CHALLENGES IN DETERMINING “IMMINENCE” IN LANDSLIDE EVALUATIONS FOR FEMA BUYOUTS

A statutory duty of the Vermont Geological Survey (VGS) is to provide geologic information and advice to the public and other State agencies on a variety of issues including geohazards. While not generally known as a landslide-prone state, the surficial geologic units of glacial origin in Vermont can pose significant slope instability problems, particularly where they were deposited over steep terrain. In 1999, the investigation of a ~25,000 m³ landslide in glacial sediments in Jeffersonville, VT by the VGS and our partners led to the first buyout of a property by the Federal Emergency Management Agency (FEMA) due to additional landslide risks. Since that time the VGS has become increasingly involved in evaluating and inventorying landslides, particularly after Tropical Storm Irene induced widespread geomorphic instability in the state in 2011. Our increased involvement led to the development of a Landslide Protocol in 2013, which was incorporated into Vermont’s State Hazard Mitigation Plan and includes a comprehensive landslide database of over 3,000 landslides.

The evaluation of landslide sites for FEMA buyouts can be challenging because of the need to place risk onto a timeline. FEMA’s Benefit-Cost Analysis procedure requires documentation of “imminent” failure; that is, it requires “proof” that the building(s) being acquired by the project are in an area facing the threat of catastrophic slope failure within the next 5 years. When evaluating parcels, we seek out indicators of slope instability including tension cracking in soils at the tops of slopes, angled trees, permeability contrasts in surficial materials, groundwater seeps (particularly along permeability contrasts), incipient block failures, proximity to bedrock, cracks in foundation walls, and site historical data. While some parcels display several of these proxies as clear evidence for imminent failure, others are more subtle and the “imminence” determination can be difficult to make and/or “prove”. With Vermont becoming ever wetter in our changing climate, we anticipate an increase in buyout evaluation requests and seek to develop and implement a process for determining imminent risk that is standardized yet accommodating of the site-specific variability of Vermont’s slopes.