Guidelines for Evaluating and Selecting Modifications to Existing Roadway Drainage Infrastructure to Improve Water Quality in Ultra-Urban Areas (2012)

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109 This section describes general retrofitting approaches and strategies. A general process for planning and implementing ultra-urban highway stormwater control retrofits is presented. 9.1 Types of BMP Retrofits Redevelopment BMP Retrofits: Redevelopment retrofits are retrofits of existing untreated highway facilities in association with highway improvement projects. For example, highway- widening projects may require BMPs to treat all highway runoff associated with the project, including the untreated pre-project highway surfaces. In redevelopment retrofits, the retrofit proj- ect location is established by the location of the improvement project. There is no need for prioritizing and selecting retrofit project locations. Because of the high cost of retrofitting ultra- urban highways, most projects will likely be redevelopment ret- rofits associated with planned highway improvement projects. Stand-alone BMP Retrofits: Stand-alone retrofits are retro- fits of existing highway infrastructure or existing BMPs for the sole purpose of improving water quality. They are independent of other highway projects. TMDL wasteload allocations are likely the most common regulatory driver for stand-alone ret- rofits, but NPDES permits can also explicitly mandate stand- alone retrofits (e.g., the North Carolina DOT permit requires 14 statewide retrofits per year) as well as requirements of CERCLA or the Resource Conservation and Recovery Act (RCRA) in contaminated sediment circumstances or ESA requirements, particularly for aquatic species. Stand-alone retrofits are usually more costly than redevelopment retrofits. Therefore, it may be more cost effective to delay and integrate stand-alone retrofits into planned highway projects when fea- sible and allowed. 9.2 Retrofit Prioritization Approaches Regulatory drivers will determine the type of retrofit proj- ect and the need for prioritizing and selecting the retrofit proj- ect location. Stand-alone water quality retrofits may require a retrofit scoping, evaluation, and prioritization process to establish the project location within specific jurisdictions, regions, or watersheds. Retrofit prioritization approaches are discussed in the following paragraphs. Benefit-Cost Approach: A benefit-cost approach for outfall prioritization requires the quantitative assessment of retro fit costs and benefits. As an example, Kalman et al. (2000) con- ducted a benefit-cost evaluation of stormwater treatment for impaired reaches of the Ballona Creek watershed in Los Angeles, California. In this evaluation, the BMP costs were estimated on the basis of treatment levels: Level 1 control was for floatables and TSS; Level 2 control provided filtration and disinfection; and Level 3 control included advanced treatment to meet all beneficial use standards. The benefits were estimated on the basis of economic value of the restored beneficial uses in the receiving waters. Weighted Scoring Approach: WSDOT developed an out- fall prioritization scheme in 1996 using numeric scoring pri- oritization (WSDOT, 1996; Barber et al., 1997). This outfall prioritization study found the highest priority outfalls were concentrated in urban areas that discharge to small streams. Landphair et al. (2001) adapted the WSDOT weighted scor- ing approach for the Texas Department of Transportation (TxDOT). The scoring categories used by WSDOT and TxDOT included: • Type and size of receiving water body, • Beneficial uses of receiving water body, • Pollutant loading, • Percentage contribution of highway runoff to watershed, • Cost/pollution benefit, and • Values tradeoff. Multiple Screening Approach: A multiple screening approach is a general prioritization process that may include quantitative and qualitative retrofit evaluation criteria. A multiple screening approach is employed by WSDOT and S e c t i o n 9 Retrofitting Strategies and Process

110 NCDOT for retrofit prioritization and is used in the CWP retrofit guidance manual (Schueler et al., 2007) for watershed restoration. The general steps include the following: 1. GIS screen/scoping. The first screen is to conduct GIS- based assessments using existing information to initially identify potential retrofit sites. Criteria may include watershed size, highway area to watershed ratio, impervi- ous area, receiving water impairments, ADT, maintenance capability, etc. GIS-based decision support tools are avail- able that can aid such efforts. For example, the Structural BMP Prioritization and Analysis Tool (SBPAT) was devel- oped to identify and prioritize potential structural BMP retrofit projects throughout Los Angeles County, as well as estimate planning-level costs and potential pol- lutant concentrations and load reductions resulting from the implementation of the prioritized projects (Geosyntec, 2008). 2. Reconnaissance: The second screen is more rigorous. In this stage, field-based reconnaissance and site-specific charac- terization studies are used to further prioritize candidate sites. Field reconnaissance could include mapping of the ROW, topography, soils, drainage system, verification of infrastructure, evaluation of receiving water conditions, and coordination with field personnel and biologists. 3. Retrofit evaluation and prioritization: The final screen involves quantitative and qualitative evaluations to select and prioritize feasible retrofit sites. The evaluation criteria may include numeric scoring of site conditions, and/or other more qualitative stakeholder and regulatory input. The goal of this level of screening is to complete a retrofit project priority list. Caltrans used a similar multiple screening process to select sites for the retrofit pilot study (Caltrans, 1998). In their approach, a general scoping process based on review of as- built drawings was used to determine initial candidate sites in target watersheds. Field reconnaissance studies were next conducted to gather site information and refine the list of candidate sites. Final retrofit site selection was determined using a weighted scoring evaluation approach, where scoring criteria were based on the BMP type. Multiple screening approaches are recommended for ret- rofit site prioritization for the following benefits: • They are designed to screen out poor candidate sites with reduced/minimal evaluation. • They take into account both receiving water conditions/ benefits, cost/benefits, and the site constraints that can limit retrofit feasibility. • They allow for direct input from stakeholders, field per- sonnel, contractors, and other design and O&M personnel. • They include opportunities for collaboration with other jurisdictions and groups. This is especially important when combining efforts may result in more cost-effective solutions, if only through economies of scale. 9.3 Attributes of Successful Retrofitting The retrofitting process requires a comprehensive and flexi- ble approach to address the challenges of complex site- specific conditions and the high costs of modifying ultra-urban high- way infrastructure. The CWP has identified the following attributes of successful retrofitting (Schueler et al., 2007). • Investigation: “Retrofitting requires a different way of thinking; it requires sleuthing skills to determine what can work at highly constrained sites.” Information gathering is more comprehensive. Retrofitting requires significant data gathering and site characterization, as well as greater coor- dination with highway maintenance crews, roadway engi- neers, BMP designers, neighboring municipalities, utility agencies, watershed stakeholders, regulators, and construc- tion managers and contractors. • Foresight: Retrofit designers need to simultaneously envision BMP possibilities and anticipate problems. Retro fitting requires significant effort to understand site conditions and greater experience with the constructabil- ity and performance of BMPs. Foresight is gained through data gathering, coordination, research, and pilot testing. • Creativity: Retrofit designers must be creative to find and design effective and affordable BMP retrofits that will pro- duce the desired treatment objectives. Retrofitting requires a willingness to develop and consider a range of approaches. This may include consideration of new, irregular, and non- standard BMP approaches, or a willingness to coordinate and partner with regulators and other watershed stakeholders. The key factor is the experience level of the DOTs, design- ers, and team members with retrofitting and with designing and constructing non-traditional and site-specific BMPs. DOTs can gain retrofit experience and BMP development through pilot testing programs, research and development initiatives, and ongoing coordination and cooperative rela- tionships with construction contractors, universities, and research institutions. 9.4 Project Planning and Coordination The planning stages for BMP siting and design are usu- ally the most critical phases of the retrofit process. Project planning can be a major cost element of retrofit projects, but

111 broad and early coordination with regulators, local officials, and personnel familiar with the site can benefit and reduce likelihood of encountering problems and increasing costs in the later project stages. Recommended project planning and coordination practices include the following: • Proactively engage regulators and local officials. Pro- actively involve regulators and local officials in a collaborative role throughout the retrofit planning and implementation. Regulators can help to identify local and regional permit requirements for the project. In addition, involving regula- tors and local officials can: (1) cultivate a common appre- ciation of the challenges of working in ultra-urban settings and (2) potentially help to develop and gain acceptance and approval of workable alternatives that provide the most practical benefit to receiving waters. • Involve field and maintenance personnel, pavement designers, biologists and environmental personnel, and construction managers early in the planning process. Lots of upfront coordination during the early planning stages will support the identification of sensible and acceptable retrofit alternatives and will reduce the likelihood of costly project changes and redirects down the road. Early coor- dination will also help to identify constraints and establish construction and maintenance schedules. • Coordinate with personnel that have knowledge of site conditions. Planners should search for and coordinate with personnel that can potentially supply information about the site conditions and site constraints. These may include the following: – DOT personnel: Seek out and coordinate with DOT planners, engineers, and construction personnel with previous project experience at the site. – Field personnel: Coordinate and conduct site visits with DOT field and maintenance crews that can provide working knowledge about site conditions, including known problems/issues, and drainage patterns, and can confirm the location and characteristic of stormwater infrastructure. – Municipalities and utility agencies: Planners should actively coordinate with adjacent municipalities, pub- lic works departments, utility agencies, and as appro- priate private landowners that have knowledge of the site and adjacent infrastructure and can help to iden- tify unmapped utilities and constraints as well as help to assess potential BMP alternatives and in some cases partnerships. • Conduct thorough site investigations. Unidentified buried utilities or other underground constraints are major causes of construction delays, construction change orders, and budget overruns. A thorough site investigation upfront can reduce uncertainties, lead to more applicable BMP designs, and potentially mitigate delays and overruns during construction stages. When a thorough site investi- gation is not possible, a preliminary excavation (pot hol- ing) should be conducted prior to excavation to confirm accuracy of as-builts and to discover project infrastructure that may need relocation. A variety of surface and borehole geophysical methods summarized in Table 9.1 can be use- ful in detecting underground utilities and objects and/or guiding test pit excavations. Subsurface utility engineering (SUE) is an engineering practice promoted by the FHWA and used by DOTs (FHWA, 2011). It combines the use of vacuum extraction and geophysical techniques to detect buried objects. If belowground issues are discovered, more detailed site investigations are likely warranted. Otherwise, Ground- Penetrating Radar Metal Detection Magnetometry Electromagnetic Methods Purpose Focused investigation Reconnaissance survey Reconnaissance survey Reconnaissance survey Typical Depth of Penetration 3 to 15 ft 10 to 12 ft (55 gal. drum) 10 to 15 ft (55 gal. drum) 8 to 10 ft Materials Detected Metal and non-metal Metal Ferrous materials Metal and non-metal Cultural Interferences Densely packed rebar, wire mesh Metal surface structures, power lines Metal surface structures, power lines Metal surface structures, power lines Natural Interferences Conductive soils (e.g., silts, clays) Mineralized soils Mineralized soils, iron deposits Highly conductive saline soils Resolution 0.1 to 4 ft 20% vertically and horizontally 10% to 15% vertically and horizontally Vertical resolution is between 4 and 12 ft; 4 ft horizontally Produces Usable Field Data Yes Yes Yes Yes Time Slow to Moderate Moderate to Fast Fast Moderate to Fast Cost Low to Moderate Low to Moderate Low to Moderate Low to Moderate Source: USEPA (1997) Table 9.1. Summary of surface geophysical method applicability.

112 costs can potentially increase significantly due to redesign and/or removal of the contamination or infrastructure during construction. • Strive for vision and creativity in developing BMP alter- natives. Do not rely solely on traditional BMP practices and as appropriate do not limit options to DOT-specific BMP guidelines. To meet the challenges of retrofitting ultra-urban highways, consider a wide range of BMP options described in Section 4 and investigate and become knowledgeable of new and innovative practices. Designers should understand the processes needed to treat target pol- lutants and develop a vision of BMP elements and treat- ment trains needed to achieve those processes. If candidate BMPs are not approved by DOTs, planners should be amenable to pursuing these approaches, possibly through BMP pilot testing, other internal approval processes, and/ or through coordination with regulators. • Use sound engineering. Attention to details and quality control can help to avoid change orders and cost overruns as well as to ensure effective performance. Some specific lessons learned from the Caltrans retrofit study follow (Currier and Moeller, 2000; Currier et al., 2001): – Quality control during surveying is critical and can help to avoid subsequent adjustments. – Material quality specifications should be included with BMP product orders (e.g., vegetation conditions), and specifications checked and confirmed before acceptance of product delivery. – Manufacturer installation instructions should be fol- lowed for proprietary BMPs; otherwise poor perfor- mance can result. • Plan for contingencies. Even with the most comprehen- sive planning, unforeseen circumstances can arise during construction, particularly in older and dense urban envi- ronments. For example, designers may want to avoid pre- cast units at sites where there are tight tolerances because as-built maps can be inaccurate. Cast in-place features would allow for adjustments that may be needed to adjust to actual field conditions or changes due to construction (Currier and Moeller, 2000). In addition, planners should allow for budget and time contingencies that are commen- surate with the degree of uncertainty about site conditions and the experience level with the construction and opera- tion of the retrofit BMPs. 9.5 Retrofitting Process Framework for Specific Retrofit Sites Table 9.2 outlines the recommended retrofitting process for specific sites that are associated with the redevelopment project or are first identified through a retrofit prioritiza- tion analysis, as discussed in Section 9.2. The retrofit process follows a general top-down planning framework based on fundamental steps commonly used in water resources plan- ning. For many retrofit projects, the process may not proceed sequentially. As appropriate, steps will be conducted concur- rently, out of sequence, and will be repeated as new informa- tion is gained. The following subsections discuss the steps of the retrofitting process. 9.5.1 Step 1: Project Scoping The first step is to define the scope of the retrofit project. Retrofit scoping includes the following tasks: • Establish the regulatory/DOT retrofit requirements. The project retrofit requirements will likely be prescribed in DOT policy manuals, TMDL implementation plans, NPDES permits, or other regulatory requirements. Com- plex projects in sensitive areas may require negotiation with regulators and stakeholders to establish regulatory require- ments. A clear understanding and agreement of the regula- tory requirements at the outset of the project is essential. • Define the scope of treatment requirement for new and existing impervious areas. Know the new and/or existing impervious areas that will require treatment or enhanced treatment. Different levels of retrofit treatment could be required depending on regulatory policy. For example, redevelopment projects may require retrofit treatment of all existing highway impervious areas associated with the highway improvement project. Alternatively, require- ments may allow exclusion of some sub-basins that are not feasible or practical to treat (e.g., pollutant reduction trading may be allowed), or planners may consider allow- able alternative mitigation measures. Stand-alone retrofits may have specific performance objectives (e.g., wasteload reductions) or may be opportunistic retrofit projects with no minimum performance requirement. • Identify the receiving waters and environmental areas. Knowledge of the project’s receiving waters (surface and potentially groundwaters) and potential environmental impact areas (sensitive areas, conservation areas) supports scoping of environmental permit requirements and BMP planning. • Understand the issues of concern. Gather and review water quality information and consult with stakeholders and biologists as necessary to understand the receiving water issues of concern. An assessment of surrounding land use within the drainage area and a pollutant loadings analysis can help characterize the DOT contributions to receiving water issues. Collectively, this information sup- ports the identification of project POCs, assessment of flow control requirements, and BMP planning. Section 2 pro- vides background information on highway runoff issues

113 and lists potential data sources. The data sources may include DOT policy documents, DOT monitoring data, and regulatory and stakeholder reports. • Determine the project team. The project team must include appropriate personnel to meet DOT requirements and to support the identification of retrofit constraints and opportunities. Personnel may include, but are not limited to, design engineers, construction personnel, maintenance departments, real estate and surveying personnel, environ- mental specialists, regulatory contacts, geotechnical engi- neers, and traffic and safety personnel. 9.5.2 Step 2: Define Retrofit Objectives The regulatory and treatment objectives will develop from the scoping process. A narrative statement of the main retro- fit objectives can help to establish a common understanding of the water quality goals. The retrofit objectives include one or more of the following: • Regulatory and/or DOT compliance requirements • Identification of the specific project pollutants and con- ditions of concern and the corresponding treatment objectives • Specification of performance criteria, such as: – BMP treatment performance requirements – Maximum discharge loads to meet TMDL wasteload allocations – BMP sizing specifications – Flow control and attenuation requirements • Other objectives such as pilot testing objectives and perfor- mance monitoring objectives Step Key Tasks Step 1. Project Scoping • Establish regulator y/ DOT requirements • Define highway facilities that require retrofit treatment • Define receiving waters and environmental areas • Gather w ater quality information • Understand receiving water issues of concern and highway contributions • Gather available site data as needed to support project scoping • Determine the planning team, budget, and schedule Step 2. Definition of Retrofit Project Objectives • State the regulatory /DOT compliance objectives • State the water quality treatment objectives • Establish performance criteria: treatment standards, sizing requirements, flow objectives Step 3. Characteriz ation of Site Conditions and Constraints • Gather available information about the site • Coordinate control with knowledgeable personnel and stakeholders • Conduct site characterization investigations and field studies (e.g., survey s, utility searches, soils and infiltration tests, etc.) • Characterize site conditions and constraints Step 4. Identification of BMP Retrofit Altern atives • Task 1: Determine applicable unit operations and treatment trains • Task 2: Identify feasible candidate BMPs that provide UOPs. Give primary consideration to aboveground alternatives ▪ 2a) ROW options ▪ 2b) Jurisdictional partnerships ▪ 2c) Pollutant trading • Task 3: Consider underground options as necessary • Task 4: Evaluate, refine, and screen concepts; select alternatives for assessment Step 5. Practicality Assessment • Hy drologic and hy draulic analys es • BMP sizing • Treatment performance assessment • Maintenance assessment • Preliminary design preparation • Cost evaluation • BMP retrofit selection Step 6. Design and Construction • Prepare final designs, specifications, O&M plan, and cost estimates • Obtain permits • Bid and contract • Manage construction Step 7. Post-Construction Operation and Evaluation • Ongoing BMP inspection and maintenance • Retrofit project evaluation • BMP performance monitoring as needed Table 9.2. Retrofitting process framework for specific sites.

114 In setting the objectives, it is important to keep in mind the potential limitations of retrofitting in ultra-urban settings. The objectives may include alternatives to retrofitting and/or reduced performance metrics as allowable or pollutant trad- ing amongst project areas (i.e., over-treating some areas and under-treating others) consistent with the ultimate goals of improving receiving water quality by cost effectively treating runoff from existing highway facilities. 9.5.3 Step 3: Characterize Site Conditions and Constraints Detailed site investigations improve the likelihood of siting and selecting appropriate and workable BMPs and will help to avoid or reduce redesigns and change orders to address unfore- seen conditions. The objective of Step 3 is to characterize the site conditions and constraints for BMP siting and design. During site characterization studies, a constraints map will help visualize BMP siting options and limitations. DOTs have well-established GIS and environmental management tools that support development constraints maps. The site investi- gation studies should include as appropriate: • Data gathering and coordination: Compile and review as-built drawings, design reports, and historical informa- tion as available. Identify and coordinate with personnel that are potentially knowledgeable of the site conditions, including field crews and adjacent municipalities. • Highway development/redevelopment plans: Review cur- rent (if applicable) and planned future uses for the site. Assess implications for BMP siting opportunities and constraints. • Land surveys: Coordinate with real estate departments and conduct surveys to confirm and identify vacant and potentially available ROW area for siting BMPs, taking in account existing and future highway construction plans. Use aerials and field surveys to investigate construction staging areas and to explore potential off-site treatment locations areas such as parks. • Topographic surveys: Use detailed topographic mapping to establish and verify elevations of existing drainage infra- structure, to delineate drainage boundaries, and to establish the available head for stormwater conveyance and treatment. Quality control during surveying is critical and can help to avoid subsequent adjustments (Currier and Moeller, 2000). • Archeological survey: A professional archeological survey is potentially needed if working in known archeological areas or artifacts are discovered during field work. Archeo- logical involvement may include literature searches, field inspections, and site excavations. • Storm drain infrastructure: Fully characterize exist- ing drainage facilities to support hydrologic analyses and assessment of connectivity options. Review as-builts and conduct field verification of the type, elevations, and con- dition of existing facilities, including ditches and open channels, catch basins, piping, and outfalls. • Existing treatment BMPs: Determine the presence of exist- ing treatment BMPs that can potentially be modified or enhanced to meet retrofit objectives. Characterize and evalu- ate any existing treatment BMPs in terms of the size, location, elevation, target pollutants, general effectiveness, mainte- nance requirements, and other benefits and limitations. • Drainage patterns and hydrology: Determine existing drainage patterns to support hydrologic analyses, BMP siz- ing, and for identifying the options for connecting BMPs to existing facilities. Analyses include the delineation of catchment areas; the identification of off-site sources, springs and seeps, outfalls, and receiving waters; and the identification of flood plains and wetlands. • Hydrologic conditions: Compile precipitation data and determine storm characteristics or design storms for siz- ing BMPs. • Soil conditions and properties: Review and conduct geo- technical investigations as necessary to evaluate soil condi- tions, soil infiltration rates, depth to groundwater, and the presence of hazardous materials. • Utilities and buried obstructions: Conduct utility searches and review and confirm as-built drawings for abandoned structures such as building foundations, historic structures, and abandoned utilities. • Maintenance access: Coordinate with designers, and maintenance and safety departments as necessary to assess potential maintenance access routes for safe access and maintenance functions without requiring lane closures or significantly impacting traffic flow. • Environmental resources: Coordinate with environmental department to identify environmental resources, wetlands, or sensitive areas that may impact BMP siting or design. • Societal issues: Investigate possible historic, archeological, or socio-economic concerns. • Traffic and safety: Work with traffic and safety depart- ments to collect traffic data and to assess potential safety issues/requirements for BMP siting and construction, for example, clear zone and slope requirements. Also, include possible safety issues for maintenance operations. • BMP design constraints: Work with project team mem- bers to establish BMP design constraints, for example, lim- its on infiltration above the subbase. 9.5.4 Step 4: Identify BMP Retrofit Alternatives Step 4 is the retrofit scoping stage. In this step, the proj- ect team identifies BMP opportunities that are potentially feasible within the site constraints and develops preliminary retrofit concepts and evaluations. The authors recommend a

115 BMP-driven approach where the project team first identifies appropriate BMPs, followed by site scoping, conceptualiza- tion, and evaluation. The process is iterative and includes the following tasks: 1. Determine candidate BMPs that provide applicable treat- ment processes in the appropriate sequence. 2. Evaluate aboveground retrofit opportunities first: a. Look for opportunities within the ROW, b. Consider jurisdictional partnerships, and c. Evaluate water quality trading. 3. Pursue underground BMPs when aboveground approaches are not practical. 4. Select BMPs for detailed evaluation. Task 1: Determine Candidate BMPs The first task is to identify candidate BMPs and BMP com- binations that provide the unit operations needed to achieve the retrofit objectives for the identified POCs. Section 5 describes the BMP unit processes, and Table 5.1 shows the unit processes inherent in retrofit categories. The intent of Task 1 is to identify appropriate and potentially effective BMP approaches and not to limit BMP options. The product of this task is a listing of applicable BMPs and BMP treatment trains for scoping and evaluation. When identifying the candidate BMPs and BMP compo- nents, the project team should consider a hierarchy or treat- ment train of BMP components that targets the more easily treated pollutants first, in most cases also reducing potential clogging or other issues for later processes, and progressively targets the more difficult to treat finer particles and dissolved pollutants as shown in Table 9.3. Tables 5.1 and 5.5 can be used to help select and configure BMP components based on the BMP unit processes, the typi- cal sequence of the BMP components in a treatment train, and expected performance for specific POCs. For example, many retrofit projects will target basic treatment of sedi- ments and associated pollutants such as total metals and oil and grease. This objective requires primary and secondary treatment processes, depending on expected sediment loads and particle sizes in highway runoff. Table 9.3 shows there are a number of candidate BMPs and BMP combinations that are potentially effective options including the following: • Conventional approaches: – Vegetated filtration swales with trash racks at outlets. Optionally include check dams to promote settling – Extended detention with various pretreatment approaches (pre-settling basins, sumped catch basins, hydrodynamic systems) – Sand filters with pre-settling basins Treatment Train Processes Retrofit Goal Candidate BMP Components 1) Hydrologic control Reduce runoff volume and/or flow control • Capture and use system • Infiltration BMPs • Detention facilities • Vegetative filtration BMPs 2) Pretreatment Remove bulk pollutants > 5 mm (trash, debris, large solids) • Screens, racks, gross solids removal device • Pretreatment settling basins, catch basins • Hydrodynamic systems • Oil-water separators • Filter strip/swales • Permeable asphalt overlays 3) Primary/ secondary treatment Remove easily settleable solids (> 50 µm) and pollutants associated with particles (metals, organics, particulate nutrients) • Extended-detention, wet basins • Filter strip/swales • Infiltration BMPs (trenches, basins, vaults) • Underground detention facilities • Hydrodynamic systems 4) Secondary treatment Remove finer solids (< 50 µm) and provide more effective treatment of pollutants associated with particles (metals, organics, particulate nutrients) • Sand filters • Bioretention, swales with underdrains • Proprietary storm filters • Media filter drains • Infiltration BMPs (trenches, basins, vaults) 5) Enhanced treatment Remove dissolved pollutants such as metals and nitrate • WSDOT media filter drain • Multi-chambered treatment train • Constructed wetlands • Media filters with amended and engineered media (proprietary and non-proprietary) Table 9.3. Treatment train processes, goals, and example components.

116 – Bioretention – Infiltration BMPs (basins, trenches) • Underground and non-traditional approaches for space- limited sections: – Various proprietary underground detention vaults for water quality treatment. Many integrate pretreatment settling areas. Underground detention could also be integrated with other pretreatment approaches includ- ing sumped catch basins, hydrodynamics systems, and permeable asphalt overlays. – Non-proprietary underground vaults with batch-mode operation. – Permeable asphalt overlays used for pretreatment to proprietary small-footprint storm filters (catch basin systems or underground inline systems) – Hydrodynamic systems used for pretreatment to pro- prietary underground storm filters – Underground infiltration systems Task 2: Give Primary Consideration to Aboveground Retrofit Opportunities Next, the project team must conceptualize and evaluate potential BMP configurations within the site using the com- ponents identified in Task 1. A core retrofitting principle is that primary consider- ation is given to finding aboveground retrofit opportunities. Aboveground BMPs are preferable to underground BMPs for the following reasons: • Typically less expensive to construct and operate • Can be easier to connect with existing conveyances and are less likely to require pumping facilities in flat terrains • Simpler to inspect, increasing the likelihood that mainte- nance needs will be identified and carried out • Easier maintenance access and require less specialized equipment and training for maintenance • Allow the use of vegetated treatment components, which typically provide better treatment performance, and can also provide volume reduction and conveyance functions. The team should fully explore all the following options for identifying aboveground BMPs before selecting under- ground BMP alternatives. Task 2a: Evaluate Opportunities for Locating Above- ground BMPs. Ultra-urban highways by definition are space constrained, but even in ultra-urban settings there are highway features that can provide opportunities for siting aboveground BMPs. Interchanges and Cloverleafs. Interchanges and clover- leafs often have landscape or vacant areas within the ROW that provide opportunities for locating aboveground BMPs (Fig- ure 9.1). These areas can be comparatively large and well iso- lated from traffic flow and can have safe maintenance access. Consequently, such areas can be ideal for locating detention, infiltration, or media filtration facilities that provide treat- ment for adjacent highway sections and elevated overpasses. ROW Strips Adjacent to Highway Sections and Ramps. Many highway sections include vacant strips adjacent to highways and ramps (Figure 9.2). ROW strips are potential opportunity areas for vegetated BMPs that are integrated into the landscaping, such as swales, filter strips, bioretention, and WSDOT media filter drains. ROW strips and landscaped areas should be exploited to the maximum extent feasible. Guardrails and concrete barriers can be used if clear zone set- backs are not adequate within the available ROW strip. Other potential opportunities for intercepting storm conveyances Figure 9.1. Interchanges and cloverleafs are opportunity areas for locating aboveground BMPs: (A) vacant areas within I-280/I-87 interchange, San Jose, California, and (B) detention facilities constructed in cloverleafs, I-405 near Bellevue, Washington.

117 in ROW strips are embankment cuts along downslope sec- tions and at down-gradient outfalls. Such areas are poten- tially suitable for aboveground filtration systems, sand filters, and GSRDs. Underneath Elevated Sections. Vacant areas beneath elevated highway sections, ramps, and stacked highway sec- tions are potential opportunity areas for locating above- ground BMPs such as detention basins, media filtration facilities, and GSRDs (Figure 9.3). Vegetated BMPs are poten- tially suitable if there is adequate light. Conveyances. Existing stormwater conveyances can be modified with aboveground treatment using vegetated bio filtration swales or biofiltration/infiltration facilities for example, which provide both effective treatment and conveyances function (Figure 9.4). Concrete ditches and road- side strips can be converted to biofiltration swales, or treat- ment functions of existing roadside ditches can be improved with check dams. Ideal locations are low-lying linear pervi- ous areas or cut sections adjacent to roadways that intercept highway drainage. The topography or graded sections should meet design criteria, typically 1% to 5%, but check dams can be used to allow for higher slopes. Lateral slopes should con- form to highway safety requirements, typically 6:1, but can be steeper if potential access is prevented (i.e., guard rails, etc.). The following list identifies items to look for when evaluat- ing vacant ROW areas in interchanges and ROW strips and underneath elevated sections: • Space: Look for usable ROW with adequate space/width, tak- ing into consideration clear zone setbacks, future planned uses of the ROW, and other constraints/obstacles identified in Step 3. Consider ways to adapt available ROW spaces by: – Using embankment cuts and retaining walls to increase BMP surface space, Figure 9.2. Two examples of ROW strips adjacent to ramps: (A) Houston, Texas, and (B) Kennedy Expressway, Chicago. Figure 9.3. Two examples of vacant areas below elevated highway sections: (A) I-405/I-105 interchange, Los Angeles, and (B) dry extended-detention basin beneath SR-125/SR-94 interchange, San Diego County.

118 – Using guard rails and concrete barriers if clear zone set- backs or slopes are not adequate, and – Modifying BMP sizing and design to fit within space and contours, or to meet design constraints. • Drainage and connectivity: Look for sufficient hydrau- lic gradient to potential connection points with existing conveyances, taking into account the BMP head require- ments. Whenever possible, look for opportunities to uti- lize existing collection and conveyance facilities in the retrofit concept to reduce costs. Potential opportunities for intercepting storm conveyances are embankment cuts along downslope sections and at down-gradient outfalls. New conveyances and outfalls must be considered when existing facilities cannot be used due to unsuitable loca- tion, insufficient capacity, or poor condition. Consider surface conveyances when there is adequate space, as they can combine treatment and conveyance functions, reduce costs of stormwater conveyance infrastructure, and have low head requirements. • Opportunities to modify and integrate existing BMPs: Exist- ing BMPs should be evaluated for potential modifications and/or integration into a treatment train. For example, existing detention basins can be modified to improve treat- ment of target conditions, such as adding GSRDs to reduce trash loads, adding/improving vegetation to enhance treat- ment, and modifying outlet structures with batch operation to improve sedimentation of fine particulates. Similarly, swales and filters can potentially be modified with check dams or amended soils to improve treatment functions. • BMP head requirement: The available head is a significant constraint in flat terrains. Tight tolerances may require additional design and construction requirements. In flat terrains, BMPs with low head requirements should be the primary siting consideration. Ideal BMPs would include surface conveyances, biofiltration swales, surface BMPs such as filter strips and bioretention within landscape areas, surface detention facilities, and porous pavements (see Section 5 for details). Avoid new pumping facilities when possible because they can significantly increase capi- tal and operation costs and are subject to possible power failure and equipment failure. Maintenance and operation of pumps were recurring problems at sites with insuf- ficient hydraulic head in the Caltrans retrofit pilot study (Caltrans, 2004). When pumping is unavoidable, design- ers should explore opportunities for using pumping facili- ties to increase BMP siting options. Where possible, utilize pumping to slowly draw down the storage within a BMP and allow gravity overflow. This will minimize pump- ing sizes and operating costs as well as reduce impacts of pump failure. • Soils and infiltration rates: For infiltration BMPs, look for sites with soil infiltration rates that more than adequately meet minimum requirements, and where there is adequate separation above the seasonal high groundwater table. As noted earlier, siting infiltration devices under marginal soil and subsurface conditions entails a substantial risk of early failure due to clogging (Caltrans, 2004). Consider the potential impacts of infiltration on the pavement base or other structures, and use liners only where there is risk to infrastructure. Also, consider the possibility that soils and sediments removed from infiltration facilities during maintenance could potentially be classified as hazardous materials, which would necessitate additional disposal requirements and costs. • Maintenance access: Look for acceptable and safe mainte- nance access routes and sufficient space for maintenance equipment without the need for lane closures or traffic controls. Figure 9.4. Highway surface conveyances: (A) Caltrans retrofit of a ROW strip adjacent to I-5 with a vegetated biofiltration swale and (B) roadside ditches adjacent to ramps, Atlanta, Georgia.

119 Task 2b: Consider Jurisdictional Partnerships. If aboveground BMPs within the ROW are not feasible or excessively costly or where a partnership would be more cost effective overall, planners should consider and investi- gate potential cross-jurisdictional partnerships with local municipalities to develop off-site aboveground retrofit solu- tions. This can provide more cost-effective treatment and can be mutually beneficial if there are common objectives such as meeting TMDL allocations. Although collaborating with local municipalities will require a greater level of planning and coordination, there are a number of potential benefits (Yu et al., 2003; Caltrans, 2004): • More flexibility in locating and designing aboveground BMPs: Local municipalities are likely to have greater options for siting aboveground BMPs on public ROWs such as parks, schools, other public properties, and public easements adjacent to roads, drainages, and utility corri- dors. Greater siting options lead to more flexibility in BMP design and more effective treatment. • Greater benefit to receiving waters: Off-site BMPs can potentially be designed as regional facilities that treat combined highway and municipal drainage areas. Regional facilities using effective aboveground BMPs would pro- vide more overall load reduction and greater benefits to receiving waters in comparison to underground retrofit options targeting only highway catchments. In some cases, the larger regional system that is treating untreated runoff from existing development may be utilized to provide pol- lutant “credits” for areas of the highway that are difficult to treat (i.e., pollutant trading). • Reduced costs: Off-site retrofits potentially generate cost savings from: – Economies of scale for the construction of facilities that treat larger areas; – Lower construction and maintenance costs with the use of aboveground BMPs in comparison to underground alternatives; – Lower probability that groundwater table issues could impact the effectiveness of the BMP and result in poten- tial project redesigns; – Avoidance or reduction of traffic-control costs and more efficient construction due to reduction of space constraints; and – Sharing of design, construction, and O&M costs between the DOT and municipalities. Task 2c: Consider Water Quality Trading. Water quality trading is a voluntary exchange of pollutant reduction cred- its. A facility with a higher pollutant control cost can buy a pollutant reduction credit from a facility with a lower con- trol cost thus reducing their cost of compliance. Thus, water quality goals can be achieved more efficiently and more eco- nomically. Water quality trading programs can potentially be used to offset costly ultra-urban retrofit mandates with less costly off-site retrofits that are more beneficial to receiving waters. The USEPA’s 2003 policy statement on water quality trad- ing supports trading of nutrients and sediment loads as well as cross-pollutant trading of oxygen-demanding pollutants. The USEPA may consider supporting trades of other pollut- ants but believes that these trades require a higher level of scrutiny. The USEPA does not support trading of persistent bioaccumulative toxics (USEPA, 2003). DOTs recognize that water quality trading credit approaches are imperative for economically complying with TMDLs in areas where BMP implementation costs are excessive (Hon et al., 2003; McGowen et al., 2010). However, water quality trading programs are not yet widely developed and trading approaches are likely to be inconsistent among states. Only a few DOTs have experience with trading programs. One exam- ple is the Maryland SHA, which is allowed to trade treatment credits (treated impervious surface) between watersheds, with a 20% “charge” each time credit is withdrawn from the “bank” (stored treatment credit) and used to offset treatment require- ments for a project (McGowen et al., 2010). Although pollutant trading programs are not yet widely used, it is likely that future programs similar to the Mary- land SHA example will be available as retrofit and TMDL requirements increasingly impact DOTs. In ultra-urban areas where BMP costs are excessive, pollutant trading may be a viable cost-effective alternative for meeting highway retro- fit requirements. As mentioned in previous tasks, “pollutant credits” that are less formal than a trading program may be possible by working with local regulators. This is particularly true for TMDL situations where an overall loading reduc- tion is specified and it is left to the DOT to determine how to achieve the reduction. Task 3: Pursue Underground BMPs Options It is necessary to consider underground BMPs for many ultra-urban retrofit projects or highway sections (Figure 9.5). The objective of this task is to identify and develop conceptual plans for potentially feasible underground BMP options when: • There is insufficient surface area in the ROW; • It is not possible or cost effective to use available ROW area; • Off-site alternatives are not feasible; and • It is appropriate and cost effective to use underground BMPs for pretreatment to other BMPs. Retrofit planners and designers have a wide variety of proprietary and non-proprietary underground BMPs from

120 which to choose. Considerations when evaluating under- ground BMPs include the following: • Existing infrastructure. To simplify construction and reduce costs, look for opportunities to integrate under- ground BMPs within existing infrastructure: – Existing conveyances: Locate underground BMPs to take advantage of existing catch basins and storm lines when practical. – Catch basins: Catch basins that are safely and conveniently accessed for maintenance (e.g., behind barriers) can be modified for pretreatment, for example, retrofits using sumped basins or proprietary catch basin devices (Section 4.2), including adding new water quality catch basins just upslope and retaining the existing basins for drainage. – Existing treatment BMPs: Cost savings can be realized when existing treatment BMPs are modified and/or inte- grated into a BMP treatment train. For example, existing hydrodynamic separators or detention facilities could be used as pretreatment devices for underground media fil- tration systems that target dissolved constituents. – Existing pumping facilities: Existing pump stations at below-grade sections may provide opportunities for conveying runoff to more feasible sites or for meeting BMP head requirements. – Pavement retrofits: Permeable asphalt overlays on existing roadbeds increase highway safety in wet weather and have shown promising water quality treat- ment benefits. Permeable asphalt overlays can be cost effectively integrated into retrofit designs as pretreat- ment components. Because performance life and main- tenance requirements are not well established, it may be appropriate to consider pilot projects to gain opera- tional experience and additional performance informa- tion or to include redundant pretreatment components. • BMP size. The project team should weigh the tradeoffs of using small-footprint proprietary BMPs. Many under- ground BMP options will be feasible because of the ben- efits they provide in terms of space requirements, simple installation, and cost. However, small-footprint propri- etary BMPs can also have significant inspection and main- tenance requirements, high maintenance costs, and poor effectiveness. Larger BMPs, such as underground deten- tion and underground media filtration, have greater capi- tal costs in comparison to small-footprint BMPs but can potentially deliver more effective treatment with less fre- quent or similar maintenance requirements. • Acceptability. Some state DOTs are limited in the types of allowable proprietary BMPs, and some states have estab- lished certification procedures. Consider and explore pilot testing for BMP options that are unapproved but thought to be effective. • Maintenance practices. Underground BMPs must have acceptable inspection and maintenance practices. Some DOTs do not allow routine use of various underground BMPs (storm filters, hydrodynamic systems) primarily because of excessive maintenance requirements. It is imper- ative to coordinate with maintenance personnel to screen BMPs with unacceptable maintenance requirements. How- ever, as water quality requirements become more stringent, DOTs may have to consider undertaking projects that require more maintenance. • Treatment performance. There must be reasonable confi- dence that the candidate BMPs can meet treatment objec- tives. Suitable performance information, however, may be limited or lacking for many proprietary BMPs. An assess- ment of the expected treatment performance of under- ground BMPs may be supported by: – DOT familiarity and experience with the BMPs, – Regulatory certification, Figure 9.5. Examples of ultra-urban highways with limited space for aboveground BMPs.

121 – Review of independent performance evaluations, and/or – Modeling of expected performance by the DOT. Consider retrofit pilot tests for promising BMP approaches with incomplete performance information. • Design and performance specifications. Some DOTs may choose to use detailed bid specifications as the means for selecting proprietary BMPs (see Case Study 5 in Section 10) that are expected to meet the desired performance. At a minimum, comprehensive specifications should include criteria for the size and/or volume, bypass capacity, treat- ment capacity and performance, maintenance access and frequency, resuspension performance, and supporting per- formance evaluations. Task 4: Evaluate and Select Primary Retrofit Alternatives The product of this task is the selection and ranking of a limited number of primary alternatives. A systematic evalua- tion of alternatives is beneficial for projects or project sections where there are no clear superior alternatives. Developing an alternatives analysis matrix will help to relate and visualize the constraints and opportunities and evaluate the tradeoffs. The following are the basic steps: • Compile and list the retrofit options identified during, and in conjunction with, the scoping and conceptual Task 3. • Define the criteria for evaluating alternatives. The most useful criteria will distinguish differences between the alternatives, such as cost, maintenance, etc. Table 9.4 pro- vides example criteria for consideration. To help narrow alternatives, identify mandatory criteria such as safety. • Construct the analysis matrix with alternatives versus cri- teria. Assess applicable criteria for each alternative with qualitative descriptions and/or numeric scoring. Weight- ing factors are often applied to the criteria based on pre- determined importance. • Evaluate the alternatives and choose the primary alternatives. 9.5.5 Step 5: Conduct Practicality Assessment In Step 5 the project team prepares preliminary designs, costing, and performance assessments of the primary alter- natives and chooses the final retrofit approach. The tasks include the following: • Hydrologic and hydraulics analysis: Detailed hydrologic and hydraulics analyses are conducted to finalize contrib- uting drainage areas and to support design and sizing of conveyances. • BMP sizing: Detailed sizing of BMPs, typically using DOT procedures and specifications based on design storm analy ses, are conducted. Such DOT approaches are suit- able for conventional and approved BMPs where there is adequate space and no design constraints. Section 6 describes BMP sizing and design using con- tinuous hydrologic simulation analyses. Alternative sizing approaches are appropriate for severely space-constrained and cost-prohibitive settings where undersized facilities may be considered. For example, undersized vaults that target first flush can provide meaningful treatment, con- sistent with the goal of cost effectively maximizing pol- lutant reduction and receiving water benefit. Continuous hydrologic simulation allows for analysis of BMP sizing based on conventional volume capture criteria, as well as other performance metrics including ideal sedimentation efficiency in volume-based BMPs (detention facilities) and average contact time in flow-based BMPs (media fil- tration BMPs). Furthermore, the approaches described in BMP Selection/Performance ▪ Regulatory/DOT compliance ▪ Expected treatment perform ance ▪ Cold weather performance ▪ Longevity ▪ DOT experience with BMPs Location/Siting ▪ Space availability, compatibility ▪ Aboveground vs. underground ▪ Areas treated ▪ Hydraulics (head, connectivity, use of existing facilities) ▪ Major construction, grading/shoring, obstructions ▪ Safety concerns ▪ Environmental issues/permittin g ▪ Aesthetic issues ▪ Site uncertainties Design/Construction ▪ BMP sizin g ▪ Cold weather design modifications ▪ Special ma terial/design requirem ents ▪ Construction/installation requirem ents ▪ Construction schedule/phasing ▪ Staging, traffic control Operation & Maintenance ▪ Inspection and maintenance requirem ents ▪ Available equipm ent/personnel ▪ Access and safety concern s ▪ Proprietary materials ▪ Major maintenance requirements Cost ▪ Capital ▪ O&M ▪ Uncertainties Table 9.4. Example criteria for alternatives evaluation.

122 Section 6 provide a means of assessing sizing tradeoffs with alternative operation strategies, such as batch mode opera- tion of detention facilities versus extended detention. For proprietary systems, manufacturer sizing criteria should be used as minimum sizing guidance. The effec- tiveness of small-footprint BMPs such as hydrodynamic separators and small underground vaults (oil-water sepa- rators) is directly associated with the size/volume of the device; better performance is obtained with increasing size, although controlled testing has shown a plateau at which further increases in size provide no additional benefit. Cold weather also diminishes performance of small-footprint BMPs. Sizing of manufactured systems should be based on thorough evaluation of available performance informa- tion, ideally including direct DOT experience. Designers may want to consider over-sizing or including a factor of safety for small-footprint BMPs to improve performance, particularly in cold climate applications. • Performance assessment: An assessment of water quality treatment and/or hydrologic performance may be required to verify compliance or to compare alternatives. In other cases, BMPs selected and designed in accordance with approved policies may have a presumptive level of perfor- mance and no formal performance assessment is needed. Section 5 describes approaches for assessing the water quality treatment performance of BMPs. Recommended performance criteria include the runoff capture efficiency of the BMPs, the ability of the BMP to reduce runoff vol- umes, and expected effluent quality of treated runoff. Designers may also need to perform pollutant loadings calculations to assess compliance with TMDL wasteload allocations or BMP efficiency standards. Hydrologic performance calculations typically include basin routing calculations for assessment of peak discharge– frequency analysis. A more comprehensive assessment of hydromodification impacts could include flow-duration analysis using continuous simulation approaches. • Preparation of preliminary designs: Preliminary BMP designs are developed to (1) obtain a clear picture of the structural elements, layout, and dimensions; (2) to dis- cover and resolve critical issues and problems; (3) to obtain material quantities for estimating costs; and (4) to use the preliminary design as a check on the final design. Considerations for BMP design include the following: – Address maintenance equipment and access in the design. Also, consider maintenance impacts on biologi- cal or environmental resources. – Avoid precast proprietary units in cases when there are tight tolerances because as-built maps can be inaccurate. Cast in-place features allow for adjustments that may be needed to match actual field conditions or changes due to construction (Currier and Moeller, 2000). – BMP designs should consider and avoid standing water that may promote mosquito breeding as necessary. • Cost estimation: Section 8 describes retrofit cost factors, available cost information, and potential cost reduction strat- egies. Cost estimates should include both capital and O&M costs, and appropriate contingency costs. They should also consider major maintenance or replacement cycle as well. • Choose retrofit approach: The project team chooses the retrofit approach based on results of the practicality assess- ment and input from DOT representatives, team members, and stakeholders. 9.5.6 Step 6: Prepare Final Design and Construction Specifications In this step, the project team prepares the final designs and specifications, obtains permits, and oversees construction. Guidance and considerations include the following: • Permitting: Obtaining and complying with local and regional permits can be a major effort and can impact con- struction and O&M. Coordinate with local officials and regulators from the outset of the project to ensure permit- ting will not delay or impact construction and O&M. • Construction phasing: Coordinate construction with other planned construction activities as feasible. This can produce savings and help to solicit and receive more bids for smaller jobs. It can also expand the scope of the retrofit by including larger drainage areas. • BMP specifications: Order materials with long lead times as soon as possible and check availability. Specifications must be very explicit and the materials must be readily available. Check and confirm specifications of ordered products. For example, the specification of media composition can be critical as sub- stitution of media components can substantially impact cost and/or result in unintended leaching of pollutants. • Installation of proprietary systems: Manufacturer instal- lation instructions should be considered as guidelines and followed; otherwise, poor performance can result. • Construction inspection: Because retrofits are likely to involve unique designs and material, construction inspec- tion is important to ensure that design specifications are met. The construction inspectors must have sufficient training in the identification of construction materials and construction practices, for example, media specifications and vegetation type (see the case study in Section 10.6). Include material quality specifications with orders (Currier and Moeller, 2000). 9.5.7 Step 7: Post-Construction Operation and Evaluation Post-construction BMP maintenance and monitoring evaluations are key activities to ensure the retrofits perform

123 at the design capacity, and for obtaining feedback about BMP design and ongoing performance. • Ongoing BMP inspection and maintenance: As discussed in various sections previously, an initial BMP O&M plan should be coordinated and developed early in the retrofit planning process. Following construction, the mainte- nance crews must implement and refine the O&M plan as necessary to ensure proper BMP performance. Ideally, the O&M plan will include more frequent early inspections to ensure proper function and to allow for early adjustments if needed. Designers and planners should proactively seek feedback from maintenance crews regarding BMP O&M practices. For new and/or unfamiliar BMPs, planners and maintenance departments should consider a detailed main- tenance monitoring/auditing program to establish and document the required maintenance practices and costs. • Water quality and BMP monitoring: Post-construction activities may include the development and implementa- tion of a formal monitoring program to meet objectives such as: – Establish BMP performance: DOTs may want to estab- lish the treatment and/or hydraulic performance of retrofit BMPs, particularly if unfamiliar, proprietary, or non-standard BMP designs with little direct perfor- mance data are selected in order to meet site constraints and/or other retrofit objectives. – Regulatory compliance: Regulators or permit condi- tions may require DOTs to conduct water quality moni- toring to establish BMP effluent quality, receiving water quality, and/or to establish if wasteload allocations are met. • Retrofit project evaluation: Post-construction coordina- tion to review and evaluate the retrofit project can provide valuable information to support future retrofit projects. A formal review should address all project phases including project planning, design, costing, construction, and post- construction activities.