Estimation of landslide risk at national scale by means of environmental indicators

The purpose of this work was the definition of a new set of environmental indicators for a fast estimation of landslide risk over very wide areas. The proposed methodology was performed in GIS environment using Italy (301.340 km²) as test case since it is a country characterized by a very high exposure to hydrogeological disasters and where landslides are very common.

The proposed indicators aim to characterize landslide risk by quantifying how much urban expansion interferes with geomorphological processes; to this end a landslide susceptibility map and a soil sealing/land consumption map were combined to derive a spatially distributed indicator over the whole Italian country (namely, Landslide Risk Index - LRI). LRI emphasizes how much anthropic elements are exposed to landslide processes, and it is a basic element which can be aggregated over larger spatial units to characterize them respect to risk. To this aim, LRI was aggregated at the municipal scale in order to define two more indexes named Average Landslide Risk (ALR) and Total Landslide Risk (TLR).

ALR was defined by the mean value of LRI for each municipality: it represents how hazardous is the area of the territory where the exposed elements have been located. TLR was defined as the sum of susceptibility values of all cells with land consumption within each municipality: it expresses how much the urbanization of a municipality involves areas which can be affected by landslides.

The highest values of ALR are located in small municipalities renowned as international holiday destinations located by the sea in rocky coasts; on the contrary, highest values of TLR are in large and densely urbanized municipalities and where large portions of the territory urbanized are located in hazardous areas. The obtained results are supported by evidence collected from other national databases of landslide hazard and risk.

Both indexes showed to be useful to evaluate if local administrations have been prudent in planning urban development or if they ignored the geomorphological hazards threatening its territory. The proposed indexes are simple to understand and they can be adapted to various contexts and at various scales (e.g provinces, districts or basins) and updated with very low efforts. Obviously, they represent an oversimplification of the complexity of landslide risk and they cannot substitute a detail quantitative risk assessment, nevertheless a thorough national-scale risk assessment is not yet feasible in Italy and this is the first time that a set of landslide risk indicators have been defined in Italy at national scale combining landslide susceptibility and land consumption maps allowing to gain preliminary insights about the landslide risk produced by the interaction between hillslope dynamics and urban expansion.