Transportation Research Center Research Reports

Document Type

Report

Publication Date

8-31-2020

Abstract

Floodplains perform many functions of value to society, including conveyance and storage of floodwaters for reduced downstream impacts, sediment and nutrient deposition to support soil formation, and maintenance of pulsed overbank flows to support diverse habitats. When constructed along Vermont’s river valleys in the mid-to-late 1800s, railroads often isolated large areas of natural floodplain, leading to decreased flood and sediment storage, and increased downstream flood stages, sediment and nutrient delivery. Where rail lines have been federally-banked and converted to recreational trails, floodplain reconnection could be achieved by modifying the rail embankment through lowering or installing cross culverts or bridges. With the Lamoille Valley Rail Trail (LVRT) in the Lamoille and Missisquoi River basins as a focal study area, this research has generated tools and planning frameworks for transportation and river managers to identify and prioritize candidate reconnection sites, and to holistically evaluate the benefits of these projects alongside potential impacts to adjacent infrastructure or land uses. Effectiveness of completed and proposed floodplain reconnection sites along the LVRT was evaluated at various spatial scales using a suite of tools. At the watershed and reach scales, a screening protocol was developed, leveraging stream geomorphic assessment data to prioritize potential floodplain reconnection sites for further vetting through field inspection. Ten out of twelve floodplain reconnection sites completed along the LVRT in 2006-2008 were predicted as a priority in a retrospective application of this screening protocol. Low-complexity (Height Above Nearest Drainage) hydraulic modeling results confirmed that most completed projects provided significant increases in the floodplain capacity for floods of 2- to 500-year recurrence intervals. Event-scale monitoring conducted at selected sites has confirmed accumulation of fine sediment and phosphorus. A conservative estimate of a half-ton of phosphorus deposited during one storm on 57 acres highlights the water quality benefits of restoring floodplains. Reconnection alternatives were evaluated in more detail using two-dimensional hydraulic modeling (2D HEC-RAS) at a demonstration reach of the Black Creek near East Fairfield spanning two completed reconnection sites and one proposed site on the LVRT. Modeled reconnection alternatives resulted in modest changes in flooding parameters due to an unexpected, existing degree of cross connection between floodplains of the Black Creek and Elm Brook tributary. Nevertheless, this research project has created a framework for more holistic analysis of floodplain reconnection opportunities at similar sites across Vermont and beyond. The hydraulic modeling products and scenarios developed for this project are being adapted to support analysis and modeling of fine sediment and phosphorus attenuation as the Vermont Agency of Transportation continues to collaborate with the Vermont Agency of Natural Resources and other stakeholders to develop a phosphorus-crediting framework for floodplain reconnection projects.

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