Intensive Course Barcelona, UPC, 1-4 September 2008 Landslide hazard and risk management in the Barcelonnette Basin: some case studies J.-P. Malet 1, A. Remaître 1, Y. Thiery 1 O. Maquaire 2, M. Peyron 3, C. Waeckel 3 1. CNRS UMR 7516, School and Observatory of Earth Sciences, Strasbourg, France 2. CNRS UMR 6554, University of Caen-Basse Normandie, Caen, France 3. ONF RTM, Office National des Forêts, Service de Restauration des Terrains en Montagne, Barcelonnette, France 1
Observed major landslide types La Valette Faucon, 1996 slow-moving mudslide Faucon, 2003 fast-moving muddy debris-flow 2
Aim of this talk 1/ Characteristics of the Barcelonnette Basin: a multi-hazard prone area - Geomorphology and geology - Landuse - Climate 2/ Some statistics on the processes creating risks 3/ Practical examples of risk management - management of debris flow risks - management of the risk associated to large mudslides - example of risk zonation plans 4/ Conclusions 3
4 STUDY SITE BARCELONNETTE BASIN (South East French Alps)
Several favourable predisposing factors for slope instability Geomorphological features 5
Several favourable predisposing factors for slope instability 6 A geological window outcropping the clay shale bedrock
Several favourable predisposing factors for slope instability 7 A susceptible sedimentary tectonized bedrock
Several favourable predisposing factors for slope instability Glacial cirque 8 The influence of glaciation in the landscape Discontinuous morainic deposits
Several favourable predisposing factors for slope instability An evolving landuse: intense deforestation in the 19 th century (agro-pastoral communities) intense gullying 9
Several favourable predisposing factors for slope instability 1893 An evolving landuse: intense deforestation in the 19 th century (agro-pastoral communities) intense landsliding 10
Several favourable predisposing factors for slope instability 1897 An evolving landuse: during the 20 th century, intense reforestation for torrential control 2003
Several favourable predisposing factors for slope instability An evolving landuse: during the 20 th century, intense reforestation for torrential control 1987 1890 12
The actual landscape: contrast South/North slopes South-facing slope: high energy relief - intense gullying discontinuous morainic cover W E North-facing slope: landsliding high thickness morainic cover E 13 W
A favourable climate for slope instability Alpine climate with Mediterranean influence - High inter annual rainfall variability (735 mm ± 400 mm; 1928-2005) - Mean annual temperature: 7.5 ± 1.3 ; 1920-2005) - Presence of a snowpack on the upper slopes (> 1900m) for 4 to 6 months - intense thunderstorms I max > 50 mm.h -1 14
Climate Landslide relationships (1928 2005) Archive investigation and dendrochronological analyses at several time scales - Annual time scale: periods of landslide activity are correlated to excess yearly rainfall amounts, but landslide events are also observed in relatively dry periods - Daily time scale: 2 types of climate situations Type A: heavy daily rain (shallow slides) Type B: heavy 30-days rain (deep-seated slides) 15
16 SOME STATISTICS ON THE HAZARDS
Observed natural hazards since 1850 Flageollet et al., 1994 (Teslec) Source: ONF-RTM database & regional archive Distribution of landslide types Distribution of flood types 17 Major problems: - exhaustivity of the database - approximative dating - wrong identification of the type of process
Inventory of landslide types ca. 60% ca. 42% Translationnal (Faucon) Active rotational (Poche torrent) ca. 75% Relict rotational (Poche torrent) 18
19 Landslide maps