranges from 0.3 up to 5.5°C and indicates the importance of complex 3D-topological effects (mountain shadow), which is reflected in the second approach but not in the first.

The frequency of failure is highest on very steep terrain with mean gradients ranging from 30 to 75°. About 65% of the rock-falls occurred on slopes with mean slope angles of more than 50°. When looking at critical factors, the maximum slope is the determining factor. The maximum slope ranges from 46 to 82°. For 90% of the events, the steepest part of the scarp has a maximum gradient of more than 50°, for 80% more than 55° and for 55% more than 60°.

The starting zone of many rock-falls that occurred during the last hundred years is located in the permafrost altitudinal belt. Despite the considerable scatter in the data, a concentration of instabilities in warm permafrost is indicated. The starting zones of 3 sites are located outside permafrost. Permafrost occurrence above the scarp might have influenced the water regime and helped to trigger the landslide. However, events situated in cold permafrost areas have been observed, too. Thus, warm permafrost alone is not a necessary factor for all slope failures in periglacial areas and colder regions, but it cannot be disregarded when investigating slope instabilities. As permafrost temperatures in the Alps show a clear upward trend, warming permafrost could lead to an increase of permafrost-related slope instabilities. In view of the melting process that might trigger a landslide, maximum radiation values seem to be the most important radiation values as they indicate maximum energy input.
Hypothèses
 

Paramètres de l'aléa
Sensibilité du paramètre de l'aléa à des paramètres climatiques et du milieu
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Rockfalls activity
Permafrost temperature
This study have undertaken a systematic parameterization of rock-fall events in order to increase information about thermal and topographic conditions under which rock instabilities develop in areas of mountain permafrost. Thermal conditions of historically documented starting zones have been parameterized by applying either empirical rules or GIS-based spatial models; slope is derived from digital terrain models (DTMs). All DTMs and cell-based calculations have a resolution of 25 m.

20 rock-fall events or slope instabilities have been documented (9 located in the Swiss Alps, 10 in the Italian Alps and 1 in the German Wettersteingebirge). They all took place between 1900 and 2002 (except one in 1717). The affected rock walls are located between 2100 and 4200 m a.s.l. and are above the tree line. Most of them are situated within the permafrost altitudinal belt.

Two approaches have been conducted to estimate thermal conditions at the scarp. Both were based on mean annual air temperature (MAAT) and potential direct solar radiation, the 2 most dominating factors governing permafrost occurrence.
In a first approach, to estimate the thermal conditions at the surface of the starting zones, the PERMAKART model has been applied. It is based on the so-called “rules of thumb” to predict local permafrost occurrence. These rules primarily consider radiation effects as related to aspect, and air temperature as related to altitude. Secondly, the altitude of the lower limit of permafrost existence has been determined for each event by applying the adjusted rules. Then the altitude difference between the mean altitude of the starting zone to the respective permafrost limit has been calculated and converted to a temperature value using temperature gradients.
The second approach has allowed another estimation by applying the GIS-based spatial model PERMAMAP. It calculates permafrost limits based on a statistical relationship between BTS-measurements, MAAT and potential direct solar radiation at the limit of the permafrost existence. The mean potential solar radiation of the summer months for each starting zone has been calculated using Funk & Hoelzle’s model.

(4) - Remarques générales
 

(5) - Préconisations et recomandations