Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide (2022)

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2022 N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP RESEARCH REPORT 1011 Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects GUIDE Roger Kilgore Kilgore Consulting and Management Denver, CO Alan Leak Paul Hummel Paul Duda Michael Barzaghi RESPEC Company, LLC Denver, CO Maura Flight Megan Sheahan Christopher Lewis Brent Boehlert IEc Cambridge, MA Blake Meinecke Lee Cook Quest Ecology, Inc. Wimauma, FL Ted Cleveland Lubbock, TX David Sample Virginia Beach, VA Subscriber Categories Environment Research sponsored by the American Association of State Highway and Transportation Ofcials in cooperation with the Federal Highway Administration

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state departments of transportation (DOTs) administrators and engineers. Often, highway problems are of local or regional interest and can best be studied by state DOTs individually or in cooperation with their state universities and others. However, the accelerating growth of highway transporta- tion results in increasingly complex problems of wide interest to high- way authorities. These problems are best studied through a coordinated program of cooperative research. Recognizing this need, the leadership of the American Association of State Highway and Transportation Officials (AASHTO) in 1962 ini- tiated an objective national highway research program using modern scientific techniques—the National Cooperative Highway Research Program (NCHRP). NCHRP is supported on a continuing basis by funds from participating member states of AASHTO and receives the full cooperation and support of the Federal Highway Administration (FHWA), United States Department of Transportation, under Agree- ment No. 693JJ31950003. The Transportation Research Board (TRB) of the National Academies of Sciences, Engineering, and Medicine was requested by AASHTO to administer the research program because of TRB’s recognized objectivity and understanding of modern research practices. TRB is uniquely suited for this purpose for many reasons: TRB maintains an extensive com- mittee structure from which authorities on any highway transportation subject may be drawn; TRB possesses avenues of communications and cooperation with federal, state, and local governmental agencies, univer- sities, and industry; TRB’s relationship to the National Academies is an insurance of objectivity; and TRB maintains a full-time staff of special- ists in highway transportation matters to bring the findings of research directly to those in a position to use them. The program is developed on the basis of research needs iden- tified by chief administrators and other staff of the highway and transportation departments, by committees of AASHTO, and by the FHWA. Topics of the highest merit are selected by the AASHTO Special Committee on Research and Innovation (R&I), and each year R&I’s recommendations are proposed to the AASHTO Board of Direc- tors and the National Academies. Research projects to address these topics are defined by NCHRP, and qualified research agencies are selected from submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Academies and TRB. The needs for highway research are many, and NCHRP can make significant contributions to solving highway transportation problems of mutual concern to many responsible groups. The program, however, is intended to complement, rather than to substitute for or duplicate, other highway research programs. Published research reports of the NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM are available from Transportation Research Board Business Office 500 Fifth Street, NW Washington, DC 20001 and can be ordered through the Internet by going to https://www.mytrb.org/MyTRB/Store/default.aspx Printed in the United States of America NCHRP RESEARCH REPORT 1011 Project 25-60 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-68731-7 Library of Congress Control Number 2022944417 © 2022 by the National Academy of Sciences. National Academies of Sciences, Engineering, and Medicine and the graphical logo are trade- marks of the National Academy of Sciences. All rights reserved. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FTA, GHSA, NHTSA, or TDC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. Cover photo credit: Courtesy of Ann M. Hartell NOTICE The research report was reviewed by the technical panel and accepted for publication according to procedures established and overseen by the Transportation Research Board and approved by the National Academies of Sciences, Engineering, and Medicine. The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research and are not necessarily those of the Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; the FHWA; or the program sponsors. The Transportation Research Board does not develop, issue, or publish standards or speci- fications. The Transportation Research Board manages applied research projects which provide the scientific foundation that may be used by Transportation Research Board sponsors, industry associations, or other organizations as the basis for revised practices, procedures, or specifications. The Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; and the sponsors of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturers’ names or logos appear herein solely because they are considered essential to the object of the report.

e National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, non- governmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. e National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. John L. Anderson is president. e National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. e three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. e National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.nationalacademies.org. e Transportation Research Board is one of seven major programs of the National Academies of Sciences, Engineering, and Medicine. e mission of the Transportation Research Board is to provide leadership in transportation improvements and innovation through trusted, timely, impartial, and evidence-based information exchange, research, and advice regarding all modes of transportation. e Board’s varied activities annually engage about 8,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. e program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation. Learn more about the Transportation Research Board at www.TRB.org.

C O O P E R A T I V E R E S E A R C H P R O G R A M S CRP STAFF FOR NCHRP RESEARCH REPORT 1011 Christopher J. Hedges, Director, Cooperative Research Programs Waseem Dekelbab, Deputy Director, Cooperative Research Programs, and Manager, National Cooperative Highway Research Program Sid Mohan, Associate Program Manager, Implementation and Technology Transfer, National Cooperative Highway Research Program Ann M. Hartell, Senior Program Officer Dajaih Bias-Johnson, Senior Program Assistant Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications Kami Cabral, Editor NCHRP PROJECT 25-60 PANEL Field of Transportation Planning—Area of Human and Natural Environment Michael Brad McManus, Georgia Department of Transportation, Atlanta, GA (Chair) Sajjad Ahmad, University of Nevada, Las Vegas, Las Vegas, NV Gillian Davies, BSC Group, Inc., Worcester, MA William B. Fletcher, Oregon Department of Transportation (retired), Portland, OR Garrett Jackson, Washington State Department of Transportation, Olympia, WA Geil E. Rios-Vidal, AECOM, Washington, DC Stephen M. Sisson, Delaware Department of Transportation, Dover, DE Michael A. Stevens, King County (WA), Seattle, WA Nicklas Tiedeken, Minnesota Department of Transportation, St. Paul, MN Joe S. Krolak, FHWA Liaison Melissa Anderson Savage, AASHTO Liaison Christine L. Gerencher, TRB Liaison

NCHRP Research Report 1011: Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide describes a watershed-level approach to identify strate- gies and techniques that can mitigate hydrologic impacts of transportation projects. The Guide will be of interest to those at state departments of transportation (DOTs) who are responsible for environmental planning, project development, stormwater management, and stormwater regulatory compliance. State DOTs are facing increasingly stringent requirements for treating not only the quality but also the quantity of stormwater runoff. Increased quantity of stormwater runoff can be the result of increased impervious surfaces and other land-use changes; stream channeliza- tion; soil compaction; or changes to the precipitation regime (frequency and intensity of storms). These changes alter the storm hydrograph, increasing runoff volumes, heighten- ing peak flows, and shortening 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. 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. In addition, where right-of-way is limited, it may be infeasible to construct basins of sufficient size to mitigate hydrologic impacts. An alter- native approach to these on-site mitigation techniques is to consider techniques that can be used outside the right-of-way that address hydrologic conditions at the watershed scale. At the watershed level, hydrologic impacts can be mitigated by techniques such as recon- necting floodplains; reforestation; and conserving, constructing, or restoring wetlands. These techniques can reduce, delay, and desynchronize peak flows. These techniques also offer valuable co-benefits such as providing recreational opportunities; habitat for fish and wild- life; and climate resiliency for nearby communities. Under NCHRP Project 25-60, “Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects,” Kilgore Consulting and Management was tasked to develop a watershed-level approach to planning and implementing strategies that mitigate hydro- logic impacts of transportation projects. The project team first defined a decision frame- work for adopting a watershed-level approach. To support the decision framework, the team developed an analysis tool that can be used to identify and assess the hydrologic effec- tiveness of mitigation techniques—as well as anticipated co-benefits—for a site or several sites in a watershed. This Guide presents the decision framework and detailed information F O R E W O R D By Ann M. Hartell Staff Officer Transportation Research Board

about how to apply the tool using three example applications. Additional project deliverables include a downloadable spreadsheet with details on how to use the tool; a set of presentation slides summarizing the project; and NCHRP Web-Only Document 333: Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Conduct of Research Report documenting the background research and technical work that underpins the Guide and the analysis tool. These deliverables are available on the National Academies Press website (www.nap.edu) by searching NCHRP Research Report 1011.

1 Chapter 1 Introduction 1 1.1 Mitigation Techniques within the Watershed 4 1.2 Guide Organization 5 Chapter 2 Watershed Mitigation Decision Framework 12 Chapter 3 Screening Analyses 12 3.1 Hydrologic Screening (Step 8A) 17 3.2 Co-Benefits Screening (Step 8B) 25 Chapter 4 Detailed Analyses (Step 11) 25 4.1 Detailed Hydrologic Analysis (Step 11A) 28 4.2 Detailed Co-Benefits Analysis (Step 11B) 30 Chapter 5 Piney Creek Example 31 5.1 Screening Analyses (Step 8) 41 5.2 Detailed Analyses (Step 11) 54 Chapter 6 Summary 57 References 59 Acronyms and Abbreviations 61 Glossary 63 Appendix A Mitigation Tables 67 Appendix B Generic Causal Chains 72 Appendix C Site-Specific Factors Related to Co-Benefits C O N T E N T S