Seuils hydro-climatiques critiques au déclenchement de mouvements de terrain le long des côtes Normandes du Calvados

Transcription

1 Seuils hydro-climatiques critiques au déclenchement de mouvements de terrain le long des côtes Normandes du Calvados Candide Lissak*, Olivier Maquaire*, Jean-Philippe Malet**, Robert Davidson* *Laboratoire GEOPHEN, Géographie Physique et Environnement, CNRS UMR 6554, Université de Caen-Basse-Normandie, Esplanade de la Paix, F Caen Cedex France ** Institut de Physique du Globe de Strasbourg, CNRS UMR 7516, Ecole et Observatoire des Sciences de la Terre, EOST/Université de Strasbourg, 5 rue Descartes, F Strasbourg, France. 1

2 Introduction 1. Research context and study area Context of this research Area affected by numerous type of hazards Global changes (climate, land use changes ) Vulnerability increasing Risk management tools to enhance the overall resilience of societies National research program Sisca Coastal landslides Floods Continental landslides 2

3 Introduction 1. Research context and study area Specificities of the Villerville & Cricqueboeuf landslides Most active landslides of the region Complex landslide motion: seasonal kinematic regime + event-type kinematic regime (4 acceleration events) Many scientific studies since 1970 Numerous element at risk: Several houses and road section destroyed and continuously damaged 1982 landslide reactivation 2001 last acceleration event Continuous activity 3 cm (2009) 4 cm (2010) 3

4 Introduction 2. Objectives Definition of rainfall & piezometric thresholds Omnipresence of water (groundwater, pounds, sources, ) Major acceleration events an seasonal activity associated to groundwater fluctuation 4

5 Methods 3. Methods Spatial & temporal distribution of the landslide velocity A new geodetic network initiated in 2008 (24 benchmarks) Permanent dual-frequency GNSS stations since 2009 (Deprez, 2011) Resolution 1. Piezometer 2. Wells 3. Inclinometer 4. Piezometer with perm. Sensor 5. Geobead probe 6. Meteo. station 7. Topographic benchmark 5 8. GNSS receivers

6 Methods 3. Methods Hydro-climatic condition for landsliding Historical data analysis and field investigation GWL = 29 observation points: 4 wells, 7 inclinometer tubes, 18 piezometers (6 automatic probes) + 1 piezometer on the plateau since 1974 Rainfall = 2 meteorological stations : Villerville (rr, wind, temp, hum, sun Day +2) since station on the plateau since Piezometer 2. Wells 3. Inclinometer 4. Piezometer with perm. Sensor 5. Geobead probe 6. Meteo. station 7. Topographic benchmark 6 8. GNSS receivers

7 Kinematic pattern of the landslide: spatial heterogeneities A concentric organization around a central medial nucleus A regressive evolution of the landslide from downhill to uphill Several morpho-structural units (or compartments) characterized by the presence of chalk blocks or panels Toe Median Edge 7

8 Kinematic pattern of the landslide: temporal heterogeneities 3 major acceleration events (1988, 1995, 2001) with several decimeters to several meters displacement rates A continuous creeping between 2 major acceleration events 8

9 Kinematic pattern of the landslide: temporal heterogeneities A continuous creeping confirmed by high resolution data analysis Since 2009 = 35cm of displacements VLRB, 12cm VLRH Seasonal behavior with acceleration phases ( amplitude) Delays between stations vary between 1 and 4 days for the different phases VLRB VLRH Aug Dec

10 Rainfall Groundwater level relationships Periods of groundwater recharge: , and Periods of drainage: 3 to 4 years Seasonal behavior of the groundwater system Landslide area Plateau 10

11 Slope hydrology slope kinematics relationships Only 4 major acceleration events Landslide triggering during unusually wet periods with several years of hydrological excess 1982 event = excessive hydrological year (July June 1982) with particularly abundant winter rainfall amount Consequence: Danestal GWL elevation >1.50 m Empirical rainfall duration threshold for Cirque des Graves landslide(bogaard et al.2011) 11

12 Slope hydrology slope kinematics relationships Only 4 major acceleration events Landslide triggering during unusually wet periods with several years of hydrological excess 1982 event = during an excessive hydrological year (July June 1982) with particularly abundant winter rainfall amount Consequence: Danestal elevation up to 1.50 m Critical GW threshold between m Warning ( 10.35) Monitoring 12

13 Slope hydrology slope kinematics relationships The seasonal behavior of the landslide associated to the periods of groundwater seasonal recharge Critical GW level by a statistical approach? 13

14 Slope hydrology slope kinematics relationships Focus on SD4 piezometers and GNSS receivers 14

15 Slope hydrology slope kinematics relationships Focus on SD4 piezometers and GNSS receivers Landslide displacement rate significantly increases when the groundwater level reaches ± 8.30 m in depth The slope stabilizes when the groundwater drops below 8 m in depth 1 jan oct oct

16 Conclusion 5. Conclusion Typical slow-moving deep seated coastal landslide Historical data + field monitoring (high spatial & temporal resolution) to highlight: - Regressive evolution of the landslide - Spatial/temporal variabilities of displacement rates in the landsldie body - GW effects 2 time scale warning system: Analysis of the Danestal piezometer allows defining two significant piezometric thresholds a seasonal acceleration of low amplitude = threshold at -13 m a large acceleration of high amplitude = threshold at m Consequently in-situ observations suggest that a change of m of the groundwater level is necessary to trigger a major acceleration. (Activité saisonnière) 16

17 Merci de votre attention 17

18 Rainfall Groundwater level relationships Lag between all piezometers in the beginning of the groundwater recharge A progressive groundwater rise associated to successive rainfall events Groundwater level response to effective rainfall: [2 and 5] days. 18