Evaluation of rockfall trends at a sedimentary rock cut near Manitou Springs, Colorado, using daily photogrammetric monitoring

Rockfall remains a prominent hazard for transportation corridors worldwide. Recent studies have shown promising results in resolving the relationships between rockfall activity and triggers, including in some cases detecting precursor activity prior to failure, which could have implications to improving safety and performance of transportation corridors. The aim of this study is to better understand rockfall failure processes and triggers for cut slopes in interbedded sedimentary rock through a long-term study using photogrammetry data with high spatiotemporal frequency. The combination of daily data, high-precision rockfall volume estimation, and 22-month monitoring duration is unique among studies that evaluate rockfall triggers and allows us to derive insights into differences in rockfall triggering between blocks of different volumes. The data collected allowed the relative frequency of rockfalls of different volumes to be well-constrained for volumes ranging from 0.01 m³ up to 76 m³ (the largest event that occurred during the monitoring period). A quantitative comparison between precipitation and rockfall activity established that precipitation was the primary trigger for rockfall at the site, with only 1.4% of 24-h photo intervals without precipitation having at least one rockfall, as compared to 25.0% of photo intervals with precipitation (and 57.1% of photo intervals with at least 5 mm of precipitation). The marginal impact of additional rainfall above 8 mm per 24-h period on rockfall probability was negligible among all rockfalls observed, whereas the probability of the largest rockfalls at the site (> 1 m³) occurring continued to increase as a function of precipitation up to 20 mm per 24-h period. Detailed analysis of change data leading up to the largest (76 m³) rockfall observed illustrated the progressive failure mechanism of the block, including observations of forward toppling motion and smaller precursor rockfalls around its perimeter. This rockfall was also used for a proof-of-concept demonstration of the potential for a spatiotemporal rockfall density metric to be used to help identify areas of potential hazard. Ultimately, the findings from this study contribute to knowledge on rockfall processes outside alpine regions, which have historically been less well-studied.