JAG 2015 – 3 et 4 septembre 2015

Estimation of landslides activities evolution due to

land–use changes in a Pyrenean valley

R. Vandromme, N. Desramaut, S. Bernardie, G. Grandjean

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1 – Introduction (1)

> Need to produce dynamic susceptibility maps in

changing contexts • Short-term prediction

• Long-term forecasting (Global Changes).

> In FP7-Safeland project, development of methods to

integrate climatic scenarios in landslide hazard

mapping• Precipitation, ETP, Temperature (distinction Rain/snow)

• Spatialization over large areas (up to department scale)

JAG 2015 – 3 et 4 septembre 2015

> But, to really represent future evolution of landslide

hazards, also necessity to address land-use changes

> SAMCO : implement risk mitigation strategies in an integrated

way

• How climate controls mountain hazards occurrence;

• How mountain risks will evolve in the Anthropocene (Land use

change);

• How the main economic, social and political stakeholders interact

for the definition of adaptation scenarios at the region scale.

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Climate

Change

Climate

ChangeHuman

Activities

Human

Activities

Land-

use

Land-

use

Landslide

activities

Landslide

activitiesJAG 2015 – 3 et 4 septembre 2015

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1 – Introduction (2)

2 - Climate change and land use change through

global socio economic scenarios

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The developed methodology for constructing prospective socio-economic

scenarios is as follows :

- Identifying national and global strong trends and driving forces likely to

influence the socio-economic evolution of the municipality up to 2050 and

2100- Downscaling national context scenarios at local scale- Story-telling of four narrative scenarios at local scale : abandonment of the

territory ; sheeps and woods ; a renowned tourism resort ; green town- Validation and spatial characterization of scenarios (LUCC model)

3. Applications to the Pyrenean site in Cauterets

17/09/2015

Nom du service émetteur

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• Superficie = 54 km²

• Altitude entre 600 et 2720 m, altitude moyenne = 1590 m

• Climat montagnard, précipitations importantes toute l’année (963 mm/an)

Localisation de la zone d’étude

Vue sur la vallée de Cauterets depuis la niche

d’arrachement du glissement des granges de Pan –

Cliché L. Cottin (avril 2014)

Relief et pentes sur la zone d’étude

Concep

tion

L. C

ott

in (

20

14

) – S

ourc

es B

D A

LTI ®

/IG

N

4

4 – Method

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Drainage Basin

Characteristics

• Area & Reservoirs• Residence Times

Meteorological

Data

• Precipitation• Temperature• PET

Hydrological

Data

• Piezometers• River Discharge

Topography

• Digital Elevation

Model (DEM)

Geomechanical

Characteristics

• Cohesion• Volumetric Weights• Friction angle

Geometry of

geologic layers

• Layers Thicknesses

Vegetation cover

• Additional cohesion• Additional weight

• Geotechnical model

associated to a GIS

interface

• Probabilistic model

> ALICE > Hydrological

model• Global model

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A - Slope stability model

ALICE software (Sedan et al., 2013)

- 2.5D approach, stability factor computed along the steepest slope

profiles

- Finite slope approach, with user defined geometry of surface

rupture(Morgenstern & Price, 1967)

- Probabilistic Approach: geomechanic parameters (cohesion, frictional

angle and volumetric weight) are given through possibility

distributions

Schematic illustration of ALICE approach

Parameter’s distribution for each soil unit

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B – Evaluation of vegetation’s influence

1. Shear strength

• Additional cohesion

2. Suction

• This suction phenomenon increases the

effective stress in the unsaturated layer of the

soil

• Not taken into account yet.

3.Weight

• Not predominant for deep-seated landslides,

but could have impacts on shallow landslides.

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B – Evaluation of vegetation’s influence

Forest cover

Probability of FS<1

for different sizes of

slopes

size = 25m

Probability of FS<1

for different

additional cveget=20kPa

with root depth = 3 m , for 25m-

landslide

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B – Evaluation of vegetation’s influence

Evolution of the stability of the slope with the consideration of 20kPa root reinforcement compared to no vegetation

Rotational landslides

Landslides’ length : 25 m

1 m > Landslides’ depth < 5 m

Land use from CLC (forest only)

C- Hydrological Model

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> Production function• Snow model

> Reservoirs model• 3 reservoirs

• Time series of tank levels

Gardenia (Thiéry, 2003 )

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« Water table

level »

D- Different climatic scenarios

> The climate change inputs : 2 scenarios of emission of

g r e e n h o u s e g a s e s . ( p o r t a l D R I A S http

://www.drias-climat.fr)

> Performed with the GHG emissions scenarios RCP 4.5

and RCP 8.5 for the ALADIN-Climate model of Météo-

France, and RCP 4.5 and RCP 8.5 for the WRF model

used by the IPSL.

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D- Different climatic scenarios

E - Climate change impact

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Landslide probability occurrence in Cauterets area

(French Pyrenees) – influence of a variation of 1m of

water table depth (2.5m vs. 3.5m)

Rotational landslides

Landslides’ length : 50 m

2 m > Landslides’ depth < 7 m

Conclusion and perspectives

> Methodology for considering global

change impact into landslide hazard

analysis

> Landslides susceptibility maps

according to socio economic and

climate scenario

> Hazard evolution and variation

> Risk variation

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Acknowledgement

This work has been funded by ANR-

SAMCO

http://www.anr-samco.com/

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Thank you

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