Réf. Bauder & al. 2008 - A

Référence bibliographique complète

BAUDER, A., BLATTER, H., DEICHMAN, N., FUNK, M., HUSS, M., LÜTHI, M., RIESEN, P., SUGIYAMA, S., WALTER, F., WERDER, M. 2008. On the outburst of glacier-dammed lakes: Gornergletscher, Valais. Bulletin für angewandte Geologie, Vol. 13/2, 2008 S. 17-21. [Etude en ligne]

Introduction: The release of water from glaciers in catastrophic floods poses an important threat to human activity. Such events are called jökulhlaups, an expression from Iceland, where spectacular outburst events originate in large water bodies impounded within ice caps. In the Alps or in glacierized mountain areas in general, glacier-dammed lakes develop in a depression resulting from a combination of topographical conditions and glacier extent. The most famous historical cases in the Swiss Alps, where such glacier-dammed lakes suddenly drained with disastrous consequences, are Glacier du Giétro, Allalin-gletscher, Gruben-gletscher and Aletsch-gletscher/Märjelensee. These outbursts represent a severe threat in mountain ranges and have caused major damage and loss of life in the past. Lakes impounded behind an ice barrier drain in a variety of ways. Among the most well known are lake outbursts associated with a catastrophic drainage due to rapid thermal enlargement of subsurface channels. But sometimes, for unknown reasons, other mechanisms occur, even at the same location, owing to the complex nature of these events. The initiation of an outburst may be of particular complexity.

Mots-clés
 

Organismes / Contact

• Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie VAW, ETH Zürich, CH-8092 Zürich, Switzerland 2
• Institute for Atmospheric and Climate Science IAC, ETH Zürich, CH-8092 Zürich, Switzerland
• Swiss Seismological Service SED, ETH Zürich, Höngger- berg, CH-8093 Zürich, Switzerland
• Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan

Summary of a presentation given at the VSP/ASP annual convention, Sion, Switzerland, June 2008.


(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

 

Observations

Results for Gornergletscher

Gornergletscher is the second largest glacier in the Alps. It consists of several tributaries and covers an area of nearly 60 km². At the confluence of Gorner- and Grenz-gletscher, Gornersee (an ice-marginal lake) has formed every spring and drained every summer for many years. In the last century Gornergletscher experienced a significant ice loss, especially in the lake area (150 m thinning since 1931). The greatest ice thickness of Gornergletscher is 450 m at the confluence area (Huss et al. 2007) and the main glacial valley is slightly over-deepened. Significant changes of glacier geometry during the last century caused changes in lake location and volume. The lake usually starts to fill in May and drains annually between June and August (Bezinge et al. 1973). Often, the lake is filled to the maximum level beyond which supraglacial outflow would occur at the start of the drainage.
A gauging station operated by the Grande Dixence hydropower company is situated 1 km downstream of the glacier terminus, recording hourly discharge since 1970, providing the unique possibility to carry out an assessment of glacier floods. An evaluation of these data has shown that each year 1 to 6 Million m3 of meltwater are impounded by the lake and drains subglacially within a few days. The peak discharges during the outburst events, measured at the glacier terminus, reaches 20 to 50 m 3 s−1, of which 40-75% is lake water. In the first half of the 20th century flood intensities of more than 100 m3.s−1 were reported, regularly causing severe damage in the valley of Zermatt (Raymond et al. 2003).
Since 1970 we identified significant drainage events every year except for 1984, 1991 and 1995. The evolution of the lake outburst timing [Fig. 5] shows an obvious trend. Between 1950 and 2005 a shift of about two months has been observed, moving the expected date of the event from late August to late June. In contrast, the temporal evolution of drainage volume does not show an uniform trend. In addition to the year-to-year variability, long-term fluctuations of drainage volumes also occurred. Since only very limited direct observations exist, we do not know to what extent the volume fluctuations are caused either by changing the lake basin geometry or different filling levels of the lake.

Triggering mechanism of the lake drainage

During the outburst event in July 2004, the ice surface moved vertically upward by up to 10 cm within a distance of 400 m from the lake. This suggests a separation of the glacier sole from the bed due to the intrusion of lake water. The largest surface upward motion was found in the zone where the ice floatation level was exceeded. This indicates that the seal broke as soon as the hydraulic potential line φ = 0 surpassed the level of the glacier bed [Fig. 1]. In addition to the aforementionned vertical displacement, the glacier surface was lifted up by 0.5-3 m within 100 m from the lake border. Moreover, the formation of a substantial englacial drainage could be observed in a borehole. This can be explained by an upward bend of the ice dam due to the buoyancy force. The englacial fracturing caused by the large upward displacement probably favoured the initiation of the observed englacial lake water drainage. It is likely that the lake outburst was initiated by this englacial drainage, after which the sub-gla- cial water flow started in the basal opening caused by the upward bend of the marginal ice (Sugiyama et al. 2008).

Modélisations

 

Hypothèses

 


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

 

 

 


(4) - Remarques générales

Jökulhlaups: Outburst floods from subglacial or ice-dammed lakes (e.g. Björnsson 1992) are commonly described to occur when the lake level has reached a critical level at which the hydrostatic water pressure maintained by the lake is equal to the ice overburden pressure of the ice dam. As soon as the line φ = 0 [in Fig. 1] just touches the bed curve, the seal breaks and the conditions for a subglacial outburst are met. As water begins to leak underneath the dam, flow typically localizes in one or a few channels forming in the ice. Such channels increase rapidly in size due to melt-back of the ice walls caused by the dissipation of potential energy (e.g. Röthlisberger 1972, Nye 1976). As lake drainage proceeds, water pressure in the channel drops and creep-clo- sure of ice counteracts melt-enlargement progressively. The rapid closure or even col- lapse of the channel can stop the flood, even if the lake is not empty. The discharge of a typical jökulhlaup-hydrograph increases as a power law of time with a finite time singu- larity (because of progressive channel-growth) and a steep falling limb, reflecting rapid closure of the conduit (e.g. Rist 1955, Björnsson 1974).


(5) - Syntèses et préconisations

 

Références citées :

Bauder, A., Funk, M., & Huss, M. 2007: Ice volume changes of selected glaciers in the Swiss Alps since the end of the 19th century. Annals of Glaciology, 46,145-149.

Bezinge, A., Perreten, J., & Schafer, F. 1973: Phénomènes du lac glaciaire du Gorner. In Symposium on the Hydrology of Glaciers, Cambridge 1969, volume 95, 65-78. Association Internationale d’Hydrologie Scientifique.

Björnsson, H. 1974: Explanation of jökulhlaups from Grímsvötn, Vatnajökull, Iceland. Jökull, 24,1-24.

Björnsson, H. 1992: Jökulhlaups in Island: Prediction, characteristics and simulation. Annals of Glaciology, 16,95-106.

Huss, M., Bauder, A., Werder, M., Funk, M. & Hock, R. 2007: Glacier dammed lake outburst events of Gornersee, Switzerland. Journal of Glaciology, 53(181), 189-200.

Nye, J.F. 1976: Water flow in glaciers: jökulhlaups, tunnels and veins. Journal of Glaciology, 17(76),181-207.

Raymond, M., Wegmann, M., & Funk, M. 2003: Inventar gefährlicher Gletscher in der Schweiz. Mitteilung 182, Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie der ETH Zürich.

Rist, S. 1955: Skeidarárhlaup 1954, The jökulhlaup of Skeidara. Jökull, 5, 30-36.

Röthlisberger, H. 1972: Water pressure in intra- and subglacial channels. Journal of Glaciology, 11(62),177-203.

Sugiyama, S., Bauder, A., Huss, M., Riesen, P., & Funk, M. 2008: Triggering and drainage mecha- nisms of glacier-dammed lake outburst in Gornergletscher, Switzerland in 2004. Journal of Geophysical Research, 113, F4.

Wilhelm, G. 1967: Gornersee. Rapport interne Grande Dixence (unveröffentlicht).