UAV-PHOTOGRAMMETRY ROCKFALL MONITORING OF NATURAL SLOPES IN GLENWOOD CANYON, CO, USA: BACKGROUND ACTIVITY AND POST-WILDFIRE IMPACTS (Invited Presentation)

Rockfall is an important hazard along US Interstate Highway 70 in Glenwood Canyon, Colorado. Because natural rock slopes source a substantial number of rockfalls in the area, there is a need to constrain typical activity levels from these slopes. In addition, the 2020 Grizzly Creek Fire in the area provided an opportunity to characterize the post-wildfire response of rock slopes. We monitored four natural rock slopes from 6 to 18 months after the start of the fire using an Un-piloted Aerial Vehicle and Structure from Motion photogrammetry. The monitored slopes included 1) a pair of slopes of different lithologies, one granite and one orthoquartzite, along the highway alignment and 2) a pair of slopes with similar geology, jointing, and topography but with different burn severities: one burned at moderate severity and one essentially unburned. Ground control points were deployed during the initial scan at each slope, and automated flight planning software was used to improve consistency between scans. A total of five rockfalls (0.08 – 0.68 m³) were detected during the monitoring interval at two slopes, while no rockfalls were detected at the other two slopes. While overall rockfall activity is relatively low, the observed activity is in good agreement with results of a previous study in the area, rockfall records for the highway, and analysis of a historical photo of one of the slopes. No increase in rockfall due to the fire was observed during the monitored period, implying either that the fire did not significantly affect the studied slopes or that any increased activity had already returned to background levels by the start of the monitoring campaign. Four of the five rockfalls detected are spatially correlated with seeps in the slope, and three of the rockfalls are temporally correlated with the late winter snowfall and spring thawing, highlighting the importance of water as a consideration in selecting rock slopes for hazard monitoring and mitigation.