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The Lake Verde Complex Landslide at Vallée Étroite,Hautes-Alpes, South France

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This paper briefy describes the study carried out in connection with the Lake Verde Complex Landslide located on the left side of the Vallée Étroite, a Valley North-South oriented in South-East France but close to the boundary with North Italy. The valley has the particularity of being the upper portion of a long valley which is developed for the most part in Italy to the south and is accessible from France crossing the Col de l’Échelle (1780 m a.s.l.) with the D1T road which is closed for 5-6 months/year in the wintertime. For these reasons this landslide hasn’t been studied until now but the tourist vocation, the presence of a dam at the entrance of the Vallée Étroite and the villages of Pian del Colle, Melezet, Les Arnauds, and Bardonecchia along the valley, makes the study of this big landslide very relevant
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ACADEMIA Letters
The Lake Verde Complex Landslide at Vallée Étroite,
Hautes-Alpes, South France
Riccardo Amici
This paper briey describes the study carried out in connection with the Lake Verde Com-
plex Landslide located on the left side of the Vallée Étroite, a Valley North-South oriented in
South-East France but close to the boundary with North Italy (Figure 1). The valley has the
particularity of being the upper portion of a long valley which is developed for the most part
in Italy to the south and is accessible from France crossing the Col de l’Échelle (1780 m a.s.l.)
with the D1T road which is closed for 5-6 months/year in the wintertime. For these reasons
this landslide hasn’t been studied until now but the tourist vocation, the presence of a dam at
the entrance of the Vallée Étroite and the villages of Pian del Colle, Melezet, Les Arnauds,
and Bardonecchia along the valley, makes the study of this big landslide very relevant.
Academia Letters, August 2021
Corresponding Author: Riccardo Amici, riccamici@gmail.com
Citation: Amici, R. (2021). The Lake Verde Complex Landslide at Vallée Étroite, Hautes-Alpes, South France.
Academia Letters, Article 3034. https://doi.org/10.20935/AL3034.
1
©2021 by the author — Open Access — Distributed under CC BY 4.0
Figure 1: The geographical framework of the study area and inside the red circle the Lake
Verde Complex Landslide
The Lake Verde Complex Landslide is a big landslide on the left side of the Vallée Étroite
which is a typical U section alpine valley of glacial origin, as shown in Figure 2 (Prol Al-
timétrique).
The estimated rock mass volume involved in the landslide is about 10 to 15 million cubic
meters.
GEOLOGICAL AND GEOMORPHOLOGICAL CHARACTERS
The Vallée Étroite, from the geological point of view (Figure 3), is located at the Sapey zone
which is the boundary between the External and Internal Briançonnais with, on the East part
of the map, Mesozoic cover rocks, folded into synclines overturned to the East with rocks
mainly dolomitic (Middle Triassic).
In this area, the east side of the valley is composed of a sequenceof carbonate and dolomitic
rocks originating the alignment of Pointe Gaspard-Pointe Melchior-Pointe Balthazar along the
boundary, all over 3000 m a.s.l. At the base of the carbonate sequence, there are tectonic rocks
like gypsum and cargneules (Figure 4 from BRGM, Notice explicative de la feuille Névache
– Bardonecchia – Modane à 1:50 000).
The geomorphological map (Figure 5) shows the characters of the landslide with a main
Academia Letters, August 2021
Corresponding Author: Riccardo Amici, riccamici@gmail.com
Citation: Amici, R. (2021). The Lake Verde Complex Landslide at Vallée Étroite, Hautes-Alpes, South France.
Academia Letters, Article 3034. https://doi.org/10.20935/AL3034.
2
©2021 by the author — Open Access — Distributed under CC BY 4.0
Figure 2: Aerial photo of the landslide area (red circle) and the cross-section viewed toward
the mount of the Vallée Étroite with the typical glacial U section
scarp at the altitude of 2300 m a.s.l. and the presence of a series of cracks drawing a zigzag
shape accordingly to the tectonic style of the area. Main Faults are aligned along the valley
and perpendicular to the same, creating a regular pattern of cracks, tension cracks, joints,
fractures.
The glacial activity which is characterized by a rapid deepening of the alpine valleys, rapid
decompression with the discharge of the forces acting on the slopes following the melting of
the ice, start of the post-glacial erosion phase, therefore proposes itself as a preparatory cause
of great landslides to which it would seem to be attributable the landslide of the Lake Verde.
Going back to the older aerial photo available on the IGN website (1955-1965) it seems
that the slope evolution is very slow. Photos are not very clear and it’s dicult to understand
if there was big evolution. It’s only probably that the Main Scarp looks higher today than in
1955-65.
The landslide has moved in the past along a lower surface but the big landslide body hasn’t
collapsed yet. The Main Scarp has a height of 25-30 m at the center and this height increases
Academia Letters, August 2021
Corresponding Author: Riccardo Amici, riccamici@gmail.com
Citation: Amici, R. (2021). The Lake Verde Complex Landslide at Vallée Étroite, Hautes-Alpes, South France.
Academia Letters, Article 3034. https://doi.org/10.20935/AL3034.
3
©2021 by the author — Open Access — Distributed under CC BY 4.0
Figure 3: Geological map from BRGM. Inside the black circle the zone of the Lake Verde
Landslide
Figure 4: Cross-section from BRGM. In the red box the zone of the Lake Verde Landslide.
The carbonate sequence on the right is in tectonic contact with the houlliere zone along the
Accident Sapey-Vallée Étroite. This tectonic alignment is characterized by the presence of
gypsum
moving toward the sites. The main body of the landslide is divided into smaller blocks from
cracks and fractures that are well visible from the frontal view.
From the frontal view of the slope (Figure 7) it is possible to see the presence of others
characteristics like the big Talus slope and a double debris cone which probably is a small part
of the landslide that collapsed at the base of the slope. At the toe of the debris cone, there is
the outcrop of Gypsum. The debris cone has created the depression for Lake Verde (Lac Vert
in French).
Academia Letters, August 2021
Corresponding Author: Riccardo Amici, riccamici@gmail.com
Citation: Amici, R. (2021). The Lake Verde Complex Landslide at Vallée Étroite, Hautes-Alpes, South France.
Academia Letters, Article 3034. https://doi.org/10.20935/AL3034.
4
©2021 by the author — Open Access — Distributed under CC BY 4.0
Figure 5: Geomorphological map
Academia Letters, August 2021
Corresponding Author: Riccardo Amici, riccamici@gmail.com
Citation: Amici, R. (2021). The Lake Verde Complex Landslide at Vallée Étroite, Hautes-Alpes, South France.
Academia Letters, Article 3034. https://doi.org/10.20935/AL3034.
5
©2021 by the author — Open Access — Distributed under CC BY 4.0
Figure 6: Cross-section of the slope. Extending down the scarps is possible to evaluate the
thickness and the internal structure of the landslide and understand the failure mechanism (the
trace of the cross-section is in Figure 5)
Academia Letters, August 2021
Corresponding Author: Riccardo Amici, riccamici@gmail.com
Citation: Amici, R. (2021). The Lake Verde Complex Landslide at Vallée Étroite, Hautes-Alpes, South France.
Academia Letters, Article 3034. https://doi.org/10.20935/AL3034.
6
©2021 by the author — Open Access — Distributed under CC BY 4.0
Figure 7: Frontal view of the landslide. The Landslide body is clearly fractured with cracks
and fractures which divide it into numerous blocks
The landslide is composed of a principal rotational slide along a deep circular surface and
debris and talus slope created by the slow and continuous mechanical decay of the rocks which
cover the mid-lower part of the slope. The presence of numerous and extensive fractures at the
top of the slope, behind the Main Scarp, suggests that the landslide will evolve in the future
with others falls of big parts of the slope collapsing toward the valley pushing the current
Main Body in this crash.
The length of the landslide front is around 600 m but it can become 1000-1200 m if the
cracks at the backside of the Main Scarp continue to open. The toe at the valley is at 1850-
1860 m a.s.l. and the top of the slope is at 2300-2310 m a.s.l. with a very high average gradient
of more than 60%. The volume of the landslide is impressive and can be estimated at 10 to
15 million cubic meters considering the current and the future evolution of the slope.
Academia Letters, August 2021
Corresponding Author: Riccardo Amici, riccamici@gmail.com
Citation: Amici, R. (2021). The Lake Verde Complex Landslide at Vallée Étroite, Hautes-Alpes, South France.
Academia Letters, Article 3034. https://doi.org/10.20935/AL3034.
7
©2021 by the author — Open Access — Distributed under CC BY 4.0
DISCUSSION AND CONCLUSIONS
The lake Verde Complex Landslide is a big mass movement on the left side of the Vallée
Étroite at the boundary between Italy and France. Due to the particular location, the landslide
has never been studied before but the presence of a dam and villages along the valley, be-
yond the Italian border, is important to study the evolution and the future developments of the
movement, to assess the risk of the sudden collapse of the slope with consequent damming of
the valley and possible formation of a dangerous lake.
On the most recent aerial photo (2018, IGN) is clearly visible the presence of fractures
and secondary scarps at the main body of the landslide (Figure 8). It seems that the complex
movement is evolving towards the subdivision of the body into numerous smaller bodies which
could move downwards independently.
Figure 8: Aerial photo from IGN (2018) where is possible to notice the fractures (in red) at
the upper part of the main body of the landslide
The landslide is a very high hazardous phenomenon and the stability of the slope appears
very precarious with a high risk for local communities.
Academia Letters, August 2021
Corresponding Author: Riccardo Amici, riccamici@gmail.com
Citation: Amici, R. (2021). The Lake Verde Complex Landslide at Vallée Étroite, Hautes-Alpes, South France.
Academia Letters, Article 3034. https://doi.org/10.20935/AL3034.
8
©2021 by the author — Open Access — Distributed under CC BY 4.0
It seems reasonable to deepen the studies on the ground, mapping the traction fractures on
the back of the crown area and activate the monitoring to observe, on a numerical database, the
evolution of the landslide, for example following intense rain events or following the spring
melting of snow. This information can allow the local authorities to prepare landslide suscep-
tibility and hazard models in the case of collapse.
REFERENCES
BRGM, Service Géologique National, France. http://infoterre.brgm.fr/
BRGM, Notice explicative de la feuille Névache – Bardonecchia – Modane à 1:50 000.
Burri M. & Grumer E. (1976) – Phénomenes d’instabilité dans les Vallées des Drances
(Valais). Eclogae Geol. Helv., 69 (1), 75-83.
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IGN. Institut national de l’information geographique et forestiere. https://www.geoportail.
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Panizza M. (1973) - Glacio-pressure implication in the production of landslides in the Dolomitic
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GIae. It., 22, 45-48.
Perna G. (1974) – Le Frane glaciali e postglaciali nel Trentino meridionale (marocche e laste).
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Schuster R.L. (1986) - Landslide Dam: processes, risk, and mitigation. Am. Soc. Civ. Eng.,
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Servizio Geologico d’Italia. Carta Geologica d’Italia in scala 1:50.000. Fogli 132-152-153.
Academia Letters, August 2021
Corresponding Author: Riccardo Amici, riccamici@gmail.com
Citation: Amici, R. (2021). The Lake Verde Complex Landslide at Vallée Étroite, Hautes-Alpes, South France.
Academia Letters, Article 3034. https://doi.org/10.20935/AL3034.
9
©2021 by the author — Open Access — Distributed under CC BY 4.0
ResearchGate has not been able to resolve any citations for this publication.
Article
Recurring large rock-slope failures of the Argentera–Mercantour massif (southern French Alps) over the past 20,000years were identified in the field and analyzed using a continuum mechanics approach based on material weakening in a three-dimensional finite-difference model. We compared a mountain-slope failure model neglecting the effect of rock strength loss with a model simulating the effect of strength loss and integrating cohesional weakening and frictional strengthening processes. The good correspondence between the relative chronology and location of observed and simulated deformations shows that (1) the gradual loss of rock mass strength related to stress release effects following the Pleistocene deglaciation can be as long as several thousands of years, and (2) the causes and triggering of the present-day active landslides are related to this long failure history. Our study suggests that the episodic nature of massive rock slides can be explained through the gradual development of tensile and shear failure, and time-dependent strength degradation. Our numerical model is a good reproduction of large landslides at the regional-scale of a mountainous massif.
Phénomenes d'instabilité dans les Vallées des Drances (Valais)
  • M Burri
  • E Grumer
Burri M. & Grumer E. (1976) -Phénomenes d'instabilité dans les Vallées des Drances (Valais). Eclogae Geol. Helv., 69 (1), 75-83.
Institut national de l'information geographique et forestiere
  • Ign
IGN. Institut national de l'information geographique et forestiere. https://www.geoportail. gouv.fr/
Fenomeni franosi connessi ad azioni di glacio pressione
  • M Panizza
Panizza M. (1974) -Fenomeni franosi connessi ad azioni di glacio pressione. Boll. Como GIae. It., 22, 45-48.
  • G Perna
Perna G. (1974) -Le Frane glaciali e postglaciali nel Trentino meridionale (marocche e laste). Boll. Como Glac. It., 22 (2), 59-66.
Landslide Dam: processes, risk, and mitigation
  • R L Schuster
Schuster R.L. (1986) -Landslide Dam: processes, risk, and mitigation. Am. Soc. Civ. Eng., Geotechnical Special Publication, 3, 162.