A decrease in rockfall probability associated with changing meteorological conditions in Germany

In this study we assess the influence of changes in the relevant meteorological conditions on the probability for rockfall in German low mountain regions. The study is based on data from a rockfall data base for Germany (Rupp and Damm, 2020) and a data set supplied by the Deutsche Bahn (German railway company) covering the periods 1838-2018 and 2015-2020, respectively. 

In a first approach, a logistic regression model for the probability of rockfall at a given location developed by Nissen et al. 2021 was applied to gridded meteorological station observations (RADOLAN and EOBS) ranging from the year 1950 to 2019. The logistic regression model quantifies the influence of daily precipitation, a proxy for pore water and freeze-thaw cycles on rockfall probability. A probability forecast was made for each day and location. The day-to-day variability in rockfall probability at the individual sites is high. Thus, the sign of the trends is site specific, but the majority of sites is showing a negative trend over the 70-year period investigated. The significance of the trends at most sites is below the 95% level. Sites at which the trend is statistically significant almost all show a negative trend, down to -4% per decade in terms of the annual number of days with a higher than climatological hazard. The mean probability decreased by as much as -2.3% per decade. 

The second approach is based on large-scale weather patterns. An analysis identified 3 weather pattern that occur on average at 9% of all days but include  19% of the days on which a rockfall event occurred. The trend in the number of these patterns was determined for the last 40 years. It suggests a decrease by -2.2% per decade and is not statistically significant.