# Modelling the thermal regime of perched talus slopes: The Gislá landslide (06/10/2020, Iceland)

Meven Philippe, Florence Magnin, Jean-Yves Josnin, Costanza Morino, Nicolas Monzie, Skafti Brynjólfsson

► To cite this version:

Meven Philippe, Florence Magnin, Jean-Yves Josnin, Costanza Morino, Nicolas Monzie, et al. Modelling the thermal regime of perched talus slopes: The Gislá landslide (06/10/2020, Iceland). 15th DACH Permafrost Conference, WSL, Jan 2025, Davos (Suisse), Switzerland. hal-04913462

HAL Id: hal-04913462

https://hal.science/hal-04913462v1

Submitted on 27 Jan 2025

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# Modelling the thermal regime of perched talus slopes

## The Gislá landslide (06/10/2020, Iceland)

**Contributors:**

Meven Philippe, Florence Magnin, Jean-Yves Josnin,
Costanza Morino, Nicolas Monzie, Skafti Brynjólfsson

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image credits: local police agents
Permafrost degradation can generate
landslides...

...which represent an **increasing hazard in mountain environments**

<figure><img src="image_5.png" /><figcaption>the Gislá landslide (map.is)</figcaption></figure>
Permafrost degradation can generate
landslides...

...which represent an **increasing hazard in mountain environments**

<figure><img src="image_6.png" /><figcaption>the Gislá landslide (map.is)</figcaption></figure>
Permafrost degradation can generate landslides...

...which represent an **increasing hazard in mountain environments**

<figure><img src="image_7.png" /><figcaption>the Gislá landslide (map.is)</figcaption></figure>
Permafrost degradation can generate
landslides...

...which represent an **increasing hazard in mountain environments**

<figure><img src="image_8.png" /><figcaption>the Gislá landslide (map.is)</figcaption></figure>
Permafrost degradation can generate landslides...

...which represent an **increasing hazard in mountain environments**

→ **predicting permafrost boundaries is crucial for prevention/protection purposes**

<figure><img src="image_9.png" /><figcaption>the Gislá landslide (map.is)</figcaption></figure>
To predict permafrost occurrence:

* statistical models

→ from topoclimatic data

→ large scale modelling

<figure><img src="image_10.png" /></figure>

Boeckli et al., 2012 (the Alps)
To predict permafrost occurrence:

* statistical models

→ from topoclimatic data

→ large scale modelling

* physic-based models

→ compute physical processes

→ at site scale

→ can predict azonal permafrost

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

Wicky et al., 2024 (in a talus slope)
In talus slope, particular air convection can take place - the 'chimney effect'

<figure><img src="image_12.png" /></figure>
In talus slope, particular air convection can take place - the 'chimney effect'

→ **winter:** suction of cold air at the base of the talus

<figure><img src="image_13.png" /></figure>
In talus slope, particular air convection can take place - the 'chimney effect'

* **winter:** suction of cold air at the base of the talus

* **summer:** gravitational flow of cold air to the base of the talus

<figure><img src="image_14.png" /></figure>
In talus slope, particular air convection can take place - the 'chimney effect'

* **winter:** suction of cold air at the base of the talus

* **summer:** gravitational flow of cold air to the base of the talus

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

Thermal dynamics of talus slopes are poorly understood

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The Gislá landslide (October 6th 2020):

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The Gislá landslide (October 6th 2020):

* source: perched talus slope

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The Gislá landslide (October 6th 2020):

* source: perched talus slope

* ice-cemented material in the deposits (molards)...

<figure><img src="image_20.png" /></figure>
The Gislá landslide (October 6th 2020):

* source: perched talus slope

* ice-cemented material in the deposits (molards)...

* ... but located outside of permafrost climatic boundaries

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

<figure><img src="image_22.png" /></figure>

Lilleøren et al. (2013)
The Gislá landslide (October 6th 2020):

- source: perched talus slope

- ice-cemented material in the deposits (molards)...

- ... but located outside of permafrost climatic boundaries

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

persistence of permafrost
due to the chimney effect ?

<figure><img src="image_24.png" /></figure>

winter

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summer
1. Can permafrost persist in the Gislá talus slope until the date of the landslide?

2. What was its thermal state at the date of the landslide?

3. What thermal properties of the Gislá talus slope are required to maintain permafrost?
* computation of heat transfer equations

* Finite-Element Method (FEFLOW commercial software)

* modelling of the thermal dynamics from -20,000 to present

* evaluation of permafrost conditions at the date of the Gislá landslide

<figure><img src="image_26.png" /></figure>
Heat transfer modelling within the
Gislá perched talus slope:

1. definition of 2D site geometry

<figure><img src="image_27.png" /></figure>
Heat transfer modelling within the
Gislá perched talus slope:

1. definition of 2D site geometry

<figure><img src="image_28.png" /></figure>
Heat transfer modelling within the
Gislá perched talus slope:

1. definition of 2D site geometry

<figure><img src="image_29.png" /></figure>
Heat transfer modelling within the
Gislá perched talus slope:

1. definition of 2D site geometry

2. construction of temperature
time series (boundary conditions)

975 m

394 m

<figure><img src="image_30.png" /></figure>
Heat transfer modelling within the
Gislá perched talus slope:

1. definition of 2D site geometry

2. construction of temperature
time series (boundary conditions)

<figure><img src="image_31.png" /></figure>
Heat transfer modelling within the
Gislá perched talus slope:

1. definition of 2D site geometry

2. construction of temperature
time series (boundary conditions)

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

975 m

394 m

surface temperature time series

(-20,000 to present)

<figure><img src="image_33.png" /></figure>

geothermal heat flux
Heat transfer modelling within the
Gislá perched talus slope:

1. definition of 2D site geometry

2. construction of temperature
time series (boundary conditions)

3. modelling approach:

- division in sections

• **meshing**

975 m

394 m

<figure><img src="image_34.png" /></figure>
Heat transfer modelling within the
Gislá perched talus slope:

1. definition of 2D site geometry

2. construction of temperature
time series (boundary conditions)

3. modelling approach:

- division in sections

• *meshing*

<figure><img src="image_35.png" /></figure>
Heat transfer modelling within the
Gislá perched talus slope:

1. definition of 2D site geometry

2. construction of temperature
time series (boundary conditions)

3. modelling approach:

* division in sections

* meshing

* parametrisation

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

<figure><img src="image_37.png" /></figure>
Heat transfer modelling within the
Gislá perched talus slope:

1. definition of 2D site geometry

2. construction of temperature
time series (boundary conditions)

3. modelling approach:

- division in sections

- meshing

- parametrisation

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

models are good to go!

<figure><img src="image_39.png" /></figure>
Final temperature/permafrost conditions mainly depend on:

1. the **porosity** and **initial ice content** of the talus slope

→ **higher porosity/ice content**
= **greater permafrost persistence**

October 6th 2020
thermal conductivity = 1.1 W/m/K

temperature (°C)

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

% ice

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

porosity =
30%

<figure><img src="image_42.png" /></figure>
October 6th 2020
porosity/initial ice content = 80%

Final temperature/permafrost conditions
mainly depend on:

1. the **porosity** and **initial ice content** of the talus slope

→ **higher porosity/ice content**
= **greater permafrost persistence**

2. the **thermal conductivity** of the rock

→ **lower thermal conductivity**
= **greater permafrost persistence**

<figure><img src="image_43.png" /></figure>
1. Can permafrost persist in the Gislá talus slope until the date of the landslide?

→ Yes - permafrost persists at the base of the talus, as expected with a chimney effect

<figure><img src="image_44.png" /><figcaption>model run
(por. = 80%
TC = 0.75 W/m/K)</figcaption></figure>

<figure><img src="image_45.png" /></figure>
**2. What was the thermal state of permafrost at the date of the landslide?**

→ **25% of ice persists** at the base, with the most ice-conservative scenario

→ **nearly no ice persists** with the least ice-conservative ones

<figure><img src="image_46.png" /></figure>
3. What thermal properties of the Gislá talus slope are required to maintain permafrost?

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

<figure><img src="image_48.png" /></figure>
3. What thermal properties of the Gislá talus slope are required to maintain permafrost?

→ ice-cemented material in the landslide deposits indicate a **relatively high ice content (~50%)**

→ the most ice-conservative scenario shows **lower ice content (25%)**

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

<figure><img src="image_50.png" /></figure>
3. What thermal properties of the Gislá talus slope are required to maintain permafrost?

→ ice-cemented material in the landslide deposits indicate a **relatively high ice content (~50%)**

→ the most ice-conservative scenario shows **lower ice content (25%)**

→ modelling air convection would **increase the final ice content**

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Thank you for your attention!

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image credits: local police agents