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.
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:
- Identify opportunities in a watershed to conserve, restore, enhance, or create landscape features for hydrologic mitigation;
- Plan, site, and design features for hydrologic mitigation;
- Quantify the hydrologic outcomes from selected features; and
- Develop strategies for implementing a watershed-based approach to hydrologic mitigation.
STATUS: Research is complete. NCHRP Research Report 1011: Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide is available here: DOI: 10.17226/26762 Accompanying the Guide is a spreadsheet tool for use in applying the approach, a conduct of research report, a set of presentation slides summarizing the project, and a memo describing potential pathways for implementation of the research.