Volume Reduction of Highway Runoff in Urban Areas: Final Report and NCHRP Report 802 Appendices C through F (2015)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

C H A P T E R 1 Background Surface runoff volume reduction can be an important element of controlling stormwater impacts in urban areas when appropriate. The 2009 National Academy of Sciences (NAS) report, Urban Stormwater Management in the United States presented shortcomings in the current National Pollutant Discharge Elimination Systems (NPDES) program and provided suggestions to correct the identified deficiencies. The NAS report notes, “Presently … the regulation of stormwater is hampered by its association with a statute that focuses primarily on specific pollutants and ignores the volume of discharges.” The report notes that in an idealized regulatory system, “Future land-use development would be controlled to prevent increases in stormwater discharges from pre-development conditions, and impervious cover and volumetric restrictions would serve as a reliable proxy for stormwater loading from many of these developments.” The report also identified roads and parking lots as the most significant land use with respect to stormwater volumes and pollution. However due to a suite of constraints and other design restrictions associated with the urban highway environment, many volume reduction approaches (VRAs) are not applicable or must be used carefully in this application. The assessment of feasibility of VRAs in the urban highway environment must consider a broad suite of factors to ensure that the approaches do not lead to negative impacts to the highway infrastructure, human safety, surrounding infrastructure, groundwater supplies, and the environment. As a result, consideration and incorporation of VRAs can add complexity to transportation project planning, design, construction, operations, and maintenance. Consequently, the National Cooperative Highway Research Program (NCHRP) initiated this study to develop practical, technically defensible, and comprehensive guidelines for transportation agencies on implementing volume reduction strategies that are specific to limited access urban highways. Specific objectives of the project included: • The Guidance Manual should be applicable across a broad range of urban highway project types, site conditions, and climate zones. • The Guidance Manual should have sound technical bases and should build from the current state of the practice relative to VRAs. • The Guidance Manual should advance beyond an assessment of the applicability of conventional VRAs and consider innovative approaches that are specifically suited to the urban highway environment. • The Guidance Manual should be user-friendly and readily transferable in order to facilitate streamlined adoption and use by intended users. These objectives were addressed through a comprehensive guidance manual development process that involved literature review, technical analysis, development of numeric tools, and development of guidance manual elements (such as worksheets and flow charts). This effort culminated in the 3

development of the Guidance Manual titled Guidance for Achieving Volume Reduction of Highway Runoff in Urban Areas. References for More Information • Section 1 and 3 of the Guidance Manual provide more information on the background and context for achieving volume reduction of urban highway runoff. This information was included in the main body of the Guidance Manual instead of this report because is useful as a direct reference by a practitioner who is seeking to gain a better understanding of the regulatory and technical background for achieving volume reduction. • Each section of the report and technical annex provides additional topic-specific background information associated with the respective topic of that section or annex, as appropriate. Note, to avoid redundancy, this report does not reiterate the findings that are presented in the Guidance Manual. Rather, this report provides a brief summary of these findings and provides references to the sections of the Guidance Manual where a more complete presentation of findings can be found. Additionally, this report does not present findings beyond those presented in the Guidance Manual, therefore technical citations are not included in the report. For detailed technical findings and citations on a given topic, see the respective sections Guidance Manual. 4