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The Cotton Brook landslide, located in Mt. Mansfield State Forest near Waterbury, Vermont is the state’s largest documented landslide. The site’s stratigraphy is characterized by glaciolacustrine sediment overlying glacial till and bedrock. When the hillslope initially failed in 2019, it mobilized up to 200,000 m3 of surficial material downstream toward the Waterbury reservoir. This study spans from 2014 to 2023 and integrates field-based and UAS-derived data to 1) identify the mechanisms of continued mass wasting following the 2019 slip and 2) develop a workflow that allows us to estimate the magnitudes and rates of topographic change linked to diverse styles of earthflow. We utilized ArcGIS, Metashape Pro and CloudCompare softwares to conduct topographic differencing techniques with DEMs and 3-dimensional point clouds. We compared their outcomes to refine the workflow and quantify uncertainty. Vertical change measurements derived from DEMs over-estimated topographic change by up to ~10% when compared to values from 3-D point cloud results. We attribute this discrepancy to errors introduced by georeferencing and interpolation of elevation values. The latest volumetric estimates detail material redistributed from the hillside to the surrounding watershed. For instance, volumes extrapolated from ArcGIS and CloudCompare for material accumulated at the toe are approximately 135,000 m3 and 126,000 m3, respectively. Calculated uncertainties ranging from 1 cm – ~50 cm from CloudCompare were mapped spatially. To ground truth our geospatial analysis results, we mapped the main active earthflow processes driving sediment movement. The predominant mechanisms contributing to mass wasting include the collapse of thick piles of glacial lake sediment bordering the main slip and deepening gullies on the slip surface. Our quantitative analyses suggest the collapse of glacial material is accelerating, in part due to recent historic flooding. Gully features began as shallow rills and have evolved to reach depths of up to 1.5 m and are responsible for channelizing sediment into Cotton Brook. Our findings provide an opportunity to quantify material displaced and make predictions about how the sediment budget in the watershed and the Waterbury reservoir is impacted by the Cotton Brook landslide.
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QUANTIFYING LANDSCAPE CHANGE AT VERMONT’S LARGEST LANDSLIDE USING TIME-LAPSE SPATIAL DIFFERENCING OF 3D SURFACE MODELS FROM DRONE AND FIELD SURVEY DATA, WATERBURY, VERMONT
On May 31, 2019, a landslide near Waterbury in central Vermont removed >200,000 m3 of glacial lake deposits from a hillside in the Mt. Mansfield State Forest and transported the material across Cotton Brook, creating a dam near its toe. In the months following the slide, the brook breached the toe, removing ≥54, 000 m3 of sediment and transporting it ~1.3 km downstream into the Waterbury reservoir where it formed a large sedimentary delta. The delta grew 243% by Fall, 2020 until it began to erode into the reservoir in 2021. A collaborative team from the Vermont Geological Survey, the Vermont Agency of Transportation, Norwich University, and the University of Vermont team began a yearly monitoring of these events in 2019 using field-based mapping of bedrock and surficial geology, photogrammetry using annual drone surveys, and two LiDAR data sets. The first LiDAR data set was collected in 2014 prior to the slide by the Vermont Center for Geographic Information and the second after the slide in 2021 by the U.S. Army Corp of Engineers. Time-lapse spatial differencing allowed us to (1) quantify changes in surface topography over time, (2) calculate sediment budgets from source (landslide) to sink (delta), and (3) determine how mechanisms of mass wasting changed over time. Through this study we have also documented the following: (a) a second slip event in 2020 that removed ~25, 000 m3 of additional material from the hillside and contributed to growth of the Waterbury reservoir delta, (b) bedrock basins defined by the intersection of bedrock foliation and orthogonal fracture sets that appear to control slip location and geometry, (c) bedrock structures that influence the subsurface hydrology of the slip, which is expressed by oxidized groundwater seeps and a preferential deepening of rills into gullies on one side, and (d) how horizontal variations in the type and thickness of glacial lake sediments influenced mass-wasting mechanisms, including catastrophic failure of the hillside, the slumping of landslide sidewalls, the formation of crescent-shaped earth fractures, channeling around slumps, and the removal of material in deepening gullies. This study shows how a large landslide evolves from a major failure phase through later erosional and colluvial adjustments and supplies sediment at an episodic rate to the surface water system.
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- Award ID(s):
- 2138734
- PAR ID:
- 10494844
- Publisher / Repository:
- Geological Society of America Abstracts with Programs
- Date Published:
- Journal Name:
- Geological Society of America Abstracts with Programs
- Volume:
- 55
- Issue:
- 2
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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