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1 1.1 Motivation and Objectives State departments of transportation (DOTs) and county and city transportation departments routinely design, con- struct, and maintain highway drainage systems to ensure safe driving conditions and to prevent downstream flood- ing. Highway runoff that is not treated or flow-managed beyond flood control has been associated with detrimental effects to the water quality and hydrologic characteristics of receiving waters. Accordingly, various federal and state environmental regulations increasingly require DOTs and municipalities to meet water quality and hydrologic dis- charge requirements for runoff that originates from their jurisdictions. The federal Clean Water Act (CWA) is the primary regu- latory framework for many DOT discharge requirements. Commonly encountered regulations include the National Pollutant Discharge Elimination System (NPDES) permit- ting requirements, the Section 303(d) water quality impair- ments designation, and discharge prohibitions established through Total Maximum Daily Loads (TMDLs). Other reg- ulations such as the Endangered Species Act (ESA), legal actions, or local watershed initiatives may also lead to spe- cific discharge requirements for highway runoff. Highway runoff contributions to contaminated sediments is also receiving increasing attention under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA; commonly known as Superfund). To mitigate potential impacts of highway runoff on receiv- ing waters and to meet discharge requirements, DOTs com- monly implement a broad array of measures that collectively are referred to as Best Management Practices (BMPs). BMPs include the following: ⢠Education BMPs: Programs intended to promote environ- mental awareness and change public behavior. Examples are highway anti-littering campaigns and adopt-a-highway initiatives. ⢠Source control BMPs: Initiatives intended to reduce pol- lutants before they are entrained in highway runoff. These include programmatic actions such as proper selection and use of herbicides for roadside vegetation control, selection of road building and associated infrastructure materials, or tangible measures such as highway litter cleanup, street sweeping, and catch basin stenciling. ⢠Treatment/flow control BMPs: Structural facilities designed to remove pollutants from stormwater run- off prior to discharge to receiving waters and/or to reduce or control runoff volumes. Common approaches include roadside swales, filter strips, detention basins, infiltra- tion systems, and sumped catch basins. Treatment BMPs are normally required during both the construction and post-construction phases of highway projects. Run- off control is typically completed for post-construction phases. Post-construction treatment/flow control BMPs are often the focus of DOT stormwater programs and regulatory over- sight. Environmental and resource agencies that are man- dated to protect water resources are necessarily focused on the effectiveness of treatment BMPs, including factors such as BMP selection, sizing, design, and maintenance. While these criteria are also important to the success of DOT storm- water programs, DOTs are additionally concerned about the cost of constructing and maintaining post-construction treatment BMPs. Costs are particularly important given that DOT budgets are often constrained by their dependence on legislative allocation, which is affected by economic and political circumstances. The regulatory requirements for treatment BMPs have his- torically been associated with the construction of new highway facilities. However, environmental regulations continue to S e c t i o n 1 Introduction
2evolve. More recently, environmental regulations increas- ingly require retrofit treatment BMPs for previously con- structed highway facilities. Some examples of highway retrofit drivers include: ⢠NPDES permit requirement (Washington State DOT, North Carolina DOT); ⢠TMDL loading allocations (California, Maryland, many other state DOTs); ⢠Endangered species protection (Oregon and Washington DOTs for endangered salmon protection); and ⢠Watershed initiatives (state DOTs in the Chesapeake Bay Watershed). Retrofitting highways with treatment BMPs is relatively straightforward when there is adequate space, hydrau- lic head, and budget. However, highway facilities that are located in dense urban areas or âultra-urban areasâ can be particularly difficult and costly to retrofit due to space limitations, high pollutant loadings, hydrologic flashiness, hydraulic constraints, and utility conflicts. In addition, ultra-urban environments are typically much more chal- lenged in terms of receiving water quality and in some cases stream stability issues. Consequently, DOTs potentially face costly and challenging BMP retrofit requirements in ultra- urban environments. Recognizing the potentially high costs and difficulty associ- ated with retrofit mandates in ultra-urban areas, the National Cooperative Highway Research Program (NCHRP) initiated NCHRP Project 25-31 with the goal of developing guidance on retrofit procedures for evaluating and selecting modifi- cations to existing drainage infrastructure. This document provides the basis for and details of this guidance. The pur- pose of this guidance is to provide planners, designers, and engineers with a basic understanding of the technical issues of BMP selection and design as applied to ultra-urban retrofit settings. This guidance is intended as a starting point for ret- rofit projects. Because the nature of retrofitting is highly site specific, this guidance cannot be a substitute for site-specific planning, permitting, engineering analysis, design, cost esti- mation, construction, and post-cost operation, maintenance, and monitoring. 1.2 What Is an Ultra-Urban Highway? An âultra-urban environmentâ is a highly urbanized area that has little or no available space for new development and where land costs are typically high. The first use of the term is attributed to city staff in Alexandria, Virginia (Shoemaker et al., 2002). In an ultra-urban environment, the use of tra- ditional treatment BMPs such as detention basins and swales is constrained by the lack of available surface area. Thus, the term âultra-urban BMPâ is associated with the use of pro- prietary treatment BMPs that have small footprints and are installed underground. By extension, an âultra-urban highwayâ is defined as a highway segment located in a highly urbanized area, with little to no right-of-way (ROW) for expansion of highway infrastructure, and where adjacent land costs are high or essentially unavailable. An ultra-urban highway has one or more of the following characteristics: ⢠Limited right-of-way: The most common and signifi- cant feature of the ultra-urban highway is the lack of available surface space in the ROW corridor for locating retrofit BMPs. ⢠High land costs: High land costs associated with highly urbanized areas may constrain the ability to acquire addi- tional ROW or to find potential off-site locations for siting retrofit BMPs. ⢠Large traffic volume: Ultra-urban highways have mul- tiple lanes designed for large average daily traffic (ADT), typically in excess of 30,000 and in many cases in excess of 100,000 or more vehicles per day. ⢠Large fraction of impervious cover: Ultra-urban high- way catchments have a high percentage of impervious surfaces, typically 75â100%. The contribution of high- way impervious area to total impervious area is small. In a national study (Tilley and Slonecker, 2006), all roads make up about 20â25% of total impervious area (TIA) as TIA increases over 25%, and highways likely make up a small fraction of the total roadways in ultra-urban areas. ⢠Potential for utility conflicts: Highly urbanized areas potentially have numerous existing and abandoned under- ground utilities. ⢠Compacted soils or unknown subsurface conditions: Soils in dense urban environments may have poor infil- tration characteristics and/or are usually compacted and less amenable for infiltration. Older highways may include unknown or unengineered fill. ⢠Underground drainage system: Drainage collection sys- tems typically include curbs and inlets to underground- piped storm sewers due to the lack of ROW for surface conveyances. 1.3 What Is a Highway Water Quality Retrofit? A highway water quality retrofit (or âBMP retrofitâ) is defined as the construction and maintenance of engineered treatment BMPs to reduce the water quality and hydrologic
3 impacts of runoff from existing highway facilities. A BMP retrofit can entail: ⢠Modification and enhancement of existing BMPs and infrastructure; ⢠Construction of stand-alone treatment/flow control BMPs for existing highway facilities; and ⢠Construction of retrofit BMPs in association with highway improvement projects, for example, highway-widening projects that include BMPs to treat existing and added travel lanes. In ultra-urban highway settings, BMP retrofits are most commonly associated with highway improvement projects, as this is usually more cost effective and affords more flex- ibility in BMP design than stand-alone retrofits. Stand-alone retrofits in highly space-constrained urban highways can be triggered by TMDLs and ESA requirements, but are less often constructed due to high costs, challenges, and localized benefit. In addition, off-site alternatives may be available that are more cost effective and provide greater benefits to receiv- ing waters. In the future, evolving water quality and quantity regulations may increasingly necessitate more stringent and costly stand-alone retrofit projects. 1.4 Characteristics of Retrofitting Highway BMP retrofitting is more complex and costly than BMP planning and construction for new highways. In retrofitting, the BMPs must be adapted to the existing high- way and drainage systems. In contrast, BMP design for new highways is more flexible as there are almost always fewer constraints and more opportunity for coordinated plan- ning and construction of the highway, drainage system, and BMPs. Table 1.1 compares BMP development for highway retrofits and new highway construction projects. 1.5 Challenges of BMP Retrofitting in Ultra-Urban Highway Environments Designing and constructing BMP retrofits of ultra-urban highways can be difficult and costly. Physical, operational, and budgetary constraints can each limit the ability to imple- ment BMP retrofits. These challenges and constraints are described below and are summarized in Table 1.2. Finding Adequate Space to Locate Aboveground Retrofit BMPs: The most common and significant constraint, other than funding in general, is the lack of surface area within the Project Component Highway BMP Retrofitting BMPs Integrated with New Highway Construction Data collection Significant data collection for characterizing constraints and identifying opportunities. Less data collection needed for existing infrastructure. Characterization studies required for highway design are usable for BMP design. BMP siting Fewer options. Physical obstructions (space, head, connectivity, utilities) are more likely to constrain BMP siting and design. BMP siting is more flexible. Conflicts with existing infrastructure are less likely and are more easily mitigated through coordinated design. Stormwater conveyance system BMPs must conform to the existing storm sewer system, or the existing system must be modified. Allows for coordinated design and construction of conveyance systems and BMPs. BMP sizing Sized to meet treatment objectives. There may be limits on the ability to meet sizing requirements due to physical and economic restrictions. Sized to meet DOT or local stormwater standards. There is less flexibility for reducing sizing below minimum criteria. Practicality assessment Fewer feasible options. Candidate BMPs are more likely to be infeasible due to physical or economic constraints. Most candidate BMPs are likely to be feasible. Planning and design costs Higher costs. More coordination, data collection, and site characterization is needed. Greater likelihood of design changes. Lower costs. More flexibility in design. Constraints more likely to be identified and mitigated. Construction cost 1.5 to 4 times greater than new construction sites. Must work around existing infrastructure. Greater likelihood of unexpected conditions and change orders. Fewer construction constraints. Less likely to encounter unexpected conditions. Source: Adapted from the Center for Watershed Protection Urban Stormwater Retrofit Practices Manual (Schueler et al., 2007) Table 1.1. Project components for BMP retrofitting versus BMPs for new highway construction.
4Retrofit Consideration Retrofit Constraints Mitigation Potential Implications Limited ROW ⢠ROW too small for aboveground BMPs ⢠Available ROW has planned uses such as future highway expansion ⢠Available ROW has poor configuration or location, limited access ⢠Adjacent land costs are high ⢠Select alternative locations ⢠Locate BMPs underground ⢠Adapt BMP design or type to fit within available ROW ⢠Use proprietary small-footprint BMPs ⢠Coordinate retrofits with future projects ⢠Greater design and construction costs ⢠More maintenance requirements and costs for underground BMPs ⢠Reduced treatment performance ⢠Project delays Obstructions ⢠Buried utilities ⢠Building foundations ⢠Landfills/contaminated soils ⢠Historic structures ⢠Archeological finds ⢠As-built drawings are not available or unreliable ⢠Select alternative location ⢠Adapt/change BMP design ⢠Dig additional test pits to identify obstructions ⢠Remove obstruction at additional cost ⢠Greater design and site characterization costs ⢠Greater construction costs ⢠Project delays and change orders ⢠Longer construction period Topography ⢠Below-grade highway sections ⢠Roadway crowned to drain away from candidate locations ⢠Available ROW has steep slopes, rocky, and uneven terrain ⢠Insufficient head in flat terrains ⢠Select alternative location or retrofit design ⢠Re-grade/excavate/fill as needed ⢠Construct retaining walls ⢠Modify existing drainage system ⢠Modify roadway crown ⢠Greater design and construction costs ⢠Longer construction period Soil and groundwater conditions ⢠Compacted soils with low permeability ⢠Shallow groundwater ⢠Overly wet soils/hydric soils ⢠Unknown or non-engineered fill ⢠Soil or groundwater contamination ⢠Select alternative location or retrofit design ⢠Amend soils at additional cost ⢠Install dewatering systems ⢠Excavate and dispose unsuitable or contaminated soil ⢠Increased design and construction costs ⢠Construction delays and change orders to address unexpected conditions ⢠Longer and more complicated construction Connection to existing drainage system ⢠Piped and underground systems ⢠Difficult to tie in BMPs due to insufficient head, conveyance capacity, and location ⢠Select alternative location or retrofit design ⢠Use pumps to compensate for elevation issues at additional cost ⢠Reconfigure existing conveyances ⢠Greater costs for design, construction, and ongoing operation Construction ⢠Space and connectivity constraints ⢠Obstructions, depth, confined-space issues ⢠Limited space for construction staging ⢠Longer distance from import/export sites ⢠Lane closures due to limited space or retrofit design ⢠Traffic delays due to high volume or lane closure ⢠Select alternative location ⢠Modify BMP design ⢠Develop designs that eliminate required lane closures, minimize connectivity issues, require no proprietary materials ⢠Schedule construction for off- peak hours/seasons ⢠Reuse exports on site ⢠Greater construction costs ⢠Longer construction periods because of traffic impacts and traffic control ⢠Potential construction delays and change orders to address unexpected conditions ⢠Worker and public safety concerns due to limited space and large traffic volume BMP treatment performance ⢠Large flowrates, runoff volumes, and pollutant loadings ⢠Space constraints limit BMP options and capacity ⢠Compacted soils restrict use of infiltration BMPs ⢠Little potential for runoff capture and reuse ⢠Cold climateârelated effects on pollutant loadings and BMP sizing ⢠Select alternative location ⢠Modify BMP design and sizing ⢠Include operational and design enhancements ⢠Provide pretreatment ⢠Use treatment trains ⢠Greater project costs ⢠Target pollutants not mitigated Maintenance burden ⢠Impacts to traffic ⢠High maintenance frequency ⢠Need for specialized equipment or materials ⢠Safety issues for road crews and drivers ⢠Maintenance access requires lane closures or traffic impacts ⢠Vegetation maintenance ⢠Select alternative location ⢠Select alternative BMP types ⢠Modify BMP design ⢠Schedule maintenance at off-peak hours/seasons at additional cost ⢠Potentially excessive maintenance requirements and costs ⢠Diminished treatment performance if maintenance needs are not met Table 1.2. Constraints and challenges of BMP retrofits for ultra-urban highways.
5 ROW for siting aboveground retrofit BMPs. The lack of ade- quate surface area requires retrofit designers to seek alternative locations or to develop designs that can fit within the available ROW including consideration of underground BMPs. Retrofit locations that are underground, off-site, or require extensive site changes are likely to be more costly, have increased main- tenance requirements, and may result in selected BMPs with poorer treatment performance than aboveground ones. The search for usable and sufficient surface storage or vegetative filtration is a primary retrofitting task. Selecting and Designing Retrofit BMPs: Identifying and designing retrofit BMPs that cost effectively achieve treat- ment objectives can be difficult. Factors that can complicate BMP selection are as follows: ⢠Physical constraints: Space limits, topography (steep slopes or flat topography), high groundwater, poorly draining soils, underground infrastructure and obstructions, includ- ing existing soil contamination. ⢠High hydraulic loadings: Hydrologic flashiness (peaky hydrographs), high flow rates, and large runoff volumes caused by large impervious fractions and small drainage catchments. ⢠High pollutant loadings: Comparatively greater pollut- ant concentrations and pollutant loadings associated with large ADT, surrounding urban land uses, and greater run- off volumes. ⢠Reduced feasibility for infiltration: Although infiltration BMPs are among the most effective measures for reducing hydrologic and pollutant loadings, their feasibility in ultra- urban highways is constrained by: â Limited surface area; â Probative infiltration rates associated with compacted soils and fill; â Geotechnical concerns for protection of the roadway subgrade or other adjacent infrastructure; â Greater likelihood of conflicts with underground infrastructure; and â Greater potential for subsurface contamination. ⢠Limited feasibility for on-site retention: Ultra-urban high- ways have limited potential for on-site retention due to reduced feasibility for infiltration, limited surface area for storage and evaporation of harvested stormwater, and few options for use of stormwater such as irrigation and non- potable water supply. Project costs generally increase when large, proprietary, and/or complex underground BMPs are used in an effort to meet treatment objectives. Alternatively, BMP designers may consider smaller and more affordable BMPs in an effort to mitigate space and budget constraints and to comply with reg- ulatory requirements. However, treatment performance can be compromised if the BMPs do not include relevant unit pro- cesses and/or have extensive maintenance requirements. A key retrofitting task is evaluating and selecting candidate BMPs and treatment trains that include appropriate unit processes. Developing Retrofit BMPs That Are Adequately Main- tained: BMP maintenance is vital to ensuring design-level treatment performance. Treatment effectiveness is compro- mised when maintenance needs are not identified, are scaled back, and/or are neglected. Without ongoing maintenance, the utility of retrofit BMPs and the investment in retrofit- ting is questionable. On the other hand, DOT maintenance departments are often under-resourced and may be reluctant to assume additional responsibilities. DOTs have noted dis- proportionate funding for BMP construction versus BMP maintenance of the nature: âthey provide money to construct BMPs, but no money to maintain them.â Consequently, maintenance departments may view BMP maintenance as onerous, especially for small-footprint, under- ground, and proprietary BMPs that can require frequent main- tenance, specialized equipment, significant health and safety Table 1.2. (Continued). Retrofit Consideration Retrofit Constraints Mitigation Potential Implications Cost ⢠High land / implementation cost ⢠High maintenance cost ⢠High replacement cost ⢠Select alternative location ⢠Select alternative BMP types ⢠Modify BMP design ⢠Seek off-site locations, pollutant trading ⢠Re-evaluate treatment objectives ⢠Potentially excessive costs needed to meet regulatory requirements ⢠Diminished treatment performance with alternative BMPs Public acceptance ⢠Aesthetics ⢠Increased traffic and public hazard from space limitations ⢠Mosquito habitat from standing water in underground BMPs ⢠Odors ⢠Use of sustainable materials ⢠Install BMPs behind guard rails ⢠Use grates and fences to reduce hazards ⢠Modify design to eliminate or reduce standing water ⢠Maintain regularly for aesthetics, odors, pests ⢠Greater maintenance costs ⢠Public concerns about vectors, aesthetics, sustainable practices, and safety ⢠Public support when aesthetics can be improved
6measures, and costly proprietary materials. As a result, BMP maintenance requirements and costs can dictate BMP selec- tion. Ongoing BMP maintenance is a principal consideration in retrofitting. Identifying and Mitigating Utility Conflicts, Obstruc- tions, and Unknown Conditions: Ultra-urban highways potentially have numerous existing and abandoned under- ground utilities or other obstructions such as foundations, old landfills, or historic structures. As-built drawings may be unreliable or unavailable, particularly in older urban areas. Retrofit costs increase when utilities or obstructions must be relocated or designs must be adapted to accommo- date utility constraints and unfavorable or contaminated soils. Retrofit costs also increase when extensive test pits are needed to locate or confirm utility locations, and con- struction change orders are needed to address unforeseen conditions. Connecting Retrofit BMPs to the Existing Drainage Sys- tems: Drainage collection systems in ultra-urban highways are typically piped and underground. The existing drain- age systems can be poorly defined, include large diameter conduits, include run-on from off-site drainage areas, and have inadequate capacity or insufficient head to accommo- date BMP retrofits. As a result, retrofit designers may need to consider system modifications or new facilities such as pumping equipment. Connectivity constraints will increase retrofit costs or can limit retrofit options. The ability to tie into existing drainage systems is a primary consideration in retrofitting. Constructing BMP Retrofits in Limited-Space, High- Traffic Conditions: Construction in ultra-urban highway environments is affected by space limitations, existing infra- structure, known and unknown obstructions, and high traffic volumes. Construction constraints can significantly increase project costs and construction periods. Traffic flow will also be impacted if lane closures are required and there are safety concerns with construction in high-traffic, space-constrained settings. Retrofit constructability should be considered early in the planning process. Identifying Cost-Effective Retrofits: Retrofitting ultra- urban highways with BMPs is potentially very costly. DOTs are very concerned about the ability to fund retrofit projects and to meet regulatory obligations or watershed initiatives. Cost is a critical factor throughout the retrofitting process, including BMP planning, design, construction, operation, and maintenance. 1.6 Document Organization Although the constraints of ultra-urban highways can be daunting, a rational approach to BMP retrofitting follows the same fundamental steps commonly used in water resources planning (Orth and Yoe, 1997). Accordingly, this document is organized about the fundamental steps of rational planning as shown in Figure 1.1. Ideally, the steps in a rational planning process are sequen- tial. In reality, retrofit planning may begin with any step and steps will be repeated to assess new information. Sections 2 through 8 of this document are organized into topics that separately support the fundamental retrofitting steps, and Sections 9 and 10 integrate the information and guidance provided in the previous sections, as follows: 1. Define the Problem ⢠Section 2, Ultra-Urban Highway Characterization: This section summarizes the characteristics of runoff from ultra-urban highways and the potential impacts on receiving waters. Runoff and receiving water character- Figure 1.1. Document organization.
7 ization supports the development of sensible treatment objectives and effective retrofit solutions. A summary of common highway pollutants and conditions of concern and their characteristics is provided in Table 2.1. 2. Statement of Retrofit Objectives ⢠Section 3, Retrofit Drivers and Practices: Regulatory compliance is usually the main retrofit objective. This section describes the regulatory requirements for BMP retrofits and DOT compliance practices. 3. Formulation of Candidate Retrofits ⢠Section 4, BMP Options for Ultra-Urban Highway Retrofits: This section describes treatment BMPs that can potentially be used in retrofitting highways. BMP options are grouped into 10 retrofit categories based upon their design characteristics, target pollutants, and applicability to surface and underground applications. A summary of retrofit categories and general BMP characteristics is provided in Table 4.1. Detailed sum- mary tables are provided throughout Section 4 for each retrofit category. 4. Practicality Assessment of Candidate Retrofits ⢠Section 5, Evaluating BMP Effectiveness: This section describes regulatory criteria and empirical data that are used to assess treatment performance of candidate BMPs. Fundamental unit processes of BMPs are dis- cussed as a primary criterion for the selection of BMPs and treatment trains. Rankings for unit operation effec- tiveness are listed in Table 5.1 for each retrofit category. A summary of median influent and effluent levels from the BMP Database is provided in Table 5.4. ⢠Section 6, BMP Sizing and Design: This section dis- cusses regulatory and performance considerations for assessing BMP sizing and design. This section describes a BMP sizing spreadsheet tool that synthesizes continu- ous simulation modeling results for evaluating deten- tion (volume-based) and media filtration (flow-based) BMPs. The purpose of the tool is to assist stormwater and highway professionals with planning-level sizing and design of detention and media filtration BMPs for ultra-urban highway runoff control. ⢠Section 7, BMP Maintenance and Monitoring: This section describes post-construction activities, which include ongoing BMP maintenance and monitoring. Maintenance practices are discussed and a summary of common maintenance practices and maintenance indicators is provided in Table 7.1. This is followed by a description of BMP monitoring and performance assessment practices and protocols. ⢠Section 8, Retrofit Costs: Ultimately, retrofit costs will be the overriding consideration for retrofit assessment. This section describes cost elements, cost factors for ultra-urban settings, and cost reduction strategies. A summary of available retrofit cost data is provided in Table 8.1. 5. Integration ⢠Section 9, Retrofitting Strategies and Process: Only a limited number of retrofit alternatives can be evaluated in detail. This section describes general strategies for identifying promising candidates, including strategies for locating and selecting BMPs and alternatives to ret- rofitting. A retrofit process is discussed. ⢠Section 10, Case Studies: This section presents seven BMP retrofitting case studies from DOTs. Case studies include aboveground and underground BMP applica- tions, and pilot studies to assess BMP design, perfor- mance, and construction procedures.
