Réf. Bader et Kunz 2000f - R: PNR 31

Référence bibliographique complète
Climate Risks - The Challenge for Alpine Region - PNR31. / ed. by BADER S., KUNZ P. Zürich : vdf Hochschuverlag AG an der ETH Zürich, 2000, 291 p.

Mots-clés
Mudflows, climatic triggering factors, evolution under changing climatic conditions.

Organismes / Contact
Partenaires
NRP 31 Programme Leaders
Thunstrasse 91, CH-3006 Berne
Tel. : +41 31 356 65 65
Fonds National Suisse; Swiss Re; Office Fédéral de l'Environnement, des Forêts et du Paysage; Direction du Développement et de la Coopération; ProClim; CLIRIS Recherche sur les Risques Climatiques; Hochschulverlag an der ETH Zürich

Principaux rapports scientifiques sur lesquels s'est appuyé le rapport
"Eisschwund und Naturkatastrophen ", Haeberli, Kaab et al, 1998.
"Stabilité des terrains meubles en zone de pergélisol et changements climatiques - Deux études de cas en Valais : le Ritigraben (Mattertal) et la moraine du Dolent (Val Ferret)", Lugon et Monbaron, 1998.
"Zur sensitivität von Wildbachsystemen", Kienholz, Keller et al, 1998.
"Murganggefahr und Klimaänderung - ein GIS-basierter Ansatz", Zimmermann, Mani et al, 1997      

(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
  Rivers Mass movements
Torrential events
Channel-type muflows, slope-type mudflows

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Switzerland Alps and PreAlpine region        

(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
The effects of global warming on the hydrological balance of the catchment area of a torrent may be summerized as fallow:
with precipitation tending to fall increasingly as rain rather than snow, increased discharge during the winter half of the year at altitudes between 1000 and 1500 m are likely. Thus pre-Alpine regions in particular are affected;
significant effects on the water cycle are observed in spring (March to April) owing to the reduced volume of melt water;
summer and autumn are still critical times, characterized by storms and concentrated heavy rainfall. This storm activity may be expected to bring an increase in peak flow rates, transportation of solids and mudflow events;
evapotranspiration will increase greatly (up to 30%) with higher air temperatures. A lack of snow in spring will also encourage premature evapotranspiration.

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

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
After analysis of the 1987 floods (which affected the whole Alpine region), half of the channel-type mudflows recorded were found to come from areas of possible or probable pemafrosts or where glaciers had melted since 1850.

There are many unresolved issues and gaps in the knowledge of the climate sensitivity of mudflows.The variable disposition parameters and triggering factors are still difficult to estimate (the basic disposition factors and the spatial extension are well understood and assessed).

It does not seem possible to relate the frequency and intensity of slope-type mudflows quantitavely to different temperatures and precipitation ; no causal relationship between climate change and triggering of slope-type mudflows. The slope-type mudflows have a tendency to increase with altitude. Extreme meteorological events are the determining factor in triggering slope-type mudflows, particulary in the stormy summer season (July and August) but their future development is uncertain.

The transition from event to non-event (of slope-type mudflow) for hourly precipitation corresponds generally to a treshold of 40 mm (Alps) and 70 mm (edge of the Alps) or 21 mm/h and 32 mm/h respectively. The "precipitation and hydrological histrory" of a location before an event cannot be clearly quantified yet, any more than the exact role of snow cover. Channel-type mudflows are not sensitive to the amount of precipitation before an event - strong precipitation does not necessarily lead to a mudflow.

There are differences in the precipitation structure leading to mudflows: torrential rains are responsible for mudflows mainly in the Pre-Alps and Northern Alps, whereas prolonged period of rain, often combined with a high 0°C isotherm, lead to mudflows in the higher region of the central and southern Alps. Mudflows are more frequent in summer and autumn, especially at high altitudes.

Mudflows events are considerably more variable than the climate changes expected for these regions. Hence it is not possible to produce scenarios for their development.
Modélisations
 
Hypothèses
The future incidence of slope-type mudflows in the high mountains will be closely connected with the glacier retreat and changing temperatures in the permafrost zone (especially the active layer). With more water circulating more freely and penetrating fissure, more loose material will be released - especially on steep slopes and crags.

The periglacial areas in the high mountains (2400-2800 m) are the places most affected. The retreat of the glacier fronts (due to gobal warming) exposes increasingly large areas of unconsolited materials which were previously protected by block of ice. Therefore, there will be a greater danger of these materials being mobilized again, particulary in connection with the retreat of permafrost in scree or moraines on steep slopes ( >30°). The permafrost then acts as an impermeable shear surface; In such situations, channel-type mudflows can take place along water channel or steep rivulets with an unprecedent intensity and frequency (e.g the Ritigraben and Dorfbach).

Any significant future change in mudflows activity in the peri-Alpine regions will probably be very limited (the effects of climate change on the mudflows activity will be hardly perceptible because of the great variability of the events).

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Triggering factors

Channel-type mudflows are not sensitive to the amount of precipitation before an event - strong precipitation does not necessarily lead to a mudflow.

The transition from event to non-event (of slope-type mudflow) for hourly precipitation corresponds generally to a treshold of 40 mm (Alps) and 70 mm (edge of the Alps) or 21 mm/h and 32 mm/h respectively.

Channel-type mudflows occur within pre-existing channels or flow paths, often with quite a steep gradient of over 15%. Slope-type mudflows are a cross between a landslide and a channel-type mudflows. They arise on the actual surface of the slope due to mass movements and without any pre-existing channel.

Comparaison between 160 mudflows events covering the last 250 years and 113 cases of heavy precipitations between 1978 and 1995. It lead to the division of precipitation events into 3 categories and made it possible to define limit values or critical precipitation thresholds in relation to the triggering of slope-type mudflows.

(4) - Remarques générales
 

(5) - Préconisations et recomandations
Destinataires et portée du rapport Policy makers, swiss citizens, teachers and students in the climatic and environmental studies
Types de recommandations et / ou préconisations
Where mudflows are a regular event, the situation seems to be weel understood and controlled; but where they only happen occasionaly, the general awardness of the risk is much more limited.

A better understanding of the mechanism involved (predisposition, variables and triggering factors) is indispensable, so that the resultant knowledge ca be used in mapping unforeseen events and hazards and planning protective measures.

Example of the protective measure on the Dorfbach torrent in the Valais canton.