Correlation between rockfall frequency and overhang geometrical attributes

The estimation of rockfall frequency for the quantitative assessment of rockfall hazard is challenging when there are not records of previous events. The aim of this study is to investigate the correlation between the rockfall frequency and the geometry of the overhanging rock blocks, at rocky slopes. The investigation takes place for seven rocky slopes along a coastal road in Northern Spain, and it is based on 15 years of rockfall data. A field survey of these slopes took place between May and June 2022, using a terrestrial laser scanner as well as ground and aerial photogrammetry from UAVs, in order to build three-dimensional digital models. Then, the geometrical attributes of the existing overhanging rocks were measured on the models. The surface area and the width of the latter were calculated. The correlation between the rockfall frequency and the sum of the overhanging areas at each slope was assessed. A good linear correlation was indicated between the normalized per slope area number of rockfalls and total overhang area (R² = 0.9013) and between the respective normalized per unit of road length parameters (R² = 0.9594). The magnitude-frequency relationship for the rockfall events that occurred at the seven slopes follows a power law distribution with exponent −0.65.