Estimation of landslides activities evolution due to land use changes in a Pyrenean valley

Transcription

1 Estimation of landslides activities evolution due to land use changes in a Pyrenean valley R. Vandromme, N. Desramaut, S. Bernardie, G. Grandjean

2 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) > 3 > 2

3 1 Introduction (2) > 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. Climate Change Human Activities Landuse Landslide activities > 4

4 2 - Climate change and land use change through global socio economic scenarios 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 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) > 5

5 3. Applications to the Pyrenean site in Cauterets Superficie = 54 km² Climat montagnard, précipitations importantes toute l année (963 mm/an) Altitude entre 600 et 2720 m, altitude moyenne = 1590 m Vue sur la vallée de Cauterets depuis la niche d arrachement du glissement des granges de Pan Cliché L. Cottin (avril 2014) Localisation de la zone d étude Conception L. Cottin (2014) Sources BD ALTI /IGN Nom du service émetteur 17/09/2015 Relief et pentes sur la zone d étude > 6 4

6 4 Method Vegetation cover Additional cohesion Additional weight Topography Meteorological Data Precipitation Temperature PET Digital Elevation Model (DEM) Drainage Basin Characteristics Geomechanical Characteristics Area & Reservoirs Residence Times Cohesion Volumetric Weights Friction angle Hydrological Data Geometry of geologic layers Piezometers River Discharge Layers Thicknesses > ALICE Geotechnical model associated to a GIS interface Probabilistic model > Hydrological model Global model >7

7 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 > 8

8 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. > 9

9 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 c veget=20kpa with root depth = 3 m, for 25m- landslide > 10

10 B Evaluation of vegetation s influence Rotational landslides Landslides length : 25 m 1 m > Landslides depth < 5 m Land use from CLC (forest only) Evolution of the stability of the slope with the consideration of 20kPa root reinforcement compared to no vegetation > 11

11 C- Hydrological Model Gardenia (Thiéry, 2003 ) > Production function Snow model > Reservoirs model 3 reservoirs Time series of tank levels «Water table level» > 12

12 D- Different climatic scenarios > The climate change inputs : 2 scenarios of emission of greenhouse gases. (portal DRIAS http :// > 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. > 13

13 D- Different climatic scenarios > 14

14 E - Climate change impact Rotational landslides Landslides length : 50 m 2 m > Landslides depth < 7 m Landslide probability occurrence in Cauterets area (French Pyrenees) influence of a variation of 1m of water table depth (2.5m vs. 3.5m) > 15

15 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 Acknowledgement This work has been funded by ANR- SAMCO > 16

16 Thank you > 17