A CENTURY OF URBAN LANDSLIDES: THE LEGACY AND CONSEQUENCES OF ALTERING OUR LANDSCAPES

Landslides in urban areas can be a deadly and costly hazard. Human actions, such as altering the steepness, composition, and tree-cover of slopes, increase the threat of mass movements. Here, we use an interdisciplinary approach to examine the spatial distribution, timing, and cause of landslides affecting buildings and roads along the top of a steep, urban riverbank in northern Vermont. Using over 100 years of mapping, air and ground photographs, and written records along with hydraulic and slope stability calculations, we demonstrate that most mass movements along this corridor occur in slopes over-steepened by the addition of unconsolidated fill – added informally and without engineering considerations.

Beginning in the 1920's, fill composed of junked cars, sand, gravel, concrete and trash, was dumped over the edge of a 20 to 30 meter tall cut bank along the region’s major river, the Winooski. Emplaced atop glacially deposited lake sediments, the fill having little to no cohesion, expanded buildable areas, but at a cost; the new infrastructure sat on potentially unstable ground. Over the following decades, repeated failures, mostly shallow and translational but including several deeper-seated rotational slides, sent buildings, trees, and the riverside roadway into the river below. Today, the people at risk of future slides include small business owners, employees, homeowners, and tenants. A recently approved, 64-unit elderly housing building could substantially grow this at-risk community if built.

The future looks increasingly unstable. Regional climate change predictions indicate an increased mean annual rainfall of 10% by 2100 over and above a nearly 20% increase since 1920. Paired with predicted increases in storm event frequency and intensity, future precipitation events could exacerbate slope hazards by raising ground water tables and thus pore pressure. The approach we use for analyzing this landslide hazard provides a framework for similar geographic settings and can inform planning and risk assessment within the context of climate change.