Subglacial lake in the glacier of Tete Rousse (Mont Blanc area, France) C. Vincent (1), A. Legchenko (2) M. Descloitres (2), S. Garambois (3), (1) Laboratoire de Glaciologie et Géophysique de l Environnement (LGGE), (2) Laboratoire d étude des Transferts en Hydrologie en Environnement (LTHE) (3) Laboratoire de Géophysique Interne et Tectonophysique (LGIT) 20 Avril 2011 Grenoble
11 July, 1892 Town of Saint Gervais devastated 175 fatalities
Origin : outburst flood from a subglacial cavity
2 cavities
3240 3220 Altitude (m) 3200 3180 3160 3140 Upper Cavité Sup. cavity 3120 Conduit Cavité inf. Lower cavity 3100 0 20 40 60 80 100 120 140 160 180 200 Distance (m)
3240 3220 Altitude (m) 3200 3180 3160 3140 Upper Cavité Sup. cavity 3120 Conduit Cavité inf. Lower cavity 3100 0 20 40 60 80 100 120 140 160 180 200 Distance (m) 200 000 m 3 of water > 1 000 000 m 3 of mud at St Gervais
Tunnels drilled in 1899 and 1904
- Is this tunnel needed to prevent from a new outburst flood? - Could this glacier cause another outburst flood?
2007/2008 Glaciological studies:. Topographic measurements. Radar measurements. Temperature measurements. Mass balance measurements
0 20 40 60 80 100 120 140 160 3170 3160 Rive Droite Rive Gauche 3170 3160 3150 3140 3130 1 402 403 404 405 406 407 408 409 410 411 412 413 414 424 415 416 417 418 419 420 421 422423 3150 3140 3130 Altitude 3120 Galerie 3120 3110 3110 3100 3100 3090 3090 3080 3070 Profil 4 3080 3070 0 20 40 60 80 100 120 140 160 Distance Galerie First conclusion: the tunnel of 1904 is probably unuseful to prevent from a new outburst flood.
Second conclusion: the radar measurements showed a zone (volume ) with an anomaly..
Water? Rocks into glacier?
Présentation au conseil municipal de Saint Gervais le 4 août 2008. Présentation des résultats. Recommandation de faire des forages pour élucider cette zone d anomalie
Sept 2009, nouvelle étude: Prospection géophysique par la méthode de résonance magnétique (Laboratoire d Hydrologie de Grenoble, LTHE)
Volume d eau de 65 000 m3 Rapport à la préfecture le 4 mars 2010
Réunion Préfecture le 24 mars 2010: - Résultats de la prospection RMP de septembre 2009 - Propositions d investigations supplémentaires (avant août 2010):. Campagnes de terrain RMP et radar pour préciser la localisation du volume d eau liquide. Réaliser des forages pour vérifier directement la présence d eau à la base du glacier
Mai 2010: nouvelles mesures radar
2105200 Glacier de Tête Rousse 2105150 23 22 2105100 2105050 2105000 21 11 18 9 8 10 4 7 8 9 20 19 7 13 3 2 1 11 12 15 14 6 5 16 10 21 17 17 18 20 19 22 2104950 12 23 11 14 10 13 9 15 16 17 18 19 20 2104900 Ancien Refuge Cabane RTM Ancienne Galerie Galerie de 1904 0 20 40 60 80 100 120 140 160 180 200 2104850 947700 947750 947800 947850 947900 947950 948000 948050 948100 948150 948200 948250 948300
20 drilling holes have been performed between the 29 of June and the 8 of July, 2010 using a thermal drilling machine,
17 Forage 2 Forage 20 2 20 7 16 Forage 16 Forage 10 5 10 Forage 17 Forage 5. 20 drilling holes. Maximum thickness is 76 m
17 Forage 2 Forage 20 2 20 7 16 Forage 16 Forage 10 5 10 Forage 17 Forage 5 - Camera -Water pressure sensors - Ice temperature sensors (Thermistors)
We found a subglacial water reservoir at the bottom of 6 boreholes (Video)
At 2 boreholes, artesian outpourings 10 cm higher than the ice surface occured when the subglacial cavity was reached
Heights of the subglacial cavity 2105200 2105150 22 23 18,7 m Glacier de Tête Rousse 29 m 15 m 2105100 2105050 2105000 21 11 18 9 8 10 4 7 8 9 20 19 7 13 3 2 1 11 12 15 14 6 5 16 10 21 17 17 18 20 19 22 2104950 2104900 8.4 m Ancien Refuge 12 23 14 11 10 13 9 15 16 17 18 19 20 Cabane RTM Ancienne Galerie 20 m Galerie de 1904 0 20 40 60 80 100 120 140 160 180 200 2104850 947700 947750 947800 947850 947900 947950 948000 948050 948100 948150 948200 948250 948300 12.2 m
3190 Surface de neige (juillet 2010) 3180 3170 3160 Altitude (m) 3150 3140 3130 3120 3110 3100 3090 0 50 100 150 200 Distance (m)
The hydrostatic pressure exceeded the ice pressure due to the weight of the ice column Snout of the glacier Lower Cavity in 1892 Upper Cavity in 1892 Longitudinal section 18 Tunnel 1904 Tunnel 1899 0 20 40 60 80 100 120 140 160 180 200 m
We could expect that the water contained in the glacier would be released suddenly Snout of the glacier Lower Cavity in 1892 Upper Cavity in 1892 Longitudinal section 18 Tunnel 1904 Tunnel 1899 0 20 40 60 80 100 120 140 160 180 200 m
Forage 17 The public authorities have been warned immediatly (13 July, 2010): It has been decided to drain the subglacial lake as soon as possible, because 3000 people were threatened in the valley.
Alert system set up to trigger off fire sirens in the villages downstream
The subglacial lake has been drained artificially using down hole pumps between August and October 2010
Risk of collapse with the decrease of water pressure?
Sonar measurements to obtain the geometry of the subglacial cavity (1st September) (see 3D image)
Elmer/Ice code (Ice flow modelling) (Gagliardini et al., 2010, Durand et al., 2009, Gagliardini and Zwinger, 2008, Zwinger et al., 2007 ) Result: max tensile stress : 0.2 MPa +/-0.1 MPa Tensile strength known for temperate ice : 0.8 +/- 0.4 MPa
The risk of cavity roof collapse was estimated to be low but could not be completely excluded
Analyse de la résistance de la voûte par modélisation numérique Résultat: Contrainte de traction modélisée proche de la valeur de la résistance à la traction de la glace Réunion à la mairie de saint Gervais le 8 septembre «on doit considérer que le risque d effondrement du toit de la cavité ou d une partie du toit de la cavité, en cours de vidange, est faible mais pas nul.»
For preventive measure, we suggested :. To measure the vertical and horizontal velocities from a network of stakes, each day, from topographic measurements. To observe the glacier surface and craks
Many questions for the scientists: -What is the cause of water accumulation inside this glacier? - Will this subglacial lake water form again in the future?
Conclusions: - The first time that a subglacial lake has been detected in an alpine glacier - The water accumulation is due to thermal regime - The rate of water accumulation remains unknown - A lot of work is needed again to reply to all questions. has
has e Thank you for your attention.