Watershed Approach to Mitigating Hydrologic Impacts of Highway Projects

Departments of transportation are under pressure to address the adverse impacts of larger stormwater volume discharges from increasing impervious surface area (aka hydromodification). Development and more impervious surfaces in a watershed results in a change to the storm hydrograph, increasing runoff volumes, heightening peak flows, and shortening the duration of the runoff event. This change increases flooding, triggers stream channel change, accelerates stream bank and bed erosion, increases sediment loads and turbidity, harms aquatic habitat, and reduces low flow discharges. While the U.S. Environmental Protection Agency (EPA) has not promulgated federal rules to regulate stormwater volume directly as a pollutant, Endangered Species Act (ESA) compliance conditions, some state-issued NPDES MS4 permits, and some state regulations do have hydromodification requirements. For example, in Washington state projects must reestablish pre-development hydrology, and in Oregon conditions of ESA Biological Opinions for highway projects require maintaining pre-project hydrology for channel forming flows. The traditional technique for addressing hydrologic impacts of highway projects is to construct detention and retention basins. While the basins are effective at reducing peak flows and flood issues, they are less well suited to maintaining or reestablishing relatively “natural” hydrology for streams, and may in fact lead to local degradation. They also require considerable right-of-way adjacent to highways, add to the maintenance burden, and can become attractive nuisances. Full infiltration of stormwater discharges on-site is frequently not feasible due to physical constraints. Other approaches, such as using compost amended vegetated filter strips for volume reduction, are promising, but still require right-of-way that may not be available. Various elements of the landscape, primarily wetlands, forests and floodplains, reduce, delay and desynchronize peak flows from precipitation, and are important elements in establishing the natural hydrology of a watershed. While individually these elements have on occasion been used to mitigate for project impacts, there has not been a broader examination of, or methodology for, an integrated approach to mitigating for highway or development impacts on surface water hydrology at the watershed scale. This project will investigate the capability of landscape modifications elsewhere within a watershed to off-set the hydrologic effects of the increase in impervious surface and changes in drainage associated with highway improvement projects. Examples include floodplain reconnection, use of strategically placed and designed created or restored wetlands, and reforestation. The goal of the research is to identify effective actions, develop techniques for quantifying the benefits of the actions and assessing their ability to compensate for project and system impacts, and develop guidance for evaluating and selecting the appropriate approach for the affected watersheds. It will be useful for project mitigation, advance mitigation and watershed enhancement efforts. This research will be a companion to NCHRP Research Report 840: A Watershed Approach to Mitigating Stormwater Impacts. The proposed project is intended to identify elements of the landscape in a watershed that can be enhanced or created to offset adverse hydromodification, assess the feasibility and effectiveness of creating or enhancing those elements, and develop a methodology for mitigating highway project and system impacts. The research is anticipated to provide a practical methodology and guidance for: (1) identifying opportunities in a watershed to enhance or create landscape features that moderate runoff or storm flows and can be used to offset hydrologic impacts of highway facilities; (2) designing created or enhanced landscape features for hydromodification mitigation; (3) quantifying hydrologic changes from creating or enhancing landscape features; (4) analyzing the effectiveness of modification of specific landscape features, singly or in concert, to offset highway hydrologic impacts. Work on the project to include identifying and evaluating hydrologic models and techniques for their suitability for the project’s goals. Testing of the methodology can demonstrate the practicality and accuracy of the results. Depending on the approach taken by the researcher, software or a model to support decision making and quantification of the hydrologic impacts of mitigation alternatives may be developed. The final product should be aimed at DOT hydraulic engineers, environmental staff, and planners, but would also be useful for city and counties responsible for watershed improvements.  


  • English


  • 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: 20180709
  • 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