Quantitative landslide hazard assessment at 1:250 000 scale for Malawi (Southeastern Africa)
Résumé
Landslide are ubiquitous phenomena affecting many countries around the world. In recent years, in the context of landslide risk reduction and management an increasing number of landslide susceptibility or hazard maps were carried out at the national scale. These analyses are generally based on: (1) an existing inventory (national database or compilation of work carried out individually); (2) empirical indirect or data-driven methods. However, few studies, at this scale of work, take into account the temporality of events and/or the triggering factors to assess landslide hazard assessment. This statement is often due to a lack of information, especially for emerging countries, where a lack of spatial and temporal information on events and on triggering factors subsist.
Thus, if landslide inventories provide the first information to assess susceptibility, at national scale of work, it is also necessary to identify and analyze the components inducing hazard. For instance, it is possible by different ways to complete the analyses with annual frequency of landside events and/or meteorological or seismic events to assess quantitatively the landslide hazard (Corominas et al., 2014). This identification can be carried out by (i) direct approaches based on analysis of temporal data on past landslides (e.g. computations of the exceedance probability of landslide occurrence estimated by Poisson or binomial distributions); or (ii) by indirect approaches based on analysis of triggering factors (e.g. rainfalls volume, intensity and duration).
This contribution focuses on the methodology adopted during the GEMMAP[*] project to assess landslide hazard at national scale (i.e. 1:250,000) for Malawi, a landlocked country in southeastern Africa. This country is characterized by a large diversity of landscape and a topography composed of mountains, hills and plateaus, crossed by the Great Rift Valley and the Malawi Lake. It is experiencing many slope instabilities (i.e. debris-slides, debris-flows, slides, rockslides, rock-falls), principally due to its complex geology (weathered metamorphic lithology, complex structure, active seismic activity) and intense rainfalls (from tropical cyclones to depressions, World Meteorological Organization, WMO, https://public.wmo.int/en).