Volume Reduction of Highway Runoff in Urban Areas: Final Report and NCHRP Report 802 Appendices C through F (2015)

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C H A P T E R 2 Research Approach To meet the objectives of the project, the research approach was divided into five tasks as introduced below. • Task 1 included characterizing the urban highway environment relative to runoff volume reduction. This characterization included both regulatory and technical considerations. This task also resulted in guidance for practitioners to use as part of characterizing a specific project site. • Task 2 included developing potential VRAs. This task involved review of existing guidance as well as recent literature. • Task 3 included evaluating these VRAs on a number of technical factors to identify a narrower menu of VRAs for inclusion in the Guidance Manual. This task also included development of metrics, criteria, and tools for practitioners to use in evaluating VRAs on a project-specific basis. • Task 4 included focused technical analyses of several key areas that the research team identified as being important as part of achieving volume reduction in the urban highway environment. The outcome of this task was four technical white papers which were included as appendices to the Guidance Manual. • Task 5 included compiling the results of Task 1 through 4 into a functional Guidance Manual. This included developing a stepwise structure to the Guidance Manual and integrating technical components into this structure in a usable and logical format. The sections below summarize the efforts that were involved in completing each task. Task 1: Characterize the Urban Highway Environment Relative to Runoff Volume Reduction The objective of this task was to define the scope of the Guidance Manual and to develop a baseline characterization of the urban highway environment for use throughout the development of the Guidance Manual. The research team reviewed literature, regulations, and other technical documentation to develop a summary of the various motivations for reducing surface runoff volumes, underlying concepts, and constraints related to volume reduction strategies in the urban highway environment. Based on this information, the research team developed the scope of the Guidance Manual and prepared an annotated outline for Project Panel review. The information developed in this task was then summarized in the Guidance Manual with the intent of providing background and introduction for users by introducing key concepts and vocabulary. The overall goals of this task were to: • Understand the regulatory and design considerations that exist in the urban highway environment as related to surface runoff volume reduction, 5

• Characterize the key types of urban highway projects and conditions that the Guidance Manual must support, • Identify the key physical parameters that are most influential in volume reduction effectiveness and feasibility, and • Identify site and watershed information and assessment needs for project planners and designers seeking to apply VRAs. Subtask 1.1: Summarize the Regulatory Context Stormwater quality regulations were identified as the main motivations for the use of stormwater VRAs. It is critical to consider that there are many other regulatory and design requirements that influence the design and construction of roadways including safety, flood management, construction site stormwater quality, and others. It was important to understand how these various considerations interact within an urban highway project to understand the context for selecting and applying VRAs. This subtask sought to characterize the general regulatory context within which the Guidance Manual is intended to be applied. The research team reviewed and summarized stormwater management regulations that directly or indirectly mandate the consideration and use of stormwater volume reduction, including current regulations and evolving regulatory trends that could potentially affect DOTs in the future. Examples include: • EPA goals and directions related to potential changes to NPDES discharge permits based on National Academy of Sciences (NAS) recommendations • Section 438 of the Energy Independence and Security Act (EISA) (not applicable to urban highways, but indicative of general trends) • Localized MS4 Stormwater Permits that require volume control to be prioritized (CA, DC, etc.) • Total Maximum Daily Loading Limits (TMDLs) that may incentivize volume reduction or use volume as a surrogate for pollutant loads The research team relied, in part, on work conducted as part of NCHRP 25-25 (Cost and Benefit of Transportation Specific MS4 and Construction Permitting) for this evaluation, as well as review of more recent regulatory information. The research team also reviewed and summarized the other regulatory considerations and design requirements that exist within the highway project development process, including highway design requirements for safety, flood control, construction site stormwater quality and others, as applicable. Subtask 1.2: Characterize Urban Highway Project Attributes, Physical Setting, and Key Constraints The types of urban highway projects that are supported by the Guidance Manual include a wide range of attributes and physical settings. For example, projects may range from new roadway construction near the urban fringe, to the addition of a new HOV lane in a depressed ultra-urban freeway section, to the construction of a freeway intersection flyover segment. The purpose of this task was to characterize the potential range of project types so that the Guidance Manual could be developed to be applicable across these ranges. 6

In addition, this task identified the key constraints for volume reduction that are expected to be encountered in the types of urban highway projects that the Guidance Manual supports. The research team reviewed and summarized the challenges for implementing volume reduction, including, but not limited to: • Physical Limitations: Constrained urban highway settings by definition have limited surface area within the right-of-way (ROW) for siting above ground storage and infiltration or use facilities. Moreover, due to the linear nature of highways, storage and infiltration or use facilities must be distributed at collection points and located in opportunity areas. • Hydraulic Loadings: Constrained urban highway settings typically have small drainage areas with a high proportion of impervious cover and little vegetated pervious area. This results in comparatively large runoff volumes compared to the opportunities that exist to mitigate runoff volume. • Feasibility of Infiltration: Infiltration practices in constrained urban highway settings and dense urban areas may be restricted by limited surface area, prohibitive infiltration rates of compacted soils, geotechnical concerns for the protection of the roadway subgrade, and/or a greater likelihood of utility conflicts and contaminated soils and/or groundwater. • Opportunities for On-site Use: Limited pervious/landscaped areas within urban highway ROW generally provide few opportunities for on-site usage of harvested stormwater; watershed- based use approaches beyond the ROW will typically need to be evaluated for potential usage of harvested water if harvest and use VRAs are employed for highway runoff. • Construction and Maintenance Costs and Practicality: Space constraints, unsuitable soils, and utility conflicts may each limit the suite of viable VRAs and/or increase costs. Additionally, the costs and operational burdens of maintaining VRAs and the ability to safely access facilities for maintenance are important considerations. • Opportunities: The unique characteristics of urban highway design and construction may provide enhanced opportunities for volume reduction if well-suited VRAs are selected. In some cases VRAs may be the most cost-effective solution. Specific opportunities for volume reduction present in the urban highway environment were introduced. Subtask 1.3: Identify Key Factors and Opportunities in Design and Application of Stormwater Volume Reduction As part of this task, the research team developed a brief technical explanation of volume reduction processes to establish the technical foundation for subsequent analyses and provide educational material for the end user. This explanation included descriptions of key factors in volume reduction performance, feasibility, and cost including: • The ability to capture and store runoff. • The ability to recover storage capacity between storms through infiltration, evapotranspiration (ET), or treatment and utilization for other uses such as irrigation and non-potable supply. • Site-specific design factors such as local climatic characteristics, watershed characteristics, geotechnical conditions, adjacent land uses, and highway designs. • Ratios between areas generating runoff and areas available to manage runoff. • Cost elements that are specific to VRAs. • Opportunities to enhance volume reduction cost effectiveness and practicability. 7

The purpose of this subtask was to develop information to educate users on basic principles of volume reduction and what levels of reduction are potentially reasonable and not. Additionally, the identification of key parameters was used to identify site and watershed assessment needs, as well as to formulate and communicate the key research needs to be addressed in subsequent detailed analyses (Task 4). References to Final Work Products The results of Task 1 formed the basis for the following sections of the Guidance Manual: • The results of Task 1.1 are provided in Chapter 1.2 and 3.1 of the Guidance Manual. • The results of Task 1.2 and Task 1.3 are provided Section 3.2 through 3.4 of the Guidance Manual. Task 2: Develop Potential Volume Reduction Approaches The objectives of this task were (1) to provide a summary of the state of the practice of volume reduction, (2) to produce a focused menu of practicable approaches specific to volume reduction in the urban highway environment, (3) to define approaches in sufficient detail to distinguish key design parameters and applicability. The results of this task formed the basis for a number of sections and an appendix of the Guidance Manual. Subtask 2.1: Literature Review and Data Compilation The research team reviewed and summarized recent domestic and international literature on runoff VRAs, with a focus on applicability to the urban highway environment. The literature survey included journals, conference proceedings, and research reports from academic institutions, as well as state and international DOTs. The focus of the literature evaluation included: • Feasibility and Design Factors: Compilation and synthesis of information and data related to feasibility and design factors, including groundwater quality protection, geotechnical hazards, soil infiltration rates and factors of safety, identifying and quantifying use demands and ET rates, and sizing and designing storage and distribution facilities. • Effectiveness Data and Factors: Compilation and synthesis of information and data related to effectiveness of volume reduction practices, including such factors as climatic patterns, runoff storage and sizing, and pathways and rates of storage recovery (i.e. infiltration, evapotranspiration, harvest and use, and release at “base flow” rates). • Construction and Maintenance Considerations: Compilation and synthesis of information and data related to construction and maintenance practices, requirements, and potential constraints in urban highway environments. • Cost Data: Compilation and synthesis of available cost information and data, including various cost elements, cost factors, and potential cost reduction strategies. • Definition of Volume Reduction Metrics: Definition of volume reduction and the metrics used to quantify volume reduction based on review of recent literature. The intent of this subtask was to provide a summary of the state of the practice, provide the basis for developing the menu of approaches, and identify key areas where additional research was needed. 8

Subtask 2.2: Develop a Preliminary Menu of Volume Reduction Approaches and Evaluation Methods The research team compiled a preliminary inventory of conventional and innovative VRAs based on results of Subtask 2.1. From this inventory, the team worked with the Project Panel to select the most promising nine (9) approaches for further evaluation and inclusion in the Guidance Manual. For each selected VRA the research team developed a description of the system components, the key processes and design parameters, the applicability to urban highway environments, and the applicability across climate zones and watershed characteristics. The approaches and strategies that were selected include above and below ground storage facilities, above and below ground infiltration practices and facilities, associated stormwater treatment practices and facilities, and stormwater conveyance and dispersal options. A fact sheet was prepared for each of the selected VRAs. Through the consideration of urban highway characteristics and construction practices, the research team identified geometries and configurations associated with each of the VRAs that are compatible with the urban highway environment, as well as design adaptations to help improve volume reduction performance. Additionally, the research team developed recommendations for project layout (i.e., site design) to enhance opportunities for VRAs. References to Final Work Products The results of Task 2 formed the basis for the following sections of the Guidance Manual: • Section 4.1 of the Guidance Manual reports the findings from the research conducted to identify the potential menu of VRAs. • Section 4.2 of the Guidance Manual identifies the menu of VRAs that were selected for inclusion in the Guidance Manual. • Appendix A of the Guidance Manual provides fact sheets for each of the selected VRAs. Task 3: Evaluate Volume Reduction Approaches The outcome of this task was the development of guidance for evaluating, comparing, and selecting applicable VRAs, where feasible. This task also resulted in the development of a spreadsheet based Volume Performance Tool for conducting quantitative comparisons between VRAs. This task included synthesis from the results of Task 1 and 2, as well as results of focused technical analysis (Task 4). The outcomes from this task formed the basis for sections of the Guidance Manual intended to provide processes and resources for: (a) selecting the most promising VRAs for the project from the menu of VRAs prepared in Task 2, and (b) comparing and prioritizing VRAs as well as the spreadsheet tool and appendix. Subtask 3.1: Develop Evaluation and Selection Criteria; Develop Selection and Feasibility Matrices The research team compiled and organized feasibility criteria, costs, operations and maintenance requirements, volume reduction performance and other criteria that can be used to compare the VRAs compiled in Task 2. Based on synthesis of these criteria and factors, a series of selection and evaluation matrices were developed to provide end users with tools (i.e., flow charts, tables, worksheets) to quickly determine if a given VRA is likely to be applicable and effective for their site. This guidance allows the user to take into account site-specific information about project characteristics, physical constraints, watershed characteristics, and climate, in determining the recommended VRAs for further consideration. 9

This guidance was also intended to provide technical basis for where VRAs may not be feasible or appropriate. The selection and evaluation matrices, flowcharts, and worksheets include considerations related to the following: Feasibility criteria - The research team investigated volume reduction implementation feasibility and provided guidance for considering the following factors in determining the applicability, feasibility, and desirability of a VRA for a given site: • Climate and local hydrology • Availability of space and elevation differential • Site soil infiltrative capacity limitations • Potential for utility conflicts and impacts to existing infrastructure • Potential impacts to ground water and • Various other potential feasibility criteria. The outcome of this investigation was the development of a process and associated criteria for evaluating the applicability, feasibility, and desirability of a given VRA. Additionally, matrices were developed to provide a relative comparison of the metrics to inform this process. Matrices included a comparison of volume reduction processes, geometric siting considerations, potential geotechnical impacts, potential water groundwater impacts, and safety considerations for the selected VRAs. Relative costs - The research team reviewed literature to develop relative cost comparisons for the selected VRAs, including relative costs for construction, operations and maintenance, and replacement/restoration. Relative costs were summarized for the cost of VRA construction as part of a larger construction project, as well as costs of VRA construction as retrofit projects where no other construction activities are being conducted. Retrofit costs tend to be higher in most cases. Costs are highly site-specific, therefore guidance was also developed for preparing site-specific whole lifecycle cost estimates. Cost reductions and synergies to be had by using green infrastructure for stormwater conveyance were considered and the cost assessment framework provided guidance for accounting for “avoided costs” in comparing different project scenarios. In other words, guidance was provided for accounting for the avoided cost of grey infrastructure in the determining the true incremental cost of VRAs. Operation and maintenance – The research team conducted a literature review and conducted informal interviews with DOT representatives to summarize the maintenance requirements for each of the selected VRAs. Operations and maintenance activities were identified for each VRA in a matrix format, including both routine maintenance activities and corrective maintenance activities. VRA-specific maintenance considerations and requirements were also summarized in each VRA fact sheet (Appendix A of the Guidance Manual). Performance – The research team investigated and compiled volume reduction performance factors for each of the VRAs. These factors were used to evaluate the applicability of each type of VRA for different site and watershed conditions. The results of this compilation were expressed in terms of the space requirements and infiltration conditions necessary to support each VRA type. The Volume Performance Tool (described in Task 3.4) was used as the quantitative basis for evaluating potential levels of expected performance for numerous different geographic and site conditions and the sensitivity of key performance factors. 10

Subtask 3.2: Summarize Feasibility-related Research Gaps and Identify Focused Analysis Topics As an outcome of the previous tasks, the research team identified areas that were considered to be significant research gaps and/or opportunities to consolidate available research to be applicable to the urban highway environment. Such research gaps and/or opportunities were documented and recommended for further study when encountered. Those topics that required further study and also were of specific importance in evaluating VRAs were identified for focused technical analysis as part of Task 4. Subtask 3.3: Develop Selection and Feasibility Matrices This subtask was originally planned as a separate step from Subtask 3.1, however this effort was combined with Subtask 3.1 to result in selection and feasibility matrices for the selected VRAs (see discussion above). Subtask 3.4: Develop Performance Evaluation Tool The purpose of this subtask was to develop a “Volume Performance Tool” (Tool) for planning level estimation of expected performance of VRAs. As part of developing the Tool, the research team used long term precipitation and potential evapotranspiration data in combination with continuous simulation hydrologic modeling for numerous specific climates and soil conditions to develop a repository of sizing/configuration/site conditions performance relationships and lookup tables (i.e. nomographs). The user interface for the Tool was developed in Microsoft Excel. The intended uses of this Tool are described in Chapter 5 of the Guidance Manual and a concise User’s Guide is provided as Appendix B of the Guidance Manual. Further details regarding the technical development of the Tool are provided in Annex A of this project report. References to Final Work Products The results of Task 3 formed the basis for several sections of the Guidance Manual: • Section 3.4 of the Guidance Manual provides guidance for site assessment activities to support evaluation of feasibility of volume reduction processes. • Section 4.3 of the Guidance Manual provides a summary of VRA attributes for a number of evaluation metrics. The discussions and matrices found in this section are intended to help support feasibility comparisons and prioritization of VRAs. • Chapter 5 of the Guidance Manual presents the overall framework for selecting and applying VRAs, much of which is informed by the results of this Task. Specifically Section 5.2 of the Guidance Manual presents guidance for evaluating feasibility and desirability of VRAs, while Section 5.3 of the Guidance Manual presents guidance for prioritizing VRAs after evaluating feasibility and desirability. Task 4: Conduct Focused Technical Analyses This task included focused analysis and white paper development for key technical topics. The objective of this task was to conduct concise technical analysis and/or synthesize and consolidate existing literature to expand the state of the practice in areas that are important for the practical application of VRAs. The outcomes of this task included findings that are of what we believe to be highly practical 11

relevance to users of the Guidance Manual. These findings were incorporated in the main body of the Guidance Manual, as appropriate, and the more detailed supporting documentation was incorporated into the Guidance Manual as White Papers. An initial list of topics was developed based on areas where: (a) research gaps exist and/or consolidation of literature was needed, and (b) the topic was of specific importance in evaluating and applying VRAs. The list of topics was provided to the Project Panel and was agreed upon as part of an interim meeting with the Project Panel. The selected topics are described in the subtask headings below. The findings of these white papers significantly influenced and informed the content of the Guidance Manual related to site investigation/characterization, feasibility and desirability evaluation, VRA selection, and conceptual design approaches. These white papers are valuable for project evaluations, but are also valuable for larger regulatory development discussions as well as State or Local DOT or MS4 guidance document development. Subtask 4.1: Infiltration Testing and Factors of Safety in Support of the Selection and Design of Volume Reduction Approaches This white paper provides guidance for assessing the infiltration capacity of a given site, including methods and concepts that are applicable at the planning and design phases of the project, as well as guidance on selecting an appropriate factor of safety on measurements (White Paper 1; Appendix C of the Guidance Manual). Subtask 4.2: Potential Impacts of Highway Stormwater Infiltration on Water Balance and Groundwater Quality in Roadway Environments This white paper provides guidance for identifying potential impacts related to water balance and groundwater quality and provides recommendations for project planners and designers with respect to assessing and avoiding and/or mitigating these potential impacts (White Paper 2; Appendix D of the Guidance Manual). Subtask 4.3: Geotechnical Considerations in the Incorporation of Stormwater Infiltration Features in Urban Highway Design This white paper provides guidance to help identify potential geotechnical and pavement impacts of stormwater infiltration, and to help guide the development of geotechnical designs with respect to assessing and/or mitigating these potential impacts (White Paper 3; Appendix E of the Guidance Manual). Subtask 4.4: Review of Applicability of Permeable Pavement in Urban Highway Environments This white paper provides guidance on potential applicability of permeable pavement technologies in the urban highway environment to help owners, project managers, and designers evaluate whether permeable pavements should be considered for a specific project (White Paper 4; Appendix F of the Guidance Manual). 12

References to Final Work Products The results of this task are presented in their entirety as White Papers #1 through 4 (Appendices C through F, respectively). Additionally, the findings of these white papers were incorporated into Guidance Manual recommendations related to site investigation/characterization (Section 3.4), feasibility and desirability evaluation (Section 4.3 and 5.2), VRA prioritization and selection (Section 5.3), and conceptual design approaches (Section 5.4). Task 5: Develop Guidance for Implementing Volume Reduction Approaches The outcome of this task was the completion of the Guidance Manual, including arrangement of the work products from previous tasks into a step-by-step approach for using the Guidance Manual. Subtask 5.1: Compile Guidance and Develop Step-by-Step Approach and Recommended Use Based on the intermediate outcomes of Task 1 through 4, the research team developed a stepwise approach to guide practitioners through the application of the recommendations contained in the Guidance Manual. This approach was discussed at an interim meeting with the Project Panel and general agreement with it was obtained. This approach was then used to structure the Guidance Manual. The stepwise approach itself is described in Chapter 2 of the Guidance Manual. Later chapters refer back to the steps identified in the stepwise approach. Subtask 5.2: Introduce Watershed Scale Approaches Recognizing the limitations of on-site VRAs (i.e., approaches within the project boundary) for many constrained urban highway environments, the research team developed a supplemental section of the Guidance Manual (Section 5.5) to introduce watershed-based alternatives for achieving volume reductions. This section introduces the topic of watershed-based alternatives, provides an inventory of the types of alternatives that may be available, and provides a list of resources related to watershed approaches. This section was informed by the draft findings of NCHRP Project 25-37, Watershed Approaches to Mitigating Stormwater Impacts. Subtask 5.3: Develop Approaches for Enhancing Feasibility of Volume Reduction Approaches Due to the highly constrained nature of the urban highway environment, many VRAs may not be feasible in many cases. The purpose of the Guidance Manual was to identify the conditions that render VRAs infeasible, but also to identify ways in which volume reduction could be feasibly achieved in a broader range of site conditions and project types. As part of this overall research effort, the research team provided various forms of guidance for enhancing the feasibility of volume reduction, including (with reference to Guidance Manual section in parentheses): • Site assessment approaches to help better characterize site conditions and thereby improve the opportunity to identify locations applicable for VRAs (Section 3.4; Appendix C). • Site design approaches to improve space availability and site conditions for achieving volume reduction and/or help mitigate site constraints (Section 4.2). 13

• VRA design adaptations to improve volume reduction performance and/or applicability to the urban highway environment (Section 4.2; Appendix A). • VRA design measures to mitigate or eliminate feasibility constraints (Appendix A, D, E, F). • Other options to improve volume reduction performance and/or reduce VRA cost to help reconcile performance and cost goals as part of conceptual design (Section 5.4.6). Subtask 5.4: “Test Drive” the Guidance and Volume Performance Tool Introduction To obtain feedback from potential users on the Guidance Manual and to facilitate its adoption, the research team conducted “test drives” of the Guidance Manual and the Volume Performance Tool with two state level DOTs. Test drives with Washington State Department of Transportation (WSDOT) and District of Columbia Department of Transportation (DDOT) were conducted on February 19, 2014, and February 27, 2014, respectively. Both test drives included an introductory presentation about the purpose of the Guidance Manual and the Tool and an explanation of their context. This was followed by a demonstration of the Volume Performance Tool for example project scenarios. Finally, the research team facilitated discussions about potential improvements to the Guidance Manual and/or the Tool. Agency staff were given the opportunity to review both elements prior to the test drive and provide their comments during and after the test drives. The following agency staff participated in these test drives: WSDOT Mark Maurer Alex Nguyen Le Nguyen Ebrahim Sahari DDOT Meredith Upchurch Reginald Arno Kyle Ohlson Alit Balk Carmen Franks Key Input and Revisions Resulting from Test Drives The following paragraphs summarize key points of discussion and the resulting changes that were made to the Guidance Manual and Tool. Guidance Manual • Both WSDOT and DDOT have already been implementing volume reduction approaches to some extent and have certain processes in place for making decisions about applying VRAs as part of project design. Therefore certain elements of the Guidance Manual were not as valuable to these agencies as they may be for a DOT that is at an earlier stage in evaluating volume reduction approaches. Specifically, several participants did not find the introduction, stepwise process, and regulatory background to be as useful for them and/or they did not understand the 14

goals of these parts of the Guidance Manual as it applies to their situation. However, they also recognized that other DOTs would potentially benefit from these parts of the Guidance Manual. To address these comments, the research team included additional explanation in the introduction regarding how the Guidance Manual is intended to be used by different audiences, including DOTs that are earlier in the process of evaluating and implementing VRAs (or stormwater management approaches in general), and those that already have an established program. • There was agreement by the test drive participants that the rest of the Guidance Manual (after the introduction, stepwise process, and regulatory background) was valuable regardless of DOT knowledge or presence of an existing program. • The White Papers included in as Appendices were considered to be valuable for topic-specific details and reference. • Overall, there were relatively few comments on the Guidance Manual from the test drive participants. Volume Performance Tool • The Tool was generally well received. Both WSDOT and DDOT personnel felt it was easy to use and quickly provided estimates of the volume reduction provided by a given VRA design. The sensitivity analysis included in the Tool was also considered be valuable. • WSDOT staff were concerned that the level of precision that the Tool allows to be entered for infiltration rate was inconsistent with precision of typical infiltration measurement techniques. The research team agreed that this is the case. It was discussed that that a user should always be aware of the uncertainty in a given input when using any type of model. Discussion of appropriate user supplied inputs to the Tool has been added to the User’s Manual accompanying the Tool. • DDOT’s current regulatory stormwater requirements are to retain the runoff from the 90th percentile, 24-hour storm via infiltration, ET, or harvest and use to the maximum extent practicable. In this local regulatory context, the Tool is not intended to be used as a regulatory compliance tool. In order to serve a regulatory compliance purpose, a tool would need to implement locally-acceptable sizing/design calculations and incorporate local feasibility criteria to demonstrate jurisdiction-specific regulatory compliance. The Tool was developed for a nationwide audience, therefore could not conform to each potential local regulation. Rather the Tool is intended to estimate the average long term volume reduction achieved by a given VRA conceptual design or set of conceptual design alternatives. The research team demonstrated that once design volumes are developed to meet the local regulatory requirements, the Tool can be used to rapidly evaluate the performance of different permutations of VRA conceptual designs that meet local standard and/or evaluate sensitivity of these parameters. DOTs that have sizing requirements based on a given level of long term capture (for example, 80 percent average annual capture efficiency) would potentially be able to use the Tool directly for sizing. • It was discussed that one potential use of the Tool was to determine the benefit a VRA can provide toward achieving TDML waste load allocations (WLAs). Follow up questions asked where these calculations were found with this Tool. It has been clarified that the Tool does not perform water quality calculations, however if data are available to describe the average runoff quality from the catchment, this can be used along with the volume of runoff reduced to determine the pollutant load reduced via volume reduction. For additional information on water 15

quality calculations, the resulting guidance and tools from NCHRP Project 25-40 provide more information and water quality calculations. • The Tool currently accepts inputs in the form of the design volume, flowrate, or tributary area ratio and translates this input (with the user-defined geometric assumptions specific to a VRA) into a VRA footprint area. Test drive participants suggested that it would be helpful if there was also an option for the Tool to allow inputs expressed in terms of the VRA footprint. In different applications, the user may have one input or the other. Within the Excel environment, it was not feasible to allow both forms of input within the project scope. However, to address this comment, the research team reorganized the input forms for VRAs so that it is easier to see how the calculated footprint changes as the primary input (design volume, flow rate, or tributary area ratio) changes. This will allow more rapid iteration to adjust VRAs input parameters to determine footprints that fit within the given available space. • Test drive participants suggested that more guidance would be helpful about the range of input parameters that could be safely selected to avoid warnings within the Tool. For example, when a given VRA scenario is outside the bounds of the underlying lookup database, a warning message is returned. To address this, we included a new column with calculations showing the “recommended range” for key input parameters. Selecting a value within these parameters prevents warnings from occurring. These ranges are dynamic and update as other parameter inputs change. • As part of the participants’ beta testing period with the Tool, a few calculation errors were identified. These errors have been corrected. • Some test driver participants wondered whether there was a way to make the Tool fit better on most monitors. In some cases, on smaller monitors, it is necessary to scroll both laterally and vertically to see the full screen while still keeping text size readable. We recognize this limitation, however this comment could not be addressed at this time because it would require changing the underlying structure of the Tool – potentially adding more tabs/steps and/or migrating out of the standard Excel interface. The Tool provides flexibility to change many inputs, provides guidance for these inputs, and provides schematics to help understand VRA conceptual design parameters – i.e., there is a lot of information to display. The user is able to hide the Guidance column of the Project Design tab (using native Excel commands), which helps the form fit into narrower or smaller monitors. • Test drive participants appreciated the guidance that was embedded within the Tool and wanted it to be kept. • Generally, the test drive participants thought the cells that contain calculations should be locked to prevent accidental overwriting. Summary In general the test drives achieved their goals of receiving feedback that we believe helped make the Guidance Manual and the Tool more effective. It was not possible to accommodate all requests; however, substantial improvements were made as a result of suggestions from test drive participants. References to Final Work Products The completion of Task 5 resulted in the preparation of: • Preliminary Draft Final Guidance as a stand-alone document. 16

• Preliminary Draft Final Report documenting the entire research approach that includes an executive summary of the research results. • Revised Final Guidance following one round of Panel comments; including response to comments. • Final Revised Report following one round of Panel comments; including response to comments. 17