THE COTTON BROOK LANDSLIDE IN THE GREEN MOUNTAINS OF NORTH-CENTRAL VERMONT

A major translational earth slide occurred on the south side of Cotton Brook in the Mount Mansfield State Forest in Waterbury, Vermont on May 30-31, 2019. Although there were no injuries, portions of a popular trail were destroyed. The Vermont Geological Survey, VT Departments of Forests, Parks & Recreation and Environmental Conservation, and university partners worked to characterize the landslide, assess risks to State Forest visitors, and identify trail closures.

The landslide is 109 m high and 175 m wide, has an overall slope of 25˚, and an area of 48,700 m². Materials are silt-clay overlain by sandy sediments deposited in glacial Lake Winooski. A dipping bed surface of silt-clay served as a failure surface and was striated by the overriding material. The slide resulted in massive sedimentation and debris along Cotton Brook and enlarged a delta at the shore of the Waterbury Reservoir. Comparing elevation data from photogrammetry acquired 2 weeks after the slide with pre-landslide lidar shows ~200,000 m³of material were excavated, ~100,000 m³ deposited at the base, at least 25,000 m³ in the delta, and ~75,000 m³ are unaccounted for (deposited along the brook, in the reservoir, or transported down the Little River). The landslide blocked Cotton Brook, creating a pond upstream and causing multiple shifts in the stream channel. Over 8,800 m² of hillside adjacent to the landslide scar are heavily fractured and are areas of future slope failure. Observations during the 6 months following the slide showed that the site remained hazardous due to fresh mud flows, falling trees and boulders, new cracks, and stream erosion at the toe. Game cameras show that the pond fluctuated due to continued earth flows and slides. The landslide dam subsequently failed during heavy rain on 10/31, draining most of the pond. The lidar topography indicates that this slide occurred on the site of a previous landslide, but a review of aerial imagery shows no major slope failures at the site in the last 60 years. Mapping by Wright showed two other landslide sites upstream suggesting a history of slope failure along the brook. Antecedent rainfall and increased pore pressure contributed to the failure, but the exact causes and triggers are unclear. The damaged trail has been permanently closed and the Cotton Brook Road at the base was temporarily closed for safety.