Probabilistic Rockfall Hazard Analysis (PRHA)

In order to perform a rigorous quantitative rockfall risk analysis to help in land planning and zonation, as well as for countermeasure design (e.g. cost/benefit and cost/efficiency analysis), it is fundamental to assess rockfall hazard quantitatively. This requires quantifying the exceedance probability for a given location of being affected by a rockfall event with a specific level of kinetic energy (local intensity), integrating the contribution of different volume scenarios (magnitude). We propose a Probabilistic Rockfall Hazard Analysis (PRHA) methodology, which presents analogies with Probabilistic Seismic Hazard Analysis (PSHA). The methodology consists of three steps: (1) modeling of onset annual frequency, (2) modeling of rockfall runout and spatially-varying kinetic energy, (3) probabilistic analysis. Onset frequency is modeled through a frequency-magnitude relationship, derived from historical event rockfall catalogues, if available, or adapted from the literature. The probability distribution of kinetic energy for a given location and volume scenario is obtained by rockfall runout modeling of non-interacting blocks through the 3D HY-STONE code. Finally, the probabilistic analysis integrates over all rockfall volume scenarios to produce rockfall hazard curves. These give an estimate of the probability of exceeding values of kinetic energy, at each site, within a reference time interval, under the common assumption that rock-falls follow a Poisson process. To produce a rockfall hazard map it is necessary to further reduce the hazard curve to a single value. This can be done by choosing a fixed annual probability of exceedance following standards or regulation requirements.