Contribution of physical modelling to landslide hazard mapping: case of the French Basque coast

International audience; Mechanical approaches represent a solution for quantitative landslide susceptibility and hazard modeling. These methods consist in, (1) generalize at regional scale geometrical and geotechnical parameters which contribute to slope stability regarding a certain type of landslide, (2) apply triggering factors to the studied area (water table level or seismic stress for example), and (3) calculate a safety factor for each point of the area. 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. For the calculation of safety factors, based on the Morgenstern and Price method, variability of mechanical parameters is handled by the Monte Carlo method. Thus a distribution of factors is obtained for each point of the studied area, instead of a single value. The probability of having the 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 pixel of the studied area. For the DO-SMS project, created in the frame of the European SUDOE partnership, this method is experimented for landslide hazard mapping on the French Basque coast. The Pays Basque, situated in the South West of France, presents numerous instabilities along its 42 km long coast. Landslides and rock falls contribute for a great part to the littoral erosion in an area with high human and economic stakes. Indeed, the Pays Basque littoral has a population density of 504 people/km², whereas it is of 81 people/km² in average on the South West littoral of France. Probabilities of occurrence are calculated for a scenario considering a two meter deep water table and a landslide size of 25m. Hazard levels are defined from ranges of probabilities and the result map is used as a base for drawing the limits of the different hazard zones. The obtained landslide hazard map is then validated by a field expertise. This method proposes a quantitative classification of landslide hazard and offers a useful tool to gain time and efficiency in hazard mapping. However, in order to take into account some influent factors in slope stability, such as heterogeneity of the geological formations or effects of anthropic interventions, an expertise approach is necessary to finalize the map.