Site-specific to large-scale landslides susceptibility assessment in Rwanda

This note deals with a pilot study that intends connecting the landslide susceptibility analysis at large scale with the slope stability analysis at single slope scale in the Rwandan context. The suggested methodology aims at putting together analyses at different scales, in order to better validate models at large scale on the basis of both field and laboratory geotechnical data at local scale. This approach, that has been already applied in other contexts such as Italy (Bordoni et al., 2015), will be applied for the first time in Rwanda. Landslides in Rwanda are very common in slopes close to urbanized areas and main roads. For this reason, they can cause significant damage to cultivation, structures and infrastructures and sometimes cause human losses. It appears extremely urgent to conduct a detailed landslide susceptibility study for Rwanda in order to overcome losses and curb the impacts. Efforts, therefore, must be in highlighting unstable zones by using suitable methodologies since it can provide sustainable response and protect lives and properties. In this perspective, landslide susceptibility study can deliver valuable information useful for hazard mitigation through appropriate planning (Nsengiyumva et al., 2018). In this work, a methodology that aims at linking field observations and laboratory tests on some sample slopes with distributed slope stability analyses at large scale is presented. To assess the occurrence of rainfallinduced landslides in a certain area, three main aspects can be considered to be of paramount importance: (1) a detailed description of the physical– mechanical triggering mechanism in relation to the site specific characteristics of the involved soils and stratigraphy; (2) the choice of the more suited slope stability model to be applied at the local scale; and (3) the definition of a reliable methodology to extend the model from the site-specific to large scale. On the other hand, it is well known that landslide triggering mechanisms are strictly linked to rainfall events. For this reason, even typical rainfall pattern in Rwanda should be taken into account. The proposed working scheme includes the implementation of well-established models and aims at finding a potential methodology for extending the analysis from site-specific to a wider area. This work gives some indications about the influence of the type of mapping unit chosen for the homogenization of the soil parameters, which are required as input data for the model’s application at large scale. The analysis of the role played by mapping units described in this research aims at identifying which zoning can be more representative of the distribution of the soil’s properties over the study area and evaluate the differences in the assessment of susceptible areas considering different types of soil mapping units.