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

Chapter: Chapter 1 - Introduction and Orientation

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Suggested Citation:"Chapter 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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 1 - Introduction and Orientation." 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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1 The purpose of this Guidance Manual is to support evaluation, selection, siting, design, and construction of infiltration best management practices (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. This Guidance Manual is intended to complement and inform local guidance and serve as a resource for plan- ners, designers, regulators, and policymakers. The goal of this Guidance Manual is to support responsible decisions about stormwater infiltration in the highway environment. The Guidance Manual contains effective system designs for projects that include infiltration. In preparing this Guidance Manual, the research team considered a broad range of issues that can limit infiltration. The research team identified approaches for assessing and overcom- ing these limits for each phase of the project delivery process. These research findings sup- ported the development of practical guidance. The Guidance Manual includes approaches (e.g., frameworks and underlying principles) to overcome conceptual limitations and also provides detailed guidance (e.g., topical guides, tools, design adaptations, construction, and checklists) that focuses on more specific limitations. This Guidance Manual is accompanied by a Project Summary Report. These documents are intended to serve complementary roles. The Guidance Manual provides technical guidance to targeted users, including transportation planners, designers, regulators, and policymakers. The Project Summary Report provides documentation of the research efforts and methods used to support development of this Guidance Manual and the associated software tools. The Project Summary Report can be found on the TRB website (www.trb.org) by searching for “NCHRP Research Report 922”. 1.1 Key Features and Uses of this Guidance Manual This Guidance Manual includes five chapters, organized into a stepwise decision-making framework. The main body of this Guidance Manual serves as an efficient resource and provides an orientation to key issues. Appendices and Microsoft Excel-based user tools provide focused technical references on selected issues. Appendix A is published herein and Appendices B through J can be found on the TRB website (www.trb.org) by searching for “NCHRP Research Report 922”. Table 1 highlights the key features of this Guidance Manual. The following paragraphs summarize the organization of the Guidance Manual and the intended uses of each section. Chapter 1 provides an overview of infiltration approaches to stormwater management and provides a summary of the key factors influencing infiltration feasibility and desirability. This chapter serves as a reference of the primary decisions and factors associated with infiltration C H A P T E R 1 Introduction and Orientation

2 Stormwater Infiltration in the Highway Environment: Guidance Manual approaches. This chapter also introduces the decision-making framework that provides struc- ture for the remaining chapters. The steps in this framework are as follows: Step 1: Perform Project Scoping and Preliminary Planning for Stormwater Infiltration Step 2: Tentatively Select BMP Locations and Types Step 3: Conduct Prioritized Site Investigations and Analyses to Confirm BMP Selection and Sizing Step 4: Design, Construct, and Maintain BMPs Section 1.7 explains these steps. Chapter 2 provides guidance for Steps 1 and 2 of the decision-making framework. The guid- ance in this chapter helps a user select a general strategy for stormwater infiltration and identify Feature Description Location Stepwise Decision-Making Framework A framework and guidance for conducting investigations, organizing data, and scoping analyses to support BMP selection, siting, design, and construction. Includes flow charts, worksheets, example criteria, and distilled guidance to support each step Overview in Section 1.7 Details in Chapters 2, 3, and 4 Examples in Chapter 5 BMP Fact Sheets Fact sheets summarizing characteristics, key considerations, design schematics, and example design criteria for 10 infiltration BMPs Appendix A Detailed Technical Guides Technical guides providing detailed information on key topics including the following: • Infiltration estimation methods (including 10 fact sheets on testing methods) • Groundwater mounding • Water balance and groundwater quality • Geotechnical issues • Cold and arid climate issues Appendix B Appendix C Appendix D Appendix E Appendix I Microsoft Excel- Based User Tools Three new software tools intended to streamline key analyses that project teams may need to conduct include the following: • Roadside BMP Groundwater Mounding Assessment Guide and User Tool • Guide for Assessing Potential Impacts of Highway Stormwater Infiltration on Water Balance and Groundwater Quality in Roadway Environments • BMP Clogging Risk Assessment Tool These can be found on the TRB website (www.trb.org) by searching for “NCHRP Research Report 922 ”. Appendix C Appendix D Appendix F Case Studies Case study applications of this Guidance Manual to three real projects Case studies of whole lifecycle cost and performance analysis tools Case studies of infiltration BMPs constructed by DOTs, weighted toward BMP failures that can serve as a learning opportunity Chapter 5 Appendix G Appendix J Table 1. Key features of this Guidance Manual.

Introduction and Orientation 3 tentative BMP types and locations. This chapter introduces screening-level methods for reach- ing preliminary determinations and helps determine the need for conclusive methods to confirm feasibility. Chapter 2 is supported by several appendices, designed to address specific topics that may be relevant for selection of an infiltration strategy: 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 Appendix E: Guide to Geotechnical Considerations Associated with Stormwater Infiltration Features in Urban Highway Design Chapter 3 supports Step 3 of the framework. This chapter provides guidance for scoping and performing site investigations and preliminary design analyses intended to result in confirma- tion of the selected BMP types, locations, and overall infiltration strategy. Appendices A through E also support Step 3 (each appendix describes both preliminary and confirmatory assessment methods). Chapter 4 provides guidance on BMP design, construction, operations and maintenance (O&M), and post-construction monitoring in support of Step 4 (Design, Construct, and Main- tain BMPs). This section supports projects that include an infiltration-based approach. This chapter is also supported by appendices that address specific topics: Appendix F: BMP Clogging Risk Assessment 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 Note that Appendices A through E also include guidance supporting Step 4 (BMP-specific design decisions). Chapter 5 provides brief summaries of how the steps in this Guidance Manual could apply to example projects. Appendix J provides BMP case study reports (with an emphasis on infiltration failures and lessons learned) that may be of interest to users. The decision-making framework and criteria presented in this Guidance Manual can be adapted to an agency’s project delivery processes and accommodate project-specific issues. 1.2 Introduction to Infiltration Approaches for Stormwater Management in the Highway Environment The infiltration approach to stormwater management involves the design, construction, and O&M of engineered systems that infiltrate stormwater runoff into soils. These systems, referred to as “infiltration BMPs,” are intended to reduce the volume of stormwater runoff and associ- ated pollutants that discharge to stormwater systems and receiving waters via surface runoff. The concept of stormwater infiltration in the built environment is inherently different from the natural rainwater/snowmelt infiltration that occurs on pervious lands. In the built environ- ment, stormwater runoff from impervious surfaces is routed to a pervious BMP area (often designed to pool water), resulting in greater levels of hydraulic and pollutant loading in this area than would occur via precipitation alone. As a result, a greater portion of stormwater percolates

4 Stormwater Infiltration in the Highway Environment: Guidance Manual to deeper groundwater and discharges to surface runoff than would occur in natural refer- ence conditions in which evapotranspiration (ET) tends to have greater influence on the water balance (Strecker et al. 2015). As a result, a range of conditions can develop in engineered stormwater infiltration systems that are less frequently observed on natural pervious lands, including ponded water, soil satura- tion, localized groundwater mounding, pollutant accumulation, and surficial clogging. Where site conditions do not support the intended level of infiltration or pose risks to infrastructure or the surface or sub-surface environment, an infiltration approach may be infeasible or not desirable. Identifying where limits exist for a given site is a key step in responsible application of stormwater infiltration. In addition to physical limits, there are regulatory limits associated with infiltration of storm- water runoff from the built environment. State regulators may classify stormwater as discharge to a receiving water (e.g., groundwater), and local groundwater management entities may estab- lish groundwater protection criteria that apply to stormwater infiltration. Project teams need to consider these issues as part of selection, siting, design, operation, and monitoring of stormwater infiltration BMPs. However, the same underlying processes that pose risks in some cases can also provide benefits. For example, in suitable conditions, the use of infiltration can help project teams efficiently comply with surface water requirements while lessening the need for downstream conveyance infrastructure. In some cases, infiltrated stormwater can also be a desirable resource for groundwater augmentation. There is not a single “infiltration approach” that has categorical benefits or limitations. Rather, this Guidance Manual considers a range of infiltration-based stormwater manage- ment approaches. These approaches target different levels of infiltration, have different levels of sensitivity to site conditions, pose different risks, and have different limitations. One of the goals of this Guidance Manual is to assist users in (a) evaluating a range of potential infiltra- tion approaches, (b) selecting and implementing one that is appropriate for the objectives and constraints that apply to a given site, and (c) identifying the need for alternative non-infiltration approaches to be considered. 1.3 Rationales for Considering Stormwater Infiltration There are numerous reasons for DOTs and project designers to consider some form of storm- water infiltration as part of a stormwater management approach. Examples include the following: • Infiltration of stormwater may need to be considered or implemented to comply with appli- cable regulations, such as National Pollutant Discharge Elimination System (NPDES) permits or Total Maximum Daily Loads (TMDLs). • Pollutant removal performance of BMPs can be improved when volume reduction is increased. • Infiltration can be more cost-effective than other stormwater management approaches under favorable conditions and can sometimes help reduce the cost of overall stormwater management design (e.g., via fewer storm inlets and less piping). In many cases, some level of infiltration occurs incidentally at no additional cost. • Multiple benefits can be realized such as groundwater augmentation and reduction of hydraulic load to streams; water that infiltrates and later enters receiving waters as interflow or baseflow tends to be cleaner and mimics natural flow regimes compared with direct surface runoff. Given these potential motivations and advantages, an approach involving some level of infiltration warrants consideration for stormwater management applications in the highway environment.

Introduction and Orientation 5 1.4 Key Limitations to Infiltration as a Stormwater Management Approach Stormwater infiltration approaches have potential limitations. There are five general catego- ries of limits to infiltration. 1. Physical Feasibility. Can you do it (feasibility)? Key limits related to physical feasibility include the following: • Soil infiltration rate at the intended infiltration surface (i.e., the interface between more per- meable media and the underlying native soil) including the effects of compaction (intentional or unintentional) on infiltration. • Capacity of the soil/groundwater receptor to receive infiltrated volume including limiting layers, potential for groundwater mounding, and associated degradation of infiltration rate. • Amount of space available for an infiltration surface within the highway environment. 2. Impacts to Infrastructure or the Environment. Should you do it (desirability)? Infiltra- tion of stormwater poses potential risks, including the following: • Geotechnical hazards related to structures, foundations, and slopes. • Roadway damage, such as impacts to the integrity of base, subbase materials, and pavements. • Deterioration of groundwater quality from stormwater-borne pollutants and mobilization of pollutants in soil or groundwater. • Unnatural water balance effects involving artificially elevated groundwater tables can result in a change of stream systems from ephemeral or intermittent to perennial (with possible habitat changes) in arid areas. 3. O&M Limits. Can performance be sustained? Infiltration BMPs can be susceptible to O&M issues including the following: • Clogging of systems as a result of sediment loading • Challenges in accessing surfaces that have become clogged • Uncertainty in what remedial efforts will be effective to restore function if clogging or other issues occur • Maintaining acceptable levels of vegetation • Other challenges to safely and consistently perform maintenance at an acceptable cost 4. Practical Limits. This category of limits pertains to practical factors associated with planning, designing, implementing, and operating infiltration BMPs, including the following: • Cost and time requirements. Assessing the feasibility and desirability of infiltration approaches can require substantial cost and time. This is particularly true if there are complex factors at a site, or if the design must ensure that a certain reliable amount of infiltration will occur. In practice, a single missed factor from one of the three categories (numbers 1 through 3 in this list) can lead to failure or unintended consequences requiring an alternative approach to be implemented. While some level of geotechnical investigation is needed for most BMP types, there are often extra costs associated with investigation for infiltration BMPs, including the need for reliable infiltration tests, greater number of tests, and longer periods of monitoring to determine seasonal hydrogeologic conditions. • Unknowns in design and construction. Even with a thorough investigation and assessment as part of the design, uncertainties remain in predicting as-built infiltration rates of full-scale facilities. This is due to limitations in infiltration measurement techniques as well as the poten- tial for changes in infiltration properties during construction and post-construction activities. Developing designs and construction plans to accommodate these inherent unknowns, while still ensuring the survivability of the system, can be more challenging.

6 Stormwater Infiltration in the Highway Environment: Guidance Manual • Project delivery and contracting methods. Successful implementation of infiltration approaches can require careful control through the design, construction, and post-construction phases of the project. This may require adjustment of typical project delivery approaches, for example, specifying “means and methods” of construction, allowing for design contingencies/ modification based on construction-phase tests, and providing for a longer warranty/bonding period for system establishment. • Unknowns in maintenance needs. Limited data are available regarding the maintenance needs of infiltration BMPs, particularly how maintenance needs are affected by site-specific factors. This can have major implications on lifecycle costs and budgeting. • Regulatory uncertainty for groundwater receptors. It can be unclear what limits apply to infiltration discharges to groundwater, and how these limits may change in the future. Addi- tionally, due to limits in available scientific understanding and contaminants of emerging concern, there are cases for which it may not be possible to quantify potential long-term effects of infiltration on groundwater quality. The framework for infiltration evaluation and implementation presented in this Guidance Manual is designed to address and overcome these practical limitations, where possible. 5. Program Management Limits. This class of limits pertains to program management issues that can be associated with the broadening use of infiltration in the highway environment. During interviews and communication with DOT program managers, several issues were identi- fied, including the following: • Long-term liability. Broader use of infiltration BMPs can increase DOT liability related to inventorying features, reporting compliance, funding long-term O&M, developing memo- randa of understanding (MOUs) with local government, and associated staffing needs. These issues apply to any stormwater control approach. However, uncertainties in the lifecycle cost and management needs of some infiltration BMPs can make it more challenging to quantify long-term liability compared with conventional stormwater management approaches that have more defined costs and operating requirements. • Legal liability. Even with a careful screening and design process, infiltration BMPs have the potential to pose legal liability related to groundwater contamination, geotechnical failures, water rights, and other issues. While DOTs can limit these risks with effective technical guid- ance and project review processes, the elimination of legal liability arising from stormwater infiltration is not realistically possible in all cases. • Compliance monitoring. Depending on types of BMPs used and applicable regulations, a compliance monitoring program may be needed to evaluate performance and impacts. • Compatibility with land use plans. The infiltration approach may be incompatible with local land use plans, such as source water protection zones and wellhead protection zones. DOT program managers should consider these factors when establishing agency policies and technical guidance. These issues differ with different classes of infiltration approaches (as described in Section 1.5). Additionally, the framework is designed to guide appropriate BMP selection, design, construction, and maintenance to reduce these organizational risks. 1.5 Classes of Infiltration Approaches The decision-making and implementation framework is organized around three overall classes of infiltration approaches: 1. Full Infiltration. This class involves infiltration BMPs that rely solely on infiltration into underlying soils. Full Infiltration does not imply that all stormwater runoff is infiltrated. The

Introduction and Orientation 7 amount of water infiltrated is a function of BMP size and site conditions. However, these BMPs do not have a design discharge to surface waters except when the system overflows or bypasses. The key distinguishing trait of these BMPs is that they depend on a certain minimum infiltration rate to meet their intended functions and avoid unintended consequences (e.g., nuisance condi- tions, vegetation mortality, vector issues, safety concerns, excessive bypass, or overflow levels). Examples include the following: • Infiltration basins • Infiltration trenches • Bioretention without underdrains • Permeable pavement and shoulders • Infiltration galleries Within this category, systems can be designed with or without features that could allow them to be adapted to a Partial Infiltration design (e.g., capped underdrains). 2. Maximized Partial Infiltration. This approach involves BMPs designed specifically to maximize infiltration of a portion of the applicable design volume while also providing other treatment mechanisms. These BMP types are not wholly reliant on infiltration to maintain an operable condition and meet water quality and flow control requirements but are expected to result in significant levels of infiltration. Examples include the following: • Vegetated filter strips with amended soils • Vegetated swales with shallow subsurface retention storage • Media filter drains • Bioretention with underdrains and internal retention storage • Permeable pavement and shoulders with supplemental drains These approaches share common design attributes: (1) subsurface storage compartments dedicated to infiltration only and (2) freely draining surface storage compartments that do not rely on infiltration to be operable. These systems can be designed to meet a specific vol- ume reduction goal if the underlying soil infiltration rates are well understood. 3. Incidental Infiltration. This approach involves the use of BMPs designed principally for treatment and flow control of stormwater but with design considerations that allow for inciden- tal infiltration of stormwater. Examples are similar to the Maximized Partial Infiltration category, but without design features specifically intended to maximize infiltration. These approaches are generally not designed for a given level of volume reduction. These classes vary principally in (1) the degree to which they rely on a certain minimum infiltration rate to remain operable, (2) the degree of infiltration provided, and (3) their design approach relative to the specificity of infiltration goals. These distinctions have a sig- nificant effect on the planning, evaluation, and design processes described in this Guidance Manual. 1.6 Menu of Infiltration BMPs This Guidance Manual presents a decision-making framework based first on the class of infil- tration approach and then on the characteristics of the individual BMP type. Knowledge of the attributes and applicability of individual BMPs can support reasonable planning-level decisions about BMP feasibility and tentative selection of BMPs. Table 2 summarizes the menu of infiltra- tion BMPs supported by this Guidance Manual (common alternative terminology is given in parentheses). Fact sheets for each are provided in Appendix A.

8 Stormwater Infiltration in the Highway Environment: Guidance Manual BMP 01 Vegetated Conveyance This BMP type includes engineered vegetated swales and other vegetated drainage features that serve the purpose of conveying stormwater runoff and can also provide treatment and significant reduction of stormwater runoff volume. Variations on this approach can include an amended soil or stone storage layer to increase storage capacity and promote infiltration. This BMP type is usually designed as an Incidental Infiltration BMP. Robust vegetative growth is important to maintain infiltration rates, slow water, and stabilize the surface to prevent scour. BMP 02 Dispersion This BMP type consists of the dispersion of runoff toward existing or restored pervious areas including road shoulders amended with compost and additional materials such as sand (if needed), designed to convey runoff as sheet flow over the surface or as shallow subsurface flow through amended soil layers. Dispersion reduces overall runoff volume by promoting infiltration and ET. Volume reduction performance can be improved with flow spreaders, shallow slopes, and soil amendments. This BMP type could qualify as Full Infiltration, Maximized Partial Infiltration, or Incidental Infiltration, depending on design and site conditions. Robust vegetative growth in dispersion areas is important to stabilize the surface and maintain good infiltration rates. BMP 03 Media Filter Drain This BMP consists of a stone vegetation-free zone, a grass strip, a storage reservoir filled with specialized media, and a conveyance system for flows leaving the reservoir. The conveyance system usually consists of a gravel-filled underdrain trench or a layer of crushed surfacing base course. The stone vegetation-free zone is intended to promote sheet to spread the water before it flows across the grass strip. It is then captured by the storage reservoir, where it infiltrates into the subsoil or is discharged through the underdrain. This BMP type is typically designed as a Maximized Partial Infiltration BMP. This BMP is typically installed between the road surface and a ditch or other conveyance located downslope. This BMP is based specifically on designs developed and applied by Washington State DOT. BMP 04 Permeable Shoulders This BMP type includes a permeable pavement surface course (asphalt, concrete, or interlocking pavers) along the shoulders of a roadway, underlain by a stone reservoir. Precipitation falling on the permeable pavement as well as stormwater flowing onto permeable pavement from adjacent travel lanes infiltrates through the permeable pavement top course into the stone reservoir where it infiltrates into the subsoil or is discharged through an underdrain and outlet control structure. With an underdrain and flow control outlet to augment infiltration capacity, permeable shoulders can be applied in a wide range of soil conditions and could also be used when soil conditions are less favorable for other infiltration BMPs. They could qualify as Full Infiltration or Maximized Partial Infiltration BMPs. Table 2. Introduction to primary menu of infiltration BMPs.

Introduction and Orientation 9 Table 2. (Continued). BMP 05 Bioretention without Underdrains Bioretention consists of a shallow surface ponding area underlain by porous soil media storage reservoirs and an optional porous stone storage layer. Captured runoff is directed to the bioretention area where it infiltrates into an engineered soil medium and then infiltrates into the subsoil. They would typically qualify as Full Infiltration BMPs. Engineered soil media is a central element of bioretention design and typically includes a mixture of sand, soils, and organic components (e.g., compost) that are designed to provide permeability, promote plant growth, and provide treatment. When infiltration is exceeded, water is conveyed to a surface discharge via an overflow riser or via an overland flow pathway. BMP 06 Bioretention with Underdrains This BMP type is similar to BMP 05 but includes an underdrain system to supplement infiltration discharge. Where soil infiltration rates permit, volume reduction can be enhanced by installing a stone reservoir beneath the underdrain discharge elevation. An upturned elbow or outlet structure can be used to create a retention storage zone (e.g., internal water storage zone). This category of BMPs is suitable for a wider range of conditions than bioretention without an underdrain and can potentially be used to mimic natural baseflows via careful control of discharges from the underdrain. These could qualify as Maximized Partial Infiltration or Incidental Infiltration BMPs. BMP 07 Infiltration Trench This BMP type consists of a stone-filled trench that provides subsurface storage of stormwater runoff and allows water to infiltrate through the bottom and walls of the trench into subsoils. These could qualify as Full Infiltration or Maximized Partial Infiltration BMPs. Pretreatment for infiltration trenches is commonly provided via vegetated conveyance such as swales or filter strips. Infiltration trenches tend to be well suited to the linear highway environment, because they are generally constructed in a linear configuration and their surface tends to be nearly flush to existing grade or slightly removed when pretreatment is included. BMP 08 Infiltration Basin Infiltration basins are relatively large, shallow basins that discharge water primarily via infiltration. Their contours appear similar to detention basins, but they do not have a surface discharge point below their overflow elevation. Infiltration basins are typically located in relatively permeable soils. They would qualify as Full Infiltration BMPs. Infiltration basins can be designed with detention surcharge above the infiltration volume to provide a combination of volume reduction and peak flow mitigation. Infiltration basins are differentiated from bioretention basins, because they are typically built on a larger scale and typically do not include an engineered soil medium. Vegetative cover may also be different. (continued on next page)

10 Stormwater Infiltration in the Highway Environment: Guidance Manual Note that this Guidance Manual does not explicitly consider drywells. The use of drywells in the highway environment is rare. Many considerations related to infiltration trenches and infiltra- tion galleries apply to drywells. Additionally, drywells are required to be registered as part of a fed- eral Underground Injection Control program. Specific state and local standards may also apply. 1.7 Overall Infiltration Assessment and Decision-Making Framework This Guidance Manual proposes a structured framework for conducting infiltration assess- ments, evaluating infiltration limits, and making decisions about infiltration approaches for a given site. This framework is intended to support efficient investigation and selection of appropriate infiltration approaches. It is designed to improve efficiency by focusing on the questions that are crucial for a given project and the site conditions. The overall objective of this framework is to match appropriate infiltration approaches to site conditions and infiltration objectives to efficiently comply with applicable regulations. Figure 1 provides an overview of this process. The steps shown in Figure 1 are further described in the following sections. Step 1: Perform Project Scoping and Preliminary Planning for Stormwater Infiltration One key to successful implementation of stormwater infiltration is early consideration of stormwater management in project planning. Ideally, this will occur as part of advanced plan- ning and environmental permitting. In this step, the project team assembles readily available information and applies efficient planning-level screening methods to reach initial decisions about the potential types of infiltration BMPs or non-infiltration alternatives that would align with infiltration objectives and site conditions. While the data to support these decisions are not typically conclusive, these preliminary decisions can guide and improve the efficiency of subsequent efforts. Information compiled and used by the project team in this step includes the following: • Regulatory requirements (e.g., infiltration requirements, applicable alternatives or “offramps,” underlying regulatory goals, and groundwater quality standards) • Other volume reduction goals (e.g., groundwater augmentation, stream protection, and cost avoidance) • Project constraints and opportunities (e.g., project type, cuts and fills, and available space) BMP 09 Infiltration Gallery Infiltration Galleries (aka underground infiltration systems) include a broad class of BMPs that consist of storage reservoirs located belowground preceded by pretreatment systems. Water is pretreated, routed into the systems, and infiltrated into the subsoil. They would typically qualify as Full Infiltration BMPs. A range of potential options are available for providing storage including use of open graded stone or a variety of engineered storage chambers (concrete, plastic, or metal). There are also a range of potential locations where infiltration galleries can be placed, such as below (a) parking areas, (b) access roads, or (c) travel lanes. Table 2. (Continued).

Introduction and Orientation 11 • Site conditions (e.g., readily available or efficiently obtained information about soil types, sensitive infiltration receptors, groundwater levels, slopes, and contamination) • Local groundwater management criteria and guidance [e.g., wellhead protection programs, source water protection programs, and sole source aquifer (SSA) designation] • Budget and schedule constraints and available sources of funding • Capacity and preferences of O&M staff related to BMP types and maintenance needs The outcomes of this step may include the following: • Refinement of stormwater management goals pertaining to infiltration (or identification of alternative non-infiltration approaches that meet project goals) • Identification of potential project areas to reserve for infiltration • Identification of potential limits that may apply • Preliminary selection of a class of infiltration approach (e.g., Full Infiltration, Maximized Partial Infiltration, or Incidental Infiltration) Step 2. Select Tentative BMP Locations and Types Step 1. Perform Project Scoping and Preliminary Planning for Stormwater Infiltration Tentatively Select BMPs Types and Locations Establish Infiltration Objectives Conduct Preliminary Assessment of Infiltration Feasibility Evaluate Other Practical BMP Selection Factors Select Tentative Class of Infiltration Approach based on Preliminary Information Step 3. Conduct Prioritized Site Investigations and Analyses to Confirm BMP Selection and Sizing Identify Supporting Analyses Needed to Confirm or Modify Selection Select Investigation Approach to Confirm Tentative BMP Selection and Siting Confirm Full Infiltration Feasibility Confirm Infeasibility Further Investigate Full Infiltration vs. Partial Infiltration Confirm Partial Infiltration Feasibility Confirm or Revise Tentative BMP Siting and Selection Decisions Step 4. Design, Construct, and Maintain BMPs Pretreatment Selection Adaptable Design Contingency Planning Other Specific Design Features Construction Phasing and Site Controls Project Delivery Model Design Features to Support O&M O&M Planning Clogging and Lifecycle Assessment Figure 1. Overview of infiltration decision-making framework.

12 Stormwater Infiltration in the Highway Environment: Guidance Manual • Identification of the primary risks and failure modes that could control decision-making for the site, and prioritization of issues for further consideration (e.g., the key factors that need to be resolved) • Scoping and prioritization of the site investigations and analytical efforts necessary to con- firm or refine the selected approach (e.g., soil infiltration testing at potential BMP locations, and groundwater and geotechnical analyses) • Adaptation of the project delivery process (e.g., project-specific delivery approaches to miti- gate risks) • Determination of BMP types and locations that can be reasonably maintained This step is intended to be relatively quick. It is not intended to be conclusive. This step is intended to promote efficiency in future steps and ensure that the decisions made will allow for infiltration options, where applicable and desired or required. This step helps focus the scope of future studies on the limits that may apply. Step 2: Select Tentative BMP Locations and Types In this step, the project team tentatively identifies BMP locations and tentatively selects the types of BMPs that will be evaluated for each location. These selections should be based on the findings from Step 1. While available data may not yet be conclusive to determine the feasibil- ity of these BMPs, this step helps the project team narrow the scope of subsequent infiltration feasibility investigations. By narrowing the scope of these investigations and prioritizing inves- tigation needs, this step helps the project team develop more reliable information about each location. Key questions in this step include the following: • For the locations where infiltration could be implemented, which BMPs are applicable? • Which BMPs will have the greatest potential to meet infiltration goals and limit risk to accept- able levels? Considerations include the following: – Overlay of infiltration feasibility category and infiltration objectives – Location, geometry, and size of available space – Adaptability needs – Whole lifecycle costs – O&M requirements and compatibility with DOT O&M capabilities The intended outcomes of this step include the locations, types, and potential footprints of the BMPs, approximate tributary areas, and the overall conceptual design of each BMP (macro- level parameters, such as approximate depth, size, and discharge pathways). These parameters will support the confirmatory-level investigations in Step 3. Step 3: Conduct Prioritized Site Investigations and Analyses to Confirm BMP Selection and Sizing In this step, the project team conducts investigations and analyses intended to confirm or revise the feasibility of the selected BMPs. This step may vary considerably depending on the results of the project scoping and preliminary planning efforts (Step 1) and the types of BMPs tentatively selected (Step 2). The key difference from Step 1 is that investigations and analyses in this step are intended to be confirmatory. Project teams may need more rigorous investiga- tion and analysis methods, particularly if Full Infiltration BMPs are under consideration. Note that depending on BMP selected, some elements may not be needed. For example, the project team may not need to determine design infiltration rates if BMPs will be designed for Partial Infiltration and will not depend on a certain minimum infiltration rate.

Introduction and Orientation 13 Key questions in this step include the following: • Is it physically feasible to infiltrate stormwater at the target levels within the identified poten- tial infiltration areas? Considerations include the following: – Design infiltration rates – BMP sizing calculations – Effect of groundwater mounding on reliable infiltration rates – Topography and space – Reasonable approaches to improve physical conditions for infiltration • Is it desirable to infiltrate stormwater at the target rates? Are there sensitive receptors or condi- tions that would be affected? Considerations include the following: – Geotechnical/pavement/utilities – Groundwater or soil contamination – Adherence to local groundwater protection criteria – Local water balance issues (particularly in arid climates) – Reasonable approaches to mitigate issues • Do these data confirm the selected infiltration strategy and associated BMPs? Or do the pre- liminarily selected BMP types and locations need to be revised based on the prioritized site investigation results? This step should result in confirmation of the selected BMP locations and types or identifica- tion of the need for revisions to this strategy. Step 4: Design, Construct, and Maintain BMPs In this step, the project team develops detailed designs and construction plans, along with maintenance and monitoring protocols, for the selected BMPs. Design, delivery, and maintenance processes will inherently vary by project type but should generally consider the following: • Development of design details to mitigate risks. The designer should consider and assess potential design variations based on site features, site conditions, project goals, and risk factors. The following are examples: – Pretreatment or isolation approaches – Design elements to improve resiliency (back-up plans and adaptability) – Supplemental treatment/drainage features built into the design (e.g., relief valves) – Design features needed to allow for maintenance of the BMP Project designers should consult with construction and O&M personnel during the develop- ment of the design to help ensure that the proposed system can be constructed and maintained. • BMP construction. What construction-phase specifications and precautions should be used to minimize risks to infiltration and other functions of BMPs during construction and estab- lishment phases? The following are examples: – What approaches can be used in designing, bidding, and contracting to reduce the risk of construction errors or construction-phase impacts to infiltration sites and infiltration BMPs? – What will be done to remediate infiltration rates if there are unavoidable or unforeseen construction impacts? – What contingency plans are needed for design adjustment based on conditions encoun- tered during construction? • BMP maintenance. How will the BMP be maintained and what specific provisions are needed to ensure that maintenance occurs? The following are examples: – How will the BMP be assessed to determine the need for maintenance? Do the design and site access support these assessments?

14 Stormwater Infiltration in the Highway Environment: Guidance Manual – How will the BMP be maintained? Does the design allow for maintenance to occur? – What are the estimated timing and cost of key activities? – Is a BMP-specific O&M plan required, or will the BMP be covered under a standard main- tenance procedure? • Post-construction monitoring. Monitoring can help DOTs improve guidance, assess main- tenance needs, evaluate performance, and assess impacts associated with infiltration BMPs. Chapter 4 of the Guidance Manual is intended to help ensure that appropriate factors are considered in design, construction, and maintenance of BMPs. However, design, contracting and maintenance processes will vary considerably by agency. Therefore, this step is less struc- tured than the previous steps. Summary of Decision-Making Framework This four-step process can serve as an overall road map to improve efficiency and reduce risks associated with evaluating and developing stormwater infiltration BMPs. This process is defined by (1) conducting early decision-making to focus the scope of subsequent investigations, (2) selecting BMPs based on their ability to meet project goals and their compatibility with site- specific conditions, (3) reserving more rigorous investigation methods for locations where they are needed, and (4) designing BMPs to reduce sensitivity to uncertain conditions (e.g., improv- ing resiliency) and allow for maintenance to be performed. The remainder of this Guidance Manual is organized around this framework.

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