MANAGEMENT OF LARGE MUDSLIDES

Transcription

1 36 MANAGEMENT OF LARGE MUDSLIDES

2 Three large and active mudslides Poche, 1860?, Super-Sauze, 1960?, La Valette, March 1982, 1 M m 3 900,000 m M m 3 37

3 Three large and active mudslides Type of development: 1/ Initial slides in the upper part 2/ Weathering of the displaced material 3/ Progressive development of a flow type landslide. The material is filling up gullies Super-Sauze

4 Three large and active mudslides Continuously active mudslides (permanent style of activity) Velocity: m.day -1 and sudden acceleration with possible fluidization Safety factor is fluctuating around Fs = 1.0 during all the year 39

5 Risk management of large mudslides Example of La Valette mudslide Main aim of the risk management: 1/ decrease the risk that debris / mudflows reach the village of St-Pons 2/ decrease the risk that the debris block the Ubaye valley 3/ maintain a road crossing the mudslide Observed debris flows: 1988, 1992, 1993, 1996 Debris-flow in

6 Risk management of large mudslides 1. Structural mitigation: construction of a deposit zone downstream Mudslide Mud/debris flow 100,000 m 3 Deposit zone Risk area 41

7 Risk management of large mudslides 2. Structural mitigation: drainage of the middle part of the mudslide Mudslide Area to drain Upper rocky landslide Bedrock - marls Main scarp Sandstone contact 42

8 Risk management of large mudslides 2. Structural mitigation: drainage of the middle part of the mudslide Area being drained 43

9 Risk management of large mudslides 2. Structural mitigation: drainage of the middle part of the mudslide Each 3-4 years, part of the drainage system has to be reconstructed! But, the system is efficient, as it has resulted: * in a decrease of 1.5m of GWL in average in the lower part * in a decrease of a factor 2 of the velocity in the lower part 44

10 Risk management of large mudslides 3. Structural mitigation: reprofiling and creation of surface drains 45

11 Risk management of large mudslides 4. Non-structural mitigation: monitoring system Web-based infra-red camera: - directly connected via high-speed internet to the Prefecture (CODIS) in Digne-lès-Bains - SMS alert if movements are detected 46

12 Risk management of large mudslides 4. Non-structural mitigation: monitoring system Contactors in the stream: - an electric circuit is switched if a certain debris height is flowing in the stream 0.5 m - SMS alert 47

13 Risk management of large mudslides 4. Non-structural mitigation: displacement monitoring * Topometry for the displacement of ca. 50 benchmarks, performed by ONF RTM each 3 months * Collaboration with research institutes (IPGS): - kinematic analysis by daily photographs and correlation techniques - continuous GPS survey 48

14 Risk management of large mudslides 4. Non-structural mitigation: displacement monitoring * Collaboration with research institutes: - terrestrial laser scan acquisition 49

15 50 RISK ZONING AND LAND USE PLANNING Shallow slides Debris flows

16 Risk zoning PPR Methodology Barnier Law (1995): Plan de Prévention des Risques Special Guide Mouvement de Terrain Philosophy * Qualitative method * Based on expert judgment of the scientist * Use of available data & reports; no specific investigation * Scale of work: 1/10,000 Procedure * Inventory of processes: type, activity, age, magnitude; * Inventory of exposed elements & major stakes; * Hazard map = interpretation of the type of processes, and their activity, magnitude and frequency; * Risk map = hazard map X inventory of major stakes 51

17 Risk zoning 1: Inventory of processes Photo-interpretation Poche mudslide Analysis of terrestrial pictures & field visits Bois Noir landslide

18 Risk zoning 2: Characterization of processes Geomorphological interpretation and identification of a degree of activity Degree of activity Active Dormant Relict 53

19 Risk zoning 3: Definition of hazard areas Hazard delineation by taking into account the possibilities of landslide development for the next 100 years (up- and downslope) Potential activity of the slide is a proxy for intensity Possible extension of the Poche mudslide in

20 Risk zoning 3: Definition of hazard areas Hazard delineation with weighting procedures (RTM approach) Depth of landslide is a proxy for intensity Landslide hazard areas for the Larche municipality 55

21 Risk zoning 3: Definition of hazard areas Hazard delineation with multi-criteria expert rules A combination of factors is used as a proxy for intensity Hazard class H0: No hazard H1: Low hazard Expert rules Environmental conditions favourable to slope stability. No possibility of landslide developments for the next one hundred years. Environmental conditions are lowly favourable to slope instability. Low possibility of landslide developments for the next one hundred years. Future human and socioeconomic developments of the area are possible and subject to specific attention. Environmental conditions SLOPE: 0-10 LANDUSE: arable land, permanent crop SLOPE: LANDUSE: pasture, grassland TOPOGRPHY: moderate presence of slope accidents H2: Moderate hazard Environmental conditions are moderately favourable to slope instability. Moderate possibilities of landslide developments for the next one hundred years. Mitigation works are essential for future human and socio-ecomonic developments of the area. SLOPE: LANDUSE: pasture, grassland, forests lowly maintained TOPOGRAPHY: high presence of slope accidents, hummocky topography 56 H3: High hazard Environmental conditions are very favourable to slope instability. High possibility of landslide developments for the next one hundred years. Future human and socio-ecomonic developments of the area are impossible. SLOPE: > 30 LANDUSE: landuse highly deteriorated, bare soils, forests not maintained TOPOGRAPHY : very hummocky topography

22 Risk zoning 4: the hazard maps Multi-Hazard map (type of processes & information on intensity) Multi-Hazard map (hazard classes) Hazard level High Moderate Low 57 Hazard map for The Thuiles municipality

23 58 CONCLUSIONS

24 What did the Barcelonnette case study learn to us? An example of long-term mountain risk management - The type of mitigation works used in the 19th century are still used today! - Maps are the firs working documents for the local risk managers - Simple evaluation techniques are used for risk mapping, identification of dam construction, identification of landslide development, etc - Simple monitoring techniques are used (eg. essentially displacement) - The local risk managers are maintainig a database on the mitigation works - The local presence of risk managers may be important for the population by increasing their perception of being proctected - The local risk managers are looking for more quantitative information coming from the scientists 59

25 What did the Barcelonnette case study learn to us? Since 2000, the local population has to face new hazards. Permafrost melting, Bérard June

26 What did the Barcelonnette case study learn to us? Since 2000, the local population has to face new hazards. Outburst at Glacier Chauvet, 17 July

27 THANKS FOR YOUR ATTENTION! 62