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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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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.

2017 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 840 A Watershed Approach to Mitigating Stormwater Impacts Neil Weinstein Emily Clifton Low Impact DeveLopment center, Inc. Beltsville, MD Marie Venner venner consuLtIng, Inc. Lakewood, CO Marc Leisenring Daniel Pankani Charlie Wisdom geosyntec consuLtants Portland, OR Scott Struck geosyntec consuLtants Lafayette, CO Scott Taylor mIchaeL Baker InternatIonaL Carlsbad, CA Kevin Halsey ecometrIx soLutIons group Polebridge, MT Dan Nees sanDy poInt envIronmentaL Arnold, MD Subscriber Categories Highways • Environment • Hydraulics and Hydrology Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed research is the most effective way to solve many problems facing highway administrators and engineers. Often, highway problems are of local interest and can best be studied by highway departments individually or in cooperation with their state universities and others. However, the accelerating growth of highway transportation results in increasingly complex problems of wide inter- est to highway 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, United States Department of Transportation. 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 Academies is an insurance of objectivity; and TRB maintains a full-time staff of specialists in high- way 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 identified by chief administrators and other staff of the highway and transporta- tion departments and by committees of AASHTO. Topics of the highest merit are selected by the AASHTO Standing Committee on Research (SCOR), and each year SCOR’s recommendations are proposed to the AASHTO Board of Directors and the 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 Acad- emies 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 http://www.national-academies.org and then searching for TRB Printed in the United States of America NCHRP RESEARCH REPORT 840 Project 25-37 ISSN 0077-5614 ISBN 978-0-309-44615-0 Library of Congress Control Number 2016963724 © 2017 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, FMCSA, FRA, FTA, Office of the Assistant Secretary for Research and Technology, PHMSA, 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. 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; or the program sponsors. 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 appear herein solely because they are considered essential to the object of the report.

The 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. The 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. C. D. Mote, Jr., is president. The 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. The 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. The 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.national-academies.org. The Transportation Research Board is one of seven major programs of the National Academies of Sciences, Engineering, and Medicine. The mission of the Transportation Research Board is to increase the benefits that transportation contributes to society by providing leadership in transportation innovation and progress through research and information exchange, conducted within a setting that is objective, interdisciplinary, and multimodal. The Board’s varied committees, task forces, and panels annually engage about 7,000 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. The 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 840 Christopher J. Hedges, Director, Cooperative Research Programs Lori L. Sundstrom, Deputy Director, Cooperative Research Programs Gary Jenkins, Senior Program Assistant Eileen P. Delaney, Director of Publications Margaret B. Hagood, Editor NCHRP PROJECT 25-37 PANEL Field of Transportation Planning—Area of Impact Analysis Richard Gersib, Washington State DOT, Olympia, WA (Chair) Shane Cherry, Shane Cherry Consulting, Inc., Fort Myers, FL William B. Fletcher II, Oregon DOT, Salem, OR Mark Hemmerlein, New Hampshire DOT, Concord, NH Rachel Herbert, US Environmental Protection Agency, Washington, DC Terry L. Johnson, Logan, UT Sonal Sanghavi, Maryland State Highway Administration, Baltimore, MD Gayle A. Unruh, Missouri DOT, Jefferson City, MO Patricia A. Cazenas, FHWA Liaison Susan Jones, FHWA Liaison Kate Kurgan, AASHTO Liaison Christine Gerencher, TRB Liaison

NCHRP Research Report 840: A Watershed Approach to Mitigating Stormwater Impacts provides a practical watershed-based decision-making framework and spreadsheet- based Watershed-Based Stormwater Mitigation Toolbox that will enable state depart- ments of transportation (DOTs) to identify and implement offsite cost-effective and environmentally beneficial water quality solutions for stormwater impacts when onsite treatment and/or mitigation is not possible within the right-of-way. The report and toolbox—which identifies other useful resources—should be of immediate use to storm- water managers and roadway drainage engineers who have to mitigate water quality impacts from roadway stormwater runoff in order to comply with state and federal water quality requirements. State DOTs are sometimes unable to meet stormwater management requirements onsite or in close proximity to the impacted area; therefore, additional approaches to accomplish- ing mitigation requirements are needed. By focusing on watershed protection, stormwater pollutant reduction, and hydrologic impairment, state DOTs and natural resource agen- cies that are interested in exploring approaches that include off-site mitigation and non- traditional techniques and strategies may find that equivalent or greater environmental benefits can be achieved in a cost-effective manner. State DOTs’ stormwater manage- ment programs require new tools in order to cost-effectively achieve compliance with water quality permits that are derived from watershed-based Total Maximum Daily Load limitations and other requirements included in stormwater permits that are issued to state DOTs. Under NCHRP Project 25-37, Low Impact Development Center, Inc. was asked to pro- vide a comparative decision-making framework and toolbox to enable stormwater man- agers to identify feasible water quality solutions within a watershed. Using the watershed approach developed by the U.S. EPA (available at https://www.epa.gov/npdes/npdes- stormwater-program), the framework accommodates varying degrees of extant watershed planning, is applicable in a variety of watershed types, and includes methods to demonstrate the net environmental benefit of potential water quality solutions. The Watershed-Based Stormwater Mitigation Toolbox considers conditions under which conventional onsite best management practices (BMPs) are infeasible or inadequate, contains conventional BMPs and mitigation options that augment conventional BMPs, offsite water quality solutions (e.g., outside of the project area or right-of-way), and solutions that mitigate water quality impacts from existing and new impervious surfaces. To find the toolbox, go to the TRB website and search for NCHRP Research Report 840. F O R E W O R D By Lori L. Sundstrom Senior Program Officer Transportation Research Board

When site constraints simply do not allow appropriate onsite treatment of highway stormwater runoff to meet water quality requirements, this guidance and toolbox provides state DOT stormwater managers and their counterparts at state and federal natural resource agencies with a means of identifying solutions that can achieve regulatory compliance and deliver superior environmental outcomes in a cost-effective manner.

1 Summary 3 Chapter 1 Introduction 3 Project Overview 4 Institutional and Cost Implications 5 Report Organization 5 About the WBSMT 6 Chapter 2 Watershed-Based Mitigation Datasets 6 Background 6 Understanding the Basic Data Needs 7 Data Selection Criteria 10 Dataset Analysis 11 Datasets Used in the Toolbox 13 Limitations of Datasets and Models 15 Chapter 3 Methods to Develop Mitigation Options 15 Defining Overall Stormwater Management Objectives 17 Criteria for Selection of In-Kind BMPs 30 Out-of-Kind Watershed-Based Approaches 36 Chapter 4 Framework for Evaluating Off-Site Mitigation Options 36 Background 36 Stormwater Impacts and Pollutant Loads from Impervious Surfaces 38 Baseline Physical Watershed Processes and Functions 39 Linkages of Stormwater Impacts and Physical Watershed Processes 43 Mitigation Options 46 Developing Mitigation Equivalencies 52 Identifying Candidate Off-Site Mitigation Options 58 Chapter 5 Using Local Planning Information to Support the Mitigation Decision Process 58 Extant Planning 59 Adequacy and Utility of Extant Planning 60 Identifying and Locating Extant Plans 63 Utilizing Extant Plans with the WBSMT 64 Chapter 6 Linking Project Impacts to Mitigation Alternatives in the WBSMT 64 Determining Off-Site Mitigation Options in the WBSMT 65 Watershed Characterization 69 Defining Load Reduction Targets 69 Conducting On-Site BMP Assessment C O N T E N T S

71 Off-Site Assessment (In-Kind Mitigation) 73 Off-Site Assessment (Out-of-Kind Mitigation) 75 Analysis of Mitigation Efforts 78 Reporting 79 Chapter 7 Watershed-Based Mitigation Toolbox Case Study 79 Case Study Example: Road Widening in Pierce County, WA 79 Assigning Project Parameters 89 Effects of Altering Project Locations 94 Chapter 8 Conclusions 95 References 100 Abbreviations and Acronyms 102 Glossary 104 Appendix A Out-of-Kind Mitigation Assessment Approach Note: Photographs, figures, and tables in this report may have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.trb.org) retains the color versions.

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TRB's National Cooperative Highway Research Program (NCHRP) Research Report 840: A Watershed Approach to Mitigating Stormwater Impacts provides a practical decision-making framework that will enable state departments of transportation (DOTs) to identify and implement offsite cost-effective and environmentally beneficial water quality solutions for stormwater impacts when onsite treatment and/or mitigation is not possible within the right-of-way.

The report is accompanied by the Watershed-Based Stormwater Mitigation Toolbox, a Microsoft Excel-based program to facilitate the characterization of the project watershed and the identification of mitigation options at the planning level.

Disclaimer - This tool is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences, Engineering, and Medicine or the Transportation Research Board (collectively "TRB") be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

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