Stormwater Infiltration in the Highway Environment: Guidance Manual (2019) / Chapter Skim
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Contents • BMP 01 Vegetated Conveyance • BMP 02 Dispersion • BMP 03 Media Filter Drain • BMP 04 Permeable Shoulders • BMP 05 Bioretention without Underdrains • BMP 06 Bioretention with Underdrains • BMP 07 Infiltration Trench • BMP 08 Infiltration Basin • BMP 09 Infiltration Gallery • Glossary of Key Terms in Infiltration BMP Fact Sheets • References for Appendix A 120 Infiltration BMP Fact Sheets A P P E N D I X A
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Vegetated Conveyance BMP 01 Alternative names: dry swale, bioswale, grassed swale, retention swale, regenerative stormwater conveyance (Photo credit: Caltrans.) VOLUME REDUCTION PROCESSES Overall Volume Reduction Potential Infiltration Evapotranspiration Consumptive Use Baseflow-mimicking Discharge URBAN HIGHWAY APPLICABILITY Ground level highways Ground level highways with restricted cross-sections Ground level highways on steep transverse slopes Steep longitudinal slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges High Moderate Low Description This category includes engineered vegetated swales and other vegetated drainage features that convey stormwater runoff and significantly reduce stormwater runoff volume.
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Volume Reduction Processes and Performance Factors Volume reduction is achieved through infiltration and evapotranspiration (ET)
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adjustment to enhance treatment and prevent groundwater contamination. Where soils allow high rates of infiltration, the use of vegetated conveyance may shift the water balance toward excess infiltration.
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Appendix A – Infiltration BMP Fact Sheets Vegetated Conveyance BMP 01 4 when sediments and metals are the main target constituents; there is adequate space along the highway shoulder, along ramps, between sidewalks and roadways, and other landscaped areas; drainage patterns and topography are suitable; and there is safe maintenance access. Use in a Treatment Train Vegetated conveyances can be used to collect and convey water downgradient of a filter strip.
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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.
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structural stormwater management practices (SMPs) to meet water quality treatment goals, including feasibility and cold climate design guidance.
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Appendix A – Infiltration BMP Fact Sheets Vegetated Conveyance BMP 01 7 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 to 10 feet Side slopes The steepness of the sides of the conveyance that connect the bottom of the swale to the ground surface.
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Figure 1. Cross-section view.
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Figure 3. Plan view.
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Example O&M Activities and Frequencies Activity Frequency GENERAL INSPECTIONS Remove trash and debris Two times per year including before and after wet season.Repair eroded facility areas Inspect and maintain access roads Inspect and resolve areas of standing water Remove minor sediment in facility bottom Provide vector control if needed Identify any needed corrective maintenance that will require site-specific planning or design ROUTINE MAINTENANCE Vegetation In arid climates, irrigate as recommended by a landscape professional, typically for the first 3 years to establish vegetation As needed Remove undesirable vegetation Annually Repair areas of thin or missing vegetation Annually Repair areas of scour, rilling, or channelization Annually Inflow Outflow Structures Check energy dissipation function and add riprap Annually Inspect inlets and outlets and remove accumulated sediment if it impairs hydraulic function Annually CORRECTIVE (MAJOR) MAINTENANCE Regrade and replace top 3 to 6 inches of topsoil layer and accumulated sediment and replace vegetation Estimated every 10 years (highly site specific)
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Dispersion BMP 02 Alternative names: natural dispersion, engineered dispersion, vegetated filter strip, compost amended vegetated filter strip, vegetated buffer area Informal dispersion to median and shoulder, Interstate 8, San Diego, California, urban area. (Credit: Google.)
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treatment benefits with very limited incremental cost. Volume Reduction Processes and Performance Factors Volume reduction is achieved through infiltration and ET.
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Applicability and Limitations Site and Watershed Considerations Dispersion to areas with high infiltration rates will result in higher rates of volume reduction. Dispersion is suitable for most soil types.
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recommended upper limits on embankment slope. Groundwater Quality and Water Balance Considerations Because water disperses in shallow depths over a broad area, dispersion poses relatively low risk of groundwater quality impacts and water balance impacts.
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into the project as design features. 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 more uniform dispersion, and/or enhancing the dispersion area, such as by amending, decompaction, leveling, and/or vegetating the area.
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within the dispersion area. Absorption capacity can be gained by using compostamended soils to disperse and absorb contributing flows to the dispersion area.
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Additional References California Stormwater Quality Association. California Stormwater BMP Handbook: New Development and Redevelopment.
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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.
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Figure 1. Cross-section view.
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Figure 3. Natural or engineered dispersion without a gravel level spreader (WSDOT 2014)
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Example O&M Activities and Frequencies Activity Frequency GENERAL INSPECTIONS Identify any needed corrective maintenance that will require site-specific planning or design Annually Inspect function of level spreader Inspect degree of degree of channelization in filter strip Inspect degree of undesirable vegetation (weeds) ROUTINE MAINTENANCE Vegetation In arid climates, irrigate as recommended by a landscape professional, typically for the first 3 years to establish vegetation As needed Reseed areas of thin or missing vegetation Annually Repair eroded areas Annually Level Spreader Fill areas of level spreader that appear to be channelized or sedimented to restore function Annually Regrade road shoulder and augment gravel periodically to restore level spreader 3 to 5 years CORRECTIVE (MAJOR)
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Media Filter Drain BMP 03 Alternative names: formerly known as "Ecology Embankment" Media Filter Drain along SR 14in Clark County, Washington. (Source: WSDOT 2011.)
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General DOT Experience This BMP is widely used by WSDOT and was formerly referred to as an "Ecology Embankment." A technology evaluation report prepared for "Ecology Embankments" for WSDOT shows both significant volume and load reductions in some cases up to 100% (Herrera Environmental Consultants 2006)
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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.
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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.
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Additional References Washington State Department of Transportation, Highway Runoff Manual.
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Figure 1. Cross-section view.
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Figure 2. Plan view.
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Activity Frequency GENERAL INSPECTIONS Identify any needed corrective maintenance that will require site-specific planning or design Annually Inspect function of level spreader Inspect degree of degree of channelization in filter strip or media filter drain Inspect degree of undesirable vegetation (weeds) ROUTINE MAINTENANCE Vegetation-Free Zone (rock level spreader)
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Permeable Shoulders BMP 04 Alternative names: permeable shoulders with stone reservoirs, permeable gutters VOLUME REDUCTION PROCESSES Overall Volume Reduction Potential Infiltration Evapotranspiration Consumptive Use Baseflow-mimicking Discharge URBAN HIGHWAY APPLICABILITY Ground level highways Ground level highways with restricted cross-sections Ground level highways on steep transverse slopes Steep longitudinal slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges High Moderate Low Description This BMP includes use of a permeable pavement surface course (typically permeable asphalt or concrete) along the shoulders of a roadway, underlain by a stone reservoir.
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Volume Reduction Processes and Performance Factors Volume reduction is achieved primarily through infiltration. The degree of allowable infiltration is a function of soil infiltration rates (after compaction)
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reservoir and would then be discharged via underdrains or routed to additional BMPs. Current applicability of permeable pavements to main roadway sections is not well established relative to structural design requirements, top course durability, and safety.
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surface clogs. Permeable shoulders function in the same way as shoulders with standard pavement and do not present any added safety hazards.
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adaptability of designs relative to water balance issues. Consider various materials and thicknesses.
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Pavements. Information Series 131, National Asphalt Pavement Association.
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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)
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Example Conceptual Design Schematics Figures 1 and 2 show two different cross-section views, Figure 3 shows the plan view, and Figures 4 and 5 show two different longitudinal profiles. Figure 1.
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Figure 3. Plan view.
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Figure 5. Longitudinal profile of an installation along a mild slope (geotextile cutoff walls)
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Example Inspection and Maintenance Activities Activity Frequency GENERAL INSPECTIONS Inspect for areas of sediment accumulation in the pavement surface If sediment accumulation is elevated, inspect for potential sources of sediment in the tributary area and determine control approaches to reduce sediment Observe and record drawdown rate via observation port following storm event Periodically (every 2 to 5 years) measure the permeability of the surface of the permeable pavement Identify any damage to pavement Inspect outlet control and overflow structures Identify any needed corrective maintenance that will require site-specific planning or design ROUTINE MAINTENANCE Permeable Surface Layer Remove sediment and leaf litter using a mechanical sweeper (e.g., regenerative air or vacuum-assisted sweeper)
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Bioretention without Underdrains BMP 05 Alternative names: rain garden, bioretention, retention swale Highway 99E Viaduct, Portland, Oregon. (Photo credit: Geosyntec Consultants.)
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Volume Reduction Processes and Performance Factors Volume reduction in bioretention cells is achieved through infiltration and ET. Efficient volume reduction performance is dependent on adequate medium and subsoil infiltration rates to ensure that captured runoff filters through the system between storm events.
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the urban highway environment. Through the use of underdrains (see BMP 06)
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considerations to both salt-tolerant and drought-tolerant situations. Peat and compost media are ineffective during the winter in cold climates.
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other soil amendments. Add surcharge detention.
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Fact Sheet C-2, Bioretention Cell.
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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 sqft; can potentially be much larger Effective footprint area The portion of the total facility footprint area that provides storage and infiltration during typical operations.
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Example Conceptual Design Schematic Figures 1 and 2 show cross-section and plan views, respectively. Figure 1.
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Example Inspection and Maintenance Activities Activity Frequency GENERAL INSPECTIONS Accumulation of trash and debris Annually or semi-annually depending on loading Eroded facility areas Sediment accumulation Extended standing water Vector or rodent issues Identify any needed corrective maintenance that will require site-specific planning or design ROUTINE MAINTENANCE General Remove trash and debris Annually or semi-annually depending on loading Repair eroded facility areas Remove minor sediment in forebay Vegetation In arid climates, irrigate as recommended by a landscape professional, typically for the first 3 years to establish vegetation As needed Remove undesirable vegetation Annually Reseed or replant areas of thin or missing vegetation Annually Mulch Remove and replace mulch in areas where significant sediment (>1 inch) has accumulated Annually Add an additional 1 to 2 inches of mulch; replace any mulch that is removed As needed Media Layer Rake to scarify media to promote infiltration while removing and replacing mulch When replacing mulch Replace media in areas that experience scour When fixing erosion Inflow, Underdrain, and Outflow Structures Check energy dissipation function and add riprap As needed Remove accumulated sediment from inlets and outlets As needed Flush underdrain As needed (less often)
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Activity Frequency Replace full depth of media and replace vegetation Estimated every 30 years (highly site specific) Replace aggregate drainage layer As needed if silted in Repair structural damage to inlets, outlets, and underdrain and/or replace these elements As needed if at end of usable life Prepare documentation of issues and resolutions for review by appropriate parties; modify O&M Plan if needed Before major maintenance Document major maintenance activities; record modified O&M Plan and asbuilt plan set if needed After major maintenance Take photographs before and after from the same vantage point Before and after Bioretention without Underdrains BMP 05 10 Appendix A – Infiltration BMP Fact Sheets
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Bioretention with Underdrains BMP 06 Alternative names: bioretention, biofiltration, retention swale I-5 Exit 298, Portland, Oregon. (Photo credit: Geosyntec Consultants.)
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controls) that is similar in many ways to shallow groundwater baseflow in undeveloped/predevelopment watersheds.
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In areas with very high soil infiltration rates or shallow groundwater tables, captured stormwater may not be sufficiently treated prior to contact with groundwater. In areas with existing groundwater contamination, bioretention cells can be lined to keep treated stormwater out of contact with groundwater and discharged only via the underdrain.
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vegetated conveyance features or a forebay. Stormwater runoff in excess of the bioretention cell's storage capacity can be conveyed to additional BMPs by use of overflow controls such as weirs.
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Resilient Design Features Bioretention with underdrains should have the underdrain placed at the bottom of the infiltration layer and tied into an adjustable outlet structure such that the amount of retained depth can be adjusted. The perforated underdrain pipe shall be placed in between drain rock to prevent fine sediments from clogging and prohibiting the functionality of the underdrain pipe.
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Additional References Massachusetts Stormwater Handbook, Volume 2, Chapter.
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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 sqft; can potentially be much larger Effective footprint area The portion of the total facility footprint area that provides storage and infiltration during typical operations.
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Example Conceptual Design Schematic Figures 1, 2, and 3 show cross-section view, an example design of outlet control structure, and plan view, respectively. Figure 1.
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Figure 3. Plan view.
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Example Inspection and Maintenance Activities Activity Frequency GENERAL INSPECTIONS Accumulation of trash and debris Annually or semi-annually depending on loading Eroded facility areas Sediment accumulation Extended standing water Vector or rodent issues Identify any needed corrective maintenance that will require site-specific planning or design ROUTINE MAINTENANCE General Remove trash and debris Annually or semi-annually depending on loading Repair eroded facility areas Remove minor sediment in forebay Vegetation In arid climates, irrigate as recommended by a landscape professional, typically for the first 3 years to establish vegetation As needed Remove undesirable vegetation Annually Reseed or replant areas of thin or missing vegetation Annually Mulch Remove and replace mulch in areas where significant sediment (>1 inch) has accumulated Annually Add an additional 1 to 2 inches of mulch; replace any mulch that is removed As needed Media Layer Rake to scarify media to promote infiltration while removing and replacing mulch When replacing mulch Replace media in areas that experience scour When fixing erosion Inflow, Underdrain and Outflow Structures Check energy dissipation function and add riprap As needed Remove accumulated sediment from inlets and outlets As needed Flush underdrain As needed (less often)
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Activity Frequency Repair structural damage to inlets, outlets, and underdrain and/or replace these elements As needed if at end of usable life Prepare documentation of issues and resolutions for review by appropriate parties; modify O&M Plan if needed. Before major maintenance Document major maintenance activities; record modified O&M Plan and asbuilt plan set if needed After major maintenance Take photographs before and after from the same vantage point Before and after Bioretention with Underdrains BMP 06 11 Appendix A – Infiltration BMP Fact Sheets
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Infiltration Trench BMP 07 Alternative names: exfiltration trench (Source: Maryland SHA.) VOLUME REDUCTION PROCESSES Overall Volume Reduction Potential Infiltration Evapotranspiration Consumptive Use Baseflow-mimicking Discharge URBAN HIGHWAY APPLICABILITY Ground level highways Ground level highways with restricted cross-sections Ground level highways on steep transverse slopes Steep longitudinal slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges High Moderate Low Description This category of BMP 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.
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General DOT Experience Infiltration trenches have been widely used across the United States. When properly designed and infiltration rates are maintained, volume reductions are high on average.
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Use of infiltration trenches to provide more infiltration than historically present or characteristic of similar sites in the region may alter a site's water balance in undesirable ways. Safety Considerations Infiltration trenches should not present a significant hazard to errant vehicles.
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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 vegetated conveyance or a sedimentation forebay.
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Design Specification No. 8: Infiltration Practices v.1.9.
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Figure 1. Cross-section view.
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Example Inspection and Maintenance Activities Activity Frequency GENERAL INSPECTIONS Identify eroded facility areas in facility or upstream Annually Observe and record drawdown rate via the observation port Estimate degree of sediment accumulation in the surface pea gravel or sand layer, thickness of surface layer or depth of penetration Identify any needed corrective maintenance that will require site-specific planning or design ROUTINE MAINTENANCE Pea Gravel/Sand Filter Layer Remove sediment via scraping of the top layers of this layer and replace with clean washed pea gravel or sand Annually or when sediment has accumulated to a depth of more than 2 inches within the surface layer Replace full depth of pea gravel When fully comingled with sediment Upstream Sediment Control Repair any eroded areas that are contributing elevated sediment to the BMP As needed Maintain pretreatment systems As needed CORRECTIVE (MAJOR) MAINTENANCE Excavate the entire facility, rehabilitate bottom and sides via overexcavation, and replace aggregate layers.
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Infiltration Basin BMP 08 Alternative names: percolation basins, recharge basins VOLUME REDUCTION PROCESSES Overall Volume Reduction Potential Infiltration Evapotranspiration Consumptive Use Baseflow-mimicking Discharge URBAN HIGHWAY APPLICABILITY Ground level highways Ground level highways with restricted cross-sections Ground level highways on steep transverse slopes Steep longitudinal slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges High Moderate Low Description Infiltration basins are relatively large, shallow basins that have relatively little vegetation. Their contours appear similar to detention basins, but they do not have a surface discharge point below their overflow elevation.
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Volume Reduction Processes and Performance Factors Volume reduction in infiltration basins is achieved through a combination of infiltration and ET. Efficient performance is dependent on adequate subsoil infiltration rates to ensure that captured runoff exits the basin between storm events.
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Use of infiltration basins to provide more infiltration than historically present or characteristic of similar sites in the region may alter a site's 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 feet from driving lanes)
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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.
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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.
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I-5 Exit 102, Tumwater, Washington. (Source: Google Earth.)
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Figure 2. Plan view.
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Example Inspection and Maintenance Activities Activity Frequency GENERAL INSPECTIONS Identify eroded facility areas Annually Observe and record drawdown rate Estimate degree of sediment accumulation Depth of sediment migration into sacrificial sand/media layer (if present) Identify areas of compromised plant health or density Identify any needed corrective maintenance ROUTINE MAINTENANCE Sediment, Trash, and Debris Remove trash from facility Each visit as needed Remove sediment from forebay if greater than 25% of the forebay volume As needed Remove sediment from pretreatment system per manufacturer's recommendations Per manufacturer recommendation Vegetation (if basin is vegetated)
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Infiltration Gallery BMP 09 Alternative names: underground infiltration systems, infiltration vaults (Photo credit: WSDOT.) VOLUME REDUCTION PROCESSES Overall Volume Reduction Potential Infiltration Evapotranspiration Consumptive Use Baseflow-mimicking Discharge URBAN HIGHWAY APPLICABILITY Ground level highways Ground level highways with restricted cross-sections Ground level highways on steep transverse slopes Steep longitudinal slopes Depressed highways Elevated highways on embankments Elevated highways on viaducts Linear interchanges Looped interchanges High Moderate Low Description Infiltration galleries include a broad class of BMPs that consist of storage reservoirs located belowground preceded by pretreatment systems.
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General DOT Experience While case studies on the effectiveness of infiltration galleries in the highway environment are currently limited, their use in some states, such as Minnesota, is increasing. Monitoring studies for several infiltration galleries around the City of St.
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Groundwater Quality and Water Balance Considerations There must be sufficient separation from the seasonally high groundwater table and water supply wells to reduce the potential for contamination. Typical separation discharges are 2 to 10 feet above groundwater and 100 to 150 feet from wells.
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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.
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Management Practices (MassDEP)
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Key Planning Level Design Parameters for Volume Reduction Conceptual Design Parameter Description Representative Range Footprint area The area covered by the infiltration gallery. Any Effective storage depth The effective depth of water stored within the infiltration gallery.
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Example Conceptual Design Schematic Figures 1 and 2 show cross-section and plan views, respectively. Figure 1.
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Figure 2. Plan view (example of siting in breakdown lane)
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Activity Frequency CORRECTIVE (MAJOR) MAINTENANCE It is not typically practical to maintain the storage reservoir or infiltrating surface; plan for overall reconstruction when infiltration falls below the design infiltration rate.
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206 Adaptable outlets: Refers to outlets or outlet control structures that can be readily adapted by O&M crews without significant construction effort or new permitting. Base: The layer of aggregate material below the road surface course.
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Outlet control structure: A structure designed to control the level and/or rate of water discharge from a BMP. Resiliency: In the context of stormwater BMPs, resiliency can be defined as the ability to tolerate, adapt to, and/or rapidly recover from adverse conditions, such as incomplete site investigations, construction impacts, elevated sediment loading, contaminant spills, extreme storm events, lack of maintenance, change in tributary area characteristics, and change in design goals.
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208 Alms, W., and Carlson, J
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Pitt, R., Clark, S., and Parmer, K
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Key Terms
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