Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects

State departments of transportation (DOTs) are facing increasingly stringent requirements for treating not only the quality but also the quantity of stormwater runoff. State DOTs are required to address project-related hydrologic volume impacts under the Endangered Species Act (ESA) and the Clean Water Act, and in some cases, state and local regulations. For example, in Oregon, ESA Biological Opinions for highway projects require maintaining pre-project hydrology for channel-forming flows; in King County, Washington, projects must reestablish pre-development hydrology. A common technique for addressing stormwater runoff impacts of highway projects is to construct on-site facilities such as detention and retention basins. While these facilities are effective at reducing peak flows and flooding, they are less suited to maintaining or reestablishing predevelopment hydrology. They also add to the transportation agency’s maintenance burden and can become attractive nuisances. Further, when available right-of-way adjacent to the highway is limited, it may not be feasible to construct basins of adequate size to fully treat the runoff. NCHRP Report 840: A Watershed Approach to Mitigating Stormwater Impacts addressed this issue with guidance on adopting a watershed-based approach, defined as “on- or off-site stormwater mitigation measures that take place within the same watershed in order to address the direct impacts of the project.” However, the guidance focuses on strategies to address water quality, not stormwater runoff quantity. Increased quantity of stormwater runoff can be a result of increased impervious surfaces and other land use changes, stream channelization, soil compaction, or changes to the precipitation regime (frequency and intensity of storms). These changes alter the storm hydrograph with increased runoff volumes, heightened peak flows, and shortened durations of the runoff event. Hence, hydrologic impacts can increase flooding, trigger and accelerate stream channel change, increase sediment loads and turbidity, alter groundwater recharge, harm aquatic habitat, and reduce low-flow discharges. There are a number of effective techniques for mitigating hydrologic change. For example, the impacts of increased impervious surface area can be mitigated by techniques such as reconnecting floodplains; reforestation; or conserving, constructing, or restoring wetlands. These techniques can reduce, delay, and desynchronize peak flows. These techniques also offer important co-benefits, such as providing recreational opportunities and fish and wildlife habitat. However, to be most effective, techniques for addressing runoff volumes require a watershed-based strategy that combines specific techniques into an overall plan for managing the hydrologic impacts from transportation facilities. Research is needed to develop a fuller array of effective strategies and techniques for managing increased stormwater runoff volumes and mitigating hydrologic impacts associated with transportation projects. As regulatory requirements increasingly address stormwater runoff quantity, state DOTs need information on a range of techniques that can support effective watershed-based strategies. The research and resulting guidance will support the efforts of engineers, environmental staff, and planners at state DOTs and their regulatory and other partners with flexible approaches to regulatory compliance and improved environmental outcomes. The objective of this research is to develop guidance for state DOTs for developing and implementing watershed-based strategies and techniques for mitigating hydrologic impacts of transportation facilities. The results of the research will be guidance on how to: (1) Identify opportunities in a watershed to conserve, restore, enhance, or create landscape features for hydrologic mitigation; (2) Plan, site, and design features for hydrologic mitigation; (3) Quantify the hydrologic outcomes from selected features; and (4) Develop strategies for implementing a watershed-based approach to hydrologic mitigation.

Language

  • English

Project

  • Status: Proposed
  • Funding: $500000
  • Contract Numbers:

    Project 25-60

  • Sponsor Organizations:

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Project Managers:

    Hartell, Ann

  • Start Date: 20190319
  • Expected Completion Date: 0
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01672566
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 25-60
  • Files: TRB, RiP
  • Created Date: Jun 18 2018 3:03PM