UAV-photogrammetry rockfall monitoring of natural slopes in Glenwood Canyon, CO, USA: background activity and post-wildfire impacts

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.