ROCKSLIDE HAZARD MITIGATION USING DRONE SURVEYS, FRACTURE MEASUREMENTS, AND VOLUMETRIC ANALYSIS OF A CLIFF FACE, WRIGHTSVILLE DAM SPILLWAY, CENTRAL VERMONT

Rockslide hazards are becoming increasingly problematic in Vermont and elsewhere as large storms and major flooding events increase in frequency due to the effects of climate change. In central Vermont, recent rockslides have occurred on a ~430 meter long cliff face that borders the spillway of an earthen dam north of the town of Wrightsville. Recently, the University of Vermont partnered with the Vermont Geological Survey to calculate the approximate volume of rock that needs to be removed from the Wrightsville Dam spillway to mitigate the effects of potential rockslides, which could block or constrict the spillway during a severe flood. To solve this problem we identified four areas of potential rockslides and conducted a combination of field-based structural analyses of fracture sets exposed on the cliff and unoccupied aerial vehicle (UAV) surveys of each site at multiple scales using a Mavic Air 2 DJI drone. Areas of highest risk correspond to regions where three dominant fracture sets intersect and form rock wedges. Each fracture set was measured using a digital compass. Raw images of the cliff face were converted to digital 3-D meshes using Agisoft Metashape Pro software. Model scaling and georeferencing to sub-centimeter accuracy was achieved using ground control points and EMLID Reach RS2 survey equipment. To calculate the volume of rock to be removed, we defined 25 polygons (rectangles or triangles) that correspond to areas of potential rockslides. The polygons display a variety of orientations in three-dimensions. The surface areas and thicknesses (i.e., depth into the cliff) of each polygon were obtained directly from the digital model using Metashape Pro measuring tools. Volumes were obtained using the surface area and depth measurements. Despite sub-centimeter accuracy of the scaled models, uncertainties of up to 20% occurred in some sections due to challenges associated with estimating the three-dimensional shapes of potential rockslides. We estimate that between 710 and 850 cubic meters of rock needs to be removed from the Wrightsville Dam Spillway to help mitigate rockslide hazards during a flood.