INTEGRATING STRUCTURAL GEOLOGY, UAV SURVEYS, AND PHOTOGRAMMETRY TO UNDERSTAND ROCKSLIDE MECHANISMS AT SMUGGLERS NOTCH, VERMONT

Rockslides occur where large pieces of rock detach from a cliff and slide, bounce, or fall down a slope. Smugglers Notch is a steep mountain gap (~300 m relief) through the Green Mountains in northern Vermont that is bisected by Route 108. Rockslides have occurred at Smugglers Notch for 1000s of years and are an ongoing hazard predicted to increase in frequency with climate change. Freeze-thaw cycles, root wedging, surface and groundwater flow, and gravity all act on the underlying bedrock structure(s) to cause rockslides. Recent rockslides (2020-21) that obstructed the road and a hiking trail prompted us to initiate annual LiDAR and optical monitoring surveys accompanied by photogrammetry and structural analysis.

The bedrock at Smugglers Notch consists of rusty weathering sulfidic gray and green albitic schists of the Neoproterozoic Hazens Notch and Fayston formations, respectively. The dominant gently dipping Ordovician composite foliation (S_1-2) is defined by alternating mica and quartz domains. A Devonian north-striking and steeply dipping crenulation cleavage (S₃) transects S_1-2. Two fracture sets were formed by parting parallel to S _1-2 and S₃, a third strikes ~E-W and dips steeply, and a fourth exfoliation/ unloading fracture set parallels topography. The intersections between these structures produce prismatic blocks that become unstable from physical weathering processes. Schist blocks with a long axis up to 15 m on the notch floor are bounded by these planar structures.

LiDAR data were acquired by a Yellowscan Mapper sensor on a DJI M300 quadrotor UAV, and processed using Applanix PosPAC and Yellowscan Cloudstation software, whereas optical surveys utilizing a DJI P1 sensor were processed with Pix4Dmapper software. LiDAR and optical data were processed using Agisoft’s Metashape Pro software to facilitate structural analysis.

Although measurements of all bedrock structures were acquired during previous field surveys and correlated with slide-prone gullies, much of Smugglers Notch is inaccessible by foot. Optical and LiDAR UAV surveys have enabled us to apply “structure from motion” techniques” to extrapolate planar data into steep areas and construct 3-D maps. We are using these data to monitor rockslide-prone areas on an annual basis in extreme detail, including mapping individual boulders.