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

Chapter: Chapter 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations

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Suggested Citation:"Chapter 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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 3 - Confirmation of BMP Selection Through Prioritized Analyses and Investigations." 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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61 Confirmation of BMP Selection Through Prioritized Analyses and Investigations This chapter provides guidance on prioritized analyses and site investigations that may be necessary to confirm or revise preliminary selection of infiltration approach, BMP types, and locations. This is Step 3 in the overall decision-making process. 3.1 BMP Confirmation Process by Planning Track The following sections describe the recommended next steps to confirm or revise the selected infiltration BMP types at the tentative locations determined in Step 2. An overview of this pro- cess is provided in Figure 9. These track numbers align with the matrix presented in Table 10. Tracks are further described in the subsections that follow. To use Figure 9, find the track number in the left column that best fits the tentatively selected BMPs types and locations. Read across to determine the analyses and investigations necessary to confirm that the tentatively selected BMPs are appropriate. The following subsections provide further explanation. 3.1.1 Track 1a: Favorable Infiltration Conditions— Infiltration BMP Selected Unless the preliminary determination of infiltration feasibility was supported by methods appropriate for BMP design, the project team should conduct additional investigations and analyses to confirm feasibility including the following: • Confirm soil infiltration rates and develop a reliable factor of safety based on appropriate methods (see Section 3.2 and Appendix B). • Evaluate sizing and performance based on the design infiltration rate determined from design- level methods (see Section 3.3). • Conduct additional characterization of soil properties and geologic conditions as needed to confirm absence of geotechnical issues and verify assumptions used in groundwater mound- ing evaluations (see Section 3.4). • Conduct more thorough or longer-term characterization of groundwater depth, seasonality, pretreatment requirements, and regional- or watershed-scale issues to verify assumptions (see Section 3.5). To reduce the risk of infiltration failures, the project team should address each of these fac- tors to confirm the selection of this approach based on the criteria presented in Sections 3.2 through 3.5 (or applicable local criteria). C H A P T E R 3

62 Stormwater Infiltration in the Highway Environment: Guidance Manual Confirmation Required – BMP design and function requires these data Confirmation Optional – BMP design/compliance does not depend on this information, but may be useful to support design and/or compliance Key Analyses and Investigations to Confirm BMP Selection 1a) Tentatively Select a Full Infiltration BMP and Confirm Full Infiltration Feasibility 3a) Confirm Infeasibility (Opportunistic Goals) and Select a Non-Infiltration-based Strategy 1b) Stringent Objectives in Marginal Conditions – Additional Assessment Needed to Determine BMP Selection 2a) Tentatively Select a Partial Infiltration BMP and Confirm Partial Infiltration Feasibility Confirm Design Infiltration Rate Confirm Feasible Sizing to Meet Applicable Requirements Confirm Hazards are Avoided or Mitigated Adequately to Support Infiltration Confirm Presence of Unavoidable Infiltration Hazards 3b) Confirm Infeasibility (Stringent Goals) and Select a Non-Infiltration-based Strategy Data needed depends on potential limiting factor(s). Obtain remaining data needed to determine Full Infiltration vs. Partial Infiltration. Then follow Track 1a or 2a, respectively. Preliminary Infiltration Strategy from Step 2 Section 3.2 Section 3.3 Sections 3.4, 3.5 Figure 9. Overview of analyses and investigations to confirm BMP selection. 3.1.2 Track 1b: Stringent Infiltration Objectives in Marginal Conditions In marginal conditions, the feasibility of infiltration can depend on a range of factors that may not have been present at the time of project design (e.g., soil conditions after grading has occurred) or may be costly to investigate (e.g., site-specific groundwater mounding in complex hydrogeologic conditions). When infiltration objectives are stringent, project teams may face a mandate to conduct these assessments and/or determine whether there are design alterna- tives that could result in adequate confidence to move forward with infiltration. This is likely to be the most costly and time-consuming planning track. Project teams should undertake this track only if mandated by the project infiltration objectives (either regulatory driven or project driven). Key elements needed to confirm selection of an infiltration approach include the following: • Confirm soil infiltration rates and develop a reliable factor of safety based on appropriate methods (see Section 3.2 and Appendix B). – Potential issues. The site was classified as marginal because of the inability to conduct field- scale tests or the use of rapid and less reliable tests. – Potential resolutions. Conduct more reliable field tests if feasible.

Confirmation of BMP Selection Through Prioritized Analyses and Investigations 63 • Evaluate ability to better control infiltration properties through the construction phase of the project. – Potential issues. Infiltration conditions were rated as marginal because of the inability to predict infiltration properties in the post-constructed conditions (e.g., significant cut and fill operations or construction traffic that affects infiltration rates). – Potential resolutions. Consider special specifications for fill material and construction site management. Consider modifications to the project delivery process, as described in Section 3.6. • Analyze selected BMP profile and footprint to verify sizing and performance analysis (see Section 3.3). – Potential issue. The combination of space constraints and marginal soil infiltration capacity make it uncertain whether BMPs will be able to meet performance goals (e.g., long-term capture efficiency) and drawdown time limits. – Potential resolutions. Analyze potential BMP footprints, depths, performance, and ground- water mounding impacts. Depending on results, consider different design alternatives, such as BMPs that are shallower and more distributed (e.g., permeable shoulders or infiltration swales) that provide a larger infiltration surface total. • Evaluation of other issues needed to confirm feasibility. If evaluation of these issues indicates that Full Infiltration may be feasible, then the remaining feasibility criteria should be assessed: – Potential issues. Feasibility determinations relative to geotechnical and groundwater issues are preliminary and need to be finalized based on actual BMP locations and types. – Potential resolutions: � Conduct more thorough characterization of soil properties and geologic conditions to confirm absence of geotechnical issues and verify assumptions used in groundwater mounding evaluations (see Section 3.4). � Conduct more thorough or longer-term evaluation of groundwater depth, seasonality, pretreatment requirements, and regional- or watershed-scale issues to verify assump- tions (see Section 3.5). A flowchart is provided in Figure 10 to support decision-making. Upon completion of the applicable investigations, the project team should apply the final decision-making criteria (see Section 3.7 or locally applicable criteria) to determine whether the project should proceed with Full Infiltration or pursue a reduced level of infiltration complemented by alternative non-infil- tration approaches. 3.1.3 Track 2a: Maximized Infiltration Objectives in Marginal Conditions When a project has maximized objectives for stormwater infiltration and is in marginal (but at least partially feasible) infiltration conditions, the project team has two alternative approaches. These are at the discretion of the project designers. Decision guidance is provided as follows. 1. Select and design BMPs to achieve Maximized Partial Infiltration. Examples of BMPs that can provide this level of treatment include the following: a. Bioretention basins or bioretention swales with underdrains can include a gravel sump below the discharge elevation of the underdrain. This type of BMP requires the gravel sump to fill before treated discharge can occur. If soils are more permeable than expected, this type of system can achieve performance similar to Full Infiltration; the underdrain would be infrequently utilized. If infiltration rates are less than expected, then greater treated discharge would occur, but the overall the system would still provide water quality functions and be able to meet applicable treatment standards.

64 Stormwater Infiltration in the Highway Environment: Guidance Manual No Are BMP sizing and depth feasible? (draw down, performance) Does level of proposed infiltration pose unacceptable and unavoidable risks? Proceed with Full Infiltration (w ith contingency plan, as applicable) Move to Track 2a – Maximized Partial Infiltration or Consider Alternative Compliance Options Conduct necessary analyses to establish design infiltration rate (Section 3.2) Can uncertainty in infiltration capacity be resolved prior to construction? Yes Conduct necessary analyses to evaluate risk and potential mitigation approaches (Section 3.4, 3.5) Size BMPs and determine required footprint/depth/profile (Section 3.3) No Yes Yes Is an adaptive design/construction process acceptable? (compliance, project delivery) Yes No Identify primary and contingency plans to be decided based on construction- phase testing (Section 3.6, 4.4) No Size BMPs and develop conceptual profile for each contingency scenario Yes Figure 10. Flow Chart Track 1b: stringent objectives in marginal conditions. b. Filter strips and other dispersion approaches provide treatment and positive overland flow paths for water that is not infiltrated. Therefore, the level of infiltration depends on the actual soil properties, but the operability of the BMP to manage and treat stormwater does not rely on a certain level of infiltration. Because these approaches include supplemental treatment pathways and do not rely on a certain minimum infiltration rate, additional infiltration testing is not critical. If this option is selected, then the project can typically proceed without further infiltration rate inves- tigations. The project team should still determine that partial level of infiltration would not pose geotechnical or groundwater quality risks.

Confirmation of BMP Selection Through Prioritized Analyses and Investigations 65 2. Refine site investigations to attempt to support Full Infiltration BMPs. In cases in which the marginal categorization is based on the preliminary nature of the investigation or in which there are data gaps, the project team should consider additional analyses to refine infiltration investigations. The project would follow the guidance presented for Track 1b. Key reasons why projects may seek this option include the following: a. The types of Maximized Partial Infiltration BMPs described above are not feasible or are cost prohibitive. Perhaps these BMPs do not fit within the project constraints, or there are not storm drains available to receive the underdrain flow. b. The use of Full Infiltration BMPs (i.e., systems that infiltrate the full water quality volume) could save money overall, even if these BMPs are costly to investigate and confirm. c. Local guidelines or policies require greater rigor to be applied in rejecting the use of Full Infiltration. For example, if soil maps were used to make initial assessments, these may not be adequate as the sole basis for rejecting Full Infiltration. Using maps to make decisions about infiltration feasibility can be subject to local discretion and requirements. If additional data collected clearly support Full Infiltration, then the project could shift to planning Track 1a and demonstrate the feasibility of Full Infiltration. If additional data con- firm that site conditions do not support Full Infiltration, then the project could shift to using a Maximized Partial Infiltration approach if feasible. 3.1.4 Track 3a: Limited or No Infiltration Feasible— Opportunistic Objectives In this case, nothing further is typically needed to justify decision-making. If there are not regulatory drivers to pursue infiltration and conditions appear infeasible, then additional effort to investigate or confirm this finding is not needed. 3.1.5 Track 3b: Limited or No Infiltration Feasible— Maximized or More Stringent Objectives The primary strategies in this case should focus on one or both of the following. The specific approach will depend on the degree of certainty in the preliminary finding of infeasibility and whether local regulations establish a minimum burden of proof for rejecting infiltration. 1. Conduct analyses to confirm or revise tentative infeasibility findings. If preliminary findings were based on limited information or a rapid analysis, then supplemental investigations may be warranted to verify these findings. If the refined findings allow the project to transition to a Maximized Partial Infiltration BMP approach, then this could help the project team accrue benefits toward compliance and other infiltration objectives. 2. Conduct analyses to present an adequate regulatory case for rejecting the use of infiltration. If local requirements or guidance prescribe the use of specific methods to determine infiltra- tion feasibility, then it may be necessary to apply these methods to justify the decision to not infiltrate. In these cases, the final infiltration feasibility criteria presented in Section 3.7 can be used to refine or confirm initial findings. 3.1.6 Summary Table 22 summarizes additional efforts that may be needed to confirm feasibility.

66 Stormwater Infiltration in the Highway Environment: Guidance Manual 3.2 Design Infiltration Rate Design-level infiltration testing is required for all Full Infiltration BMPs and for Partial Infil- tration BMPs that are being designed to meet a specified infiltration performance. Design-level infiltration testing is used to confirm preliminary planning infiltration thresholds and develop design infiltration rates. Design-phase testing may not be required for Partial Infiltration BMPs that do not rely on a specific infiltration rate and do not need to be designed to achieve a specific quantify of infiltration. 3.2.1 Refinements from Preliminary Assessment The scope of the investigation should include the following activities as explained further in Appendix B: • Conduct infiltration testing at the location and elevation of the proposed infiltrating surface consistent with an acceptable design-phase testing method for the anticipated BMP type. • Complete necessary infiltration tests, corresponding with method type selection, to adequately characterize infiltration surface spatial variability. • Interpret infiltration testing results based on site conditions and other available data (e.g., groundwater levels and bore logs). • Develop a design infiltration rate using an appropriate factor of safety to account for uncer- tainty and clogging. Appendix B provides guidance on method selection and interpretation. If groundwater mounding has been identified as a potential issue, the project team or hydro- geologic professional should further evaluate mounding. This analysis could potentially be per- formed using Appendix C: Roadside BMP Groundwater Mounding Assessment Guide and User Tool. At this phase, data should be site specific wherever feasible, particularly for parameters found to have an appreciable influence on results. Additionally, the project team or hydro- geologic professional should review the simplifying assumptions in this tool and verify them to be acceptable for the project site. If these simplifications are not applicable to the site, and groundwater mounding is potentially an issue, then a site-specific groundwater mounding assessment (potentially including modeling) may be needed. Prioritized Investigation Supporting Resources Applicability and Purpose Develop design infiltration rate Section 3.2 and Appendix B Track 1a, 1b: verification of Full Infiltration feasibility Verify sizing and performance feasibility Section 3.3, Appendix B, and Appendix C Track 1a, 1b: verification of Full Infiltration feasibility Confirm geotechnical findings and recommendations Section 3.4 and Appendix E Track 1a, 1b, 2a, 3b: confirm or revise findings as needed Confirm groundwater findings and recommendations Section 3.5 and Appendix D Track 1a, 1b, 2a, 3b: confirm or revise findings as needed Evaluate alternative project delivery options and needs Section 3.6 Primarily 1b where construction- phase control and decision-making are critical for success Finalize infiltration feasibility findings Section 3.7 All except Track 3a Table 22. Prioritized site investigations to confirm or revise infiltration approach.

Confirmation of BMP Selection Through Prioritized Analyses and Investigations 67 If the capacity of the infiltration receptor may limit reliable infiltration, the project team should consider reasonable mitigation approaches. Example mitigation approaches are as follows: • Adapt site design to locate BMPs in areas with fewer limitations (e.g., more permeable soils, greater separation to groundwater). • Reduce the loading rate to the BMP. • Reduce the width and depth of the BMP (i.e., narrower and shallower BMPs tend to result in less mounding for a given set of loading, soil, and groundwater conditions). These cases may also warrant further evaluation of groundwater conditions to verify or improve assumptions used in these analyses. 3.2.2 Underlying Criteria A reliable, long-term design infiltration rate is required for design of Full Infiltration BMPs. This should be used as part of sizing and performance feasibility analyses (Section 3.3) to verify that this design infiltration rate is adequate to support the selected BMP type and profile. There is not a fixed threshold that applies to all BMP types. The design infiltration rate should be adequate to drain the BMP in an acceptable amount of time and meet performance goals. 3.2.3 Example Criteria Manuals often establish certain minimum infiltration rate thresholds, such 0.3 in./h or 1 in./h, to determine the potential feasibility (or infeasibility) of infiltration BMPs. Unless this is necessary to satisfy local regulations, this Guidance Manual recommends avoiding the use of specific thresholds. As illustrated in Section 3.3, the threshold needed to support infiltration varies depending on the available space and the selected BMP. This Guidance Manual recom- mends that the reliable infiltration rate, the available space, and the applicable BMP types be considered to determine if infiltration is feasible; however, if there are local requirements, these should be followed, or permission should be obtained to deviate from them. 3.3 Sizing and Performance Feasibility Design infiltration rates have an important influence on the types of BMPs that can be sup- ported on a site, the allowable ponding depth for these BMPs, and the resulting footprint required to capture a certain design stormwater runoff volume (e.g., the 85th percentile storm) or achieve long-term performance criteria (e.g., infiltrate or treat 80% of long-term runoff volume). As the available space for BMPs becomes more limited, the ponding depth of the BMP must typically increase to provide similar volumes of stormwater storage. This can increase the loading ratio, which in turn can increase drawdown time and increase the risk of groundwater mounding and clogging. As a result, in sites with both limited space and moderate infiltration rates, it can be infeasible to rely solely on infiltration to meet sizing and performance goals. Table 23 summarizes the range of minimum design infiltration rates typically needed to sup- port various types of Full Infiltration BMPs and the associated loading ratios needed to capture and store the runoff from a representative 1-in. storm. 3.3.1 Refinements from Preliminary Assessment As part of the preliminary feasibility assessment described in Step 2, site conditions were con- sidered independently from available space, sizing requirements, and selected BMP. Integration

68 Stormwater Infiltration in the Highway Environment: Guidance Manual of these factors is critical to confirm feasibility. Where it appears that the combination of avail- able space and infiltration rate could be marginal, then Full Infiltration may not be feasible even if preliminary feasibility criteria are met. The following sections introduce key questions associated with this analysis and supporting resources. Key Questions The following questions may apply at this stage: How long will BMPs take to drain? This fundamental question is of critical importance. It can affect the long-term performance (e.g., accounting for back-to-back storms), the viability of the BMP (e.g., plant mortality), and nuisance issues (e.g., vectors, odors, wildlife usage, etc.). Do the proposed BMPs achieve the project objectives? When project objectives are expressed in terms of the performance of BMPs, designers can use models (or modeling-derived tools) to evaluate whether a proposed suite of BMPs achieves these objectives. Project objectives could take the form of the following examples: • Example 1: Capture runoff from a design event and subsequently recovery (via infiltra- tion) of the storage volume within a specific time (e.g., retain the 1.2-in. storm event, and recover storage within 48 hours following the end of rainfall; note that many MS4 permits do not address storage recovery, which is crucial for BMP performance as well as nuisance issues). • Example 2: Reduce the long-term runoff volume by a certain percent (e.g., 80% long-term volume reduction) compared with the developed condition without BMPs. • Example 3: Limit discharge volume to a certain long-term runoff coefficient (e.g., reduce runoff volume to 10% of long-term rainfall volume). Infiltration BMP Type Typical Effective Ponding Depth Typical Design Target Drawdown Time and Controlling Factor Minimum Design Infiltration Rate for Full Infiltration Maximum Loading Ratio to Store of 1-in. Storm Runoff (% of Impervious Area) Mounding Potential Shallow Flow Infiltration BMPs • Shallow Infiltration Swale • Filter Strip/Dispersion • Media Filter Drain 0.2 to 0.5 ft 12 to 24 hours (plant survival; aesthetics) 0.1 to 0.5 in./h 2:1 to 5:1 (20% to 50%) Low Subsurface Infiltration BMPs with Shallow Storage • Permeable Shoulders 0.4 to 0.8 ft 24 to 48 hours (long-term performance in sequential events) 0.1 to 0.4 in./h 4:1 to 8:1 (13% to 25%) Low Surface Ponding Infiltration BMPs with Shallow Storage • Distributed Bioretention without Underdrains • Infiltration swales 0.5 to 1.5 ft 12 to 24 hours (plant survival) 0.5 to 2.0 in./h 10:1 to 16:1 (6% to 10%) Moderate Surface Ponding Infiltration BMPs with Deep Storage • Infiltration Trenches • Infiltration Basins/ Centralized Bioretention Basins • Infiltration Galleries 3.0 to 6.0 ft 24 to 72 hours (vector issues; long- term performance in sequential event) 0.75 to 3.0 in./h 20:1 to 50:1 (2% to 5%) Moderate to High Table 23. Infiltration screening thresholds by BMP type.

Confirmation of BMP Selection Through Prioritized Analyses and Investigations 69 • Example 4: Match the long-term volume of surface runoff that is estimated to have occurred in the pre-project condition. • Example 5: Reduce the frequency of discharge from the site by a certain percentage compared with the developed condition without BMPs. How do different BMPs compare in terms of relative performance? When multiple BMPs are under consideration, designers can compare the relative performance, costs, and associated cost–benefit ratio of these BMPs. How do sizing and design parameters influence volume reduction and capture performance? This can be a critical question when some parameters remain uncertain, such as infiltration rate. Designers can conduct an evaluation of the sensitivity of BMP performance to uncertain parameters to evaluate the range of BMP performance uncertainty that could be expected. Tools Available to Support Analysis In addition to local modeling and analysis tools that may be applicable to a project, several tools with nationwide coverage are available to support this evaluation. Whole Lifecycle Cost and Performance Tools (NCHRP Report 792). Supports bio- retention (with and without underdrains), infiltration basins (as a variation of bioretention), swales, and filter strips including long-term volume reduction performance, pollutant load reduction, and lifecycle costing. Wet ponds and sand filters are also supported by this report and tool, which provides comparison with non-infiltrating systems. Estimates are based on long-term continuous simulation modeling at 344 long-term precipitation stations (one for each climate division). Volume Reduction Tool (NCHRP Report 802). Supports dispersion/filter strips, vegetated conveyance/swales, media filter drains, bioretention (with and without underdrains), permeable shoulders, infiltration trenches, infiltration basins, and infiltration galleries, including long-term volume reduction for individual BMPs and BMPs in series. Estimates are based on long-term continuous simulation modeling at 344 long-term precipitation stations (one for each climate division). This tool was based on the same hydrologic modeling as the NCHRP Report 792 tool. Roadside BMP Groundwater Mounding Assessment Guide and User Tool (Appendix C). Returns estimates of the magnitude and duration of mounding and accounts for reduction in effective infiltration rate as a function of groundwater mounding. This tool can be used to pro- rate the findings of the tools listed previously for cases in which mounding appears to influence long-term performance. BMP Clogging Risk Assessment Tool (Appendix F). While this Guidance Manual addresses clogging as a design decision in Chapter 4, it may be advantageous to assess clogging risk as part of developing design infiltration rates and assessing how loading ratios influence BMP feasibility. Appropriate Level of Detail The analysis of sizing and performance to confirm feasibility is not intended to be based on detailed designs. It should be rapid enough to be useful in alternatives evaluation, but also rep- resentative in macro-level design parameters, such as footprint and ponding depth. Figure 11 shows an example of the schematic design exhibits contained in the BMP fact sheets. Figure 12 provides an example of a preliminary conceptual site plan for a hypothetical BMP retrofit, illustrating that several options can be efficiently considered as part of a single conceptual design development process.

70 Stormwater Infiltration in the Highway Environment: Guidance Manual Infiltration Engineered soil medium thickness Optional stone storage layer thickness Overflow Mulch Inflow via surface flow or pipe inlet Energy dissipation stone Perforated underdrain Optional upturned elbow Ponding depth Figure 11. Example schematic design profile appropriate for use in feasibility analysis. Legend Storm drain receiving BMP discharge Tributary area to BMP Potential dispersion footprint (option 1) Bioretention footprint, vertical walls (option 2) Bioretention footprint, sloped sides (option 3) Figure 12. Example conceptual site plan appropriate for use in feasibility analysis. 3.3.2 Underlying Criteria A Full Infiltration BMP needs to be able to meet the applicable stormwater management objectives solely through infiltration. Sizing and performance analyses must be based on the reliable design infiltration rate and must demonstrate the following three underlying criteria: • The BMP will drain in an amount of time that does not compromise the integrity of the system. • The BMP will not pose hazards to public health related to mosquitos or other vectors. • The BMP will meet applicable sizing and performance requirements.

Confirmation of BMP Selection Through Prioritized Analyses and Investigations 71 3.3.3 Example Criteria Sizing and performance criteria should typically be derived from local requirements or guidance. Example criteria include the following: • The storage volume should be recovered via drawdown in not less than 48 hours or a locally acceptable alternative to ensure adequate long-term performance. • If the BMP is vegetated, water should drain below the root zone of plants in 24 hours or a locally acceptable alternative to support plant survival. • BMP should provide storage for the applicable design storm and long-term performance adequate to meet local standards. 3.4 Geotechnical Findings and Recommendations When infiltration of stormwater is near or within the highway environments, a geotechnical report that evaluates infiltration is typically a standard practice. Appendix E provides guidance on factors to assess and recommended contents of the geotechnical report on infiltration. 3.4.1 Refinements from Preliminary Assessment After other factors have been considered as part of the preliminary infiltration feasibility assessment, the locations of potential BMPs and the characteristics of these BMPs (size, depth, loading ratios) can be better defined. This supports more site-specific assessment of conditions and potential hazards. The scope of the refined geotechnical evaluation should be commensurate with the level of risk posed by the BMP and the applicable burden of proof to reject the use of infiltra- tion. For example, for shallow distributed infiltration BMPs located outside and especially downgradient of the highway prism, the level of risk may be limited and could be addressed with a brief report in the form of a letter to confirm that risks have been assessed but found to be negligible. This may require limited additional effort compared with the preliminary screening step. For more complex conditions, such as infiltration within the roadway embankment, adjacent to structures, or in more centralized locations (e.g., highway median), the geotechnical report should be more detailed and may need to include supporting analyses, such as slope stability, buoyancy, and groundwater mounding. The initial findings and assessments from the prelimi- nary screening step should be used to establish the necessary scope of this evaluation. In marginal cases with stringent infiltration objectives (Track 1b), the geotechnical report may need to consider and assess the practicality of mitigation measures to improve the feasibility of infiltration (e.g., underdrains in the base layer to protect the pavement). A range of potential mitigation measures are described in Appendix E. 3.4.2 Underlying Criteria At a minimum, the geotechnical analysis and recommendations should be adequate to address the following feasibility criteria: • Recommendations must be pertinent to the actual locations and types of BMPs proposed. • Recommendations must establish the maximum level of infiltration that allowed in each BMP location without posing risks (this could range from zero to unrestricted). • If infrastructure or structures exist near BMP locations, recommendations must establish the minimum setbacks from these features.

72 Stormwater Infiltration in the Highway Environment: Guidance Manual • Recommendations must be supported by and include documentation of soil investigations and infiltration testing if performed by the geotechnical engineer. • Recommendations must identify any construction-phase oversight or monitoring required. 3.4.3 Example Criteria The following sections identify example criteria that could be applicable as the basis for geotechnical recommendations. Geotechnical Risk Factors Preventing Any Infiltration (Result = Infeasible) • Soils with potential for volume change from wetting (e.g., expansive soils) or freeze/thaw, in which volume change (soil moisture) could result in impacts to pavement or structures • Slopes in which stability is sensitive to soil water content that cannot be reasonably designed to allow for any amount of soil wetting • Soils that exhibit a significant loss of strength when wetted, in cases where loss of strength cannot be reasonably accommodated in design • Utilities or existing infrastructure that cannot be designed to avoid or accommodate some intrusion of infiltrated water • Other factors as determined by a geotechnical professional Geotechnical Risk Factors Preventing Some Infiltration (Result = Some Limitations) • Soils that require a high degree of compaction to serve structural functions (e.g., compacted fill, roadbed), thereby reducing infiltration rates • Slopes or fill structures that can allow some soil wetting but cannot be reasonably designed to allow for Full Infiltration • Utilities that would potentially be susceptible to impacts in the case of Full Infiltration • Potential for significant mounding or lateral dispersion if infiltration exceeds allowable amount • Other factors as determined by a geotechnical professional Geotechnical Mitigation Approaches If a geotechnical risk factor is identified, the geotechnical analysis should document consideration of reasonable mitigation approaches. Example geotechnical mitigation approaches are as follows: • Attempt to locate BMPs in areas where they are outside applicable setbacks and in areas with- out fill or with lower depths of fill. • Over-excavate and backfill with more permeable materials in cases where the depth of fill is relatively shallow (less than approximately 5 ft below the invert of the BMP). • Use a more robust foundation or retaining wall design of the same type as otherwise pro- posed such that some infiltration can be allowed; it would be unreasonable to require a project to utilize a different type of foundation or retaining wall design solely to accom- modate infiltration. • Use underdrains or drain tiles to limit groundwater levels below infrastructure. 3.5 Groundwater Quality Findings and Recommendations 3.5.1 Refinements from Preliminary Assessment Depending on the complexity of local groundwater and the applicable groundwater water quality standards, refinements could range from relatively little effort to considerable effort. As part of preliminary screening (Section 2.2.5), project teams should identify the need for addi- tional assessment.

Confirmation of BMP Selection Through Prioritized Analyses and Investigations 73 The following topics may need to be assessed further to determine feasibility. Hydrogeologic Conditions. This includes depth to groundwater, groundwater properties (thickness, gradient, and direction of flow). Long-term monitoring to inform the project design may be needed in some cases, particularly where natural fluctuations are considerable. For example, the thickness of the unconfined aquifer and gradient of the water table are particularly important for assessing acute salt impacts in cold climates. Acute Salt Contamination. If the project is in a cold climate that utilizes salts, the potential for acute contamination of nearby wells should be evaluated (if not previously assessed). The Groundwater Quality Assessment Tool (found in Appendix D) can support this evaluation. This tool performs a relatively simply evaluation of advection and dispersion of salt, accounting for user-defined salt loading, BMP dimensions, and groundwater flow properties. Soil Properties and Pollutant Attenuation. If stormwater pollutants are identified as a potential risk to groundwater quality (as part of Step 2), then the project team may need to investigate soil properties and pollutant attenuation effects. Very sandy soils can lack the attenu- ation capacity to protect groundwater from stormwater pollutants at ordinary levels. The project team can analyze samples for cation-exchange capacity (CEC) and organic content to assess the pollutant attenuation capacity of soils and determine the need for soil amendments and pre- treatment to protect groundwater quality. Soil and Groundwater Contamination. If present, the limits of contamination and groundwater flow direction should be determined. The hydrogeologic investigation should evaluate whether infiltration would pose a risk of exacerbating contamination or complicating cleanup of contamination. Consultation with Applicable Groundwater Management Agencies. In general, it is a best practice for DOTs to coordinate with agencies responsible for local groundwater manage- ment whenever infiltration is considered for a project. These agencies have a vested interest in protecting groundwater supplies and underground infrastructure and usually have extensive knowledge about these resources. This consultation should ideally start as part of establishing infiltration objectives and preliminary constraints. Consultation with Sanitary Sewer Collection System Operators. It may be appropriate to consult with sewerage agencies to determine if inflow and infiltration (I&I) has been identified as a concern in the area. Stormwater infiltration has the potential to raise groundwater levels and increase I&I. Spill Containment. Local groundwater quality protection policies or wellhead protec- tion ordinances may specify the need for spill containment. Spill containment can require space and may not be compatible with all BMP types. Project teams should confirm that selected BMP types and locations can be designed to feasibly provide adequate spill contain- ment. Where spill containment is mandated, this may be an overriding consideration in BMP selection. Pollutant Fate and Transport Modeling and Calculations. While most projects should not require project-specific modeling of pollutant fate and transport modeling, this could be needed to support large-scale or highly constrained cases. Groundwater Monitoring. Depending on the severity of potential issues and the strin- gency of infiltration requirements, there could be cases where the use of infiltration must be accompanied by monitoring to confirm absence of impacts and the need to alter operation of the BMP (e.g., uncap underdrains).

74 Stormwater Infiltration in the Highway Environment: Guidance Manual 3.5.2 Underlying Criteria At a minimum, the groundwater quality analyses and recommendations should address the following feasibility criteria at the BMP locations: • The selected infiltration BMPs (including the use of pretreatment and soil amendments) pro- vide adequate pollutant attenuation to avoid unacceptable impacts to groundwater quality. • Infiltration does not mobilize existing soil or contaminate groundwater. • Infiltration BMPs incorporate features to comply with any applicable spill containment requirements. • Infiltration BMPs are used in a manner that complies with local criteria for groundwater resource management. • The level of infiltration proposed does not violate water rights. • The level of infiltration proposed does not create potential water balance modifications that could impair natural streamflow regimes (e.g., ephemeral streams) or elevate groundwater levels that impact other infrastructure. 3.5.3 Example Detailed Criteria The following sections identify example criteria that could be applicable as the basis for groundwater-related recommendations. Groundwater Quality Risk Factors That Prevent Any Infiltration (Result = Infeasible) • The infiltration facility is within 100 ft of a public or private water supply well, non-potable well, drain field, or spring (or is prohibited by locally applicable guidance or requirements). • Groundwater standards are determined to be very stringent (perhaps based on anti- degradation of high-quality groundwater or connectivity to a sensitive receiving water), such that impacts cannot be avoided. • The infiltration facility is on or adjacent to a brownfield site where infiltration of any appre- ciable amount would result in a significant risk of mobilizing or moving contamination that cannot be reasonably avoided. • A groundwater pollutant plume (constructed or natural) is under or near the site and any level of stormwater infiltration would contribute to plume movement that cannot be reasonably avoided. • Other critical factors have been identified as part of site assessment activities. Groundwater Quality Factors That Prevent Some Infiltration (Result = Some Limitations) • There are soils with limited attenuation capacity and shallow groundwater, but groundwater quality standards can be reasonably protected via the use of pretreatment or soil amendments. • There is soil or groundwater contamination in the vicinity of the project, in which a poten- tial rise in groundwater table associated with Full Infiltration could exacerbate contami- nant mobilization, migration, and cleanup efforts, but where Partial Infiltration would have acceptable effects. • Other factors have been identified as part of site assessment activities. Groundwater Quality Mitigation Approaches If a groundwater quality risk factor is identified, the documentation of infiltration infeasibility should consider reasonable mitigation approaches. Example groundwater quality mitigation approaches are as follows: • Remediate minor areas of contaminated soil on a site. • Design infiltration BMPs with spill containment, pretreatment, and soil augmentation.

Confirmation of BMP Selection Through Prioritized Analyses and Investigations 75 • Hydrologically isolate areas of the site that have higher risk of stormwater contaminants so that infiltration can be more feasibly applied to a lower-risk area. • Consider pretreatment or soil amendment if the following criteria are not met [adapted from (Washington State Department of Ecology 2012)]: – CEC of the treatment soil is least 5 milliequivalents CEC/100 g dry soil. – Organic content is at least 1.0% dry weight. – CEC and organic content encompass all distinct layers below the base of the facility to a depth of at least 2.5 times the maximum design water depth, but not less than 6 ft. – Other locally applicable guidance at the discretion of the project engineer. • Use BMP types that have lower risk of groundwater contamination or are more compatible with available groundwater separation (e.g., using shallower bioretention rather than deeper infiltration trenches). Water Balance and Water Rights Criteria While less common, infiltration of stormwater could change the existing flow regime of ephemeral streams or violate downstream water rights. If concerns are identified, the project team should perform a site-specific evaluation to determine whether water balance impacts or water rights violations would occur as a result of infiltration, including the following factors: • Infiltration levels exceeding pre-developed conditions could cause impairments to down- stream beneficial uses because of discharge of contaminated groundwater or changes in base- flow regimes to ephemeral streams. This is generally only a concern when infiltration rates are high (observed infiltration rates above 1 in./h), surface waters are proximate to the infil- tration BMP, and groundwater depths are relatively shallow. The level of allowable increase in infiltration should be documented in a site-specific study. This could also be the case in areas experiencing widespread conversion to urban development in which infiltration is being increased on a large scale. • Infiltration of runoff from the project would violate downstream water rights. Site-specific evaluation of water rights laws should be conducted if this is believed to be a potential issue in the project location. • ET from vegetated infiltration BMPs would violate downstream water rights. Site-specific valuation of water rights law should be conducted if this is believed to be a potential issue at the project location. 3.6 Alternative Project Delivery Project delivery refers to the approach for designing, bidding, contracting, and constructing a project, including bonding of the contractor, construction oversight, and transfer of main- tenance responsibility at the termination of the contract. The typical project delivery process (known as “design-bid-build”) involves development of plans, then bidding, then construction by a contractor that does not have a formal relationship to the designer. Potential limits of this contracting process that pertain to infiltration approaches include the following: • DOT design team is not able to receive input about BMP constructability from the contractor during the design process. • DOT construction manager may not be able to prescribe “means and methods” unless they are specifically defined in contract documents. This may limit the ability of the DOT construc- tion manager to prescribe construction methods and construction phasing. • Standard bonding and warranty periods may not be long enough for vegetation establishment. • Construction-phase design modifications may not be feasible unless specific contingencies are included in the design and bid package.

76 Stormwater Infiltration in the Highway Environment: Guidance Manual Alternate project delivery could give the DOT more control over construction-phase deci- sions and site management. Alternative project delivery options could include the following: • Development of special specifications or special contract provisions within a standard design- bid-build process. Distinct specifications could dictate construction methods, construc- tion phasing, vegetation acceptance criteria, and other issues relevant for infiltration BMPs construction. • Development of contingency design alternatives within a standard design-bid-build approach. This could include multiple versions of a design component, including specific with triggers for when a certain version would be activated. For example, the design drawings could include a version of the BMP that would be built if at-grade infiltration rates exceed a threshold and a different version that would be built if this threshold is not met. • Use of a design-build or construction-manager-at-risk delivery model. In these models, the contractor is responsible for developing the design or the contractor works as part of a collab- orative team with the DOT and the designer. These delivery models offer greater opportunity for design-phase input on constructability and phasing. They can also be more conducive to contingency plans, because site information could be collected during early phases of con- struction while detailed designs are still under development. The following are examples of cases in which some form of alternate project delivery could potentially improve the implementation of infiltration BMPs. Stringent Infiltration Requirements in Which Feasibility Depends on Construction-Phase Measurements. Examples could include projects that will involve considerable earthwork (cut or fill) in BMP locations such that it is not possible to obtain reliable measurements before construction. In this case, the design could proceed with two alternatives: an infiltration- based approach and a back-up plan that is based on partial infiltration and partial treatment. Construction-phase testing could be used as the ultimate deciding factor to determine which approach to construct. The project delivery approach and permitting processes would need to support this contingency. See additional guidance on adaptable designs in Section 4.4. Unavoidable Construction-Phase Compaction or Clogging in Infiltration Areas. This may compromise infiltration rates and warrant remediation of the area at the end of the construction period to support infiltration. This too, could justify construction-phase testing to confirm that infiltration rates have been adequately restored. It could also warrant specific requirements for construction-phase approaches, including directing the means and methods of construction. Vegetation Establishment Period for Site-Stabilization. Case studies have shown that elevated sediment loads during the post-construction vegetation establishment phase can pose risks to BMPs. This suggests that alternative phasing to allow vegetation establishment prior to the finish grading of infiltration facilities would reduce risks. This type of alternative phasing may require modifications to project delivery such as specifying phasing or requiring longer bonding/warranty periods. This could apply to the use of any infiltration or filtration BMP. For sites where these types of risks are present, the ability to use an alternative project delivery model could be an important factor in whether there is adequate confidence to proceed with infiltration. 3.7 Step 3 Results—Feasibility Findings Table 24 provides a template for confirming the feasibility determinations of the methods and criteria described in this section. Local guidance should be consulted to determine the degree to which infiltration is supported for each metric. This may require consultation with local

Confirmation of BMP Selection Through Prioritized Analyses and Investigations 77 3 Would infiltration pose significant risks for groundwater quality that cannot be reasonably mitigated to an acceptable level? (Section 3.5 and Appendix D) Provide basis: BMP Summary Drainage Area ID BMP Type Infiltration Feasibility Class Design Infiltration Rate Tributary Area Impervious Fraction/Area Loading Ratio (Tributary Impervious Area: BMP Area) Infiltration Sizing Criteria Directions: Answer each screening question below to determine the supported level of infiltration for that factor. Provide the basis for each selection by summarizing findings of site investigations and providing a narrative discussion of study and data source applicability. If applicable, describe risk mitigation approaches taken. Attach additional information as needed or provide reference to studies, calculations, maps, data sources, and so forth. Row Screening Question Level of Infiltration Supported Infeasible Partial Infiltration Supported Full Infiltration Supported 1 Do the design infiltration rates and available space support the selected BMP without negative consequence? (Sections 3.2, 3.3 and Appendices B and C) Provide basis: 2 Would infiltration increase risks of geotechnical hazards that cannot be mitigated to an acceptable level? (Section 3.4 and Appendix E) Provide basis: Table 24. Infiltration infeasibility screening criteria worksheet. (continued on next page)

78 Stormwater Infiltration in the Highway Environment: Guidance Manual regulatory authorities and technical experts to determine site-specific requirements. As a start- ing point, example feasibility criteria have been summarized in Section 3.2 through 3.5 and can a serve as a reference in the absence of local criteria. In evaluating each factor, the design team should consider reasonable approaches for miti- gating risks through site design, BMP design, or other project development aspects. Section 3.2 through 3.5 provide examples of reasonable mitigation approaches for improving the feasibility of infiltration. Table 24 is intended to serve as the method for documenting decision-making for issues that do not apply to a site or cannot be addressed with simple explanation. It is intended to serve as the method for documenting decision-making. For issues that warrant more site-specific evalu- ation, Table 24 can be completed with reference to applicable reports or studies. Row Screening Question Level of Infiltration Supported Infeasible Partial Infiltration Supported Full Infiltration Supported 4 Would infiltration pose impacts to local or regional water balance that could impact infrastructure or environmental resources? (Section 3.5 and Appendix D) Provide basis: 5 Would infiltration conflict with water rights and/or water balance? (Section 3.5 and Appendix D) Provide basis: Result Based on the screening criteria, what is the infiltration risk categorization? Infeasible Partial Infiltration Acceptable Full Infiltration Acceptable If any answer from row 1 through 5 is infeasible, this factor limits infiltration and the overall designation is Infeasible. If one or more factors support Partial Infiltration and no factors are infeasible, the overall designation is Partial Infiltration. If all answers are Full Infiltration, then Full Infiltration BMPs are acceptable. Table 24. (Continued).

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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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