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11 C H A P T E R 3 This chapter provides a baseline characterization of the urban highway environment as that relates to achieving surface runoff volume reduction. It is intended to orient the user to the regu latory context for achieving volume reduction, the basic concepts central to volume reduction, and the general considerations that exist for applying volume reduction approaches within the urban highway context. It also provides guidance for site investigations recommended to support volume reduction design. This chapter supports Step 1âEstablish Volume Reduction Goals and Step 2âCharacterize Project Site and Watershed as identified in Section 2.1. 3.1 Regulatory Context This section introduces the current regulatory requirements for reduction of urban storm water runoff, including those that are applicable to highways and other land uses. Trends in regu lations and potential regulations that may apply to highways in the future are introduced. This section also introduces other regulations and design principles that are important in highway project development and that may influence the application of volume reduction approaches. This section will help the user answer the following questions: ⢠Why is volume reduction being considered for urban highway runoff? ⢠What are the benefits of volume reduction? ⢠What volume reduction mandates are currently in place or may be applicable for roadways in the future? ⢠How do safety, roadway design (e.g., geometrics, subbase), and flood control regulations (e.g., detention) interact with stormwater volume management on an urban highway project? ⢠How do overlapping design goals affect the application of volume reduction approaches? 3.1.1 Current Volume Reduction Mandates and Trends in Stormwater Management Regulations Benefits of Volume Reduction in Stormwater Management Urban Stormwater Management in the United States (National Academy of Sciences, 2008) identified a number of recommendations for improving stormwater management approaches and regulations. Among these recommendations is a greater emphasis on hydrology (e.g., runoff volumes, flow rates) in managing stormwater runoff. Reducing runoff volumes has a number of key benefits as part of a stormwater management approach, including: ⢠Reducing the loading of pollutants to receiving waters. Volume reduction is among the most effective treatment mechanisms for removing pollutants (Strecker et al., 2005; Oregon State University et al., 2006). In comparison to treatÂandÂrelease systems, volume reduction tends to achieve more complete removal of pollutants. Volume Reduction in the Urban Highway Environment
12 Volume Reduction of Highway Runoff in Urban Areas ⢠Reducing the potential for channel erosion. Volume reduction reduces the cumulative energy of stormwater discharged to stream channels, which can reduce the potential for channel ero sion. While channel erosion can also be controlled via careful flow control (with or without volume reduction), the use of volume reduction can result in a better match to the total flow volume than a stream received from the project site in the natural condition and, therefore, better mimic natural habitat and sediment transport regimes (U.S. EPA, 2009; Santa Clara Valley Urban Runoff Pollution Prevention Program, 2005). ⢠Augmenting water supply and base flow. Infiltration of stormwater can potentially increase groundwater recharge and augment water supply or stream base flow. When a site is developed and water is infiltrated in VRAs, this tends to result in a shift in the water balance involving a reduction in evapotranspiration (ET) and an increase in deeper percolation and groundwater recharge (see Appendix D, published as part of NCHRP Web-Only Document 209). This can be advantageous when an aquifer is used for water supply or contributes to base flow to streams, and the additional volume of recharge would be beneficial. Augmenting base flows can also help address water quality impairments, such as dissolved oxygen or eutrophication problems, by reducing stagnation and increasing flux through the system. ⢠Potentially reducing peak runoff flow rates. When volume reduction approaches are designed for the purpose of controlling the volume or peak flow rate of runoff in severe events, they can be part of an overall peak flow control strategy. These benefits have been well demonstrated in literature and practice. The Clean Water Act and NPDES Permits The major regulatory framework for stormwater management from highways in the urban environment stems from the Clean Water Act (CWA), which was established in 1972 to regulate the discharge of pollutants to the surface waters of the United States. In 1987, the CWA was amended to regulate stormwater discharges as point sources through the implementation of a permitting program using NPDES (U.S. Senate, 2002). Discharges from MS4s are included under the NPDES permit requirements. Municipalities, roadways, educational institutions, and other public works can all be considered MS4s. Phase I of the NPDES implementation covers specified industrial facilities, larger construction sites (greater than 5 acres), and MS4s that serve populations of 100,000 or greater. Phase II of NPDES implementation covers smaller MS4s and construction sites that are at least 1 acre in size. The permitting of DOTs differs by states, as discussed in the following and also as discussed in detail in Cost and Benefit of Transportation Specific MS4 and Construction Permitting (Austin, 2010). MS4 Permits and DOTs DOT projects are different from typical municipal and private projects in a number of important ways. First, they are typically characterized by their elongated linear nature, high degree of imper viousness, and tendency to cross multiple waterways, watersheds, and jurisdictions. Consequently, stormwater runoff from transportation projects typically discharges to a larger number of distrib uted points than typical construction projects in other land uses and may need to be managed at a greater number of locations along the length of the roadway. Second, in urban environments, there are typically impervious surfaces directly adjacent to highways, such as buildings, walkways, and local roads, which can produce runoff that drains directly onto or into the highway rightÂofÂway (ROW). Highway permitting and project development must take into consideration how these flows are handled. Third, stormwater runoff from highways includes pollutants specific to transportation land uses and tends to have characteristics that differ significantly from the runoff from other or mixed land uses. Differences in stormwater characteristics may influence the selection of treatment processes. Fourth, projects in transportation corridors typically face many constraints for storm water management, such as limited flexibility in geometric design, safety considerations, and space constraints due to longÂreinforced design standards.
Volume Reduction in the Urban Highway Environment 13 Because of these factors, it is common for states to address the permitting of DOTs differently than other permittees and to include requirements in DOT permits that differ from permits for other entities. The treatment of DOTs under the NPDES system by each state can generally be classified into six categories (Austin, 2010): 1. DOTs that are covered by a statewide Phase II MS4 general permit (permittees may include the entire state DOT, individual DOT districts, or DOTs within a specific region). 2. DOTs that have specific individual permits (including combination MS4Âconstruction general permits). 3. DOT districts that are permitted individually within the state under various permit types. 4. DOTs that are coÂpermitted with surrounding Phase I and/or Phase II areas. 5. DOTs that have a combination of permit types. 6. DOTs that are not currently covered by an NPDES permit. For the most part, individual states are responsible for writing both individual and general permits, with the exceptions of Alaska, Idaho, Massachusetts, New Hampshire, New Mexico, the District of Columbia, tribal lands, and U.S. territories, where the U.S. EPA is the responsible party. TMDLs and DOTs Section 303(d) of the CWA requires that states identify âimpairedâ waters that fail to meet their designated uses related to habitat and recreational activities. Specific plans to improve the water quality in these waters are required, including the determination of TMDLs of specific pollutants that can be discharged to a receiving water (U.S. Senate, 2002). U.S. EPA regulations require that a TMDL include WLAs, which identify the portion of the loading capacity allocated to individual existing and future point source(s). In some cases, WLAs may cover more than one discharger. As permittees that discharge to water bodies with TMDLs, DOTs are typically assigned WLAs. WLAs are typically incorporated into DOT permits, which may mandate stormwater retrofits to meet the assigned WLA. Currently, TMDL implementation is the primary regulatory driver for retrofit of urban highways. In the future, MS4 permits may require retrofit of best management practices (BMPs) into highways as part of a longÂterm plan for implementation. Volume reduction approaches can be important elements of a DOTâs strategy for meeting WLAs. Reducing the runoff volume from existing roadways via VRA retrofits can reduce pollut ant loads to TMDL water bodies and also reduce the potential for inÂstream sources of pollution, such as by erosion. Other Water Quality Regulations Potentially Applicable to State DOTs Other sections of the CWA that may pertain to stormwater management in the urban roadway environment are Section 401, which requires that construction projects comply with state water quality standards and other provisions, and Section 404, which requires mitigation of wetlands damaged by discharge or fill materials associated with construction activities. The 401/404 per mitting process may provide a pathway for the federal government or states to issue additional requirements on DOT projects (U.S. Senate, 2002). Construction general permits issued at the statewide level are primarily intended to regulate construction site stormwater runoff but may also provide a pathway for states to impose postÂconstruction (i.e., permanent) stormwater control requirements on projects. For example, the California Construction General Permit (Order No. 2009Â0009ÂDWQ) includes postÂconstruction requirements for projects to mimic the predevelopment water balance of the site (California State Water Resources Control Board, 2009). This permit requires projects to retain stormwater up to the 85thÂpercentile, 24Âhour precipitation event unless proven infeasible. These postÂconstruction (i.e., permanent) stormwater control requirements apply to development projects that would
14 Volume Reduction of Highway Runoff in Urban Areas otherwise not be required to install permanent BMPs as part of the Phase I or Phase II MS4 permitting program. Trends Toward Volume Control in MS4 Permits Traditionally, the CWA has not been interpreted to regulate stormwater runoff volumes; how ever, trends in MS4 permitting are moving toward incorporating runoff volume control into permit requirements to a greater degree. Channel protection standards are evolving from solely focusing on peak runoff rates to standards that call for more closely mimicking predevelop ment hydrology in terms of both peak runoff rates and the total stormwater volume discharged or flowÂduration requirements for certain portions of the flow regimes. Likewise, criteria for the selection of BMPs for water quality control are shifting toward a preference for BMPs that provide volume reduction versus those that primarily address pollutant concentrations through treatment and release. In 2010, the U.S. EPA developed the MS4 Permit Improvement Guide (U.S. EPA, 2010) to pro vide suggestions to states and municipalities for ways to strengthen the effectiveness of MS4 per mit requirements. It recommends establishing stormwater management performance standards that emphasize the use of stormwater controls that infiltrate, evapotranspire, and/or harvest stormwater in order to minimize the volume of stormwater discharged. It also suggests adopting requirements that explicitly address the modification of hydrologic cycles that occur when a site is developed through maintaining or restoring the predevelopment hydrology. The MS4 Permit Improvement Guide also acknowledges Urban Stormwater Management in the United States, which recommends an emphasis on hydrology. The approaches described in the Permit Improvement Guide have been implemented in numerous areas. An example is the North Orange County MS4 permit issued by the Santa Ana Regional Water Quality Control Board (Order No. R8Â2009Â0030) in 2009. This permit required âpriority development projectsâ to âretainâ stormwater onÂsite (with no surface discharge) using infiltration, evapotranspiration, and/or rainwater harvesting BMPs to the maximum extent practicable (MEP) based on a ârigorousâ feasibility analysis (California Regional Water Quality Control Board, 2009). Similar requirements are found in MS4 permits across California as well as in other regions. For example, the District of Columbia was issued an MS4 permit in 2011 that required onÂsite retention of 1.2 in. of stormwater from a 24Âhour storm event. The retention standard may be achieved using a combination of infiltration, evapotranspiration, and storm water harvesting (U.S. EPA, 2011). Volume control measures have already been incorporated into a number of NPDES permits for state DOTs. For example, in 2012, the California Department of Transportation (Caltrans) was issued a renewed statewide NPDES permit, which requires the selection and sizing of BMPs to retain the 85thÂpercentile, 24Âhour storm event. BMPs that incorporate infiltration, capture and use, or evapotranspiration of runoff are given preference (California State Water Resources Control Board, 2012). Similarly, the District of Columbia permit mentioned previously encom passes road projects sponsored by the District of Columbia Department of Transportation (DDOT) and applies the same 1.2Âin. retention standard to these projects. Trends Toward Volume Control in TMDLs Historically, TMDLs have been developed for specific pollutants or pollutant groups. However, due to the extreme variability in source characterization, pollutant type, storm water loadings, and the increasingly intensive land use seen across the country, it is difficult to establish TMDLs for many pollutants individually. In 2010, the U.S. EPA MS4 Permit Improvement Guide adopted guidance that recommends the use of surrogate parameters, such as volumetric stormwater flows, for evaluating TMDLs rather than specific pollutant
Volume Reduction in the Urban Highway Environment 15 discharge limits. This practice has already been implemented in U.S. EPA Regions 1 and 3, where flow has been used to track sediment loading and has been used in TMDLs as a proxy for pollutant loading (U.S. EPA, 2003). At the time of writing, the future of flowÂbased surrogates in TMDLs was uncertain due to a recent successful court challenge of the Accotink Creek TMDL (Virginia DOT et al., v. U.S. EPA et al., 12ÂCVÂ775, U.S. District Court for Eastern Virginia, 2013). However, volume reduction may be an important option for DOTs in meeting WLAs as part of implementing TMDLs, par ticularly for pollutants that may be more challenging to address with treatÂandÂrelease stormwater management approaches, such as bacteria indicators. The Endangered Species Act and Volume Reduction The Endangered Species Act (ESA) was established in 1973 to protect threatened and endan gered species and their habitats (U.S. Fish and Wildlife Service, 1973). Activities that may adversely affect these species and their habitats, such as stormwater discharges, are restricted under this act. A number of negative impacts associated with stormwater discharges, includ ing erosion, hydromodification, and pollutant loading, can be reduced through implementing volume reduction strategies. While the application of the ESA varies widely depending on loca tion, sensitive species, and project type, it is possible that volume reduction mandates could be imposed as part of complying with the ESA for transportation projects. MAP-21 and Volume Reduction The MAPÂ21 Act was passed in 2012 to reauthorize federally aided highway and highway safety construction programs through a more performanceÂbased framework. One of the performance goals established is environmental sustainability (U.S. DOT, 2012), which requires that metro politan planning organizations develop transportation plans that include provisions for storm water management planning. Some of the approved planning approaches include watershedÂbased management strategies and mitigation banking. Additionally, the MAPÂ21 Act recognizes the role that volume reduction can play in minimizing environmental impacts by encouraging the adoption of permeable, pervious, or porous paving materials or systems designed to reduce environmental impacts, stormwater runoff, flooding, and/or pollutants by allowing the infiltration of stormwater in a manner mimicking predevelopment hydrology. Other Regulatory Trends The Energy Independence and Security Act passed in 2007 includes a provision in Section 438 that states that the âsponsor of any development or redevelopment project involving a Federal facility with a footprint that exceeds 5,000 square feet shall use site planning, design, construction, and maintenance strategies for the property to maintain or restore, to the maxi mum extent technically feasible, the predevelopment hydrology of the property with regard to the temperature, rate, volume, and duration of flowâ (U.S. EPA, 2007a). Although this regulation does not cover highways, this mandate is indicative of the trends toward volume reduction observed in the municipal sector. The U.S. EPAâs Technical Guidance on Imple menting the Stormwater Runoff Requirements for Federal Projects Under Section 438 of the Energy Independence and Security Act (U.S. EPA, 2009) states that stormwater control measures that implement volume reduction, such as harvesting, infiltration, and evapotrans piration, are essential for reducing runoff volumes and pollutants loadings associated with small storms. Also at the federal level, the U.S. EPA initiated a national stormwater rulemaking process in 2009 to establish a program that may require reduction of stormwater discharges from new and redeveloped sites. The new program may regulate reduction in discharges from exist ing developments as well. Some anticipated improvements include developing performance
16 Volume Reduction of Highway Runoff in Urban Areas standards for new development and redevelopment to better address stormwater manage ment in the planning and construction process, expanding the MS4 program to include mini mum requirements and more comprehensive protection for all MS4s, and the establishment of specific requirements for transportation facilities. The schedule for release of draft rules is not known as of this writing. It is possible that performance standards established as part of this rulemaking process will include volume reduction standards either specifically or gener ally applicable to roadway projects. Examples of Other Trends Toward Volume Reduction There are a variety of examples that demonstrate trends toward stormwater volume reduc tion based on drivers other than stormwater quality management. For example, in Tucson, Arizona, a citywide ordinance requires implementation of rainwater harvesting for all new com mercial developments for the explicit purpose of water conservation (Jackson, 2012). Storm water harvesting systems, as well as infiltration of stormwater, are acceptable means of meeting this requirement. More recently, the city began offering rebates to residential customers that implement rainwater harvesting. While these approaches are implemented explicitly for water conservation, they also have benefits for stormwater management. Similar examples of water conservation drivers exist in other states facing issues of water scar city. For example, in Los Angeles County, California, the Council for Watershed Health has led the development of the Los Angeles Basin Water Augmentation Study. This study is a partner ship between local water, public works, and wastewater agencies, the State of California, and the U.S. Bureau of Reclamation to evaluate the capacity and feasibility of stormwater management practices to augment water supplies (Council for Watershed Health, 2013). Flood control goals have also motivated the use of volume reduction practices in some areas, most commonly achieved in large flood control basins that rely on infiltration for all or part of achieving peak flow and volume control. For example, the Los Angeles County Antelope Valley interim drainage criteria stipulate that the proposedÂcondition clear runoff volume match the existingÂcondition clear runoff volume for the 25Âyear storm (Los Angeles County Department of Public Works, 1987). 3.1.2 Other Design Objectives Within the Highway Project Development Process A number of other design objectives and standards govern the development of projects within the urban highway environment; those with the highest significance are highway safety, geomet ric design, drainage, and flood control. In some cases, these design objectives and standards may restrict the type or extent of volume reduction approaches that can feasibly be implemented. In other cases, they may share mutual goals and benefit from the implementation of volume reduction approaches. Highway Safety Standards and Volume Reduction Highway safety laws, which are variable among states, remain a top priority when considering volume reduction practices. Based on the highway type and design speeds, safety regulations specify minimum safety criteria, such as shoulder widths and roadway slopes. Safety standards that are relevant to volume reduction approaches include: ⢠Geometric design standards, ⢠Vegetation and landscaping standards, ⢠Drainage standards. These are considered in greater detail in the following sections.
Volume Reduction in the Urban Highway Environment 17 Highway Geometric Design Standards Highway geometric design refers to the layout of highways, both horizontally and vertically. AASHTO has published its âGreen Bookâ (A Policy on Geometric Design of Highways and Streets) in various forms since the late 1930s, with the most recent edition issued in 2011 (AASHTO, 2011b). The Green Book provides a series of guidelines for geometric design within which the designer is afforded a range of flexibility. In order for the design criteria in the Green Book to become a stan dard, it must be adopted by a particular state (or may be set by court decision). The key requirements for minimum geometric design standards are related to safety (e.g., site distance, stopping distance, designs speed) and serviceability (e.g., land widths, overpass heights). Because volume reduction practices require space and are typically located within the highway rightÂofÂway, geometric design standards are important constraints in the application of vol ume reduction approaches. Geometric design standards can limit the flexibility of the designer in adjusting site designs to accommodate volume reduction. Geometric design standards also limit the features that can be located within the portions of the roadway that may be traversed by errant vehicles. Features associated with VRAs, such as slopes, depressions, inlet and outlet structures, soils with low structural strength, and vegetation, may have safety considerations. Safety considerations of VRAs are further discussed in Chapter 4. Vegetation and Landscaping Standards AASHTOâs Roadside Design Guide (AASHTO, 2011a) and the FHWAâs Vegetation Control for Safety: A Guide for Street and Highway Maintenance Personnel (FHWA, 2008) provide guidance for the types of vegetation that can be used in the road rightÂofÂway. These guid ance documents are incorporated in various ways into state highway regulations. In terms of safety, the proximity of landscaping and vegetation to a roadway can obscure or limit a driverâs view of traffic control devices, other vehicles, wildlife, and pedestrians and bicycles. Larger vegetation, such as trees or hedges, which are often near highway shoulders or inter changes, may become obstacles if not maintained and placed properly. These criteria are particu larly important to consider in evaluating and applying vegetated volume reduction approaches near travel lanes. Drainage and Flood Control Efficient and reliable drainage of stormwater from travel lanes is a critical safety consideration in the design of roadways. FHWA provides guidance for highway drainage design in Highway Hydrology, 2nd Edition, and the Urban Drainage Design Manual, Hydraulic Engineering Circular 22, Third Edition (FHWA, 2002 and FHWA, 2009). State DOTs typically adopt drainage criteria that specify acceptable hydrologic and hydraulic methods and minimum levels of service for travel lanes. The design of volume reduction practices must comply with these regulations and not interfere with the level of service needed for the drainage of travel lanes. State and/or local agencies also typically regulate the rates and/or volumes of stormwater runoff discharging from highways to offÂsite receiving waters or conveyance systems to ensure that the discharge volumes do not cause or contribute to flooding of downstream areas. Typi cally this is accomplished through maintaining runoff volumes or flow rates within an acceptable percentage of predevelopment values for one or a set of design storm events. Additionally, when a highway is located within a federally established base or a 100Âyear floodplain, National Flood Insurance Program regulations will need to be met [Federal Emergency Management Agency (FEMA), 2012]. In general, volume reduction practices that are designed to manage runoff from smaller storm events (i.e., 85th percentile) have relatively little effect on peak flow rates for large, infrequent events; however, they may provide some benefit. Facilities may also be combined to provide volume reduction of smaller storms and peak flow reduction of large, infrequent storms (via detention).
18 Volume Reduction of Highway Runoff in Urban Areas 3.2 Key Technical Considerations in Applying Stormwater Volume Reduction Practices This section is intended to familiarize the user with volume reduction processes and introduce key technical factors in achieving volume reduction. It is intended to help the user answer the following questions: ⢠What is meant by âvolume reductionâ? What metrics are used for assessing performance rela tive to volume reduction? ⢠By what processes is the volume of stormwater runoff reduced? ⢠What are the most important technical factors in achieving volume reduction? ⢠How do the project type and the physical setting of the project influence opportunities and constraints for volume reduction? ⢠What are the obstacles to incorporating volume reduction approaches? 3.2.1 Volume Reduction Metrics In a general sense, âvolume reductionâ refers to reducing the amount of stormwater runoff volume discharged to receiving waters via overland flow or a stormwater conveyance system. When assessing the performance of a certain practice or project in achieving volume reduction, it is necessary to use more specific metrics. Table 1 summarizes various metrics that may be used to describe volume reduction perfor mance as a function of resource protection goals and regulatory requirements. Metric/Description Potential Applicability Percent volume reduction â What is the relative change in long-term surface runoff volume? For example, how much less surface runoff volume does the site produce on a long-term average basis compared to the same site without controls? Compared to the site prior to the project or in the predevelopment condition? Quantify the benefit of VRAs to reduce long⢠⢠⢠⢠⢠⢠⢠⢠- term pollutant loading as part of a TMDL or other opportunistic retrofit action. Demonstrate compliance with a project performance standard based on mimicking predevelopment discharge or managing a certain fraction of runoff (i.e., 80%). Retention design storm â What is the largest storm event that can be retained by the VRA such that it does not cause surface runoff from the site? How does this compare to regulatory design-storm retention requirements? Compliance metric for design-stormâbased retention standards (i.e., 85th-percentile storm based on a measured precipitation record). Note that a full description of the metric should also state the inter-event drawdown time assumed to ensure effectiveness. Simple surrogate for long-term reduction in pollutant loads. Simple surrogate for long-term reduction in stream energy and channel erosion. Frequency of discharge â How does the change in the site and the use of VRAs change the frequency of site surface discharge? For example, how much less frequently does the water discharge from the site compared to the same site without controls? Compared to the site prior to the project or in the predevelopment condition? Evaluate how well VRAs mitigate increases in the frequency of runoff, which is a common impact of development. Flow duration â What is the change in flows and durations of surface runoff? How does the use of VRAs influence the flow rates and durations of flows from the site compared to the same site without controls? Compared to the site prior to the project or in the predevelopment condition? Evaluate how well VRAs address flows that cause stream erosion. Evaluate how VRAs may influence flow- dependent biological processes. Table 1. Summary of volume reduction metrics potentially applicable in the urban highway environment.
Volume Reduction in the Urban Highway Environment 19 3.2.2 Volume Reduction Processes Reduction of highway runoff can be achieved through using practices that incorporate infil tration, ET, and/or beneficial uses of captured runoff (U.S. EPA, 2010). Infiltration and evapo transpiration occur naturally to some degree in most conditions but may occur at different relative magnitudes when reducing runoff volume in the postÂproject condition from those that were present in preÂproject or predevelopment hydrology (see Appendix D). The sections that follow discuss the role that each runoff reduction process plays in predevelopment, natural hydrology, and in reducing runoff volume in postÂproject conditions, both with and without VRAs. Predevelopment Hydrology Predevelopment or natural conditions refer to the undeveloped land left to naturally evolve in a given climate and geologic setting. The major types of natural vegetation in the United States are shrubs, grasslands, softwood forests, and hardwood forests (Kuchler, 1966; U.S. EPA, 2007b). The predevelopment hydrologic cycle is defined by several interrelated fluxes of water (Maidment, 1992). As precipitation falls, it is first subjected to interception by leaves and stems of vegetation, from which the intercepted water either evaporates or falls through to the ground surface. Precipitation reaching the ground surface is divided between infiltration (i.e., move ment of water into the ground surface), surface runoff (i.e., movement over the ground surface), and evaporation. Water that moves into the ground surface is further subdivided between deep percolation to groundwater (i.e., vertical migration through the unsaturated zone of the sub surface soil), wicking and evaporation from the soil surface, uptake and transpiration by plants (i.e., in the root zone), and throughflow or interflow (i.e., lateral migration of water back to the ground surface down gradient from where it entered). The water that percolates to groundwater is divided between that which emerges as base flow in streams and that which flows to an aquifer (i.e., an underground layer of waterÂbearing permeable rock, gravel, sand, silt, or clay). Substan tial transient storage exists in the predevelopment hydrologic cycle in the form of ponded and flowing water on the ground surface, soil moisture in unsaturated soil layers, and groundwater. Figure 3 illustrates the elements of the natural hydrologic cycle. Predevelopment hydrologic response refers to the relative magnitude of the elements of the predevelopment hydrologic cycle. The unique response of each watershed is controlled by com plex interactions between climate, vegetation, soils, topography, and geology. Definitions of pre development hydrology vary both by jurisdiction and location. In addition, some jurisdictions reference the existing conditions, or preÂproject conditions, as the hydrologic baseline rather than predevelopment or natural conditions. General Effects of Land Development on Hydrologic Response Land development activities, including highway construction, tend to result in an increase in the amount of impervious cover, a decrease in vegetative cover, and the compaction of soils (incidental or intentional). Development may also result in the importing of water to the water shed via potable or nonÂpotable municipal supplies. The typical effects of these changes on the hydrologic cycles include: ⢠Reduction in rainfallÂderived infiltrated volume and the corresponding reduction in rainfall derived deep percolations volumes, primarily as a result of the increase in impervious surface and reduction in soil infiltration rates due to compaction; ⢠Reduction in rainfallÂderived evapotranspiration, primarily due to the removal of vegetation, removal of the duff layer (i.e., plant litter or dead plant material such as leaves and bark that have fallen to the ground and partially decayed) and surface interception storage, and the reduction in the storage capacity of the root zone via compaction;
20 Volume Reduction of Highway Runoff in Urban Areas ⢠Increase in surface runoff, as a result of the increase in impervious surfaces and increased degrees of compaction, and also as a natural effect of reductions in the other two key elements of the hydrologic cycle; and ⢠Potential for increases in infiltration and percolation due to the introduction of irrigation water, as well as the potential for increases in dryÂseason ET as a result of a change in vegeta tion type and introduction of irrigation (Maidment, 1992). Similar to the predevelopment hydrologic cycle, the postÂdevelopment hydrologic response to development is also controlled by complex interactions between climate, vegetation, and soils and geology. Effects and Roles of Volume Reduction Processes in Mitigating Changes in Hydrology Each volume reduction process available to VRA planners and designers has specific attributes relative to its effects and roles in mitigating changes in the hydrologic cycle caused by land devel opment. An understanding of these processes helps set reasonable expectations for their level of achievable performance and helps identify potential negative consequences that each could introduce. Evapotranspiration occurs whenever water is present on the surface or in the root zone, at a rate controlled by the climatic conditions, the type of vegetation, and soil moisture condi tions. ET follows seasonal trends, most strongly influenced by temperature but also influenced by wind speed, humidity, solar insolation, and plant life cycle. Rates of ET can range from near Figure 3. Major components of the natural hydrologic cycle. Source: http://snobear.colorado.edu/IntroHydro/geog_hydro.html. Notes: In this manual, throughflow is synonymous with interflow; as shown in this figure evaporation is intended to include evaporation and transpiration, collectively evapotranspiration.
Volume Reduction in the Urban Highway Environment 21 zero during cold, wet weather to more than 10 in. per month in hot, arid parts of the country (Vogel and Sankarasubramanian, 2005); however, even at peak ET rates, the rates of ET tend to be slower than precipitation rates and soil infiltration; at 10 in. per month, this corresponds to approximately 0.3 in. per day, or about 0.01 in. per hour on average. Of the factors that a VRA designer can control, the surface area of the VRA is the most important with respect to ET, followed by the storage provided in the soil and the type of vegetation selected. Limitations exist when trying to match preÂproject ET flux rates when removing, compacting, or covering vegetated and soiled areas. This can result in an increase in recharge volume compared to pre project conditions (discussed in Appendix D). The most effective VRAs for ET are those that cover a large area (e.g., a filter strip on a roadway shoulder) and have significant volumes of soil such that water is held in storage between precipitation events. Infiltration into the soil occurs when water is present on the ground surface and is controlled by the infiltration rates of the soil, the land slope, and the rate of precipitation or melting of frozen precipitation (or inflow to a VRA). Initial infiltration rates into soil tend to taper off as additional water is infiltrated, until a steadyÂstate, saturated infiltration rate is reached (Maidment, 1992). Soils can have saturated infiltration rates ranging from near zero for clays and bedrocks to an excess of 100 in. per hour for coarse sands and gravels (Maidment, 1992; Rawls and Brakensiek, 1983). Percolation below the root zone occurs when rates of input exceed the capillary storage of the soils in the root zone and rates of ET over a sufficient period of time such that the mois ture in the root zone exceeds the capillary suction storage (Maidment, 1992). In the natural hydrologic condition, peak infiltration capacities of porous soils are rarely approached because rates and quantities of rainfall are limited and a large percentage of precipitation is stored in the root zone. However, in the developed condition, VRAs typically receive runoff from areas much larger than their own footprint, which has the effect of applying higher flow rates and vol umes of water over the area of infiltration, thereby approaching the infiltration capacity of the underlying soils more frequently and for longer durations. The effect is that infiltrationÂbased VRAs, such as infiltration basins, infiltration trenches, bioretention/rain gardens, and pervious pavements, have the potential to effectively mitigate increases in surface runoff volume within a relatively small footprint. Actually, in many cases, because of the reduction in ET surface area of a site, they often tend to result in an increase in percolated volume when compared to natural or preÂproject conditions. Increased infiltration over natural conditions may be advantageous for groundwater replenishment or, in some cases, may be detrimental (discussed in greater detail in Appendix D). Harvest and use is a nonÂnatural process in which stormwater is captured, held, and used for beneficial purposes, such as irrigation and nonÂpotable water supply. The application of harvest and use for reducing stormwater runoff volumes is an emerging practice that has been applied in certain project types where stormwater can be used for irrigation, flushing of toilets, vehicle washing, cooling tower makeÂup water, or other uses. The rate of demand for harvested water is analogous to ET rates and infiltration rates in other systems. The ultimate fate of water that is used for beneficial purposes depends on the useâfor example, water that is used for irrigation becomes ET or infiltration, and water that is used to flush toilets is conveyed to a wastewater treatment plant or septic system. Hydrologically referenced discharge refers to the controlled release of stored water in such a way that flow rates and timing of discharge mimic natural surface hydrologic response. Research has suggested that controlled discharges from certain stormwater control measures may mimic the receding limb of the natural streamÂflow hydrographs in some conditions (DeBusk et al., 2011). The implication of this finding is that the definition of what is âretainedâ may not be limited to only water that is discharged to deeper infiltration, ET, or harvest and use. Where VRA
22 Volume Reduction of Highway Runoff in Urban Areas discharges are released to surface waters at rates that mimic natural baseÂflow rates following storm events, some or all of this treated discharge may also be considered as retained or reduced rather than as a direct surface discharge. This may have significant implications on the practicabil ity of volume reduction in constrained environments, where means for draining BMPs between storms would otherwise be very limited or would introduce potential negative consequences, such as overÂinfiltration and the creation of dryÂweather seeps. This concept is discussed further in Chapter 4. A more comprehensive discussion of the VRAs applicable to the urban highway setting and their relative reliance on ET, infiltration, beneficial uses, and hydrologically referenced discharge is presented in Chapter 4. 3.2.3 Physical Setting and Site Design Factors Influencing Volume Reduction Effectiveness, Feasibility, and Desirability The effectiveness, feasibility, and desirability of volume reduction approaches are strongly dependent on the physical setting of the project and the project site design. These key terms are defined in the context of this manual in the following. Effectiveness refers to how well VRAs achieve their overall goal of reducing surface runoff volumes, based on one of more volume reduction metrics. Feasibility refers to whether it is physically and financially practicable to implement a volume reduction approach. Desirability refers to whether the outcome of the approach would be the most advan tageous to address the underlying issues, and whether negative consequences may result that outweigh the volume reduction benefits. An approach could be feasible but not desir able if, for example, a potential impact to other environmental media or infrastructure may result. Physical setting refers to the physical characteristics of the project site, including climate characteristics, topography, soil, groundwater, and watershed properties (as well as the location of the project within the watershed). Project site design refers to the project layout and earthwork, including vertical and horizontal alignment, slopes, location of landscaping relative to travel lanes, and alignment of storm drains. The proposed location and layout of VRAs are integral elements of site design. The following sections introduce the key physical setting and site design factors that influ ence the effectiveness, feasibility, and desirability of volume reduction approaches. Evaluation and feasibility criteria for VRAs are described in greater specificity in Chapter 5. General Factors Influencing Effectiveness of VRAs 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 VRAâs volume reduction processes (i.e., infiltration, ET, and harvest and use) to recover the storage capacity of the VRA during and between storm events. VRA storage capacity. The capacity of a VRA to capture and store runoff is controlled, in part, by the size of the storm that the VRA is designed to address. The volume of storage pro vided in the VRA can include ponded water storage (either above or below ground and surface exposed or in a tank) and the pore capacity of soils or stone reservoirs. The capacity of a VRA to capture runoff is also a function of the ability to convey water to the VRA, which may be limited
Volume Reduction in the Urban Highway Environment 23 by topography or conveyance system design. Clearly, if storage capacity is provided in a location where water cannot be conveyed to it or is much larger than the volume of runoff that could be conveyed to it, then this storage capacity would have more limited effectiveness. Storage recovery rates. The capacity of volume reduction processes to recover the stor age capacity of a VRA is important for achieving longÂterm volume reduction. Systems that drain more quickly tend to allow greater capacity on average for subsequent storm events as well as greater volume reduction during events, which results in a greater fraction of long term runoff volume being retained. The recovery pathways for a particular BMP depend on the facility configuration, siteÂspecific soil conditions, and local climate. For example, the depth of storage in a VRA and the underlying infiltration rate control the time it would take for the VRA to drain completely (i.e., the drawdown time). Drawdown of stored water can be achieved through a combination of infiltration of captured stormwater into subsoil, slow release via an outlet (i.e., underdrains, after treatment in soil media), ET, and beneficial uses of captured water. The minimum storage recovery rate for a VRA should be set to ensure that project goals are met on a longÂterm basis. Therefore, the storage recovery rate is a critical factor in determining whether a VRA is feasible. Where the storage recovery rate is lower than project goals, some level of volume reduction may still be feasible, but it may need to be augmented with other discharge pathways (e.g., treated surface discharge) to provide reliable longÂterm performance, or sig nificant additional storage may need to be provided over and above required sizing. Typically, a drawdown time in the range of 24 to 72 hours is acceptable [Orange County Public Works, 2011; Water Environment Federation (WEF)/ASCE, 1998]. However, the sensitivity of drawdown time on longÂterm volume reduction is strongly dependent on local precipitation patterns. Therefore, drawdown criteria should be established on a locationÂspecific basis to achieve intended design goals. For example, Orange County established a maximum drawdown time of 48 hours for BMPs (Orange County Public Works, 2011). This limit was set based on the results of a con tinuous simulation analysis, which demonstrated that this drawdown time in combination with storage set at the water quality design volume would result in at least 80% capture of average annual runoff volume. However, Orange County also allows longer drawdown times to be used if compensatory increases in storage volume are also provided. Similarly, systems that can drain more quickly may be able to provide less storage volume. Figure 4 provides an example nomograph from the Volume Performance Tool (on the CD ROM that accompanies this report) to illustrate the dual roles of storage volume and drawdown time in longÂterm performance. Various combinations of storage volume and drawdown time can result in the same level of longÂterm volume reduction performance. This nomograph is the result of a large number of continuous simulation runs in the U.S. EPA Storm Water Management Model (SWMM). Figure 5 is also developed from the Volume Performance Tool and illustrates the trend between drawdown time and longÂterm performance for various cities representing distinctly different climates. It can be seen that different sensitivities to drawdown time exist in different regions. Where rainfall tends to have lower intensities and extended events (e.g., Portland, OR and west of the Cascade Range in the Pacific Northwest in general), the sensitivity to drawdown time is high, while in areas where events are typi cal shorter with more space between consecutive events (e.g., Austin, TX), the sensitivity to drawdown time is less. In all cases, the role of drawdown time on longÂterm performance is appreciable and must be considered in feasibility analyses. Chapter 5 describes the Volume Performance Tool, which has been developed to estimate volume reduction effectiveness based on selected siteÂspecific factors. Table 2 and Table 3 pro vide an introduction to how physical setting and project site design influence volume reduction effectiveness.
24 Volume Reduction of Highway Runoff in Urban Areas 40% 45% 50% 55% 60% 65% 70% 75% 80% 85% 90% 95% 100% 0 24 48 72 96 120 144 168 192 216 240 Av er ag e An nu al C ap tu re E ffi ci en cy , % Drawdown Time, hours 215435 - Minneapolis, MN - 0.8" 305811 - New York City, NY - 1.0" 356751 - Portland, OR - 0.63" 410428 - Aus n, TX - 1.2" 45114- Los Angeles - 1.0" COOP ID - City - 85th Percen le Storm* *For each city, the unit storage volume was set to the runoff from the 85th percen le, 24-hour precipita on depth Figure 5. Example chart illustrating the influence of drawdown time on long- term capture efficiency (and volume reduction)âfive cities. Developed from the Volume Performance Tool. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0.00 0.50 1.00 1.50 2.00 2.50 3.00 Av er ag e An nu al C ap tu re E ffi ci en cy , % Compartment Unit Storage Volume, as fracon of runoff volume from 85th percenle, 24-hour storm event, unitless 2 3 6 12 24 48 72 120 180 360 Drawdown Time, hours Figure 4. Example nomograph illustrating the influence of storage volume and drawdown time on long-term capture efficiency (and volume reduction)âexample Portland, ORâCOOP ID: 356751. Developed from the Volume Performance Tool.
Volume Reduction in the Urban Highway Environment 25 Factor Indicators/Metrics Influence on Effectiveness Influence on Feasibility and Desirability Local climate ⢠Average precipitation intensity, depth, duration, and seasonal characteristics ⢠Typical inter-event dry periods ⢠ET rates and patterns of ET relative to precipitation ⢠Freeze/thaw cycles ⢠A given VRA design will achieve better volume reduction in climates with lower intensities and depths of rainfall (holding inter-event times fixed) and where precipitation occurs more evenly throughout the year. ⢠A given VRA design will achieve better volume reduction in climates where average inter-event dry periods are longer (holding intensities and depths of rainfall fixed). ⢠High ET rates (high temperatures, sunny, low humidity) allow for increased reduction via ET, particularly when high ET conditions are present between rainfall events in the wet season. Climates that receive the majority of rainfall when ET is low tend to have lower potential for ET-based volume reduction. ⢠Areas that experience frequent freeze/thaw cycles may present challenges for design of some VRAs. ⢠Roads in cold climates may require additional salting for safety reasons, which may pose an elevated risk of groundwater contamination. ⢠Arid or semi-arid regions that have longer dry periods may not be able to sustain VRAs with certain vegetation or permanent pools. In addition, the tendency for concentrated precipitation periods reduces the effectiveness of harvest-and-use systems. Soil and geologic characteristics and conditions ⢠Soil type ⢠Infiltration rate ⢠Level of compaction ⢠Depth to bedrock ⢠Sandy, loamy soils with good infiltration rates can allow infiltration VRAs to drain more quickly than would tighter soils. ⢠Soils that are uncompacted or lightly compacted tend to have higher infiltration rates than similar soils that are compacted. ⢠Shallow depth to bedrock may result in groundwater mounding that limits the reliable rate of infiltration. ⢠Clay soils tend to have low infiltration rates, and their structural strength tends to be more sensitive to moisture content than sandy soils; the combination of these factors may greatly reduce feasibility of infiltration in clay soils. ⢠Compaction of fine-grained soils may be necessary for structural stability but may greatly reduce the infiltrating capacity of soils. ⢠Expandable and collapsible soils pose challenges for volume reduction. ⢠Sandy or gravelly soils with low organic content and high infiltration rates may pose an elevated risk of groundwater quality impacts. Groundwater conditions ⢠Depth to seasonally high groundwater table ⢠Potential for groundwater mounding ⢠Karst aquifers1 ⢠Shallow groundwater may result in groundwater mounding that limits the reliable rate of infiltration or causes infiltration VRAs not to drain during high groundwater conditions. ⢠Infiltration in karst aquifer regions may lead to the development of sinkholes in VRAs or down gradient. ⢠Shallow groundwater may render infiltration infeasible or undesirable due to potential for groundwater contamination, geotechnical stability issues, or groundwater table impacts. ⢠Karst aquifers provide a direct pathway for groundwater contamination and can result in catastrophic subsidence (sinkholes). Topography ⢠Longitudinal slope ⢠Cross-slopes ⢠Spacing between low points ⢠Highways with longitudinal slopes may provide more opportunity to route stormwater to preferred locations than flat sections; they also may allow fewer and more centralized VRAs. ⢠Spacing of low points may dictate locations and spacing of VRAs. ⢠Highways on steeper cross-slopes tend to have steeper embankments and less space in rights-of- way for VRAs. ⢠Steep slopes may limit the types of vegetated conveyance systems that can be used (higher velocities, more potential for erosion). ⢠Highways on steep cross-slopes tend to have greater quantities of cut and fill and tend to have greater potential for geotechnical issues, including impacts from saturated soils that result in landslides. ⢠Very flat areas may have limited groundwater flow gradient and more potential for mounding. Watershed characteristics, project location in watershed, and adjacent land uses ⢠Watershed topography/ slope ⢠Degree of development of watershed ⢠Upstream drainage area ⢠Proximity to adjacent structures ⢠Off-site drainage to project ⢠Presence of ephemeral streams ⢠Where watershed-scale approaches are available in coordination with adjacent landowners, more effective volume reduction may be achieved than with approaches within the right-of-way (see Section 5.5 for more information). ⢠Off-site drainage into the right-of-way may dilute highway runoff and reduce effectiveness for addressing highway pollutants; however, projects may show net benefits if off-site drainage can be addressed in project VRAs. ⢠If off-site drainage into the right-of-way has high sediment loading, clogging of infiltration VRAs may occur more quickly. ⢠Buildings, utilities, or roadways in close proximity to the right-of-way may pose feasibility constraints for infiltration. ⢠Smaller watersheds with significant development may be more sensitive to potential water balance impacts of VRAs (i.e., if upstream development has already increased percolation compared to natural conditions and additional percolation may cause increases in groundwater elevations or base-flow discharges and resulting impacts on ephemeral streams). ⢠Off-site drainage into the highway from pollutant hot spots, such as industrial land uses, may pose risks to groundwater if water is infiltrated. 1 â Karst aquifers are geological formations that are composed of soluble bedrock that conduct water through larger conduits created by the dissolution of rock, often connecting directly to groundwater reservoirs. They are often characterized by large interconnected caves and can contain sinkholes. Table 2. Influence of physical setting on effectiveness, feasibility, and desirability of volume reduction approaches.
Factor Indicators/Metrics Influence on Effectiveness Influence on Feasibility and Desirability Project type ⢠New project ⢠Modification of existing roadway (e.g., lane addition) ⢠Retrofit project (e.g., rebuilding of existing lanes) ⢠New projects may have more flexibility to allow space for VRAs than modifications or retrofits. ⢠New projects may provide better ability to protect soils from compaction during construction. ⢠Modifications and retrofits provide opportunities to address existing roadway runoff and can result in net reduction in runoff volume compared to existing conditions. ⢠Impacts on existing utilities, structures, and other infrastructure from infiltration may be more difficult to avoid in modification and retrofit projects. ⢠Existing rights-of-way can have greater potential for existing soil or groundwater contamination. ⢠Incremental costs may be lower in new projects and modifications than retrofits, particularly when (1) VRAs include elements that offset traditional design costs (i.e., permeable pavement in place of traditional pavement), or (2) VRA costs are heavily dependent on excavatio n, which can be balanced as part of a new project. Highway type Highway segment type can be used as a composite indicator of many of the other factors listed in this table. See highway-type fact sheets in Section 3.3 for a description of typical highway segment types and key opportunities and constraints specific to eight standard urban highway types. Amount of open space in medians and shoulders ⢠Ratio of tributary area to receiving area ⢠Slope of open space ⢠Future proposed development restrictions (i.e., median reserved for future land additions) ⢠Sediment generation ⢠Higher volume reduction is possible when ratios of VRA area to tributary area are higher. ⢠Shallower slopes tend to provide more opportunity for volume reduction (wider suite of and larger VRAs; slower velocities). ⢠Open space further from travel lanes may allow a broader suite of VRAs to be used; may allow lesser compaction below VRAs. ⢠Reduction in long-term effectiveness if part or all of VRA is removed as part of future development plans. ⢠Sediment loading in VRA tributary area may reduce effectiveness of infiltration VRAs (clogging). ⢠Open space in shoulders and medians may have a dedicated role in errant vehicle recovery; restrictions in usage may apply. ⢠Open space further from travel lanes may allow infiltration with lower risk of geotechnical issues. ⢠In good soil conditions, bioretention and other infiltration-based VRAs have the capability to capture and retain stormwater runoff from areas that are much larger than their respective footprint areas. ⢠VRAs that rely primarily on ET for volume dissipation require large surface areas to maximize the extent of contact with the atmosphere. Shoulder width and usage ⢠Expected traffic on shoulders ⢠Shoulder width ⢠Wide shoulder widths may allow for larger storage reservoirs for permeable shoulders (see Chapter 4). ⢠Lower traffic loading on shoulders may allow for more cost-effective implementation of permeable shoulders (see Chapter 4). Interchange spacing and type ⢠Spacing ⢠Type (diamond, cloverleaf, etc.) ⢠Alignment with outlet points ⢠Intersections may provide large open spaces conducive for infiltration into soils with less compaction. ⢠Intersections may allow for greater ponding depths and a wider range of vegetation selection. ⢠Where water can be effectively routed to intersections, the regional scale of VRAs may allow for more effective maintenance. ⢠New crossings and interchanges may present opportunities to manage existing runoff from existing roadway. ⢠See interchange-type fact sheets in Section 3.3 for key opportunities and constraints specific to diamond and cloverleaf intersections. Proposed grading and drainage ⢠Elevation differentials between travel lanes and adjacent land ⢠Depth of cut and fill ⢠Embankment slopes ⢠Drainage pathways ⢠Storm-drain alignments ⢠The ability to route runoff to shallow vegetated slopes improves volume reduction performance compared to steeper slopes or more concentrated inputs. ⢠Fill areas tend to be highly compacted, which tends to reduce infiltration rates. ⢠Embankment slopes influence the depth of ponding that can be provided; more challenging to provide storage volume. ⢠Infiltration in the vicinity of fill structures and embankments may increase the potential for geotechnical hazards. ⢠Connections to off-site drainage can restrict placement of VRAs. ⢠Grading can present an opportunity for volume reduction in new projects if VRAs are accounted for in the project balance of cut and fill (i.e., negligible incremental cost for excavation). Highway landscaping/ vegetation ⢠Type, location, and density of vegetation and landscaping ⢠Location of soil amendments ⢠The use of vegetation in medians or shoulders can enhance infiltrating capacity, operate as natural pretreatment to VRAs, and increase visual attractiveness. ⢠Vegetation has the potential to interfere with a driverâs line of sight or cause collision hazards; use should be limited by these considerations. ⢠Soil amendments may not be permitted in errant vehicle recover zones if strength of soil would be significantly reduced. Maintenance access ⢠Proximity to travel lanes ⢠Space for parking maintenance vehicles ⢠Slopes leading to VRA areas ⢠Maintenance is critical for long-term effectiveness of VRAs; designing to allow for efficient maintenance tends to improve long-term effectiveness. ⢠Where maintenance would require lane shutdowns, the cost of maintenance can increase significantly. Table 3. Influence of project type and layout on effectiveness, feasibility, and desirability of volume reduction approaches.
Volume Reduction in the Urban Highway Environment 27 General Feasibility and Desirability Factors The feasibility and desirability of VRAs is strongly influenced by siteÂspecific and watershed factors as well as the nature of the specific VRA. In general, feasibility and desirability are assessed by asking three fundamental questions: 1. Is it physically possible to implement a certain VRA based on the site conditions? For example, do soil or geologic (e.g., bedrock) conditions render infiltration rates negligible? Does the site layout present no opportunity for a specific type of VRA? 2. Would the use of a certain VRA have the potential to result in undesirable physical consequences to the project or the site environs? For example, would the use of a VRA pose an unacceptable elevated risk of groundwater contamination? Or would infiltration in excess of natural conditions potentially cause geotechnical issues or downÂgradient habitat concerns? 3. Does the cost required to construct the VRA 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 result ing benefit may not warrant this added project expense of additional area consumed or costs. Chapter 5 provides a framework for addressing these questions. 3.3 Urban Highway Types Urban highways vary greatly in their attributes and physical settings relative to achieving volume reduction. The general implications of physical setting and project layout are intro duced in the previous section. While each urban highway project will have unique attributes, the exercise of classifying highway types into categories can be a useful tool for understanding potential opportunities and constraints in different highway types. This section of the guid ance manual identifies eight different urban highway types and describes the characteristics that are common to each. Overall, this section is intended to help the user answer the follow ing questions: ⢠What is the range of highway project types covered by this guidance manual? ⢠How does highway type influence opportunities and constraints for volume reduction? 3.3.1 Project Attributes and Types This manual categorizes urban highways into eight different representative highway types based on geometric design variations typical of urban freeway design as described in Chap ters 8 and 10 of AASHTOâs A Policy on Geometric Design of Highways and Streets (AASHTO, 2011b). Each of these highway types is intended to encompass a variety of geometric con figurations that are common in urban highway environments and to represent these grouped configurations in a manner that allows for cohesive and effective guidance on the constraints and opportunities for volume control particular to each type. Some highways may contain more than one of the highway types described, in which case guidance found in the fact sheets should be combined. For example, a single highway project may include a groundÂlevel high way segment, a looped interchange, and a depressed highway segment. Guidance from the three fact sheets associated with these highway types could each be used for the respective sections.
28 Volume Reduction of Highway Runoff in Urban Areas Six of the highway types describe linear highway sections, and the remaining two types describe common interchange types. The eight representative highway types are: ⢠GroundÂlevel highway segments ⢠GroundÂlevel highway segments with restricted crossÂsections ⢠Highway segments on steep transverse slopes ⢠Depressed highway segments ⢠Elevated highway segments constructed on embankments ⢠Elevated highway segments constructed on viaducts ⢠Linear interchanges ⢠Looped interchanges The sections that follow include fact sheets that contain the basic geometry, important ele ments of the physical setting, opportunities for stormwater volume reduction, and typical plan views and crossÂsections of each representative highway type. The fact sheets also present the key constraints that may limit the implementation of certain VRA types due to feasibility and desirability factors. For example, many VRAs have space and slope requirements for safety and efficiency purposes that could limit their application in certain highway applications. 3.3.2 Ground-Level Highway Segments GroundÂlevel highway segments are found in both urban and rural settings but are more com mon in rural settings because of lower expense, high design speeds, greater availability of space, fewer conflicts with surface streets (underÂcrossings and overpasses), and less concern about highway noise. Where groundÂlevel freeways are found in urban areas, these types of segments are most often found in suburban areas and the urban fringe. Defining Physical Features ⢠Wide medians and greater outer separations and borders are designed to provide aesthetically pleasing greenbelts and to insulate the freeway from the surrounding areas. ⢠GroundÂlevel highway segments are often slightly elevated above adjacent areas based on drainage and earthwork considerations. ⢠Curbs and concrete barriers are uncommon. ⢠Medians and shoulders are generally constructed with shallow slopes designed to allow for errant vehicle recovery. ⢠Access roads and ramps may run parallel to travel lanes. ⢠Dispersed overland flow to ditches and inlets is common. Key Constraints for VRAs Related to Highway Type ⢠The errant vehicle recovery purpose of medians and shoulders must be maintained after VRAs are installed; this may limit landscape selection, the potential use of compost amendments, and the use of depressedÂbasinâtype VRAs. ⢠Elevation differentials between the roadway and potential VRA locations may be limited with respect to routing of stormwater and design of VRAs. ⢠LongÂterm plans may include subsequent paving of areas that would otherwise be used for VRAs. Key Opportunities for VRAs Related to Highway Type ⢠Shallow shoulder slopes can be constructed to enable dispersion of stormwater. ⢠Long stretches of uninterrupted shoulder and median area provide opportunities for VRAs with linear geometry. ⢠Wide shoulders may allow VRAs to be located well away from travel lanes; a broader suite of VRAs may be used.
Volume Reduction in the Urban Highway Environment 29 ⢠Vegetated conveyance features are common in standard sections, reducing the incremental costs of VRAs. 3.3.3 Ground-Level Highway Segments with Restricted Cross-Sections GroundÂlevel highway segments with restricted crossÂsections are common in the urban highway environment as a result of the common need to provide high traffic capacity within a limited right ofÂway width. RightÂofÂway area is commonly influenced by the limits of existing development or natural topographic features. GroundÂlevel highway segments can evolve to become more con strained as subsequent projects add lanes or other infrastructure within an existing rightÂofÂway. Defining Physical Features ⢠Segment crossÂsections are often entirely paved, including the median. Some pervious area may remain on the shoulders or in the median. Example Plan and Profile Not to Scale
30 Volume Reduction of Highway Runoff in Urban Areas ⢠Curbs commonly collect and convey stormwater runoff to stormÂdrain inlets rather than to open swales. ⢠Frontage roads may be present. ⢠The highway is usually slightly elevated above adjacent ground as dictated by drainage and earthwork considerations. Key Constraints for VRAs Related to Highway Type ⢠Space for vegetated VRAs may be limited. ⢠Access for construction and maintenance activities may be limited or require lanes to be shut down. ⢠Buildings, foundations, retaining walls, or sound walls outside of the rightÂofÂway may be located closer to the travel lanes than in many other highway types. Example Plan and Profile Not to Scale
Volume Reduction in the Urban Highway Environment 31 ⢠Concrete barrier dividers are common. ⢠Where lane addition/expansion projects eliminate preÂexisting stormwater measures, the new projects may need to provide control for the total affected area (existing and proposed). Key Opportunities for VRAs Related to Highway Type ⢠Shoulders or medians may present an opportunity for narrow vegetated VRAs or permeable pavement surfaces. Shoulders and medians are generally located down gradient from roadway travel lanes. ⢠Piped conveyances may allow water to be transported to regional VRA opportunity locations at interchanges with minimal incremental conveyance cost. ⢠The potential for adjacent slope failures or lateral water migration is reduced compared to elevated sections because roadway elevations are roughly atÂgrade. 3.3.4 Highway Segments with Steep Transverse Slopes Highway segments with steep transverse slopes are common where highways traverse hilly or mountainous terrain. This may occur in both urban and rural areas. These types of segments can have various degrees of restriction, as influenced by rightÂofÂway width and the steepness of the crossÂslope. This category is generally reserved for segments with crossÂslopes of greater than about 10%. Segments with shallower crossÂslopes may be better covered by guidance for groundÂlevel segments (Sections 3.3.2 and 3.3.3). Defining Physical Features ⢠CrossÂsections are typically restricted; acceptable horizontal and vertical alignments are gen erally obtained by creating cut slopes and fill slopes. ⢠Because the roadway width has significant implications on the height of the cutÂandÂfill slopes, the widths of shoulders and medians are typically minimized in these types of segments. ⢠Interceptor drains may be installed to limit stormwater flowing along or over the roadway from uphill areas and to limit collected water flowing toward the uphill embankment from crowned or superÂelevated sections. ⢠Where downslopes are not excessively steep, water from the roadway may be allowed to sheet flow. However, drainage must be intercepted before draining to private property. Key Constraints for VRAs Related to Highway Type ⢠Space limitations related to restrictive crossÂsections (Section 3.3.3) and embankment cross sections (Section 3.3.6) are commonly applicable in these segment types. ⢠Space for vegetated VRAs may be limited. ⢠Access for construction and maintenance activities may be limited or require lanes to be closed. ⢠Slope stability and retainingÂwall issues may be of specific concern in this highway segment type. ⢠Much of the roadbed typically exists as compacted fill. Key Opportunities for VRAs Related to Highway Type ⢠Elevation differences associated with the topography may allow water to be piped to regional VRA opportunity locations at interchanges. ⢠Where the crossÂslope is shallow or moderate, there may be opportunity to disperse water to vegetated areas downslope or use vegetated conveyance elements.
32 Volume Reduction of Highway Runoff in Urban Areas 3.3.5 Depressed Highway Segments In a depressed highway segment, the roadway is depressed below adjacent ground levels. Either sloped embankments or vertical retaining walls may be used to tie the cross section into the existing grade. Depressed highway segments are typically found in loca tions where surface streets cross frequently (as overpasses) or where road roadway noise is an issue. Defining Physical Features ⢠Depressed segments are usually found in highly urbanized areas. ⢠Segments can be depressed to varying degrees, typically controlled by minimum vertical clear ance criteria for overpasses (16 ft; FHWA, 2007). ⢠CrossÂsections are often highly restricted; the toe of embankments or retaining walls tends to be located very close to the shoulder. Example Plan and Profile Not to Scale
Volume Reduction in the Urban Highway Environment 33 ⢠Concrete barrier dividers are common. ⢠In less restricted settings, medians and vegetated shoulders may exist. ⢠Ramps are used to connect to surface streets. Key Constraints for VRAs Related to Highway Type ⢠Opportunities for dispersion and vegetated conveyance are limited because the road tends to be the lowest part of the section; storm drains are typically used for conveyance. ⢠Extra precautions are commonly taken in drainage design to remove stormwater from the road surface quickly and efficiently (i.e., tighter spacing of inlets than other types). ⢠Because these segments are commonly found in highly developed areas, the section is usually restricted, and little or no vegetation exists in the shoulder or median. ⢠Access for construction and maintenance activities may be limited and require lanes to be shut down. Example Plan and Profile Not to Scale
34 Volume Reduction of Highway Runoff in Urban Areas Key Opportunities for VRAs Related to Highway Type ⢠Geotechnical considerations associated with infiltration may be mitigated, in part, because the infiltrating surface would be at a lower elevation than slopes and adjacent structures/infrastructure. ⢠Pumping may be required for drainage purposes in cases where runoff cannot be conveyed to an outlet via gravity flow. This may present an enhanced opportunity for routing of storm water to captureÂandÂuse systems. 3.3.6 Elevated Highway Segments on Embankments Elevated highway segments on embankments are found mostly in suburban areas where crossing streets are widely spaced and where grading designs provide adequate material for fill. However, they are also found in many urban areas. The total widths of elevated roadway sections vary considerably; however, the total width required is comparable to the total width needed for depressed highways (AASHTO, 2011b). Example Plan and Profile Not to Scale
Volume Reduction in the Urban Highway Environment 35 Defining Physical Features ⢠The earth embankment is usually of sufficient height to permit intersecting surface roads to pass below, based on minimum vertical clearance criteria (16 ft, FHWA, 2007). ⢠Earthen embankments are typically 6H:1V to 3H:1V (AASHTO, 2011b) and are usually not designed to allow errant vehicle recovery; guardrails are common. ⢠If desired, earthen sloped embankment areas are available for planting of smaller trees, pro vided that lineÂofÂsite criteria are maintained. ⢠Linear ramps or cloverleaf ramps may be used to traverse slopes to connect to surface streets. ⢠Embankments are normally constructed on compacted fill. Key Constraints for VRAs Related to Highway Type ⢠Slope stability and retainingÂwall issues may be of specific concern in this highway segment type. ⢠Where retainingÂwall embankments are used, very limited vegetation may be present. ⢠Embankment slope tends to limit applicable VRAs. ⢠Access for construction and maintenance activities may be limited for VRAs installed on an embankment. Key Opportunities for VRAs Related to Highway Type ⢠If space is available at ground level, infiltrationÂbased VRAs may be practical at locations away from the toe of slope or footing of the foundation. ⢠Where slopes are relatively shallow, dispersion to an amended road shoulder may be possible. ⢠When separate embankments are built for each direction of traffic, opportunities may exist for VRAs in the median. ⢠Geotechnical design may be able to accommodate some infiltration in roadway or embankments. ⢠Open space at interchanges tends to be lower in elevation than the highway surface. 3.3.7 Elevated Highway Segments on Viaducts Elevated highway segments on viaducts (also known as aerial segments) are found primarily in densely developed urban areas where space is limited and significant constraints exist for a surfaceÂlevel highway. Elevated highways on viaducts tend to be very expensive. Aerial segments are commonly found at interchanges as well, and share many characteristics of linear aerial seg ments. Aerial segments present unique considerations for achieving volume reduction. Defining Physical Features ⢠The degree of elevation varies greatly but is not important for volume reduction considerations. ⢠Supporting columns of the viaduct are positioned to provide reasonable clearance on each side and to leave much of the groundÂlevel area free for other urban uses. ⢠Space under the structure may be used for a variety of urban needs, such as surfaceÂstreet traffic, parking, or even buildings and playgrounds. ⢠Where rightÂofÂway widths are highly limited, a twoÂlevel structure may be designed in place of the conventional twoÂway, oneÂlevel structure. ⢠Constraints and opportunities may be more strongly influenced by the conditions that exist below the viaduct. Key Constraints for VRAs Related to Highway Type ⢠There are no opportunities for infiltration in the aerial segment. ⢠Infiltration VRAs in the area below the roadway may pose specific considerations related to geotechnical stability of the viaduct support columns. ⢠Land ownership (if different below the roadway) may present issues that limit volume reduc tion opportunities.
36 Volume Reduction of Highway Runoff in Urban Areas Key Opportunities for VRAs Related to Highway Type ⢠At the ground level, substantial opportunities for volume reduction may be present; highly dependent on groundÂlevel conditions. ⢠Aerial segments may not result in a net addition of imperviousness; may be able to use existing VRAs or coordinate on joint stormwater management projects. ⢠Storage tanks may be incorporated into viaduct design to provide baseÂflowâmimicking flow control, equalization for groundÂlevel VRAs, or storage for direct beneficial uses. ⢠Open space at interchanges tends to be at a lower elevation than the highway surface. 3.3.8 Diamond Interchanges Interchanges between highways or between highways and local roads often vary in cross section and combine a number of the highway types previously discussed. Interchanges present unique considerations for volume reduction design. Diamond interchanges include those where the leg connects one highway to another in a moreÂorÂless linear fashion, creating long, narrow wedges of open space. There are a range of variations on diamond interchanges. Example Plan and Profile Not to Scale
Volume Reduction in the Urban Highway Environment 37 Defining Physical Features ⢠The crossÂsectional geometry of interchanges is highly variable as a function of siteÂspecific factors. ⢠Degree of crossÂsectional restriction varies greatly. ⢠Diamond interchanges often enclose narrow wedgeÂshaped areas of unused space, providing a number of opportunities for stormwater retention. Key Constraints for VRAs Related to Highway Type ⢠Constraints vary throughout an intersection, depending on the type of highway segment, per Sections 3.3.2 through 3.3.7. ⢠Where interchanges connect roadways at very different grades or in tight rightsÂofÂway, veg etated slopes may be relatively steep or may be replaced by vertical retaining walls. ⢠Interchanges for depressed roadway sections tend to be located at a higher elevation than the main travel lanes. Example Plan View Not to Scale
38 Volume Reduction of Highway Runoff in Urban Areas ⢠Access for maintenance may be limited and require lane closures in some cases. These closures may have significant traffic and maintenance labor implications. ⢠At the tip of wedges, plant selection may be limited by lineÂofÂsite and collision considerations. Key Opportunities for VRAs Related to Highway Type ⢠WedgeÂshaped areas may provide substantial open space for construction of vegetated conveyance and basinÂtype VRAs. ⢠Geotechnical considerations may be partially mitigated as a result of adequate setbacks from compacted road base. ⢠Trees or bushes may be allowable within certain parts of interchange wedges and may increase water retention on site. ⢠Interchanges for elevated roadway sections tend to have open space located at a lower eleva tion than the main travel lanes. ⢠Where access is considered in roadway design, maintenance may be possible without requir ing lane closures. 3.3.9 Looped Interchanges (Also Known as Cloverleaf Intersections) Interchanges between highways or between highways and local roads often vary in cross section and combine a number of the highway types previously discussed. Interchanges present unique considerations for volume reduction design. Looped interchanges include those in which Example Plan View Not to Scale
Volume Reduction in the Urban Highway Environment 39 legs connect highways via a combination of external arcs and internal loops. There are a range of variations on looped interchanges. Defining Physical Features ⢠Looped interchanges typically include a combination of segment types, including embank ment segments, aerial segments, and atÂgrade segments. ⢠Available space dictates the radius of loops and curves, which influences design speeds and the degree of restriction of crossÂsections. ⢠Looped interchanges enclose central circular areas of open space that are frequently unused. Key Constraints for VRAs Related to Highway Type ⢠See constraints associated with diamond interchanges (Section 3.3.8) Key Opportunities for VRAs Related to Highway Type ⢠See opportunities associated with diamond interchanges (Section 3.3.8). ⢠Because of their unique geometry, central loops tend to be less restricted (relative to top ographic, geotechnical, and safety considerations) than wedgeÂshaped sections formed by diamond interchanges. 3.4 Site Assessment Activities to Support Volume Reduction Planning and Design As discussed in Section 3.2, 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 vol ume reduction approaches requires the review of existing information and may include the collection of siteÂspecific measurements, especially after VRAs are determined to be feasible. Available information regarding site characteristics, such as impervious cover, slope, soil characteristics, local groundwater conditions, and geotechnical conditions, should be assessed as part of site characterization efforts. In addition, specific explora tions, such as soil and infiltration testing and groundwaterÂlevel measurements, may be necessary to determine and confirm if stormwater infiltration is feasible and to deter mine the appropriate design parameters for VRAs. Focused analyses, such as estimating the effects of the project on water balance and estimating the potential for groundwater mounding, may help supplement site investigation and data review efforts. This overall process is outlined in Figure 6. Local planning and design requirements in effect for a project may describe minimum siteÂassessment requirements applicable to specific project types. In addition, certain activi ties are recommended (even if not required) to help ensure that opportunities for volume reduction are identified and potential volume reduction issues are considered. The follow ing subsections are intended to provide recommendations for site assessment activities to support the incorporation of volume reduction into overall project planning and design. The recommendations are not intended to prevent the consideration of siteÂspecific factors or substitute for the need to exercise sound engineering judgment. In addition, the recom mendations are intended to be applied only to the extent that they are necessary to meet minimum siteÂassessment requirements. They are not intended to imply that each of these assessments must be conducted for every project if an equally reliable source of informa tion is available in place of any of these analyses or if the analysis outcome is obvious and can be documented based on simpler analysis methods. For example, if groundwater is known to be very deep based on regional surveys or other available information, it is not Identify constraints and opportunities dictated by highway and project type Review available desktop information Conduct initial site assessments for feasibility screening and prioritization purposes Coordinate with applicable agencies and parties Conduct additional focused assessments to support prioritization and design Define site assessment goals Figure 6. Example approach for site assessment.
40 Volume Reduction of Highway Runoff in Urban Areas necessary to conduct an evaluation of the exact water table depth or the potential for ground water mounding. What specific types of findings must be supported from site assessment activities? The answer to this question varies depending on site conditions, the phase of the projects, and what types of VRAs are proposed. Section 3.4.1 provides guidance for determining what information is needed at which phase of a project. Sections 5.2, 5.3, and 5.4 provide guidance for how findings from site investigation activities can be used for determining the feasibility and desirability of VRAs and then for developing conceptual designs. Therefore, it is recommended that these sections be reviewed as part of planning for site assessment activities so that the scope of these activities is adequate to answer the key questions that designers will face at each phase. Additionally, users should familiarize themselves with the types of VRAs that are available/approved for use, so that the attributes of these practices are considered in site assessment (see Chapter 4 and Appendix A or local guidance for the applicable menu of VRAs). 3.4.1 Phasing of Site Assessment Activities To improve the efficiency of site assessment, specific efforts should be phased, with consider ation of the information needed to inform decisions at each point in the project. The types of information and assessment methods that may be applicable to screen and prioritize potential VRAs at the planning phase may be inadequate to support designÂlevel efforts. Conversely, the degree of rigor needed for designÂlevel investigations may be costÂprohibitive for planning phase assessments. In general, planning phase screening methods are used to identify VRAs and poten tial VRA locations that can be definitively dropped from consideration and to help prioritize VRA types and locations among those that remain. DesignÂlevel assessment methods tend to be more focused on precisely quantifying conditions relevant to the specific VRAs and locations selected. Guidance for conducting site assessment activities related to soil infiltration capacity, ground water considerations, and geotechnical considerations is provided in Appendices C, D, and E, respectively. These resources provide a systematic guide for phasing assessment activities by helping to describe the methods that are applicable at the planning and design phases. A general discussion of appropriate planningÂlevel and designÂlevel siteÂassessment methods is provided in the following sections. At both the planning and design phases, the specific screening and feasibility criteria appli cable to the project should be considered in scoping siteÂassessment activities. For example, if the feasibility criterion for depth to groundwater is in the range of 10 ft below the ground surface, then it may not be necessary to conduct borings to 50 ft to characterize groundwater levels as they specifically relate to VRA selection and design. Specific feasibility criteria and approaches for interpreting the results of site assessment efforts in the context of feasibility screening and prioritization are described in Chapter 5. Additionally, the availability of data from other sources should be considered in scoping field assessment efforts. Planning Phase Site Assessment Site assessment efforts should ideally be initiated early in the design process so that volume reduction approaches can be incorporated into the project layout as it is developed. At this phase, it may still be possible to adjust the project layout to preserve areas that provide good opportunities for VRAs and configure project grading and drainage so that water can be routed to these areas. There are many factors that influence highway design and result in lesser oppor tunities for adjustment to layout (e.g., alignment) than other types of projects; however, adjust ments to grading and drainage routing to improve volume reduction opportunities may still be
Volume Reduction in the Urban Highway Environment 41 possible if this is initiated early in the design process. At this phase of project development, the project team is faced with two key questions: ⢠Where within my project area are VRAs feasible? ⢠What VRAs are potentially suitable for my project? The amount of information available to answer these questions at the planning phase of proj ect development may vary. For example, at this phase, project planners may have access to exten sive geotechnical investigation reports from previous projects, the results of early investigations for the project of interest, and other information, or may be faced with much more limited data, such as county soil maps and anecdotal evidence about groundwater levels in the vicinity. Generally, the burden of proof is lower at this phase of project assessment, and, therefore, simpler and more efficient screening methods may be appropriate. Methods that are used at this phase are commonly referred to as âscreening methodsâ or âprioritization methods.â These methods do not seek to definitively establish feasibility or establish final design parameters; however, their efficiency allows screening to be conducted over a greater spatial extent and for a broader menu of potential VRAs than would be feasible using more detailed siteÂassessment methods. Programmatic analysis may be possible at some DOTs or in some states, where ready data are available. PlanningÂlevel screening methods do tend to contain more potential for error than more detailed methods; hence, discretion of the design team is needed to balance the cost of data acquisition with the level of certainty needed at this phase. In the absence of specific local guid ance, the decision of what data to collect at the project planning phase, and at what resolution, should be based on projectÂspecific factors and questions, such as: ⢠How variable in space and time are the conditions (e.g., groundwater elevations) at the site? Can I reliably interpolate between a lesser number of data points? ⢠What are the project goals relative to volume reduction? How important is it to provide an exhaustive investigation and quantification of volume reduction opportunities? Do applicable regulations require a rigorous demonstration that volume reduction is conducted to the maxi mum degree possible? ⢠How much would more rigorous investigation methods cost as a portion of the project budget? Could other design costs potentially be reduced (e.g., for conveyance, flood con trol) if increased budgets were allocated to thoroughly investigating volume reduction opportunities? Finally, planningÂlevel screening may be the only evaluation needed to inform selection and placement of potential VRAs. If a certain site characteristic conclusively rules out a certain type of VRAs or VRA locations, then it is not necessary to conduct additional siteÂassessment activi ties to further support this finding. For example, if contaminated soils rule out infiltration in a certain area of a site, then it is likely not necessary to conduct infiltration rate assessments in that area. Design-Phase Site Assessment At the VRA design phase, a more detailed and accurate assessment is typically needed to (1) establish specific design parameters (e.g., infiltration rate), (2) demonstrate that the selected VRAs can be safely implemented in the selected locations (e.g., via slope stability calculations), and (3) demonstrate that potential negative consequences have been addressed (e.g., by char acterizing and mitigating potential for groundwater contamination). At this phase of project development, the project team is faced with three key areas of questions: ⢠What design parameters should I use to design volume reduction facilities? What factors of safety should I apply?
42 Volume Reduction of Highway Runoff in Urban Areas ⢠Is the design safe? How does the design mitigate unacceptable levels of risk? ⢠Is the design protective of potential unintended consequences for other media? Are risks of impacts mitigated to acceptable levels? Site assessment activities and design calculations conducted at this phase bear a greater burden of proof to definitively answer these questions; however, they are typically focused on specific designs and specific VRA locations, which helps mitigate the need for more rigorous analyses. Additionally, some analyses, such as for slope stability, may be required regardless of whether VRAs are proposed and may not represent a significant incremental cost. Similarly to planning phase assessments, tradeÂoffs exist between the costs of investigations and analyses and the qual ity and resolution of the data available to support design decisions. The value obtained from each designÂlevel assessment activity can be improved by: ⢠Using a tiered approach for investigation (i.e., planningÂlevel screening in advance of design level testing), such that more rigorous designÂlevel tests are conducted only in areas where VRAs are likely to be placed; ⢠Using proven assessment methods that are acceptable to local jurisdictions and provide reli able information; and ⢠Selecting methods that are applicable for the project conditions. This guidance manual provides general recommendations for designÂlevel siteÂassessment meth ods. Because of the important role of siteÂspecific conditions in designÂphase analyses, it is expected that professional judgment and discretion will play a large role in planning and conducting the assessment activities needed at the design phase. The site assessment categories discussed include: ⢠Topography and drainage patterns, ⢠OffÂsite drainage and adjacent land uses, ⢠Soil and geologic conditions, ⢠Local weather patterns, ⢠Groundwater considerations, ⢠Geotechnical considerations, ⢠Existing utilities, ⢠HarvestedÂwater demand, ⢠Responsible agencies and other stakeholders, ⢠Local ordinances, and ⢠WatershedÂbased and other joint planning opportunities. Sections 3.4.2 through 3.4.12 provide guidance related to each of these categories. Sections 5.2, 5.3, and 5.4 provide guidance for how findings from site investigation activities can be assimi lated into determining the feasibility and desirability of VRAs and then developing conceptual designs. Table 4 provides a checklist of typical goals for site assessment at each phase of project planning and summarizes potential site assessment activities. 3.4.2 Topography and Drainage Patterns The siteâs topography should be assessed to evaluate surface drainage, identify topographic high and low points, and identify the current and future presence of steep slopes, all of which have an impact on the type of VRAs that may be most applicable and beneficial for a given project site (as summarized in Table 2). Topography and drainage patterns are also key factors in identifying potential locations for VRAs. Topographic assessment and mapping should docu ment existingÂcondition impervious areas, drainage patterns, the interface of site topography with adjacent parcels/rightsÂofÂway (e.g., manufactured slopes), and any other topographic fea tures of interest to site layout or stormwater management. Assessment of site topography relative to volume reduction design can generally be accomplished via review of the topographic survey conducted at the outset of the project. At early planning phases, it may be necessary to use more approximate data sources, such as the United States Geological Survey (USGS) quadrangle maps or digital elevation models available
Volume Reduction in the Urban Highway Environment 43 from the National Elevation Dataset. Many resources are available as part of the USGS National Map (http://nationalmap.gov/viewer.html) to facilitate approximate assessment of site topogra phy. Project design schematics can be used to assess postÂproject topography. 3.4.3 Off-Site Drainage and Adjacent Land Uses OffÂsite drainage is an important factor in the layout of the project site and in determining appropriate VRAs. OffÂsite flows that enter or cross the project area may pose challenges for Planning Phase Underlying Goals of Site Assessment Addressed? (Y/N)1 Have the volume reduction goals for my project been identified? Have VRAs that are potentially suitable for my project been identified? Have areas within my project area where VRAs are feasible been identified? Is there additional information that I will need to obtain as part of design-phase assessments to confirm these findings and complete the design? Site Assessment Activities Potentially Applicable Assessed2 N/A2 Topography and drainage patterns â Section 3.4.2 Off-site drainage and adjacent land uses â Section 3.4.3 Preliminary infiltration capacity assessment â Section 3.4.4, Appendix C Preliminary water balance and groundwater quality screening â Section 3.4.6, Appendix D Geotechnical risk assessment (major issues) â Section 3.4.7, Appendix E Existing utilities â Section 3.4.8 Harvested-waterâdemand assessment â Section 3.4.9 Responsible agencies and other stakeholders â Section 3.4.10 Local ordinances â Section 3.4.11 Watershed-based and other joint planning opportunities â Section 3.4.12, 5.5 Other, as determined via professional judgment of design team Design Phase Underlying Goals of Site Assessment Addressed? (Y/N)1 Have I identified design parameters that I should use to design volume reduction facilities? Have appropriate factors of safety been identified? Is the design safe? Has the design mitigated unacceptable levels of risk? Is the design protective of potential unintended consequences for other media? Are risks of impacts mitigated to acceptable levels? Is additional information or oversight needed at the construction phase to confirm design assumptions? Additional Site Assessment Activities Potentially Applicable Assessed2 N/A2 Design phase infiltration rate evaluation and factors of safety, as needed â Section 3.4.4, Appendix C Focused groundwater-related analyses, as needed â Section 3.4.6, Appendix D Geotechnical design parameters and mitigation measures, as needed â Section 3.4.7, Appendix E Other design-level analyses and refinements, as determined via professional judgment 1 â See Sections 5.2 and 5.3 for more guidance on documenting findings of suitability and feasibility; See Section 5.4 for guidance on developing conceptual designs. 2 â Include results of site assessment activities or explanation of why they are not applicable as part of project submittal documentation. Table 4. Checklist of site assessment goals and activities.
44 Volume Reduction of Highway Runoff in Urban Areas implementing VRAs, such as excessive flow rates, high sediment loadings of interest that can lead to clogging of VRAs, and high pollutant loadings relative to potential impacts on groundwater quality. Opportunities to keep offÂsite flows separate from onÂsite flows should be assessed. Off site flows may present opportunities for a project to provide additional volume reduction or provide volume reduction in alternative ways. For example, a project could have flows managed from offÂsite and show a net benefit with respect to the hydrologic impact. It may also be possible to address offÂsite flows in one portion of the project to compensate for lack of volume reduction opportunities in other portions. Locations and sources of offÂsite runÂon onto the project site should be identified as part of early siteÂassessment efforts. Assessment efforts should include characterization of the locations of offÂsite flows, the relative magnitude of these flows, and the land uses and potential pollut ant sources associated with these flows. Key pollutants relevant to volume reduction feasibil ity include sediment, nutrients, pathogens, and salts. More guidance on potential impacts on groundwater quality is provided in Section 3.4.5 and Appendix D. 3.4.4 Soil and Geologic Conditions The siteâs soil and geologic conditions should be determined to evaluate the capacity of a site for stormwater infiltration and to identify suitable and unsuitable locations for siting infiltration based VRAs. Among volume reduction processes, infiltration has the greatest potential to achieve substantial volume reduction in spaceÂconstrained highway settings. However, site soil condi tions influence the rate at which water can physically enter the soil and determine the amount of infiltration that can be feasibly and desirably achieved with consideration of geotechnical issues, groundwater quantity and quality, and utilities. Site assessment approaches for soil and geologic conditions may consist of: ⢠Review of available geologic or geotechnical reports on local geology to identify relevant features, such as depth to bedrock, rock type, lithology, faults, and hydrostratigraphic or confining units; ⢠Review of previous geotechnical investigations of the area; and ⢠SiteÂspecific geotechnical or geologic investigations, such as borings and infiltration tests. These geologic investigations may also identify shallow water tables and past groundwater or soil contamination issues that are important for BMP design (see Section 3.4.5). Geologic inves tigations should seek to provide an assessment of whether soil infiltration properties are likely to be uniform or variable across the project site. A wide range of potential methods for characterizing soil and geologic conditions are discussed in greater detail in Appendices C and E. These range from planningÂlevel methods to characterize approximate levels of infiltration potential and areas that may be most suitable for infiltration, to designÂphase methods for establishing infiltration rates and assessing geotechnical issues. Finally, in areas where fill will be important, it is important to understand the characteristics of the fill material. Where there are cuts, it is important to understand soil conditions at the cut elevation. 3.4.5 Local Weather Patterns As introduced in Section 3.2.3, local weather patterns have an important influence on the natural hydrologic regime of a site as well as on the suitability, design, and performance of VRAs. Key information regarding local weather patterns is as follows: ⢠Typical storm sizes (e.g., 85thÂpercentile, 24Âhour storm depth). ⢠Peak storm intensities (e.g., 2Âyear peak flow) that would influence energy dissipation requirements.
Volume Reduction in the Urban Highway Environment 45 ⢠Relative seasonal patterns of rainfall and ET (i.e., is ET typically high between storm events? For how many months is ET greater than precipitation, and vice versa?). ⢠Average annual rainfall depth. ⢠Typical length of dry season(s). ⢠Portion of precipitation that occurs as snow. ⢠Continuous time series of precipitation and ET, if available, to support siteÂspecific modeling. The Volume Performance Tool can be used to obtain some of this information for nearby gages (although not always the most local gage). Other information can be obtained from local or nationwide resources. The National Climatic Data Center (http://www.ncdc.noaa.gov/) and the Oak Ridge National Laboratory Distributed Active Archive Center (http://www.daac.ornl. gov) provide free data downloads and publications of climatic data at a nationwide scale. 3.4.6 Groundwater Considerations Site groundwater conditions should be considered prior to the siting, selection, sizing, and design of VRAs. Specific guidance on evaluating groundwater considerations is provided in Appendix D. Site assessment activities related to groundwater generally include: Groundwater levels. The depth to seasonally high groundwater tables (normal high depth during the wet season) beneath the project site may preclude infiltration. Depth to seasonally high groundwater levels can be estimated based on wellÂlevel measurements or redoximorphic methods. For sites with complex groundwater tables, longÂterm studies may be needed to un derstand how groundwater levels react in wet and dry years. Groundwater and soil contamination. In areas with known groundwater and soil pollu tion, infiltration may need to be avoided if it could contribute to the movement or dispersion of soil or groundwater contamination or adversely affect ongoing cleanup efforts, either on site or down gradient of the project. Mobilization of groundwater contaminants may also be of concern where contamination from natural sources is prevalent (e.g., marine sediments, seleniumÂrich groundwater), to the extent that data is available. If infiltration is under consid eration in areas where soil or groundwater pollutant mobilization is a concern, a siteÂspecific analysis should be conducted to determine where infiltrationÂbased VRAs can be used without adverse impacts. Stormwater pollutant sources. Certain pollutants found in stormwater have the potential to have impacts on groundwater quality. Research conducted by Pitt et al. (1994) on the effects from stormwater infiltration on groundwater found that the potential for contamination due to infiltration of stormwater is dependent on a number of site factors, including the local hydrogeology and the chemical characteristics of the pollutants of concern (as well as the level of treatment that runoff receives prior to infiltration or as it is infiltrating). Chemical characteristics of stormwater that influence the potential for groundwater impacts include high mobility (low absorption potential), high solubility fractions, and abundance of pollutants in urban runoff. The chemical characteristics of the subsurface soils relate to how mobile pollutants are in the vadose zone. The depth to groundwater has also been found to be an indicator of the potential for contamination. Site assessment efforts specific to potential impacts of stormwater infiltration on groundwater quality include: ⢠Identification of pollutant hot spots in tributary areas within the project or in drainage from offÂsite areas, ⢠Characterization of soil properties, and ⢠Characterization of depth to groundwater.
46 Volume Reduction of Highway Runoff in Urban Areas Appendix D provides more information on evaluating the risk of groundwater contamination from stormwater sources. Coordination with resource agencies. Infiltration activities should be coordinated with the applicable groundwater management agency, such as groundwater providers or resource protection agencies, to ensure that groundwater quality is protected. It is recommended that coordination be initiated as early as possible during the planning process to determine whether specific siteÂassessment activities apply or whether these agencies have data available that may support the planning and design process. Groundwater recharge. Infiltration of stormwater can provide the benefit of recharging groundwater; however, groundwater recharge is not an implicit benefit of infiltration in all cases. Some areas of a site may provide pathways for water to recharge groundwater, while other areas may have a less efficient connection to groundwater or connect to a perched groundwater aquifer that is not used for water supply purposes. If groundwater recharge is desired, the site charac terization should attempt to identify areas where infiltration would have the greatest benefit for groundwater recharge. Generally, a greater fraction of infiltrated water reaches groundwater in cases where there is a relatively direct hydrogeologic connection between the surface and an aquifer. Groundwater/surface water interactions. Groundwater discharge to surface water is gen erally a primary source of dryÂweather base flows in perennial stream systems. Intermittent and ephemeral systems are often characterized by groundwater discharge during some portions of the year and streams losing flow to groundwater during other portions of the year. These systems may be sensitive to minor changes in groundwater levels, which could result from increased infiltration compared to the existing condition. In such systems, increases in groundwater levels could potentially increase the duration of dryÂweather base flows in intermittent and ephemeral drainages. These changes may have significant impacts on riparian habitat and geomorphol ogy and affect species that favor these drier habitats. If intermittent or ephemeral drainages are located adjacent to or down gradient of the project, an assessment of the site water balance and potential impacts on groundwater/surface water interactions may be warranted. 3.4.7 Geotechnical Considerations Infiltration of stormwater can cause geotechnical issues, including: (1) impacts on utili ties, (2) settlement and volume changes, (3) slope instability, and (4) impacts on foundations or retaining walls. Stormwater infiltration temporarily raises the groundwater levels or soil mois ture near the infiltration facility, such that the potential geotechnical conditions are likely to be of greatest significance near the area of infiltration and diminish with distance. If infiltration is considered, a geotechnical investigation should be performed for the infiltration facility to identify potential geotechnical issues and geological hazards that may result from infiltration and potential mitigation measures to reduce risks to acceptable levels. Appendix E provides guidance for evaluating potential geotechnical issues at the planning phase and the design phase. In general, assessment activities include: ⢠Assessment of topography and drainage (see Section 3.4.2), ⢠Assessment of soil and geologic conditions (see Section 3.4.4), and ⢠Assessment of groundwater conditions (see Section 3.4.5). These assessments provide initial information to assess potential geotechnical issues associ ated with stormwater infiltration. Focused siteÂassessment activities and analyses may be needed to assess specific issues identified in planningÂphase screening efforts. A licensed geotechnical
Volume Reduction in the Urban Highway Environment 47 engineer should determine recommendations for focused investigation and analysis of geo technical issues based on soil boring data, drainage patterns, and the current requirements for stormwater management. Implementing the geotechnical engineerâs requirements is essential to prevent damage associated with infiltration in the roadway environment. 3.4.8 Existing Utilities The locations of existing subsurface utilities may limit the possible locations of certain VRAs and may constrain site design. Additionally, the condition of utilities is relevant. For example, trenches that do not contain lateral cutoff walls (to prevent concentrated flow within trenches) may present a more substantial risk for infiltration in their vicinity (e.g., sinkhole formation). The location and condition of utilities can generally be obtained as part of the topographic survey. 3.4.9 Harvested-WaterâDemand Assessment If harvestÂandÂuse approaches are under consideration for a project, a site assessment should include an assessment of the reliable demand for harvested water during the times of the year when precipitation and runoff occurs. A phased assessment method is recommended. First, at the planning level, the assessment should seek to answer the following types of ques tions to determine if harvest and use of stormwater is potentially applicable: ⢠Is irrigation used for landscaping in the rightÂofÂway? ⢠Are there any facilities in the rightÂofÂway (e.g., maintenance yards) that could make use of nonÂpotable water for toilet flushing, vehicle washing, cooling tower makeÂup water, or other uses? ⢠Are there any adjacent land owners that have expressed interest in harvesting roadway runoff? If the answer to any of these questions is yes, then additional assessments may include: ⢠Evaluation of the magnitude of the demand during the times of year when precipitation occurs. Is the demand present during the wet season? Would it be possible to store water to be available for use during the times of year when it is in demand? ⢠Evaluation of the legality and desirability of using harvested water. Is harvesting of runoff legal based on water rights laws? What uses of harvested water are allowable per appli cable public health codes? Is reclaimed water available in the area, such that the use of reclaimed water during the wet season would have a higher priority than use of harvested stormwater? Finally, if harvested water is selected for use (per decision criteria described in Section 5.3), then it may be necessary for site assessment activities to quantify the actual demand profile for the site via measurements or other methods. 3.4.10 Responsible Agencies and Other Stakeholders Early siteÂassessment activities should include the identification of responsible agencies and other stakeholders that may influence decision making for the project. Potential information obtained through coordination with these parties may include: ⢠Identification of local watershedÂbased stormwater management planning efforts or other joint planning opportunities (see Section 3.4.11); ⢠Understanding of local minimum criteria for site assessment, feasibility determination, and design;
48 Volume Reduction of Highway Runoff in Urban Areas ⢠Acquisition of data related to soil, groundwater, utilities, foundations, and so forth that may inform the planning and design process; and ⢠Identification of real or perceived potential impacts of stormwater infiltration on environ mental resources or adjacent land owners. 3.4.11 Local Ordinances Early siteÂassessment activities should include the identification of local ordinances that may influence the goals and constraints related to achieving volume reduction for a given project. As introduced in Section 3.1.1, stormwater runoff from DOT projects is typically permitted under the CWA/NPDES system at a state or regional level; however, in some cases, local ordinances for stormwater management (e.g., TMDL plans, flood control) may apply to urban highway proj ects. Additionally, local ordinances related to resource protection (e.g., groundwater protection ordinances) may prescribe specific limitations or methods related to discharges to other media (e.g., groundwater). Local ordinances may also relate to geotechnical design, flood control, or other aspects of project development. 3.4.12 Watershed-Based and Other Joint Planning Opportunities WatershedÂbased approaches may provide greater opportunity for volume reduction than can be safely and reliably achieved in the rightÂofÂway. Early siteÂassessment activities should seek to identify potential watershedÂbased opportunities or other joint planning opportunities that may be applicable for the project. Section 5.5 provides more information about identifying and evaluating these options.
