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

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1 C H A P T E R 1 This chapter introduces the purpose and origin of NCHRP Report 802: Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual, the regulatory and policy drivers for designing for volume reduction, the intended users and uses of this guidance, and the organization of the remainder of the document. The major questions addressed in this chapter are: • What does this manual seek to achieve? • What is the scope of this manual? • What regulatory issues and national and local policies are intended to be addressed and informed by this manual? • Who are the intended users? • How should this manual be used? • How is this manual organized? 1.1 Statement of Purpose Volume reduction of surface runoff is an important element of controlling stormwater impacts on water quality and stream functions in urban areas. Key benefits of volume reduction for stormwater management are: • Load reduction of pollutants to receiving waters, • Decreased potential for channel erosion by reducing the cumulative energy of stormwater discharged to stream channels, • Potentially increasing groundwater recharge and augmenting water supply or stream base flow, and • Reduced peak runoff flow rates. Due to various constraints and other design objectives associated with the urban highway environment, many conventional volume reduction approaches commonly applied in other land uses are not applicable or require careful application in this space-constrained environ- ment. In addition, the assessment of feasibility of volume reduction approaches (VRAs) in the urban highway environment must consider a broad suite of factors. As a result, incorporation of volume reduction approaches can add complexity to transportation project planning, design, construction, operations, and maintenance. Finally, limited information about cost, mainte- nance requirements, and life span of VRAs in the highway environment is available to support decision making. The intent of this guidance manual is to provide practical, technically defensible, and compre- hensive guidance for the evaluation and implementation of volume reduction practices in a wide range of urban highway environments. Scenarios, such as varying project types, site conditions, and climate zones, are included, along with practical recommendations on conditions specific Introduction

2 Volume Reduction of Highway Runoff in Urban Areas to urban highway projects. The current state of the practice for achieving volume reduction is incorporated. Meanwhile, this manual also introduces potential innovative approaches specifi- cally tailored to the urban highway environment. In the preparation of this guidance manual, the research team recognized that some state departments of transportation (DOTs) have already been implementing volume reduction approaches to some extent and have certain processes in place for making decisions about apply- ing VRAs, while other DOTs are earlier in the process of evaluating and implementing VRAs (or stormwater management approaches in general). The purpose of this guidance manual is to serve all DOTs at a uniform level of detail, regardless of the state of each DOT’s current program. As a result, different DOTs may use this guidance manual in different ways. 1.2 Regulatory and Policy Background Reduction of runoff volumes has long been recognized as an effective method for controlling the impacts of urbanization on water resources. In areas with suitable soil conditions, infiltration systems have been widely applied to manage stormwater runoff for flood control purposes. For example, the City of Portland operates approximately 9,000 infiltration sumps in the eastern part of the city that capture the runoff from storm events and have eliminated the need for a regional stormwater conveyance system in this area (City of Portland, 2013). Eliminating the need for such hard infrastructure can have substantial cost savings. Other regions, such as the Sun Valley water- shed in Los Angeles County, California, have voluntarily embraced volume control approaches as part of addressing flood control, water quality, and water supply objectives (Los Angeles County Department of Public Works, 2013). The term “low-impact development” (LID) refers to an approach for managing runoff from new development projects [U.S. Environmental Protection Agency (U.S. EPA), 2013a]. This approach emphasizes controlling the rates and volumes of stormwater runoff close to its source in order to reduce the impacts of development on downstream receiving waters and conveyance systems. This approach does not necessarily include relying on infiltration for flood control, but for reducing water quality and hydromodification impacts on receiving waters. LID approaches have seen varied degrees of adoption in different applications across the United States, most notably in state and local regulations governing stormwater management from new development projects. However, the paradigm for stormwater management in many areas of the country included minor emphasis on runoff volumes; instead it focused on capturing, treating, and releasing stormwater runoff to remove target pollutants, while simultaneously controlling peak rates of runoff to address flooding risks, where necessary (National Academy of Sciences, 2008). The National Research Council (NRC) report Urban Stormwater Management in the United States (National Academy of Sciences, 2008) identified a number of recommendations for improving stormwater management approaches and regulations, including a greater emphasis on hydrology (i.e., runoff volumes, flow rates) in managing stormwater runoff. Consistent with this report’s recommendations, municipal separate storm sewer system (MS4) permits issued to state and municipal permittees, including DOTs, as part of the National Pollutant Discharge Elimination System (NPDES) have increasingly emphasized volume reduction for new develop- ment and redevelopment projects. Similarly, Section 438 of the Energy Independence and Secu- rity Act (EISA) and accompanying U.S. EPA guidance includes requirements to control volumes and rates of stormwater runoff from projects that involve development or redevelopment of federal facilities (U.S. EPA, 2007a). While EISA is not applicable to highways, it is indicative of overall trends in stormwater management. Finally, the U.S. EPA has published the “MS4 Permit Improvement Guide” (U.S. EPA, 2010), based in part on the NRC’s 2008 recommendations, which includes guidance for state and federal regulators to develop MS4 permits that include

Introduction 3 greater emphasis on surface runoff volume control. Volume control is already encouraged in some MS4 permits issued to DOTs—for example, Caltrans (California State Water Resources Control Board, 2012) and District of Columbia Department of Transportation (U.S. EPA, 2011)—and volume control provisions may apply on a broader scale to roadway projects as part of the NPDES/MS4 permit system in the near future. Other drivers for incorporation of volume reduction into highway projects are the Moving Ahead for Progress in the 21st Century (MAP-21) Act, which uses a performance-based approach for the reauthorization of federal aid highway and highway safety projects as well as trends toward sustainability rating systems such as the Institute for Sustainable Infrastructure Envisio Rating system (U.S. DOT, 2012), and FHWA’s Sustainable Highways Self-Evaluation Tool, INVEST. Drivers for retrofits of highways to reduce runoff volumes are primarily found as part of the implementation of total maximum daily loads (TMDLs), which includes the development of waste-load allocations (WLAs) and subsequent identification of management actions by various dischargers within the watershed to achieve these WLAs (U.S. EPA, 2012). For DOTs, actions to achieve WLAs may include retrofits of existing roadways or participation in regional approaches with other dischargers to address their apportioned share of the required reduction in loading. Runoff volume is not commonly identified as a pollutant in TMDLs [for example, see the recent successful court challenge of the Accotink Creek TMDL (Virginia DOT et al., v. U.S. EPA et al., 12-CV-775, U.S. District Court for Eastern Virginia)]; however, the enforcement and implementa- tion of TMDLs may encourage management actions involving volume reduction. Additionally, there are examples of TMDLs that include volume of runoff as a surrogate for pollutant loads and impairments (U.S. EPA, 2013b). In the future, volume control retrofits may also be required within the MS4 permits as indicated in the U.S. EPA MS4 Permit Improvement Guide (U.S. EPA, 2010). A more comprehensive presentation of these regulations and their applicability to volume control methods is included in this guidance in Section 3.1. 1.3 Intended Users and Uses Incorporating VRAs into the highway project development process can add complexity and introduce additional feasibility and desirability considerations when compared to a stan- dard highway stormwater management design processes. However, in appropriate conditions, the incorporation of VRAs can also result in reduced infrastructure requirements and accrue important water quality protection benefits. As such, the need was identified for practical guidance to facilitate safe and effective incorporation of volume control into highway designs, where feasible and appropriate. This guidance manual is intended to be used by a range of user types, for different purposes, in the planning and design process for new projects, lane addition projects, and retrofit projects. At the early project planning level and program management level, this manual can be used to facilitate a mutual understanding among the design team regarding volume reduction goals and the way that volume reduction considerations will be incorporated into the project development process. Simi- larly, this manual can be used to scope the additional or alternative analyses that may be needed in the design process as part of achieving volume reduction or demonstrating its infeasibility. At later stages of planning, this manual may assist in identifying feasible VRAs and conduct- ing early site investigations to identify project-specific opportunities and constraints to allow for refined estimates of achievable levels of volume reduction. This manual is also intended to support designers by contributing site-specific approaches to prioritize, select, evaluate, and apply VRAs. Last but not least, this manual may serve as a resource for permit writers and compliance staff when considering the level of volume reduction that may be achievable in the urban highway

4 Volume Reduction of Highway Runoff in Urban Areas environment and potential adverse impacts associated with volume reduction designs. Figure 1 summarizes the range of users and potential uses, organized in approximate chronological order from planning-level to design-level uses. As introduced in Section 1.1, the use of this manual may also vary depending on whether a DOT (or regulatory agency) is earlier in the process of evaluating and implementing VRAs (or stormwater management approaches in general) or already has an established program. For users earlier in the process of evaluating and implementing VRAs, this manual could be used as the technical basis and associated guidance for developing a program; the majority of the manual may be of interest, including the stepwise process described in Chapter 2 and the supporting technical information in later chapters and appendices. For users working within an established program, this manual could be used as a technical reference for specific issues encountered in the administration of the program or on specific projects or as a potential basis for improving their existing process or criteria; however, certain sections, such as the stepwise approach and background, may be of limited value. The latter suite of users may need to be more selective about how the contents of this manual are used within their programs and may need to identify topic areas in which their program-specific processes and criteria supersede those in the manual. While this manual provides detailed and methodical guidance for selecting and implementing VRAs, supplemented with checklists and schematics consistent with a conceptual design level of detail, it does not provide criteria for the detailed design of specific volume reduction facilities. This manual considers operations and maintenance activities and costs as key considerations in the feasibility and prioritization of VRAs; however, it does not provide detailed guidance for operations and maintenance. Instead, this manual provides references to other documents that provide information in these areas. 1.4 Organization of the Guidance Manual This manual approximately parallels the example stepwise process described in Section 2.1. It is intended to provide supporting guidance as the user proceeds through each general step. Intended Users DOT managers Permit writers Consultants and planners DOT project staff and design engineers Potential Uses Understanding the technical basis of volume reduction requirements and setting volume reduction goals Identifying approaches available to achieve volume reduction in the urban highway environment Scoping the analyses necessary for incorporating VRAs into the project design process Understanding potential design implications of volume reduction goals Refining project-specific estimates of achievable volume reduction Conducting site assessment and feasibility analyses Prioritizing, selecting, and applying VRAs Pl an ni ng a nd P ro gr am M an ag em en t Pr oj ec t D es ig n an d Im pl em en ta tio n Figure 1. Intended users and potential uses.

Introduction 5 Chapter 2 describes a step-by-step model approach for incorporating volume reduction tech- niques into an urban highway project and describes how this manual can be used to support planning and analysis at each project phase. Chapter 3 provides a baseline characterization of the urban highway environment related specifically to achieving surface runoff volume reduction. The intent of this chapter is to orient the user to the regulatory context for achieving volume reduction, the basic concepts central to volume reduction, and the general considerations that exist for applying volume reduction approaches within the urban highway context. This chapter primarily supports Step 1 of the stepwise process (establish volume reduction goals) and Step 2 (characterize project site and watershed). Chapter 4 provides a focused menu of potentially practicable approaches specific to volume reduction in the urban highway environment and describes these approaches in sufficient detail to distinguish key conceptual design parameters and applicability. VRAs are described in a series of fact sheets that are included in Appendix A. This chapter is intended to serve as a resource for Step 3 (identify potentially suitable VRAs) and Step 4 (select VRAs and develop conceptual designs). Chapter 5 provides the user with information to help evaluate, compare, and ultimately select applicable volume reduction approaches for a project. This chapter includes evaluation criteria, an approach for screening and selecting VRAs, and an introduction to the Volume Performance Tool (available on the CD-ROM that accompanies this report) to support site-specific quantita- tive analysis of volume reduction approaches. It is also intended to serve as a resource for Step 3 (identify potentially suitable VRAs) and Step 4 (select VRAs and develop conceptual designs). Chapter 5 also introduces watershed-scale approaches for achieving volume reduction. Technical appendices include content that is directly relevant to the application of this manual but focuses on a specific technical topic area. The appendices include Appendices A and B, which are published as part of this report, and Appendices C through F, which are included in NCHRP Web-Only Document 209. • Appendix A: Volume Reduction Approach Fact Sheets—provides three- to five-page fact sheets for each of the nine VRAs identified in Chapter 4, including VRA-specific information for selection and conceptual design of VRAs. • Appendix B: User’s Guide for the Volume Performance Tool—provides instructions for using the Volume Performance Tool to conduct site-specific assessment of potential volume reduction performance. 1.5 Limitations This manual has a variety of potential uses and includes guidance on many technical areas; however, a number of important limitations should be understood in the use of this manual: • While this manual may be useful to provide technical support to the development and imple- mentation of local stormwater management ordinances, it is not intended to establish policy or criteria related to stormwater management or volume control. The user should refer to local ordinances for specific requirements and criteria. • Due to the importance of considering and the variability of site-specific conditions in various technical analyses (e.g., infiltration feasibility assessment, geotechnical studies and design), it is not possible for this manual or its appendices to provide specific numeric or other rec- ommendations that are universally applicable. Where possible, this manual provides typical

6 Volume Reduction of Highway Runoff in Urban Areas numerical ranges and guidance. However, professional expertise and judgment must be exer- cised by qualified professionals based on site-specific information to develop project plans and designs. The project team provides no warranty, expressed or implied, for the recommenda- tions provided in this manual or its appendices. • The primary audience for this manual is professionals involved in stormwater management. While this manual includes topic areas that overlap with related disciplines (e.g., geotechni- cal engineering, hydrogeology), the discussions of these topic areas are primarily intended to facilitate a common understanding between disciplines on key concepts rather than to elabo- rate on, much less cover, discipline-specific topics. This manual is not intended to serve as a detailed guide for professionals in other disciplines (e.g., geotechnical engineers, hydrogeolo- gists) to conduct their analyses. These limitations apply to the entire manual and its appendices; to avoid unnecessary redun- dancy, they are not repeated in other locations.