Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual (2015) / Chapter Skim
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From page 133... ...
A-1 A p p e n d i x A Volume Reduction Approach Fact Sheets VRA 01 Vegetated Conveyance VRA 02 Dispersion VRA 03 Media Filter Drain VRA 04 Permeable Shoulders with Stone Reservoirs VRA 05 Bioretention Without Underdrains VRA 06 Bioretention with Underdrains VRA 07 Infiltration Trench VRA 08 Infiltration Basin VRA 09 Infiltration Gallery Additional information about VRAs is provided in Chapters 4 and 5.
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From page 134... ...
A-2 Volume Reduction of Highway Runoff in Urban Areas Vegetated Conveyance VRA 01 Description This category includes engineered vegetated swales and other vegetated drainage features that serve the purpose of conveying stormwater runoff and can also provide significant reduction of stormwater runoff volume. Some variations on this approach include an amended soil or stone storage layer to increase storage capacity and promote infiltration.
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From page 135... ...
Volume Reduction Approach Fact Sheets A-3 Volume Reduction Processes and Performance Factors Volume reduction is achieved through infiltration and evapotranspiration. Volume reduction can be enhanced by including a stone or amended soil storage layer, providing shallow retention in the conveyance, and using a broader, flatter cross-section.
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From page 136... ...
A-4 Volume Reduction of Highway Runoff in Urban Areas to be adjusted to enhance treatment and prevent groundwater contamination. Where soils allow high rates of infiltration, the use of a vegetated conveyance may shift the water balance toward excess infiltration.
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From page 137... ...
Volume Reduction Approach Fact Sheets A-5 Proper functioning requires maintaining dense plant cover to prevent scouring. Patches of thin or missing vegetation should be repaired right away.
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From page 138... ...
A-6 Volume Reduction of Highway Runoff in Urban Areas Washington Department of Ecology. Stormwater Manual for Western Washington.
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Volume Reduction Approach Fact Sheets A-7 Example Conceptual Design Schematics Figure 1. -- Cross-section view. Figure 2. -- Longitudinal profile.
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From page 140... ...
A-8 Volume Reduction of Highway Runoff in Urban Areas Figure 3. -- Plan view. RO AD RO AD Op onal check dam Discharge to secondary VRA Op onal piped inlets Swale boom Stone inlet protec on
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From page 141... ...
Volume Reduction Approach Fact Sheets A-9 Alternative names: amended n vegetated atural dispe filter strip, v rsion, engin egetated bu eered dispe ffer area rsion, vegetated filter strip, compostDispersion VRA 02 Description This category consists of the dispersion of runoff toward existing or restored pervious areas for the purpose of reducing stormwater runoff volumes and achieving incidental treatment. This includes road shoulders amended with compost and additional materials such as sand (if needed)
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From page 142... ...
A-10 Volume Reduction of Highway Runoff in Urban Areas Volume Reduction Processes and Performance Factors Volume reduction is achieved through infiltration and evapotranspiration. The quantity of volume reduction expected is dependent on the site's soils, topography, and hydraulic characteristics (e.g., storage capacity, hydraulic retention time)
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From page 143... ...
Volume Reduction Approach Fact Sheets A-11 Applicability and Limitations Site and Watershed Considerations Dispersion to areas with high infiltration rates will result in higher rates of volume reduction. Dispersion is suitable for most soil types.
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From page 144... ...
A-12 Volume Reduction of Highway Runoff in Urban Areas Long-term stability and reduction in erosive flow potential can be enhanced with robust plant growth, effective dispersion, and adhering to recommended upper limits on embankment slope. Groundwater Quality and Water Balance Considerations Due to its extensive nature (i.e., water is dispersed in shallow depths over a broad area)
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From page 145... ...
Volume Reduction Approach Fact Sheets A-13 Retrofit of dispersion may include modifying the current drainage pathway, such as by removing a curb and gutter to allow dispersion to occur or providing for more uniform dispersion, or enhancing the dispersion area, such as by amending, decompaction, leveling, or vegetating the area. In either case, an incremental benefit in treatment and volume reduction capabilities can be claimed through this retrofit.
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From page 146... ...
A-14 Volume Reduction of Highway Runoff in Urban Areas Sources of Additional Information California Stormwater Quality Association. California Stormwater BMP Handbook: New Development and Redevelopment.
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From page 147... ...
Volume Reduction Approach Fact Sheets A-15 Infiltration rate The infiltration rate of the underlying soils within the dispersion area Any. Higher infiltration rates will achieve greater volume reduction.
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A-16 Volume Reduction of Highway Runoff in Urban Areas Figure 2. Plan view.
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From page 149... ...
Volume Reduction Approach Fact Sheets A-17 Alternative names: formerly known as "ecology embankment" Media Filter Drain VRA 03 Description This VRA consists of a stone vegetation-free zone, a grass strip, a media filter storage reservoir filled with specialized media, and a conveyance system for flows leaving the reservoir. This conveyance system usually consists of a gravel-filled underdrain trench or a layer of crushed surfacing base course.
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From page 150... ...
A-18 Volume Reduction of Highway Runoff in Urban Areas Volume Reduction Processes and Performance Factors Runoff volume is reduced through infiltration and evapotranspiration. Water is treated as it moves over the grass strip and through the media within the reservoir.
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From page 151... ...
Volume Reduction Approach Fact Sheets A-19 Groundwater Quality and Water Balance Considerations Due to its extensive nature and the degree of treatment provided by the media, this VRA poses relatively low risk of groundwater quality impacts and water balance impacts. Risks of water balance impacts may be elevated in areas with very high soil infiltration rates and hydrogeologic conditions that are sensitive to increases in infiltration volume.
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From page 152... ...
A-20 Volume Reduction of Highway Runoff in Urban Areas Increase footprint area at intersections and wider portion of right-of-way. Drainage can be routed to media filter drains with broader footprints in the open space formed by intersections and at wider sections of the right-of-way to help increase the dispersion area that is provided.
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From page 153... ...
Volume Reduction Approach Fact Sheets A-21 Example Conceptual Design Schematic Figure 1. -- Cross-section view. Figure 2. -- Plan view.
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From page 154... ...
A-22 Volume Reduction of Highway Runoff in Urban Areas Alternative names: permeable shoulders, permeable gutters Permeable Shoulders with Stone Reservoirs VRA 04 Description This VRA includes use of a permeable pavement surface course (typically permeable asphalt or concrete) along the shoulders of a roadway, underlain by a stone reservoir.
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From page 155... ...
Volume Reduction Approach Fact Sheets A-23 conditions or other factors, permeable pavement systems can be enhanced with underdrains to provide flow control and augment infiltration discharge. When designed with adequate storage, permeable pavement systems can provide temporary detention of storm flows and controlled release, discharging flows at rates similar to natural base flows with the use of underdrains and flow controls.
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A-24 Volume Reduction of Highway Runoff in Urban Areas Current applicability of permeable pavements to main roadway sections is not well established relative to structural design requirements, top course durability, and safety. Research is ongoing.
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From page 157... ...
Volume Reduction Approach Fact Sheets A-25 Regional Applicability Permeable pavement can be used across a wide range of climates; however, designs must account for differences in climate (specifically precipitation) , peak temperatures, freeze/thaw cycles, and solar irradiation.
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A-26 Volume Reduction of Highway Runoff in Urban Areas surface clogging and help reduce the migration of fines into the subbase, permeable shoulders should be cleaned regularly with a high-efficiency vacuum sweeper. Enhancements and Variations Add storage.
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Volume Reduction Approach Fact Sheets A-27 Key Planning-Level Design Parameters for Volume Reduction Conceptual Design Parameter Description Representative Range Footprint area The area covered by permeable shoulder N/A Tributary area ratio The footprint of the permeable shoulder as a fraction of the total tributary area (including the permeable shoulder itself) Typically limited to 5:1, but may be increased with effective maintenance Stone reservoir thickness The thickness of the stone storage layer Typically 1 to 3 ft Porosity The effective void space within the stone storage layer Typically 0.35 to 0.45 (unitless)
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From page 160... ...
A-28 Volume Reduction of Highway Runoff in Urban Areas Example Conceptual Design Schematics Figure 1. -- Cross-section view. Figure 2. -- Plan view.
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Volume Reduction Approach Fact Sheets A-29 Figure 3. -- Longitudinal profile of an installation along a mild slope (earthen berms)
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From page 162... ...
A-30 Volume Reduction of Highway Runoff in Urban Areas Alternative names: rain garden, bioretention, retention swale Bioretention Without Underdrains VRA 05 Description Bioretention consists of a shallow surface ponding area underlain by porous soil media storage reservoirs and an optional porous stone storage layer. Captured runoff is directed to the bioretention area where it infiltrates into an engineered soil medium and then infiltrates into the subsoil.
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From page 163... ...
Volume Reduction Approach Fact Sheets A-31 Volume Reduction Processes and Performance Factors Volume reduction in bioretention cells is achieved through infiltration and evapotranspiration. Efficient volume reduction performance is dependent on adequate medium and subsoil infiltration rates to ensure that captured runoff filters through the system between storm events.
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From page 164... ...
A-32 Volume Reduction of Highway Runoff in Urban Areas shoulders, and interchanges tend to provide the best opportunities for bioretention in the urban highway environment. Through the use of underdrains (see VRA 06)
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From page 165... ...
Volume Reduction Approach Fact Sheets A-33 situations, bioretention may require additional excavation and hauling costs as well as additional piping costs. Use in a Treatment Train Pretreatment of runoff to reduce particulate matter and suspended solids will increase the life of the bioretention cell and reduce required maintenance.
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A-34 Volume Reduction of Highway Runoff in Urban Areas Additional Sources of Design Information Low Impact Development Center, Inc. Low Impact Development Manual for Southern California: Technical Guidance and Site Planning Strategies.
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Volume Reduction Approach Fact Sheets A-35 Key Planning-Level Design Parameters for Volume Reduction Conceptual Design Parameter Description Representative Range Footprint area The area covered by the surface of the bioretention cell Typically 100 to 2,000 ft2; potentially to be much larger Effective footprint area The portion of the total facility footprint area that provides storage and infiltration during typical operations. For planning-level design efforts, the effective footprint can be considered to be the ponded water area when the system is at half of its design ponding depth.
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A-36 Volume Reduction of Highway Runoff in Urban Areas Example Conceptual Design Schematic Figure 1. -- Cross-section view. Figure 2. -- Plan view.
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From page 169... ...
Volume Reduction Approach Fact Sheets A-37 Alternative names: bioretention, biofiltration, retention swale Bioretention with Underdrains VRA 06 Description Bioretention with underdrains consists of a shallow surface ponding area underlain by porous soil media storage reservoirs, an underdrain layer, and optional porous stone storage layers below the underdrain layer. Runoff is captured within and directed to the bioretention area, infiltrates into the soil medium, and is discharged through an underdrain.
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From page 170... ...
A-38 Volume Reduction of Highway Runoff in Urban Areas similar in many ways to shallow groundwater base flow in undeveloped/predevelopment watersheds. Volume Reduction Processes and Performance Factors Volume reduction in bioretention with underdrains is achieved through infiltration below the underdrains of the system (unless lined)
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From page 171... ...
Volume Reduction Approach Fact Sheets A-39 Groundwater Quality and Water Balance Considerations In areas with very high soil infiltration rates or shallow groundwater tables, captured stormwater may not be sufficiently treated prior to contact with groundwater. In areas with existing groundwater contamination, bioretention cells can be lined to keep treated stormwater out of contact with groundwater and discharged only via the underdrain.
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From page 172... ...
A-40 Volume Reduction of Highway Runoff in Urban Areas VRA-Specific Maintenance Considerations (see Section 4.3.6 for additional maintenance information in common with other VRAs) Plant types and landscaping techniques may differ from traditional roadside vegetation, but do not require specialized equipment; mowing is not appropriate.
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From page 173... ...
Volume Reduction Approach Fact Sheets A-41 Sources of Additional Information Low Impact Development Center, Inc. Bioretention Specification.
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A-42 Volume Reduction of Highway Runoff in Urban Areas Total storage depth The effective depth of water stored within the bioretention cell. It is a function of ponding depth, sump storage, bioretention medium thickness and porosity, and the thickness and porosity of the optional stone storage layer.
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From page 175... ...
Volume Reduction Approach Fact Sheets A-43 Figure 2. -- Plan view. RO AD r Op onal piped inlet Op onal vegetated swale Energy dissipa on stone Op onal vegetated filter strip Max ponded area Bioreten on media footprint Overflow Perforated underdrain
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From page 176... ...
A-44 Volume Reduction of Highway Runoff in Urban Areas Alternative names: Exfiltration trench Infiltration Trench VRA 07 Description This category of VRA consists of a stone-filled trench that provides subsurface storage of stormwater runoff and allows water to infiltrate through the bottom and walls of the trench into subsoils. Pretreatment for infiltration trenches is commonly provided via vegetated conveyances such as swales or filter strips.
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Volume Reduction Approach Fact Sheets A-45 General DOT Experience Infiltration trenches have been widely used across the United States. When properly designed and infiltration rates are maintained, volume reductions are high, on average.
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A-46 Volume Reduction of Highway Runoff in Urban Areas Regional Applicability Infiltration trenches have been applied successfully across a broad range of climates. Urban Highway Opportunities Infiltration trenches can be readily applied to shoulders with low slopes and medians.
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From page 179... ...
Volume Reduction Approach Fact Sheets A-47 Enhancements and Variations Increase storage capacity. Storage capacity can be enhanced by increasing the depth of the stone reservoir, provided that sufficient depth to, and distance between, groundwater is maintained.
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A-48 Volume Reduction of Highway Runoff in Urban Areas Figure 1. -- Cross-section view. Effective storage depth The effective depth of water stored within the infiltration trench.
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Volume Reduction Approach Fact Sheets A-49 RO AD Oponal compost amended filter strip pretreatment Infiltraon Trench Oponal piped inlet Oponal sedimentaon basin pretreatment Overflow Figure 2. -- Plan view.
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From page 182... ...
A-50 Volume Reduction of Highway Runoff in Urban Areas Alternative names: percolation basin, recharge basin Infiltration Basin VRA 08 Description Infiltration basins are relatively large, shallow basins that generally have relatively little vegetation. Their contours appear similar to detention basins but do not have a surface discharge point below their overflow elevation.
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From page 183... ...
Volume Reduction Approach Fact Sheets A-51 Volume Reduction Processes and Performance Factors Volume reduction in infiltration basins is achieved through a combination of infiltration and evapotranspiration. Efficient performance is dependent on adequate subsoil infiltration rates to ensure that captured runoff exits the basin between storm events.
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From page 184... ...
A-52 Volume Reduction of Highway Runoff in Urban Areas Urban Highway Opportunities Infiltration basins have relatively straightforward applications to shoulders with low slopes and medians where sufficient space is available. Because infiltration basins generally capture runoff from larger areas than other BMPs, they may be difficult to apply to urban highway settings with limited space or constrained rights-of-way.
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From page 185... ...
Volume Reduction Approach Fact Sheets A-53 Provide backup flow-control outlet. Including an underdrain (normally closed)
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A-54 Volume Reduction of Highway Runoff in Urban Areas Example Conceptual Design Schematic Figure 1. -- Cross-section view. I-5 Exit 102, Tumwater, Washington.
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From page 187... ...
Volume Reduction Approach Fact Sheets A-55 RO AD Basin floor Piped inlet Oponal sedimentaon forebay Overflow Figure 2. -- Plan view.
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From page 188... ...
A-56 Volume Reduction of Highway Runoff in Urban Areas Alternative names: infiltration galleries, infiltration vaults Underground Infiltration Systems VRA 09 Description Underground infiltration systems include a broad class of VRAs that consist of storage reservoirs located below ground and preceded by pretreatment systems. Water is pretreated, is routed into the systems, and infiltrates into subsoil.
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From page 189... ...
Volume Reduction Approach Fact Sheets A-57 General DOT Experience While case studies on the effectiveness of underground infiltration systems in the highway environment are currently limited, their use in some states, such as Minnesota, is increasing. Monitoring studies for several underground infiltration systems around St.
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A-58 Volume Reduction of Highway Runoff in Urban Areas Safety Considerations Underground infiltration systems are installed beneath standard paved shoulders and should not pose any additional hazards to drivers. Inlet grates should be flush with the road surface and fully traversable.
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From page 191... ...
Volume Reduction Approach Fact Sheets A-59 potential for clogging. Advanced pretreatment methods such as cartridge media filters, bioretention with underdrains, and other advanced filtration systems should be considered.
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From page 192... ...
A-60 Volume Reduction of Highway Runoff in Urban Areas Example Conceptual Design Schematic Figure 1. -- Cross-section view (example of arch gallery sited in breakdown lane)
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