An integrated approach coupling physically based models and probabilistic method to assess quantitatively landslide susceptibility at different scale: application to different geomorphological environments

International audienceLandslide hazard assessment is the estimation of a target area where landslides of a particular type, volume, runout and intensity may occur within a given period. The first step to analyze landslide hazard consists in assessing the spatial and temporal failure probability (when the information is available, i.e. susceptibility assessment). Two types of approach are generally recommended to achieve this goal: (i) qualitative approach (i.e. inventory based methods and knowledge data driven methods) and (ii) quantitative approach (i.e. data-driven methods or deterministic physically based methods). Among quantitative approaches, deterministic physically based methods (PBM) are generally used at local and/or site-specific scales (1:5,000-1:25,000 and >1:5,000, respectively). The main advantage of these methods is the calculation of probability of failure (safety factor) following some specific environmental conditions. For some models it is possible to integrate the land-uses and climatic change. At the opposite, major drawbacks are the large amounts of reliable and detailed data (especially materials type, their thickness and the geotechnical parameters heterogeneity over a large area) and the fact that only shallow landslides are taking into account. This is why they are often used at site-specific scales (> 1:5,000). Thus, to take into account (i) materials' heterogeneity , (ii) spatial variation of physical parameters, (iii) different landslide types, the French Geological Survey (i.e. BRGM) has developed a physically based model (PBM) implemented in a GIS environment. This PBM couples a global hydrological model (GARDENIA ®) including a transient unsaturated/saturated hydrological component with a physically based model computing the stability of slopes (ALICE ® , Assessment of Landslides Induced by Climatic Events) based on the Morgenstern-Price method for any slip surface. The variability of mechanical parameters is handled by Monte Carlo approach. The probability to obtain a safety factor below 1 represents the probability of occurrence of a landslide for a given triggering event. The dispersion of the distribution gives the uncertainty of the result. Finally, a map is created, displaying a probability of occurrence for each computing cell of the studied area. In order to take into account the land-uses change, a complementary module integrating the vegetation effects on soil properties has been recently developed. Last years, the model has been applied at different scales for different geomorphological environments: (i) at regional scale (1:50,000-1:25,000) in French West Indies and French Polynesian islands (ii) at local scale (i.e.:10,000) for two complex mountainous areas; (iii) at the site-specific scale (1:2,000) for one landslide. For each study the 3D geotechnical model has been adapted. The different studies have allowed : (i) to discuss the different factors included in the model especially the initial 3D geotechnical models; (ii) to precise the location of probable failure following different hydrological scenarii; (iii) to test the effects of climatic change and land-use on slopes for two cases. In that way, future changes in temperature, precipitation and vegetation cover can be analyzed, permitting to address the impacts of global change on landslides. Finally, results show that it is possible to obtain reliable information about future slope failures at different scale of work for different scenarii with an integrated approach. The final information about landslide susceptibility (i.e. probability of failure) can be integrated in landslide hazard assessment and could be an essential information source for future land planning. As it has been performed in the ANR Project SAMCO (Society Adaptation for coping with Mountain risks in a global change COntext), this analysis constitutes a first step in the chain for risk assessment for different climate and economical development scenarios, to evaluate the resilience of mountainous areas

Caractérisation Hydrologique de l’Extension du Milieu INondable

National audienceLogiciel d'aide à la cartographie de l'aléa inondatio

Seismic Vulnerability Assessment from Earthquake Damages Historical Data Using Constrained Optimization Technique

International audienc

Weighing the importance of model uncertainty against parameter uncertainty in earthquake loss assessments

Epistemic uncertainties can be classified into two major categories: parameter and model. While the first one stems from the difficulties in estimating the values of input model parameters, the second comes from the difficulties in selecting the appropriate type of model. Investigating their combined effects and ranking each of them in terms of their influence on the predicted losses can be useful in guiding future investigations. In this context, we propose a strategy relying on variance-based global sensitivity analysis, which is demonstrated using an earthquake loss assessment for Pointe-à-Pitre (Guadeloupe, France). For the considered assumptions, we show: that uncertainty of losses would be greatly reduced if all the models could be unambiguously selected; and that the most influential source of uncertainty (whether of parameter or model type) corresponds to the seismic activity group. Finally, a sampling strategy was proposed to test the influence of the experts' weights on models and on the assumed coefficients of variation of parameter uncertainty. The former influenced the sensitivity measures of the model uncertainties, whereas the latter could completely change the importance rank of the uncertainties associated to the vulnerability assessment step

Findings on the Earthquake Risk Preparedness of the City of La Paz, Bolivia

International audienceLocated on the Bolivian altiplano and built on the slopes of the canyon network eroded by the Choqueyapu river and its main tributaries, the city of La Paz is no stranger to natural disasters, having suffered catastrophic floods, hailstorms and landslides in recent memory, such as the hailstorm in 2002 that caused at least 69 deaths and 100 disappearances or the 2011 Pampahasi-Cayapa mega-landslide, which destroyed more than 800 homes. However, despite its location in the South American subduction context and evidence of paleoseismicity in local fault networks, historical and instrumental seismic activity has been relatively low and therefore not a priority in local settlement and construction practices. As part of an ongoing effort by local authorities to increase risk preparedness, a preliminary diagnosis of the earthquake risk preparedness of the city was commissioned by the Municipality of La Paz, with funding from the European Commission DIPECHO programme. This study, whose main findings are presented in this paper, is based on a review and analysis of seismic hazard literature as well as a series of interviews with local actors in the fields of catastrophe management, geological research, seismographic surveillance, land-use planning and structural design. An assessment of the seismogenic potential of local faults and a simple deterministic seismic hazard analysis are first carried out to give an idea of possible hazard. This information is then completed with an analysis of existing vulnerabilities from a legislative and urban planning point of view, as well as commentary on current risk management efforts. Finally, the earthquake hazard is analyzed in the context of a high landslide hazard, analyzing the interactions between risk management strategies for both types of hazards particularly with respect to earthquake-induced landslides

Limits on the potential accuracy of earthquake risk evaluations using the L'Aquila (Italy) earthquake as an example

International audienceThis article is concerned with attempting to ‘predict’ (hindcast) the damage caused by the L’Aquila 2009 earthquake (Mw 6.3) and, more generally, with the question of how close predicted damage can ever be to observations. Damage is hindcast using a well-established empirical-based approach based on vulnerability indices and macroseismic intensities, adjusted for local site effects. Using information that was available before the earthquake and assuming the same event characteristics as the L’Aquila mainshock, the overall damage is reasonably well predicted but there are considerable differences in the damage pattern. To understand the reasons for these differences, information that was only available after the event were include within the calculation. Despite some improvement in the predicted damage, in particularly by the modification of the vulnerability indices and the parameter influencing the width of the damage distribution, these hindcasts do not match all thedetails of the observations. This is because of local effects: both in terms of the ground shaking, which is only detectable by the installation of amuch denser strong-motion network and a detailed microzonation, and in terms of the building vulnerability, which cannot be modeled usinga statistical approach but would require detailed analytical modeling for which calibration data are likely to be lacking. Future studies shouldconcentrate on adjusting the generic components of the approach to make them more applicable to their location of interest. To increase thenumber of observations available to make these adjustments, we encourage the collection of damage states (and not just habitabilityclasses) following earthquakes and also the installation of dense strong-motion networks in built-up areas

A review of historical lahars, floods, and landslides in the Precheur river catchment (Montagne Pelee volcano, Martinique island, Lesser Antilles)

International audienceThe Precheur river catchment belongs to the Montagne Pelee volcano massif located in the north of Martinique island, in the Lesser Antilles arc. In June 19-20, 2010 two high-discharge lahars overflowed and partially damaged the Precheur bridge, interrupting the road traffic for several days, and inundating part of the Abymes quarter. These non-eruptive, rain-triggered lahars were the consequence of the mobilization of loose volcanic material accumulated in the river bed by a massive landslide of the Samperre cliff that occurred on May 11, 2010. In order to improve lahar risk assessment for the Precheur river, we review the existing knowledge on lahars and floods for the period 1851-2011. We improved the database mainly on the basis of geophysical (seismic and acoustic) data for the most recent period (1980-present). We show that: (1) the largest event ever observed in Precheur is the eruption-related lahar of May 8, 1902 at 06:00 UTC, (2) high-discharge, non-eruptive lahars able to partially destroy or submerge the bridge occur every 10.3 years in average since 1950, (3) geophysical records show that there are more low magnitude lahars than previously recognized because witness accounts report the larger events and therefore filter out the smaller ones; by contrast geophysical instruments are able to detect both high and low magnitude events, (4) the recent paroxysms of 2009-2010 belong to a landslide-lahar crisis which started in 2004; the onset of the landslide-lahar crisis was unnoticed by the population due to the low magnitude of the events. The results of this compilation stress the importance of the presence of geophysical instrumentation sufficiently close to the river for monitoring small scale landslides and lahars. Indeed, these low magnitude events are likely to be unnoticed by the local population and therefore likely to be absent from the collective memory and local newspapers. However, they can inform about the imminence of more significant events, as observed in 1980 and 2010

Contribution of physical modelling to climate-driven landslide hazard mapping: an alpine test site

EGU2012-2445The aim of this work is to develop a methodology for integrating climate change scenarios into quantitative hazard assessment and especially their precipitation component. The effects of climate change will be different depending on both the location of the site and the type of landslide considered. Indeed, mass movements can be triggered by different factors. This paper describes a methodology to address this issue and shows an application on an alpine test site. Mechanical approaches represent a solution for quantitative landslide susceptibility and hazard modeling. However, as the quantity and the quality of data are generally very heterogeneous at a regional scale, it is necessary to take into account the uncertainty in the analysis. In this perspective, a new hazard modeling method is developed and integrated in a program named ALICE. This program integrates mechanical stability analysis through a GIS software taking into account data uncertainty. This method proposes a quantitative classification of landslide hazard and offers a useful tool to gain time and efficiency in hazard mapping. However, an expertise approach is still necessary to finalize the maps. Indeed it is the only way to take into account some influent factors in slope stability such as heterogeneity of the geological formations or effects of anthropic interventions. To go further, the alpine test site (Barcelonnette area, France) is being used to integrate climate change scenarios into ALICE program, and especially their precipitation component with the help of a hydrological model (GARDENIA) and the regional climate model REMO (Jacob, 2001). From a DEM, land-cover map, geology, geotechnical data and so forth the program classifies hazard zones depending on geotechnics and different hydrological contexts varying in time

Integration of landslide hazard maps into probabilistic risk assessment in context of global changes: an alpine test site

The aim of this work is to develop a methodology to integrate global changes scenarios into quantitative risk assessment. This paper describes a methodology to take into account effects of changing climate on landslides activity and impacts of social changes on exposure to provide a complete evaluation of risk for given scenarios. This approach is applied for demonstration purpose on a southern alpine test site. Mechanical approaches represent a solution to quantify landslide susceptibility and to model hazard on unprecedented conditions, as it is likely to occur. However, as the quantity and the quality of data are generally very heterogeneous at a regional scale, it is necessary to take into account their uncertainty in the analysis. In this perspective, a new hazard modeling method has been developed and integrated in a GIS-based software called ALICE®. To go further, climate change scenarios have been computed for the alpine test site (Barcelonnette area, France) using the REMO-COSMO-LM. From the precipitation time series, a daily index of the soil water content has been computed thanks to a reservoir-based model (GARDENIA®). Hence, the program classifies hazard zones depending on the several spatial data (lithological, DEM, etc...) and different hydrological contexts varying in time. The probabilistically initiated landslides are then propagated thank to a semi-empirical model (BORA) to provide real hazard maps. Different scenarios of land-use have been developed using an automate cellular model to cover the probable range of development of potential elements at risks in the future. These exposure maps are then combined with the aforementioned hazard maps to obtain risk maps for the different periods and the different land-use development scenarios. Potential evolutions of landslide risks are then evaluated, with a general increase in the 7 communes. This methodology also allows the analysis of the contributions of both considered global changes (climate and land-use) to the evolution of risk. This communication, realized within the framework of Safeland project, is supported by the European Commission under the 7th Framework Programme for Research and Technological Development, Area "Environment", Activity 1.3.3.1 "Prediction of triggering and risk assessment for landslides" (grant agreement n°226479)

Development of a scenario builder tool for volcanic risk assessment and application to Mount Cameroun

One of the objectives of the MIAVITA project is to develop a conceptual frame for the risk assessment of inhabited areas exposed to various volcanic hazards. The present communication aims at presenting a scenario-building tool that would take into account the succession of volcanic, seismic, gravity and hydro-geological events and, consequently, analyse the impact of such events on people, physical elements (e.g. buildings, agriculture) and various functional systems composing the exposed community. As several scenario software tools are available in the field of seismic risk, such an approach proves less straightforward in the case of volcanic risk: the main difficulty resides in the crossover of several types of geological phenomena and exposed elements, each combination of them usually relying on specific damage mechanisms. Therefore, before building a risk assessment tool, the first step comprises the careful definition of all damaging phenomena, damage mechanisms and exposed elements that may be potentially involved in a volcanic eruption. For the analysis of complex systems of components, the object-oriented paradigm is a convenient approach and enables to clearly represent the hazard phenomena and the vulnerable elements as a set of well-structured classes that are assigned specific attributes (i.e. properties) and methods (i.e. functions): such an approach has been previously used in the frame of a seismic risk analysis (Cavalieri et al., 2012) and has been adapted to the case of volcanic, by adding new hazard classes and damage mechanism corresponding to the specificities of volcanic risk. The definition of classes of objects is then used to draw UML (Unified Modified Language) diagrams that represent the successive steps of a risk scenario computation, from the definition of the hazard phenomena to the estimation of physical and functional damage of the exposed elements. A software tool developed by Cavalieri et al. (2012) in the frame of the SYNER-G FP7 project has been used as the "core engine" for the MIAVITA scenario-builder tool: the changes that were implemented enable to compute the impact of several volcanic events on a wide range of exposed elements (e.g. buildings, lifelines, road network, cultivated areas, emergency centres...). A non-negligible feature relies also in the ability to build a scenario composed from different successive volcanic events (e.g. tephra fall, pyroclastic density current, debris flow, lahar...), thus adding a temporal dimension in the computation. A few probable scenarios have been elicited for the Mount Cameroun area and they were implemented in the risk assessment tool in order to get a robust and quantitative estimation of the impacts of different volcanic events. Using sets of fragility curves previously compiled by Jenkins & Spence (2009), the software tool yields probabilistic results for some indicators such as the number of casualties or collapsed buildings, the area of damaged cultivated fields or the connectivity loss in the road network