Rockfall causes a large number of accidents and fatalities in steep environments. A realistic quantification of rockfall risk is thus crucial for an effective prevention of damages and loss of lives. The estimation of rockfall and block volumes for different return periods thereby remains a major challenge. In this paper, we present a straightforward rockfall frequency model (RFM: Rockfall Frequency Model) and its application at 8 different sites at 7 locations in the Swiss Alps. The RFM assumes that the magnitude-frequency relationships of rockfall events and blocks follow a power law. The parameters of this distribution are estimated based on a simple classification of rock structures and on field inventories. Beside the block volume frequency, which is very sensitive to the consideration of large rockfall events, the frequency of rockfalls with at least one block with a minimum volume, is determined. The block size distributions measured in this study were well captured by power laws. The rockfall and block volumes calculated with the RFM were generally slightly higher than the scenarios of the official hazard assessments. The uncertainty analysis, however, revealed a high variability of the release scenarios with respect to the parameters of the RFM, increasing with the return period. Both, the rockfall volumes and the block volumes, are particularly sensitive to the estimated exponent of the power law distribution of the rockfall events. Nevertheless, the proposed RFM provides an objective and transparent approach to derive magnitude-frequency relationships of rockfall events and individual blocks even if historical inventories are missing or insufficient and is thus a promising alternative to merely expert-based approaches.
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