Les mouvements de terrain

Transcription

1 Centre Européen sur les Risques Géomorphologiques Les mouvements de terrain Études de cas : Glissements du Pays d Auge Prof. Olivier MAQUAIRE Faculté de Géographie, Université de Caen Basse-Normandie Laboratoire Géophen, LETG UMR 6554 CNRS Centre Européen sur les Risques Géomorphologiques Version : 07 décembre D après : Case histories on landslide hazard and risk assessment: Coastal landslides in Normandy (France) O. Maquaire (1) J.-P. Malet (2) A. Puissant (3) M. Guillemette (1) C. Lissak-Borges (1) (1) CNRS UMR 6554, University of Caen Basse-Normandie, Caen, France (2) CNRS UMR 7516, School and Observatory of Earth Sciences, Strasbourg, France (3) CNRS UMR 7011, Image et Ville, Strasbourg, France European Centre on Geomorphological Hazards, Strasbourg, France 2

2 Illustration of Risk assessment by using mainly direct methodology (heuristic approach) by field geomorphological analysis, completed by information provided from survey network and geotechnical investigation. 1. INTRODUCTION 2. MORPHOLOGICAL CONTEXT 3. MAJOR LANDSLIDES CRISES 4. LANDSLIDES MECHANISM 5. RISK ASSESSMENT AND RISK MANAGEMENT 6. CONCLUSION: 3 1. INTRODUCTION Location of the Villerville-Cricqueboeuf landslides and of the monitoring networks. Aerial view of the Villerville-Cricqueboeuf landslide in In Normandy, along 12 km, Pays d Auge coast is periodically affected by landslides. In January 1982, major landslides major damages (roads, destroyed houses): Cirque des Graves & Fosses du Macre. 4

3 2. MORPHOLOGICAL CONTEXT Morphological setting of Pays d Auge cliffs (modified from Flageollet & Helluin, 1987). Pays d'auge Plateau is bordered on the North by high cliffs of up to 140 m. Topography and geology of the cliffs are various ( dip to the Est) MORPHOLOGICAL CONTEXT Morphological setting of Pays d Auge cliffs (modified from Flageollet & Helluin, 1987). Main scarp is composed of Cenomanian chalk overlying glauconitic sands. Below, a thick layer of marls is on top of the sandy limestone of Hennequeville Étude de cas which Pays shapes d Auge the O. Maquaire cliff toe and 07 constitutes décembre 2008 a reef flat 6

4 2. MORPHOLOGICAL CONTEXT Morphological setting of Pays d Auge cliffs (modified from Flageollet & Helluin, 1987). Below the scarp, the slope is more gentle and composed of an accumulation of superficial heterogeneous materials MORPHOLOGICAL CONTEXT Heterogeneous materials are composed by huge blocks and debris of chalk and flints, loamy sands). These formations have been placed during the Upper Pleistocene period. Geological profile of the present coastal slope of Villerville (modified from Flageollet & Helluin, 1987). 8

5 2. MORPHOLOGICAL CONTEXT Lower part of the profile presents characteristic herited morphology with counterslopes, and scarps. Slope is affected by successive rotational slide. Several main blocks of chalk with sandy layer have slipped in the marl substratum. Quaternary deposits are between blocks. Geological profile of the present coastal slope of Villerville (modified from Flageollet & Helluin, 1987). 9 Evolution of coastal slope of Villerville (from Flageollet & Helluin, 1987). 10

6 3. MAJOR LANDSLIDES CRISES First time failure: on 10/11 January 1982, a major landslide destroyed totally or partially some thirty houses and damaged the road in two places. At the top, crown of the landslide consists in a 3 m high scarp. Villa destroyed in 1982 Cirque des Graves MAJOR LANDSLIDES CRISES First time failure: on 10/11 January 1982, a major landslide destroyed totally or partially some thirty houses and damaged the road in two places. At the top, crown of the landslide consists in a 3 m high scarp. Trescartes Villa (1982) Cirque des Graves 12

7 3. MAJOR LANDSLIDES CRISES First time failure: on 10/11 January 1982, a major landslide destroyed totally or partially some thirty houses and damaged the road in two places. At the top, crown of the landslide consists in a 3 m high scarp. Road damaged (Mirella) Cirque des Graves MAJOR LANDSLIDES CRISES First time failure (10/11 January 1982) was followed by three major reactivations: Reactivation - First crisis: 12/13 February 1988, landslide has been reactivated. The crisis caused several damages, and extended the main scarp (Fosses du Macre) both laterally, uphill and downhill. Reactivation Second crisis: beginning of February 1995 (precise date of onset is unknown), major displacements occurred. Reactivation Third crisis: on 23/24 March 2001, after several weeks of warnings signs such as the opening of cracks in two houses, a recession and a subsidence of the main scarp occurred (in Fosses du Macre). Illustration of the recession of the main scarp and main damages for the Fosses du Macre (Villerville & Cricqueboeuf). 14

8 Recession of the main scarp and main damages: Fosses du Macre (Villerville - Cricqueboeuf) Recession of the main scarp and main damages: Fosses du Macre (Villerville). Tennis March 2001 March

9 Recession of the main scarp and main damages: Fosses du Macre (Cricqueboeuf). Les Ecorres February 1988 March Regcession of the main scarp and main damages: Fosses du Macre (Cricqueboeuf). Les Préfailles March

10 Recession of the main scarp and main damages: Fosses du Macre (Cricqueboeuf). Les Prefailles Villa (1) February Recession of the main scarp and main damages: Fosses du Macre (Cricqueboeuf). Les Prefailles Villa (2) March

11 Recession of the main scarp and main damages: Fosses du Macre (Cricqueboeuf). Les Prefailles Villa (3) February Recession of the main scarp and main damages: Fosses du Macre (Cricqueboeuf). Les Prefailles Villa (4) March 2001 March

12 Recession of the main scarp and main damages: Fosses du Macre (Cricqueboeuf). Les Troenes March 2001 March Recession of the main scarp and main damages: Fosses du Macre (Cricqueboeuf). La colline & Le Clos des Renards March 2001 March 2001 March

13 4. LANDSLIDES MECHANISM 4.1. Monitoring network Since 1984, a monitoring network is installed on the site. It is composed of : 80 cemented benchmarks (which positions are regularly measured with a tacheometer and an distancemeter (EDM), and since 2003 by GPS, 3 inclinometers in boreholes and, 21 wells and piezometers to monitor groundwater levels. Climate data of the Saint-Gatien-des-Bois station are used to correlate effective rainfall to the groundwater fluctuations. Monitoring network at Cirque des Graves, Villerville (benchmarks, micro-levelling) LANDSLIDES MECHANISM 4.2. Slip surface Inclinometer data indicate a concave slip surface located at around -14 m in the marls. Location of inclinometers. Geometry of the Cirque des Graves landslide with the location of the slip surface identified in the inclinometer. 26

14 4. LANDSLIDES MECHANISM 4.2. Slip surface & surficial displacement Relation between rainfall and groundwater levels 28

15 4. LANDSLIDES MECHANISM 4.3. Relation between rainfall and groundwater levels LANDSLIDES MECHANISM 4.3. Relation between rainfall, groundwater levels and landslide velocity Displacements reached 4 to 7 m for the points located at the bottom of the slope. 30

16 4. LANDSLIDES MECHANISM 4.3. Relation between rainfall, groundwater levels and landslide velocity landslide mechanism is controlled by the hydrology. Rainfall plays a decisive part in the temporal variability of the observed movements LANDSLIDES MECHANISM 4.3. Relation between rainfall, groundwater levels and landslide velocity Analyses at different time scales (daily to yearly) have outlined hydrological thresholds triggering the crises. Development of the Villerville-Cricqueboeuf landslide in relation to the groundwater table and effective annual rainfall data. High groundwater level observed in 1982 is in phase with the onset of the major movement of January Same characteristics can be observed for the crises of February 1988 and 1995, and January 2001 which occurred after several hydrologic years of rainfall amounts higher than the average. 32

17 4. LANDSLIDES MECHANISM 4.4. Influence of triggering factors on slope stability Slope stability calculations and deformation analyses were performed with the FLAC 2D geotechnical software in order to (1) back-calculate the geomechanical characteristics of the slip surface, and to (2) quantify the influence of pore pressure variations and geometrical changes of the slope on slope stability. With the observed slip surface and a groundwater level at around ca. -2m below the topography, the material characteristics back-calculated (FS=1) are c r = 0 kpa and φ r = 10.4 ) LANDSLIDES MECHANISM 4.4. Influence of triggering factors on slope stability Slope stability calculations and deformation analyses were performed with the FLAC 2D geotechnical software in order to (1) back-calculate the geomechanical characteristics of the slip surface, and to (2) quantify the influence of pore pressure variations and geometrical changes of the slope on slope stability (i.e. toe recession). A sensitivity analysis has been performed in order to quantify the impacts of 2 triggering factors on slope stability. it is demonstrated that a rise in pore pressure of 20 kpa lowers the safety factor of 10% (FS = 0.89), while a modification of the slope geometry (toe recession of 5m) lowers the safety factor of less than 5% (FS = 0.97). 34

18 5. RISK ASSESSMENT AND RISK MANAGEMENT Landslide is subjected to a quasi permanent activity, and the associated landslide hazard is high. Landslide stability is controlled by the hydroclimatic conditions which play a main role in the onset or the crisis of these landslides. No specific measures have been yet adopted to reduce the hazard. Only, some owners have installed a surficial drainage. At Fosses du Macre, sea wall has been constructed in the middle of 80 s, but landslides crisis continued at three times (1988, 1995 & 2001). A project of mitigation has been defined with deep drainage (holes, trenches, siphons drainage, ) and sea fence but municipalities have not found the financial support by the state and by the regional & departmental councils. Today, only an expert risk map has been produced using the French Methodology for Landslide Risk Zoning (Plan de Prévention des Risques, PPR) RISK ASSESSMENT AND RISK MANAGEMENT 5.1. The French Methodology: PPR (Law February 1995) Philosophy: Qualitative method Easier to prepare than the PER (law 1982), Based on expert judgment of the scientist, Use of available data & reports (no specific investigation), Identification of a reference event, Zoning for the next 100 years, Scale of work: 1/10,000 1/25,000. Procedure: Inventory of major stakes Inventory of exposed elements & major stakes, Inventory of processes: type, activity, age, magnitude, Hazard map = interpretation of the types of processes, and their activity, magnitude and frequency, Risk map = hazard map X inventory of major stakes. 36

19 5. RISK ASSESSMENT AND RISK MANAGEMENT 5.1. The French Methodology: PPR Hazard & Risk maps: R1: Area without specific restriction. R2: Area with low restriction. R3: Area with specific restrictions. 37 Evaluation du risque mouvement de versant : cadre réglementaire r (France) Carte d inventaire Carte d aléacarte des enjeux Carte de zonage PPR MATE/MATL,

20 5. RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf Hazard mapping: Villerville town is limited by two very active areas, with an extension of these active areas, mainly at the Est (Fosses du Macre). Why Villerville town is still (or always!) stable? What is the future of the Villerville town? This stability will be permanent or existing a possibility, in the future, that Villerville town could be partially or totally destroyed by the extension of the active landslides RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf Hazard assessment Predisposing factors: - x - x - x - x, - x - x Susceptibility of landslide Triggering factors (see previously): Main influence of: -x, -x xxxxx. 40

21 5. RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf Hazard assessment Predisposing factors: - Lithology (marl, sand & chalk aquiferus layers-), - Weak mechanical characteristics of marls, - Tectonic (weak dip (< 1 ) to the NE), - Slope (DTM): steep slope to moderate steep slope, - Geomorphololy: scarps, open & active cracks, - Land cover: forest, bush, pond, Susceptibility of landslide Triggering factors (see previously): Main influence of: - GWT variations, - sea erosion at the base of the hillslope (toe unloading). Acceleration triggered by GWT above a certain threshold RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf Hazard assessment: case of Villerville town Could you describe the location of the Villerville town? 1. Town is located in the axis of the small valley at 20m a.s.l., 2. Slope is protected by the seawall, 3. Material of the slope is mainly eolian loam, sand & gravels (head) permeable material without watertable 42!!

22 43 5. RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf Hazard assessment: case of Villerville town 1. Town is located in the axis of the small valley at 20m a.s.l., 2. Slope is protected by the seawall, 3. Material of the slope is mainly eolian loam, sand & gravels (head) permeable material without watertable 44!!

23 5. RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf Hazard assessment: case of Villerville town 1. Town is located in the axis of the small valley at 20m a.s.l., 2. Slope is protected by the seawall, 3. Material of the slope is mainly eolian loam, sand & gravels (head) RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf Hazard assessment: case of Villerville town Position of the slope in 1840 (cadastre) Paleo-valley was fullfilled by colluvial-alluvial materials (flints, eolian loamy sands & gravels). These formations have been placed during the Upper Pleistocene period when the see level was low (max -100m). Materials are very permeable without watertable, Low hazard!! 46

24 2. MORPHOLOGICAL CONTEXT Lower part of the profile presents characteristic herited morphology with counterslopes, and scarps. Slope is affected by successive rotational slide. Several main blocks of chalk with sandy layer have slipped in the marl substratum. Quaternary deposits are between blocks. Geological profile of the present coastal slope of Villerville (modified from Flageollet & Helluin, 1987). 47 Recession of the main scarp and main damages: Fosses du Macre (Cricqueboeuf). Les Troenes March 2001 March

25 5. RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf Hazard assessment and mapping Hazard High G3 Moderate G2 Low G1 Null G0 Criteria -x -x -x -x -x -x -x -x -x -x -x RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf Hazard assessment and mapping Landslide inventory map 50

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27 5. RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf Hazard assessment and mapping Hazard High G3 Moderate G2 Low G1 Null G0 Criteria - Active landslide: open cracks, scarps, counter slopes with ponds, - steep slope (> 30 ), - geology: blocks of chalk, glauconic sands & marls, IV deposits, - buffer zone of security around active landsilde. - No activity. Smooth topography +- hummocky, - same geologic conditions of G3, - steep to gentle slope. - No activity. No indices. - same geologic conditions of G2 or colluvial/alluvial permeable materials, - gentle to low slope. - Flat topography (plateau) RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf Hazard assessment and mapping In progress, draft version Landslide inventory map Hazard zoning map G1: Low hazard. G2: Moderate hazard. G3: High hazard. 54

28 5. RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf In progress, draft version Comparison of PER (1988) and revised PPR (2007) Hazard maps Hazard zoning map PER 1988 Hazard zoning map PPR 2007 G1: Low hazard. Main scarp in 2007 (limit of active zone) G2: Medium hazard. Limit G3 of the PER hazard map 55 G3: High hazard. 5. RISK ASSESSMENT AND RISK MANAGEMENT 5.2. The PPR of Villerville - Cricqueboeuf In progress, draft version Comparison of PER (1988) and revised PPR (2007) Hazard maps Hazard zoning map PER 1988 Hazard zoning map PPR 2007 G1: Low hazard. Main scarp in 2007 (limit of active zone) G2: Medium hazard. Limit G3 of the PER hazard map 56 G3: High hazard.

29 57 6. CONCLUSION: Towards QRA (Quantitative Risk Assessments) In the frame of the PhD thesis of Candide Lissak-Borges (> Sept. 2007) Better understanding of the process at the local scale - implementation of high temporal resolution monitoring on selected sites (at Villerville & Cricqueboeuf landslides, Bennerville, etc.): - inclinometer monitoring, continuous GPS, - identification of water circulation by geophysical techniques, etc. - spatial modelling & site-scale hydro-mechanical modelling Better understanding of the distribution of processes at the regional scale - multi-date Lidar and hyperspectral survey - potential of VHR satellite imagery (quantification of landslide displacement by image correlation) - probabilistic analysis of landslide susceptibility - effects of Global Change (and sea heights) on coastal hazards Consequences and risks assessments - identification of damages - Inventories of elements at risk, of their attributes & their historical propensity to be damaged - vulnerability analysis (buildings, roads, protective works) - Estimation of the level of social, environmrntal & economic losses - Societal perception of risk? 58

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