Mapping Alluvial Fans to Inform Flood Hazards for Vermont’s Rural Communities

Vermont’s rural communities have experienced increasing frequency of flash flooding events in recent years leading to substantial property damages resulting from streambank erosion, river-connected landslides and deposition of sediment and debris along steep, small streams. Often these damage sites are on alluvial fans, where the channel gradient and valley confinement abruptly transition to join a wider-valley setting along a larger, low-gradient river.  The erosional and depositional hazards of alluvial fans are not currently represented on planning or regulatory maps (e.g., FEMA floodplain maps or VTANR River Corridor maps).  Historically, development has expanded on alluvial fans in rural communities, putting homes, businesses, roads and infrastructure in harm’s way.

In this project, we will address a critical gap for Vermont’s rural communities to produce statewide mapping of alluvial fans.  While the VT Geological Survey (VGS) has been systematically mapping alluvial fans as part of a broader surficial geology mapping effort for a handful of communities each year for a decade, most of the state remains unmapped at this detailed scale. We propose to use an innovative machine learning technique to accelerate the identification and mapping of alluvial fans. For rural communities, the project will produce maps that inform planning and zoning, to avoid future flood risks and economic losses by regulating development in these alluvial fan reaches with enhanced erosion/deposition hazard.  At county and basin scales, alluvial fan mapping will help regional and watershed planners to prioritize restoration and conservation projects that mitigate erosion/deposition hazards.  At the state level, alluvial fan mapping will inform updates to River Corridor mapping and rule-making required under Act 121, and support ongoing geologic mapping efforts of the VGS.  Ultimately, these products will contribute to economic wellbeing of Vermont communities by avoiding unnecessary flooding losses, thereby enabling repeat flood recovery expenses to be allocated instead toward other community needs. The partnership formalized through this project will strengthen community flood resilience and continue to co-create tools and guidance to meet Vermonters’ needs.

This project formalizes an ongoing partnership with the Vermont Geological Survey and VT Agency of Natural Resources Rivers Program to integrate alluvial fans more explicitly within community and regional hazard assessment and planning.  This project also engages undergraduate geography, geology and engineering students in service-learning projects to develop a workforce conversant in flood hazards and the prevalence of erosional/ depositional hazards across rural Vermont.