Characterization of rockfall activity and identification of weather-rockfall relationships using high temporal resolution remote sensing methods

Abstract

Relationships between rockfall and weather have been the focus of considerable research over the last several decades, yet variations in these relationships have been identified in different studies. Many of these studies have also been limited with respect to the quantification of rockfall volume and in their capacity to study multi-year trends and conditioning factors due to relatively short study periods. As such, this study utilizes high temporal frequency remote sensing methods to develop rockfall databases for two slopes West of Denver, Colorado. Data sources include 5.5 years of terrestrial lidar data collected every two to three weeks at a rock cut slope named Floyd Hill and several months of daily SfM photogrammetry models developed from a fixed-site photogrammetry station, named the Idaho Springs Site. These data were processed into rockfall databases for each rock slope, which were then used to analyze the relationship between rockfall and weather at a variety of time scales.

Based on the analysis of the Floyd Hill data, an annual rockfall cycle was interpreted where surficial rock material is weakened each Winter by freezing processes, large amounts of rockfall occur in the Spring as a result of the slope thawing out, elevated rockfall continues over the Summer as surficial rocks destabilized by freezing processes in the previous Winter are fully dislodged by triggering factors (e.g. precipitation), and rockfall activity significantly reduces in the Fall because there are fewer unstable blocks available to fall. This proposed cycle is supported by the air freezing index recorded for a given Winter correlating well with the logarithm of the arithmetic mean of rockfall volume in the following Spring-Fall, and a lack of correlations between the precipitation and the logarithm of total rockfall volume within the same scanning intervals during Summers. The importance of precipitation as a triggering factor for individual rockfall events over the Summer, however, is clear from the analysis of the Idaho Springs data, where each interval during which rockfall was detected was also found to contain precipitation.