Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual (2015)

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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 REPORT 802 Volume Reduction of Highway Runoff in Urban Areas Guidance Manual Eric Strecker Aaron Poresky Robert Roseen Ronald Johnson Jane Soule Venkat Gummadi Raina Dwivedi Adam Questad Geosyntec consultants Portland, OR Neil Weinstein Emily Ayers low Impact Development center Beltsville, MD Marie Venner venner consultInG Littleton, CO Subscriber Categories Design  •  Environment  •  Hydraulics and Hydrology TRANSPORTAT ION RESEARCH BOARD WASHINGTON, D.C. 2015 www.TRB.org  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 provides the most effective approach to the solution of 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 develops increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of cooperative research. In recognition of these needs, the highway administrators of the American Association of State Highway and Transportation Officials initiated in 1962 an objective national highway research program employing modern scientific techniques. 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Research projects to fulfill these needs are defined by the Board, and qualified research agencies are selected from those that have submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Research Council and the Transportation Research Board. The needs for highway research are many, and the National Cooperative Highway Research Program can make significant contributions to the solution of 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 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 at: http://www.national-academies.org/trb/bookstore Printed in the United States of America NCHRP REPORT 802 Project 25-41 ISSN 0077-5614 ISBN 978-0-309-30845-8 Library of Congress Control Number 2015932942 © 2015 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, FTA, or Transit Development Corporation 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 project that is the subject of this report was a part of the National Cooperative Highway Research Program, conducted by the Transportation Research Board with the approval of the Governing Board of the National Research Council. The members of the technical panel selected to monitor this project and to review this report were chosen for their special competencies and with regard for appropriate balance. The 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 Governing Board of the National Research Council. 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 Research Council, or the program sponsors. The Transportation Research Board of the National Academies, the National Research Council, 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.

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 REPORT 802 Christopher W. Jenks, Director, Cooperative Research Programs Christopher Hedges, Manager, National Cooperative Highway Research Program Sheila A. Moore, Program Associate Eileen P. Delaney, Director of Publications Doug English, Editor NCHRP PROJECT 25-41 PANEL Area Twenty-Five: Transportation Planning—Impact Analysis G. Michael Fitch, Virginia DOT, Charlottesville, VA (Chair) Daryoush D. “David” Ahdout, New Jersey DOT, Trenton, NJ Mark W. Maurer, Washington State DOT, Olympia, WA Kristin A. Schuster, Michigan DOT, Lansing, MI Matthew J. “Matt” Sunderland, Illinois DOT, Springfield, IL Scott Taylor, RBF Consulting, Carlsbad, CA Meredith Upchurch, District of Columbia DOT, Washington, DC Paul R. Wirfs, Oregon DOT, Salem, OR Brian L. Beucler, FHWA Liaison Stephen F. Maher, TRB Liaison AUTHOR ACKNOWLEDGMENTS This guidance manual was developed under NCHRP Project 25-41 by Geosyntec Consultants, with assis- tance from the Low Impact Development Center and Venner Consulting. Primary authors are Eric Strecker (Principal Investigator) and Aaron Poresky (Co-Principal Investigator) of Geosyntec Consultants, and Neil Weinstein and Emily Ayers of the Low Impact Development Center. The primary authors would like to recognize the contributions of the following additional authors and reviewers: Adam Questad, Andrea Braga, Jane Soule, Lucas Nguyen, Raina Dwivedi, Renee Bordeau, Robert Roseen, Ron Johnson, and Venkat Gummadi of Geosyntec Consultants; Lilantha Tennekoon of the Low Impact Development Center; and Marie Venner of Venner Consulting. The authors would also like to acknowledge the Washington Department of Transportation (Mark Maurer, Alex Nguyen, Le Nguyen, and Ebrahim Sahari) and the District of Columbia Department of Transportation (Meredith Upchurch, Reginald Arno, Kyle Ohlson, Alit Balk, and Carmen Franks) for their assistance with trial applications of the manual.

The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. C. D. Mote, Jr., is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Victor J. Dzau is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. C. D. Mote, Jr., are chair and vice chair, respectively, of the National Research Council. The Transportation Research Board is one of six major divisions of the National Research Council. The mission of the Transporta- tion Research Board is to provide 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 activities 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 individu- als interested in the development of transportation. www.TRB.org www.national-academies.org

This guidance manual provides practical, research-based evaluation and implementation practices for the reduction of stormwater volumes in urban highway environments. The manual outlines a five-step process for the identification, evaluation, and design of feasible solutions for runoff volume reduction based on site-specific conditions. It is accompanied by a CD-ROM containing a Volume Performance Tool to assist the user in efficiently esti- mating the performance of volume reduction approaches and understanding the effects and sensitivity of local climate patterns, design attributes, and site conditions. The manual also includes a set of volume reduction approach fact sheets and a user guide for the Volume Performance Tool. This guidance manual will be useful to DOT managers, project staff and design engineers, permit writers, consultants, and planners. Reduction of stormwater has long been recognized as an effective method for control- ling the impacts of urbanization on water resources. Key benefits of reducing stormwater volume can include: (1) reducing pollutant loads to receiving waters, (2) reducing potential for channel erosion, (3) increasing groundwater recharge and augmenting water supply and stream base flow, and (4) reducing peak runoff flow rates. Implementing volume reduc- tion approaches (VRAs) in a highly urban setting presents a number of challenges and constraints due to the limited space and lack of appropriate soils for typical stormwater management practices such as infiltration, evapotransiration, on-site use, and flow control. Under NCHRP Project 25-41, a research team led by Geosyntec Consultants developed a five-step process for runoff volume reduction. The steps include: (1) establish volume reduction goals, (2) characterize the project site and watershed, (3) identify potentially suitable VRAs, (4) prioritize VRAs, and (5) select VRAs and develop conceptual designs. The manual was developed based on an extensive literature review, synthesis of available information, and focused technical analysis. The accompanying Volume Performance Tool is an Excel-based spreadsheet application that calculates an estimate of long-term volume reduction based on user-provided location and planning-level project information. The project final report and appendices are avail- able electronically on the TRB website as NCHRP Web-Only Document 209. By Christopher Hedges Staff Officer Transportation Research Board F O R E W O R D

1 Chapter 1  Introduction 1 1.1 Statement of Purpose 2 1.2 Regulatory and Policy Background 3 1.3 Intended Users and Uses 4 1.4 Organization of the Guidance Manual 5 1.5 Limitations 7 Chapter 2   Stepwise Approach for Incorporating Volume  Reduction in Urban Highway Projects:  How to Use This Manual 7 2.1 Example Approach and Corresponding Manual Resources 10 2.2 Advantages of a Systematic Approach for Incorporating Volume Reduction 11 Chapter 3   Volume Reduction in the Urban Highway  Environment 11 3.1 Regulatory Context 11 3.1.1 Current Volume Reduction Mandates and Trends in Stormwater Management Regulations 16 3.1.2 Other Design Objectives Within the Highway Project Development Process 18 3.2 Key Technical Considerations in Applying Stormwater Volume Reduction Practices 18 3.2.1 Volume Reduction Metrics 19 3.2.2 Volume Reduction Processes 22 3.2.3 Physical Setting and Site Design Factors Influencing Volume Reduction Effectiveness, Feasibility, and Desirability 27 3.3 Urban Highway Types 27 3.3.1 Project Attributes and Types 28 3.3.2 Ground-Level Highway Segments 29 3.3.3 Ground-Level Highway Segments with Restricted Cross-Sections 31 3.3.4 Highway Segments with Steep Transverse Slopes 32 3.3.5 Depressed Highway Segments 34 3.3.6 Elevated Highway Segments on Embankments 35 3.3.7 Elevated Highway Segments on Viaducts 36 3.3.8 Diamond Interchanges 38 3.3.9 Looped Interchanges (Also Known as Cloverleaf Intersections) 39 3.4 Site Assessment Activities to Support Volume Reduction Planning and Design 40 3.4.1 Phasing of Site Assessment Activities 42 3.4.2 Topography and Drainage Patterns 43 3.4.3 Off-Site Drainage and Adjacent Land Uses 44 3.4.4 Soil and Geologic Conditions 44 3.4.5 Local Weather Patterns C O N T E N T S

45 3.4.6 Groundwater Considerations 46 3.4.7 Geotechnical Considerations 47 3.4.8 Existing Utilities 47 3.4.9 Harvested-Water–Demand Assessment 47 3.4.10 Responsible Agencies and Other Stakeholders 48 3.4.11 Local Ordinances 48 3.4.12 Watershed-Based and Other Joint Planning Opportunities 49 Chapter 4   Volume Reduction Approaches 49 4.1 Identification of Potential VRAs from Stormwater Guidance and Literature 49 4.1.1 Inventory of SCMs in Highway Guidance and Literature 51 4.1.2 Relative Frequency of Application of SCMs by State DOTs 52 4.1.3 Recent Research and Emerging Concepts in Volume Reduction Approaches for Urban Highways 60 4.2 Menu of Volume Reduction Approaches 61 4.2.1 Primary Volume Reduction Approaches 61 4.2.2 Other Potential Volume Reduction Concepts and Approaches 65 4.2.3 Site Planning Approaches to Reduce Runoff Volume 69 4.3 Summary of VRA Attributes and Considerations 70 4.3.1 Summary of Relative Volume Reduction Mechanisms and Potential Water Balance Issues by VRA 70 4.3.2 Summary of Geometric Siting Opportunities and Footprint Requirements by VRA 72 4.3.3 Summary of Potential Geotechnical Impacts Associated with Classes of VRAs 73 4.3.4 Summary of Relative Potential Risk of Groundwater Quality Impacts Associated with VRAs 76 4.3.5 Summary of Safety Considerations by VRA 76 4.3.6 Summary of Maintenance Activities by VRA 78 4.3.7 Summary of Relative Whole Life-Cycle Costs by VRA 83 4.4 Additional References for VRA Design and Maintenance Information 83 4.4.1 Selected Nationwide Guidance 84 4.4.2 Selected State-Specific DOT Guidance 86 Chapter 5   Selecting and Applying Volume  Reduction Approaches 86 5.1 Framework for Selecting and Applying Volume Reduction Approaches 86 5.1.1 Considerations in Adopting a Custom Planning Framework Versus a Uniform Planning Framework 87 5.1.2 Overview of Framework 89 5.2 Initial Screening to Identify Potential VRAs (Step 3) 90 5.2.1 Step 3a—Develop Site Layout and Geometric Design 90 5.2.2 Step 3b—Evaluate VRA Applicability for Project Type and Site Design 91 5.2.3 Step 3c—Feasibility and Desirability Screening 107 5.2.4 Combined Results of Initial Screening Processes 107 5.3 Prioritizing Approaches from the Screened Menu of VRAs 108 5.3.1 Overview of Prioritization Method 108 5.3.2 Considerations in Prioritizing VRAs 111 5.3.3 Semi-Quantitative Prioritization of VRAs

111 5.4 Conceptual Design Development 112 5.4.1 Overview of Framework for Conceptual Design Development and Evaluation 113 5.4.2 Developing Initial Conceptual Designs 114 5.4.3 Using Modeling Tools for Decision Support and Conceptual Design Adaptation 116 5.4.4 Introduction to the Volume Performance Tool 118 5.4.5 Estimating Whole Life-Cycle Costs of Conceptual Designs 123 5.4.6 Adapting Conceptual Designs to Converge with Project Goals and Constraints 125 5.5 Watershed-Scale Approaches 128  References A-1 Appendix A  Volume Reduction Approach Fact Sheets B-1 Appendix B  User’s Guide for the Volume Performance Tool C-1 Appendices C–F

BMPs: Best management practices CONUS: Conterminous United States CWA: Clean Water Act DCIA: Directly connected impervious area DDOT: District of Columbia Department of Transportation DOT: Department of transportation EISA: Energy Independence and Security Act ESA: Endangered Species Act ET: Evapotranspiration ETo: Reference evapotranspiration FS: Factor of safety IWS: Internal water storage LID: Low-impact development MAP-21 Act: Moving Ahead for Progress in the 21st Century Act MCTI: Multi-chamber treatment train MEP: Maximum extent practicable MS4: Municipal separate storm sewer system NCDC: National Climatic Data Center NPDES: National Pollutant Discharge Elimination System NPV: Net present value NRC: National Research Council O&M: Operations and maintenance ODOT: Oregon Department of Transportation PFC: Permeable friction course ROW: Right-of-way SCMs: Stormwater control measures SWMM: Storm Water Management Model TMDLs: Total maximum daily loads U.S. EPA: United States Environmental Protection Agency USGS: United States Geological Survey VRAs: Volume reduction approaches WEF: Water Environment Federation WERF: Water Environment Research Foundation WLAs: Waste-load allocations WLC: Whole life-cycle costs WSDOT: Washington State Department of Transportation Acronyms and Abbreviations

Agronomic Demand—The amount of irrigation required to meet plant water needs. In con- trast, irrigation that exceeds agronomic demand would be expected to evaporate as standing water, infiltrate below the root zone of plants, or run off via overland flow. Average Annual Capture Efficiency (also known as capture efficiency)—The estimated percent of long-term average annual runoff volume that is managed/controlled by a stormwater control measure. Base flow—The portion of stream flow that comes from the sum of deep subsurface flow and delayed shallow subsurface flow. Base flow tends to dominate discharge during dry weather and small storm events. In contrast, elevated flows during large storm events tend to be derived primarily from overland flow or rapid shallow subsurface flow. Best Management Practice (BMP), also known as stormwater control measure (SCM)— Although best management practice is the more commonly used term, stormwater control mea- sure may be a better or more accurate term since “best” may be arbitrary, ill-defined, and have no true superlative meaning. Bypass—Runoff that is routed around an SCM or passes through the SCM with minimal treat- ment. Bypass generally occurs when the inflow volume or flow rate has exceeded the capacity of the SCM. Catchment (also known as subcatchment, drainage area, drainage basin, subwatershed)— The land area that drains to a specific point of interest. A catchment is typically a portion of a watershed. Clean Water Act (CWA)—Federal legislation (1972) that established the basic structure for regulating discharges of pollutants into the waters of the United States and regulat- ing quality standards for surface waters. The CWA authorized the U.S. EPA to implement pollution control programs such as the National Pollutant Discharge Elimination System (NPDES). Climate Divisions (also known as climate zones)—Defined for the purpose of this report as the 344 climate divisions in the conterminous United States (CONUS), defined by the National Climatic Data Center (NCDC), and analogous zones defined for U.S. land outside of the CONUS. Compaction—The densification, settlement, or packing of soil in such a way that the bulk density of the soil increases. Compaction tends to result in reduction in soil permeability. Com- paction may be intentional, as in the preparation of a site for construction, or incidental, as in the movement of machinery or foot traffic over an area. Glossary of Key Terms

xiv Volume Reduction of Highway Runoff in Urban Areas Continuous Simulation Modeling—A method of hydrological analysis in which a continu- ous time series (e.g., a period of years) of precipitation and climatic data are used as input, and infiltration, evapotranspiration, and runoff are calculated on a continuous basis. The outputs of continuous simulation models are typically continuous time series of watershed and SCM responses that can be analyzed sequentially or continuously. Cost-Effectiveness—Defined in general as the ratio of effectiveness of a control for a given metric versus the cost of the control. A greater cost-effectiveness results when the ratio of effectiveness to cost is higher. Crop Coefficient—The crop coefficient is a dimensionless number that is multiplied by the reference evapotranspiration (ETo) value to arrive at the rate of evapotranspiration (ET) for a given type of vegetation. The resulting ET can be used to help an irrigation manager schedule when and how much irrigation should occur. It can also be used to estimate the amount of ET likely to be lost from an SCM. Crop coefficients vary by vegetation type, stage of growth of the vegetation, season, and other factors. Design Criteria—In this context, design criteria refers to the set of requirements that serve as the basis for designing an SCM to achieve its intended performance. For example, design criteria for a filter strip may include the slope, length, vegetation density, amended soil thick- ness, maximum flow depth, and other criteria. Design Parameters—The qualitative and quantitative physical characteristics that are used in the design process to describe and analyze a given SCM design. Design criteria are com- monly expressed in terms of allowable bounds on design parameters. Design Storm—A prescribed precipitation distribution (hyetograph) and the total precipita- tion amount that is used as part of the design process of SCMs. Design storms may be statisti- cally derived hypothetical events or real events that have been observed. Directly Connected Impervious Area (DCIA)—Impervious areas that are hydraulically connected to the conveyance system and to the basin outlet point without being routed across a pervious surface, such as landscaping, a soft-bottomed conveyance element, or a soft-bottomed SCM. Most roadways may be categorized as DCIAs. Discharge Rate—In this context, discharge rate refers to the rate at which water is discharged from an SCM. Disconnection (also known as dispersion, disconnected impervious area)—A stormwater drainage pattern that routes flow from impervious areas across pervious surfaces prior to dis- charging to a storm drain or receiving water. There are various degrees of disconnection, such as disconnection that attempts to fully mitigate hydrologic impacts and disconnection that may attempt to provide only a portion of total control needed to mitigate impacts. Drawdown Rate—The rate at which the storage volume in an SCM is recovered as a result of water discharging from the SCM, making storage volume available for subsequent storm events. Drawdown Time—The time required for an SCM to drain and return to its dry-weather con- dition. For example, the drawdown time of an infiltration basin is the time it takes for the basin to drain from brim full to empty following the end of inflow. For detention facilities, drawdown time is a function of basin volume and outlet orifice size. For infiltration facilities, drawdown time is a function of basin volume and infiltration discharge rate. Effectiveness—A measure of how well an SCM system meets its goals for all stormwater flows reaching the SCM, including flow bypasses. For example, effectiveness is a function of

Glossary of Key Terms xv capture efficiency, percent volume reduction, and effluent pollutant concentration. See perfor- mance and efficiency for complementary definitions. Efficiency—A measure of how well an SCM or SCM system removes pollutants. See per- formance and effectiveness for complementary definitions. Evaporation—The change of phase of a liquid into a vapor at a temperature below the boil- ing point, taking place at the liquid’s surface. Evapotranspiration (ET)—The loss of water to the atmosphere by the combined pro- cesses of evaporation (from water, soil, and plant surfaces) and transpiration (from plant tissues). Factor of Safety (FS)—A factor applied to a specific system design parameter that is intended to make the design of the system more robust in the event that conditions are different than analyzed, conditions change with time, or other factors are present that are not explicitly considered or are not foreseen in the design process. Feasibility Criteria (and infeasibility criteria)—Specific qualitative or quantitative criteria that are used to identify conditions under which a given stormwater management approach is considered to be feasible or infeasible. Flood Control Regulations—In this manual, flood control regulations are considered to be requirements in place to reduce the risk of damage to public property or hazards to public safety resulting from runoff from large storm events. For example, flood control regulations may require peak runoff flow rates be matched pre-project to post-project for a specific large design-storm event (e.g., 25-year, 24-hour event). In contrast, water quality regulations typi- cally focus on smaller, more frequent events that are of specific interest to protection of receiv- ing water quality. Flow Duration—A statistical approach for evaluating continuous hydrographs that consists of quantifying the cumulative duration of flows within a given range of flows or above a given flow rate. Flow Duration Control—A hydrologic control strategy including specialized detention and discharge structures designed to reduce excess post-project flow durations for a designated range of flows based on continuous simulation models of runoff from both pre-project and post- project site conditions, comparing flow durations for the designated range of flows, in order to mitigate development-caused hydromodification. Geotechnical Considerations—In this manual, geotechnical considerations refer specifi- cally to factors related to geotechnical design and performance of soil structures when consider- ing infiltration of stormwater. Considerations are landslides, liquefaction, settlement, and other factors. Green Infrastructure—Open spaces, natural areas, and functional landscaping that manage stormwater using natural and engineered functions to reduce flooding risk and improve water quality. Groundwater Recharge—The process by which surface water infiltrates into permeable soil and ultimately contributes additional water volume to groundwater sources. Harvest and Reuse (also known as rainwater harvesting)—The process of capturing rain- water or stormwater runoff, storing it, and making it available for subsequent use. Head—In hydraulics, energy represented as a difference in elevation. In slow-flowing open systems, the difference in water surface elevation (e.g., between an inlet and outlet).

xvi Volume Reduction of Highway Runoff in Urban Areas Hydraulic Loading—The ratio of stormwater inflow (volume/time) to an SCM divided by the surface area of the SCM that receives flow; this can be a specific value, expressed in terms of a length per unit time (e.g., ft/s), or it can be used in a more qualitative sense to compare between SCM configurations. Hydrocollapse—A sudden collapse of granular soils caused by a rise in groundwater dissolving or deteriorating the inter-granular contacts between the sand particles. Hydrograph—A time series of flow discharge (i.e., runoff rate, inflow rate, outflow rate) versus time. Hydromodification—Changes in runoff and sediment yield caused by land use modifications. Hydromodification Control—Management techniques that reduce the potential for impacts caused by hydromodification. Hydromodification Impact—The physical response of stream channels to changes in runoff and sediment yield caused by land use modifications. Hyetograph—A time series of rainfall intensities versus time. Impervious Surface—Surface area that allows little or no infiltration. Impervious surfaces include pavements, roofs, and similar surfaces. Highly compacted gravel and earth can behave as impervious surfaces. Infiltration—The movement of water from the surface into the soil. Movement from shallow surface layers to deeper surface layers is referred to as “percolation.” Infiltration Rate—The rate at which water moves into the soil, expressed as length per unit of time. Infiltration rate is a bulk measurement in that it describes the overall rate, not the veloc- ity of water through pores, which would tend to be faster. In-Stream Control—Modification of a receiving channel as a technique for managing hydromodification impacts or improving water quality. Interflow (also known as shallow interflow)—The flow of water through the upper soil zones into a stream. In comparison to base flow, which tends to originate from lower soil zones, inter- flow tends to have a shorter travel time and quicker response. However, interflow tends to have a longer, more attenuated response than sheet flow and concentrated overland flow. International BMP Database—A publicly available research database that contains results of SCM studies independently conducted and submitted by researchers throughout the United States and several other countries. www.bmpdatabase.org. Irrigation Efficiency—The ratio of plant irrigation needs met to the amount of irrigation water applied. A value of 0.75 refers to a condition in which 1 in. of irrigation water must be applied to satisfy 0.75 in. of plant water needs. Surplus water may be lost to evaporation, runoff, or deeper percolation. Liquefaction—A seismically induced geological hazard that can result in damage to structures as a result in reduction in bulk volume of saturated granular soils during shaking of the earth. Liq- uefaction results in the loss of a soil’s ability to support a structure. It is specifically associated with saturated granular soils. Low-Impact Development (LID)—LID is an approach to land development (or redevelop- ment) that seeks to manage stormwater as close to its source as possible and minimize down- stream discharges. LID employs principles such as preserving and recreating natural landscape

Glossary of Key Terms xvii features, minimizing effective imperviousness to create functional and appealing site drainage that treats stormwater as a resource rather than a waste product. Maximum Extent Practicable (MEP)—A standard, established by the 1987 amendments to the Clean Water Act, for the implementation of municipal stormwater pollution prevention programs. According to the act, municipal stormwater NPDES permits “shall require controls to reduce the discharge of pollutants to the maximum extent practicable, including manage- ment practices, control techniques and system, design and engineering methods, and such other provisions as the Administrator or the State determines appropriate for the control of such pol- lutants.” Maximum extent practicable is not defined by the CWA. Municipal Separate Storm Sewer System (MS4)—A conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, manmade channels, and storm drains) designed for collecting or conveying stormwater, which is not a combined sewer, which is not part of a publicly owned treatment work, and is owned by a public body approved under Section 208 of the Clean Water Act that discharges into the waters of the United Sates. National Pollutant Discharge Elimination System (NPDES)—A provision of the Clean Water Act that prohibits point-source discharges of pollutants into waters of the United States unless a special permit is issued and administered by states or the U.S. EPA. Off-Line SCM or Volume Reduction Approach (VRA)—Off-line SCM or VRA sys- tems receive flow from a flow-splitter structure such that the maximum inflow to the system is restricted and peak flows are designed to bypass around the system without treatment. On-Line SCM or VRA—On-line SCM or VRA systems receive all of the stormwater run- off from a drainage area. Flows above the water quality design flow rate or volume are passed through the system, generally via an overflow device or structure. On-Site SCMs or VRAs—SCMs or VRAs that are implemented within the boundary of a project site. In contrast, see regional SCMs or VRAs. Operation and Maintenance (O&M)—Refers to inspection of SCMs, operation of the SCMs (if actively operated), and implementation of preventative and corrective maintenance into per- petuity. O&M represents a continuing cost associated with the SCM after the initial capital cost of construction. Overland flow—Flow of water across the land surface in a down-gradient direction. Sheet flow, shallow concentrated flow, and channelized flow are forms of overland flow. Partially Feasible—The concept of partial feasibility refers to a condition in which it is feasible to achieve a portion of the established design goals, but in which it would be infeasible to achieve the entire design goal based on constraining factors. For example, if it is feasible to retain 0.3 in. of runoff, but the design goal is 1.0 in. of runoff, then it would be considered to be partially feasible to meet the design goal. Performance—A measure of how well an SCM meets its goals for the stormwater that flows through or is processed by it. In comparison to effectiveness, assessment of BMP performance does not account for bypass of flows since these flows are beyond the design goal of the system. See effectiveness and efficiency for complementary definitions. Performance Criteria—A specific measurable or verifiable set of requirements against which the performance of a system is compared to assess conformance with regulatory require- ments. For example, reduction of a certain percentage of average annual runoff volume is a common form of a performance criterion established for volume reduction approaches.

xviii Volume Reduction of Highway Runoff in Urban Areas Pervious Surface—Surface area that allows infiltration of water. Physical Setting—The physical aspects of a project site that may affect project design and performance relative to volume reduction, including the site-specific climate, geology, soils, and vegetation. Precipitation—Water that falls to the earth in the form of rain, snow, hail, or sleet. Precipitation Event—A period of precipitation separated from other events by established inter-event criteria, such as a dry period of a certain length. Project Attributes—The aspects of a project design that may affect performance relative to volume reduction, including planimetric geometry, topography, utilities, regulatory overlays, and construction methods. Reference Evapotranspiration (ETO)—The evapotranspiration that occurs from a stan- dardized plot of vegetation that has been studied extensively, generally consisting of a well- irrigated, dense grass that completely shades the soil surface. Regional SCMs or VRAs (also known as watershed-scale SCMs)—SCMs implemented within the local subwatershed, typically outside and downstream of the project boundary or treating nearby areas. In contrast, see on-site SCMs or VRAs. Right-of-Way (ROW)—For the purpose of this manual, defined as the legal parcel within which the urban roadway project is constructed. Roadway Design Regulations—For the purpose of this manual, refers to regulations related to roadway geometrics, public safety, drainage, and other aspects of roadway design, inclusive of water quality and volume reduction, as applicable. Root Zone—The depth to which the major vegetation draws water through a root system in soil. Runoff Volume—The volume of water that flows off of a surface during a period of interest. SCM System (also known as BMP system)—A system including the SCM/BMP and any related bypass or overflow. VRAs are a type of SCM. Sheet Flow—An overland flow, downslope movement of water taking the form of a thin continuous film over a generally smooth surface. Shoulder—A reserved open area located at the edge of a roadway consisting of pavement or pervious surface. Site Design—A stormwater management strategy that emphasizes conservation and use of existing site features as well as incorporation of strategic drainage patterns to reduce the amount of runoff and pollutant loading that is generated from a project site. Sizing Criteria—Specific design criteria related to SCM sizes that serve as a presumptive basis for meeting performance criteria. Stormwater Control Measure (SCM), also known as a best management practice—A device, practice, or method for removing, reducing, retarding, or preventing targeted storm- water runoff quantity, constituents, pollutants, and contaminants from reaching receiving waters. Total Maximum Daily Load (TMDL)—The calculation of the maximum amount of a pol- lutant that a water body can receive and still meet water quality standards, and an allocation of that load among the various sources of that pollutant. Pollutant sources are characterized as either point sources that receive a waste-load allocation or nonpoint sources that receive a load allocation.

Glossary of Key Terms xix Travel Lane—A portion of a road or highway that is primarily dedicated to conveying automobile travel. Urban Highway—Refers to a range of limited access roadway and freeway types described within this manual. Volume Reduction—The process by which the volume of runoff that discharges directly to receiving waters is reduced through the use of volume reduction approaches that include infiltra- tion, evapotranspiration, and harvest for beneficial use. Volume Reduction Approach (VRA), also known as volume reduction practice, volume reduction SCM, volume reduction BMP—An approach, inclusive of structural SCMs, source controls, and site design practices, which is intended to achieve volume reduction of stormwater runoff. Water Balance (also known as water budget)—The accounting of a system’s state of water storage and flux, considering the total flow of water into and out of a system and the change in storage conditions in the system. For example, water balance can refer to the flux of water in and out of a specific SCM system, a local groundwater system, or a regional groundwater system. Water Balance Analysis—In the context of this manual, water balance analysis refers to the consideration of the ultimate fate of retained stormwater (e.g., percolation, interflow, ET, beneficial use) such that potential adverse effects on local systems can be evaluated. Watershed Characteristics—Characteristics of the watershed in which a project is located that may influence goals for volume reduction and/or the amount of volume reduction that can be achieved. For example, topography, regional groundwater table, and regional water balance. Whole Life-Cycle Costs—An economic assessment, expressed in monetary value, consid- ering all significant and relevant cost flows over a period of analysis (project life expectancy). Project costs include those needed to achieve defined levels of performance, including reliabil- ity, safety, and availability. Included are capital and O&M costs.