European Geosciences Union General Assembly 2008, 14-18 April, Vienna Considerations on debris-flow hazard analysis, risk assessment and management. A. Remaître 1, J.-P. Malet 1, O. Maquaire 2 1. CNRS UMR 7516, School and Observatory of Earth Sciences, University Louis Pasteur, Strasbourg. 2. CNRS UMR 6554, GEOPHEN, University of Caen Basse-Normandie, Caen.
Introduction Debris flows are a common type of rapid gravitational mass movement. They consist mostly of a mixture of water, sediment and debris that can travel in a series of surge long distances;
Introduction Debris flows are a common type of rapid gravitational mass movement. They consist mostly of a mixture of water, sediment and debris that can travel in a series of surge long distances; Debris flows are widely recognize as one of the dominant geomorphic processes in mountainous areas and may constitute a dangerous natural hazard for the inhabitants and infrastructures located downslope.
Debris flows temporal characteristics modified from Leroueil, 2002 Kinematical characteristics of successive events in a single watershed (especially for large watershed with various geo(morpho)logical settings) can be completely different! modified from Remaître 2006
Debris-flow risk in a torrential watershed Triggering (1 or several sources): - landslide(s); - runoff; - combination of several mechanisms.
Debris-flow risk in a torrential watershed Triggering (1 or several sources): - landslide(s); - runoff; - several mechanisms. Runout: - Increasing of debris flow volume, discharge (magnitude) according scouring phenomena; - Evolution of the behaviour according surficial formations involved in the flow.
Debris-flow risk in a torrential watershed Triggering (1 or several sources): - landslide(s); - runoff; - several mechanisms. Runout: - Increasing of debris flow volume, discharge (magnitude) according scouring phenomena; - Evolution of the behaviour according surficial formations involved in the flow. Spreading: - Stopping and spreading of the material strongly influenced by channel/fan detailed morphology and behaviour of the debris flow.
Debris-flows risk assessment and management Risk assessment: - where (how) it will be triggered? - frequency? - intensity (magnitude) at the torrential fan? - elements at risk (and their vulnerability)?
Debris-flows risk assessment and management Risk assessment: - where (how) it will be triggered? - frequency? - intensity (magnitude) at the torrential fan? - elements at risk? Risk management: - Warning system? - Mitigation (torrential correction)? - Displacement of the population at risk?
Debris-flows analyses: multidisciplinary approach
Example a torrential risk basin: Barcelonnette basin
Runout modelling (1D): Run-out distance (m) Flow height (m)
Runout modelling (1D): calibration and validation
Runout modelling (1D): hazard analysis Influence of check dams (number, location) on debris-flow intensity
Runout modelling (1D): hazard analysis Influence of check dams (number, location) on debris-flow intensity Sensitivity analysis show that the location of check dams as a greater influence on the intensity than their number. Check dams are more efficient when they are located in the upper part (early energy breaking); Therefore, a small number of check dams but located near the triggering area may decrease significantly the debris-flow intensity on the alluvial fans.
Runout modelling (1D): hazard analysis Definition of minimal volumes to produce overflowing (Faucon) Bridge 3 Bridge 2 Bridge 1 Identification of critical overflowing points
Concluding remarks: Regarding their complexity, debris-flow hazard analysis, risk assessment and management needs a multidisciplinary approach (quantitative geomorphology, engineering geology, soil and fluid mechanics, economy, landuse planning, ); Numerical models (if they are calibrate and validate both at laboratory and field scales) can be used for hazard analysis according realistic scenarios nevertheless, well documented event database concerning all the aspects of debris-flows kinematics and consequences are needed for further development of numerical models and risk assessment and management methodologies. Thank you for your attention