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C H A P T E R 3 Findings and Applications Summary of Findings The purpose of this project was to develop guidelines for transportation agencies on implementing volume reduction practices that are specific to limited access urban highways and specific to the conditions encountered as part of each project. As such, it was not the intent of this project to reach categorical conclusions about volume reduction or define a single preferred approach for achieving volume reduction. Rather, this project focused on developing a process, with supporting information and user tools, to assist highway professionals with identifying, evaluating, and designing feasible solutions for runoff volume management based on site-specific factors. The following paragraphs summarize key findings of this research and provide reference to how these findings were incorporated into the Guidance Manual. These summaries are not intended to be comprehensive; full detail is provided in the Guidance Manual. Regulatory Conditions There are various current mandates and emerging trends for volume reduction of stormwater runoff from urban highways, including: ⢠Total Maximum Daily Loads (TMDLs) ⢠Municipal separated storm sewer system (MS4) permits ⢠The Endangered Species Act ⢠Moving Ahead for Progress in the 21st Century Act ⢠Local flood control and/or channel protection requirements ⢠Water conservations and groundwater augmentation initiatives Additionally, there are a variety of other design considerations that apply in the urban highway environment, including: ⢠Highway geometric design standards ⢠Vegetation and landscaping standards ⢠Highway safety standards and volume reduction ⢠Highway drainage and flood management The combination of regulatory requirements that apply to a given project will vary by state, locality, and receiving water. More information and guidance on regulatory conditions is provided in Section 1.2 and 3.1 of the Guidance Manual. 18
Key Factors for Volume Reduction Volume reduction of highway runoff refers to reduction of the amount of highway and right-of-way runoff that discharges directly either via overland flow or channelized flow or subsurface pipe flow to streams, rivers, lakes, the ocean or other water bodies. Therefore, to achieve volume reduction, water must be discharged to other hydrologic pathways. These pathways include: ⢠Evapotranspiration of water from the vegetation canopy or the ground surface (evaporation) and/or through the respiration cycle of plants (transpiration). ⢠Infiltration into the ground surface, followed by transpiration via plant uptake, return flow into streams, and/or percolation to deeper groundwater. ⢠Harvest and use of stormwater to meet demand for non-potable water on-site or in nearby locations. ⢠Controlled discharge of treated stormwater at rates that mimic baseflow recession characteristics of local streams (referred to as âhydrologically-referenced dischargeâ). The effectiveness of a VRA for achieving volume reduction is primarily a function of (1) the capacity of the VRA to capture and store stormwater runoff and (2) the ability of the volume reduction processes associated with the VRA (i.e., infiltration, ET, and/or harvest and use) to recover the storage capacity of the VRA during and between storm events. In addition to these fundamental performance factors, the effectiveness, feasibility, and desirability of volume reduction is influenced by both site characteristics and project characteristics. Site characteristics influencing volume reduction effectiveness, feasibility, and desirability include: local climate, soil and geologic characteristics and conditions, groundwater conditions, topography and existing site grading, watershed characteristics, project location in watershed, and adjacent land uses. Project characteristics influencing volume reduction effectiveness, feasibility, and desirability include: project type, highway type, amount of open space in medians and shoulders, shoulder width and usage, interchange spacing and type, proposed grading and drainage, highway landscaping/ vegetation, and maintenance access. Section 3.2 of the Guidance Manual provides more information about these factors. Section 3.4 provides guidance for conducting site characterization efforts related to these factors. Physical Characteristics of the Highway Environment Urban highways vary significantly in their attributes and physical settings relative to achieving volume reduction. For the purpose of identifying key physical attributes, the Guidance Manual categorizes urban highways into eight (8) different representative highway types based upon geometric design variations typical of urban freeway design. 1. Ground-level highway segments 2. Ground-level highway segments with restricted cross-sections 3. Highway segments on steep transverse slopes 4. Depressed highway segments 5. Elevated highway segments constructed on embankments 6. Elevated highway segments constructed on viaducts 7. Linear interchanges 8. Looped interchanges 19
The definition of these highway types for the Guidance Manual was intended to help the user identify the constraints and opportunities for volume reduction that are specific to the standard highway types that are most similar to the conditions on a given project. More information about these highway types and their attributes in relation to volume reduction is provided in Section 3.3 of the Guidance Manual. Site Assessment Activities to Support Volume Reduction Planning and Design Site conditions have an important influence on the amount of volume reduction that may be achievable as well as the types and locations of VRAs that may be applicable. Assessing the potential of a site for the implementation of VRAs requires the review of existing information and may include the collection of site-specific measurements. A fundamental purpose of the Guidance Manual is to provide users with guidance on what information to collect and how to interpret this information. This information is critical for developing a site-specific approach for volume reduction, or determining and demonstrating when volume reduction is not appropriate. Site assessment efforts should ideally be initiated early in the planning and design process so that VRAs can be incorporated into the project layout as it is developed. To improve the efficiency of site assessment, efforts should be phased as appropriate, with consideration of the information needed to inform decisions at each point in the project. Types of investigation methods tend to be different for early planning efforts compared to more detailed design efforts. Planning phase activities are typically intended to provide an overall characterization of volume reduction potential for the project as well as to prioritize areas where volume reduction may be feasible and effective. The role of design level investigations are to refine the results of planning level investigations, where necessary, to provide more robust information regarding the feasibility and desirability of a VRA in a given as-designed location, as well as support design-level parameters such as the long term reliable design infiltration rate. Section 3.4 of the Guidance Manual provides guidance for conducting site assessments to support volume reduction planning and design. This section includes guidance on phasing of site assessment activities as well as guidance on the following site assessment topics: ⢠Topography and drainage patterns ⢠Off-site drainage and adjacent land uses ⢠Soil and geologic conditions ⢠Local weather patterns ⢠Groundwater considerations ⢠Geotechnical considerations ⢠Existing utilities ⢠Harvested water demand ⢠Responsible agencies and other stakeholders ⢠Local ordinances ⢠Watershed-based and other joint planning opportunities Additionally, Appendix C, D, and E provide more detailed topic-specific guidance on infiltration rate evaluation and factors of safety (Appendix C), water balance and groundwater quality issues (Appendix D), and geotechnical issues (Appendix E). 20
Menu of Volume Reduction Approaches Section 4.1 of the Guidance Manual provides a summary of existing stormwater control measures (SCMs) in use in the highway environment, including an evaluation of which of these SCMs provide significant volume reduction potential. Additionally, Section 4.1 reviews new and emerging concepts in volume reduction. Based on this review, a primary menu of VRAs was developed and described in Chapter 4.2 of the Guidance Manual, including: ⢠VRA 01 Vegetated Conveyance ⢠VRA 02 Dispersion ⢠VRA 03 Media Filter Drain ⢠VRA 04 Permeable Shoulders with Stone Reservoirs ⢠VRA 05 Bioretention without Underdrains ⢠VRA 06 Bioretention with Underdrains ⢠VRA 07 Infiltration Trench ⢠VRA 08 Infiltration Basin ⢠VRA 09 Infiltration Gallery Other potential volume reduction concepts and approaches were also identified and described in Section 4.2 of the Guidance Manual, including: ⢠Harvest and use and land application ⢠Incidental volume reduction in other SCMs ⢠Real-time control of outlets for enhanced volume reduction performance and/or performance monitoring ⢠Hydrologically-referenced discharge to mimic natural hydrology Finally, site planning approaches to reduce runoff volume were identified. These include approaches intended to reduce the amount of runoff that occurs, as well as approaches intended to complement the primary menu of VRAs (above) by creating opportunities to site these VRAs within projects. Site planning approaches described in Section 4.2 of the Guidance Manual include: ⢠Early identification of VRA opportunity locations ⢠Develop drainage, grading, and utility configurations to accommodate VRA opportunity locations ⢠Limit footprint of disturbance ⢠Minimize non-essential impervious surface ⢠Conserve and/or amend topsoil Comparison of VRA Attributes Each VRA has a distinct set of attributes and considerations that are inherent in its design and function. Fact sheets provided in Attachment A of the Guidance Manual provide an extended summary of each of the primary VRAs. Section 4.3 of the Guidance Manual provides summaries of key attributes of each primary VRA to facilitate comparison between VRAs as well as to serve as a concise reference. Section 4.3 provides guidance regarding: ⢠Volume reduction mechanisms and potential water balance issues by VRA ⢠Geometric siting opportunities and footprint requirements by VRA ⢠Relative potential geotechnical impacts by VRA 21
⢠Relative potential risk of groundwater quality impacts by VRA ⢠Safety considerations by VRA ⢠Maintenance activities by VRA Additionally, Section 4.4 of the Guidance Manual provides additional references for VRA design and maintenance information, including selected nationwide guidance and selected state-specific DOT guidance. Framework for Selecting and Applying VRAs The Guidance Manual (Section 5.1) describes an overall framework for volume reduction planning and conceptual design. This framework was developed with the intent of assimilating a number of factors, including (1) project goals, (2) site and watershed information, and (3) the available menu of VRAs, to yield a preferred plan for achieving volume reduction. The outcome of this framework is intended to yield a volume reduction plan that meets the following goals: ⢠Is applicable to the project type ⢠Is feasible and desirable given site and watershed conditions ⢠Utilizes VRAs that are compatible with the project site as well as the future redevelopment projections for the project area ⢠Can be reliably and safely operated and maintained over the long term ⢠Is consistent with project economic constraints ⢠Meets project volume reduction goals In general, the process of developing a volume reduction plan can be considered in three phases: ⢠Initial screening to identify potential VRAs that are potentially applicable, feasible and desirable (Section 5.2) ⢠Prioritization of these VRAs on a relative basis (Section 5.3) ⢠Conceptual design evaluation relative to project goals and constraints (Section 5.4) VRA Screening (Applicability, Feasibility, and Desirability) The Guidance Manual (Section 5.2) describes a tiered evaluation process for identifying potential VRAs for a project. This process involves screening to evaluate applicability, feasibility, and desirability. In general, applicability, feasibility, and desirability can be assessed by asking four fundamental questions: ⢠Is a certain VRA applicable to the project? For example, determining the applicability of a VRA can include geometric requirements for VRAs, availability of space, presence of a storm drain system, presence of demand for harvested water, and other factors. ⢠Is it physically possible to implement a certain VRA based on the site conditions? For example, do soil or geologic (i.e., bedrock, etc.) conditions render infiltration rates negligible? Does the site layout present no opportunity for a specific type of VRA? ⢠Would the use of a certain VRA have the potential to result in undesirable physical consequences on the project or the site environs? For example, would the use of a VRA pose an unacceptable elevated risk of groundwater contamination or movement of a plume? Or 22
would infiltration in excess of natural conditions potentially cause geotechnical issues or downgradient habitat concerns? ⢠Does the cost required to construct the VRA and/or mitigate potential risks posed by the VRA outweigh the volume control benefits it would achieve? For example, it may be physically possible to infiltrate water into clay soils at some small level and possible to mitigate soil stability issues associated with infiltration, however the resulting benefit may not warrant the added project expense of additional area consumed and/or costs. Section 5.2 of the Guidance Manual provides guidance for evaluating and documenting the answers to each of these questions. Assimilate Screening Results to Identify Potentially Suitable VRAs Based on the results the initial screening of VRAs (Section 5.2), the Guidance Manual provides guidance for selecting VRAs that are compatible with the feasibility and desirability screening conditions that are identified. The feasibility of infiltration was used as the primary basis for determining which types of VRAs may be applicable, including classifying infiltration conditions into the following general management categories: ⢠Full Infiltration Feasible: Select and design VRAs with emphasis on providing reliable infiltration. ⢠Partial Infiltration Feasible, volume reduction supported by processes other than infiltration: Select and design VRAs to promote allowable level of infiltration and maximize ET. ⢠Limited or No Infiltration Feasible, volume reduction achieved primarily through other processes: Select VRAs to limit infiltration and provide ET, harvesting, and/or treated baseflow mimicking discharge, as applicable. Further discussion of these categories of screening conditions is provided in Section 5.2 of the Guidance Manual with associated recommended VRA types for each category. VRA Prioritization After identification of potentially feasible and applicable VRAs, the Guidance Manual describes a systematic approach to prioritization of VRAs. This process generally consists of weighting and scoring VRAs based on the following factors: ⢠Relative whole life cycle costs ⢠Relative O&M impacts to agencies ⢠Relative reliability ⢠Relative safety ⢠Potential performance relative to volume reduction goals Guidance for prioritizing VRAs based on each of these factors is provided in Section 5.3 of the Guidance Manual VRA Conceptual Design Volume reduction performance is a function of many factors, including site and watershed conditions, climate patterns, VRA sizing and design parameters, and other factors. Similarly, costs are site-specific as 23
a function of project type (i.e., new vs. redevelopment vs. retrofit), existing infrastructure in place, material costs, construction methods and other factors. As such, a conceptual design level analysis is necessary to provide a reliable evaluation of performance and cost in comparison to other VRAs and in comparison to project goals and constraints. The conceptual design development process may be an iterative process that evaluates a range of scenarios and adapts these scenarios iteratively to identify the conceptual design that best balances volume reduction goals with site and cost constraints. Section 5.4 of the Guidance Manual provides guidance for conceptual; design development, consisting of the following general steps: ⢠Developing initial conceptual designs ⢠Using modeling tools (e.g., the Volume Performance Tool) for decision support and conceptual design adaptation ⢠Estimating whole lifecycle costs of conceptual designs ⢠Adapting conceptual designs to converge with project goals and constraints Watershed-scale Approaches A final section of the Guidance Manual (Section 5.5) was prepared to provide users with an introduction to watershed approaches for achieving volume reduction goals for urban highway runoff. This section was also intended to help identify when a watershed approach may be more appropriate than SCMs or VRAs located within the project site (referred to as âon-siteâ VRAs or SCMs). Alternatives introduced in Section 5.5 include: ⢠Watershed-scale management of project runoff â VRA or SCM located outside of the project boundary, but receiving runoff from the project and potentially additional areas; project runoff is managed before discharge to a receiving water. ⢠In-kind management of other highway runoff â VRAs or SCMs installed to manage runoff from nearby section of roadway, but outside of project site; does not manage runoff from the project. ⢠In-kind management of non-highway runoff â VRAs or SCMs installed to manage runoff from nearby non-roadway land uses, outside of project site; does not manage runoff from the project. ⢠Out of kind mitigation banking - A general category used to refer to other approaches for achieving equivalent watershed protection benefits compared to on-site VRAs or SCEM, but possibly using different approaches/mechanisms. Intended Applications Incorporating VRAs into the highway project development process often add complexity and introduces additional feasibility and desirability considerations when compared to a standard highway stormwater management design. Highway professionals require practical information that will help them identify, evaluate, and design feasible solutions for runoff volume management. The Guidance Manual is intended to provide a process and supporting information to identify controls that are effective, reliable, and applicable for the site, as well as provide a clear technical basis as to why some or all volume reduction controls will not meet these criteria. The Guidance Manual is intended to be immediately applicable to practice, for use by DOTs and other highway environmental professionals. The Guidance Manual is intended to be used by a range of user types, for different purposes in the planning and design process for new projects, lane addition projects, and retrofit projects. At the early project planning level and program management level, the Guidance Manual can be used to help facilitate 24
a mutual understanding amongst the design team regarding volume reduction goals and the way that volume reduction considerations will be incorporated into the project development process. Similarly, the Guidance Manual can be used to help scope the additional or alternative analyses that may be needed in the design process as part of achieving volume reduction or demonstrating its infeasibility. At later stages of planning, the Guidance Manual is intended to assist in identifying potentially feasible VRAs and conducting early site investigations to identify project-specific opportunities and constraints to allow for refined estimates of achievable levels of volume reduction. Finally, the Guidance Manual is intended to support designers with site-specific approaches to prioritize, select, evaluate and apply VRAs. The Guidance Manual may also serve as a resource for permit writers and compliance staff when considering the level of volume reduction that may be achievable in the urban highway environment and potential adverse impacts associated with volume reduction designs. While the Guidance Manual provides detailed and methodical guidance for selecting and implementing VRAs, supplemented with checklists and schematics consistent with a âconceptual designâ level of detail, it does not provide criteria for detailed design of specific volume reduction facilities. Additionally, the Guidance Manual considers operations and maintenance activities and costs as key considerations in the feasibility and prioritization of VRAs, but it does not provide detailed guidance for operations and maintenance. The Guidance Manual provides references to other documents that provide more information in these areas. More information about the intended applications of the Guidance Manual can be found in Section 1.3 and Chapter 2 of the Guidance Manual. 25
