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78 9.1 Introduction The BMP selection and conceptual design methodology presented in this report is driven by well-defined stormwater management goals and a solid understanding of site charac- teristics, constraints, and water-quality conditions. As opposed to other design approaches that recommend the selection of typical BMPs based solely on documented per- formance factors such as percent removal, effluent quality and/or percent capture, the approach presented herein is to first select the unit treatment processes that address the pollu- tants of concern and stormwater management goals and then individually select the components of a BMP treatment sys- tem based on those unit processes. The steps of the BMP selec- tion and conceptual design methodology include the following: (1) problem definition, (2) site characterization, (3) identification of fundamental process categories, (4) selection of BMPs, LID elements, and other treatment options, (5) prac- ticability assessment of candidate treatment systems, (6) siz- ing the conceptual BMP design, and (7) development of a performance monitoring and evaluation plan. The sections that follow briefly describe each component of the methodol- ogy; these components are presented in detail in the Guidelines Manual. At the end of Chapter 9, the methodology is summa- rized in a detailed flow chart in Figure 9-1 (chapter and sec- tion numbers in Figure 9-1 refer to the Guidelines Manual). 9.2 Problem Definition The design of any engineering system requires a clear defi- nition of the problem. Without clear descriptions of the high- way runoff issues that need to be addressed at a particular site, including the desired results, it is difficult (if not impossible) to evaluate the steps needed to select and design a practicable and cost-effective runoff treatment system. Typical stormwater objectives in urban and highway areas are listed in Table 9-1. The title of the table, âUrban runoff management objectives checklist,â reflects the fact that runoff can be generated from several sources not directly connected to rainfall, such as base- flow, car washing, etc. Hence, stormwater control must be inte- grated with overall urban or highway runoff control. The problem definition should include a detailed description of the overall project, including whether the project is solely a stormwater retrofit project or is part of a larger highway- improvement or construction project. All project objectives (including those unrelated to stormwater) should be carefully identified and ranked. Since project objectives often conflict (e.g., water quality and flood control, cost, and safety), clearly defining the relative importance of these objectives is essential to the coordination of planning and design activities among the various project managers, subcontractors, and stakeholders. A worksheet template for such an evaluation is included in Appendix D of the Guidelines Manual. 9.3 Site Characterization After the project has been described and the objectives iden- tified, the next step in any highway construction, improve- ment, or retrofit project is to characterize the site conditions and constraints. This step is critical for the assessment and identification of feasible solutions to the runoff management problem. Site conditions may significantly influence the treatability and manageability of highway runoff.With careful characterization of the hydrologic, geologic, and anthro- pogenic factors that affect runoff quantity and quality, appro- priate and feasible selection and design alternatives can begin to be identified. Assessing site characteristics should involve identifying opportunities and, to a certain extent, constraints that influence the selection and design of highway runoff treatment systems. Opportunities for incorporation of LID techniques that emphasize pollutant source control through infiltration, interception, ET, and reduction of directly connected impervious area should also be identified when characterizing the site conditions. The highway engineer must C H A P T E R 9 BMP Selection Guidance Methodology
79 PROBLEM DEFINITION (Chapter 2) Site Characterization Data, and List of Target Pollutants, Refined Goals and Objectives Project Description (2.1) Identify and Rank Project Objectives (2.2) Overall Project Goals and Objectives SITE CHARACTERIZATION (Chapter 3) Roadway Classification (3.1) (Functional Classification, Level of Access Classification, Amenities Classification, Jurisdictional Classification, Geometric Classification, Drainage Classification, Landuse Based Classification) Stormwater Conveyance Systems (3.2) Identify drainage system elements and evaluate drainage area imperviousness (Roadside Ditches, Curb and Gutter, Street inlets, Pipes and Culverts, Bridge Drainage) Site Hydrologic Conditions (3.3&3.4) Gather representative hydrologic data (Precipitation, Seasonality) Runoff Water Quality (3.5) Gather representative water quality data to characterize highway runoff constituents, concentration estimates and factors affecting runoff quality IDENTIFICATION OF APPLICABLE FUNDAMENTAL PROCESS CATEGORIES (FPCs) (Chapter 4) Set of Applicable Unit Operations and Processes Biological Processes (4.3) (Microbially Mediated Transformations, Uptake and Storage) Chemical Processes (4.4) (Sorption Processes, Flocculation/ Precipitation, Chemical Agent Disinfection) Physical Treatment Operations (4.2) (Particle Size Alteration, Size Separation, Density Separation, Aeration, Volatilization, Physical Agent Disinfection) Hydrologic/Hydraulic Operations (4.1) (Peak Attenuation, Volume Reduction, Flow-Duration, LID Practices) Figure 9-1. Conceptual stormwater treatment system design methodology flow chart. be open to alternative designs that do not significantly alter the safety or structural integrity of the project. 9.4 Identification of FPCs After site conditions, constraints, and influent water qual- ity and hydrology have been determined and/or estimated, the unit processes that are available to reduce or treat the pol- lutants of concern should be identified and qualitatively ranked on a scale according to how well those processes reduce runoff volumes/rates or treat the pollutants of concern. Site-specific information on soils and associated infiltration rates is especially critical because of their high variability and their importance as major components of LID.
80 Figure 9-1. (Continued). Hydraulic Controls (5.6) (Energy Dissipators, Inlet Structures, Outlet Structures, Berms, Check Dams, Weirs, Sluice Gates) Candidate Treatment System Alternatives IDENTIFICATION OF BMPs, LID ELEMENTS AND OTHER TREATMENT OPTIONS (Chapter 5) Identify Highway Hydrologic Controls and LID Practices (5.1) (Evaporation Enhancement Systems, Storage Facilities, Curb Cuts, Porous Pavement, Vegetated Medians and Shoulders, Roadside Infiltration/Exfiltration Practices, Vegetated Roadside Ditches, Soft Shoulders, Narrow Pavement Designs, LID Construction and Management Techniques) Pre-Treatment Devices (5.2) (Inlet Devices, In-line Devices, Floating Traps) Primary Treatment BMPs (5.3) (Tanks and Vaults, Oil/Water Separators, Hydrodynamic Devices, Sedimentation Ponds and Forebays, Surface Filters, Vegetated Swales and Filter Strips) Secondary Treatment BMPs (5.4) (Advanced Biological Systems, Flocculent/Precipitant Injection Systems, Aeration and Volatilization Devices, Disinfection Systems) Tertiary Treatment BMPs (5.5) (Soil Amendments, Microbes, Vegetated Systems, Disinfection Systems) Many unit treatment processes applicable to stormwater treatment have been previously developed in the fields of water and wastewater engineering. UOPs can be divided according to four FPCs (see Sections ): (1) hydrologic operations, (2) phys- ical operations, (3) biological processes, and (4) chemical processes. Hydrologic operations, which are essentially a sub- set of physical operations, include the principles of flow atten- uation (e.g., peak shaving and detention) and volume reduction (e.g., infiltration and ET). These are the two funda- mental principles of LID. Physical operations, as referred to in this report, include the principles of particle size alteration (e.g., comminution), size separation and exclusion (e.g., screening and filtration), density separation (e.g., sedimenta- tion and flotation), aeration and volatilization, and natural dis- infection (e.g., ultraviolet light and heat). Biological processes include the principles of microbially mediated transformations (e.g., redox reactions resulting from microbial respiration) and uptake and storage (e.g., bioaccumulation). Chemical processes include the principles of sorption (e.g., ion exchange and surface complexation), coagulation and flocculation (e.g., particle agglomeration and precipitation), and chemical agent disinfection (e.g., chlorine and ozone). The selection of any one of these UOPs should be based on the characteristics of the tar- get pollutants in relation to specific stormwater management goals. Such a characterization is presented for a variety of pol- lutants in the pollutant fact sheets in Appendix A of the Guide- lines Manual. 9.5 Selection of BMPs, LID Elements, and Other Treatment Options Once the UOPs available for addressing the runoff man- agement goals have been identified, individual BMPs, LID facilities, and other BMP treatment systems can be selected. Some BMPs may include a higher level of runoff treatment or control than other BMPs, so it is important to understand, at
81 PRACTICABILITY ASSESSMENT OF CANDIDATE TREATMENT SYSTEMS (Chapter 6) Physical and Social Constraints (6.1-6.7) (Space Availability, Existing Infrastructure, Hydraulic Gradient, Soil Properties, Groundwater Considerations) Hydrologic and Water Quality Evaluation (6.8) (Preliminary Hydrologic Assessment, BMP Database Summaries, Other Reported Performance Estimates) Relative Cost of Candidate Treatment System Alternatives (6.9) (Volume-Based Costs, Flow Rate-Based Cost, Area-Based Costs) Selected Treatment System Alternative to be Sized and Further Evaluated SIZE AND DEVELOP CONCEPTUAL DESIGN OF SELECTED TREATMENT SYSTEM (Chapter 7) END Completed Conceptual Treatment System Design Sizing Methodologies (7.1-7.13) (LID Sizing and implementation, Hydrologic/Hydraulic Control sizing, Flow Based and Volume Based Control Sizing) Flexibility Design / Adaptive Management (7.14) (Active Inlet/Outlet Controls, Offline and Parallel Systems) Note: Chapter and section numbers refer to the Guidelines Manual. least at a fundamental level, the relationship between BMP design and UOPs. With an understanding of the physical (including hydrologic), biological, and chemical processes that typically occur in stormwater BMPs, supported by BMP performance monitoring data, candidate BMPs can be com- pared and selected. If a single BMP does not provide all of the desired UOPs, then a BMP treatment system or treatment train may need to be developed. Table 5-1 in the Guidelines Manual provides a mapping of FPCs, UOPs, associated pol- lutants, and common stormwater BMPs. An examination of this table reveals that several BMPs and combinations of BMPs may provide similar levels of stormwater treatment and control. Therefore, the next step is to consider the project constraints and site conditions that may favor one BMP alter- native (or BMP combination) over another. 9.6 Practicability Assessment of Candidate Treatment Systems Once alternative BMPs or BMP/LID systems have been selected, the practicability of implementing each alterna- tive should be considered. Determining the practicability of Figure 9-1. (Continued).
82 candidate BMPs for a site is not a decision based specifically on performance or hydrologic measures, but rather on feasibility of design, installation, and implementation.A variety of factors exist that must be taken into account by engineers when initially selecting a particular BMP system design to pursue, including performance for target pollutants, hydrology and hydraulics, surface and subsurface space availability, maintenance, costs, and aesthetics.Other factors include safety, regional constraints, downstream impacts, land use allocations, safety and human health concerns, regional and climatic concerns, and overall project budget (cost considerations). Most of these factors are discussed in Chapters 5, 6, and 7 of this report. Each BMP alter- native should be evaluated according to these practicability fac- tors, and at least one alternative should be selected for preliminary sizing. Such factors are included in practicability assessment tables in Appendix D of the Guidelines Manual. 9.7 Sizing the Conceptual BMP Design The design of a selected BMP treatment system must address the project goals and objectives as well as the design require- ments of the regulating authority. Several methods exist for hydrologic design including the following: flow-attenuation design, volume-reduction design, and flow-duration design. Flow attenuation, also referred to as peak shaving, is typically achieved with storage and controlled release, but increasing the flow path may also be feasible. Volume reduction is possi- ble through infiltration and ET, both of which are highly dependent on site-specific conditions including soils, vegeta- tion, and climate. Flow duration seeks to reduce both the mag- nitude and the time period of flow by incorporating the principles from flow attenuation and volume reduction. The applicability of any of these design methods depends on whether the system is volume-based, such as detention basins, or flow-based, such as swales. These facilities can be sized using a hierarchy of procedures including simple design storm approaches, rainfall frequency analyses, and continuous runoff simulation. Continuous simulation is generally the preference for water-quality-based design because it permits optimization for design on the basis of minimum cost, minimum down- stream discharge, minimum downstream pollutant load, and a variety of other possibilities using a long-term hydrologic record that is presumably representative of the life of the BMP. This may be done heuristically with models such as SWMM or in a more integrated fashion (i.e., incorporating optimization) with spreadsheet models. The continuous simulation approach is described in Chapter 10 of this report and in Chapter 7 of the Guidelines Manual. Continuous simulations can be performed to develop general sizing and design criteria on a subregional basis, the results of which could then be used to direct the engi- neer to more detailed design procedures. An analytical hierar- chy is described in Section 10.5.4. 9.8 Development of a Performance Monitoring and Evaluation Plan Stormwater BMP monitoring projects are initiated to address a broad range of programmatic, management, Category Typical Objectives of Urban Runoff Management Projects Hydraulics Manage flow characteristics upstream, within, and/or downstream of treatment system components Hydrology Mitigate floods; improve runoff characteristics (peak shaving) Reduce downstream pollutant loads and concentrations of pollutants Improve/minimize downstream temperature impact Achieve desired pollutant concentration in outflow Water Quality Remove litter and debris Reduce acute toxicity of runoff Toxicity Reduce chronic toxicity of runoff Comply with NPDES permit Regulatory Meet local, state, or federal water quality criteria Implementation Function within management and oversight structure Cost Minimize capital, operation, and maintenance (life cycle) costs Aesthetic Improve appearance of site and avoid odor or nuisance Operate within maintenance and repair schedule and requirements Maintenance Design system to allow for retrofit, modification, or expansion Longevity Achieve long-term functionality Improve downstream aquatic environment/erosion control Improve wildlife habitat Resources Achieve multiple-use functionality Function without significant risk or liability Function with minimal environmental risk downstream Safety, Risk, and Liability Contain spills Public Perception Clarify public understanding of runoff quality, quantity, and impacts on receiving waters Table 9-1. Urban runoff management objectives checklist.
83 regulatory, and research goals. Monitoring goals are often focused on the achievement of objectives (including hydrol- ogy/hydraulics and water quality) downstream of the facil- ity. This monitoring and evaluation effort (not shown on the flow chart in Figure 9-1) may be used to determine the degree to which these objectives are met. Multiple methods, all with different cost and time structures, can be used for sampling including manual and automated methods for collecting grab samples as well as time-weighted and flow- weighted composite samples. Depending on the specific treatment system and the goals of the monitoring program, several samples may need to be collected at multiple loca- tions during multiple storm events to obtain data useful for determining the actual performance.
