REPEAT STRUCTURE-FROM-MOTION SURVEYING OF A HAZARDOUS MASS WASTING FEATURE IN STEEP TERRAIN NEAR DURANGO, CO

The Missionary Ridge rockfall complex initiated on a steep hillslope north of Durango, CO in 1998. It has continued to evolve since then, growing in size and occasionally sending mud and debris flows downslope. These events have caused road closures and property damage, and present an ongoing hazard to residents at the base of the slope. However, traditional surveying methods had failed to capture those changes at a useful resolution.

To better understand how the mass wasting feature changes over time and what hazard it may present in the future, a repeat-surveying campaign utilizing a cheap, consumer-grade UAV paired with RTK GPS and structure-from-motion processing was launched in Oct 2016. Five surveys have been conducted in total, each taking place in April (post-snowmelt) or October (post-monsoon).

Although challenging to conduct on the steep slopes of the study area, the surveys yielded digital terrain models with spatial resolution as high as 3 cm/pixel and remarkably precise georeferencing. Comparison of successive surveys in GIS reveals a zone of progressive scarp failure near the main rockfall mass, and multiple zones of material accumulation downslope. Nearly all movement occurred during spring snowmelt, while the monsoon exerted a surprisingly minimal effect. During the study period, a newly-formed, boulder-dammed debris field was identified as a future hazard; that dam broke in 2019, sending the largest, coarsest debris flow yet downslope and burying the roadway at the base of the slope in up to 5 m of debris.

These results support the utility of using UAVs paired with structure-from-motion to monitor rapidly-changing geohazards, despite the extreme terrain and ever-evolving survey strategies.