Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual (2015) / Chapter Skim
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From page 11... ...
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.
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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.
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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)
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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.
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Volume Reduction in the Urban Highway Environment 15 discharge limits. This practice has already been implemented in U.S.
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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.
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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 "Green Book" (A Policy on Geometric Design of Highways and Streets)
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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 "volume reduction"?
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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.
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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 dryseason ET as a result of a change in vegeta tion type and introduction of irrigation (Maidment, 1992)
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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)
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22 Volume Reduction of Highway Runoff in Urban Areas discharges are released to surface waters at rates that mimic natural baseflow 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 overinfiltration and the creation of dryweather seeps.
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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.
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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*
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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.
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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)
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Volume Reduction in the Urban Highway Environment 27 General Feasibility and Desirability Factors The feasibility and desirability of VRAs is strongly influenced by sitespecific 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.
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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: • Groundlevel highway segments • Groundlevel highway segments with restricted crosssections • 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 crosssections of each representative highway type.
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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 Groundlevel highway segments with restricted crosssections are common in the urban highway environment as a result of the common need to provide high traffic capacity within a limited right ofway width.
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30 Volume Reduction of Highway Runoff in Urban Areas • Curbs commonly collect and convey stormwater runoff to stormdrain inlets rather than to open swales. • Frontage roads may be present.
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Volume Reduction in the Urban Highway Environment 31 • Concrete barrier dividers are common. • Where lane addition/expansion projects eliminate preexisting stormwater measures, the new projects may need to provide control for the total affected area (existing and proposed)
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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.
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Volume Reduction in the Urban Highway Environment 33 • Concrete barrier dividers are common. • In less restricted settings, medians and vegetated shoulders may exist.
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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.
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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)
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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 groundlevel 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.
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Volume Reduction in the Urban Highway Environment 37 Defining Physical Features • The crosssectional geometry of interchanges is highly variable as a function of sitespecific factors. • Degree of crosssectional restriction varies greatly.
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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.
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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.
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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?
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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?
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42 Volume Reduction of Highway Runoff in Urban Areas • Is the design safe? How does the design mitigate unacceptable levels of risk?
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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)
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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 offsite flows separate from onsite flows should be assessed.
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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?
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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.
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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 engineer's requirements is essential to prevent damage associated with infiltration in the roadway environment.
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From page 48... ...
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 siteassessment activities should include the identification of local ordinances that may influence the goals and constraints related to achieving volume reduction for a given project.
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