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Stormwater Infiltration in the Highway Environment: Guidance Manual (2019)

Chapter: Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection

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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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Suggested Citation:"Chapter 2 - Planning Framework for Early Decision-Making and Tentative BMP Selection." National Academies of Sciences, Engineering, and Medicine. 2019. Stormwater Infiltration in the Highway Environment: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/25705.
×
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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.

15 C H A P T E R 2 This chapter provides guidance to support decisions about infiltration approaches. The chapter supports the first two steps of the overall infiltration and assessment and decision- making framework: (1) goal-setting and preliminary planning investigations to support pre- liminary selection of an infiltration approach, and (2) tentative selection of BMP types (see BMP Fact Sheets in Appendix A) and locations. While this chapter introduces a wide range of concepts that should be considered, this process is intended to be efficient and can be conducted primarily using “desktop” methods in most cases. The principal components of this process and associated evaluation tools are summarized in Table 3. The decision-making tools in this chapter provide a means for organizing information to document initial decision-making. Figure 2 shows the relationship of these components to the infiltration assessment and decision-making process flowchart. The framework described in this chapter emphasizes early project scoping and preliminary planning efforts including planning-level site assessments as the first steps in evaluating and developing an infiltration-based approach. The remaining steps build on these preliminary planning decisions. Conducting preliminary desktop investigations as part of the preliminary planning phase may deviate from typical project delivery. However, the advantages to under- taking these steps earlier include the following: • Early identification can preserve potential high-quality infiltration areas when it is still pos- sible to do so. • Early identification of overriding constraints can eliminate the need for extraneous and costly site investigations. • Preliminary screening can focus the scope of more rigorous design-phase assessments to only those areas where infiltration BMPs are likely to be placed, mitigating the necessity of per- forming detailed investigations over a larger scale. • Early selection of tentative BMP types can focus the scope of design-phase assessments to answer questions that are specific to determining the feasibility of the selected BMP. A phased site assessment framework may not be appropriate for all projects. The project team should consider project size, budget, timeline, soil variability, and existing information as part of scoping site assessments. In certain cases, a one-time mobilization may be appropriate to collect information that supports both preliminary screening and design-phase data needs. Project teams should adapt the recommendations in this chapter based on project- specific factors and local criteria. Planning Framework for Early Decision-Making and Tentative BMP Selection

16 Stormwater Infiltration in the Highway Environment: Guidance Manual 2.1 Establishment of Infiltration Objectives Planning and design teams should begin with an evaluation of the underlying objectives associated with infiltration. This can inform selection of BMP strategies and guide the level of effort of infiltration investigations in subsequent steps. Where objectives related to infiltration are more stringent, or there are considerable cost savings associated with successfully utiliz- ing infiltration, greater effort may be justified for infiltration investigations. Where objectives are more flexible, or could be met with alternative approaches besides infiltration, it may be appropriate to use more efficient approaches for site investigation and BMP selection. 2.1.1 Categories of Project Objectives Related to Infiltration Project objectives and requirements related to stormwater infiltration can originate from regulatory mandates or other stormwater management objectives, such as NPDES storm- water permits, TMDL implementation plans or watershed plans, water quality credit frame- works, local resource protection policies, capital improvement programs, and groundwater augmentation policies or incentives. Based on these drivers, project objectives associated with stormwater infiltration can fit within the following categories: 1. Opportunistic. Opportunistic objectives are those in which infiltration may be used as one option to meet stormwater management objectives such as permit compliance, water quality improvement, flood mitigation, and groundwater recharge. In these cases, regulatory requirements may not drive decision-making about whether to use infiltration. Rather, the rela- tive cost-effectiveness of infiltration approaches (i.e., whether the use of infiltration can achieve objectives more cost-effectively than alternative approaches) is a primary driver in selecting an infiltration approach. Examples scenarios include the following: • Infiltration BMPs are one class of BMP in a menu of acceptable stormwater quality treatment approaches for meeting regulatory obligation. There is no hierarchy specified in this menu. Component Description Evaluation Tools Step 1a: Establish Infiltration Objectives (Section 2.1) Users determine volume reduction objectives based on review of applicable regulations and site-specific goals. • Table 4. Infiltration objectives checklist Step 1b: Preliminary Infiltration Feasibility (Section 2.2) Users perform initial site assessments to determine possible locations for infiltration practices, risk factors, constraints, or prohibitions associated with infiltration, and the potential physical capacity of the site for infiltration. • Table 5. Checklist for preliminary review of infiltration conditions Step 1c: Select Preliminary Infiltration Approach (Section 2.3) Users select a preliminary infiltration approach based on the results of Steps 1a and 1b: • Full Infiltration, • Maximized Partial Infiltration, or • No/Incidental Infiltration. Users identify the need for additional investigation(s) if applicable. • Table 10. Tentative infiltration approach Step 2: Tentatively Select BMP Locations and Types (Section 2.4) Users apply the findings from Step 1 to identify the following: • Tentative locations for BMPs and tributary areas, • Types of BMPs tentatively selected at each location, and • Conceptual design parameters for these BMPs. • Section 2.4 Table 3. Description of preliminary infiltration site assessment and decision-making components and tools.

Planning Framework for Early Decision-Making and Tentative BMP Selection 17 • Infiltration is being considered for flow control to reduce flooding and protect streams, but this could also be achieved by an extended-detention basin (flow-duration control). • Infiltration could be used as a retrofit to make progress toward required load reductions or to secure water quality credits as part of TMDL implementation, but other options for achieving these load reductions or credits are also available. • A local policy or incentive is in place that gives preference for stormwater management approaches that provide groundwater recharge in favorable areas, but this is not a mandate that applies to all projects. 2. Maximized Per Site Conditions. In this case, the project is required to evaluate and apply retention of stormwater runoff to a maximized level [e.g., “maximum extent practicable (MEP)”] based on site conditions, before considering other treatment methods. Under this framework, designers must work to maximize infiltration (or other surface runoff volume technique such Tentative Selection of Infiltration Approach (Section 2.3) [Full Infiltration | Partial Infiltration | No Infiltration] Establish Infiltration Objectives (Section 2.1) Regulatory Context Other Infiltration Objectives Preliminary Constraints Survey Preliminary Assessment of Infiltration Feasibility (Section 2.2) Preliminary Groundwater and Geotechnical Feasibility Factors Preliminary Infiltration Capacity AssessmentStep 1. Perform Project Scoping and Pre-Planning for Stormwater Infiltration Step 2. Tentatively Select BMP Locations and Types Other BMP Selection Factors (BMP-specific risk, cost, O&M, climate compatibility) (Section 2.4) Select Tentative BMPs Types and Locations (Section 2.4) Result: Tentative BMP locations and types, including conceptual description of drainage area and BMP design attributes Step 3: Prioritized Analyses and Site Investigations to Confirm Feasibility of BMP Selection and Sizing (See Chapter 3) Figure 2. Preliminary infiltration assessment and decision-making process flow chart (Steps 1 and 2).

18 Stormwater Infiltration in the Highway Environment: Guidance Manual as ET or harvest and use) within the site constraints, but the project is not required to achieve a certain minimum level of infiltration to comply. Often, stormwater permits that require con- sideration of infiltration approaches also include options such as biotreatment, biofiltration, conventional treatment, flow control, or alternative compliance that can be used to augment or replace infiltration, when justified. Examples scenarios include the following: • Infiltration BMPs are one class of BMP on a menu of acceptable stormwater quality treat- ment approaches that includes other BMPs. However, regulations require consideration of infiltration (or volume reduction overall) as the first priority and require that project-specific documentation be provided to justify decision-making, particularly if a “lower priority” class of BMP is deemed to be more appropriate (e.g., infiltration is infeasible). • Applicable regulations require the use of infiltration if feasibility criteria are met but allow the level of infiltration to be reduced if infiltration rates are lower than a certain threshold (i.e., below a certain infiltration rate, BMPs do not need to be designed to fully infiltrate a design volume). • Infiltration is identified to be a superior option to achieve project-specific goals, regardless of regulatory requirements. As a result, the project-specific policy direction is to attempt to find areas where infiltration will work because it would result in greater benefit, lower cost, or both than alternative approaches. Note, this is not based on compliance but has a similar “burden of proof” to exhaust opportunities for infiltration before evaluating alternative approaches. • A water quality credit system is in place, but it only allows quantifications based on the volume of infiltration, so project teams are motivated to utilize approaches that achieve infiltration to accrue credits. However, accrual of credits is not mandated for a given project or location (i.e., credits could be accrued elsewhere if a site is not suitable). 3. Specified Performance Level: In this case, the regulatory framework requires the project to achieve a certain minimum level of volume reduction of surface runoff. This may also be applicable when very rigorous standards for BMP selection demand a high burden of proof for rejecting the use of Full Infiltration BMPs as well as when infiltration is the only viable method of drainage and water quality treatment. These cases tend to be relatively rare. Examples scenarios include the following: • The applicable stormwater permit requires projects to infiltrate stormwater as the only on-site option for compliance. If this is not feasible, the project must pursue a form of alternative compliance (e.g., off-site treatment or fee-in-lieu) or the project may not be able to proceed. • An applicable TMDL is based on a volume-reduction surrogate, such that the only way to make progress toward TMDL implementation is through a volume reduction approach (note, this may not mandate infiltration on a specific project but can greatly increase the pressure to identify areas suitable for infiltration). • A flat roadway segment and adjacent areas have no available storm drain pipe and not enough room along the side for a swale or not enough grade to drain stormwater to receiving waters. The most viable approach for water quality treatment and conveyance is to infiltrate. These infiltration objective categories can be thought of as a continuum ranging from the least to the most stringent requirement or objective. 2.1.2 Guidance for Identifying Project Objectives Related to Infiltration In most cases, the project team will be able to classify the project-specific objectives based on these definitions and examples. Table 4 provides set of questions that can be used to establish the underlying objectives.

Planning Framework for Early Decision-Making and Tentative BMP Selection 19 2.1.3 Implications of Infiltration Objectives on Subsequent Steps The applicable infiltration objectives have several implications on subsequent steps: • The project team can use an understanding of infiltration objectives to determine how rigorously and aggressively the project should pursue the assessment of infiltration feasibility. In other words, what is the burden of proof that infiltration assessments will need to support? • The project team can use this information as part of scoping site investigations and inter- preting site data. Step Response Guidance Regulatory Requirements for Roadway Construction Projects (e.g., new road, lane addition, interchange expansion) a. Do post-construction BMP regulations require infiltration to be considered and/or used at a certain minimum level? If “a” and “b” are No, then this is likely an Opportunistic scenario. If “a” or “b” is Yes, and “c,” “d,” and “e” are also Yes, then this is likely a Maximized Per Site Conditions scenario. If “a” or “b” is Yes, and the answer to “c,” “d,” or “e” is No, then, then this may be a “Specified Performance Level” scenario. More research may be justified to determine what would happen if infiltration is found to be infeasible. b. Are there other regulatory drivers that encourage or require infiltration? c. Are feasibility constraints recognized in the applicable regulations? d. Do other options exist if infiltration is not feasible? e. Are other options viable (i.e., available, not cost-prohibitive, compatible with the site)? Regulatory Objectives for BMP Retrofit Projects What are the regulatory motivations for the BMP retrofit? This can vary greatly by region or watershed, including watershed plans, TMDL implementation, local resource protection ordinances, or other considerations. What classes of BMPs can meet retrofit objectives? Is infiltration the only way to meet the objectives? This can influence how rigorously infiltration needs to be considered. How limited are the siting opportunities to meet these objectives? The number of siting opportunities may dictate how important it is to try to achieve infiltration at a certain location or project. Other Infiltration Objectives Are there other regulatory reasons [besides the Clean Water Act (CWA)] to consider infiltration? Examples include the following: • Groundwater augmentation • Flood reduction • Avoiding new stormwater infrastructure • Endangered Species Act • National Environmental Policy Act (NEPA) or state environmental policy acts Are there other means by which (i.e., other locations or projects where) these objectives could be achieved? The number of siting opportunities may dictate how important it is to try to achieve infiltration at a certain location or project. Establish Volume Reduction Objectives Select infiltration objective category. [This is relevant as part of the decision-making process described in Section 2.3.] Opportunistic Maximized Per Site Conditions Specified Performance Level Summarize rationale(s) for selection of infiltration objective category. Table 4. Infiltration objectives checklist.

20 Stormwater Infiltration in the Highway Environment: Guidance Manual • As outlined in Figure 2, the project team should combine the results of preliminary feasibility analyses with the established infiltration objectives to help make preliminary decisions about infiltration strategies for the project. Section 2.3 provides guidance for integrating Steps 1 and 2 into preliminary decision-making. 2.2 Preliminary Feasibility Analyses to Support BMP Selection The preliminary planning or preliminary stage refers to the early stages of project develop- ment, ideally prior to or concurrent with environmental permitting and clearance. At this stage, the project team knows the general scope of the project but is still working to determine project constraints, lay out the site, and determine stormwater management approaches. The project team can improve stormwater management outcomes by beginning evaluations of infiltration feasibility at this phase, including the following: • Reserve space where conditions are most suitable. • Focus subsequent investigations on relevant data needs to support and confirm decision-making. • Determine the need for alternative approaches and alternative project delivery methods. The purpose of this section is to outline planning-level infiltration-feasibility investigations that can be applicable at this phase. During this stage, the project team gathers information based on reviews of existing site information and low-effort site assessment techniques to sup- port characterization of physical constraints, groundwater and geotechnical feasibility, and infiltration capacity. The project team then uses this information in Section 2.3, along with the established infiltration objectives, to select a preliminary infiltration approach for the project. This section relies primarily on desktop methods and rapid field methods, where feasible. Field-level data may not always be feasible at this stage because of timing and site access limita- tions. If the project team can obtain field data to support initial decisions, this can reduce the potential that these decisions will need to be revised. 2.2.1 Categories of Constraints Planners and designers can organize infiltration feasibility assessments into three categories of constraints. Physical Constraints and Project Layout. Within the project area, where are infiltration BMPs potentially feasible? Can the site layout be adapted to support BMPs in these locations? The project team compiles and summarizes physical constraints and determines where (a) BMPs can be located and (b) infiltration could be feasible. This supports decision-making on the adaptation of site layouts to preserve areas with good infiltration opportunities. Factors such as structures, slopes, highway types, and drainage patterns may limit the locations where an infil- tration practice can be located. At this phase, designers may be able to adjust the project layout and conceptual drainage plan to support infiltration objectives. Infiltration Capacity. Can water be infiltrated reliably at an appreciable rate considering soil permeability and groundwater conditions? What effect would infiltration have on the local- ized groundwater table? The project team estimates the infiltration rate of the in-situ soils and the capacity of the infiltration receptor (groundwater depth and mounding) via desktop methods or rapid field methods to determine a preliminary infiltration capacity designation. At the planning phase, the focus is on using cost-effective methods to compare potentially feasible locations for initial assessment of infiltration approaches that can be supported.

Planning Framework for Early Decision-Making and Tentative BMP Selection 21 Groundwater and Geotechnical Feasibility. Can water be infiltrated without introducing undesirable consequences or elevating risks to infrastructure or the environment? The project team coordinates with applicable agencies and conducts desktop research into groundwater protection criteria, site contamination, and other factors. The team also uses available data and tools to assess groundwater mounding, geotechnical, and other associated risks of infil- tration. This can be potentially supported by low-effort site investigations at a level of detail adequate to support the tentative determination of infiltration feasibility. The team conducts these investigations for all portions of the site where infiltration BMPs could reasonably be located and considered. The following sections describe these categories of constraints in more detail. Table 5 provides a preliminary assessment checklist. Key resources for these steps include the following: • Appendix B: Infiltration Estimation Method Selection and Interpretation Guide • Appendix C: Roadside BMP Groundwater Mounding Assessment Guide and User Tool • Appendix D: Guide for Assessing Potential Impacts of Highway Stormwater Infiltration on Water Balance and Groundwater Quality in Roadway Environments • Appendix E: Guide to Geotechnical Considerations Associated with Stormwater Infiltration Features in Urban Highway Design Notes on Phasing and Scoping Site Investigations This section and the supporting appendices are organized by distinct categories of constraints. However, the research team does not intend to imply a priority between these assessments. In practice, project teams may choose to investigate these constraints simultaneously as part of a single preliminary feasibility evaluation. The project team should determine the scope of investigations necessary to adequately consider these factors. This can vary by project. For example, if the project team believes that soil contamination may affect infiltration feasibility, then it may be appropriate to investigate this issue first. If contamination is found to be present, then this could be the overriding factor in decision-making. Therefore, it would be unnecessary to conduct other investigations of infiltration feasibility. 2.2.2 Physical Constraints and Project Layout Assessment In this step, the project team determines where infiltration practices could potentially be installed based on constraints related to project layout, topography, grading, drainage patterns, safety considerations, O&M access, and other factors. The primary goal of this step is to determine what limitation may exist and identify locations that can potentially support infiltration BMP while avoiding design conflicts. If the project team identifies constraints and opportunities during preliminary planning, then it can work to reserve areas that may be suitable for infiltra- tion. Potential opportunities for land acquisition can also be considered. The following sections summarize factors that should be considered in assessing physical constraints and project layout. Project Location and Watershed Characteristics Preliminary site investigations should identify the location of the project and its connection to other watershed features.

22 Stormwater Infiltration in the Highway Environment: Guidance Manual Connection to Receiving Water. Create a map to show the connection from the project site to each outfall location based on existing drainage infrastructure, including the following information: • Receiving waterbody name • Location of existing or new outfalls • Proximity of project to existing or new outfalls • Land ownership and space availability along flow path Characteristics of Receiving Water. Because of the highly linear nature of highway projects, multiple receiving waters are often potentially impacted by the project. Assess whether different Step Summary of Findings (if applicable) Conduct Physical Constraints and Project Layout Assessment (2.2.2) Identify receiving water body connections and environmentally sensitive areas. Identify portions of project layout that are inflexible versus flexible. Identify potential opportunities for land acquisition. Create conceptual drainage map with potential watershed bounds, flow directions. Identify potential BMP opportunity areas (a simple map with grading and topographic information as well as project layout is highly useful). Conduct Preliminary Infiltration Capacity Assessment (2.2.3) (See Appendix B and Appendix C) Estimate infiltration rate and capacity, including evaluation of whether groundwater (GW) mounding could limit infiltration. Create infiltration capacity site map. Investigate Geotechnical Feasibility Factors (2.2.4) (See Appendix E) Describe and map site soil conditions (texture, hydrologic soil group, etc.). Estimate underlying geology (depth to confining layer, soil stratification, etc.). Identify possible soil stability concerns. Identify structural setback requirements on infiltration opportunities map. Evaluate potential for formation of a groundwater mound where it would pose a geotechnical hazard or limit drawdown time to a point where vector issues could be a concern. Investigate Groundwater Feasibility Factors (2.2.5) (See Appendix D) Research applicable groundwater quality standards. Estimate depth to seasonal high groundwater table. Determine if groundwater protection criteria apply or if there are drinking water wells in the project vicinity. Determine if existing soil/groundwater contamination is a potentially concern for infiltration. Assess risk of groundwater contamination due to infiltrating runoff. Assess whether formation of a groundwater mound could reduce pollutant attenuation effects. Table 5. Checklist for preliminary review of infiltration conditions.

Planning Framework for Early Decision-Making and Tentative BMP Selection 23 objectives apply to different parts of the project based on different receiving waters and their specific conditions and regulatory status. Environmentally Sensitive Areas. Identify environmentally sensitive areas within the project area and along the downstream flow path. Determine if environmentally sensitive areas impact where infiltration practices can be reliably located. Highway Type The highway type can have inherent impacts on opportunities and limitations for infiltra- tion BMPs. Highway segments can be characterized into eight representative types based on common geometric design variations for urban highways as described in AASHTO’s A Policy on Geometric Design of Highways and Streets (Green Book) (AASHTO 2011a). Ground-Level Highway Segments. Slightly elevated roadways with wide vegetated medians and shoulders (common in suburban and rural areas). Ground-Level Highway Segments with Restricted Cross Sections. Slightly elevated road- ways with narrow medians and shoulders because of topographic and development constraints (common in urban areas). Highway Segments on Steep Transverse Slopes. Cross-sectional slopes are greater than 10% because of traversing hilly or mountainous terrain resulting in restricted cross sections. Highway Segments with Steep Longitudinal Slopes. Longitudinal slopes are greater than 5% because of traversing hilly or mountainous terrain. Adjacent land inside and outside of the ROW also tends to be relatively steep. Depressed Highway Segments. Roadways are depressed below adjacent ground surfaces to allow for overpassing surface streets common in urban areas. Sloped embankments or vertical side walls result in restricted cross sections. Elevated Highway Segments Constructed on Embankments. Roadways built on earthen fill material creating embankments with slopes between 3:1 and 6:1. Common in suburban areas where surface streets are widely spaced, and grading designs provide adequate fill material. Elevated Highway Segments Constructed on Viaducts. Aerial highway areas found primarily in dense urban areas where the space under the roadway is used for a variety of urban needs. Diamond Interchanges. Linear roadway connections resulting in long narrow wedges of open space. Looped Interchanges. Roadways are connected using arcs and loops (cloverleaf configura- tion) of various sizes resulting in circular areas of open space. Table 6 provides a summary of infiltration opportunities and constraints based on high- way type. Multiple highway types may be present within a single project. Additionally, future planned projects can effectively change the highway type. For example, a ground-level highway could evolve over time to have a more restricted cross section as lanes are added. Project Type Project types include new roadways, enhancement of an existing roadway through the addition of lanes or other improvements, and projects solely to retrofit the highway with BMPs. The project type has important ramifications for infiltration opportunities summa- rized as follows.

24 Stormwater Infiltration in the Highway Environment: Guidance Manual Highway Type Opportunities for Infiltration Constraints on Infiltration Ground-level highways • Infiltration BMPs can be integrated into vegetated conveyances present in the typical cross section. • Wide shoulders and long stretches allow for flexibility in practice selection and siting. • BMPs located in the median and shoulder must allow for errant vehicle recovery. • Future lane expansion or other widening into available space may impact BMP siting. Where lane additions are anticipated, BMP placement in these areas should be avoided. • Shallow slopes may limit routing flexibility. Ground-level highways with restricted cross sections • Narrow vegetated BMPs or permeable shoulders can be integrated into the right of way (ROW). • Piped conveyance may allow for regional scale BMPs at interchange locations. • Limited space due to adjacent structures. • Construction and maintenance activities may require lane closures. • Geotechnical considerations may be amplified due to proximity to urban structures. Highways on steep transverse slopes • Infiltration practices can be integrated into areas with shallow slopes or routed to downslope areas. • Piped conveyance may allow for regional scale BMPs at interchange locations. • Creating space for flat-bottomed or level pool basins would tend to increase earthwork requirements. • Construction and maintenance activities may require lane closures. • Underlying soil likely includes compacted fill in some parts of the section. • Stability and erosion concerns are amplified when using surface conveyances on steep slopes. Highways with steep longitudinal slopes • Piped conveyance may allow for regional scale BMPs at interchange locations. • Creating flat-bottomed or level pool areas for infiltration can require the BMP to be segmented by cutoff walls or berms, increasing cost. This applies to linear systems such as permeable pavement shoulders, vegetated swales, and linear bioretention or infiltration trenches. • Stability and erosion concerns are amplified when using surface conveyances on steep slopes. Depressed highways • Geotechnical concerns about adjacent infrastructure are lessened because infiltrating surface is at a lower elevation than adjacent slopes and structures. • Limited space due to adjacent urban areas. • Opportunities for vegetated conveyance and dispersion may be limited because of topography. • Groundwater and highway geotechnical concerns are amplified because of installation in low lying areas. • Construction and maintenance activities may require lane closures. Elevated highways on embankments • Space for infiltration may be available at toe of slope or footing of retaining wall. • Infiltration practices can be integrated into areas with shallow slopes or routed to downslope areas. • Interchange locations likely at lower elevations allowing for routing. • Limited space due to steep slopes. • Geotechnical concerns amplified because of erosion on steep slopes and stability of retaining walls. • Construction and maintenance activities may require lane closures. Elevated highways on viaducts • Installations may not increase net imperviousness allowing for coordination with existing controls. • Available space possible at ground level. • Interchange locations likely at lower elevations allowing for routing. • No infiltration opportunities on aerial segment. • Geotechnical stability concerns amplified when infiltrating below viaduct columns. • Land ownership may limit areas in which runoff can be managed. Table 6. Infiltration opportunities and constraints based on highway type.

Planning Framework for Early Decision-Making and Tentative BMP Selection 25 New Projects. These types of projects include construction of new roadways. When infiltra- tion is considered early in the project design, the project team can identify opportunities to allow space for infiltration practices, integrate BMPs into the drainage and grading design resulting in cost savings, and protect soils in infiltration areas from compaction during construction. Lane Addition or Redevelopment Projects. These types of projects involve the addition of lanes within an existing ROW. These projects tend to have less flexibility in their site design for improving infiltration opportunities. There tend to be existing utilities and structures as part of the projects that cannot be relocated; however, because these projects typically include grading and drainage modifications, project teams may have the flexibility to accommodate stormwater infiltration if this is considered early in project’s planning. BMP Retrofit Projects. These types of projects involve retrofitting BMPs into an existing roadway. Infiltration opportunities will depend on opportunities within the existing drainage configuration, including location of existing stormwater controls or feasible modifications to this drainage configuration. Impacts of grading and construction activities on infiltration fea- sibility will tend to be simpler; however, the project team may not be able to avoid impacts to existing utilities, structures, and other infrastructure. Topography, Drainage Patterns, and Infrastructure Topography and drainage patterns are key factors in identifying potential locations for infil- tration BMPs. The project team can assess surface constraints relative to infiltration planning via review of the topographic survey conducted at the outset of the project or the existing infrastruc- ture data. At early planning stages, prior to the completion of a site survey, the team can consult desktop-based methods such as digital elevation models, topographic maps, land cover or land use datasets, and local parcel datasets. The project team can obtain these datasets from national databases such as The National Map or local planning departments. The following information is recommended to support infiltration planning: • Elevation contours showing topography and slope • Surface drainage patterns and points of concentrated flow onto and off the site • Location of steep slopes (greater than 10%) • Existing impervious surfaces and structure • On-site or adjacent utilities (within 100 ft) • Existing storm drain infrastructure and points of connection Highway Type Opportunities for Infiltration Constraints on Infiltration Diamond Interchanges • Wedge areas provide substantial open space that can be used as an infiltration surface or provide temporary storage upstream of an infiltration system. • Flexibility in vegetation if adequate setbacks from roadway are provided. • Geotechnical concerns lessened if adequate setbacks from roadway are provided. • Constraints dependent on highway type. • Steep slopes may be required when interchanges connect roadways at very different grades. • Construction and maintenance lane closures have added traffic management costs. Looped Interchanges • Central loops provide substantial open space that can be used as an infiltration surface or provide temporary storage upstream of an infiltration system. • Topography, geotechnical, and safety considerations are reduced compared with diamond interchanges because of large space and even grade. • Constraints dependent on highway type. • Steep slopes may be required when interchanges connect roadways at very different grades. • Construction and maintenance lane closures have added traffic management costs. Table 6. (Continued).

26 Stormwater Infiltration in the Highway Environment: Guidance Manual While infiltration capacity, groundwater quality, and geotechnical issues are addressed in separate sections, the following information may be useful to include on a topographic map: • Known soil and groundwater contamination from state environmental agency or other applicable data source • Known areas of sanitary sewer inflow and infiltration issues from local sewerage agency • Groundwater elevation data (either available contour maps or well data), potentially available from a local government or groundwater management agency • Soil type(s) such as from the Web Soil Survey (https://websoilsurvey.sc.egov.usda.gov) The team can use conceptual design schematics to assess proposed conditions, including esti- mated locations of proposed structures, topography, and drainage pathways. This includes pro- posed changes to the roadway and adjacent land uses. As the project develops, assessments of drainage areas and catchment hydrology will impact sizing and selection of infiltration practices. Off-Site Drainage Through the Site and Treatment of Off-Site Areas The project team should identify off-site drainage areas that enter the ROW, characterize the relative magnitude of the flow from these areas, and identify the land uses and potential pollutant sources associated with these areas. Off-site flows that enter or cross the project area may pose challenges for implementing infiltration practices because of excessive flowrates, high sediment loading (either chronic or episodic events), or high land use pollutant loading (posing a possible liability related to pollutant accumulation in the BMP and groundwater quality protection). In general, it is preferred to keep off-site flows separate from on-site flows. There can be overall environmental benefits and potential additional funding sources if a DOT elects to design a BMP to treat off-site runoff. Some DOT policies and state regulations may encourage treatment of off-site flows where feasible. If a BMP will treat off-site flows, the project team should characterize pollutant levels from the tributary area and potential hot spots that could contribute to elevated groundwater quality or sediment loading issues. Additionally, the DOT should develop appropriate agreements with the owner of adjacent land to (1) ensure upkeep of source controls within the watershed, (2) define responsibility for O&M of the facility and associated cost sharing, and (3) allocate liability for potential cleanup or remediation in the event of contamination of the BMP. If a framework exists for water quality trading or watershed-based compliance, off-site flows could also present opportunities for a project to provide additional water quality and flow control benefits to achieve watershed planning goals, possibly as part of a credit program. For example, a project could choose to manage flows from off-site and show a net benefit with respect to the hydrologic and water quality impact of the project. The project could also consider addressing off-site flows in one portion of the project to compensate for lack of opportunities to treat project runoff from other portions of the project. The existence and structure of water quality trading and watershed-based compliance options vary greatly among states and jurisdictions. Safety Considerations Highway safety laws, which vary between states, are a top priority when considering feasible siting opportunities for infiltration BMPs. The following safety considerations that are relevant to infiltration approaches. Highway Geometric Design Standards. Highway geometric design refers to the layout of highways, both horizontally and vertically. Geometric design standards vary by state and are typically derived from AASHTO’s Green Book (AASHTO 2011a). The key requirements for

Planning Framework for Early Decision-Making and Tentative BMP Selection 27 minimum geometric design standards are related to safety (e.g., site distance, stopping distance, design speed, etc.) and serviceability (e.g., land widths, overpass heights, etc.). Geometric design standards can influence infiltration BMP placement including the following: • Limit the flexibility of the designer to adjust site designs to accommodate infiltration BMPs. • Limit the features that can be located within the portions of the roadway (e.g., shoulders and medians) that may be traversed by errant vehicles. • Establish locations where it is acceptable to have depressions, inlet and outlet structures, soils with low structural strength, and vegetation. Vegetation and Landscaping Standards. AASHTO’s Roadside Design Guide (AASHTO 2011b) and the Federal Highway Administration’s (FHWA’s) Vegetation Control for Safety— A Guide for Local Highway and Street Maintenance Personnel (FHWA 2007) provide guidance for the types of vegetation that can be used in the road ROW. Vegetation and landscaping standards can influence infiltration BMP placement including the following: • Limit BMPs with mature vegetation to areas outside of lines of site and outside of errant vehicle recovery zones. • Ensure BMP placement allows vegetation maintenance to be performed safely. Drainage and Flood Control. Efficient and reliable drainage of stormwater from travel lanes is a critical safety consideration in the design of roadways. State DOTs typically adopt drainage criteria that specify acceptable hydrologic and hydraulic methods and minimum levels of service for travel lanes. The design of infiltration BMPs must comply with these regulations and not interfere with the level of service needed for the drainage of travel lanes including the following: • Analysis of BMPs to ensure that they do not increase the risk of flooding. Designers should consider cases in which infiltration rates are overwhelmed by intense rainfall, BMPs drain slowly at the end of their maintenance cycle, or both. For example, in evaluating permeable pavement shoulders, consider a case in which the permeable pavement is clogged and ensure that there is still a positive drainage pathway for water to drain from the travel lanes. • If infiltration is used as part of a flood control strategy, then designers should apply appropri- ate factors of safety to ensure that the target level of operation is reliably provided, even if the BMP is near the end of its maintenance cycle. Access for O&M. The project team should consult with O&M personnel to confirm that they can safely access BMP locations to perform O&M activities. If an area would require significant lane closure or unsafe access conditions, then it may not be feasible for BMP siting. Land Acquisition At this phase of the project, it may also be feasible to consider opportunities outside of the project footprint. This could include land acquisition to create more room for BMPs. It could also involve establishment of a regional treatment approach that manages runoff from an area greater than the project footprint. Potential benefits of these options are as follows: • Expand the range of sites considered, potentially allowing more suitable areas for infiltration. • Create more room for infiltration outside of the grading limits of the project. This can allow the project to better preserve the natural infiltration capacity and protect the area from construction-phase impacts. • Improve stormwater management system efficiency by treating water at a more regional scale. The BMP performance calculation tools available as part of NCHRP Report 792: Long-Term Performance and Life-Cycle Costs of Stormwater Best Management Practices and

28 Stormwater Infiltration in the Highway Environment: Guidance Manual NCHRP Report 802: Volume Reduction of Highway Runoff in Urban Areas can be used to help assess these options. • Centralize operation and maintenance activities. 2.2.3 Infiltration Capacity Assessment An understanding of the infiltration capacity of the site is critical to determine appropriate infiltration approaches and plan the site layout. Estimation of infiltration rates and potential for groundwater mounding can also influence assessment of geotechnical groundwater quality issues. The following are objectives of preliminary planning infiltration rate estimation: • Estimate potential reliable, long-term infiltration rates of soils (semi- quantitative) to determine the feasibility of achieving volume reduc- tion goals. • Evaluate the groundwater level, magnitude of potential groundwater mounding, and associated influence on geotechnical and groundwater quality issues. • Compare relative infiltration capacity of potential installation locations. As part of this step, the project team may use methods that are more efficient and less accu- rate than design-level methods. To support preliminary decision-making, the project team does not need to conclusively demonstrate feasibility of infiltration; rather it needs to under- stand the relative capacity at different BMP opportunity locations and classify opportunity sites into general bins (e.g., ideal, favorable, marginal, infeasible) (see Table 7). The level of effort required for preliminary planning infiltration rate assessment will depend on existing data availability, size of area considered for infiltration, and variations of soil type. Note: If there are overriding geotechnical or groundwater quality issues that do not depend on infiltration capacity, skip to Sections 2.2.4 or 2.2.5. In these cases, it may not be necessary to estimate infiltration capacity to support decision-making. Conduct Preliminary Assessment of Soil Infiltration Rate Preliminary planning phase investigation methods should yield adequate information to classify infiltration capacity per the general ranges in Table 7. The project team should conduct this investigation at locations within the project boundaries, where it is reasonable to site infil- tration BMPs and other feasibility factors do not preclude infiltration. Desktop methods using soil maps and available data may be adequate, particularly if soil is relatively uniform. For larger or more variable conditions, this Guidance Manual recommends some form of preliminary field verification, such as simple test pits or review of boring logs if available. Infiltration rate estimation and measurement methods are described in Appendix B, including applicability for preliminary screening, and confirmatory- and design-level investigation. Preliminary methods most appropriate at this step include the following: • Review of available reports and data • Review of soil maps • Estimates based on soil texture and other properties • Simple pit testing • Rapid infiltrometer and permeameter methods The project team may elect to use more rigorous tests if they are feasible and not cost- prohibitive. In this case, the results of these tests may also be suitable for subsequent feasibility confirmation as described in Step 3. Key Resources Appendix B: Infiltration Estimation Method Selection and Interpretation Guide Appendix C: Roadside BMP Groundwater Mounding Assessment Guide and User Tool

Planning Framework for Early Decision-Making and Tentative BMP Selection 29 The project team should consider the following factors when selecting methods for prelimi- nary infiltration assessments. Results of Other Groundwater and Geotechnical Investigations. Preliminary geotechni- cal and groundwater data collection (Sections 2.2.4 and 2.2.5) may provide useful information to inform this assessment. Soil type, soil variability, confining layers, and groundwater depths will all impact the selection of methods for infiltration assessments. This information could be available from geotechnical investigations. Infiltration Capacity Preliminary Infiltration Capacity (in./h)1 Qualitative Metrics2 Implications for Infiltration Feasibility Potential BMP Retention Depth3 Potential Sizing Factor for Full Infiltration4 Ideal > 5 HSG5 A soils and no indication of shallow groundwater or confining layer within 15 ft Ideal areas have the highest potential to achieve Full Infiltration and least potential to form a significant mound, even if there is limited space. 3 ft or greater 2% to 5%(level pool) Favorable 1 to 5 HSG A or B soils with sandy loam or coarser texture class, and no indication of shallow groundwater or confining layer within 10 ft or HSG A soils with groundwater > 5 ft These areas could support Full Infiltration BMPs. The feasibility may be contingent on confirmation of infiltration rate via design-level testing. 1 to 3 ft 5% to 10% (level pool) Marginal 0.1 to 1 HSG B or C soils with loamy or silty texture class (note some HSG D soils could fit into this class if they have low clay content), or confining layer or shallow groundwater conditions within 5 ft These areas can support Maximized Partial Infiltration but are unlikely to support full infiltration unless there is space to design shallow BMPs with low loading ratios. 0.2 to 1 ft (often complemented by treatment) 10% to 40%. Partial Infiltration BMPs are more likely to be suitable. Infeasible Less than 0.1 HSG D soils with significant clay content or very shallow groundwater or confining layer These areas likely do not support appreciable levels of infiltration; incidental infiltration may occur but may be too small to reliably estimate. Less than 0.2 ft Not feasible 1 The preliminary infiltration capacity is based on the raw estimate (no factor of safety) with adjustment to account for. limiting groundwater conditions if present. The provided ranges are recommended values in the absence of local guidance. 2 Quantitative metrics can be used to complement testing results or can be used in lieu of testing as part of preliminary. feasibility evaluation. 3 Potential BMP depth is based on 24- to 48-hr target drawdown time witha design infiltration factor of safety of 2 to 4. 4 Sizing factorrefers to the ratio of BMP footprint to the tributary impervious surface. Sizing ranges are based on a 0.8- to 1.6-in.water quality storm event and arange of allowable BMP depths. 5 HSG = Hydrologic Soil Group. Table 7. Preliminary infiltration capacity designation based on estimated reliable infiltration rate.

30 Stormwater Infiltration in the Highway Environment: Guidance Manual Fill Conditions. If BMPs will be installed in fill conditions, in situ measurements are not possible until grading has been completed. This can be an overriding limit on the use of Full Infiltration approaches that rely on a certain design infiltration rate. Cut Conditions. BMPs installed significantly below current grade require careful atten- tion to the presence of confining layers and interpretation of borehole methods as described in greater detail in Appendix B. Spatial scope and soil variability. The level of effort placed into preliminary infiltration assessments will depend on the availability of existing data and the size of the area considered. If a relatively small area is being considered, it may be cost-effective to proceed to more detailed, confirmation-phase investigations early in the project; however, for large sites, variable soil con- ditions, or projects with flexible layouts, project teams could use lower-intensity methods as the first phase of investigation, allowing a greater part of the site to be assessed efficiently. Ultimately, the selection of infiltration assessment method will require project-specific judg- ment. The key underlying goal at this phase is to support an initial planning-level assessment and allow comparisons between potential BMP sites. Determine Depth to Groundwater The depth to seasonal high groundwater table (normal high depth during the wet season) beneath a project may preclude infiltration. An elevated groundwater table can reduce the capacity of the aerobic vadose zone soil to attenuate pollutants. An elevated water table can also reduce the capacity of the soil to receive infiltrated water, which can lead to extended drawdown times and premature bypass or overflow of the system. The water table at a site often varies over time. Variations can occur diurnally (e.g., from tidal influence), seasonally (e.g., wet versus dry season), or over a longer period (e.g., from wetter versus drier conditions over several years). Therefore, obtaining longer-term records or measurements (one to several years) can be important if groundwater depth may limit infiltration. The project team should consider groundwater levels during preliminary site investigations to determine if groundwater limits may apply. Groundwater levels may vary across the project area, which can influence site layout considerations. Additionally, if groundwater may limit infiltration, the project team may choose to begin collecting groundwater level data early in the planning process to characterize temporal variations. Applicable methods for initial screening may include some or all of the following: • Test pits or bore holes. Use soil borings or pit investigations to measure the depth to ground- water. This provides measurement of the water table level at a given point in time. Consider precipitation conditions (wet, normal, or dry) and season in evaluating representativeness. • Hydric soils. Use redoximorphic indicators in test pits or boreholes to estimate seasonal high groundwater levels (U.S. Department of Agriculture 2016). • Desktop methods. Estimate seasonal high groundwater table based on review of available well data, regional groundwater elevation maps, soil maps, and geologic reports. For sites in which groundwater depth and hydrogeologic condition may limit infiltration, the project may require more rigorous studies to characterize the site hydrogeology and understand how groundwater levels react during wet and dry periods. The project team should initiate this investigation and monitor the project process to provide a long-term record. When long- term well measurements are available, the project team can estimate the depth to seasonal high groundwater level as the average of the shallowest measurement for all years on record (or alter- native local method if applicable).

Planning Framework for Early Decision-Making and Tentative BMP Selection 31 In addition to characterizing current conditions, the project team may also need to consider future rise in the groundwater table, such as from sea level rise, wider spread use of infiltration in the vicinity of the project, nearby water impoundments, or other factors. Groundwater level rise that could potentially interfere with BMP operation in the future could influence feasibility determinations. Assess Potential Groundwater Mounding These soil infiltration rate assessment methods introduced seek to understand the rate at which water will enter the ground surface (at the planned infiltration surface level) when unimpeded. However, the capacity of the groundwater receptor (i.e., the fate of water after it enters the surface) can also be a limiting factor in overall infiltration capacity of a site and can introduce concerns related to geotechnical hazards and groundwater quality protection. Some degree of groundwater mounding will inherently occur below stormwater infiltration BMPs. The formation of a groundwater mound is the response to a concentrated loading of water. The mound creates a local gradient in the groundwater table that allows the infiltrated water to dissipate from below the BMP. However, the degree and frequency of mound forma- tion relative to the vertical distance of separation to groundwater table can vary greatly by BMP type, design, and site conditions. This is important for assessing whether mounding may limit infiltration rates, introduce geotechnical concerns, or reduce pollutant attenuation capacity for groundwater quality protection. For preliminary screening purposes, mounding will very likely not be an issue when all the following criteria are met: • Depth to groundwater is 15 ft or greater, • The BMP is not located in the embankment, • Utilities, foundations, and retaining walls are not present within 20 ft of the BMP, • Soils above the water table are sandy loam, loamy sand, sand, or more permeable (approxi- mately 1 in./h or greater), • Loading ratios are less than 20:1 (tributary impervious area to BMP footprint area), and • Narrow dimension of the BMP is 10 ft or less. If these conditions are not met, then there is some elevated potential for mounding. Infiltra- tion may still be feasible, but it is recommended that the project team further assess site-specific conditions and their impact on mounding. Appendix C provides guidance for assessing groundwater mounding and describes how project planners and designers can use the User Tool (developed as part of this Guidance Manual) to support preliminary assessment. To use the Groundwater Mounding Assessment Tool, the user should collect or estimate as much of the following information as readily available: • Soil texture and estimated infiltration rate of limiting soil layers • Approximate depth to groundwater • Approximate ratio of roadway tributary area to BMP area (i.e., hydraulic loading ratio) • Proposed roadway cross section at the BMP location Using this tool, designers and planners can perform a sensitivity analysis to evaluate the mag- nitude of potential mounding. Users can then rapidly screen whether the mounding would possibly impact infiltration rates or compromise the separation between the BMP and the groundwater table (see Figure 3). Prolonged saturation and reduced separation to groundwater could reduce pollutant attenuation effects in the vadose zone. Geotechnical engineers could also use the output from the tool as part of geotechnical assessments.

32 Stormwater Infiltration in the Highway Environment: Guidance Manual Interpret Results of Infiltration Capacity Rate Assessment The primary objective for preliminary site investigations is to determine the semi-quantitative infiltration capacity at potential BMP locations within a project site. Table 7 provides general guidance for interpreting preliminary assessments. Designers should consult appropriate local regulations and design guidance to determine the applicability of the screening thresholds reported in Table 7 for a specific site. Note, if there are other factors that preclude infiltration in an area (slope, landslides, contami- nation), it is not necessary to quantify the physical infiltration capacity. These factors could rule out infiltration in any quantity. Prepare Infiltration Capacity Site Map Infiltration capacity site maps can be a useful tool for site planning. Project teams can con- struct these maps by overlaying infiltration rate assessments with other site constraints and feasibility criteria. Maps should build on the identified preliminary infiltration opportuni- ties map (Section 2.2.2). The infiltration capacity site map and corresponding documentation should describe the following: Imp.– Impervious. Figure 3. Example of groundwater mounding nomographs of selected BMP for Birmingham, Alabama.

Planning Framework for Early Decision-Making and Tentative BMP Selection 33 • Location of BMP opportunity areas (Section 2.2.2) • Locations of infiltration testing conducted at the project site • Boring log locations and results relevant to infiltration feasibility determination • Raw results and professional interpretation of infiltration test results • Categorization of areas into infiltration feasibility classes (see Table 7): ideal, favorable, marginal, and infeasible • Key geologic and groundwater features identified per other investigations (Section 2.2.4 and 2.2.5) Design teams can use the infiltration capacity site map to compare the infiltration capacity across the project site to aid in site layout decision-making. For smaller projects with less opportunity for grading or design changes, the infiltration capacity site map will be of less use in comparing project areas. However, maps can still be useful in supporting selection of an infiltration feasibility designation and selecting appropriate BMPs that fit within spatial constraints. 2.2.4 Preliminary Geotechnical Feasibility Factors and Investigation Methods Infiltration of stormwater can contribute to geotechnical issues that result in impacts to adjacent structures, utilities, or graded surfaces. Storm- water infiltration temporarily raises the soil moisture and groundwater levels below and adjacent to the infiltrating area. Geotechnical risks are greatest near the BMP and typically diminish with lateral distance. Accurate geotechnical assessments and supporting analyses are essential to prevent damage associated with infiltration in the roadway environment. Prelimi- nary geotechnical assessments can serve an important purpose in helping to site BMPs and form appropriate infiltration goals at an early phase. The role of preliminary site geotechnical investigations is to identify geologic or geotechnical hazards that would clearly or potentially limit infiltration. Preliminary geotechnical investigations should involve review of several desktop data sources including the following: • Available soil maps • Geological reports • Available site investigations (such as previous borings) • Regionally applicable data (such as testing of similar soil units from different projects) • Rough grading plans The following subsections explain key aspects of the preliminary geotechnical feasibility evaluation and the associated planning-level methods and feasibility screening criteria that may be appropriate. Evaluate Limiting Geotechnical and Geologic Factors Relevant geological and geotechnical factors that should be reviewed at this phase. Depth to Confining Layer and Slope of Confining Layer. Does a shallow confining layer pose a potential risk of lateral water migration and mounding-related hazards if infiltration is to be used? A depth of 15 ft or less is considered shallow and would warrant further investigation. A sloping confining layer could also indicate potential issues, because infiltrated water could travel along this face and result in a landslide or other issue. Presence of Karst Geology. Karst can be prone to sinkhole formation. It also has ground- water quality implications, as described in Section 2.2.5. The presence of karst geology is an issue that would likely preclude infiltration. Key Resources Appendix C: Roadside BMP Groundwater Mounding Assessment Guide and User Tool Appendix E: Guide to Geotechnical Considerations Associated with Stormwater Infiltration Features in Urban Highway Design

34 Stormwater Infiltration in the Highway Environment: Guidance Manual Proposed Fill Conditions. Compacted fills have important influence on infiltration feasi- bility. While infiltration into fill does not inherently pose unacceptable risks, it is not possible to determine the physical properties or infiltration rates in fill soil at the preliminary planning or design phases, unless the origin and type of the fill is known and tightly specified. Even so, there would be considerable uncertainty in this estimate. As a result, these areas may be inherently unsuitable to support infiltration systems that rely on a certain minimum infiltration rate. If the depth of fill (including any remedial excavation and compaction) exceeds 5 ft, it is generally not reasonable to install a deeper profile BMP to achieve infiltration. If fill is less than 5 ft, then it is possible that the BMP could be extended into more permeable underlying soil. Collapsible Soils. Collapsible soils are loosely deposited sediments that are separated by coatings or clay/carbonate particles. Hydrocollapse occurs when soil saturation results in the deterioration of the soil structure. Preliminary desktop assessments should evaluate the potential for hydrocollapse, especially in areas near proposed infiltration practices and potential mitigation measures. This could rule out any level of infiltration. Expansive Soils. Expansive soil is defined as soil or rock material that has a potential for shrinking or swelling under changing moisture conditions. Expansive soils contain clay miner- als that expand in volume when water is introduced and shrink upon drying. Expansive soil movement can affect nearby structures, such as foundations and roadways. Preliminary desktop assessment should evaluate whether expandable materials are present near possible infiltration facilities and potential mitigation measures. This could rule out any level of infiltration. Potential for Liquefaction. Soil liquefaction is a phenomenon in which soil strength is lost as a result of an earthquake or other rapid loading that occurs within a saturated granu- lar soil, resulting in the soil behaving temporarily as a liquid. This can cause lateral spreading of embankments and areas of sloping ground. If soil types and groundwater levels show the potential for liquefaction, the geotechnical engineer should consider the effect of stormwater infiltration within these areas as part of geotechnical analyses. This could rule out infiltration if it results in prolonged elevated water tables that significantly increase liquefaction risk. Shorter- term fluctuations in the groundwater table caused by episodic infiltration would pose a lower risk, because liquefaction requires that ground shaking happen concurrently with saturation. Establish Planning-Level Setbacks from Pavement, Structures, and Utilities Decreased soil strength because of elevated soil moisture levels near infiltration BMPs can make foundations more susceptible to settlement and slopes more susceptible to failure. Infil- tration BMPs must be set back an adequate distance from building foundations or steep slopes. At the preliminary planning stage, the project team should consult with the project geotechnical professional to determine appropriate setbacks from pavement, slopes, and structures. Assess Risks Related to Groundwater Mounding and Near-Roadway Soil Saturation The development of a groundwater mound could be an important factor in geotechnical risk. For example, saturation of soils or fluctuations of groundwater level near slopes could reduce soil strength and slope stability. The potential magnitude of mound formation depends on many factors but tends to be greatest in finer grained soils and for more centralized BMPs with larger dimensions and loading ratios. Appendix C presents guidance and an Excel-based tool for estimating potential mounding response based on a preliminary understanding of site conditions and potential BMP types. The geotechnical engineer can use the output from this tool to support a preliminary evalu- ation of the shape and extent of the groundwater mound. The mound can be overlaid with

Planning Framework for Early Decision-Making and Tentative BMP Selection 35 the proposed roadway cross section and adjacent infrastructure or utilities to determine if the mound would contribute to geotechnical issues. Figure 4 shows a subset of an example output from the Groundwater Mounding Assessment Tool. Should the results from this tool indicate that mounding or increase in soil moisture may be an issue, the project team can use the tool as a sensitivity analysis to test which parameters may have the greatest influence on mounding. This can focus the scope of subsequent site investigation efforts. (Note: See Appendix C for tool description and documentation. Figure 4 shows select tool output for a permeable shoulder in Denver, Colorado, with a loading ratio of 8:1 and an initial groundwater depth of 4 ft. Green lines in the upper right and upper left parts of the figure depict maximum groundwater elevations during a 6-month simulation. Points A and B are model monitoring nodes beneath the edge of the pavement. Green and red colored points in the upper right indicate points that were never saturated during the 6-month simulation and points that were saturated at least once during the 6-month simulation, respectively. The bottom part of the figure depicts time-series data for precipitation as blue bars, groundwater levels as green lines, and surface ponding as blue lines.) Figure 4. Example output visualization from Groundwater Mounding Assessment Tool.

36 Stormwater Infiltration in the Highway Environment: Guidance Manual Summarize Planning-Level Findings Investigations of soil and geological properties at the preliminary planning stage should be only as detailed as needed to support site layout planning and initial selection of BMP strategies. Using results from these investigations, the project team can overlay soil and geological features on the physical constraints map to identify areas where (1) no issues exist, (2) potential issues exist that require further re-evaluation or may limit the amount of infiltration allowed, and (3) clear limiting conditions exist that would preclude infiltration. If, based on this evaluation and other feasibility factors, the team selects infiltration as a potential stormwater management approach, then more detailed investigations may be needed at the areas where infiltration is proposed, as described in Chapter 3. 2.2.5 Preliminary Groundwater Quality Feasibility Factors This Guidance Manual recommends researching applicable ground water quality standards and local groundwater protection requirements, then researching the physical setting of the project including depth to ground- water, pollutant sources, and existing soil or groundwater contamination. The following sections provide recommended steps to conduct prelimi- nary screening. Appendix D provides additional guidance and supporting technical information related to groundwater quality considerations. Research State and Local Standards That Apply to Stormwater Discharges to Groundwater Applicable groundwater standards, regulatory frameworks, and ground- water protection criteria vary by state and locality, depending on the beneficial uses of the groundwater and state or local agency implementation of ground- water regulations and programs. As a result, applicable water quality criteria can vary greatly and, in some cases, can limit stormwater infiltration approaches. Key questions for the project planners to research at a project or regional level include the following: • Does the local groundwater management agency have a wellhead protection plan, source water protection plan, or similar plan for protecting groundwater quality? If so, does it include infiltra- tion prohibitions, siting criteria, pretreatment criteria, water quality criteria, or other guidance? • Is the aquifer designated an SSA (https://www.epa.gov/dwssa)? • Are there other aquifer-specific plans that apply, such as salt and nutrient management plans, that govern discharges from the project? • What water quality criteria apply to discharges to groundwater? This can depend on the local regulatory framework, the beneficial use of the groundwater, and the current quality of the groundwater. • Are water quality criteria based on specified concentration limits? If so, what is the basis for these limits? Examples could include maximum contaminant levels (MCLs) derived from the Safe Drinking Water Act (SDWA) or limits based on the CWA for protection of surface waters that receive groundwater discharges. • Are groundwater water quality protection requirements based on anti-degradation (i.e., the discharge shall not deteriorate existing water quality)? If so, what is the current water quality of the groundwater that must be preserved, and what parameters are used to evaluate this? • Where do these limits apply (i.e., point of compliance)? This can vary including the point where infiltrated water discharges to groundwater (immediately below the BMP), a plane where ground- water leaves a site (e.g., the ROW boundary in a highway), the point of extraction (e.g., the nearest down-gradient well), or other location established by the applicable regulatory authority. Key Resources Appendix C: Roadside BMP Groundwater Mounding Assessment Guide and User Tool Appendix D: Guide for Assessing Potential Impacts of Highway Stormwater Infiltration on Water Balance and Groundwater Quality in Roadway Environments

Planning Framework for Early Decision-Making and Tentative BMP Selection 37 • What is the separation to private or public water wells? Do these wells draw from a shallow unconfined aquifer or a deeper confined aquifer? This research should typically involve review of rules, guidance, and policies adopted by state environmental quality agencies and local groundwater management agencies. Conduct Preliminary Screening of Potential Groundwater Quality Impacts For planning-level assessment, the project team should determine whether applicable ground- water standards preclude infiltration, require specific considerations for infiltration (e.g., spill containment, pretreatment), or do not limit infiltration. Two primary categories of sites are relevant for interpreting the influence of groundwater quality limits. Sites with Groundwater Quality Standards Based on Drinking Water MCLs. As summa- rized in Appendix D, infiltration of highway runoff poses limited risk to groundwater in cases in which the primary beneficial use is municipal water supply, and water quality standards are based on MCLs. The following are exceptions: • In areas where deicing salts are applied, salt can accumulate and form plumes that exceed MCLs, particularly where points of compliance (e.g., wells) are near the highway. The Groundwater Quality Assessment Tool (found in Appendix D) can be used to evaluate acute impacts of deicing salts on nearby groundwater quality. • Pathogenic bacteria and viruses can be mobile and persistent in groundwater. The presence of human pathogens is primarily an issue in urban areas where human waste may be present in stormwater. In these areas, groundwater is nearly always treated before being used in water supplies, mitigating this risk. In rural areas, however, the project team should consider the potential for pathogen contamination of water wells. A setback of 100 ft from drinking water wells is common, but local ordinances may prescribe much more stringent criteria to protect wells, such as 1-year or 10-year time of travel zone (i.e., the area that could enter a well within a 1-year or 10-year period, for example). • Where groundwater is very shallow, soil is low in organic matter, or both, the vadose zone may have inadequate pollutant attenuation to prevent breakthrough of organic compounds, such as polyaromatic hydrocarbons (PAHs), to groundwater. This can be mitigated through use of organic soil amendments, observation of minimum separation from groundwater of at least 5 ft (or more if required by local regulations), and evaluation of groundwater mounding to ensure that there are not extended periods of diminished vadose zone thickness (City of Portland 2008; Brody-Heine et al. 2011). • Water soluble pesticides such as neonicotinoid pesticides are highly mobile in soil and ground- water. These have the potential to pose risks to human health if wells are nearby; however, these parameters are infrequently detected in untreated stormwater at levels of concern to human health and can be managed via selection of pest control products in the ROW. • In areas of karst topography, there can be limited or no attenuation, and direct stormwater inflows should be avoided. In each case, the proximity to a public or private drinking water well and connectivity between surface infiltration and the production aquifer are important in classifying risk. Other highway runoff contaminants are unlikely to approach MCLs or are not very mobile in soils under most conditions. Observations of metal buildup and breakthrough have been noted in some research, particularly in sandy soils that lack attenuation capacity (Pitt et al. 1999; Weiss et al. 2008). However, untreated metals concentrations in highway runoff tend to be much lower than MCLs (10 times or more), indicating relatively low risk to human health, even if no treat- ment occurred in the vadose zone (Table 8). Note that aquifers used for municipal drinking water supply may have local ordinances related to aquifer or well-head protection. This may prohibit stormwater infiltration or require specific

38 Stormwater Infiltration in the Highway Environment: Guidance Manual approaches to protect groundwater quality, including pretreatment, spill containment, separa- tion distance, or other approaches. In general, accidental contaminant spills, such as solvents or petroleum products, are the principal concern for groundwater quality protection in these areas. Sites with Groundwater Quality Criteria Based on Anti-Degradation Policy or Surface Water Standards. Groundwater quality criteria based on anti-degradation policy or surface water standards can be much more stringent than drinking-water-based limits for some con- taminants. If this regulatory framework applies, the applicable water quality criteria depend on the existing quality of the groundwater, the beneficial uses of the groundwater, the existing water quality, and beneficial uses of the surface waters that receive groundwater discharges. If groundwaters are degraded, stormwater infiltration can improve groundwater quality. For example, monitoring in Fresno, California, has shown that widespread use of stormwater infiltra- tion over more than 40 years has had the effect of reducing nitrate concentrations in their SSA, which has also experienced impacts from agriculture (http://www.rechargefresno.com/groundwater/). If groundwater is relatively clean and stormwater is discharged to groundwater on a long-term basis, it may be impossible to avoid groundwater quality deterioration. Certain soluble contami- nants in stormwater, such as nitrate, could exceed background levels if the groundwater is espe- cially clean. Additionally, over time, metals, PAHs, soluble pesticides, and other partially mobile contaminants could break through soil layers and cause detectible increases in groundwater concentrations. Breakthrough can be mitigated with soil amendments and periodic removal of surface soils; however, this risk cannot be eliminated over long periods of operation. Periodic removal of surface soils/media may also be part of an operations plan to dispose of materials before they build up contaminates such that they become classified as hazardous wastes. Parameter Typical Influent Quality1 Typical Sand Filter Effluent Quality1 Drinking Water MCL Representative Surface Water Quality Standard E. Coli, count/100 mL 6,025 1,805 Zero (not present) 235 Fecal Coliform, count/100 mL 8,700 2,488 Zero (not present) 400 Total Copper, ug/L 42 19 1,000a 10 to 50c Total Lead, ug/L 44 5 15b 20 to 130 c Total Zinc, ug/L 190 26 5,000a 50 to 200 c Nitrate-N [NO3-N], mg/L 1.06 1.06 10 Narrative (d) Total Nitrogen, mg/L 3.59 2.40 N/A Narrative(d) Total Phosphorus, mg/L 0.44 0.20 N/A Narrative (d) Total Suspended Solids (TSS), mg/L 139 15 N/A (Turbidity < 1) Narrative (50 to 100 mg/L is typical) Chloride Less than 20 without deicing Can exceed 1,000 with deicing Same as influent (no removal) 250 a 300 1 NCHRP Report 792 (Taylor et al. 2014). a Secondary MCL. b Action level based on 1991 Lead and Copper Rule. MCL is zero. c Metal standards are hardness dependent or based on the Biotic Ligand Model. Ranges are representative for chronic contaminant levels for illustration purposes only. d Except where waterbody specific criteria exist, water quality standards for nutrients are typically narrative, based on biostimulatory effect. Limits can vary greatly based on water body sensitivity and limiting nutrients. Table 8. Typical highway runoff concentrations and filtration BMP effluent quality.

Planning Framework for Early Decision-Making and Tentative BMP Selection 39 Sensitive surface waters may have water quality standards considerably lower than drinking water standards. For example, the drinking water MCL for copper is 1 mg/L, but toxicity-based standards (for fish and other aquatic biota) for copper in receiving waters can be less than 0.02 mg/L. Where anti-degradation or a direct connection to a sensitive surface water is present, a specific evaluation of the local regulatory framework, applicable standards, existing groundwater quality, and highway runoff quality is warranted to determine if any level of infiltration is allowable. This Guidance Manual does not contain categorical conclusions about these conditions. Determine Depth to Groundwater and Assess Vadose Zone Thickness for Pollutant Attenuation In addition to posing a physical limitation, an elevated groundwater table can reduce the capacity of the aerobic vadose zone soil to attenuate pollutants. Groundwater mounding can further reduce the thickness of the aerobic vadose zone. Section 2.2.3 provides guidance for assessing the depth to groundwater and the potential for mound formation. Project teams should refer to local criteria for minimum separation to seasonal high, mounded groundwater to protect groundwater quality. Minimum separation may be specified in ground- water protection ordinances, Municipal Separate Storm Sewer System (MS4) permits, or other local criteria. In the absence of local criteria, planners and designers should observe a minimum separation of 2 to 4 ft from the seasonally high, mounded groundwater table to the infiltrating surface. This is intended to maintain an unsaturated aerobic vadose zone to support pollutant attenuation and retention. Investigate Pollutant Attenuation Properties of Soil Soil properties can influence the ability of soils to attenuate pollutant loads to protect ground- water quality. Sandy soils with high permeability and low organic content can have limited pollutant attenuation capacity, especially for dissolved constituents and those bound to very small particles. At the preliminary assessment phase, this is not typically a controlling factor in decision-making because pretreatment and soil amendments can be used to augment treatment capacity if soils are too coarse or inert; however, if the project team collects available information about soil texture and organic content at this phase, this information can be used later to assess whether amendments are needed. This information could be available from soil maps, bore logs, soil studies from nearby projects, or other sources. Investigate Existing Soil and Groundwater Contamination In areas with known or potential groundwater or soil contamination, the project team may need to avoid infiltration if it would contribute to the movement or dispersion of soil or groundwater contamination or adversely affect ongoing clean-up efforts. The presence of groundwater or soil contamination can preclude any level of infiltration. Pollutant mobilization can occur on-site or down-gradient of the project. Mobilization of groundwater contaminants may also be of concern where contamination from natural sources (e.g., marine sediments, groundwater naturally high in phosphorus, selenium rich groundwater, etc.) is present. In some situations, infiltration BMPs may positively impact existing contamination issues because of dilution effects. For example, if ground- water is high in total dissolved solids, infiltrating stormwater might benefit groundwater quality. As part of preliminary site investigations, the project team should review available site data, state databases of contaminated sites, regional guidance, and other sources to determine if exist- ing soil or groundwater contamination is a concern. If the team is considering infiltration in areas where soil or groundwater pollutant mobilization is a concern, then decisions should be supported by a site-specific analysis to determine where infiltration-based BMPs can be used without causing or contributing to adverse impacts. Appendix D provides specific guidance on assessing groundwater and soil contamination to ensure that project drainage plans do not contribute to movement or dispersion of groundwater contamination.

40 Stormwater Infiltration in the Highway Environment: Guidance Manual 2.3 Selection of Preliminary Infiltration Approach 2.3.1 Overview This step guides users in selecting a preliminary infiltration approach that aligns with the established infiltration objectives (Section 2.1) and the preliminary infiltration feasibility (Section 2.2). The primary elements of this step include the following (see also Figure 5): 1. Compile site assessment data to determine preliminary feasibility and desirability of infiltration. 2. Interpret preliminary site assessment data in the context of infiltration objectives. 3. Tentatively select an infiltration strategy: (1) Full Infiltration, (2) Maximized Partial Infiltra- tion, (3) Incidental Infiltration, or (4) an alternative non-infiltration approach. The following sections provide guidance for each of these steps. There are two key concepts to consider at the outset of this process: • Infiltration should not be a “Yes” or “No” decision. While some conditions exist that can limit any amount of infiltration, this decision is not often a simple “Yes” or “No.” More often, the practical decision facing project proponents is whether to attempt to rely fully on infiltra- tion to meet applicable BMP sizing requirements (e.g., Full Infiltration) or use practices that promote partial or incidental infiltration while also providing supplemental non-infiltration processes to meet sizing requirements. • The tentative selection may not be final. This is particularly true if rapid or coarse methods were used to determine that infiltration appears viable. The project team will often need to confirm preliminary findings with testing at specific BMP locations. Tentative selection of BMP types can and should change if more refined methods yield different findings about feasibility. 2.3.2 Integrated Assessment of Planning-Level Feasibility The outcome of this step is a determination of the level of infiltration feasibility and desirability at the locations where BMPs can be reasonably located. Project teams can use Table 9 to organize preliminary site assessment information and document the preliminary level of infiltration feasibility. This table is intended to be used for each BMP location or for relatively uniform segments of the project. If all conditions in Section 1 apply, then the rating is “favorable.” If one or more conditions in Section 1 do not Select Preliminary Class of Infiltration Approach (Section 2.3) What class of approach best aligns with objectives and conditions? [Full Infiltration | Partial Infiltration | No Infiltration] Infiltration Objectives (Section 2.1) How rigorously should infiltration be investigated? Preliminary Infiltration Feasibility (Section 2.2) How likely is it that infiltration will be feasible? Figure 5. Overview of methodology for preliminary selection of infiltration class.

Planning Framework for Early Decision-Making and Tentative BMP Selection 41 apply, but all conditions in Section 2 apply, then the rating is “marginal.” If any criteria in Section 3 apply, then the rating is “infeasible.” 2.3.3 Identification of Tentative Infiltration Approach To support tentative decision-making, the project team should review the feasibility findings summarized with the infiltration objectives established in Section 2.1: • Where conditions are clearly favorable for infiltration, the decision to proceed with an infil- tration approach may be obvious, regardless of infiltration objectives. Infeasible: Any condition in Section 3 is met. Applicable? Section 1: Favorable Conditions (check all that apply) Infiltration capacity is rated as favorable or ideal (see Table 7). The BMP location can be tested prior to construction and can be protected from construction impacts (Section 2.2.2). The depth to groundwater table including temporal variations can be reasonably assessed (Section 2.2.3). Groundwater mounding does not limit infiltration rates or come within 2 ft of the bottom of the BMP (Section 2.2.3). Geotechnical hazards to structures, slopes, pavement, or other infrastructure that preclude infiltration are not identified (Section 2.2.4). Applicable groundwater protection criteria do not preclude stormwater infiltration and infiltration is considered to pose a low risk to groundwater quality (Section 2.2.5). Soil and groundwater contamination are not present (Section 2.2.5). There is adequate space that can be used for a level-bottomed BMP (see ranges of potential space requirements in Table 7). Section 2: Marginal Conditions (check all that apply) One or more conditions in Section 1 are not met or could not be adequately assessed at the time of preliminary decision-making. Infiltration capacity is rated as marginal or better (Section 2.2.3). Infiltration does not pose unavoidable geotechnical or groundwater quality issues that preclude infiltration (Section 2.2.4 and 2.2.5). Applicable groundwater protection criteria include limitations on infiltration or potential groundwater quality impacts may be present and require further assessment (Section 2.2.5). The project has some amount of space available for infiltration but may not meet the required space for Full Infiltration (see ranges in Table 7). Section 3: Infeasibility Factors (check any that apply) Any condition is identified that poses an unavoidable geotechnical risk that limits any level of infiltration. Applicable groundwater protection criteria prohibit infiltration, or any condition is identified that poses an unavoidable risk to groundwater quality or sensitive receptors. Soil infiltration rates are rated as infeasible and do not support an appreciable level of intentional infiltration. Section 4: Integrated Summary (identify category that applies) Favorable: All conditions in Section 1 are met. Marginal: All conditions in Section 2 are met, but one or more conditions in Section 1 are not met. Table 9. Worksheet for rating preliminary feasibility conditions.

42 Stormwater Infiltration in the Highway Environment: Guidance Manual • If infiltration is clearly infeasible, then the stringency of infiltration objectives may not be rel- evant for making decisions about infiltration. Regardless of the level of stringency, infiltration should not be used. However, the stringency of requirements may be relevant for document- ing these decisions and identifying an acceptable alternative. • Infiltration objectives have the most relevance when sites have marginal feasibility condi- tions. In this “middle ground,” it is possible that a range of infiltration approaches could be used but each would have different tradeoffs regarding effectiveness, the need for addi- tional assessment, space requirements, and risk of failure. The stringency of infiltration objectives can be an important deciding factor in selecting a tentative infiltration approach and identifying the need for additional site investigations and analyses. When conditions are marginal, the project team should avoid Full Infiltration BMPs unless further consid- eration is mandated by project objectives. Table 10 provides a matrix to help project teams select tentative infiltration approaches based on the infiltration objective category and tentative infiltration condition. Each of these approaches has different implications for BMP selection, which are described in Sec- tion 2.4. The remaining efforts needed to confirm the selected approach also vary by track (see Chapter 3). 2.4 Tentative BMP Selection 2.4.1 Overview This section supports Step 2 in the decision-making process (see Figure 6). This step involves tentatively selecting the BMP types and locations for the project. This should be based on the preliminary findings from Sections 2.2 and 2.3 (collectively Step 1 in the decision-making process). The purpose of this step is to narrow down the potential BMP locations and types of BMPs so that appropriate confirmatory investigations and analyses can be scoped if needed. The results of this step may be tentative. Key questions include the following: • Which BMPs are most appropriate given the overlay of preliminary infiltration feasibility category and infiltration objectives? (Section 2.4.2) Tentative Infiltration Condition (Section 2.2 and 2.3) Infiltration Objective Category (Section 2.1) Opportunistic Maximized Specified Performance Level Favorable Track 1a Full Infiltration Track 1a Full Infiltration Track 1a Full Infiltration Marginal Track 2a Maximized Partial Infiltration Track 2a Maximized Partial Infiltration Track 1b Full Infiltration or Partial Infiltration (additional data required to support decision) Infeasible Track 3a Incidental/No Infiltration Track 3b Incidental/No Infiltration with additional supporting documentation (if needed) Track 3b Incidental/No Infiltration with additional supporting documentation Table 10. Tentative infiltration approach selection matrix.

Planning Framework for Early Decision-Making and Tentative BMP Selection 43 • For the site areas where infiltration could be implemented, which BMPs are applicable or suitable (considering location, geometry, and size of available space)? (Section 2.4.3) • Which BMPs are compatible with local climate? (Section 2.4.4) • If multiple BMPs are available, how do they compare in relation to other decision factors? (Section 2.4.5) – Relative level of geotechnical risks – Relative level of groundwater quality risk – Relative safety – Relative whole lifecycle costs – Relative O&M requirements 2.4.2 BMP Suitability for Infiltration Planning Track The overlay of infiltration objectives and infiltration feasibility categories has a strong influence on the BMP types that may be suitable for the project. Table 11 identifies a narrower menu of potential BMPs based on the categorization conducted in Section 2.3.3. 2.4.3 BMP Suitability by Roadway Project Type and Site Features The roadway project type and site features strongly influence what types of BMPs could be reasonably sited to receive roadway runoff. Table 12 provides a summary of the relative suit- ability of various BMPs for different highway opportunities considering the typical space and inherent shape associated with each opportunity. Figure 7 provides an example illustration of common geometric siting opportunities that may be present in the highway environment. BMP Suitability for Infiltration Planning Track (2.4.2) Tentatively Selected BMP Locations and Types (2.4.6) Other BMP Selection Factors (2.4.5) Compatibility with Local Climate (2.4.4) Compatibility with Roadway Geometry (2.4.3) Figure 6. Overview of BMP selection approach (Step 2).

44 Stormwater Infiltration in the Highway Environment: Guidance Manual Screening Condition Potential BMPs and BMP Adaptations Best Suited to Infiltration Screening Conditions1 Track 1a – Favorable Infiltration Conditions • Select BMPs to provide Full Infiltration. • Confirm planning-level feasibility findings. • Include adaptable features if desired. BMP 02 Dispersion/Filter Strips (where soils are permeable, and enough dispersion area can be provided to reliably infiltrate the full sizing criteria) BMP 03 Media Filter Drain (without underdrain) BMP 04 Permeable Shoulders (optionally with capped underdrain) BMP 05 Bioretention without Underdrains (optionally with capped underdrain) BMP 07 Infiltration Trench BMP 08 Infiltration Basin BMP 09 Infiltration Gallery Track 1b – Stringent Infiltration Objectives in Marginal Conditions • Conduct further investigation to support BMP selection. • Select BMPs with emphasis on adaptability and resiliency. If infiltration feasibility improves with further investigation, the BMPs described for Track 1a may be feasible. If Full Infiltration cannot be supported based on the results of further investigation, then refer to Track 2a for available BMPs. If it is not possible to achieve adequate confidence in the infiltration assessment, but infiltration must still be attempted, then select BMPs that can be readily adapted as part of the construction and post- construction process, such as the following: BMP 03 Media Filter Drain (typical design with underdrain but with the ability to cap or uncap the underdrain depending on actual in- situ conditions) BMP 05 and BMP 06 Bioretention without and with Capped Underdrains, respectively,(can be uncapped if rates do not support Full Infiltration) Track 2a – Maximized Infiltration Objectives in Marginal Conditions • Select BMPs to maximize level of infiltration and ET within site constraints. • Design BMPs such that they do not rely on a certain minimum infiltration rate to remain operable. BMP 01 Vegetated Conveyance (including shallow sump or check dams) if possible BMP 02 Dispersion/Filter Strips (with amended soil) BMP 03 Media Filter Drain (typical design with underdrain) BMP 04 Permeable Shoulders (with elevated underdrains, creating a gravel reservoir for Partial Infiltration) BMP 06 Bioretention with Underdrains (with elevated underdrains, creating a gravel reservoir for Partial Infiltration) Track 3a and 3b – Limited or No Infiltration Feasible • Select BMPs to limit infiltration and provide ET and supplemental treatment processes. • Collect additional information to support decision, as necessary. BMP 01 Vegetated Conveyance (with amended soil and positive drainage) BMP 02 Dispersion/Filter Strips (with amended soil) BMP 03 Media Filter Drain (underlain by low permeability soil) BMP 06 Bioretention with Underdrains (lined or with underdrains at bottom of facility) Non-infiltration approaches 1 See Appendix A for BMP Fact Sheets. Table 11. Summary of infiltration BMPs potentially suitable for each planning track. Table 13 provides a checklist for organizing findings about the applicability of BMPs to a given site: • Part 1 summarizes screening of project geometric design features and BMP siting opportunities. • Part 2 summarizes screening of overall project attributes such as available space and presence of storm drains. • Part 3 summarizes how other project-specific factors can influence BMP selection. • Page 4 summarizes the applicability and suitability screening.

Geometric Siting Opportunity BM P 01 V eg et at ed C on ve ya nc e BM P 02 D is pe rs io n BM P 03 M ed ia F ilt er D ra in BM P 04 P er m ea bl e Sh ou ld er s BM P 05 B io re te nt io n w /o U nd er dr ai ns BM P 06 B io re te nt io n w ith U nd er dr ai ns BM P 07 In fil tra tio n Tr en ch es BM P 08 In fil tra tio n Ba si ns BM P 09 In fil tra tio n G al le rie s Narrow Medians X X X X X X Wide Medians X X X X X X X X X Shoulders including breakdown lane and area within the clear zone (less than approx.15% or 6H:1V) X X X X X X Shoulders outside of the clear zone (less than approx.15% or 6H:1V) X X X X X X X Moderately Steeper Shoulders (steeper than approx.15% or 6H:1V but less than approximately 25% or 4:1) X ROW Locations with Limited Uses (e.g., wide spots, irregular geometries) X X X X X X X X Adjacent Natural Areas X Looped Interchange Medians X X X X X X X X Diamond Interchange Medians X X X X X X X X Low Traffic Areas, Maintenance Yards, etc. X X X 1 X X X X X 1 Permeable pavement in general; shoulders not present. Table 12. Summary of geometric siting opportunities by BMP type. Looped Interchanges Diamond Interchanges Irregular ROW Shoulder Outside of Clear Zone Shoulder Inside of Clear Zone or Breakdown Lane Median or Inside Breakdown Lane Figure 7. Key to common geometric opportunities within urban highway environment (hypothetical opportunities shown).

46 Stormwater Infiltration in the Highway Environment: Guidance Manual Part 1: Screening of Project Geometric Design Features and BMP Siting Opportunities Instructions: 1. Enter Y (for Yes) or N (for No) in the “Project Attribute” row to indicate project attribute that is present in the project. 2. Match opportunity to BMPs that are potentially applicable in that location. 3. Enter result: Is there potentially a location where each BMP could be sited? BMP M ed ia ns Sh ou ld er s, in cl ud in g br ea kd ow n la ne a nd a re a w ith in c le ar z on e Sh ou ld er s, o ut si de o f c le ar zo ne St ee pe r S ho ul de rs R O W L oc at io ns w ith L im ite d U se s (e .g ., w id e sp ot s, irr eg ul ar g eo m et rie s) Ad ja ce nt N at ur al A re as Lo op ed In te rc ha ng e M ed ia ns D ia m on d In te rc ha ng e M ed ia ns Lo w T ra ffi c Ar ea s, M ai nt en an ce Y ar ds , e tc . Project Attribute: X X X X X X X (if wide) X X X X X X BMP 02 Dispersion (outside of ROW) X BMP 03 Media Filter Drain X X X X X X X BMP 04 Permeable Shoulders (if paved) X X 1 BMP 05 Bioretention w/o Underdrains X X X X X X BMP 06 Bioretention w Underdrains X X X X X X BMP 07 Infiltration Trench (if wide) X X X X BMP 08 Infiltration Basin (if wide) X X X X BMP 09 Infiltration Gallery (if wide) X – Potential BMP opportunity when geometric project feature is present. 1 Permeable pavement in general; shoulders not typically present. Key for Table 13 (Part 1) Headings User Input Guidance No Meaningful Nexus with Site Geometric Design Features Result: Opportunity to Site BMP? BMP 01 Vegetated Conveyance BMP 02 Dispersion (within ROW) Table 13. Checklist of site applicability.

Planning Framework for Early Decision-Making and Tentative BMP Selection 47 (continued on next page) Part 2: Screening of Overall Project Attributes Instructions: 1. Enter project information in the “Project Value” row to indicate project attribute that is present. 2. Determine if BMP is compatible project-entered value. 3. Enter result in last column: Is the overall project compatible with the BMP? Y (for Yes) or N (for No) Overall Project Attributes Typical Ratio of BMP Infiltration Surface Area to Impervious Area Needed Presence of Storm Drain System Undeveloped Adjacent Land Use Acceptable for Dispersion or Land Application? Result: Potential for BMP Based on Project Attributes? Project Value: Vegetated Conveyance 0.01 to 0.10 Dispersion (within ROW) 0.10 to 0.50 Dispersion (outside of ROW) 0.10 to 0.50 Critical criteria for dispersion outside of the ROW Media Filter Drain 0.10 to 0.25 Permeable Shoulders 0.10 to 0.25* Bioretention with Underdrains 0.02 to 0.10 Important unless grades allow underdrains to daylight Bioretention w/o Underdrains 0.02 to 0.10 Infiltration Basin 0.02 to 0.10 Infiltration Trench 0.02 to 0.10 Infiltration Gallery 0 (BMP within impervious footprint) Guidance Values shown indicate approximate minimum value to achieve meaningful volume reduction performance. Underground systems and systems with underdrains must generally discharge to a storm drain system; additionally, storm drain systems allow pretreatment upstream of underground facilities. Applicable to determining if dispersion is possible in the event that space is not available in the ROW See Part 1e for Geometric Opportunity Screening. * Note, constructed within pavement footprint. Key for Table 13 (Part 2) Headings User Input Guidance No Meaningful Nexus Important to enable pretreatment and discharge Table 13. (Continued).

48 Stormwater Infiltration in the Highway Environment: Guidance Manual Key for Table 13 (Part 3) Headings User Input Guidance Part 3: Other Project-Specific Factors Instructions: 1. Review guidance relative to project attributes. 2. Enter screening results (i.e., which BMPs are not applicable based on the respective factor) and supporting rationales in last column. Screening Factor Guidance Screening Result BMP 01 – Vegetated Conveyance BMP 02 – Dispersion BMP 03 – Media Filter Drain BMP 05 and 06 – Bioretention Vegetated BMPs may require irrigation of some sort during establishment or over long-term operations in some climates. If plants cannot be identified that are compatible with irrigation that can be practically applied, then these BMPs may not be applicable. Locally available materials Does the BMP require materials that are not available locally? This will be uncommon but, for example, could include specialized binders required for permeable pavement designed for heavy traffic loadings. Local jurisdiction acceptance Do the local jurisdictions with responsibility for approving plans accept the BMP type? Can barriers to approval be overcome? Local contractor experience For specialized installations, such as permeable pavements, do local contractors have the experience needed to ensure successful installation? Do local contractors or the agency have experience maintaining these systems? Plants are a critical element of the performance of the following: Planting requirements and irrigation needs Table 13. (Continued).

Planning Framework for Early Decision-Making and Tentative BMP Selection 49 Part 4: Summary of Applicability and Suitability Screening Instructions: 1. Review results of Parts 1 through 3. 2. Enter screening results: Y (for Yes) or N (for No). 3. Provide summary of rationale for screening result. BMP Screening Results: Applicability Summary of Rationales Vegetated Conveyance Dispersion (within ROW) Dispersion (outside of ROW) Media Filter Drain Permeable Shoulders Bioretention with Underdrains Bioretention w/o Underdrains Infiltration Trench Infiltration Basin Infiltration Gallery Key for Table 13 (Part 4) Headings User Input Guidance Table 13. (Continued). 2.4.4 Compatibility with Local Climate Cold and arid climates pose specific issues for BMP design and may require design adapta- tions (see Appendix I). The purpose of this step is to identify overriding issues related to climate that could limit the menu of applicable BMPs. Potential Overriding Arid Climate Issues Vegetation Establishment and Maintenance. It can be impractical to supply irrigation in the highway environment, particularly for more distributed BMPs such as vegetated swales and filter strips. The effectiveness of these BMPs relies on establishing grasses with adequate cover to stabilize soils, prevent rill erosion, and facilitate filtering and infiltration. In the arid southwest United States, it may be impractical to use these BMPs, particularly if soils are erosive. Bioretention BMPs may also have limited applicability in arid climates without irrigation (see Figure 8). Erosive Soils in Tributary Area. Arid climates can experience high rates of erosion from open space areas. If open space areas cannot be adequately stabilized and hydraulically

50 Stormwater Infiltration in the Highway Environment: Guidance Manual Figure 8. Example Phoenix, Arizona, freeway with sparsely vegetated shoulders. isolated from the BMP, this can be a fatal flaw for the use of infiltration and filtration-based BMPs. The BMP Clogging Risk Assessment Tool (Appendix F) can be used to evaluate poten- tial risks. Project teams can also make use of site-specific knowledge of soil types and poten- tial erosive loads. Potential Overriding Cold Climate Issues Clayey Soils and Sodium Effects. Sodium in road salts can change the ratio of sodium to calcium and magnesium, which can result in the dispersion of soil clays. This can greatly impair infiltration rate. If soils have measurable clay content and sodium-based deicers are used, then this may preclude infiltration. Similarly, if bioretention is used, then the amended media should be free of clay. Permeable Pavements and Studded Tires. Permeable pavement can be damaged by studded tires, resulting in premature failure. NCHRP Report 802 concluded that this was an overriding factor in determining the feasibility of permeable pavement. Permeable Pavement and Deicing Salts. Permeable pavement can also be damaged by deicing salts. NCHRP Report 802 concluded that design and construction approaches could limit these issues, but there was still limited experience with these approaches. Permeable Pavements in Shady Wet Areas. Permeable pavement shoulders (e.g., in low traffic areas) could become occluded by moss in shady areas in some wet climates. This has been observed by the research team in Portland, Oregon, in shady locations with low traffic. The project team should review the exposure of the project and consult local practitioners to determine whether this has been observed in the project region. Permeable Pavement and Frost Heave. Frost heave within pavement structures can be particularly damaging. If it is not possible to maintain the water storage reservoir below the frost line, then this can render permeable pavement infeasible. Salt-Induced Corrosion of Steel and Reinforced Concrete Culverts and Structures. Salt can induce or enhance corrosion, which can deteriorate steel structures and steel rebar within

Planning Framework for Early Decision-Making and Tentative BMP Selection 51 reinforced concrete features. For infiltration in cold climates, project teams should consider whether infiltrated water would pose elevated corrosion risks to steel culverts, steel structures, or other intrastate containing steel (either external or as internal reinforcement). Note that this issue is not limited to infiltration BMPs. 2.4.5 Comparison of Other Decision Factors by BMP Type The following sections provide guidance to help project teams compare different BMP types that may be feasible for a site. All ratings are relative and are not intended to be con- firmatory. They are intended to help support BMP selection in cases were multiple BMP types may be suitable and compatible with infiltration objectives and site conditions. Geotechnical Risk Factors Project teams can conduct a relative assessment of geotechnical feasibility based on the unit treatment processes and typical installation geometry for different BMPs (see Table 14). This is not a replacement for geotechnical feasibility analyses described in Chapter 3 but can be used to evaluate how BMPs compare with one another. Groundwater Quality Table 15 presents factors related to BMP design that can influence potential for groundwater quality impacts. Pretreatment is a general term that refers to providing an initial level of treat- ment provided to stormwater before it enters a BMP, such as filtering through grass, settling, centrifugal separators, media filters, or other devices. Table 16 provides a synthesis of relative risk posed by each of the nine primary BMPs based on the information provided in Table 15. Note that a higher-risk ranking in Table 15 does not necessarily imply that the BMP should not be used; however, the BMP may be less favor- able than lower-risk BMPs when site conditions indicate a higher potential for groundwater quality issues. Roadway and Maintenance Safety Several key safety considerations may relate to the siting and design of BMPs including the following: • Limitations on grading and structures within the clear zone along the road shoulders to allow errant vehicle recovery and reduce collision hazards • Vegetation management to maintain line-of-site requirements as well as to eliminate collision hazards within the clear zone • Adequate supplemental drainage as needed to avoid flooding of travel lanes • Lane closures to allow maintenance activities within the ROW • Other potential issues Based on their respective locations within the highway environment and their inherent design attributes, each BMP has a different suite of applicable factors. Safety considerations that may apply to specific BMPs are described in the respective BMP Fact Sheets and are summarized in Table 17. These factors do not necessarily result in BMPs being considered infeasible but should be a factor in selection, siting, and design. Maintenance Activities and Requirements Maintenance of BMPs ranges from regular highway maintenance activities (e.g., trash con- trol or vegetation management) that may be done whether or not a BMP is in place, to more

52 Stormwater Infiltration in the Highway Environment: Guidance Manual Category of BMP Characteristic Properties Example Opportunities and Constraints Related to Geotechnical Issues1 Opportunities Constraints Direct infiltration into roadway subgrade Example: BMP 04 Permeable Pavement loading ratio2 Road subgrade Relatively low has important structural considerations, particularly for flexible pavement design. Broad footprint of permeable pavement may allow infiltration in relatively impermeable and compacted soils. Standard roadway designs typically account for wetting of subgrade. Rigid pavement design (e.g., concrete) is less sensitive to subgrade strength. Utilities and infrastructure in the ROW Settlement and volume change processes (e.g., consolidation, frost heave, swelling, liquefaction) Reduction in strength of subgrade material from increase in moisture content Mounding and effects on nearby infrastructure Infiltration in breakdown lane and near shoulders Example: BMP 03 Media Filter Drain BMP 04 Permeable Shoulders Outside of main travel lanes; significantly less traffic loading Relatively low loading ratio2 Shoulders designed to accommodate less traffic loading than travel lanes Well-distributed inflow Linear configuration less susceptible to groundwater mounding than basin configurations An underdrain can control the amount of water infiltrated and limit the maximum water level in the reservoir. Typically, shoulder must be compacted to same degree as mainline roadway. Potential for water to migrate laterally into mainline subgrade rock or nearby development Settlement or volume change Potential reduction in slope stability for embankment or depressed sections Mounding and effects on nearby infrastructure Infiltration and surface dispersion in the clear zone Example: BMP 02 Dispersion BMP 03 Media Filter Drain Allows incidental infiltration over relatively broad area; also provides ET Typically coupled with vegetated conveyance at toe of filter strip Drainage over shoulder is a typical design feature. Higher proportion of losses to ET than other BMPs Relatively little mounding expected May lead to erosion issues if applied on slopes that are too steep or lack stabilizing vegetation. Slopes may need to be compacted to same degree as mainline roadway; surficial soils need to be strong enough for errant vehicle recovery. In some cases, settling or volume change could damage roadway. Subject to frozen ground issues Channels, trenches, and other linear depressions offset parallel to roadway Example: BMP 01 Vegetated Conveyance BMP 05 and 06 Linear Bioretention Variation BMP 07 Infiltration Trenches Tends to be located 10 or more ft from travel lanes Typically, effective water storage depth is between 6 in. and 36 in. Typically have a relatively high loading ratio May be fully or partially infiltrated Channels with positive grade are common drainage features; have relatively limited increase in risk. Due to horizontal separation, features have less potential to damage roadway. Some settlement may be tolerable. Deeper designs may avoid frost impacts. Greater potential for impacts out of the ROW due to proximity to the ROW line. Greater potential for mounding due to concentration of infiltrating footprint. May reduce stability of slopes if located near top or toe. Table 14. Summary of relative geotechnical opportunities and constraints for specific categories of BMPs.

Planning Framework for Early Decision-Making and Tentative BMP Selection 53 Category of BMP Characteristic Properties Example Opportunities and Constraints Related to Geotechnical Issues1 Opportunities Constraints Basins Example: BMP 05 and 06 Bioretention (more centralized variation) BMP 08 Infiltration Basins BMP 09 Infiltration Galleries Typically located in more centralized locations Typically have a relatively high loading ratio Typically, effective water storage depth is between 12 in. and 60 in. Centralized areas, such as wide spots in ROW or interchanges, may allow ample setbacks from foundations, slopes, and structural fill. May be possible to preserve natural soil infiltration rates through construction Impacts of potential settlement may be minor. Deeper ponding depths may result in substantial groundwater mounding and lateral water migration; greater setbacks may be needed than would be applied for more distributed systems. Surface systems subject to frozen ground issues 1 Examples provided to identify typical opportunities and constraints of the infiltration design feature. Additional. opportunities and constraints may be present at a given site. 2 “Loading ratio” refers to is the ratio of the impervious tributary area to the footprint of the infiltrating surface. A high loading ratio indicates that the infiltrating footprint is relatively small compared with the impervious tributary area. and vice versa. This Guidance Manual defines the following general categories for loading ratios: high: > 20:1; medium: 5:1 to 20:1; low: < 5:1. Table 14. (Continued). Risk Factor Discussion Lower-Risk Indicators Higher-Risk Indicators Hydraulic loading ratio The relative footprint of the system influences the pollutant loading per unit area and the potential for natural assimilative capacity to be overwhelmed. Systems with broader, shallower footprint such as dispersion Systems with deeper profiles and smaller footprints such as infiltration trenches Layer at which infiltration occurs When infiltration occurs below organic soil and/or closer to the groundwater table, there tends to be less pollutant attenuation capacity. Systems infiltrating near the surface where soils have higher organic content and biologic activity (or are amended to provide this) Systems infiltrating below the organic strata and not providing a treatment layer such as imported amended soil (or other pretreatment) Amount of infiltration occurring Potential for groundwater impacts tends to be higher when there is more infiltration. Systems with less infiltration, such as vegetated conveyance Systems with more infiltration, such as infiltration galleries Potential for pretreatment or treatment within the BMP Pretreatment is important to reduce potential for clogging as well as to address groundwater quality. Systems providing a treatment layer such as an engineered soil media layer or an amended soil layer Systems where pretreatment cannot be practically provided and treatment processes within and below the BMP are limited such as permeable pavement Spill risk and spill containment options Spills are infrequent events but have the potential to cause major groundwater quality issues. The most problematic pollutants are solvents and other phase non-aqueous dense liquids. Systems that have a pretreatment/ containment system Systems that drain lower traffic roadways Systems with lower hydraulic loading ratio Systems where it is not possible to provide pretreatment or containment Systems that drain higher traffic roadways Systems with higher hydraulic loading ratio Table 15. Summary of relative BMP-related risk factors for groundwater quality impacts.

54 Stormwater Infiltration in the Highway Environment: Guidance Manual BMP Relative Risk of Groundwater Impacts1 Key Characteristics Influencing Ranking BMP 01 Vegetated Conveyance L More limited infiltration, shallower ponding, and soil filtration of infiltrating runoff BMP 02 Dispersion L Shallower ponding, high soil contact ratio, amended/organic/biologically active soils BMP 03 Media Filter Drain L Shallow ponding, specialized media with high treatment capacity BMP 04 Permeable Shoulders M Can have a relatively small footprint area, some pretreatment provided in base material but additional pretreatment not practical, can infiltrate water below organic soil strata BMP 05 Bioretention without Underdrains L to M Provide treatment for most constituents within media, can have relatively small footprint and deeper infiltrating surface BMP 06 Bioretention with Underdrains L Provide treatment for most constituents within media, infiltrate less water than bioretention w/o underdrain BMP 07 Infiltration Trenches M to H Deeper profile typically below surface soil strata, pretreatment options may be limited BMP 08 Infiltration Basins M Deeper profile and typically smaller tributary area ratio but soil can be amended to improve water quality. BMP 09 Infiltration Galleries M to H Deeper profile typically below surface soil strata, pretreatment may not address all pollutants of concern. 1 Rankings are relative to other BMPs. This is not a ranking of total risk because that would also be a function of pollutant sources, site conditions, and applicable groundwater quality criteria. L, M, and H – Low, Medium, and High, respectively. Quality Table 16. Relative ranking of potential groundwater quality risk by BMP type. BMP-specific maintenance activities that are needed to maintain the intended function of the systems. These activities can be categorized into routine maintenance, which includes normally scheduled inspections and activities needed on a regular basis, and corrective maintenance, which includes as-needed activities triggered by observations of damage, fail- ure, pending issues, or other factors that require action to return the facility to its intended function. Table 18 and Table 19 provide an inventory of routine maintenance activities and corrective maintenance activities, respectively, that may apply to each of the primary BMPs. These tables were developed based on review of guidance manuals and interviews with DOT maintenance staff; however, it is important to note that information on main- tenance requirements of BMPs in the highway environment is still limited to informing decision-making. NCHRP Report 792 included assessments of maintenance needs and devel- oped whole lifecycle cost estimating tools for a variety of stormwater control measures, including common infiltration BMPs. Whole Lifecycle Costs Whole lifecycle cost estimation is not the focus of this Guidance Manual; however, tools are available from NCHRP Report 792 to support this assessment for multiple infiltration BMPs, and

Planning Framework for Early Decision-Making and Tentative BMP Selection 55 Potential Safety Consideration B M P 0 1 V eg et at ed C on ve ya nc e B M P 0 2 D is pe rs io n B M P 0 3 M ed ia F ilt er D ra in B M P 0 4 P er m ea bl e S ho ul de rs B M P 0 5 B io re te nt io n w /o U nd er dr ai ns B M P 0 6 B io re te nt io n w ith U nd er dr ai ns B M P 0 7 In fil tr at io n T re nc h B M P 0 8 In fil tr at io n B as in B M P 0 9 In fil tr at io n G al le rie s Limitations on side slopes and berms within the clear zone, including check dams, etc. X X X X X X X Limitations on drainage structure design within the clear zone (e.g., pipe inlets and outlets flush to slope) X X X X X X Stability of soil within the clear zone, particularly if compost amended X X X X X Vegetation management to remove collision hazards X X X Vegetation management to maintain line of site X X X X X Supplemental drainage to ensure free drainage of travel lanes in the event of clogging X Low speed vehicle maintenance activities and lane closures X X X X X X = indicates that the safety consideration may apply to the BMP. Table 17. Summary of potential safety considerations by BMP. Routine Maintenance Activities B M P 0 1 V eg et at ed C on ve ya nc e B M P 0 2 D is pe rs io n B M P 0 3 M ed ia F ilt er D ra in B M P 0 4 P er m ea bl e S ho ul de rs B M P 0 5 B io re te nt io n w /o U nd er dr ai ns B M P 0 6 B io re te nt io n w ith U nd er dr ai ns B M P 0 7 In fil tr at io n T re nc he s B M P 0 8 In fil tr at io n B as in s B M P 0 9 In fil tr at io n G al le rie s Mowing Maintain-Level Spreading Functions Landscaping and Weeding Routine Woody Vegetation Management Sediment Removal and Management Vacuum Sweeping Trash and Debris Removal Erosion Repair Rodent Hole or Beaver Dam Repair Fence or Access Repair Key: Primary maintenance activity; Minor maintenance activity (may not apply in some cases or may be limited); Not usually applicable. Table 18. Summary of potential routine maintenance activities by BMP.

56 Stormwater Infiltration in the Highway Environment: Guidance Manual Corrective Maintenance Activities B M P 0 1 V eg et at ed C on ve ya nc e B M P 0 2 D is pe rs io n B M P 0 3 M ed ia F ilt er D ra in B M P 0 4 P er m ea bl e S ho ul de rs B M P 0 5 B io re te nt io n w /o U nd er dr ai ns B M P 0 6 B io re te nt io n w ith U nd er dr ai ns B M P 0 7 In fil tr at io n T re nc he s B M P 0 8 In fil tr at io n B as in s B M P 0 9 In fil tr at io n G al le rie s Re-grading to maintain level spread function X X Re-grade to remove sediment or fix erosion X X X X Repair berms, inlets, outlets, or other structures X X X X X X X X Cleaning of underdrain pipes X X X Decompaction/re- amendment X X X X Partial removal of surface material to remediate clogging or pollutant buildup X X X X X X Complete replacement of system components X X X X X X Re-seeding to provide needed coverage X X X X Significant re-vegetation to provide needed coverage X X X X X Remediate contamination from acute or chronic loadings (oil, gas, or other contaminants) X X X X X X X X X X = Potentially applicable. Table 19. Summary of potential corrective maintenance activities by BMP. local cost estimation frameworks can be used. It is challenging to assign an average or typical whole lifecycle cost to an entire category of BMPs because of variability in design and con- struction as a result of site-specific factors. Additionally, information on whole lifecycle costs and lifespan is still limited to informing decision-making. For purposes of initial decision- making about BMP selection, Table 20 represents the relative costs of selected BMPs based on a typical application, with notes to identify key site-specific factors that may influence these rankings. Because relative capital costs can be significantly different in new roadway projects and lane additions as opposed to retrofit projects, a separate column is provided for these two categories of projects. 2.4.6 Step 2 Results: Selection of BMP Locations and Types Step 2 should yield three primary outcomes: • Tentative selection of BMP type and variation if applicable (e.g., presence of underdrains) • Determination of available space and tentative feasibility conditions at the BMP location • Delineation of the tentative tributary area to the tentative BMP locations These key parameters are needed to support the confirmation of BMP feasibility described in Chapter 3. Table 21 is an example of a template that can be used to summarize these parameters. The process for BMP selection may vary based on local criteria, the preferences of the design team, and other factors.

BMP Capital Costs – New Roadway or Major Redevelopment Capital Costs – Retrofit Projects or Minor Redevelopment O&M and Replacement/ Reconstruction Costs Effective Life Span BMP 01 Vegetated Conveyance Low to Moderate Can typically be easily incorporated into grading plans for non-ultra-urban settings. Provides conveyance function that can offset need for pipes and structures. Low to Moderate Modifications to existing swales to improve volume reduction may be inexpensive. Can add significant cost if regrading and rerouting must be done to accommodate BMP. Low to Moderate Requires more frequent maintenance than a typical vegetated or concrete ditch without water quality functions. Erosion/scour must be addressed. 20 to 50 years Regrading of conveyance. Decompact underlying soils, potentially add new amendments. Correct major erosion. BMP 02 Dispersion Low Assuming no acquisition costs for the ROW; land acquisition can render this option cost prohibitive. Provides conveyance function that can offset pipes and structures. Low to Moderate Assuming no acquisition costs for the ROW; land acquisition can render this option cost prohibitive. Depends on extent of routing and grading improvements needed to utilize dispersion area. Low Requires minimal maintenance of vegetation that would be similar to vegetated ROW. Reconstruction costs are typically lower than original construction. 20 to 50 years Regrade level spreader. Decompact underlying soils, potentially add new amendments. Correct major erosion. BMP 03 Media Filter Drain Low to Moderate Assumes no acquisition costs for land needed for conveyance or storage system. Shared grading/excavation costs with project. Moderate Requires minor excavation and removal of soil. May require modifications to drainage patterns. Can fit on existing shoulders. Low to Moderate Requires infrequent maintenance to remove sediment and maintain conveyance if tributary watershed is stabilized. Periodic maintenance possibly needed to replace media. 5 to 20 years* Regrade level spreader. Replace media if exhausted. Shorter than BMP 01 and 02 because footprint tends to be smaller and more specialized media is used. Table 20. Typical BMP costs per volume of stormwater managed [adapted from Washington State Department of Transportation (WSDOT) (2014)]. (continued on next page)

BMP Capital Costs – New Roadway or Major Redevelopment Capital Costs – Retrofit Projects or Minor Redevelopment O&M and Replacement/ Reconstruction Costs Effective Life Span BMP 04 Permeable Shoulders with Stone Reservoirs Low to Moderate Assumes new development or lane additions in which the cost of permeable pavement offsets traditional pavement costs that would have been used. High Cost to add permeable shoulders to an existing roadway are much greater than building these as part of a new roadway. Requires excavation and hauling of previous roadway; import of new material. Equipment, labor, and installation costs are directly associated with BMP. Moderate to High Requires regular vacuum sweeping. Surface replacement may be required more frequently than traditional pavement. Full depth replacement may cost more than initial construction. If water routed directly to subbase via inlets, sweeping not needed, but earlier clogging of the subbase layer may occur. 15 to 25 years** Replace top course of permeable pavement because of structural wear. Fully excavate to restore infiltration capacity of subgrade. Dependent on sediment loading, traffic loading, and other factors. Not well established. BMP 05 and 06 Bioretention Moderate Specialized planting and soil, so net cost increase should be considered over areas that would have been planted. Assumes grading and conveyance in conjunction with overall project. Use of an underdrain results in greater cost and less volume reduction; however, it can reduce the risk of failure. Moderate to High Cost of rerouting flows to specific areas. Some aspects of site investigation and construction not shared with overall project. Possibility of additional land acquisition. Moderate Regular maintenance of vegetation and trash similar to baseline landscape maintenance. May require restoration of surface infiltration capacity and replanting at regular intervals. Periodic removal of top layer to prevent contamination build-up and maintain infiltration. 5 to 12 years (partial) Dependent on effectiveness of pretreatment. Restoration of surface infiltration capacity and replanting. Intervals may be longer if vegetation is robust. 25 to 50 years (complete) Replacement of media/ structures/piping at less frequent intervals. BMP 07 Infiltration Trench Moderate to High Requires several additional construction materials. Volume is based on porosity of gravel, so bulk volume is greater than effective volume. Assumes no land acquisition. Can be incorporated into excavation plans. May also need to construct a swale for pretreatment. High Increased equipment, construction, and labor costs. Additional excavation costs. High Requires maintenance of debris and sediment removal to maintain infiltration. Failures have been common. Replacement cost similar to new construction, because infiltration surface is not exposed. 5 to 15 years Dependent on effectiveness of pretreatment. Excavate rock and rework trench to maintain infiltration rates; backfill with existing rock after removing fines. May only be able to restore capacity a limited number of times before moving the facility location. Table 20. (Continued).

BMP 08 Infiltration Basin Moderate Assumes no acquisition costs for land. Assumes potential additional excavation and infrastructure to convey water to centralized location. Basins can offset pipes or reduce size of downstream conveyance. High Cost of rerouting flows to specific areas. Aspects of site investigation and construction not shared with overall project. Possibility of additional land acquisition. Costs can be lower if existing detention basin can be converted to infiltration. Moderate Requires maintenance of debris removal to maintain infiltration. Maintenance of any conveyance systems. Periodic removal of top layer to prevent contamination build-up and maintain infiltration. 5 to 10 years (partial) Dependent on effectiveness of pretreatment. Restoration of surface infiltration capacity can be longer if deep rooted plants are used. May only be able to restore capacity a limited number of times before moving the facility location. 25 to 50 years (complete) Replacement of structures/piping and deep restoration of subgrade at less frequent intervals (25 to 50 years). Eventually may need to move facility location if possible. BMP 09 Infiltration Gallery Moderate to High Excavation and piping can be incorporated into construction plans. Assumes robust pretreatment system High Cost of rerouting flows to specific areas Aspects of site investigation and construction not shared with overall project Assumes robust pretreatment system High Below grade is difficult to maintain Requires maintenance of debris and sediment removal to maintain infiltration Requires regular maintenance of pretreatment system 10 to 25 year*** Rough estimate, assuming robust pretreatment; could be much less without pretreatment. If gallery is accessible, may be possible to restore capacity a limited number of times before reconstruction. *Based on WSDOT best professional judgment; systems have not been in place for full lifecycle. **Not provided by WSDOT; estimated from Houle et. al. 2013. *** Best professional judgment; highly site specific and dependent on pretreatment methods used.

60 Stormwater Infiltration in the Highway Environment: Guidance Manual An infiltration feasibility exhibit can also be useful to document preliminary BMP selection and siting. Potential content of this exhibit includes the following: • Topographic and drainage feature elements (Section 2.2.2) • Proposed project elements (e.g., roadway alignments, embankments, structures) • Infiltration feasibility constraints and tentative categorization • Tentative BMP locations, footprints, and types • Tributary areas to BMP • Locations of field soil sampling, infiltration testing, and groundwater monitoring if applicable. The content of this exhibit may vary by project. BMP ID BMP Type and Variation Tentative Feasibility Condition at BMP Location Available BMP Footprint Area Anticipated BMP Depth Tributary Area % Impervious Table 21. Example table to summarize outcome of BMP selection.

Next: Chapter 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations »
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This report from earlier in 2020 is relevant to the latest issue of TR News (#328, on stormwater management).

The infiltration approach to stormwater management involves the design, construction, and operation of engineered systems that infiltrate stormwater runoff into soils. These systems, referred to as “infiltration best management practices (BMPs),” are intended to reduce the volume of stormwater runoff and associated pollutants that discharge to stormwater systems and receiving waters via surface runoff.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 922: Stormwater Infiltration in the Highway Environment: Guidance Manual supports evaluation, selection, siting, design, and construction of infiltration BMPs in the highway environment. It is also intended to identify limitations on the use of infiltration and determine the need for alternative non-infiltration-based stormwater management approaches.

Additional resources for the guide include:

  • A Power Point presentation summarizing the project
  • Appendix A: Infiltration BMP Fact Sheets
  • Appendix B: Infiltration Estimation Method Selection and Interpretation Guide
  • Appendix C: Roadside BMP Groundwater Mounding Assessment Guide and User Tool (Excel-based tool)
  • Appendix D: Guide for Assessing Potential Impacts of Highway Stormwater Infiltration on Water Balance and Groundwater Quality in Roadway Environments (Excel-based tool)
  • Appendix E: Guide to Geotechnical Considerations Associated with Stormwater Infiltration Features in Urban Highway Design
  • Appendix F: BMP Clogging Risk Assessment Tool (Excel-based tool)
  • Appendix G: Whole Lifecycle Cost and Performance Example
  • Appendix H: Example Construction-Phase Checklists for Inspector and Contractor Training
  • Appendix I: Summary of Infiltration Issues Related to Cold and Arid Climates
  • Appendix J: BMP Case Study Reports
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