Are hydrometeorological thresholds useful for regional-scale rockfall early warning systems? A preliminary analysis of the hydrometeorological conditions leading to rockfalls in Norway

Norway's high-relief landscape is susceptible to gravity-driven natural hazards including snow avalanches, landslides, debris flows, and rockfalls. Rockfalls are the most numerous geohazard in Norway. There are currently over 35 000 rockfall events registered in Norway's national hazard database, accounting for nearly 50% of the total number of events for all hazard types. Rockfalls commonly impact the functioning of infrastructure assets such as roads and railways, and occasionally damage buildings and result in death.

The relationship between rockfall events and weather conditions is recognised but not straightforward. Several hydrometeorological variables are significant for rockfall triggering including precipitation, snow melt, freezing and thawing, temperature, insolation, and soil or rock moisture. The highest frequency of rockfall activity in Norway is observed in spring, a period of snowmelt and freeze-thaw cycling. Given the links to meteorological variables, rockfall frequency is expected to change with climate, altering the exposure of population and infrastructures to rockfalls.

Rockfall risk mitigation at regional scale is challenging. Early warning systems are a helpful tool to depict the time and location of future rockfall events so that emergency managers can act in advance. At present, most existing rockfall early warning systems (REWS) are based on the monitoring and analysis of seismic signals to determine the movement of boulders or the cracking of joints. Little previous research has been conducted to analyse the meteorological conditions that could trigger rockfalls. There is currently no REWS in Norway.

The main objective of this work is to investigate the feasibility of using hydrometeorological thresholds for regional scale rockfall warning. To do so rainfall, temperature, and soil moisture data from SeNorge.no, and the rockfall inventory contained in the Norwegian national hazard database have been analysed to find relations between the hydrometeorological conditions and the triggering of rockfalls in Norway.