Réf. Gruner 2008 - P

Référence bibliographique complète

GRUNER, U. 2008. Climatic and meteorological influences on rockfall and rockslides. Proceedings 11th Interpraevent Congress “Protection of populated territories from floods, debris flows, mass movements and avalanches“, Dornbirn, Austria, 26–30 Mai 2008, Vol. 2, 147–158. [PDF EN + PDF DE]

Abstract: Climatic and meteorological influences on rockfall and rockslide events are treatetd from a historical and a rockmechanical view. The analysis of numerous events indicates, that warm periods do not induce an accumulation of events, neither of rockslides nor of rockfall. On the other hand, an accumulation of rockfall events during spring times seems to be influenced by meteorological conditions such as frequent freezing and thawing cycles, snowmelt and the first intensive precipitations. The historically documented triggering of rockfall and rockslide events is discussed from a rock-mechanical point of view. Due to accurate and automated measurement of displacements in different rock areas, a destabilization and decomposition of rock masses during cold periods can be identified. Warm periods, even if they are associated with intensive or rather extreme precipitation, do not induce displacements of smaller or median rock masses. Large rockfall and rockslide events (over 100'000m3) however are pre-dominantly influenced by mountain water conditions and thus by precipitation.

Mots-clés

Rockfall - Meteorological Influences - Global Warming - Rockslide

 

Organismes / Contact

Geschäftsführer KELLERHALS + HAEFELI AG, Kapellenstrasse 22, CH-3011 Bern - E-Mail: ueli.gruner@k-h.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

 

 

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

 

 

 

 

 

 

 

(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

The first part discusses the climatic and meteorological boundary conditions, under which in the recent past smaller and larger rockfall und rockslide events have occurred. [see methodology}

The historical analysis of the rockslides shows the impossibility to correlate large events with warm climatic periods. Furthermore, the global warming since 1850 does not coincide with a significant increase of rockslide events. A majority of the large historical rockslides (since approximately 2000 years) are related to intensive and long-lasting precipitation during summer and autumn months. Looking at the examined 800 mainly smaller- or middle-sized rock-fall events, the following conclusion can be drawn: winter- and spring-events are more frequent during generally cold periods than they are during warm periods. In the latter there is no reference to an accumulation of events; moreover it can be shown that during warm years the frequency of rockfall events is more likely to decrease during summer months. Besides there is no correlation between well-known extended humid periods and a accumulation of rockfall events.

Observations

In the second part, the historically documented climatic and meteorological influences on the triggering and mechanical processes are discussed from a rock-mechanical point of view.

Due to precise automatic deformation measurements in different rock areas, the mechanism of destabilization and decomposition of rock masses can be identified and correlated with meteorological data. For smaller- and middle-sized potential rockfall volumes such measurements show, that the clefts and fissures in the rock masses open continuously during cold periods, as a result of rock contractions, while they stagnate or even decrease during a warm-period.

This cyclic process leads to a destruction of rock bridges, to an increase of the cleft depth and to a destabilization of the rock masses. The main movement thus takes place in the cold season. Snowmelt and the first intensive precipitation during spring causes water pressure in the cleft system to increase, which may trigger a rockfall. A frequent change of freezing and thawing can accelerate the process. Intensive or even extreme precipitation during warm periods as in August 2005 did not cause increased movements on most measuring points. This behaviour may explain, why such rockfall processes occur more frequently during cold seasons and spring time than during warmer seasons. A conclusion which corresponds also with dendrochronological findings on tree trunks, located on an alpine slope in the Valais, where it could be proved, that falling rocks arose almost exclusively during the winter growth phase of the trees (between October and end of May) and were not at all influenced by summer thunderstorms. Otherwise deformation measurements taken on larger rock portions with impeded runoff (e.g. where high hydrostatic pressure can build up) show, while they are rather "cooling insensitive", also clear movements due to strong and/or long continuing precipitation.

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

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

 

 

The first part discusses the climatic and meteorological boundary conditions, under which in the recent past smaller and larger rockfall und rockslide events have occurred. To do so, approximately 230 more or less well documented rockslides since the late Pleistocene Epoch in the Alpine region have been analysed. Additionally, approximately 800 rockfall and rockslide events, which occurred during the last 500 years for the most part on the north side of the Alps of Switzerland, have been investigated, focussing mainly on the influence of temperature and precipitation on the event frequency. The results of these analyses can be found in Gruner (2004, 2006).

 

(4) - Remarques générales

 

 

(5) - Syntèses et préconisations

In the third part some conclusions, which are to be considered e.g. for the evaluation and management of a rockslide danger, are given:

♦ The most important cause for small- or middle-sized rockfall events is the extent of a cold period (cold climatic periods, cold winters, colds snap in the spring/autumn, frequent freezing and thawing cycles), which results in a destabilization of the rock masses (see figure 1).

♦ The rockfall events are often triggered by precipitation and/or snowmelt following a cold period. Warm periods, especially warm summer months, cause however a sedation of the rock masses. During these periods generally less rockfall is to be expected. An exception are rock masses in permafrost zones, where during very hot summer months usually rather small rockfall from north walls of the high mountain may occur. They however hardly reach residential areas and infrastructure.

♦ In case of large rockslide and rockfall events (over 100'000 m3) however, predominantly mountain water conditions ("mountain water pressure") and thus precipitation influences those events (see figure 2).

Références citées :

Gruner U. (2004): "Klima und Sturzereignisse in Vergangenheit und Zukunft." Bull. angew. Geol., Vol. 9/2; 23-37.

Gruner U. (2006): "Bergstürz und Klima in den Alpen - gibt es Zusammenhänge?" Bull. an- gew. Geol., Vol. 11/2; 25-34.