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A U T H O R AC K N O W L E D G M E N T S The research reported herein was performed under NCHRP Project 25-41 by Geosyntec Consultants, with assistance from The Low Impact Development Center and Venner Consulting. Primary authors of this report were: Eric Strecker, Principal Investigator Aaron Poresky Geosyntec Consultants Portland, OR Neil Weinstein The Low Impact Development Center Beltsville, Maryland Marie Venner Venner Consulting Littleton, CO The authors would like to recognize the contributions of the following additional authors: Geosyntec Consultants Adam Questad Andrea Braga Jane Soule Lucas Nguyen Raina Dwivedi Renee Bordeau Robert Roseen Ron Johnson Venkat Gummadi The Low Impact Development Center Emily Ayers Lilantha Tennekoon iii
A B S T R A C T This report and the accompanying Guidance Manual summarize the research and resulting guidance developed for achieving surface runoff volume reduction of highway runoff in urban areas. Literature review, synthesis, and focused new analyses were conducted to inform and develop guidance for identifying, evaluating, selecting, and applying volume reduction approaches (VRAs). A stepwise approach was developed and included in the Guidance Manual that guides the user through the evaluation and selection of VRAs. Regulatory and physical considerations were evaluated related to volume reduction in the urban highway environment, and key factors influencing the ability to achieve volume reduction were identified. Recommendations for project- specific site assessment efforts to support volume reduction planning and design were researched and included. Nine primary VRAs were identified that are specifically applicable to the urban highway environment, as well as accompanying site planning and evaluation approaches. A framework for evaluating the applicability, feasibility, and desirability of these VRAs based on site-specific factors was developed. Processes were also developed for prioritizing VRAs and developing conceptual designs. In support of the Guidance Manual, a spreadsheet based Volume Performance Tool was developed to provide planning level estimates the performance of VRAs, and four white papers on key technical topics were developed. The findings of this research suggest that site-specific conditions are critical in determining the applicability, feasibility, desirability, and effectiveness of VRAs. Additionally, maintaining VRAs is critical for long term effectiveness. iv
Summary DOTs are facing increasing requirements to reduce the volume of stormwater runoff from highways. However, implementing stormwater volume reduction approaches (VRAs) in highly constrained urban environments presents a number of challenges and constraints due to the limited space and appropriate soils typically available in the right location in the right-of-way for infiltration, evapotranspiration, on-site use, and/or flow control. Additional constraints include road safety requirements, geotechnical and structural concerns associated with saturated soils in the highway environment, costs to construct VRAs, cost and ability to maintain VRAs, water balance issues and other factors. Finally, the effectiveness of VRAs is an important factor in determining whether costs of VRAs justify the benefit of reducing runoff volume. The purpose of this project was to develop guidance for transportation agencies on implementing surface runoff volume reduction practices that are specific to limited access urban highways and specific to the conditions encountered as part of each project. The primary result of this project was the development of the Guidance for Achieving Volume Reduction of Highway Runoff in Urban Areas (Guidance Manual). The intended audience of the Guidance Manual included project designers, project planners, program managers and regulators. The Guidance Manual includes text, as well as flow charts, worksheets, and other user tools to facilitate its implementation. It is accompanied by a number of technical appendices as well as a Volume Performance Tool in Microsoft Excel. The Guidance Manual was developed based on literature review, synthesis of available information, and focused technical analysis, including consideration of both established and innovative approaches for achieving volume reduction. One of the overarching findings of this research was that achieving volume reduction of urban highway runoff is dependent on many site-specific factors and considerations. The overwhelming importance of these factors means that it was not possible to reach categorical conclusions about the feasibility, effectiveness, and/or cost of achieving volume reduction that are applicable across site and watershed conditions, projects types, design goals and various other site-specific factors. Project planning and design decisions must be supported by careful evaluation of site-specific factors. As such, the primary outcome of this project was the development of an evaluation process with supporting references (as provided in the Guidance Manual and appendices) by which practitioners can identify, evaluate, and design feasible solutions for runoff volume reduction based on project-specific conditions. This process is presented in the Guidance Manual as a five-step process, including: Step 1 â Establish Volume Reduction Goals. Step 2 â Characterize Project Site and Watershed. Step 3 â Identify Potentially Suitable VRAs â Preliminary Screening and Site Planning. Step 4 â Prioritize VRAs. Step 5 - Select VRAs and Develop Conceptual Designs. 1
For each step in this process, the Guidance Manual provides extensive supporting information as well as user tools (e.g., worksheets, flow charts, the Volume Performance Tool). Highlights of the contents of the Guidance Manual include: ⢠Background on regulatory requirements for volume reduction as well as the benefits and limitations to a volume reduction strategy. ⢠Unit process-based description and understanding of volume reduction processes, including the factors that make VRAs more or less effective. ⢠Guidance for conducting site assessment activities in support of volume reduction planning and design, including guidance on how to phase site assessment activities to improve the efficiency of project development. ⢠Detailed descriptions of nine (9) primary VRAs, including supporting fact sheets for each, as well as site planning approaches to facilitate volume reduction. ⢠A comprehensive framework (with accompanying worksheets and flowcharts) for evaluating the applicability, feasibility, and desirability of VRAs to help identify VRAs that are compatible with project and watershed conditions. ⢠A semi-quantitative approach for prioritizing VRAs based on relative life cycle costs, relative operations and maintenance (O&M) impacts to agencies, relative reliability, relative safety, and potential performance relative to volume reduction goals. ⢠Guidance for developing and analyzing conceptual designs, including design schematics, a whole lifecycle cost framework, the Volume Performance Tool, and guidance for adapting project plans to help improve effectiveness and/or reduce cost. ⢠Four (4) White Papers on key technical topics related to volume reduction including 1) infiltration testing, 2) groundwater issues, 3) geotechnical issues, and 4) permeable pavements. The contents of the Guidance Manual are also intended to support users who require âa la carteâ information on specific technical topics related to volume reduction planning or design -- the Guidance Manual represents a rigorous compilation and synthesis of technical information on the key information that is important for volume reduction. However, the full potential of the Guidance Manual can be realized by using it as part of a systematic approach for achieving volume reduction. By applying a methodical process to volume reduction planning and design (such as described in the Guidance Manual), it may be possible to identify more opportunities to achieve volume reduction while also more carefully identifying conditions in which volume reduction is not appropriate and should be limited or avoided due to potential negative impacts. 2
