USING REPEAT PHOTOGRAMMETRY TO MONITOR EVOLUTION OF AN ACTIVE MASS MOVEMENT NEAR DURANGO, CO

Advances in accessibility of structure-from-motion (SfM) photogrammetry is revolutionizing geoscientists’ ability to measure and monitor changes in Earth’s surface topography. By constructing high-resolution, 3D models using photos collected from an unmanned aerial vehicle (UAV), users can engage in repeat surveying of changing surface topography like mass movements and dynamic river systems without the heavy financial burden of alternative methods like repeat LiDAR. In this contribution SfM is used to document changes in the topography of a complex, hazardous mass movement near Durango, CO.

In 1998 a large mass of sandstone, shale, and mudstone failed along Missionary Ridge, north of Durango, CO, sending boulders, trees, debris, and water downslope toward the valley of the Animas River. Over the following 19 years, the slide has continued to evolve, occasionally sending debris flows and slurries downslope, causing frequent road closures and significant property damage.

In order to document the evolution of the slide during the 2016-2017 winter, its topography was surveyed in September 2016 and April 2017 using SfM - a low-cost UAV was used to collect photos and RTK GPS to provide ground control points. Pix4D software was used to generate high-resolution digital terrain models (DTMs) from the collected photos and GPS control points. These DTMs and corresponding orthomosaics were compared in a GIS environment to document locations and volume of mass transfer.

This comparison captured a major shift in mass along the slide path, as ~13,000 cubic meters of rock and debris slipped up to 200 m downslope, where it now sits poised for its next failure, which could present significant hazard to residents and infrastructure below. Some difficulties encountered include: imperfect alignment of pre- and post DTMs, changing photo resolution with respect to the ground due to the steeply sloping surface, and dangerous traversing to place and recover ground control point targets. Despite these challenges, this case study demonstrates the value of repeat SfM toward monitoring geohazards that are otherwise not feasible to study using traditional survey methods.