PREDISPOSING, TRIGGERING AND RUNOUT
CONDITIONS OF TWO ROCK SLOPE FAILURES IN
THE FRENCH ALPS: ÉTACHE VALLEY AND
CRÊTE DES GRANGETTES

Maëva Cathala, Josué Bock, Laurent Astrade, Matan Ben-Asher, Xavier Bodin, Guillaume Chambon, Thierry Faug, Kim Génuite, Stéphane Jaillet, Jean-Yves Josnin, et al.

► To cite this version:

Maëva Cathala, Josué Bock, Laurent Astrade, Matan Ben-Asher, Xavier Bodin, et al. PREDISPOSING, TRIGGERING AND RUNOUT CONDITIONS OF TWO ROCK SLOPE FAILURES IN THE FRENCH ALPS: ÉTACHE VALLEY AND CRÊTE DES GRANGETTES. 15th DACH Permafrost Conference, WSL Davos, Jan 2025, Davos (Suisse), Switzerland. **hal-04901177**

HAL Id: hal-04901177

https://hal.science/hal-04901177v1

Submitted on 20 Jan 2025

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# PREDISPOSING, TRIGGERING AND RUNOUT CONDITIONS OF TWO ROCK SLOPE FAILURES IN THE FRENCH ALPS: ÉTACHE VALLEY AND CRÊTE DES GRANETTES

<figure><img src="image_2.png" /></figure>

Authors: Cathala M., Bock J., Abdulsamad F., Astrade L., Ben Asher M., Bodin X., Chambon G., Deline P., Faug T., Génuite K., Jaillet S., Josnin J-Y., Ravanel L., Revil A., Richard J., Verroust F., Magnin F.

<figure><img src="image_3.png" /></figure>

UNIVERSITÉ
SAVOIE
MONT BLANC

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

laboratoire
edytem

environnements dynamiques fettoloires montagneuses

<figure><img src="image_6.png" /></figure>

ART
ASSOCIATION NATIONALE
RECHERCHE TECHNOLOGIE

INRAE

<figure><img src="image_7.png" /></figure>
CONTEXT AND STUDY SITES

<figure><img src="image_8.png" /></figure>

• 2020 June 18

• 225 000 m³

• 3126 m

• North West face

S. Rouenitska
CONTEXT AND STUDY SITES

<figure><img src="image_9.png" /></figure>

* 2020 September 17 (35 800 m³)

* 2020 December 20 (220 000 m³)

* 2021 July 23 (unknow volume)

* 3080 m

* North face
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

Role of permafrost in
the triggering
mechanisms?

<figure><img src="image_12.png" /></figure>

Ph. PGHM
Briançon
Combining methods to improve fundamental knowledge of the processes involved
in the **predisposition, triggering and propagation** of rockfalls and rock avalanches
in **permafrost-affected steep slopes**

<figure><img src="image_13.png" /></figure>
RESULTS & DISCUSSIONS

1) Thermal conditions

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

<figure><img src="image_14.png" /></figure>

Assessment of the
site's thermal state
1) Thermal conditions

In-situ temperature measurements

<figure><img src="image_15.png" /></figure>

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

ASSESSMENT OF THE SITE'S THERMAL STATE

Ground Surface Temperature - c. 10 cm depth – 1 measurement /hour)

Gentle slope

<figure><img src="image_16.png" /><figcaption>Possible snow
accumulation</figcaption></figure>

Rockwall

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /><figcaption>Direct contact with
the atmosphere</figcaption></figure>
## 1) Thermal conditions

In-situ temperature measurements

<figure><img src="image_19.png" /></figure>

## 2) Meteorological / hydrological factors

## 3) Spatial and temporal evolution of permafrost

## 4) Propagation mechanisms

### ASSESSMENT OF THE SITE'S THERMAL STATE

#### Ground Surface Temperature
#### Étache Valley

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

Température annuelle moyenne de la proche surface (-10 cm) 2021-2022

→ Favourable conditions for permafrost occurrence on the north face, ≠ south face
## 1) Thermal conditions

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

<figure><img src="image_22.png" /><figcaption>In-situ temperature measurements</figcaption></figure>

<figure><img src="image_23.png" /></figure>

## ASSESSMENT OF THE SITE'S THERMAL STATE

<figure><img src="image_24.png" /><figcaption>### Ground Surface Temperature
#### Grangettes crest</figcaption></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

→ Conditions favourable for permafrost occurrence on the north face and the crest (in superficial formations)
## 1) Thermal conditions

Geoelectrical surveys and petrophysical analyses

<figure><img src="image_27.png" /></figure>

## 2) Meteorological / hydrological factors

## 3) Spatial and temporal evolution of permafrost

## 4) Propagation mechanisms

<figure><img src="image_28.png" /></figure>

### ASSESSMENT OF THE SITE'S THERMAL STATE

#### Geophysical survey

##### Étache Valley

<figure><img src="image_29.png" /><figcaption>Transition freeze/no freeze</figcaption></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

Profile P1-P3-P4 : 5m spacing between the electrodes

Profile P2 : 20m spacing between the electrodes

<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /></figure>
1) Thermal conditions

Geoelectrical surveys and petrophysical
analyses

<figure><img src="image_34.png" /></figure>

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

<figure><img src="image_35.png" /></figure>

ASSESSMENT OF THE SITE'S THERMAL STATE

Geophysical survey

Grangettes crest

<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /></figure>

Profile P1 : 240m - 5m spacing between
the electrodes

Profile P2 : 400m - 20m spacing
between the electrodes

<figure><img src="image_38.png" /></figure>
<figure><img src="image_39.png" /></figure>

Preconditionning and triggering factors
Meteorological analysis

<figure><img src="image_40.png" /></figure>

## METEOROLOGICAL ANALYSIS
### Étache Valley

<figure><img src="image_41.png" /></figure>

## Positive air temperature anomalies compared to 1991-2020:

* Summer 2019 : +2.1°C (The 2nd warmest)

* Winter 2020 : +2.9°C (The 2nd warmest)

* Spring 2020 : +1.9°C (The warmest)

<figure><img src="image_42.png" /></figure>
1) Thermal conditions

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

<figure><img src="image_43.png" /><figcaption>Meteorological analysis</figcaption></figure>

METEOROLOGICAL ANALYSIS
Étache Valley

<figure><img src="image_44.png" /></figure>

Significant precipitation in winter and spring 2020

<figure><img src="image_45.png" /></figure>

→ Heavy rainfalls the week prior to the event

→ Water supply
1) Thermal conditions

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

Meteorological analysis

<figure><img src="image_46.png" /></figure>

**METEOROLOGICAL ANALYSIS**

**Grangettes Crest**

**Springs and summers exceptionally warm since 2015**

* Winter 2020 : the 4th warmest

* Spring 2020 : the warmest

* Summers 2015-2020 : the warmer summers since 2003

<figure><img src="image_47.png" /></figure>

<figure><img src="image_48.png" /></figure>
1) Thermal conditions

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

Meteorological analysis

<figure><img src="image_49.png" /></figure>

Heavy (but not exceptional)
precipitations 20 days prior to the event

→ Triggering factor?

METEOROLOGICAL ANALYSIS
---
Grangettes Crest

<figure><img src="image_50.png" /></figure>

<figure><img src="image_51.png" /></figure>

Grangettes
1) Thermal conditions

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

Hydrological and energy balance modelling

<figure><img src="image_52.png" /></figure>

THERMAL AND HYDROLOGICAL CONDITIONS OF THE GROUND BEFORE THE EVENT

ENERGY AND HYDROLOGICAL BALANCE MODELLING

(CRYOGRID; WESTERMAN ET AL., 2013)

* **Atmospheric data:** air temperature, precipitation + snow, wind, humidity, solar radiation

→ Météo France data/ Safran + Crocus models

* **Surface data:** evolution of snow cover → Crocus model

→ Ben Asher et al., 2023 (set for steep slopes)

* **Model outputs:**

→ Surface temperature

→ Water supply (available for infiltration)

→ At the point scale (1D)

<figure><img src="image_53.png" /></figure>
1) Thermal conditions

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

<figure><img src="image_54.png" /><figcaption>Hydrological and energy balance modelling</figcaption></figure>

THERMAL AND HYDROLOGICAL CONDITIONS OF THE GROUND BEFORE THE EVENT
Étache valley

<figure><img src="image_55.png" /></figure>

<figure><img src="image_56.png" /></figure>

**Exceptionally High Ground Temperatures**
(significantly exceeding air temperatures)

* Winter 2020 : +5.5 °C (2.9°C > air temperature anomaly)

* Spring 2020 : +4.1 °C (2.6°C > air temperature anomaly)

→ Ground surface temperature are unprecedented since the start of the modeled record (1958)

→ **Permafrost warming**: A preconditioning factor for slope instability
1) Thermal conditions

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

<figure><img src="image_57.png" /><figcaption>Hydrological and energy balance modelling</figcaption></figure>

## THERMAL AND HYDROLOGICAL CONDITIONS OF THE GROUND BEFORE THE EVENT
### Étache valley

<figure><img src="image_58.png" /></figure>

Spring 2020 : the highest water supply available for
infiltration since 1958 (+120 mm)

→ Heavy precipitation and snow melt

→ **Role of water infiltration:**

• Increase hydrostatic pressures

• Erosion of ice-filled fractures

<figure><img src="image_59.png" /></figure>
<figure><img src="image_60.png" /><figcaption>Assessing the conditions and spatio-temporal dynamics of permafrost</figcaption></figure>
<figure><img src="image_61.png" /><figcaption>Heat transfert modelling</figcaption></figure>

<figure><img src="image_62.png" /></figure>

## ASSESSING THE CONDITIONS AND SPATIO-TEMPORAL DYNAMICS OF PERMAFROST
### Étache Valley

<figure><img src="image_63.png" /><figcaption>Simulation 1 : Forced with rock surface temperature deduced from
air temperature</figcaption></figure>

<figure><img src="image_64.png" /><figcaption>Simulation 2 : Forced with rock surface temperature modelled with CryoGrid (energy balance modeling)</figcaption></figure>

<figure><img src="image_65.png" /></figure>
<figure><img src="image_66.png" /><figcaption>Heat transfert modelling</figcaption></figure>

<figure><img src="image_67.png" /></figure>

ASSESSING THE CONDITIONS AND SPATIO-TEMPORAL DYNAMICS OF PERMAFROST
Étache Valley

<figure><img src="image_68.png" /><figcaption>Simulation 1 : Forced with rock surface temperature deduced from
air temperature</figcaption></figure>

<figure><img src="image_69.png" /><figcaption>Simulation 2 : Forced with rock surface temperature modelled with CryoGrid (energy balance modeling)</figcaption></figure>

<figure><img src="image_70.png" /></figure>

-> Warming permafrost

• Progressive since the 1990's and very sharp after 2012

- Transition from cold to warm permafrost

<figure><img src="image_71.png" /></figure>
<figure><img src="image_72.png" /><figcaption>Heat transfert modelling</figcaption></figure>

<figure><img src="image_73.png" /></figure>

ASSESSING THE CONDITIONS AND SPATIO-TEMPORAL DYNAMICS OF PERMAFROST
Étache Valley

<figure><img src="image_74.png" /><figcaption>Simulation 1 : Forced with rock surface temperature deduced from
air temperature</figcaption></figure>

<figure><img src="image_75.png" /><figcaption>Simulation 2 : Forced with rock surface temperature modelled with CryoGrid (energy balance modeling)</figcaption></figure>

<figure><img src="image_76.png" /></figure>

-> Warming permafrost

• Progressive since the 1990's and very sharp after 2012

- Transition from cold to warm permafrost

<figure><img src="image_77.png" /></figure>

-> The rock avalanche occurred under the warmest modeled conditions
Heat transfert
modelling

<figure><img src="image_78.png" /></figure>

ASSESSING THE CONDITIONS AND SPATIO-TEMPORAL DYNAMICS OF PERMAFROST

**Grangettes crest**

<figure><img src="image_79.png" /></figure>

Ref simulation

Warm simulation

<figure><img src="image_80.png" /></figure>

<figure><img src="image_81.png" /></figure>

* Progressive warming permafrost since the 1990's
1) Thermal conditions

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

<figure><img src="image_82.png" /><figcaption>Heat transfert modelling</figcaption></figure>

ASSESSING THE CONDITIONS AND SPATIO-TEMPORAL DYNAMICS OF PERMAFROST

**Grangettes crest**

<figure><img src="image_83.png" /><figcaption>Ref simulation</figcaption></figure>

<figure><img src="image_84.png" /><figcaption>Warm simulation</figcaption></figure>

<figure><img src="image_85.png" /></figure>

* Progressive warming permafrost since the 1990's

* Sharp increase since 2015
Heat transfert
modelling

<figure><img src="image_86.png" /></figure>

ASSESSING THE CONDITIONS AND SPATIO-TEMPORAL DYNAMICS OF PERMAFROST

**Grangettes crest**

<figure><img src="image_87.png" /></figure>

Ref simulation

Warm simulation

<figure><img src="image_88.png" /></figure>

<figure><img src="image_89.png" /></figure>

- Progressive warming permafrost since the 1990's

- Sharp increase since 2015

- Warm permafrost approaching 0°C
Heat transfert
modelling

<figure><img src="image_90.png" /></figure>

ASSESSING THE CONDITIONS AND SPATIO-TEMPORAL DYNAMICS OF PERMAFROST

**Grangettes crest**

<figure><img src="image_91.png" /></figure>

Ref simulation

Warm simulation

<figure><img src="image_92.png" /></figure>

<figure><img src="image_93.png" /></figure>

* Progressive warming permafrost since the 1990's

* Sharp increase since 2015

* Warm permafrost approaching 0°C

→ **The rockfall occurred under the warmest modeled conditions**
# RESULTS & DISCUSSIONS

1) Thermal conditions

2) Meteorological / hydrological factors

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

<figure><img src="image_94.png" /></figure>

Propagation
modelling
1) Thermal conditions

<figure><img src="image_95.png" /></figure>

2) Meteorological / hydrological factors

<figure><img src="image_96.png" /></figure>

Propagation
modelling

Evaluation

3) Spatial and temporal evolution of permafrost

UNDERSTANDING PROPAGATION MECHANISMS
3D modelling (photogrammetry)

4) Propagation mechanisms

<figure><img src="image_97.png" /></figure>

Étache

Rasters comparison before/after:
volume calculation

<figure><img src="image_98.png" /></figure>

→ Drone survey (2021 and 2022)

<figure><img src="image_99.png" /></figure>

Point cloud
(3D)

High
resolution
DEM

<figure><img src="image_100.png" /></figure>
# RESULTS & DISCUSSIONS

1) Thermal conditions

<figure><img src="image_101.png" /></figure>

High-resolution
volume reconstruction

<figure><img src="image_102.png" /></figure>

2) Meteorological / hydrological factors

<figure><img src="image_103.png" /></figure>

3) Spatial and temporal evolution of permafrost

Propagation
modelling

4) Propagation mechanisms

<figure><img src="image_104.png" /></figure>

Étache

## UNDERSTANDING PROPAGATION MECHANISMS

### Simulation D :

<figure><img src="image_105.png" /></figure>

→ depth-averaged model with a
Voellmy friction law

<figure><img src="image_106.png" /></figure>

• Complex flow process

<figure><img src="image_107.png" /></figure>

• Evolution of the effective
rheological properties
along the flow: **transport**
**of snow by the rock**
**avalanche**

<figure><img src="image_108.png" /></figure>
<figure><img src="image_109.png" /></figure>

## ÉTACHE VALLEY

Cold permafrost

<figure><img src="image_110.png" /></figure>

→ Predisposing factors:

Permafrost warming

(transition from cold to warm permafrost)

<figure><img src="image_111.png" /></figure>

→ Triggering factors: Water infiltration (hydrostatics pressures and/or erosion of ice-filled fractures)

<figure><img src="image_112.png" /></figure>

→ Propagation modelling = Several phases due to snow incorporation

<figure><img src="image_113.png" /></figure>

## GRANETTES CREST

Warm permafrost

<figure><img src="image_114.png" /></figure>

→ Predisposing factors:

Permafrost warming

(approaching to 0°C)

= Mechanical weakening of ice-filled joints

<figure><img src="image_115.png" /></figure>

→ Rockfall deposit barely visible = Limitation for propagation modelling

## Perspectives :

* Better integrate water infiltration processes in permafrost modelling

* Better understand snow and water effects in the triggering and propagation mechanisms of rock slope failures

Earth Surface
Processes and Landforms

<figure><img src="image_116.png" /></figure>

CASE STUDY

<figure><img src="image_117.png" /></figure>

Open Access

<figure><img src="image_118.png" /></figure>

Predisposing, triggering and runout processes at a permafrost-affected rock avalanche site in the French Alps (Étache, June 2020)

Maëva Cathala ❁ Josué Bock, Florence Magnin, Ludovic Ravanel, Matan Ben Asher, Laurent Astrade, Xavier Bodin, Guillaume Chambon, Philip Deline, Thierry Faug, Kim Genuite ... See all authors v

Géo
morphologie

RELIEF, PROCESSUS, ENVIRONNEMENT

Recherche

<figure><img src="image_119.png" /></figure>

INDEX

* Auteurs

* Mots-clés

Articles sous presse

Assessing the role of permafrost in the preconditioning and triggering factors of the September 2020 Crête des Grangettes rockfall (southern French Alps)
THANK YOU

maeva.cathala.pro@gmail.com

Looking for a Post-Doc ;-)

<figure><img src="image_120.png" /></figure>
SUPPLEMENTS

<figure><img src="image_121.png" /></figure>
# Characteristics of the event

* The rock avalanche occurred on June 18 2020 from the crest line (3126 m).

* H/L: 0,56 (ELA 29°)

* Total drop height: 728 m

* Total runout length: 1296 m

* The deposit extends 600 m above the farm, the parking area, and the hiking trails.

* Ice in the rockfall scars suggests the presence of permafrost, but its local distribution remains to be confirmed.

<figure><img src="image_122.png" /></figure>
3D reconstruction

<figure><img src="image_123.png" /></figure>

<figure><img src="image_124.png" /></figure>

The scar:

* Width: 180 m

* Max depth: 46.5 m

* Average depth: 14.7 m.

* Estimated volume: c. 225 000 m³

The deposit:

* Estimated volume: c. 330 000 m³

* Expansion coefficient: 1.37

* Surface: 175 535 m²

* Average thickness: 1.8 m

* Maximum thickness: 12.6 m

* Maximum observed in the upper/central part which concentrates most of the volume (230 000 m³)
# Geomorphological and geological setting

<figure><img src="image_125.png" /></figure>

* Glacial and periglacial processes strongly shaped the landscape of the Vallon d'Etache (e.g. moraines, gelifluction lobes)

* A fault near the scar

<figure><img src="image_126.png" /></figure>

Permafrost
Favorability Index
(Marcer et al., 2017)

Rock wall
permafrost model
(Boeckli et al., 2012)

* Permafrost Favorability Index (PFI) suggests that the rock slope failure occurred in an area favourable to permafrost in all conditions

(PFI tends to overestimate permafrost favourability in current climatic conditions because the active rock glaciers used for the model calibration are inherited of the LIA climate conditions)

* The permafrost model by Boeckli et al. (2012a; 2012b) suggests that it occurred in an area of permafrost occurrence only in favourable conditions

→ None of the two maps are fully representative of permafrost conditions
# Meteorological context

<figure><img src="image_127.png" /></figure>

<figure><img src="image_128.png" /></figure>

## Air temperature anomalies:

* 2020 annual average air temperature: the warmest in the 1985-2014 period

* The 6 month period prior to the event: the second warmest winter and the warmest spring since 1985 at Bessans

## Snow anomalies:

* Average snow depth over the winter period: 186 cm = 48% (60 cm) more snow than the 2010-2019 winter periods at Bonneval-sur-Arc
# Meteorological context

Seasonal cumulatives precipitation anomalies (mm)

<figure><img src="image_129.png" /></figure>

Air temperature

(°C)

<figure><img src="image_130.png" /></figure>

Daily precipitation

(mm)

18-06-2020
Rock avalanche

Date (dd.mm.yyyy)

## Precipitation anomalies:

* The cumulative precipitation records during the 2020 winter (271.2 mm) is +41.6 % (80 mm) higher than the average of 1991-2020

* The spring records (224 mm) show that the cumulative precipitation was only +4.4 % (10 mm) higher than the average of 1991-2020

## Precipitation prior to the rock avalanche:

* Significant rainfall the week prior to the event (15 % of the spring 2020 rainfall in Bessans)

* Significant input of water (snow melt and rainfall, or rain on snow) suggested by positive air temperatures at the scar location
# Ground temperature measurements

<figure><img src="image_131.png" /></figure>

## Field investigation

* Sensors on gentle slope (MAGST (°C))

* Sensors on rock outcrop (MAGST (°C))

* ERT profile and profile number (electrod spacing 5 m)

* ERT profile and profile number (electrod spacing 20 m)

* Linear section for heat transfer modeling

* Rock sample for petrophysical analysis

* 12 loggers installed (c. 10 cm below the surface)

* elevation range from 2675 m to 3116 m asl.

* different ground conditions and sun-exposure.

* data recorded from 2021/10/01 to 2022/09/30.
Ground temperature measurements

<figure><img src="image_132.png" /></figure>

<figure><img src="image_133.png" /></figure>

<figure><img src="image_134.png" /></figure>

<figure><img src="image_135.png" /></figure>

* MAGST: ranges from -2.3 to 6.1°C

* Sensors affected by snow (dampened daily oscillation and persistent sub-zero temperature in winter and spring).

* Effect of permafrost at depth (suggested by low temperature in winter for ETA_4 & ETA_5)
Energy balance modelling

Temperature anomaly (°C)

<figure><img src="image_136.png" /></figure>

* **Winter 2020:** +5.5 °C warmer compares to 1991-2020 (2.9°C higher than the air temperature anomaly)

* **Spring 2020:** +4.1 °C warmer 2.9°C (2.6°C higher than the air temperature anomaly)

* **Spring 2020:** the largest available water supply (rainfall and snowmelt) since 1958 (+120 mm compared to 1991-2020).

Water supply anomalies (mm)

<figure><img src="image_137.png" /></figure>

→ Outstandingly high GST (well higher than the air temperature)

→ The GST records are unprecedented since the start of the modelled time series (1958)

Winter

Spring

Summer

Autumn
# Energy balance modelling

<figure><img src="image_138.png" /><figcaption>Simulated water supply (mm) and snow depth (cm)</figcaption></figure>

* Significant water supply available for infiltration during May 2020, extending up to 14 days prior to the event

* Substantial precipitation and snowmelt.

→ The rock avalanche also occurred during the spring with the greatest water supply ever simulated since 1958.
Heat transfert simulation

<figure><img src="image_139.png" /></figure>

Simulation 1: **Forced** with rock surface temperature deduced
from **air temperature** (does not account for the presence of
snow)

= Strong horizontal gradient of temperature (4.5 °C difference between the SE and NW aspects)

<figure><img src="image_140.png" /></figure>

Simulation 2: Forced with Cryogrid output data

= Smoother patern:

* South: cooling effect (due to albedo increase & delay of seasonal thaw because of snow cover).

* North: warming effect (the insulating effect of snow cover reduces winter cooling).

<figure><img src="image_141.png" /></figure>

→ **Heat transfer simulations confirm the presence of permafrost**
**below the NW face**
Heat transfert simulation

<figure><img src="image_142.png" /></figure>

* Significant warming trend over the past decade

* Potential transition from cold to warm permafrost occurring from a few years to a few months prior to the event

* Simulation 2: Increasing permafrost temperature since the 1990s, with a particularly sharp increase observed since 2012 at the depth of failure (reaching up to +0.06 °C y-1 at 30 m depth)

→ **Both the presence of an ice-rich layer and the steady warming of permafrost has certainly set preconditions for the rock avalanche.**
# Geoelectrical survey

<figure><img src="image_143.png" /></figure>

<figure><img src="image_144.png" /></figure>

* High resistivity values (> 10 kΩ m) confirms the presence of permafrost in the rock avalanche scar location

* Values exceeding those measured in the laboratory (> 250 kΩ m) likely reveal a relatively high ice content around the rock avalanche scar.

**Temperature-converted tomograms limitation:**

As laboratory petrophysical measurements are carried out on intact and saturated samples, they are not representative of field conditions (unsaturated rock, fractures, variable ice content)
# Propagation modelling : Method

<figure><img src="image_145.png" /></figure>

Flow-like propagation: continuous hydraulic was used and the flowing material was assumed to behave as a granular material (e.g., Pirulli and Mangeney, 2008; Sosio et al., 2012, Peruzzetto et al., 2022)

→ depth-averaged model with a Voellmy friction law

→ The material is characterized by the two rheological parameters $\mu$ (dry friction coefficient) and $\xi$ (turbulent friction coefficient, in m s-2)
# Propagation modelling : Method

<figure><img src="image_146.png" /></figure>

* Simulation A ($\mu$ = 0.35 and $\xi$ = 1,000 m s⁻²)

* Simulation B (0.25 and $\xi$ = 100 m s⁻²) correspond to a different balance between the dry and turbulent friction terms

* Simulation C ($\mu$ = 0.5 and $\xi$ = 10,000 m s⁻²) corresponds to an almost purely frictional rheology

* Simulation D:

* Upper part: a volume of 230,000 m³ was considered. (In order to reproduce the relatively homogeneous deposit thickness along the upstream gully, the Voellmy law was enriched by adding a cohesive stress $\tau_c$ to the frictional contributions). $\tau_c/\rho$ = 16.5 m² s⁻², $\mu$ = 0.1 and $\xi$ = 1,000 m s⁻²

* Lower part: a volume of 90,000 m³ was considered (with a simple Voellmy law). $\mu$ = 0.25 and $\xi$ = 1,000 m s⁻² for the lower part

= Simulation D corresponds to the merged deposits
# Propagation modelling : Results

<figure><img src="image_147.png" /></figure>

**Simulations A, B, C:** the thickest parts of the deposits are always located in the two flat zones, especially in the distal lobe whose simulated thickness can reach 8 m

→ This inability of the simulations to reproduce the actual distribution of the deposit can be interpreted as an indication that the **flow model is oversimplified**, and fails to account for important **physical processes** that were at play during the avalanche propagation

→ Such complex mechanisms would require more sophisticated modelling tools based, typically, on multiphase approaches

**Simulation D:** thickness distribution appears to be better reproduced. The simulated deposit is thicker in the upper-central area as well as at the lower front.

→ Even if the physical basis of this composite simulation remains debatable, the result seems to confirm that the propagation of the Étache rock avalanche involved distinct materials characterised by markedly different rheological properties

→ Numerical simulations allow to highlight a relatively complex flow process

→ Voellmy friction law appears insufficient to fully reproduce the observations = need for multiphase approaches

→ The results suggest an evolution of the effective rheological properties along the flow, which could be due to the **entrainment of snow by the rock avalanche**
CARACTÉRISER L'ÉTAT THERMIQUE DU SITE
Étache valley

<figure><img src="image_148.png" /><figcaption>Mesures de température in-situ</figcaption></figure>

<figure><img src="image_149.png" /><figcaption>1- Carte de permafrost dans les formations superficielles</figcaption></figure>

<figure><img src="image_150.png" /><figcaption>2- Carte du permafrost de paroi</figcaption></figure>

<figure><img src="image_151.png" /></figure>

Permafrost présent dans presque toutes les conditions

Permafrost présent principalement dans les conditions froides

Permafrost présent uniquement dans les conditions très favorables

→ Présence de permafrost sous-estimée en paroi, sur-estimée dans les formations superficielles
1) Thermal conditions

2) Preconditionning mechanism

3) Spatial and temporal evolution of permafrost

4) Propagation mechanisms

Hydrological and energy balance
modelling

<figure><img src="image_152.png" /></figure>

<figure><img src="image_153.png" /></figure>

Grangettes

## THERMAL AND HYDROLOGICAL CONDITIONS OF THE GROUND BEFORE THE EVENT
### Grangettes crest

No modelling with CryoGrid

→ **Air circulation between blocks** controlling surface temperature

<figure><img src="image_154.png" /></figure>

<figure><img src="image_155.png" /></figure>
# Crête des Grangettes

## Mesures de Températures in-situ

<figure><img src="image_156.png" /></figure>

<figure><img src="image_157.png" /></figure>
Crête des Grangettes

<figure><img src="image_158.png" /></figure>

08/2022

<figure><img src="image_159.png" /></figure>

08/2009

<figure><img src="image_160.png" /></figure>

07/2003

<figure><img src="image_161.png" /></figure>

07/2000

<figure><img src="image_162.png" /></figure>

08/1993

<figure><img src="image_163.png" /></figure>

08/1952