Subglacial lake in the glacier of Tete Rousse (Mont Blanc area ...

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
3150
3140
3130
1 402 403 404 405 406 407 408 409 410 411 412 413
414
3140
3130
Altitude
3120
415 416 417 418 419 420 421 422423 424 3070
Galerie
3120
3110
3110
3100
3100
3090
3090
3080
3070
Profil 4
3080
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
18
Longitudinal section
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
18
Longitudinal section
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….
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Thank you for your attention….