Rebetez 1997

Réf. Rebetez & al 1997 - A

Référence bibliographique complète

REBETEZ M., R. LUGON and P.A. BAERISWYL. Climatic change and debris flows in high mountain regions : the case of study of the Ritigraben torrent (Swiss Alps), Climatic Change, 1997, vol. 36, pp. 371–389.

Abstract: Debris flows in the region of Ritigraben (Valais, Swiss Alps), which generally occur in the months of August and September, have been analyzed in relation to meteorological and climatic factors. The principal trigger mechanisms for such debris flows are abundant rain on the one hand, and snow-melt and runoff on the other hand, or a combination of both. Debris flows linked to rain are likely to be triggered when total rainfall amount over a three-day period exceeds four standard deviations, i.e., a significant extreme precipitation event. An analysis of climatological data for the last three decades in the region of Ritigraben has highlighted the fact that the number of extreme rainfall events capable of triggering debris flows in August and September has increased. Similar trends are observed for the 20th Century in all regions of Switzerland.

Mots-clés

Debris flows, extreme precipitation events, standard deviation threshold.

Organismes / Contact

Swiss Federal Institute for Forest, Snow, and Landscape Research, Lausanne
Department of Geography,University of Fribourg

(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
Precipitation, temperature		Debris flows

Pays / Zone	Massif / Secteur	Site(s) d'étude	Exposition	Altitude	Période(s) d'observation
Swiss Alps	Pennine Alps (Valais)	Ritigraben area (Mattertal valley)	West / North West	1050-3100 m	1966-1994

(1) - Modifications des paramètres atmosphériques
Reconstitutions
Observations	At the foot of the Ritigraben, warming trends of both minimum and maximum temperatures have been particularly marked in the last two decades. Warming is particularly pronounced at Grächen, not only for annual averages but also specifically in July and August, and to a lesser degree in September, both for maximum and minimum temperatures. This warming has been particularly important since 1980, the period which has experienced the highest global temperature increases this century.
Modélisations
Hypothèses

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

An analysis of precipitation data of several climatological stations in August and September since 1966 (which is the observation period available) has been carried on. The evolution of minimum and maximum temperatures in July, August and September at Grächen between 1966 and 1994 has been looked at too.

(2) - Impacts 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) - Impacts du changement climatique sur l'aléa
Reconstitutions
Observations	Considering only the Grächen data (the closest climatological station to the study area), results showed that the coincidence with the triggering of debris flows was highest for the 4 standard deviations (SD) threshold with a period of three days taken into consideration. Note that 1% of cases exceeded this threshold. (Other good relationships existed for different periods and thresholds but the best was found for these parameters). Comparaison between dates at which precipitation exceeded 4 SD for the sum of 3 consecutive days and dates of large debris flows shows a relatively good correlation. One single debris flow event was not linked to exceptional precipitation event. Here the melting of large quantities of early snow played an important role, whereas rainfall was only of secondary importance. On the other hand, one very high rainfall event without triggering of debris flows has been observed. The determining factor in this particular case seems to be the lack of sufficient sediments for a debris flow to occur (previous flow 2 years ago). This event underlines the importance of the availability of sediments as an essential factor in the triggering of debris flows. The number of extreme rainfall events capable of triggering debris flows in August and September has increased in the last three decades in the region of Ritigraben. (Similar trends are observed for the 20th Century in all regions of Switzerland.) This increase in frequency of extreme rainfall may explain the higher frequency of debris flows over the past few years in Ritigraben.
Modélisations
Hypothèses	The availability of sedimentary material could be a completely local phenomenon linked to a morphodynamic crisis of the Ritigraben torrent system. However, this crisis could also be linked to global climate warming. In the eastern Swiss Alps, borehole observations indicate an increase of permafrost temperatures during the last decade at a rate of 0.1 K/year. The general rise in temperature in a region of permafrost may also play a role in the response of slope stability to extreme precipitation.

Paramètre de l'aléa	Sensibilité du paramètres de l'aléa à des paramètres climatiques	Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Debris flows triggering	Critical threshold of precipitation	The critical threshold of precipitation above which a debris flow is triggered in the Ritigraben was determined by choosing a statistical parameter valid for all measurement sites, whatever the average values usually attained in each particular case, i.e., a relative value instead of an absolute threshold. Standard deviation was chosen, as it constitutes a statistical parameter which allows the determination of relative extreme values. An analysis of precipitation data of several climatological stations in August and September since 1966 (which is the observation period available) has been carried on in order to determine a threshold best linked with the triggering of debris flows. This threshold had to be defined as a standard deviation (SD) level and a duration, i.e., the time for which the precipitation sum is taken into account. Precipitation sums for one day and for series of between two and five days (all intervals in the record, including overlapping ones) have been compared with data concerning debris flows at Ritigraben. Then the evolution of extreme precipitation events (sum of precipitation exceeded 4 SD during 3 consecutive days) at Grächen, between 1966 and 1994, and for 7 Swiss stations, between 1901 and 1994, has been analysed. The evolution of minimum and maximum temperatures in July, August and September at Grächen between 1966 and 1994 has been looked at too.

(4) - Remarques générales

(5) - Syntèses et préconisations