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

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7 C H A P T E R 2 As introduced in Chapter 1, the incorporation of volume reduction approaches into urban high- way projects can yield significant benefits, but VRA analysis can also increase the complexity of the project development process. Planning and designing for volume reduction can introduce new ele- ments that need to be coordinated with other aspects of the design. Additionally, planning and designing for volume reduction can require additional site and watershed information. With careful planning, the acquisition of this information can have a relatively minor impact on project cost and schedule; however, if not anticipated, the addition of these steps may result in project delays and more significant incremental costs. Like other design goals, volume reduction goals may need to be adjusted during the course of the project as designs evolve and additional information becomes available. To help simplify this process, this chapter describes an example step-by-step approach for incorporating volume reduction techniques into an urban highway project. This chapter is intended to help answer the questions: • How should I approach incorporating volume reduction into my project? • What resources are provided in this manual to help me at each step? • What are the advantages of taking a systematic approach? 2.1 Example Approach and Corresponding Manual Resources The example stepwise approach described in this section is based on a typical project develop- ment process that progresses from project planning, to site investigation and preliminary design activities, to project design and implementation. This process has been developed based on the following considerations: • What types of project decisions are made at each phase of the project? • What information is needed to support decisions at each phase? • What information is practicable to obtain at each phase? • What other investigation or design activities are typically ongoing at each phase? The example stepwise approach is illustrated schematically in Figure 2 and described in the paragraphs that follow. Step 1—Establish Volume Reduction Goals. The first step in this example process is to estab- lish volume reduction goals for the project. Volume reduction goals can be informed by: • Regulatory requirements such as MS4 permits, TMDL implementation plans, or other motivations (see Sections 1.2 and 3.1); Stepwise Approach for Incorporating Volume Reduction in Urban Highway Projects: How to Use This Manual

8 Volume Reduction of Highway Runoff in Urban Areas • Resource protection needs, such as groundwater recharge, protection of sensitive receiving waters, and participation in a voluntary watershed planning process (see Section 3.4); • Resource protection constraints, such as groundwater protection criteria or sensitive hydro- geologic systems that could reduce the desirability of infiltration in certain areas (see Section 3.4); • Other highway design goals, such as mandates for traffic capacity and safety (see Section 3.1); and • The anticipated amount of volume reduction that is feasible given understanding about site conditions (Section 3.4), technical factors influencing volume reduction (Section 3.2), and factors implicit in the project and highway type (Section 3.3). Because certain additional information relevant to volume reduction goals may be obtained or may arise throughout the planning and design processes, the establishment of volume reduc- tion goals is an iterative process that may need to be refined as the project proceeds. Nonethe- less, the early establishment of goals is an important step as it influences the types and scope of investigations and analyses that need to be planned. Such goal establishment also allows for an initial screening to identify the VRAs that may be the most applicable. Step 2—Characterize Project Site and Watershed. Characterization of the project site is an iterative process that can be initiated early in the planning process and may continue throughout Manual Reference Example Stepwise Process Step 1 Establish Volume Reduction Goals Step 3 Identify Potentially Suitable VRAs Step 5 Select VRAs and Develop Conceptual Designs Step 2 Characterize Project Site and Watershed Understand regulatory context Identify resource protection needs Understand factors influencing volume reduction Conduct preliminary screening of applicability Develop preliminary site plans Select VRAs Develop conceptual designs Estimate performance and cost; compare to goals Identify highway and project type and related factors Obtain available information and coordinate with applicable parties Conduct site assessments 1.2 and 3.1 – Regulatory Setting 3.2 – Key Factors in Volume Reduction 3.3 – Urban Highway Types 3.4 – Site Assessment 3.4 – Site Assessment 4.0 – Volume Reduction Approaches Appendix: VRA Fact Sheets 5.2 – Initial Screening to Identify Potential VRAs 5.4 – Conceptual Design Development 4.4 – Additional Resources for VRA Design and Maintenance Information 3.2 – Key Factors in Volume Reduction 3.3 – Urban Highway Types 3.4 – Site Assessment R ef in e go al s ba se d on n ew in fo rm at io n Step 4 Prioritize VRAs from Screened Menu Consider additional factors to differentiate between potentially suitable VRAs Weight and score factors to identify high priority VRAs 5.3 – Prioritizing Approaches from Screened Menu of VRAs Conduct preliminary screening of feasibility and desirability Figure 2. Example stepwise process for incorporating volume reduction into project development.

Stepwise Approach for Incorporating Volume Reduction in Urban Highway Projects: How to Use This Manual 9 the development of project designs. Site characterization may take a number of forms, including desktop data review, field investigations, and coordination with applicable agencies. Initial information about the site, watershed, and project type known at the onset of planning is useful for the establishment of volume reduction goals. As part of project planning, additional site characterization efforts may be needed to support the early assessment of VRA feasibility. For example, when attempting to achieve volume reduction goals, a general assessment of infil- tration rates and groundwater conditions is usually needed early in the process to estimate the extent to which volume reduction is possible. Guidance for general site-assessment activities at the planning phase is described in Section 3.4 and the appendices. Site investigation activities tend to become more focused as the design process progresses. For example, once a specific volume reduction facility is selected and sited, focused infiltration testing and groundwater-level measurements can be obtained in the specific vicinity of that facility to establish design infiltration rates and identify potential depth to groundwater issues. Section 3.4 and the appendices provide guidance for focused site-assessment methods at dif- ferent design phases. Because site characterization requires time and budgeting, investigation of site conditions is only recommended to the extent that information is needed to support decision making at the current project phase. For example, if it is known that infiltration for a site is not practicable based on sensitive geotechnical conditions, then there would be limited value in also conduct- ing an analysis of infiltration rates or groundwater quality considerations. Section 3.4 and the appendices provide guidance related to the role of site investigation at different design phases. Step 3—Identify Potentially Suitable VRAs—Preliminary Screening and Site Planning. Chapter 4 describes a fairly broad menu of VRAs, including systems that are located within different zones of the highway environment and that rely on different mechanisms for volume reduction. Chapter 4 also describes the properties of these VRAs and how they compare on a relative basis. The goal of Step 3 is to narrow the broader menu to a focused list of VRAs that appear to be most compatible with the specific project type, site conditions, and volume reduction goals. Sec- tion 5.2 describes a screening process that is intended to help efficiently rule out approaches that are clearly unsuitable and to assist with narrowing the list to support project planning and design. The other key element in this step is the development of the preliminary project layout. Because many VRAs are integrated within roadway features, early site planning can simultane- ously assist with the preliminary screening and selection of VRAs. By considering site design as an interrelated element of volume reduction planning, a greater degree of volume reduction may be achieved while still meeting multiple other site design requirements through optimized place- ment and design. The degree of flexibility within roadway layouts varies and may be constrained by overriding factors such as grades/topography, utilities, connection with existing roadways, and geometric design. However, at the early planning phase, there are typically greater opportu- nities to adjust layout than in later phases of design. Step 4—Prioritize VRAs from Screened Menu. After conducting the screening and site plan- ning activities discussed in Step 3, multiple VRAs may be identified as feasible and desirable for a site. The goal of this step is to prioritize the list so that it can be further narrowed down from the list of potentially suitable VRAs to a reduced suite that is selected specifically for more detailed evaluation for the project. Section 5.3 provides a systematic approach for prioritizing the VRAs that passed the initial screening process in Step 3. Factors that may be relevant for prioritization include: • Relative life-cycle costs, • Relative operations and maintenance (O&M) impacts on agencies,

10 Volume Reduction of Highway Runoff in Urban Areas • Relative reliability, • Relative safety, and • Potential performance relative to volume reduction goals. By ranking systems based on these factors, the most potentially favorable options can be identified and then evaluated and selected in Step 5. Step 5—Select VRAs and Develop Conceptual Designs. Based on the results of Steps 3 and 4 and designer preferences, VRAs are selected and incorporated into project designs. The final step covered by this manual is the development of conceptual designs that incorporate the screened and prioritized menu of VRAs. Guidance related to this step is provided in Section 5.4, including guidance for developing conceptual site and VRA designs, using modeling tools for decision support and design refinements, estimating costs of conceptual designs, and adapting conceptual designs to converge with project goals. Developing and analyzing VRAs at a conceptual design level can facilitate analyses of benefits and costs of VRAs. These analyses can help refine volume reduction goals with respect to what is achievable for a given site and help identify design adaptations that would allow performance and cost to better converge with project goals. Conceptual design analysis can also help the designer understand the sensitivity of design parameters to help focus further testing or investigation as part of developing final designs. For example, if infiltration rate is found to be a sensitive design parameter, then more focused testing can be planned to help reduce uncertainty in this parameter. 2.2 Advantages of a Systematic Approach for Incorporating Volume Reduction Highway project development includes a variety of diverse conditions and constraints that must be evaluated and considered to arrive at a final design. Many DOTs use a structured project development process. While no two design processes are the same, a similar systematic approach typically underlies the overall development process (i.e., initial site characterization precedes planning and environmental clearance, which is sought before full-scale detailed design activi- ties commence). A systematic approach for design development helps ensure that the necessary information is collected at the appropriate time to help avoid project delays and added expenses. Similarly, a systematic approach for incorporating volume reduction in highway design has a number of advantages. First, it helps ensure that volume reduction goals are considered early in the process, in concert with the other goals of the project. Second, it helps ensure that informa- tion is collected at the appropriate time, and the analyses that are scoped are suited to the level of information available and the decisions that need to be made at each phase. Third, taking a systematic approach helps identify opportunities for volume reduction that may not be identi- fied in a more ad-hoc process. This is especially useful when the design team has relatively little experience with volume reduction design and does not have a good feel for the opportunities available. Finally, a systematic approach helps ensure that constraints and unintended conse- quences are considered and applied appropriately in informing volume reduction approaches and reasonable volume reduction goals. A systematic identification of constraints may be espe- cially important in demonstrating to regulators or reviewing authorities why volume reduction goals may need to be refined or waived for a particular project or portions of a project. While this manual is written to facilitate a systematic approach, as described in Section 2.1, the manual does not require users to follow this approach in sequential order. In each section, the underlying technical bases and considerations are introduced alongside of the more struc- tured processes and associated tools (e.g., checklists, worksheets, flowcharts) associated with a systematic process.