Bridge Stormwater Runoff Analysis and Treatment Options (2014)

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50 BMP Evaluation Tool The BMP Evaluation Tools are a set of spreadsheets that have been provided as a supplement to this Guide that can be used for planning-level estimates of BMP treatment perfor- mance and whole life costs. The tools are Excel© applications, one for each of the various BMPs selected for bridge deck runoff treatment (Section 5.3), which allow users to input BMP design configurations and easily evaluate stormwater volume and pollutant load removal and cost implications of the BMP sizing without extensive modeling or calculations. This chapter discusses the use of the tool. One or more tools should be used to optimize BMP selection if the practitioner will be installing treatment BMPs for bridge deck runoff. To illustrate how the tool works, it will be applied to an example site in this chapter. The worked example will show how the tool can be used to quickly optimize BMP selection for the given project and assess the performance and cost of candi- date BMPs. 6.1 BMP Evaluation Tool Overview This section provides an overview of the functions, calcu- lation methodology, inputs, and results and interpretations that are common to each tool. 6.1.1 Tool Assessment Functions The tool assessment functions are to provide stormwater volumes, stormwater pollutant loads and concentrations, and costs. Stormwater volumes. Provide an estimate of key storm- water volumes including: • Annual stormwater runoff volume generated by the bridge drainage area to the BMP • Stormwater runoff volume that bypasses the BMP • Stormwater runoff that is captured, reduced, and released as treated effluent by the BMP • Total combined stormwater volume discharged to the receiv- ing water body Figure 6-1 illustrates a typical BMP and the relationship of these key stormwater volumes to the BMP. Stormwater pollutant loads and concentrations. Provide an estimate of key stormwater pollutant loads and concentra- tions including: • Annual stormwater runoff pollutant load generated by the bridge drainage area to the BMP • Stormwater runoff pollutant load that bypasses the BMP • Stormwater runoff pollutant load captured, reduced, and released as treated effluent by the BMP • Total combined stormwater pollutant load discharged to the receiving water body • Total annual stormwater pollutant load reduction • Annual influent, treated, and combined effluent concen- trations Costs. Provide an estimate of whole life costs including: • Direct and associated capital costs of designing and install- ing the BMP • Regular and corrective maintenance costs of the BMP • Annualized whole life costs per annual load removed 6.1.2 Tool Calculation Methodology Four primary calculations provide the estimations required to serve the tool volume, pollutant and cost assessment func- tions including: (1) annual stormwater runoff volume to the BMP, (2) amount of runoff captured and reduced by the BMP, (3) BMP influent and effluent pollutant loading, and (4) BMP material quantities. Summarized information regarding these four calculations is provided in the following sections. Detailed information for volume and pollutant load C H A P T E R 6

51 modeling methods can be found in Appendix E: BMP Evalu- ation Tool Modeling Methodology. 6.1.2.1 Average Annual Runoff Volume Average annual runoff volume to the BMP in the tool is based on the average annual rainfall depth, a computed volu- metric runoff coefficient, and the tributary drainage area. A volumetric runoff coefficient equation that is a function of imperviousness is used to estimate the fraction of annual rain- fall that becomes runoff. The general form of the volumetric runoff coefficient equation is based on Granato (2006). The computed volumetric runoff coefficient is then used as the basis for estimating the average annual runoff volume from a particular drainage area. Detailed information on the average annual runoff volume modeling can be found in Appendix E: BMP Evaluation Tool Modeling Methodology. 6.1.2.2 BMP Volume Capture and Loss The amount of runoff captured by the BMP in the tool was estimated using the EPA Storm Water Management Model (SWMM) Version 5.0.022 continuous simulation model. An array of unit-area hydrologic models was developed to rep- resent various climatic regions for the contiguous United States, soil types, and imperviousness. The models were used to evaluate the hydrologic and hydraulic performance of the BMP types selected for bridge deck runoff treatment. Nor- malized performance curves were developed for estimating the percentage of the annual runoff volume captured by a site- specific BMP type, configuration, and outflow rates (infiltra- tion, ET, and controlled release). The tool interpolates between the results of continuous simulation runs within the range of the BMP design parameters to produce an estimate of average annual capture efficiency and percent volume loss. An advan- tage to continuous simulation modeling for the BMP volume capture analysis was the ability to account for the variability in the frequency and magnitude of storm events at a particular climatic region/sub-region in relation to a given BMP design. 6.1.3 Pollutant Loading The pollutants of concern selected for the tool calculations were based on the types of pollutants commonly monitored and observed in highway runoff and identified in NPDES per- mits and other regulatory requirements. Pollutant load calcu- lations in the Tool were completed using different methods for inflows to the BMP and treated effluent from the BMP. To provide representative bridge stormwater runoff qual- ity inflows for BMP treatment analysis, highway runoff mean concentrations developed through statistical analyses of all sites within the Highway Runoff Database (HRDB) (Smith and Granato 2010) and highway land use sites in the National Stormwater Quality Database (NSQD) (Pitt 2008) were used. These mean concentrations were multiplied by the estimated annual runoff volume to estimate the total load to the BMP. The annual bypass load is similarly estimated by using the runoff volume minus the captured volume. For tool effluent loading, the expected BMP load removal was calculated using BMP performance curves developed from a regression of influent versus effluent mean concentra- tions from the International Stormwater Best Management Practice Database (BMPDB). The default highway runoff influent concentrations were used as input for the calculated BMPDB influent/effluent relationship. The estimated efflu- ent concentrations are multiplied by the estimated discharge volume to predict the average annual load discharging from the BMP. The total load is finally computed as the sum of the discharged load and the bypassed load. Detailed information on the pollutant loading analysis methodology used in the calculations can be found in Appendix E: BMP Evaluation Tool Modeling Methodology. Figure 6-1. General BMP stormwater volumes schematic.

52 6.1.3.1 Material Quantities Material quantities calculations in the tool were based on the BMP configurations with typical default assumption for design values such as side slopes and length-to-width ratio to estimate excavation volumes, BMP component lengths and volumes, and grading and restoration areas for capital cost calculations. As discussed in Appendix B: Simple and Complex Assessment Methods and Worked Example, many user inputs are customizable to represent desired BMP design configurations for optimized assessment of performance and costs. 6.1.4 Tool Inputs The tool inputs include user-specific climate data based on closest available rain gage, bridge deck tributary area character- istics, and the treatment BMP design features/configuration. Rain gages are selected based on groupings of the National Climatic Data Center (NCDC) climate divisions (Figure 6-2) to provide a list of gages in a specific region. User-friendly features of the Tools include a navigation bar to navigate to key input forms via a one-button click, a color-coded key to identify cell content application (i.e., instructions, headings, user data, and reference data), drop-down menus for select inputs, and built-in guidance information located directly adjacent to design values for ease of customization. Default values for climate and BMP design parameters are provided for ease of use. Appendix B: Simple and Complex Assessment Methods and Worked Example, discusses how most defaults are customizable by the user to adapt to site- specific needs. Appendix E: BMP Evaluation Tool Modeling Methodology provides detailed information on Tool organi- zation, project set up, entering project data, and general infor- mation such as saving, editing, and printing multiple scenarios. 6.1.5 Tool Results and Interpretations The tool results are presented in a single worksheet and include the following: • Summary of the modeled scenario (tributary area, BMP type, rain gage location, and precipitation depth) • Summary of design parameters (BMP type and configura- tion data) • Summary of whole life costs (capital and maintenance costs as well as WLC per load removed). Note, whole life costs presented in the BMP Evaluation Tool do not account for the cost associated with drainage conveyance systems Figure 6-2. NCDC climate division groupings for tool rain gage selection.

53 within the deck of the bridge. That cost can be estimated separately by using a stand-alone deck drain cost tool cre- ated as part of this work effort. Details associated with use and interpretation of the deck drain cost tool are discussed in Appendix D. • Tabular and graphical summary of volume performance (see Appendix B: Simple and Complex Assessment Meth- ods and Worked Example) • Tabular and graphical summary of pollutant load perfor- mance (see Appendix B: Simple and Complex Assessment Methods and Worked Example) • Tabular summary of water quality concentrations (see Appendix B: Simple and Complex Assessment Methods and Worked Example) Appendix D: User’s Guide for the BMP Evaluation Tool provides detailed information on viewing and interpreting results. 6.1.5.1 Volume Performance Results The following volume performance results are provided by the tool: • Baseline average annual runoff volume: the total volume of annual runoff for the site (bridge deck) based on climatic region/sub-region, drainage area, imperviousness, and soil type. • BMP captured volume: the volume of annual runoff cap- tured by the BMP. • BMP effluent volume: the volume of annual runoff that is treated and released from the BMP through controls such as underdrains, orifices, weirs, etc. • Runoff bypassed (overflow) volume: the volume of annual runoff not captured by the treatment BMP that bypasses or overflows directly to the receiving water body. Note that the tool conservatively assumes that overflow receives no treat- ment even though some limited treatment of this volume may occur. • Total discharge volume: the volume of annual runoff dis- charged to the receiving water body. This is calculated by adding the bypassed and effluent volumes. • Total volume reduction: the volume of annual runoff lost by the BMP through infiltration and ET. 6.1.5.2 Pollutant Load Performance Results The following pollutant load performance results are pro- vided by the tool: • Baseline average annual runoff load: the total annual pol- lutant load for the site (bridge deck). This is calculated by multiplying total annual runoff volume by the character- istic highway runoff mean concentration. • BMP captured load: the annual pollutant load captured by the treatment BMP. This is calculated as the difference between the baseline average annual runoff load and the bypassed load. • BMP effluent load: the annual pollutant load from the BMP to the receiving water body. This is calculated by mul- tiplying the BMP effluent volume by the treatment BMP pollutant mean effluent concentration (computed based on influent-effluent concentration relationship). • BMP load reduction: the total annual pollutant load removed by the BMP. This is calculated by subtracting the BMP effluent load from the BMP captured load. • Bypassed load: the annual pollutant load not captured by the treatment BMP and discharged directly to the receiv- ing water body. This is calculated by multiplying the BMP bypassed volume by the characteristic highway runoff mean concentration. • Percent annual BMP load removal: the percentage of annual pollutant load removed by the BMP. This is calculated by dividing the total BMP load reduction by the baseline average annual runoff load. • Total discharge load: the total annual pollutant load to the receiving water body. This is calculated by adding the bypassed load to the BMP effluent load. • Total volume reduction load: the annual pollutant load removed via infiltration and ET. This is calculated by mul- tiplying the baseline average annual runoff load by the per- centage of total annual volume lost. • Treatment reduction load: the annual pollutant load removed by the BMP by non-volume loss treatment pro- cesses that reduce concentrations including adsorption, fil- tration, settling, decomposition and plant uptake. This is calculated by subtracting both the total volume reduction load and the BMP effluent load from the BMP captured load. 6.1.5.3 Water Quality Concentrations The tool provides the following water quality concentrations: • Influent concentration: the pollutant concentration in the BMP influent, given as default highway runoff concentra- tions unless modified by the user. • Treated effluent concentration: the pollutant concentra- tion in the BMP effluent calculated using influent/effluent performance curves. • Whole effluent concentration: the pollutant concentration for the total discharge to the receiving water body, calcu- lated by dividing the total discharge load by the total dis- charge volume.

54 The data and methods used to calculate these concen- trations are provided in Appendix E: BMP Evaluation Tool Modeling Methodology. 6.1.6 Tool Supporting Data The tool provides underlying supporting data used to pro- duce the hydrologic and water quality estimates. For example, nomographs that summarize the long-term continuous sim- ulation model results specific to the user-selected rain gage are provided. These nomographs could be used outside of the tool for additional BMP sizing and assessment purposes. Appendix E: BMP Evaluation Tool Modeling Methodology provides information on viewing supporting data. 6.2 Worked Example of Tool This section provides a worked example of the Bioreten- tion tool, using the Marquam Bridge in Portland, Oregon, as an example. The purpose of this example is to demonstrate the tool input requirements. A comprehensive worked exam- ple, using the BMP assessment procedure outlined in this guide, is provided in Appendix D. The Marquam Bridge is a double-deck, steel truss cantile- ver bridge across the Willamette River that was designed by the Oregon Department of Transportation (ODOT) and was open to traffic in 1966. Figure 6-3 is a picture of the bridge under construction in 1964. 6.2.1 Project Locations and Climate Selection The project information was first entered into the tool on the first worksheet as shown in Figure 6-4. For this worked exam- ple, a portion of the I-5 eastbound entrance ramp and bridge deck of the Marquam Bridge was routed to a bioretention basin adjacent to the entrance ramp for treatment (Figure 6-5). The Portland International Airport rain gage was selected for this project, which has an 85th percentile, 24-hr storm Figure 6-3. Marquam Bridge under construction in 1964. Figure 6-4. Entering project information into the tool. depth of approximately 0.63 inches and 36.7 inches of aver- age annual precipitation (Figure 6-6). 6.2.2 Project Options In the “Project Options” worksheet, under “Pollutant Loads,” highway runoff concentrations were left at their default values. For “Cost Inputs,” the only change that was made for this project was to change the “Local Sales Tax” value to zero because the state of Oregon does not have sales tax. No edits were made to capital or maintenance cost inputs (Figure 6-7). 6.2.3 Project Design In the project design worksheet, the following information for the project was used: • Tributary Area = 32 ft wide × 2,050 ft long = 65,600 ft2 = 1.5 ac • Impervious Area = 100%

55 Figure 6-5. Marquam Bridge drainage routing for worked example. Figure 6-6. Entering rain gage information into the tool. • Maximum Bioretention Basin Footprint = 80 ft x 360 ft = 28,800 ft2 • Soil Type (Hydrologic Soil Group) for Bioretention Basin area = Sandy Clay Loam (C); assumption based on NCRS Web Soil Service data indicating soils as “50A-urban land, 0 to 3% slopes” Because it was assumed that the soils underlying the bio- retention basin were C soils, an underdrain was used for this bioretention design. Additionally, because footprint area was available for the bioretention basin, a shallower ponding depth was chosen and a higher length-to-width ratio of the basin was chosen to fit with the linear nature of the basin area. It was desired that a minimum 6-inch under- drain would be fully embedded in the stone reservoir layer. Based on these design considerations, the following changes were made to the default design parameter information in the tool: • Ponding depth = 0.5 feet • Stone reservoir thickness = 1.5 feet • BMP length/width ratio = 4

56 The storage volume sizing for the bioretention basins was designed to meet the City of Portland’s stormwater regulations of 90% average annual runoff volume capture. To accomplish this, the Goal Seek function under Data → What-If Analy- sis → Goal Seek was used. In the Results Summary Report worksheet, cell C45 (Percent of Baseline Runoff Volume, % for BMP Captured) was selected set to a value of 0.9 (for 90%— this was entered as 0.901 for this example to ensure the full capture volume) in the Goal Seek function by changing cell B31 (Storage Volume) in the “Project Design” worksheet. For the assumptions indicated above, the storage volume required was 2,081 cubic feet, which resulted in a total footprint of 1,610 square feet (approximately 6% of the maximum avail- able footprint area). Figure 6-8 and Figure 6-9 show the pri- mary and additional design parameters. 6.2.4 Results The following sample results were taken from the “Results Summary Report” Volumes Figure 6-10 shows the following volume results: Figure 6-7. Entering project options into the tool. Figure 6-8. Entering primary bioretention design parameters into the tool.

57 • Total volume reduction = 23% (ET plus infiltration) • Runoff bypassed = 10% (BMP captured= 90%) • BMP effluent = 67% Pollutant Loads and Concentrations • Copper: annual load reduction = 65%; total discharge = 0.141 lbs/yr; cost per lb removed = $13,657; treated efflu- ent concentration = 15.7 µg/L • TP: annual load reduction = 23%; total discharge = 3.28 lbs/yr; cost per lb removed = $3,676; treated effluent concentration = 0.44 mg/L • TSS: annual load reduction = 82%; total discharge = 241 lbs/yr; cost per lb removed = $3.26; treated effluent concentration = 16.91 mg/L Whole Life Costs Costs are summarized in Figure 6-11. Figure 6-9. Entering additional bioretention design parameters into the tool. Figure 6-10. Volume results from tool.

58 6.3 Tool Customization The tool has been designed to be customizable to allow for overwriting of much of the default data so users can use the best available project information for their sites. It is recog- nized that customization will allow for each DOT to input information based on localized rainfall statistics and water quality data, as well as BMP construction and maintenance specifications, practices, and costs. Default data that is user editable includes precipitation information (85th percentile storm event depth and annual average rainfall depth), pollutant concentrations, BMP design parameters, and cost inputs. It is recommended that, for design purposes, local precipitation gage and site-specific informa- tion be used to increase the accuracy of volume and pollutant loading results. Editable cost inputs include the following: • Location adjustment factor for unit costs • Expected level of maintenance • Design life (the expected lifespan in years) • Discount rate • Inflation rate • Percent local sales tax • Capital cost quantities and unit costs, including the addi- tion of a bridge deck conveyance system capital cost from the separate conveyance system cost spreadsheet • Maintenance frequency, hours, labor crew size, labor rates, machinery rates, and incidental costs 6.4 Tool Intended Uses The tool treatment performance results together with the whole life cost estimates are intended to provide DOTs with planning level information useful for evaluating receiving water protection benefits and the magnitude of costs asso- ciated with BMP installation efforts. This type of feedback can have a number of potential applications in BMP selec- tion and design for various direct and indirect uses that are described in the following sections. 6.4.1 Direct Tool Uses Evaluate volume and pollutant load reduction in com- parison to baseline conditions and/or performance targets/ standards. The tool can be used to estimate the volume and pollutant load reduction (i.e., percent reduction of runoff volume and loads compared to the baseline condition with- out controls) for a wide range of potential BMP configura- tions. The results from the tool can also be compared directly to project goals or regulatory requirements such as TMDL implementation plans or volume reduction goals. Design parameters can be adjusted in the Tool to improve BMP per- formance and meet project goals. Quickly compare several BMPs for a given drainage area. Once project location and tributary area have been established, the tools can be used to evaluate different BMP types, configurations, performance, and costs to provide an understanding of the varying sizing and pollutant removal capabilities of the BMP types and to aid in choosing the most appropriate, cost-effective BMP for a given site. Evaluate performance relationships and sensitivities of design parameters. The tool provides the ability to adjust design parameters and obtain near-immediate estimates of long-term performance (i.e., without requiring delay required to setup and run a continuous simulation model). This func- tionality can be used to evaluate performance relationships and sensitivities as well as understand how changing design parameters affect project costs. For example, the water quality Figure 6-11. Whole life cost results from tool.

59 benefits of increasing BMP sizing to provide 90% average annual runoff capture instead of 80% can be compared along- side the BMP costs to assess if there is a proportional benefit to increasing the average annual runoff capture. Additionally, BMP sizing can be adjusted to assess the volume and pollut- ants being captured and treated by the BMP versus the volume and pollutants that bypass or overflow the BMP. 6.4.2 Indirect Tool Uses Aid in development of stormwater programs. The tool can be used to identify and establish needs and resources as part of DOT stormwater program development including, for example, BMP land requirements, BMP costs per drain- age area to meet local regulatory requirements, and mainte- nance requirements and costs. The ability to customize input in the tool allows for easy year-to-year changes such as infla- tion and tax increases. Quantify local precipitation statistics. The tool contains the results of an analysis of 347 precipitation gages across the conterminous United States. Key precipitation statistics, including the 85th percentile and 95th percentile, 24-hour precipitations depths and average annual precipitation depths are provided after the user selects the gage that best represents the project. These statistics can be useful as part of design development. Establish planning-level sizing targets. At the start of the planning process it may be useful to hold certain parameters fixed and simply vary storage volume or footprint over a repre- sentative range to develop general relationships between BMP size and the expected performance. This can help identify how much space may be needed within a site to achieve a certain goal and provide early feedback on what goals are reasonable. The percent capture nomographs can be used to evaluate the BMP sizing impacts of a higher annualized capture volume. Evaluate potential regional variability in performance associated with a given design standard. By holding all other parameters fixed and changing the project location attributes, the user can quickly determine how much variability would be expected in performance as a function of project location if a uniform design standard were to be adopted across an entire jurisdiction (e.g., a single design storm depth across a state).