Integrated Landslide Susceptibility Analysis and Hazard Assessment in the Principality of Andorra

In the paper we present the procedure for hazard assessment that has been used to prepare the landslide hazard map of the Principality of Andorra at 1:5,000 scale. The main phases of the hazard assessment are discussed. Susceptibility analysis has involved the location of the potential slope failures, and the estimation of both landslide volume and runout distance. In the susceptible areas, landslide magnitude and frequency has been determined in order to produce the Hazard Zoning Map. Data required for hazard assessment have been introduced into a GIS or derived directly from available Digital Terrain Models. Data handling and treatment with the GIS has allowed the performance of the landslide hazard assessment and mapping in a fast and reproducible way.     

Quantitative assessment of the residual risk in a rockfall protected area

Quantitative Risk Assessment (QRA) has become an indispensable tool for the management of landslide hazard and for planning risk mitigation measures. In this paper we present the evaluation of the rockfall risk at the Solà d’Andorra slope (Andorra Principality) before and after the implementation of risk mitigation works, in particular, the construction of protective fences. To calculate the risk level we have (i) identified the potential rockfall release areas, (ii) obtained the volume distribution of the falling rocks, (iii) determined the frequency of the rockfall events, and (iv) performed trajectographic analysis with a 3D numerical model (Eurobloc) that has provided both the expected travel distances and the kinetic energy of the blocks. The risk level at the developed area located at the foot of the rock cliff has been calculated taking into account the nature of the exposed elements and their vulnerability. In the Forat Negre basin, the most dangerous basin of the Solà d’Andorra, the construction of two lines of rockfall protection fences has reduced the annual probability of loss of life for the most exposed person inside the buildings, from 3.8×10⁻⁴ to 9.1×10⁻⁷ and the societal risk from 1.5×10⁻² of annual probability of loss of life to 1.2×10⁻⁵.     

Detailed debris flow hazard assessment in Andorra: A multidisciplinary approach

In many mountainous areas, the rapid development of urbanisation and the limited space in the valley floors have created a need to construct buildings in zones potentially exposed to debris flow hazard. In these zones, a detailed and coherent hazard assessment is necessary to provide an adequate urban planning. This article presents a multidisciplinary procedure to evaluate the debris flow hazard at a local scale. Our four-step approach was successfully applied to five torrent catchments in the Principality of Andorra, located in the Pyrenees. The first step consisted of a comprehensive geomorphologic and geologic analysis providing an inventory map of the past debris flows, a magnitude–frequency relationship, and a geomorphologic–geologic map. These data were necessary to determine the potential initiation zones and volumes of future debris flows for each catchment. A susceptibility map and different scenarios were the principal outcome of the first step, as well as essential input data for the second step, the runout analysis. A one-dimensional numerical code was applied to analyse the scenarios previously defined. First, the critical channel sections in the fan area were evaluated, then the maximum runout of the debris flows on the fan was studied, and finally simplified intensity maps for each defined scenario were established. The third step of our hazard assessment was the hazard zonation and the compilation of all the results from the two previous steps in a final hazard map. The base of this hazard map was the hazard matrix, which combined the intensity of the debris flow with its probability of occurrence and determined a certain hazard degree. The fourth step referred to the hazard mitigation and included some recommendations for hazard reduction. In Andorra, this four-step approach is actually being applied to assess the debris flow hazard. The final hazard maps, at 1 : 2000 scale, provide an obligatory tool for local land use planning. Experience achieved during the study showed that the collaboration between geologists, geomorphologists, engineers, and decision makers is essential and that only a multidisciplinary approach allows for solving all the problems of such a complex process as debris flows. Finally, we propose that our approach may be applied to other mountainous areas, adapting the hazard matrix to new local conditions.