Réf. Lugon & Stoffel 2010 - A

Référence bibliographique complète

LUGON, R, STOFFEL, M. 2010. Rock-glacier dynamics and magnitude–frequency relations of debris flows in a high-elevation watershed: Ritigraben, Swiss Alps. Global and Planetary Change, 73, 202–210, doi:10.1016/j.gloplacha.2010.06.004. [Etude en ligne]

Abstract: A widespread risk in high mountains is related to the accumulation of loose sediments on steep slopes, which represent potential sources of different types of geomorphic processes including debris flows. This paper combines data on 50 yr of permafrost creep at the Ritigraben rock glacier (Valais, Swiss Alps) with magnitude–frequency (M–F) relationships of debris flows recorded in the Ritigraben torrent originating at the rock-glacier front. Debris production and volumetric changes at the rock-glacier front are compared with debris-flow activity recorded on the cone and potential couplings and feedbacks between debris sources, channel processes and debris sinks. The dataset existing for the Ritigraben rock glacier and its debris-flow system is unique and allows prime insights into controls and dynamics of permafrost processes and related debris-flow activity in a constantly changing and warming high-altitude environment. Acceleration in rock-glacier movement rates is observed in the (1950s and) 1960s, followed by a decrease in flow rates by the 1970s, before movements increase again after the early 1990s. At a decadal scale, measured changes in rock-glacier movements at Ritigraben are in concert with changes in atmospheric temperatures in the Alps. Geodetic data indicates displacement rates in the frontal part of the rock glacier of up to 0.6–0.9 m yr−1 since the beginning of systematic measurements in 1995. While the Ritigraben rock glacier has always formed a sediment reservoir for the associated debris-flow system, annual horizontal displacement rates of the rock-glacier body have remained quite small and are in the order of decimeters under current climatic conditions. Sediment delivery from the rock-glacier front alone could not therefore be sufficient to support the 16 debris flows reconstructed on the cone since 1958. On the contrary, debris accumulated at the foot of the rock glacier, landslide and rockfall activity as well as the partial collapse of oversteepened channel walls have to be seen as important sediment sources of debris flows at Ritigraben and would represent 65–90% of the material arriving on the Ritigraben cone. There does not seem to exist a direct coupling between displacement rates of and sediment delivery by the rock-glacier body and the frequency of small- and medium-magnitude debris flows. In contrast, a direct link between source and sink processes clearly exists in the case of active-layer failures. In this case, failure processes at the rock-glacier snout and debris-flow events in the channel occur simultaneously and are both triggered by the rainfall event.

Mots-clés

Permafrost - Debris flows - Geodesy - Photogrammetry - Dendrogeomorphology - Tree rings - Climate change - Swiss Alps

 

Organismes / Contact

Laboratory of Dendrogeomorphology (dendrolab.ch), Institute for Geological Sciences, University of Berne, Baltzerstrasse 1+3, CH-3012 Berne, Switzerland (markus.stoffel@dendrolab.ch)
Climatic Change and Climate Impacts, Institute for Environmental Sciences, University of Geneva, route de Drize 7, CH-1227 Carouge-Geneva, Switzerland

 

(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

Swiss Alps

Valais

Ritigraben basin

 

 

 

 

(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

Acceleration in rock-glacier movement rates is observed in the (1950s and) 1960s, followed by a decrease in flow rates by the 1970s, before movements increase again after the early 1990s. At a decadal scale, measured changes in rock-glacier movements at Ritigraben are in concert with changes in atmospheric temperatures in the Alps. Geodetic data indicates displacement rates in the frontal part of the rock glacier of up to 0.6–0.9 m yr−1 since the beginning of systematic measurements in 1995.

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

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

 

Variations in superficial horizontal rock-glacier movement rates were assessed with high-resolution geodetic surveys and low-resolution photogrammetric analysis. Highly resolved displacement measurements with measurement accuracy in the range of a few millimeters were performed with annual geodetic surveys of 15 blocks located in the terminal part of the rock glacier since 1995. In 2003, the number of surveyed blocks was augmented to 21. In addition, a low-resolution photogrammetric analysis was performed to assess mean movement rates for three periods of the last 50 yr, namely 1958–1975, 1975–1993 and 1993–2005. In this approach, the position of boulders was assessed on stereo pairs of analogue black-and-white aerial photographs (Swisstopo).

 

(3) - Effets du changement climatique sur l'aléa

Reconstitutions

 

Observations

While the Ritigraben rock glacier has always formed a sediment reservoir for the associated debris-flow system, annual horizontal displacement rates of the rock-glacier body have remained quite small and are in the order of decimeters under current climatic conditions. Sediment delivery from the rock-glacier front alone could not therefore be sufficient to support the 16 debris flows reconstructed on the cone since 1958. On the contrary, debris accumulated at the foot of the rock glacier, landslide and rockfall activity as well as the partial collapse of oversteepened channel walls have to be seen as important sediment sources of debris flows at Ritigraben and would represent 65–90% of the material arriving on the Ritigraben cone. There does not seem to exist a direct coupling between displacement rates of and sediment delivery by the rock-glacier body and the frequency of small- and medium-magnitude debris flows. In contrast, a direct link between source and sink processes clearly exists in the case of active-layer failures. In this case, failure processes at the rock-glacier snout and debris-flow events in the channel occur simultaneously and are both triggered by the rainfall event.

Modélisations

 

Hypothèses

As a result of warming mean temperatures, the movement rates of the rock-glacier body at Ritigraben have increased by more than 70% over the last few decades, resulting in larger sediment production rates at the snout of the permafrost body. The frequency of events has, in contrast, decreased, as above-threshold rainfall events were largely missing over the same period (Stoffel and Beniston 2006). While there does not seem to exist a direct coupling between rock-glacier and debris-flow processes at Ritigraben, it also becomes obvious that immediate feedbacks occur when extensive precipitation events lead to active-layer detachment failures at the snout of the rock glacier. The temporal scarcity of events over the last few years will ultimately result in increased sediment recharge rates and in a higher potential for very large debris flows in the case of future heavy summer rainfall events. In this sense and in spite of the scarcity of very direct coupling effects, it becomes obvious that there exist “delayed” feedbacks of the sediment routing system following perturbations in the climatic driving forces, as stated e.g., by Humphrey and Heller, 1995 and Densmore et al., 2007. Results presented in this paper allow first insights into the complex nature of process interactions and feedbacks in high-elevation environments, but they also clearly call for more research in this field.

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

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

 

 

This paper combines data on 50 yr of permafrost creep at the Ritigraben rock glacier with magnitude–frequency relationships of debris flows recorded in the Ritigraben torrent originating at the rock-glacier front. Debris production and volumetric changes at the rock-glacier front are compared with debris-flow activity recorded on the cone and potential couplings and feedbacks between debris sources, channel processes and debris sinks. The dataset existing for the Ritigraben rock glacier and its debris-flow system is unique and allows prime insights into controls and dynamics of permafrost processes and related debris-flow activity in a constantly changing and warming high-altitude environment.

 

(4) - Remarques générales

 

 

(5) - Syntèses et préconisations

 

Références citées :

Densmore, A.L., Allen, P.A., Simpson, G., 2007. Development and response of a coupled catchment fan system under changing tectonic and climatic forcing. J. Geophys. Res. 112, F01002.

Humphrey, N., Heller, P.L., 1995. Natural oscillations in coupled geomorphic systems: an alternative for cyclic sedimentation. Geology 23, 499–502.

Stoffel, M., Beniston, M., 2006. On the incidence of debris flows from the early Little Ice Age to a future greenhouse climate: a case study from the Swiss Alps. Geophys. Res. Lett. 33, L16404.