ANALYSIS OF ROCKFALL HAZARDS USING UNOCCUPIED AERIAL VEHICLE (UAV) AND FIELD SURVEY DATA AT THE WRIGHTSVILLE DAM SPILLWAY, CENTRAL VERMONT

Landslides, rockfalls and other mass-wasting events are expected to increase in frequency in New England as large storms, rising annual precipitation, and other patterns linked to climate change intensify. In 2021, we integrated UAV and detailed field structural surveys of a ~430 m long cliff face that forms the east wall of the Wrightsville Dam Spillway near Montpelier. The purpose of the surveys is to identify potential rock fall zones along this cliff face that could block parts of the spillway and impede flow during a major flooding event. The UAV surveys were conducted at multiple scales using DJI Matrice 210 RTK V2 and Mavic Air 2 DJI drones. Raw images were compiled into orthomosaics and converted into georeferenced digital 3-D meshes. The meshes were draped with optical photos to create textured 3-D maps, which we used to calculate the orientations and dimensions of fracture sets, foliations, and rock wedges. The accuracy of the georeferenced maps was evaluated by comparing the calculated structural attitudes with a closely spaced network of field measurements.

Two areas where recent rock falls have occurred allowed us to identify the mechanisms of rock failure. Three main fracture sets occur along the cliff face east of the dam, which trends a few degrees east of north. The first set forms a conjugate pair that dips steeply (>70°) to the north and south. The second set forms a conjugate pair that dips gently to moderately (25-35°) to the west and northwest. A third set consists of spaced fractures (parting) that parallel rock foliation and dips moderately (55°-70°) to steeply (>70°) to the west. This latter set acts as a slip plane to large rock falls on the cliff face. The orientations of all three sets are variable along the length of the cliff. Fracture-bounded rock wedges with the highest risk of failure occur where all three fracture sets are present, are long enough (>1-10 m) to intersect one another, and display all three of the following characteristics: 1) the wedges are bounded by intersecting fractures from the two conjugate sets, 2) the wedges lie on top of, and in contact with, a foliation-parallel fracture slip plane, 3) the bottom face of the wedges is either wholly or partially a free surface (i.e. an open face). Intermediate risk blocks show criteria 1 and 2 but not 3. Low risk wedges show only one or none of the three criteria.