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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.
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. A critical element of this VRA is that it must be designed to sustain robust plant growth so that infiltration rates are maintained and regenerated via root structure, and the conveyance system itself does not contribute to sediment loading from scour. In contrast to a linear variation of bioretention, this approach is generally designed with a positive surface slope toward an outlet located at the surface grade. Where check dams or step pools provide significant ponded storage volume in the system that is infiltrated between precipitation events, it may be more appropriate to consider the system as a linear bioretention area VRA for the purpose of design and performance evaluation. Alternative names: stormwat w er conveyan et swale, dr ce y swale, bioswale, grassed swale, retention swale, regenerative Low Photo credit: Caltrans. VOLUME MANAGEMENT POTENTIAL/PROCESSES Overall volume reduction potential Infiltration Evapotranspiration Consumptive use Base-flowâmimicking discharge URBAN HIGHWAY APPLICABILITY Ground-level highways Ground-level highways with restricted cross-sections Ground-level highways on steep transverse slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges High Moderate
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. Soil infiltration rates, longitudinal slopes, and the relative ratio of VRA bottom area to tributary area are believed to be the most important factors in volume reduction effectiveness. General DOT Experience In many cases, vegetated conveyances may be a standard highway design feature that would be installed regardless of water quality and volume reduction benefits. Therefore, these features can be used at very low incremental costs (for example, some minor additional bottom width may be what is needed to achieve volume reduction goals). In addition to a standard conveyance feature in many highway systems, vegetated conveyances have been implemented by DOTs to achieve water quality treatment benefits and volume reductions of highway runoff. A review of volumetric measurements from swale studies in the International BMP Database (Water Environment Research Foundation, 2011) shows moderate volume reduction on average. Applicability and Limitations Site and Watershed Considerations Vegetated conveyances are suitable for most soil types. Soil infiltration rates will determine whether the swale can be designed to achieve significant infiltration or will serve primarily as conveyance with incidental volume reduction. Longitudinal slopes must be positive but not too steep (typically 1% to 6%) in order to provide positive drainage but avoid the creation of high-velocity flows that will result in erosion. For slopes within the upper end of this range (about 4% or more), better performance can sometimes be achieved through the use of check dams. Vegetated conveyances are relatively narrow and linear in profile, which allows them to fit into constrained spaces. They are suitable for use on shoulders and in medians. Geotechnical Considerations Vegetated conveyances must be located a sufficient distance from the roadway that infiltration will not compromise its structural integrity. Vegetated conveyances are a standard design feature in ground-level highway types and therefore should not pose significant incremental geotechnical risks. Use of vegetated conveyances along steep transverse slopes may require enhanced protection of slope integrity. Groundwater Quality and Water Balance Considerations Vegetated conveyances do not generally pose elevated risks to groundwater quality or water balance. 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 these situations, designs may need pretreatment (for example, addition of filtration media in the design) or
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. Safety Considerations For vegetated conveyances to be located within the clear zone (typically in the range of 22 to 32 ft from driving lanes), vegetated conveyances should either be constructed with side slopes of 3H:1V or flatter, or a barrier should be used between the road and the conveyance (parallel to road). If a piped inlet is used, the pipe openings should be cut flush with the transverse slope in order to reduce the potential that the pipe will be struck head-on by an errant vehicle. Pipes with diameters greater than 24 in. should be covered with traversable grates. Regional Applicability Vegetated conveyances are used across a broad range of climates. As a result, plants must be selected to be compatible with the local climate. Salt loadings in cold climates may influence plant selection. Irrigation is typically required for robust plant establishment, especially in arid climates. Highly arid climates without some irrigation may be more challenging New Projects, Lane Additions, and Retrofits Vegetated conveyances may have small incremental cost in new projects with sufficient right-of-way widths because grading can be balanced and landscaping would otherwise be installed; incremental costs may be greater in lane additions and retrofits where a swale did not previously exist. Retrofitting an existing vegetated conveyance to improve volume reduction processes, such as by adding check dams, amending soils, or increasing plant density, can be an effective method of providing an incremental improvement in volume reduction for relatively minimal investment. Use in a Treatment Train Vegetated conveyances can be used to collect and convey water down gradient of a filter strip. Vegetated conveyances can be used to pretreat, achieve some volume losses, and convey stormwater to centralized VRAs such as bioretention areas, infiltration trenches, or infiltration basins. VRA-Specific Maintenance Considerations (see Section 4.3.6 for additional maintenance information in common with other VRAs) Generally requires maintenance activities similar to those already needed for maintenance of roadside vegetation and ditches.
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. Vegetation may need to be mowed or cut back regularly to maintain optimal plant height. Enhancements and Variations Add storage below the surface outlet. Vegetated conveyances may be underlain by storage areas composed of stone and/or amended soils in order to increase storage capacity and promote infiltration and ET. Where this storage becomes the defining feature of the system, the VRA may be more appropriately categorized and designed as a linear bioretention area. Slow the velocity of flow. Vegetated conveyances may be planted with densely growing native/non-invasive vegetation (turf not preferred) to slow flows, promote more infiltration, and therefore allow greater volume reduction. Check dams can also help slow and more evenly distribute flow as well as prevent erosion (assuming that downstream of the check dam is protected). Provide low-flow outlet. Water can be held and released at a slow rate via a low-flow outlet, such as slotted weir, located at the downstream end of the system. This can provide detention and added volume reduction benefits. Stabilize the surface. A stabilization approach may be included in vegetated conveyances, such as reinforcement matting, to enable higher flows to be conveyed without scour. This has the benefit of reducing scour pathways where water moves more quickly with less potential for volume reduction. It also helps prevent sediment loading from scour. Create permanent pools for water quality improvement. Wetland-type systems, often referred to as wet swales, make use of check dams to create a series of impoundments where wetland conditions are allowed to develop. These systems can achieve high pollutant removal. However, they typically display low volume removal performance because typically their construction relies on impermeable soils and thus evapotranspiration is the primary mechanism for volume removal. Wetland-type systems can also provide areas for vector establishment and reproduction, possibly resulting in the need for abatement measures. Additional Sources of Design Information California Stormwater Quality Association. California Stormwater BMP Handbook: New Development and Redevelopment. TC-30, Vegetated Swale. 2003. http://www.cabmphandbooks.com/Development.asp. Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 3: Grass Channel v.1.9. 2011. http://chesapeakestormwater.net/category/publications/design-specifications/ . Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 10: Dry Swale v.1.9. 2011. http://chesapeakestormwater.net/category/publications/design-specifications/ . Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 11: Wet Swale v.1.9. 2011. http://chesapeakestormwater.net/category/publications/design-specifications/ .
A-6 Volume Reduction of Highway Runoff in Urban Areas Washington Department of Ecology. Stormwater Manual for Western Washington. BMP RT.04: Biofiltration Swale. 2012. https://fortress.wa.gov/ecy/publications/summarypages/1210030.html. Washington Department of Transportation. Highway Runoff Manual. BMP RT.04: Biofiltration Swale. 2011. http://www.wsdot.wa.gov/Environment/WaterQuality/Runoff/HighwayRunoffMa nual.htm. Key Planning-Level Design Parameters for Volume Reduction Conceptual Design Parameter Description Representative Range Bottom width The width of the level bottom of the conveyance feature 1 to10 ft Side slopes The steepness of the sides of the conveyance that connect the bottom of the swale to the ground surface 3H:1V or flatter Longitudinal slope The slope of conveyance in the direction of flow 1% to 5% Storage layer thickness The depth of the stone or amended soil storage reservoir 0 to 24 in. (not required) Effective sump storage depth The effective depth of water retained (in media or stone pores or behind check dams) that does not freely drain to surface drainage. (If storage is in pores, the depth is the effective depth accounting for pore space.) 0 to 6 in. (not required) Water quality flow depth The water level above the bottom of the swale during small storms that is considered to provide treatment 0 to 6 in. Maximum flow depth The maximum water level above the bottom of the swale under peak storm design conditions 1 to 2 ft Design infiltration rates The rate at which water is assumed to infiltrate into the subsurface soils for the purpose of design and benefit evaluation. This should be the rate of infiltration below the amended soil layer or stone reservoir. Can be used in any soil conditions On-line versus off-line configuration Vegetated conveyance that is on-line is designed to provide conveyance for all storm events; treatment functions are considered to cease or be minimal when the water quality flow is exceeded. However, volume reduction would be expected to continue to occur at higher rates based on higher head values. A vegetated conveyance that is off-line receives only water quality design flows; peak storm flows are bypassed around the system, while treatment and volume reduction processes Highway vegetated conveyance is typically on-line because of the challenge of providing flow splitter diversion at continue. various diffuse locations.
Volume Reduction Approach Fact Sheets A-7 Example Conceptual Design Schematics Figure 1.âCross-section view. Figure 2.âLongitudinal profile. Infiltraon Max flow depth Road Water quality flow depth Oponal stone/media storage reservoir Dense vegetaon Topsoil Oponal taller vegetaon H V Side slope Storage layer thickness Infiltraon Oponal piped inlet Oponal check dam Flow to secondary VRA Oponal stone inlet protecon topsoil Oponal stone/media storage reservoir H V Longitudinal slope
A-8 Volume Reduction of Highway Runoff in Urban Areas Figure 3.âPlan view. RO AD RO AD Op onal check dam Discharge to secondary VRA Op onal piped inlets Swale boom Stone inlet protec on
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, compost- Dispersion 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) designed to convey runoff as sheet flow over the surface or as shallow subsurface flow through amended soil layers. Dispersion reduces overall runoff volume by means of infiltration and evapotranspiration. Volume reduction performance can be improved with the use of flow spreaders, soil amendments, and re-vegetation. A critical element of this VRA is ensuring that dispersion areas support robust vegetative growth to stabilize the surface and maintain good infiltration rates. Dispersion involves making use of existing design features such as vegetated medians, road shoulders, and buffers by routing water to these areas and/or improving their ability to accept water. For example, dispersion could include removing curb/gutter sections where this would enable the flow of water to a pervious area that is acceptable. Additionally, the benefits of an existing dispersion pathway can be enhanced through minor investments in modification of drainage patterns (i.e., improve uniformity of dispersion) or restoration of degraded areas. In many cases, the buffers and medians that would otherwise be constructed as part of standard roadway design can provide volume reduction and treatment benefits Informal dispersion to median and shoulder, Interstate 8, San Diego urban area (Credit: Google). VOLUME REDUCTION PROCESSES Overall volume reduction potential Infiltration Evapotranspiration Consumptive use Base-flowâmimicking discharge URBAN HIGHWAY APPLICABILITY Ground-level highways Ground-level highways with restricted cross-sections Ground-level highways on steep transverse slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges LowHigh Moderate with very limited incremental cost.
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 siteâs soils, topography, and hydraulic characteristics (e.g., storage capacity, hydraulic retention time). Highly permeable soils have the capacity to infiltrate large volumes of stormwater, and small depressions can capture and store stormwater runoff, which can then infiltrate, evaporate, or be consumed by vegetation between events. Because of the extensive nature (i.e., larger footprint) of dispersion-type approaches, the ET fraction of the water balance tends to be significant. General DOT Experience Dispersion, as a VRA, is commonly used for management of stormwater runoff from highways, particularly in more rural areas. The approach of allowing water to sheet flow over shoulders tends to be compatible with standard highway designs where shallow gradient medians and shoulders would be otherwise constructed. The benefits of dispersion for reducing runoff volumes and treating stormwater are increasingly recognized by DOTs. While DOTs have made these land and design investments for transportation and safety purposes, they also provide water quality and volume reduction benefits. Taylor et al. (2014a) conclude: âFor swales and filter strips, water quality benefits can effectively be considered free when compared to conventional drainage systems and when the maintenance is performed by the property owner.â Additionally, by amending roadway shoulders with compost and other materials, there is potential to improve the ability of existing road shoulders to reduce runoff volumes and provide treatment, thereby allowing incremental benefit to be claimed for relatively low investment. In more constrained situations, DOTs have found that current design standards for highway construction do not always align with applicable design guidelines for filter strips and dispersion. For example, Reister and Yonge (2005) note that the Washington State Highway Runoff Manual recommends a maximum side-slope of 7:1 for dispersion practices while most roadway embankments fall between 2:1 to 6:1 where space or topography constrains designs. For steeper slopes, specific attention should be given to effective spreading of flow and maintaining sheet flow. Alternatively, a more engineered approach, such as a media filter drain (see VRA 03), may be more appropriate for steeper shoulders than simple dispersion over a naturally vegetated area. DOTs have also found that vegetative cover and regenerative growth are critical to maintaining long-term infiltration rates. A monitoring case study on vegetated filter strips in Texas by Glick et al. (1993) also highlights the importance of infiltration capacity to vegetative cover, with more natural and wooded areas having greater capacity to infiltrate runoff. Studies of filter strips reported to the International Stormwater BMP Database, mostly in California, showed moderate levels of volume reduction (Water Environment Research Foundation, 2011). In addition, DOTs have considerable experience using compost amendment of road shoulders as an initial treatment following construction to promote stabilization and vegetation growth (U.S. EPA, 2013c).
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. Where soils are silty or clayey, a sand or compost amendment may be needed to provide adequate long- term permeability for water to flow into the soil. Dispersion practices rely on sheet flow over a relatively large distance (typically at least 10 to 15 ft) to achieve significant volume reduction. They may therefore not be suitable for roads with very restricted rights-of-way. Embankment slopes should provide positive drainage away from the roadway, but not be steeper than approximately 6H:1V. Longitudinal slopes must not be too steep (typically less than 5%) in order to allow more uniform dispersion and avoid the creation of high-velocity flows that may result in erosion. Large drainage areas (i.e., roadways wider than approximately 2 to 3 lanes) may increase the potential for flow to concentrate during high- intensity storm events and produce high-velocity flows with the potential to create erosive conditions. Because of the importance of maintaining sheet flow into dispersion areas, site-specific calculations are recommended to account for local precipitation intensities, design geometries, and soil conditions. Sheet flow conditions can be encouraged by the use of a gravel area between the road shoulder and the dispersion area (see schematic design of media filter drain). Urban highways are not typically surrounded by undeveloped area; however, patches of natural vegetation sometimes exist, particularly in the centers of interchanges and in wide spots in the right-of-way. Therefore, the opportunity for dispersion is dependent on specific site conditions and availability of vegetation in the vicinity of the project. The dispersion area should be owned by the project owner or located in a permanent easement dedicated for water quality purposes. Geotechnical Considerations Generally, dispersion poses relatively limited incremental risk for slope stability and settlement because standard design practices help mitigate risks, including (1) accounting for surficial wetting in geotechnical calculations, (2) design of near-highway areas with positive drainage away from the highway, and (3) design features to prevent surficial erosion (i.e., flow spreading, shallow slopes, vegetated cover). The most significant geotechnical issue is the potential for rill erosion to form and progress along the roadside shoulder if soil is not stabilized or concentrated flow paths develop. Where a design modification will result in significant infiltration occurring in a concentrated area, such as ponding more than a few inches deep, analysis of slope stability and other geotechnical factors should be considered within the vicinity of this area.
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), dispersion poses relatively low risk of groundwater quality impacts and water balance impacts. Risks may be elevated in areas with very high soil infiltration rates or shallow groundwater tables. In these situations, soil amendments may be warranted to provide better treatment of infiltrated water and better soil water retention. Safety Considerations Dispersion areas should be free from trees and other obstacles within the clear zone (typically in the range of 22 to 32 ft from driving lanes). Cross- slopes within the clear zone should not exceed 4H:1V. If maintaining these conditions is not possible, a barrier should be placed between the road and the dispersion area, parallel to vehicular travel. Soil amendments that are used within the clear zone to improve permeability or vegetation growth should be selected to provide a finished surface that is adequately stable for errant vehicle recovery. If a vegetated conveyance is used to convey water to dispersion areas, it should be constructed with side slopes of 3H:1V or flatter. Any piped inlets should have openings cut flush with the slope in order to reduce the potential that the pipe will snag an errant vehicle. Pipes with diameters greater than 24 in. should be covered with traversable grates. Regional Applicability Dispersion can be applied across a broad range of climates, but would differ in nature in terms of vegetation. Dispersion approaches require dense and robust vegetation for proper function. In arid regions, drought-tolerant species should be selected to minimize irrigation needs and reduce the potential for seasonal die-off. In cold climates where salt is used, vegetation should be selected to be tolerant of elevated salt levels. Regional rainfall intensities and characteristic patterns should be considered during the design process to ensure that road shoulder sections will not be hydraulically overloaded and that sheet flow conditions will be maintained to the extent practicable. Where adjacent natural land covers are highly erosive (such as in arid areas), the elevated potential for rill erosion may present challenges for the application of this approach. New Projects, Lane Additions, and Retrofits Dispersion may have small incremental costs in new projects since suitable areas such as vegetated shoulders are often already incorporated into the project as design features.
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. The feasibility of retrofitting an existing embankment would be influenced by the amount of import/export of material that would be needed (i.e., soil amendment versus soil replacement). Use in a Treatment Train A vegetated conveyance can be used to convey runoff to a dispersion area. Dispersion can be used to pretreat and convey stormwater to secondary VRAs. VRA-Specific Maintenance Considerations (see Section 4.3.6 for additional maintenance information in common with other VRAs) Typical roadside maintenance activities apply. In addition, maintenance activities should seek to maintain dense, robust vegetative cover and correct erosion issues before they progress. The need for corrective maintenance can be reduced by good dispersion design practices. Enhancements and Variations Slow the velocity of flow. Areas of dispersion may be planted with densely growing native/non-invasive vegetation to slow flows and allow greater volume reduction. Minor regrading to level the surface can also help slow and more evenly distribute flow. Check dams and berms may be constructed on steeper slopes to slow flows and create small ponding areas to encourage infiltration and treatment. Spread out the flow equally. Equal distribution of flows can help ensure that all the available area is being utilized, thereby improving both volume reduction and treatment capacity. Equal dispersion can be achieved by leveling the surface and using shoulder treatments such as stone spreading trenches that promote more even inflow. Maintenance may be needed to avoid the development of concentrated flow pathways. Landscaping/restoration. Planting or restoring areas of dispersion can be used to establish and promote higher and stable infiltration rates while also providing increased roughness to slow overland flows. Establishing and retaining dense/natural vegetation will help ensure that infiltration rates are maintained over the long term. Vegetated conveyance dispersion area. Where road shoulders are not conducive to overland flow or the dispersion area is a distance from the roadway, a vegetated conveyance can be used to convey flow to the dispersion area. Improve infiltration rates. Where site soils are silty or clayey, sand may be incorporated into the soil along with compost to improve infiltration and flow through the media. Where site soils are plastic and would not sustain long-term permeability, the topsoil layer can be removed and replaced with a compostâsand mixture.
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. TC-31, Vegetated Buffer Strip. 2003. http://www.cabmphandbooks.com/Development.asp. Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 2: Sheet Flow to a Vegetated Filter Strip or Conserved Open Space v.1.9. 2011. http://chesapeakestormwater.net/category/publications/design-specifications/. Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 4: Soil Compost Amendment v.1.9. 2011. http://chesapeakestormwater.net/category/publications/design-specifications/. Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 2: Sheet Flow to a Vegetated Filter Strip or Conserved Open Space v.1.9. 2011. http://chesapeakestormwater.net/category/publications/design-specifications/. Washington Department of Ecology. Stormwater Manual for Western Washington. BMP FC.01: Natural Dispersion. 2012. https://fortress.wa.gov/ecy/publications/summarypages/1210030.html. Washington Department of Ecology. Stormwater Manual for Western Washington. BMP FC.02: Engineered Dispersion. 2012. https://fortress.wa.gov/ecy/publications/summarypages/1210030.html. Washington Department of Ecology. Stormwater Manual for Western Washington. BMP RT.02: Vegetated Filter Strip. 2012. https://fortress.wa.gov/ecy/publications/summarypages/1210030.html. Key Planning-Level Design Parameters for Volume Reduction Conceptual Design Parameter Description Representative Range Footprint area The area that will receive stormwater. No practical limit, larger areas will tend to provide greater volume reduction. Contributing area The area draining to the footprint area No practical limit; however, inflows should be distributed as sheet flow or multiple diffuse inflow points to avoid concentrating flows. Site-specific analysis of overland flow hydraulics and soil properties should be conducted at the design phase to ensure that potential for erosion and scour have been addressed.
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. Width of amended shoulder The width of the shoulder in the direction of flow (i.e., perpendicular to the roadway edge) 10 to 15 ft typical; however, there is no practical limit; larger areas will tend to provide greater volume reduction. Cross-slope The final grade of the road shoulder surface (perpendicular to the roadway edge) as a ratio of vertical distance to horizontal distance (i.e., 12%, or 8H:1V) 4H:1V or flatter Amendment thickness The depth to which amendments are incorporated into the soil 6 to 12 in. Effective depth of depression storage Including pore storage added through soil decompaction/amendment or naturally occurring depressions where ponding is expected; expressed as depth 1 to 6 in. Example Conceptual Design Schematics Figure 1.âCross-section view. Infiltraon Exisng or restored vegetated area Road Oponal compost amended filter strip H V Cross slope Amendment thickness Oponal stone level spreader
A-16 Volume Reduction of Highway Runoff in Urban Areas Figure 2. Plan view. RO AD Dispersion area Oponal flow to dispersion area via vegetated filter strip on road shoulder Oponal flow to dispersion area via vegetated swale
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. The stone vegetation- free zone produces sheet flow, which is pretreated as it flows across the grass strip and is then captured by the storage reservoir, where it infiltrates into the subsoil or is discharged through the underdrain. This VRA is typically installed between the road surface and a ditch or other conveyance located downslope. While this approach shares many similarities to VRA 02 â Dispersion, its engineering design features allow it to be sited in more constrained areas and on steeper cross-slopes where dispersion would not be as viable. Media Filter Drain Along SR 14 in Clark County, WA. Source: Washington State DOT, 2011. VOLUME REDUCTION PROCESSES Overall volume reduction potential Infiltration Evapotranspiration Consumptive use Base-flowâmimicking discharge URBAN HIGHWAY APPLICABILITY Ground-level highways Ground-level highways with restricted cross-sections Ground-level highways on steep transverse slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges LowHigh Moderate
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. The relative volume reduction potential is a function of the underlying infiltration rate and the local wet-season ET rates. The primary flow pathway through the media tends to provide flow attenuation, which may partially mimic base flow in some environments. General DOT Experience This VRA is widely used by WSDOT, and was formerly referred to as an âecology embankment.â A technology evaluation report prepared on ecology embankments for WSDOT (Herrera Environmental Consultants, 2006) shows both significant volume and load reductions up to, in some cases, 10%. Other stormwater design manuals have begun to incorporate elements of the media filter drain design; however, widespread application outside of Washington State has not occurred to date. Some design standards for filter strips employed by other DOTs include elements that resemble the media filter drain. Applicability and Limitations Site and Watershed Considerations Media filter drains are suitable for most soil types. Where soils are silty or clayey, an underdrain may be required to convey excess runoff. Media filter drains are one of the few VRAs that can be constructed directly on roadside embankments up to a 4H:1V slope and incorporated into conventional highway design. They may be quite useful in situations where roadway embankments are the only vegetated area within the right-of-way. Media filter drains work best on low to moderately longitudinal slopes (less than 5%). Greater longitudinal slopes present greater difficulties for evenly spreading water. Large drainage areas (i.e., wider roadways) may increase the potential for flow to concentrate during high-intensity storm events and produce high- velocity flows with the potential to create erosive conditions. Sheet flow conditions can be encouraged by the use of a dispersion trench or other approach intended to spread and slow flows. Media filter drains can be sited in confined rights-of-way, on shoulders, and in narrow medians, and are suitable in many confined urban highway settings. Geotechnical Considerations Generally, use of media filter drains introduces relatively limited incremental risk for slope stability and settlement because standard highway design practices help mitigate risks, including (1) accounting for surficial wetting in geotechnical calculations, (2) design of shoulder with positive drainage away from the highway, and (3) design features to prevent surficial erosion (e.g., flow spreading, shallow slopes, vegetated cover). Site-specific infiltration rates and physical make-up of the soil (i.e., soil class) will determine what design features are needed for effective volume reduction and treatment. 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.
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. Safety Considerations Media filter drains are usually located within the clear zone, but their low cross-slopes and lack of fixed obstacles make them safely traversable, and no barriers are required. Regional Applicability Media filter drains require dense and robust vegetation for proper function. In arid regions, drought-tolerant species should be selected to minimize irrigation needs and reduce the potential for seasonal die-off. If a regionally adapted species cannot be identified to provide surface stabilization without irrigation, then this VRA may not be applicable. In cold climates where salt is used, vegetation should be selected that is tolerant of elevated salt levels. Regional rainfall intensities and characteristic patterns should be considered during the design process to ensure that road shoulder sections will not be hydraulically overloaded and sheet flow conditions will be maintained to the extent practicable. New Projects, Lane Additions, and Retrofits Media filter strips can be incorporated into conventional highway design or can be constructed on existing roadside embankments. Retrofitting an existing embankment would involve export of existing soils, installation of an underdrain, and import of the specialized media filter mix. As such, retrofits are expected to be more expensive than when constructed as part of a new project or lane addition. Use in a Treatment Train Media filter drains can be used to pretreat and convey stormwater to secondary VRAs. VRA-Specific Maintenance Considerations (see Section 4.3.6 for additional maintenance information in common with other VRAs) Maintenance consists of routine roadside management. Enhancements and Variations Apply on internal as well as external embankments. If the roadway has a median, then a dual media filter drain design can be used to capture runoff from both of the internal embankments. Use an underdrain to improve hydraulic conveyance where infiltration rates are limited. Where site soils are silty or clayey, an underdrain may be used to improve hydraulic conveyance of stormwater through the media. Treated runoff would be conveyed to a downstream VRA or stormwater outfall.
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. Sources of Additional Information Herrera Environmental Consultants. Technology Evaluation and Engineering Report, WSDOT Ecology Embankment, Prepared for Washington State Department of Transportation. 2006. http://www.wsdot.wa.gov/NR/rdonlyres/3D73CD62-6F99-45DD- B004-D7B7B4796C2E/0/EcologyEmbankmentTEER.pdf. Washington Department of Ecology. Stormwater Manual for Western Washington. BMP RT.07: Media Filter Drain. 2012. https://fortress.wa.gov/ecy/publications/summarypages/1210030.html. Key Planning-Level Design Parameters for Volume Reduction Conceptual Design Parameter Description Representative Range Footprint area The area covered by the surface of the media filter drain Any Maximum flow path The maximum distance runoff should travel as sheet flow to the media filter drain (i.e., maximum width of travel lanes) Up to 150 ft Tributary area ratio The footprint of the media filter drain as a fraction of the total tributary area (including the media filter drain itself) Up to 10:1 may be typical of urban roadways Cross-slope The slope of the embankment perpendicular to the roadway 4H:1V or flatter Longitudinal slope The slope running parallel to the roadway Typically limited to less than 5% Stone strip width The width of the stone strip used to create sheet flow 1 to 3 ft Grass strip width The width of the grass strip used for pretreatment 3 to 5 ft Media filter depth The depth of the filter media storage reservoir 12 in. Design soil infiltration rate The rate at which water is assumed to infiltrate into the subsurface soils for the purpose of design and benefits evaluation. This should be the rate of infiltration below the amended soil layer or stone reservoir. Any
Volume Reduction Approach Fact Sheets A-21 Example Conceptual Design Schematic Figure 1.âCross-section view. Figure 2.âPlan view. Infiltraon Road Stone strip width Flow to secondary VRA Grass strip width Media filter drain width Oponal underdrain in stone trench Media filter thickness Topsoil H V Cross slope RO AD Grass strip Stone strip Media filter drain Oponal underdrain
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. Precipitation falling on the permeable pavement as well as stormwater flowing onto the permeable pavement from adjacent travel lanes infiltrates through the permeable pavement top course into the stone reservoir, from which it infiltrates into the subsoil or is discharged through an underdrain and outlet control structure. Through the use of an underdrain and flow-control outlet to augment infiltration capacity, permeable shoulders can be applied in a wide range of soil conditions. This VRA is most effective for volume reduction when soils are suitable for infiltration or where outlet control can be provided to mimic base-flow discharge. In contrast to permeable pavements applied in parking lots, parking strips, streets, and walkways in other land uses, permeable road shoulders tend to be characterized by a higher ratio of tributary impervious area (i.e., travel lanes) to pervious area (shoulders). Additionally, more stringent requirements may apply to the structural design and subbase drainage design than apply to permeable pavements in other land uses. Volume Reduction Processes and Performance Volume reduction is achieved primarily through infiltration. The degree of allowable infiltration is a function of soil infiltration rates (after compaction), degree of subbase wetting that is allowable in design, and the presence of other factors Photo credit: Pike Industries. VOLUME REDUCTION PROCESSES Overall volume reduction potential Infiltration Evapotranspiration Consumptive use Base-flowâmimicking discharge URBAN HIGHWAY APPLICABILITY Ground-level highways Ground-level highways with restricted cross-sections Ground-level highways on steep transverse slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges LowHigh Moderate Factors such as slope stability and utility issues. Where infiltration is limited due to soil
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. General DOT Experience Permeable pavement shoulders are increasingly being considered for implementation within the highway environment. DOTs have found permeable pavement shoulders to be an effective method to not only improve roadway safety (by reducing surface flow and splash/spray effects) but also to reduce overall stormwater volumes generated from the linear roadway environment. Volume reductions from permeable pavement shoulders are generally moderate to high. Runoff reduction estimates derived from various case studies summarized by Hirschman et al. (2008) range from 45% when incorporating underdrains to 75% when not using underdrains and assuming adequate pretreatment and soil testing, although it should be noted that in some studies (Van Seters et al., 2006; Legret and Colandini, 1999; Bean et al., 2007, Collins et al., 2008 and Brattebo and Booth, 2003), volume reductions ranged from 94% to 100%. Ongoing research with methods including full-depth permeable shoulders is also being conducted with specific applications to the highway environment. The University of California Pavement Research Center concludes that permeable shoulders are technically feasible and economically advantageous compared to other BMPs and can be used where infiltration rates are as low as 0.014 in. per hour (Chai et al., 2012). Guidance on design, construction, and maintenance of permeable shoulders with stone reservoirs has been developed under NCHRP Project 25-25(82). For information, please go to http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=3315. An additional summary of permeable pavement experiences is provided in Appendix F, which is published as part of NCHRP Web-Only Document 209. Applicability and Limitations Site and Watershed Considerations Permeable pavements are especially well suited to areas with granular soils, such that infiltration rates are relatively high, and subgrade strength is not significantly diminished by wetting. Roadways with flat to shallow longitudinal slopes (less than 1%) are most suitable for permeable shoulders because the volume of the storage reservoir is best utilized. Where longitudinal slopes are steeper, cutoff walls and intermediate outlet points are needed at a greater frequency, and there is greater potential for water to flow longitudinally below the roadway. Permeable pavements can be used on road shoulders and in medians. They can be useful in constrained areas where there is insufficient space for vegetated VRAs. A fully lined version of permeable pavement with an underdrain could be used on elevated highways or viaducts. Stormwater could be stored within the stone reservoir and would then be discharged via underdrains or routed to additional BMPs.
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. Geotechnical and Pavement Design Considerations Use of a permeable shoulder without a liner increases moisture content below the shoulder and may also increase moisture content below the main-line road segment; this should be accounted for in subgrade strength calculations. A greater subbase depth may be required to account for reduced subgrade-bearing capacity. The bearing strength of granular soils tends to be less sensitive to moisture content than are fine-grained soils. The strength of fine-grained soils such as clays can be significantly reduced when the subgrade is wetted. Infiltration may also result in settlement, slope stability, utility issues, or other issues that may damage pavements. Impermeable barriers can be used between the permeable pavement installation and the roadway (i.e., a separation wall) in order to avoid compromising road integrity from excess infiltration or saturated conditions. However, this may require a supplemental drainage upstream of the separation wall to prevent accumulation of water below the main- line road section. While flow water into traditional pavement is less than into permeable pavement, water still enters the subgrade from incidental wetting through cracks, potholes, and other imperfections. 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 these situations, designs may need to be adjusted to enhance treatment and prevent groundwater contamination. Examples of design adjustments are providing an amended soil layer below the storage reservoir and providing greater separation to groundwater. Impermeable liners between the pavement subbase and subgrade soils can be used to prevent infiltration, where needed. Permeable shoulders can result in substantially greater groundwater recharge than predevelopment conditions; the use of underdrains with adaptable outlet elevation can help provide a contingency for water balance impacts. Safety Considerations Permeable shoulders function in the same way as shoulders with standard pavement and do not present any added safety hazards. Studies have found that, in cold-weather climates, less salt application is needed to address ice formation than is needed on traditional pavements (see Appendix F). Supplemental drainage may be needed in critical cross-sections, such as sags and depressed sections, to ensure that peak flows can be conveyed from the roadway in the event that the permeable surface clogs.
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. Freeze/thaw cycles should be considered in cold climates, particularly when permeable pavement is designed with storage capabilities. Expansion and contraction of stored water can have implications to long- term pavement structure and stability. Permeable shoulders should not be used where roads are sanded during the winter. Additionally, salting of roadways may pose groundwater quality issues but may have a net benefit if total salt usage can be reduced. Permeable pavement can be effective for controlling temperature impacts associated with roadway runoff in humid areas. New Projects, Lane Additions, and Retrofits Permeable shoulders tend to be more practicable and cost-effective in new construction and lane additions than as a retrofit; in new construction, the cost of the permeable shoulder can be offset in part by the avoided cost of a traditional shoulder that would otherwise be constructed. Additionally, the drainage of the main-line roadway subbase can be coordinated with the drainage of the permeable shoulder. In contrast, retrofitting existing roadways with permeable pavement requires complete removal of the existing shoulder pavement and subbase, modification of the subbase drainage, and interfacing of the new permeable shoulder with the main roadway. If an impermeable liner is needed between the main-line roadway and the permeable shoulder, a portion of the main-line roadway may need to be excavated to provide secondary drainage for the upstream side of the liner. However, permeable shoulder retrofits may be one of the only options available in space-constrained highway segments. Use in a Treatment Train It is not typically practicable to provide pretreatment prior to discharge to permeable pavement; however, a sand filter layer within the permeable pavement can serve as pretreatment prior to water entering the subsurface reservoir. Permeable pavement can be designed with an underdrain that can be used to convey stored and partially treated runoff to secondary VRAs. An amended soil layer below the stone reservoir can be used to improve the level of treatment of infiltrated water before it reaches groundwater. VRA-Specific Maintenance Considerations (see Section 4.3.6 for additional maintenance information in common with other VRAs) Permeable shoulders should include regular maintenance procedures to ensure that overall permeability and infiltration are maintained. To reduce
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. Increasing the depth or porosity of the stone subbase can be done to significantly increase the storage capacity of permeable pavement systems. Structural implications should be considered in alterations to stone properties. Incorporate an underdrain and outlet controls. The use of underdrains in permeable pavement systems can provide a means of controlled and directed release of stored and partially treated stormwater. This variation can be used to direct effluent to secondary VRAs/BMPs or mimic natural base-flow conditions. It can also help provide adaptability of designs relative to water balance issues. Consider various materials. Several different surface materials are available for permeable pavement (e.g., permeable concrete, permeable asphalt, permeable pavers). Different materials can be selected to tailor the design to specific applications and requirements. Sources of Additional Information AASHTO. Guide for Design of Pavement Structures, Washington, D.C. 1993. ACI. âSpecification for Permeable Concrete Pavements.â 522.1-08, Committee 522, American Concrete Institute. 2008. ACPA . American Concrete Paving Association, Pervious Pave â Background, Purpose, Assumptions and Equations, Washington, D.C. 2012. ASCE. Recommended Design Guidelines for Permeable Pavements. Manual of Practice on Recommended Design Guidelines for Permeable Pavements, B. Eisenberg, K. Lindow, and D. Smith, eds., American Society of Civil Engineers, The Permeable Pavements Technical Committee, Low Impact Development Standing Committee, Urban Water Resources Research Council, Environment and Water Resources Institute. Low Impact Development Center, Inc. Low Impact Development Manual for Southern California: Technical Guidance and Site Planning Strategies. 2010. http://www.casqa.org/LID/SoCalLID/tabid/218/Default.aspx. NAPA. âDesign, Construction, and Maintenance Guide for Permeable Asphalt Pavements.â Information Series 131, National Asphalt Pavement Association. 2008. NCHRP Project 25-25 (Task 82), âPermeable Shoulders with Stone Reservoirs.â http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=3315. Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 7: Permeable Pavement v.1.9. 2011. http://chesapeakestormwater.net/category/publications/design-specifications/. Washington State Department of Transportation. Highway Runoff Manual. BMP IN.06: Permeable Pavement Surfaces. 2011. http://www.wsdot.wa.gov/Environment/WaterQuality/Runoff/HighwayRunoffManua l.htm. See Appendix F for additional references on permeable pavements and shoulders.
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) Effective reservoir storage depth The effective depth of water stored within the permeable pavement system; function of the depth and porosity of the permeable stone storage layer and the elevation of the overflow Up to about 1 ft Longitudinal Slope Slope along the axis of the road and associated slope along the bottom of the infiltration bed Preferably less than 2%; possibly up to 5% with cutoff walls/berms. Top course permeability The rate at which water is assumed to flow through the permeable top course above the storage layer; note that permeability typically does not control volume reduction design for shoulders that are maintained Typically greater than 100 in./hr, up to more than 1,000 in./hr (not typically assumed to control design) Subbase design infiltration rates The rate at which water is assumed to infiltrate into the subsurface soils for the purpose of design and benefits evaluation. This should be the rate of infiltration below the stone reservoir. At least 0.3 to 0.5 in./hr for full infiltration systems without underdrains; systems with partial infiltration possible down to approx. 0.01 in./hr Surface outlet stage The stage at which the system begins to discharge to the surface conveyance system via the underdrain and outlet control features, if provided At least 6 in. below pavement Surface outlet discharge drawdown time The time it takes for the storage volume above the surface outlet stage to drain from brim full if extended detention is provided Typically, 24 to 48 hours for extended detention treatment
A-28 Volume Reduction of Highway Runoff in Urban Areas Example Conceptual Design Schematics Figure 1.âCross-section view. Figure 2.âPlan view. Infiltraon Stone reservoir thickness Road Impermeable barrier Permeable pavement surface course Oponal filter course Oponal underdrain Compacted subbase Oponal flow control outlet Oponal impermeable liner Discharge stage RO AD Permeable pavement shoulder
Volume Reduction Approach Fact Sheets A-29 Figure 3.âLongitudinal profile of an installation along a mild slope (earthen berms). Figure 4.âLongitudinal profile of an installation along a mild slope (geotextile cutoff walls). Infiltraon H V Longitudinal slope Permeable pavement surface course Oponal filter course Oponal flow control outlet Discharge stage Earthen berm separang level beds Stone reservoir Infiltraon H V Longitudinal slope Permeable pavement surface course Oponal filter course Discharge stage Geotexle fabric to create internal baï¬e walls Stone reservoir Oponal flow control outlet(s)
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. Engineered soil media are central to bioretention design and typically include a mixture of sand, soils, or organic elements that are designed to provide permeability, promote plant growth, and provide treatment; guidance for media design varies by region. Vegetation is also a characteristic element of bioretention design and typically includes grasses, sedges, and small woody plants and shrubs. Storage capacity is a function of the ponding depth, media/stone porosity, and the footprint of the facility. Additional storage can be gained by adding a stone storage layer beneath the soil medium. The shape of a bioretention area is not critical to its function, and it is common for facilities to be roundish, irregular, or linear. Overall volume reduction potential depends on infiltration rates and storage capacity, with some losses to evapotranspiration. POT Credit: Geosyntec Consultants Highway 99E Viaduct, Portland, OR. VOLUME MANAGEMENT ENTIAL/PROCESSES Overall volume reduction potential Infiltration Evapotranspiration Consumptive use Base-flowâmimicking discharge URBAN HIGHWAY APPLICABILITY Ground-level highways Ground-level highways with restricted cross-sections Ground-level highways on steep transverse slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts (if space below viaduct is available for VRAs) Linear interchanges Looped interchanges LowHigh Moderate
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. Vegetation and roots play an important role in maintaining and regenerating infiltration and evapotranspiration rates as well as supporting a healthy biological community in the soil media for treatment purposes. General DOT Experience Bioretention facilities have seen widespread use in other land uses and are increasingly being found in DOT stormwater design manuals across the country. They have been successfully implemented within the linear highway environment in many locations. Edmonston, Maryland, incorporated bioretention facilities were shown to successfully capture 1.33 in. of rainfall (90% of storm events) without overtopping (Low Impact Development Center, 2010) while Maryland State Highway Administration (SHA) is installing 80 permanent BMPs, mostly rain gardens in one interchange. Case studies along the eastern United States have shown volumetric reductions from 47% to 69% in the urban highway environment (Hunt et al., 2010). Various studies summarized by Hirschman et al. (2008) estimate volume reduction from bioretention ranging from 40% with underdrains to 80% when using an infiltration based design. MnDOT, ODOT, and WSDOT also have considerable experience with bioretention (with or without underdrains) in the urban highway environment. Applicability and Limitations Site and Watershed Considerations Use of bioretention without an underdrain requires soils with infiltration rates high enough to ensure that the bioretention cell drains fully between storm events. Proper infiltration of captured stormwater from bioretention cells requires that the groundwater table be at least several feet below the bottom of the bioretention cell. Bioretention can be used in many urban applications where available space exists and site characteristics meet or can be modified to design requirements. It can be readily applied on shoulders, interchanges, and medians with low slopes. Bioretention can be incorporated into narrower linear spaces by using vertical side walls as barriers between the bioretention cell and the road instead of shallow slopes. Appropriate safety considerations, such as guardrails, are necessary. Terraced bioretention cells can be constructed in areas with steeper longitudinal slopes. In linear configurations, bioretention can serve a conveyance purpose and allow reduction in piping requirements. Watersheds with high sediment loads (such as from disturbed open spaces) may result in premature clogging of the system. Geotechnical Considerations Bioretention without underdrains is primarily an infiltration measure and, therefore, must be cited and designed accordingly. Wide medians, wide
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), geotechnical considerations can be reduced while still providing some volume reduction. Groundwater Quality and Water Balance Considerations The amended media layer in bioretention provides a relatively high level of treatment of particulate-bound pollutants, dissolved metals, petroleum hydrocarbons, and pesticides and therefore results in a relatively low risk of groundwater quality impacts from these constituents if separation to groundwater is observed. Like other infiltration VRAs, bioretention is not generally effective for controlling salts or viruses. Media with excessive compost or poor controls on sources of media elements can leach nutrients, specifically nitrate and dissolved phosphorus, as well as metals and pathogens. This can be mitigated through careful media design. In soils with high infiltration rates, bioretention can result in greater recharge than with natural conditions. If water balance issues would potentially result from an increase in groundwater recharge, this can be mitigated by including an underdrain to reduce the amount of infiltrated water (see VRA 06). Safety Considerations Bioretention soils are intentionally porous and uncompacted; therefore, bioretention should be located out of the clear zone, or barriers oriented parallel to traffic should be used to prevent errant vehicles from entering the bioretention cell. Regional Applicability Bioretention has been applied successfully across a broad range of climates; plant and soil media must be selected to be compatible with the local climate. Salt loadings in cold climates may influence plant selection and may necessitate the use of an underdrain if groundwater quality issues would result from infiltration of salts. If roads are sanded, providing a pretreatment system to settle sands is recommended. Irrigation is typically required for plant establishment in most climates in North America. New Projects, Lane Additions, and Retrofits For retrofit applications, existing compaction of subgrade may limit application; restoration of infiltration rates may be possible with decompaction. Cut and fill can typically be balanced in new construction, and drainage can be configured to account for bioretention areas. In contrast, in retrofit
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. Pretreatment can be provided prior to the bioretention cell by the use of vegetated conveyance features. Stormwater runoff in excess of the bioretention cellâs storage capacity can be conveyed to additional VRAs by use of overflow controls such as weirs. 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 not appropriate. Facilities should be checked periodically for evidence of erosion or excess sediment deposition. Remediation of surface clogging may be needed if watershed sediment loading exceeds the assimilative capacity of the bioretention cell. Enhancements and Variations Slow flow velocities and provide level pools. Bioretention can be used wherever there is open, fairly level space. When slopes exceed 6%, intermediate berms can be used to create level ponding areas within the bioretention area. Adaption to narrow spaces. Bioretention cell geometry is flexible and is easily adapted to the narrow linear spaces commonly available in the urban highway right-of-way, such as: Linear bioretention/retention swales. A bioretention area constructed in a linear configuration such that it provides retention and also serves as a conveyance feature when its capacity is exceeded. This configuration is likely well suited to linear segments of urban highway projects, whereas traditional bioretention may be better suited to interchanges. Bioretention planters. In constrained urban areas, it may be necessary to construct bioretention with vertical concrete retaining walls, such as a typical stormwater planter used on residential and commercial streets. Additional safety features such as a guardrail or barrier may be needed to allow for vertical retaining walls. Increase storage capacity. A variety of factors can be adjusted to increase storage capacity. A stone layer can be included beneath the bioretention medium. The depth of the bioretention medium can be adjusted. Additionally, the composition of the bioretention medium can be adjusted to increase porosity. This can be accomplished through the addition of sand, expanded shale, compost, or other soil amendments. Add surcharge detention. Perimeter berms or site topography can be used to provide additional storage capacity above the maximum infiltrated ponding depth to provide enhanced flow-control performance; it may be possible to meet flow control and volume control objectives with one facility.
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. 2010. http://www.casqa.org/LID/SoCalLID/tabid/218/Default.aspx. Low Impact Development Center, Inc. Bioretention Specification. 2003. http://www.lowimpactdevelopment.org/epa03/biospec.htm. North Carolina State University. Bioretention at North Carolina State University BAE. http://www.bae.ncsu.edu/topic/bioretention/index.html. Prince Georges County Bioretention Manual. 2009. http://www.princegeorgescountymd.gov/Government/AgencyIndex/DER/ESG/Bior etention/pdf/Bioretention%20Manual_2009%20Version.pdf. Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 9: Bioretention v.1.9. 2011.http://chesapeakestormwater.net/2012/03/design-specification-no-9- bioretention/. Washington Department of Ecology. Stormwater Manual for Western Washington. BMP T7.30: Bioretention Cells, Swales, and Planter Boxes. 2012. https://fortress.wa.gov/ecy/publications/summarypages/1210030.html. Photo credit: Philip Jones.
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. Slightly smaller than total footprint area Ponding depth The maximum water depth above the surface of the bioretention medium prior to overflow Typically 0.5 to 1.5 ft; can potentially be increased if plant selection and soil infiltration rates are suitable Engineered soil medium thickness The thickness of the engineered soil medium layer Typically 1 to 4 ft Stone storage layer thickness The thickness of the optional stone storage layer, if provided Not typically provided in bioretention design; may be any depth if used for supplemental storage Total storage depth The effective depth of water stored within the bioretention cell. Total storage depth is a function of ponding depth, bioretention medium depth and porosity, and the depth and porosity of the optional stone storage layer. Typically 0.5 to 3 ft Available pore storage capacity The effective void space of engineered soil media or stone reservoirs that is available for water storage 0.2 to 0.35 (unitless) Media filtration rate The rate at which water filters into the media layer from the surface storage area Typically designed to be greater than 1 in./hr Design infiltration rate The rate at which water infiltrates into the subsurface soils for the purpose of design and benefits evaluation. This should be the rate of infiltration below the amended soil layer or stone reservoir. Typically limited to underlying soils with greater than 0.3 to 0.5 in./hr for full infiltration design
A-36 Volume Reduction of Highway Runoff in Urban Areas Example Conceptual Design Schematic Figure 1.âCross-section view. Figure 2.âPlan view. Infiltraon Engineered soil medium thickness Oponal stone storage layer thickness Ponding depth Overflow Mulch Inflow via surface flow or pipe inlet Energy dissipaon stone RO AD r Potenal piped inlet Potenal vegetated swale Energy dissipaon stone Potenal vegetated filter strip Max ponded area Bioretenon media footprint Overflow
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. Vegetation is a critical element of bioretention design and typically includes grasses, sedges, and small woody plants and shrubs. Storage capacity is dependent on ponding depth and media and stone porosity. Where soil infiltration rates permit, storage can be enhanced by installing a stone reservoir beneath the underdrain. This category of VRA is suitable for a wider range of conditions than bioretention without an underdrain and can be used to mimic natural base flows. Additional reductions in volume are possible from infiltration into subsoil, where conditions permit. Bioretention designs with underdrains typically include a stone layer below the amended media layer, with an underdrain that discharges at an elevation above the bottom of the stone layer. This creates a sump of water that leaves the system by infiltration only. When the capacity of the sump layer is exhausted, treated water discharges via the underdrain. Between storm events, runoff captured in the bioretention medium above the sump layer slowly discharges via the underdrain, Photo credit: Geosyntec Consultants, I-5 Exit 298, Portland, OR. VOLUME REDUCTION PROCESSES Overall volume reduction potential Infiltration Evapotranspiration Consumptive use Base-flowâmimicking discharge URBAN HIGHWAY APPLICABILITY Ground-level highways Ground-level highways with restricted cross-sections Ground-level highways on steep transverse slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges LowHigh Moderate producing a long-duration, low-volume flow (depending on outlet controls) that is
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), evapotranspiration, and base- flowâmimicking discharge, where applicable. Volume reduction performance is dependent on subsoil infiltration rates, vegetation, and underdrain flow controls to ensure that captured runoff exits the cell between storm events. Vegetation and plant roots play an important role in maintaining and regenerating infiltration and evapotranspiration rates as well as supporting a healthy biological community in the soil medium for treatment. General DOT Experience Bioretention facilities have been successfully implemented within the highway and roadway environments in various locations across the United States. With the regulatory trend toward volume control and dispersed treatment, some DOTs are installing larger numbers of these types of VRA. For example Maryland SHA is installing more than 20 bioretention cells in one interchange project. Studies summarized by Hirschman et al. (2008) estimate volume reduction from bioretention with underdrains of from 20% to 65%, with an average estimated reduction of 40%. In studies in the International BMP Database, bioretention systems with underdrains have shown moderate to high reductions in stormwater volumes on average (Water Environment Research Foundation, 2011), even when underdrains were present. Applicability and Limitations Site and Watershed Considerations Bioretention with an underdrain is suitable for all soils provided the system medium has sufficient permeability. Bioretention can be used in many urban applications where water can be routed to a depressed area. The shape of a bioretention area is not critical to its function, and it is common for facilities to be roundish, irregular, or linear; therefore, bioretention tends to be more flexible to a wide variety of sites than many other VRAs. Bioretention with underdrains can be incorporated into narrower spaces by using vertical retaining walls as the bioretention cell edges. Terraced bioretention cells can be constructed on shoulders and areas with steeper slopes. In linear configurations, bioretention can serve a conveyance purpose and allow reduction in piping requirements. Watersheds with high sediment loads (such as from disturbed open space) may result in premature clogging of the system. Geotechnical Considerations Bioretention with underdrains may still allow lateral and vertical flow of water from the system unless lined with an impermeable barrier; related considerations apply. The underdrain outlet structure controls the relative amount of infiltration that occurs (and associated geotechnical risk) and can be adaptively managed as necessary.
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. Safety Considerations Bioretention soils are highly porous and uncompacted; therefore, barriers should be used, were appropriate, to prevent errant vehicles from entering the bioretention cell, or bioretention cells should be located out of the clear zone. Regional Applicability Bioretention has been applied successfully across a broad range of climates; plant and soil media must be selected to be compatible with the local climate. Salt loadings in cold climates may influence plant selection. Irrigation is typically required for plant establishment. New Projects, Lane Additions, and Retrofits Given suitable soil, space, and groundwater conditions, bioretention cells are relatively straightforward designs that can be incorporated into new projects. Retrofit projects will be similar in relative costs for bioretention systems, provided that there is adequate space and suitable site conditions, particularly if depressions exist. Additional costs of excavation and possible amendments may be incurred during construction. Prefabricated bottomless planters are widely available, and can be installed in more narrow applications with moderate costs, assuming sufficient conditions are met. Retrofitting an existing bioretention system with underdrains will involve significant excavation, piping, controls, and possible amendments to the medium and/or stone. Including underdrains in new construction is recommended if there is a possibility that they will be needed to supplement infiltration. 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. Pretreatment can be provided prior to the bioretention cell by use of vegetated conveyance features or a forebay. Stormwater runoff in excess of the bioretention cellâs storage capacity can be conveyed to additional VRAs by use of overflow controls such as weirs.
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. Facilities should be checked periodically for evidence of erosion or excess sediment deposition. Remediation of surface clogging may be needed if watershed sediment loading exceeds the assimilative capacity of the bioretention cell. In the event of decline in surface drainage, check underdrains for obstructions. Enhancements and Variations Slow flow velocities and mitigate steep slope effects. Bioretention can be used wherever there is open, fairly level space. When slopes exceed 6%, check dams can be used to create level ponding areas within bioretention features. Adaption to narrow spaces. Bioretention cell geometry is flexible and is easily adapted to the narrow spaces commonly available in the urban highway right-of- way. Vertical impermeable liners can be used in tight areas to prevent road base stability from being compromised. Increase storage capacity. A variety of factors can be adjusted to increase storage capacity. A stone layer can be included beneath the underdrain. The depth of the bioretention medium can be adjusted. Additionally, the composition of the bioretention medium can be adjusted to increase porosity. This can be accomplished through the addition of sand, zeolite, expanded shale, compost, or other soil amendments. Research is ongoing to determine which mixtures provide the highest porosity without compromising pollutant removal performance. Provide overflow. Stormwater runoff in excess of the bioretention cellâs storage capacity can be conveyed to additional VRAs/BMPs by use of overflow controls such as weirs. This variation can provide a means to effectively deal with bypass flows and mitigate possible flooding effects. Energy dissipation. Deflection weirs, obstructions, and stone may be used to dissipate energy of influent flows and help prevent scour and possible additional loading of sediment to downstream facilities. Extended detention. Perimeter berms or site topology can be used to provide additional storage capacity above the maximum ponding depth. If extended detention is implemented, multiple overflow controls should be considered to reduce flooding potential and ensure proper drainage. Active control. Internet-based technology has recently allowed more widespread deployment of forecast-enabled, real-time active controls for systems with underdrains. This approach can help improve the applicability and performance of these systems by making intelligent decisions about when and at what rate to release stored water based on storage conditions and forecasted rainfall.
Volume Reduction Approach Fact Sheets A-41 Sources of Additional Information Low Impact Development Center, Inc. Bioretention Specification. 2003. http://www.lowimpactdevelopment.org/epa03/biospec.htm. Low Impact Development Center, Inc. Low Impact Development Manual for Southern California: Technical Guidance and Site Planning Strategies. 2010. http://www.casqa.org/LID/SoCalLID/tabid/218/Default.aspx. North Carolina State University. Bioretention at North Carolina State University BAE. http://www.bae.ncsu.edu/topic/bioretention/index.html. Prince Georges County Bioretention Manual. 2009. http://www.princegeorgescountymd.gov/Government/AgencyIndex/DER/ESG/Bior etention/pdf/Bioretention%20Manual_2009%20Version.pdf. Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 9: Bioretention v.1.9. 2011. http://chesapeakestormwater.net/2012/03/design-specification-no-9-bioretention/. Washington Department of Ecology. Stormwater Manual for Western Washington. BMP T7.30: Bioretention Cells, Swales, and Planter Boxes. 2012. https://fortress.wa.gov/ecy/publications/summarypages/1210030.html. 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. Slightly smaller than total footprint area Ponding depth The maximum water depth above the surface of the bioretention medium prior to overflow Typically 0.5 to 1.5 ft; can be increased if plant selection and soil infiltration rates are suitable Engineered soil medium thickness The thickness of the engineered soil medium layer Typically 1 to 4 ft Stone storage layer thickness The thickness of the optional stone storage layer, if provided Typically 0 to 2 ft Available pore storage capacity The effective void space of engineered soil media or stone reservoirs that is available for water storage Typically 0.2 to 0.35 (unitless)
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. Typically 0.75 to 4 ft Design media filtration rate The rate at which water is assumed to enter and move through the engineered filter media Typically greater than 2 in./hr and less than 12 in./hr Design soil infiltration rate The rate at which water is assumed to infiltrate into the subsurface soils for the purpose of design and benefit evaluation. This should be the rate of infiltration below the amended soil layer or stone reservoir. Any; partial infiltration (upturned elbow design) can be used as low as approximately 0.01 in./hr Underdrain discharge stage The stage at which water begins to discharge from the underdrains (typically controlled via upturned elbow) Typically 0.5 to 2 ft above the bottom of the storage reservoir, if internal water storage is provided Sump storage The effective depth of water stored within the sump layer below the outlet elevation of the underdrain (typically controlled via upturned elbow) Typically 0.2 to 0.8 ft, accounting for porosity of stone below underdrain discharge stage Example Conceptual Design Schematic Figure 1.âCross-section view. Infiltraon Engineered soil medium thickness Oponal stone storage layer thickness Overflow Mulch Inflow via surface flow or pipe inlet Energy dissipaon stone Perforated underdrain Oponal upturned elbow Ponding depth
Volume Reduction Approach Fact Sheets A-43 Figure 2.âPlan 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
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. Infiltration trenches tend to be well suited to the linear highway environment as they are generally constructed in a linear configuration and their surface tends to be nearly flush to the existing grade. They tend to be located away from the travel lanes and shoulders but may be within the clear zone dedicated for errant vehicles to recover. Volume Reduction Processes and Performance Factors Volume reduction in infiltration trenches is achieved through infiltration into the surrounding subsoil. Efficient performance is dependent on storage capacity and adequate subsoil infiltration rates to ensure that enough captured runoff exits the trench between storm events. Variation of infiltration trenches by including underdrains can provide additional volume reduction performance and operational flexibility in the form of base-flowâ mimicking discharge. Source: Maryland SHA. VOLUME REDUCTION PROCESSES Overall volume reduction potential Infiltration Evapotranspiration Consumptive use Base-flowâmimicking discharge URBAN HIGHWAY APPLICABILITY Ground-level highways Ground-level highways with restricted cross-sections Ground-level highways on steep transverse slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges LowHigh Moderate
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. The most common problem incurred with infiltration trenches is clogging. A BMP retrofit pilot program final report by Caltrans (2004) notes that for events smaller than the design storm used to size the features, volume reduction for infiltration trenches was 100%. The Virginia Department of Conservation and Recreation (2011) notes that when designs incorporate less pretreatment and involve soils with lower infiltration rates, volume reduction estimates should be reduced to 50%. Proper design and maintenance of infiltration trenches is critical to their performance. The Maryland Department of the Environment found in an early study that 53% of the infiltration trenches they inspected were not operating as designed (Lindsey et al., 1991). This high failure rate has been attributed to clogging resulting from lack of pretreatment, inadequate maintenance, and insufficient subsoil infiltration rates. Applicability and Limitations Site and Watershed Considerations Use of infiltration trenches requires soils with infiltration rates high enough to ensure proper drainage between storm events. Without significant amendments, this is critical to infiltration trenches being considered feasible. Proper exfiltration of captured stormwater from infiltration trenches requires that the groundwater table be at least several feet below the bottom of the trench. Geotechnical Considerations Infiltration trenches must be located a sufficient distance from the roadway such that infiltration will not compromise its structural integrity. Use of infiltration trenches along steep transverse slopes may require enhanced protection of slope integrity. 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 these situations, designs may need pretreatment or to be adjusted to enhance treatment and prevent groundwater contamination. Use of infiltration trenches to provide more infiltration than was historically present or is characteristic of similar sites in the region may alter a siteâs water balance in undesirable ways. Safety Considerations Infiltration trenches should not present a significant hazard to errant vehicles. If a filter strip is used for pretreatment, the cross-slope should be less than 4H:1V. Observation wells and overflows should not protrude more than a few inches above the trench surface. If a piped inlet is used, the pipe openings should be cut flush with the transverse slope in order to reduce the potential that the pipe will be struck head-on by an errant vehicle. Pipes with diameters greater than 24 in. should be covered with traversable grates.
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. The linear nature of infiltration trenches makes them useful in tight spaces common to urban highways. Pretreatment can be included with a vegetated conveyance or the use of an in-line sedimentation forebay. Impermeable liners can be used to protect the integrity of the road base. New Projects, Lane Additions, and Retrofits Infiltration trenches may have small incremental cost in new projects because grading and fill can be balanced, and landscaping would otherwise be installed; incremental costs may be greater in lane additions and retrofits. Retrofitting existing roadways to include infiltration trenches can be an effective method for reducing runoff volumes and impermeable surface area. Incremental costs may be higher in retrofit situations since there may likely be a need for excavation and fill operations. Retrofitting an existing infiltration system with underdrains will involve significant excavation, piping, controls, and possible amendments to the medium and/or stone. Including underdrains as a backup option in new construction is recommended. Use in a Treatment Train Pretreatment of runoff to reduce particulate matter and suspended solids is recommended to prevent clogging. Pretreatment can be provided as a vegetated conveyance or a sedimentation forebay. Additional BMPs could also be located prior to infiltration trenches, provided sufficient routing is incorporated. Stormwater runoff in excess of the infiltration trenchâs storage capacity can be conveyed to additional VRAs/BMPs by the use of overflow controls such as weirs. VRA-Specific Maintenance Considerations (see Section 4.3.6 for additional maintenance information in common with other VRAs) Infiltration trenches have been observed to have high potential for failure from clogging. Pretreatment swales or filter strips should be maintained per guidelines for those types of VRAs. Scour in pretreatment systems may exacerbate potential for clogging of trenches. Check trench periodically for evidence of clogging. If trench becomes clogged, stone may need to be replaced, and infiltrating surface may need to be restored via over-excavation or scarification.
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. Storage capacity can also be increased with the selection of stone materials that have higher effective porosity. Provide robust pretreatment to extend the life of the system. Clogging is the principal cause of infiltration trench failure and resulting maintenance requirements. Pretreatment to remove sediments and particulate matter prior to entering the infiltration basin can significantly improve system performance and reduce the potential for clogging. Provide backup outlet where feasible. Including an underdrain (normally closed) can provide a low-cost backup in the event that the infiltration rate declines with time. If infiltration rates decline, the outlet can be opened and flow can be controlled to achieve a combination of volume reduction and flow control until the system infiltration rate can be restored. Reduce compaction during construction. The highest infiltration rates will be achieved if care is taken to avoid compaction of the bottom of the trench during construction. Laying a 6-in. layer of sand on the bottom of the trench will help to avoid compaction as the trench is filled with stone. Sources of Additional Information California Stormwater Quality Association. California Stormwater BMP Handbook: New Development and Redevelopment. TC-10, Infiltration Trench. 2003. Lindsey, G., L. Roberts, and W. Page. Storm Water Management Infiltration. Maryland Department of the Environment, Sediment and Storm Water Administration. 1991. Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 8: Infiltration Practices v.1.9. 2011. http://chesapeakestormwater.net/category/publications/design-specifications/. Washington Department of Ecology. Stormwater Manual for Western Washington. BMP IN.03: Infiltration Trench. 2012. https://fortress.wa.gov/ecy/publications/summarypages/1210030.html. Key Planning-Level Design Parameters for Volume Reduction Conceptual Design Parameter Description Representative Range Footprint area The area covered by the surface of the infiltration trench Typically 100 to 2,000 ft2; can be any size with appropriate flow distribution Stone storage layer thickness The thickness of the stone storage layer Typically 2 to 10 ft Porosity The effective void space of the stone storage layer Typically 0.3 to 0.4 (unitless)
A-48 Volume Reduction of Highway Runoff in Urban Areas Figure 1.âCross-section view. Effective storage depth The effective depth of water stored within the infiltration trench. It is a function of the depth and porosity of the stone storage layer. Typically 0.5 to 4 ft Side wall to bottom area ratio The ratio of system surface area in the side walls versus the bottom area Depends on geometry, for narrow deep systems, side wall area may equal more than 5 times the bottom area Design infiltration rates The rate at which water is assumed to infiltrate into the subsurface soils for the purpose of design and benefit evaluation. This should be the rate of infiltration below the stone reservoir layer. Typically require at least 0.3 to 0.5 in. per hour for sufficient drawdown of storage Example Conceptual Design Schematic Infiltraon Stone storage reservoir thickness Overflow Road Compost amended filter strip pretreatment Oponal sand Oponal pea gravel
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.âPlan view.
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. Infiltration basins are typically located in relatively permeable soils. While all infiltration systems may cause geotechnical hazards if inappropriately sited, infiltration basins may pose a higher risk because they tend to capture runoff from a larger area than most BMPs and concentrate infiltrated volume in a localized area. Infiltration basins can be designed with detention surcharge above the infiltration volume to provide a combination of volume reduction and peak flow mitigation. Infiltration basins are differentiated from bioretention basins because they typically do not include an engineered soil medium, and vegetation is either absent or consists of a simple grass ground cover. They are also typically constructed at a larger scale, although it may be possible for bioretention to be constructed at similar scales in some cases. Photo credit: Google Earth. VOLUME REDUCTION PROCESSES Overall volume reduction potential Infiltration Evapotranspiration Consumptive use Base-flowâmimicking discharge URBAN HIGHWAY APPLICABILITY Ground-level highways Ground-level highways with restricted cross-sections Ground-level highways on steep transverse slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges Gravel storage reservoir LowHigh Moderate
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. Pretreatment to prevent clogging is important for the longevity of infiltration basins and can be provided via a vegetated conveyance or a sedimentation forebay. Additional mechanical pretreatment measures exist, including cartridge filtration or centrifugal separation where hydraulic and grade constraints allow. General DOT Experience Infiltration basins have been widely used across the United States. When properly designed and infiltration rates are maintained, volume reductions are high, on average. The most common problem incurred with infiltration basins is clogging. A BMP retrofit pilot program final report by Caltrans (2004) notes that if properly designed, volume reduction should be 100% due to complete infiltration. One of the two basins monitored by Caltrans was observed to not be draining within the design maximum of 72 hours, most likely due to poor soil characteristics. Applicability and Limitations Site and Watershed Considerations Use of infiltration basins requires soils with infiltration rates high enough to ensure proper drainage between storm events. Proper infiltration of captured stormwater from infiltration basins requires that the groundwater table be at least several feet below the bottom of the basin. Geotechnical Considerations Infiltration basins must be located a sufficient distance from a roadway to maintain the roadwayâs structural integrity. Use of infiltration basins along steep transverse slopes should be minimized and will likely require enhanced protection of slope integrity. 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 these situations, designs may need to be adjusted to enhance treatment and prevent groundwater contamination. Use of infiltration basins to provide more infiltration than was historically present or is characteristic of similar sites in the region may alter a siteâs water balance in undesirable ways. Safety Considerations Because infiltration basins involve fixed obstacles and side slopes that may exceed 3H:1V, they should ideally be located outside of the clear zone (typically in the range of 22 to 32 ft from driving lanes). If this distance cannot be achieved, a barrier parallel to the direction of traffic should be used between the road and the VRA. Regional Applicability Infiltration basins have been applied successfully across a broad range of climates.
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. New Projects, Lane Additions, and Retrofits Because of their large footprint and setback requirements, infiltration basins are more easily considered for new construction projects in the highway setting. Where available space exists, however, retrofit opportunities are possible and can provide significant volume reduction. Retrofitting an existing infiltration system with underdrains will involve significant excavation, piping, controls, and possible amendments to the medium and/or stone. Including underdrains in new construction is recommended. Use in a Treatment Train Pretreatment to reduce particulate matter and suspended solids will increase the life of the infiltration basin and system efficiency, and will reduce required maintenance. Pretreatment can be provided as stormwater through a vegetated conveyance to the system, by the use of a sedimentation forebay, or by mechanical devices such as cartridge filtration. Stormwater runoff in excess of the infiltration basinâs storage capacity can be conveyed to additional VRAs/BMPs by the use of overflow weirs. VRA-Specific Maintenance Considerations (see Section 4.3.6 for additional maintenance information in common with other VRAs) Maintenance is generally similar to that of flood control or extended detention basins. Check periodically for excess sediment accumulation or scour. If vegetated, maintain vegetation to avoid line-of-site issues. Check periodically for signs of clogging; periodic maintenance such as tilling or scraping of the surface may be needed to restore surface infiltration rates. Enhancements and Variations Provide robust pretreatment to improve efficiency and extend the life of the system. Clogging is the principal cause of infiltration basin failure and maintenance requirements. Pretreatment to remove sediments and particulate matter prior to it entering the infiltration basin can significantly improve system performance and reduce the potential for clogging of the media and subsoils.
Volume Reduction Approach Fact Sheets A-53 Provide backup flow-control outlet. Including an underdrain (normally closed) can provide a low-cost backup in the event that the infiltration rate declines with time. If infiltration rates decline, the outlet can be opened, and flow can be controlled to achieve a combination of volume reduction and flow control until the system infiltration rate can be restored. Distribute inflow. Spreading the flow into infiltration basins can reduce the potential for scour and heavy sediment accumulation in certain areas. Sources of Additional Information California Stormwater Quality Association. California Stormwater BMP Handbook: New Development and Redevelopment. TC-11, Infiltration Basin. 2003. City of Portland, Oregon. Stormwater Management Manual. 2008. http://www.portlandonline.com/bes/index.cfm?c=47953&. Virginia Department of Conservation and Recreation. Virginia DCR Stormwater Design Specification No. 8: Infiltration Practices v.1.9. 2011. http://chesapeakestormwater.net/category/publications/design-specifications/. Washington Department of Transportation. Highway Runoff Manual. BMP IN.02: Infiltration Pond. 2011. http://www.wsdot.wa.gov/Environment/WaterQuality/Runoff/HighwayRunoffManua l.htm. Key Planning-Level Design Parameters for Volume Reduction Conceptual Design Parameter Description Representative Range Footprint area The area covered by the surface of the infiltration basin Can be up to 0.5 acre or greater; commonly less in urban highway environment Effective footprint area The effective area of the infiltration basin for storage and drawdown estimates; typically assumed to be measured as the water surface area at mid-ponding depth Typically somewhat smaller than the total footprint area Ponding depth The distance between the floor of the basin and the overflow elevation Typically 2 to 4 ft; may be higher if infiltration rates allow Design infiltration rates The rate at which water is assumed to infiltrate into the subsurface soils for the purpose of design and benefit evaluation At least 0.5 in. per hour; higher infiltration rates needed for higher ponding depths Amend soil and plant with deep-rooted vegetation. Deep-rooted plants can help maintain infiltration pathways, soil aeration, and healthy soil processes. Soil amendments can also help better capture pollutants in infiltrating water.
A-54 Volume Reduction of Highway Runoff in Urban Areas Example Conceptual Design Schematic Figure 1.âCross-section view. I-5 Exit 102, Tumwater, Washington. Source: Google Earth. Infiltraon Overflow Inflow Oponal sedimentaon forebay Max ponding depth Oponal emergency underdrain (normally closed) Oponal amended topsoil
Volume Reduction Approach Fact Sheets A-55 RO AD Basin floor Piped inlet Op onal sedimenta on forebay Overflow Figure 2.âPlan view.
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. A range of potential options are available for providing storage, including use of open- graded stone or a variety of engineered storage chambers (concrete, plastic, or metal). There are also a range of potential locations where underground infiltration systems can be placed, including below parking areas, below access roads, and below travel lanes. Volume Reduction Processes and Performance Factors Volume reduction is achieved solely through infiltration. The degree of volume reduction achievable is a function of the subsoil infiltration rates and effective depth of the storage reservoir. Because of the potential for decline in performance as a result of clogging of subsurface systems, long-term volume reduction is also a function of the level of pretreatment provided. Photo credit: WSDOT. VOLUME REDUCTION PROCESSES Overall volume reduction potential Infiltration Evapotranspiration Consumptive use Base-flowâmimicking discharge URBAN HIGHWAY APPLICABILITY Ground-level highways Ground-level highways with restricted cross-sections Ground-level highways on steep transverse slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges LowHigh Moderate
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. Paul, Minnesota (Alms and Carlson, 2012) found that runoff volumes were reduced by 60% to 100% and often by above 90% (including snowmelt). An important point to note is that depending on design, there is a possibility that these facilities meet the U.S. EPA definition for class V injection wells (Minnesota DOT, 2012). It should be taken into account that without adequate pretreatment, underground injection systems have relatively high potential for groundwater contamination (Pitt et al., 1994). If properly designed, underground infiltration systems have the ability to reduce runoff volumes by 98%. Applicability and Limitations Site and Watershed Considerations Underground infiltration systems are suitable for sites with sufficiently permeable subsoils and where significant amounts of infiltration will not result in water balance or geotechnical issues. The subbase must be level for proper functioning and stability while still maintaining permeability. On sloped sites, underground infiltration systems can be constructed as a series of level benches. Underground infiltration systems can be used on road shoulders, on medians, and under roadways. They can be favorable in constrained areas where there is insufficient space for vegetated VRAs. Geotechnical Considerations Where underground infiltration is used in areas that supports traffic (e.g., breakdown lanes, travel lanes, parking lots), the system and its associated subgrade preparation must be designed with adequate load- bearing capacity and must not cause negative impacts on adjacent pavement structures. Impermeable vertical barriers can be used between the underground infiltration installation and the roadway to avoid compromising road integrity from excess infiltration, but drainage systems should allow the adjacent subbase to drain freely. Use of underground infiltration along steep transverse slopes may require enhanced protection of slope integrity. Groundwater Quality and Water Balance Considerations In general, infiltration galleries represent a higher risk of groundwater contamination than do other VRAs, and pretreatment should be provided unless underlying soils are determined to provide adequate pollutant attenuation capacity. 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 these situations, designs may need additional pretreatment. Use of underground systems allows negligible ET; therefore, the use of these systems has the potential to alter the water balance of a site compared to natural conditions (i.e., more infiltration).
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. Regional Applicability Underground infiltration can be used across a wide range of climates. Underground systems will generally continue to function under normal freezing conditions. New Projects, Lane Additions, and Retrofits Because underground infiltration systems are generally large and require significant grading, excavation, and geotechnical/structural requirements, they are more easily incorporated into new construction. Retrofit projects will likely incur significant costs since they would essentially contain many of the elements of new construction and additional removal of existing constraints. In both new and retrofit situations, designs of underground infiltration systems should take into careful consideration the U.S. EPA classification of underground injection wells to avoid additional permit requirements. Use in a Treatment Train Pretreatment is strongly recommended to improve long-term system efficiency and reduce the potential for failure and maintenance related to clogging. Pretreatment also reduces the potential for groundwater contamination. Stormwater runoff in excess of the infiltration systemâs storage capacity can be conveyed to additional VRAs/BMPs if sufficient hydraulic grade lines exist or if pumps are included. VRA-Specific Maintenance Considerations (see Section 4.3.6 for additional maintenance information in common with other VRAs) Underground vaults require periodic inspection to ensure continued performance. Confined-space entry permits may be required to inspect and maintain vaults. Accumulated sediment and debris must be regularly removed from pretreatment filters and settling vaults. Designs that provide access to the infiltrating surface (i.e., vaults rather than aggregate storage beds) may allow remediation of clogging without significant excavation and replacement costs. Enhancements and Variations Advanced pretreatment to extend life and protect groundwater quality. Clogging is the principal cause of infiltration gallery failure and resulting maintenance requirements. Underground infiltration galleries may also pose the highest level of risk of groundwater contamination among stormwater VRAs. Pretreatment to remove sediments and particulate matter prior to it entering the infiltration basin can significantly improve system performance and reduce the
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. Storage geometry. Dry wells can be considered as a variation of this VRA. They are typically deeper than wide, such that these systems tend to be deeper than typical infiltration galleries and infiltrate primarily from their walls instead of from their bottoms. Dry wells may be advantageous if permeable soil layers are located at a significant depth. Storage materials. Reservoir chambers can be filled with rock or can be constructed of arch sections, plastic matrices, or perforated pipes. Storage in road subbase. Storage in the pore space of an open-graded road subbase may have a high degree of opportunity in the urban highway environment. This would essentially be a variation on permeable pavement but with flows routed to the subbase via a conveyance system rather than through a permeable wearing course. This could reduce the cost of the system compared to permeable pavement and may address concerns about durability and maintenance of the permeable wearing course. However, the ability to provide pretreatment and effective flow distribution may be challenges associated with this variation. Sources of Additional Information Massachusetts Highway Department. The Mass Highway Stormwater Handbook for Highways and Bridges. 2004.http://www.mhd.state.ma.us/downloads/projDev/2009/MHD_Stormwater_Ha ndbook.pdf. Washington Department of Transportation. Highway Runoff Manual. BMP IN.04: Infiltration Vault. 2011. http://www.wsdot.wa.gov/Environment/WaterQuality/Runoff/HighwayRunoffManual.htm. Key Planning-Level Design Parameters for Volume Reduction Conceptual Design Parameter Description Representative Range Footprint area The area covered by the underground infiltration system Any Effective storage depth The effective depth of water stored within the underground infiltration system. It is a function of the depth and porosity of the storage layer and/or dimensions of the chambered reservoir. Typically 6 in. to more than 8 ft deep, as a function of system type and underlying infiltration rate Design infiltration rates The rate at which water is assumed to infiltrate into the subsurface soils for the purpose of design and benefit evaluation. This should be the rate of infiltration below the reservoir layer. Typically requires at least 0.5 in. per hour Filter course A bed of sand or small stone placed at the bottom of the excavation in order to provide bedding and storage and to help reduce the need for compaction of the subsoil during construction. 6 to 12 in.
A-60 Volume Reduction of Highway Runoff in Urban Areas Example Conceptual Design Schematic Figure 1.âCross-section view (example of arch gallery sited in breakdown lane). Figure 2.âPlan view (example of siting in breakdown lane). Infiltraon Road Impermeable barrier Paved shoulder/median Filter course Overflow Pretreatment filter or seling vault Storage reservoir RO AD Shoulder Storage reservoir under pavement Inlets (pretreatment located inside)
