Abstract
Rockfall is a common hazard along US Interstate Highway 70 in Glenwood Canyon, Colorado. While natural rock slopes source a substantial number of rockfalls, there remains a need to better understand typical rockfall frequencies from these slopes and to relate these frequencies to available rockfall records. In addition, the 2020 Grizzly Creek Fire in the area presents an opportunity to characterize the post-wildfire response of granitic rock slopes. To address these needs, we monitored 4 natural rock slopes of different lithologies and burn severities from 6 to 18 months after the start of the fire using an Unpiloted Aerial Vehicle and Structure from Motion photogrammetry. A total of five rockfalls (0.08–0.68m3) were detected during the monitoring interval at two slopes, while the other two showed no rockfalls. While overall rockfall activity is relatively low, the observed activity is in good agreement with results of previous study in the area, rockfall records for the highway, and analysis of a historical photo of one slope. No increase in rockfall due to the fire was observed during the monitored period, implying that the fire did not significantly affect the studied slopes or that any increase in activity had already returned to background levels by the start of monitoring. Four of the five rockfalls detected were spatially correlated with seeps in the slope, and three were temporally correlated with late winter snowfall and spring thawing, highlighting the importance of water as a consideration in selecting rock slopes for hazard monitoring and mitigation.
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Acknowledgements
We thank the Colorado Department of Transportation and the Association of Environmental and Engineering Geologists Foundation for funding this research. We also thank Matt Tello of the Colorado Department of Transportation Geohazards Program and Todd Parker of the US Forest Service for coordinating access to areas closed by the Grizzly Creek Fire and the Summer 2021 debris flow cleanup. We would also like to thank those who assisted with fieldwork for the monitoring, including Kyle Radach, Claire Graber, and James Graber. Additional thanks go to Kellen Cutsforth of Denver Public Library for assistance with historical photos and to colleagues at Colorado School of Mines who helped with establishing confidence levels for historical rockfalls: Cassidy Grady, Austin Donaldson, Ashton Killen, Chandan Kumar, Lauren Miller, and Nikhil Prakash. Finally, we thank two anonymous reviewers for their comments, which helped to improve the manuscript.
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Primary funding for this work was provided by the Colorado Department of Transportation. Additional fieldwork funding was provided by the Association of Environmental and Engineering Geologists Foundation.
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Graber, A., Santi, P. UAV-photogrammetry rockfall monitoring of natural slopes in Glenwood Canyon, CO, USA: background activity and post-wildfire impacts. Landslides 20, 229–248 (2023). https://doi.org/10.1007/s10346-022-01974-9
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DOI: https://doi.org/10.1007/s10346-022-01974-9