Development of a GIS-based methodology for quantitative risk analysis of fragmental rockfalls in transport infrastructures

Rockfall events can cause important direct losses such as partial or total damage to transport infrastructures, traffic disruptions, and casualties, but also other indirect consequences that alter the normal running of society, its mobility, and economic activity. A quantitative risk analysis (QRA) for rockfall hazard can help to improve investment in mitigation measures of the infrastructure in terms of benefits, time, and cost savings.  The fragmentation process during rockfall propagation determines the number and size of the fragments and their trajectories, energies, and run-out. Therefore, fragmentation has a significant but contrasting effect on the reach probability at transportation corridors, which should be considered in QRA.

This study, conducted in the research Georisk project (Ref. PID2019-103974RB-I00, funded by MCIN/AEI/10.13039/501100011033), presents the development of an objective and a GIS-based automatable methodology for quantitative risk analysis on linear structures due to fragmentary rockfalls. The annual frequency of events is established for each source point from the photointerpretation of deposits using historical orthophotographs. Some different source points maps are created from a digital elevation model, one for each maximum volume interval that could be detached from the cliff. This expected maximum volume considering the outcrop geometry is estimated automatically for each point based on the number of neighbor source points, using GIS tools. The impact energies are calculated using the RockGIS rockfall model, a 3D simulator able to reproduce the fragmentation phenomenon by using the Rockfall Fractal Fragmentation Model. The impact energies of each fragment are registered in raster maps. The exposure and vulnerability of each of the exposed elements are estimated through the corresponding developed fragility curves, which define the relationship between the impact energy and the estimated damage based on the particular characteristics of the different elements exposed.

The resulting risk values are presented in a georeferenced way along the road and for different dynamic exposed elements such as vehicles and people, in economic terms or the number of fatalities, for some given scenarios. The methodology and GIS techniques proposed have been implemented in a 27 km road corridor of great social and political interest in Catalonia (Spain).