BMP T7.30: Bioretention


Ecology accepts bioretention as having the potential to meet I-3.4.5 MR5: On-Site Stormwater Management, I-3.4.6 MR6: Runoff Treatment and I-3.4.7 MR7: Flow Control for the tributary drainage areas depending upon site conditions and sizing.

The purpose of bioretention is to provide effective removal of many stormwater pollutants, and provide reductions in stormwater runoff quantity and surface runoff flow rates. Where the surrounding native soils have adequate infiltration rates, bioretention can provide both Runoff Treatment and Flow Control. Where the native soils have low infiltration rates, underdrain systems can be installed and the bioretention BMP can still be used as a Runoff Treatment BMP. However, designs utilizing underdrains provide less Flow Control benefits.


Bioretention areas are shallow landscaped depressions, with a designed soil mix (the bioretention soil mix) and plants adapted to the local climate and soil moisture conditions, that receive stormwater from a contributing area.

Bioretention uses the imported bioretention soil mix as a treatment medium. As in infiltration, the pollutant removal mechanisms include filtration, adsorption, and biological action. Bioretention BMPs can be built within earthen swales or placed within vaults. Water that has passed through the bioretention soil mix (or approved equivalent) may be discharged to the ground or collected and discharged to surface water.

The term, bioretention, is used to describe various designs using soil and plant complexes to manage stormwater. The following terminology is used in this manual:

See Figure V-5.12: Typical Bioretention, Figure V-5.13: Typical Bioretention w/Underdrain, Figure V-5.14: Typical Bioretention w/Liner (Not LID), and Figure V-5.15: Example of a Bioretention Planter for examples of various types of bioretention configurations.

Note: Ecology has approved use of certain manufactured treatment devices that use specific, high rate media for treatment. Such systems do not use bioretention soil mix, and are not considered a bioretention BMP (even though marketing materials for these manufactured treatment devices may compare them to bioretention). See V-10 Manufactured Treatment Devices as BMPs for more information on manufactured treatment devices.

Figure V-5.12: Typical Bioretention

Cross section of a typical bioretention BMP.

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Figure V-5.13: Typical Bioretention w/Underdrain

Cross section of a typical bioretention BMP with an underdrain.

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Figure V-5.14: Typical Bioretention w/Liner (Not LID)

Cross section of a typical bioretention w/liner.

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Figure V-5.15: Example of a Bioretention Planter

Plan view and cross-section of a typical bioretention planter.

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Applications and Limitations

Because bioretention BMPs use an imported soil mix that has a moderate design infiltration rate, they are best applied for small drainages, and near the source of the stormwater runoff. Bioretention cells may be scattered throughout a subdivision; a bioretention swale may run alongside the access road; or a series of bioretention planter boxes may serve the road. In these situations, they can but are not required to fully meet the requirement to treat 91% of the stormwater runoff file (the Water Quality Design Volume, as described in III-2.6 Sizing Your Runoff Treatment BMPs) from pollution-generating surfaces. The amount of stormwater that is predicted to pass through the bioretention soil mix is treated, and may be subtracted from the 91% volume that must be treated to meet I-3.4.6 MR6: Runoff Treatment. Downstream Runoff Treatment BMPs may be significantly smaller as a result.

Bioretention BMPs that infiltrate into the ground can also provide significant Flow Control. They can, but are not required to fully meet the Flow Control Performance Standard of I-3.4.7 MR7: Flow Control. Because they typically do not have an orifice restricting overflow or underflow discharge rates, they typically don’t fully meet I-3.4.7 MR7: Flow Control. However, their performance contributes to meeting the standard, and that can result in much smaller additional Flow Control BMPs at the bottom of the project site. Bioretention can also help achieve compliance with the LID Performance Standard of I-3.4.5 MR5: On-Site Stormwater Management.

Bioretention constructed with imported composted material should not be used within one-quarter mile of phosphorus-sensitive waterbodies if the underlying native soil does not meet the criteria for Runoff Treatment per V-5.6 Site Suitability Criteria (SSC). Preliminary monitoring indicates that new bioretention BMPs can add phosphorus to stormwater. Therefore, they should also not be used with an underdrain when the underdrain water would be routed to a phosphorus-sensitive receiving water.

Applications with or without underdrains vary extensively and can be applied in new development, redevelopment and retrofits. Typical applications include:

Infeasibility Criteria

The following infeasibility criteria describe conditions that make bioretention infeasible when applying The List Approach within I-3.4.5 MR5: On-Site Stormwater Management. If a project proponent wishes to use a bioretention BMP even though one of the infeasibility criteria within this section are met, they may propose a functional design to the local government.

Criteria with setback distances are as measured from the bottom edge of the bioretention soil mix.

Any of the following circumstances allow the designer to determine bioretention as "infeasible" when applying the The List Approach within I-3.4.5 MR5: On-Site Stormwater Management:

Design Criteria

General Design Criteria

Determining the Native Soil Infiltration Rates

Determining infiltration rates of the site soils is necessary to determine feasibility of designs that intend to infiltrate stormwater on-site. It is also necessary to estimate flow reduction benefits of such designs when using a continuous runoff model.

The certified soils professional or engineer can exercise discretion concerning the need for and extent of infiltration rate (saturated hydraulic conductivity, Ksat) testing. The professional can consider a reduction in the extent of infiltration (Ksat) testing if, in their judgment, information exists confirming that the site is unconsolidated outwash material with high infiltration rates, and there is adequate separation from ground water.

The following provides recommended tests for the soils underlying bioretention BMPs. The test should be run at the anticipated elevation of the top of the native soil beneath the bioretention BMP.

Refer to V-5.4 Determining the Design Infiltration Rate of the Native Soils for further guidance on the methods to determine the infiltration rate of the native soils.

If the site subsurface characterization, including soil borings across the development site, indicate consistent soil characteristics and depths to seasonal high ground water conditions or a hydraulic restriction layer, the number of test locations may be reduced to a frequency recommended by a geotechnical professional.

After concluding an infiltration test, infiltration test sites should be over-excavated 3 feet below the projected bioretention BMP's bottom elevation unless minimum clearances to seasonal high ground water have or will be determined by another method. This overexcavation is to determine if there are restrictive layers or ground water. Observe whether water is infiltrating vertically or only spreading horizontally because of ground water or a restrictive soil layer. Observations through a wet season can identify a seasonal ground water restriction.

If a single bioretention BMP serves a drainage area exceeding 1 acre, a ground water mounding analysis may be necessary in accordance with V-5.2 Infiltration BMP Design Steps.

Assignment of Appropriate Correction Factors to the Native Soil

If the design requires determination of a long-term (design) infiltration rate of the native soils (for example, to demonstrate compliance with the LID Performance Standard and/or the Flow Control Performance Standard), refer to V-5.4 Determining the Design Infiltration Rate of the Native Soils and the following additional guidance specific to bioretention BMPs:

Determining the Bioretention Soil Mix Design Infiltration Rate

  1. Determine the initial saturated hydraulic conductivity (Ksat) based on the type of bioretention soil mix, as follows:

    • If using Ecology's default bioretention soil mix (detailed below), the initial Ksat is 12 inches per hour (30.48 cm/hr).

    • If using a custom bioretention soil mix (per the guidance for custom mixes below), use ASTM D 2434 Standard Test Method for Permeability of Granular Soils (Constant Head) with a compaction rate of 85 percent using ASTM D1557 Test Method for Laboratory Compaction Characteristics of Soil Using Modified Effort. See the additional guidance below for specific procedures for conducting ASTM D 2434. The designer must enter the derived Ksat value into the continuous modeling software.

  2. After determining the initial Ksat, determine the appropriate safety factor:

    • If the contributing area to the bioretention BMP is equal to or exceeds any of the following limitations:

      • 5,000 square feet of pollution-generating impervious surface;

      • 10,000 square feet of impervious surface;

      • ¾ acre of lawn and landscape,

      use 4 as the Ksat safety factor.

    • If the contributing area is less than all of the above areas, or if the design includes a pretreatment BMP for solids removal, use 2 as the Ksat safety factor.

  3. The continuous runoff model has a field for entering Ksat and the appropriate safety factor.

Recommended Modifications to ASTM D 2434 When Measuring Hydraulic Conductivity for Bioretention Soil Mixes

Proctor method ASTM D1557 Method C (6-inch mold) shall be used to determine maximum dry density values for compaction of the bioretention soil sample. Sample preparation for the Proctor test shall be amended in the following ways:

  1. Maximum grain size within the sample shall be no more than ½ inches in size.

  2. Snip larger organic particles (if present) into1/2 inch long pieces.

  3. When adding water to the sample during the Proctor test, allow the sample to pre-soak for at least 48 hours to allow the organics to fully saturate before compacting the sample. This pre-soak ensures the organics have been fully saturated at the time of the test.

ASTM D2434 shall be used and amended in the following ways:

  1. Apparatus:

    1. 6-inch mold size shall be used for the test.

    2. If using porous stone disks for the testing, the permeability of the stone disk shall be measured before and after the soil tests to ensure clogging or decreased permeability has not occurred during testing.

    3. Use the confined testing method, with 5- to 10-pound force spring

    4. Use de-aired water.

  2. Sample:

    1. Maximum grain size within the sample shall not be more than ½ inch in size.

    2. Snip larger organic particles (if present) into ½-inch long pieces.

    3. Pre-soak the sample for at least 48 hours prior to loading it into the mold. During the pre-soak, the moisture content shall be higher than optimum moisture but less than full saturation (i.e., there shall be no free water). This pre-soak ensures the organics have been fully saturated at the time of the test.

  3. Preparation of Sample:

    1. Place soil in cylinder via a scoop.

    2. Place soil in 1-inch lifts and compact using a 2-inch-diameter round tamper. Pre-weigh how much soil is necessary to fill 1-inch lift at 85% of maximum dry density, then tamp to 1-inch thickness. Once mold is full, verify that density is at 85% of maximum dry density (+ or – 0.5%). Apply vacuum (20 inches Hg) for 15 minutes before inundation.

    3. Inundate sample slowly under a vacuum of 20 inches Hg over a period of 60 to 75 minutes.

    4. Slowly remove vacuum ( > 15 seconds).

    5. Sample shall be soaked in the mold for 24 to 72 hours before starting test.

  4. Procedure:

    1. The permeability test shall be conducted over a range of hydraulic gradients between 0.1 and 2.

    2. Steady state flow rates shall be documented for four consecutive measurements before increasing the head.

    3. The permeability test shall be completed within one day (one-day test duration).

Default Bioretention Soil Mix (BSM)

Projects which use the following requirements for the bioretention soil mix do not have to test the mix for its saturated hydraulic conductivity (Ksat). See Determining the Bioretention Soil Mix Design Infiltration Rate.

Mineral Aggregate for Default BSM

Percent Fines: A range of 2 to 4 percent passing the #200 sieve is ideal and fines should not be above 5 percent for a proper functioning specification according to ASTM D422.

Aggregate Gradation for Default BSM

The aggregate portion of the BSM should be well-graded. According to ASTM D 2487-98 (Classification of Soils for Engineering Purposes (Unified Soil Classification System)), well-graded sand should have the following gradation coefficients:

Table V-5.2: General Guideline for Mineral Aggregate Gradation provides a gradation guideline for the aggregate component of the default bioretention soil mix (Hinman, 2009). The sand gradation below is often supplied as a well-graded utility or screened. With compost this blend provides enough fines for adequate water retention, hydraulic conductivity within recommended range (see below), pollutant removal capability, and plant growth characteristics for meeting design guidelines and objectives.

Table V-5.2: General Guideline for Mineral Aggregate Gradation

Sieve Size

Percent Passing













Where existing soils meet the above aggregate gradation, those soils may be amended rather than importing mineral aggregate.

Compost to Aggregate Ratio, Organic Matter Content, and Cation Exchange Capacity for Default BSM

Compost for Default BSM

To ensure that the BSM will support healthy plant growth and root development, contribute to biofiltration of pollutants, and not restrict infiltration when used in the proportions cited herein, the following compost standards are required.

Custom Bioretention Soil Mix

Projects which prefer to create a custom bioretention soil mix rather than using the default bioretention soil mix described above must demonstrate compliance with the following criteria using the specified test method:

Flow Entrance and Presettling

Flow entrance design will depend on topography, flow velocities and volume entering the pretreatment and bioretention area, adjacent land use and site constraints. Flow velocities entering bioretention should be less than 1.0 ft/second to minimize erosion potential. Flow entrances should be placed with adequate separation from outlets to ensure that the influent stormwater is treated prior to reaching the overflow. Five primary types of flow entrances can be used for bioretention:

Woody plants can restrict or concentrate flows and can be damaged by erosion around the root ball and should not be placed directly in the bioretention entrance flow path.

Bottom Area and Side Slopes

Bioretention areas are highly adaptable and can fit various settings such as rural and urban roadsides, ultra urban streetscapes and parking lots by adjusting bottom area and side slope configuration. Recommended maximum and minimum dimensions include:

Ponding Area

Ponding depth recommendations:

For design on projects subject to I-3.4.5 MR5: On-Site Stormwater Management, and choosing to use The List Approach of that requirement, the bioretention BMP shall have a horizontally projected surface area below the overflow which is at least 5% of the area draining to it.

The ponding area provides surface storage for storm flows, particulate settling, and the first stages of pollutant treatment within the bioretention BMP. Pool depth and draw-down rate are recommended to provide surface storage, adequate infiltration capability, and soil moisture conditions that allow for a range of appropriate plant species. Soils must be allowed to dry out periodically in order to: restore hydraulic capacity to receive flows from subsequent storms; maintain infiltration rates; maintain adequate soil oxygen levels for healthy soil biota and vegetation; provide proper soil conditions for biodegradation and retention of pollutants. Maximum designed depth of ponding (before surface overflow to a pipe or ditch) must be considered in light of drawdown time.

For bioretention areas with underdrains, elevating the drain to create a temporary saturated zone beneath the drain is advised to promote denitrification (conversion of nitrate to nitrogen gas) and prolong moist soil conditions for plant survival during dry periods (see the Underdrain (optional) section below for details).

Surface Overflow

Surface overflow can be provided by vertical stand pipes that are connected to underdrain systems, by horizontal drainage pipes or armored overflow channels installed at the designed maximum ponding elevations. Overflow can also be provided by a curb cut at the down-gradient end of the bioretention area to direct overflows back to the street. Overflow conveyance structures are necessary for all bioretention BMPs to safely convey flows that exceed the capacity of the BMP and to protect downstream natural resources and property.

The minimum freeboard from the invert of the overflow stand pipe, horizontal drainage pipe or earthen channel should be 6 inches unless otherwise specified by the local jurisdiction’s design standards.

Soil Depth

The bioretention soil mix depth must be 18 inches to provide Runoff Treatment and good growing conditions for selected plants. Ecology does not recommend bioretention soil mix depths greater than 18 inches due to preliminary monitoring results indicating that phosphorus can leach from the bioretention soil mix.

Filter Fabrics

Do not use filter fabrics between the subgrade and the bioretention soil mix. The gradation between existing soils and bioretention soil mix is not great enough to allow significant migration of fines into the bioretention soil mix. Additionally, filter fabrics may clog with downward migration of fines from the bioretention soil mix.

Underdrain (optional)

Where the underlying native soils have a measured initial Ksat between 0.3 and 0.6 inches per hour, bioretention BMPs without an underdrain, or with an elevated underdrain directed to a surface outlet, may be used to satisfy The List Approach of I-3.4.5 MR5: On-Site Stormwater Management. Underdrained bioretention BMPs must meet the following criteria if they are used to satisfy The List Approach of I-3.4.5 MR5: On-Site Stormwater Management:

Figure V-5.13: Typical Bioretention w/Underdrain depicts a bioretention BMP with an elevated underdrain. Figure V-5.14: Typical Bioretention w/Liner (Not LID) depicts a bioretention BMP with an underdrain and a low permeability liner. The latter is not considered a low impact development BMP. It cannot be used to implement The List Approach of I-3.4.5 MR5: On-Site Stormwater Management.

The volume above an underdrain pipe in a bioretention BMP provides pollutant filtering and minor detention. However, only the void volume of the aggregate below the underdrain invert and above the bottom of the bioretention BMP (subgrade) can be used in the continuous runoff model for dead storage volume that provides Flow Control benefit. Assume a 40% void volume for the Type 26 mineral aggregate specified below.

Underdrain systems should only be installed when the bioretention BMP is:

The underdrain can be connected to a downstream bioretention swale, to another bioretention cell as part of a connected treatment system, daylight to a dispersion area using an effective flow dispersion practice, or to a storm drain.

Underdrain Pipe

Underdrains shall be slotted, thick-walled plastic pipe. The slot opening should be smaller than the smallest aggregate gradation for the gravel filter bed (see Underdrain Aggregate Filter and Bedding Layer below) to prevent migration of the material into the drain. This configuration allows for pressurized water cleaning and root cutting if necessary.

Underdrain pipe recommendations:

Perforated PVC or flexible slotted HDPE pipe cannot be cleaned with pressurized water or root cutting equipment, are less durable and are not recommended. Wrapping the underdrain pipe in filter fabric increases chances of clogging and is not recommended. A 6-inch rigid non-perforated observation pipe or other maintenance access should be connected to the underdrain every 250 to 300 feet to provide a clean-out port, as well as an observation well to monitor dewatering rates.

Underdrain Aggregate Filter and Bedding Layer

Aggregate filter and bedding layers buffer the underdrain system from sediment input and clogging. When properly selected for the soil gradation, geosynthetic filter fabrics can provide adequate protection from the migration of fines. However, aggregate filter and bedding layers, with proper gradations, provide a larger surface area for protecting underdrains and are preferred.

Table V-5.3: Mineral Aggregate Gradation for Underdrain Filter and Bedding Layer

Sieve size

Percent Passing

¾ inch


¼ inch


US No. 8


US No. 50


US No. 200


Note: The above gradation is a Type 26 mineral aggregate as detailed for gravel backfill for drains in the City of Seattle Standard Specifications for Road, Bridge, and Municipal Construction (Seattle Public Utilities, 2014).

Orifice and Other Flow Control Structures

The minimum orifice diameter should be 0.5 inches to minimize clogging and maintenance requirements.

Check Dams and Weirs

Check dams are necessary for reducing flow velocity and potential erosion, as well as increasing detention time and infiltration capability on sloped sites. Typical materials include concrete, wood, rock, compacted dense soil covered with vegetation, and vegetated hedge rows. Design depends on Flow Control goals, local regulations for structures within road right-of-ways and aesthetics. Optimum spacing is determined by Flow Control benefit (modeling) in relation to cost consideration. See the Low Impact Development Technical Guidance Manual for Puget Sound (Hinman and Wulkan, 2012) for displays of typical designs.

UIC Discharge

Stormwater that has passed through the bioretention soil mix may also discharge to a gravel-filled dug or drilled drain. Underground Injection Control (UIC) regulations are applicable and must be followed (Chapter 173-218 WAC). See I-4 UIC Program.

Hydraulic Restriction Layers:

Adjacent roads, foundations or other infrastructure may require that infiltration pathways are restricted to prevent excessive hydrologic loading. Two types of restricting layers can be incorporated into bioretention designs:

Plant Materials

In general, the predominant plant material utilized in bioretention areas are species adapted to stresses associated with wet and dry conditions. Soil moisture conditions will vary within the facility from saturated (bottom of cell) to relatively dry (rim of cell). Accordingly, wetland plants may be used in the lower areas, if saturated soil conditions exist for appropriate periods, and drought-tolerant species planted on the perimeter of the facility or on mounded areas. See the Low Impact Development Technical Guidance Manual for Puget Sound (Hinman and Wulkan, 2012) for additional guidance and recommended plant species. See also City of Seattle's ROW bioretention plant lists found in Seattle's GSI Manual, Appendix G, at the following web address:

The side slopes for the bioretention facility (vertical or sloped) can affect the plant selection and must be considered. Additionally, trees can be planted along the side slopes or bottom of bioretention cells that are unlined.

Mulch Layer

You can design bioretention areas with or without a mulch layer. When used, mulch shall be:

Mulch shall not be:

In bioretention areas where higher flow velocities are anticipated, an aggregate mulch may be used to dissipate flow energy and protect underlying bioretention soil mix. Aggregate mulch varies in size and type, but 1 to 1 1/2 inch gravel (rounded) decorative rock is typical.

Runoff Model Representation

Note that if the project is using bioretention to only meet The List Approach within I-3.4.5 MR5: On-Site Stormwater Management, there is no need to model the bioretention in a continuous runoff model. Size the bioretention as described above in Ponding Area.

The guidance below is to show compliance with the LID Performance Standard in I-3.4.5 MR5: On-Site Stormwater Management, or the standards in I-3.4.6 MR6: Runoff Treatment, I-3.4.7 MR7: Flow Control, and/or I-3.4.8 MR8: Wetlands Protection.

Continuous runoff modeling software include modeling elements for bioretention.

The equations used by the elements are intended to simulate the wetting and drying of soil as well as how the soils function once they are saturated. This group of LID elements uses the modified Green Ampt equation to compute the surface infiltration into the amended soil. The water then moves through the top amended soil layer at the computed rate, determined by Darcy’s and Van Genuchten’s equations. As the soil approaches field capacity (i.e., gravity head is greater than matric head), the model determines when water will begin to infiltrate into the second soil layer (lower layer). This occurs when the matric head is less than the gravity head in the first layer (top layer). The second layer is intended to prevent loss of the amended soil layer. As the second layer approaches field capacity, the water begins to move into the third layer – the gravel underlayer. For each layer, the user inputs the depth of the layer and the type of soil.

Within the WWHM continuous runoff model, for the Ecology-recommended soil specifications for each layer in the design criteria for bioretention, the model will automatically assign pre-determined appropriate values for parameters that determine water movement through that soil. These include: wilting point, minimum hydraulic conductivity, maximum saturated hydraulic conductivity, and the Van Genuchten number.

For bioretention with underlying perforated drain pipes that discharge to the surface, the only volume available for storage (and modeled as storage as explained herein) is the void space within the aggregate bedding layer below the invert of the drain pipe. Use 40% void space for the Type 26 mineral aggregate specified in Underdrain (optional) (above).


It is preferable to enter each bioretention device and its drainage area into the approved computer models for estimating their performance.

However, where site layouts involve multiple bioretention facilities, the modeling schematic can become extremely complicated or not accommodated by the available schematic grid.

In those cases, multiple bioretention facilities with similar designs (i.e., soil depth, ponding depth, freeboard height, and drainage area to ponding area ratio), and infiltration rates (Ecology suggests within a factor of 2) may have their drainage areas and ponded areas be combined, and represented in the runoff model as one drainage area and one bioretention device. In this case, use a weighted average of the design infiltration rates at each location. The averages are weighted by the size of their drainage areas.

For bioretention with side slopes of 3H:1V or flatter, infiltration through the side slope areas can be significant. Where side slopes are 3H:1V or flatter, bioretention can be modeled allowing infiltration through the side slope areas to the native soil. In WWHM, modeling of infiltration through the side slope areas is accomplished by switching the default setting for “Use Wetted Surface Area (sidewalls): from “NO” to “YES.”

Installation Criteria


Soil compaction can lead to bioretention BMP failure; accordingly, minimizing compaction of the base and sidewalls of the bioretention area is critical. Excavation should never be allowed during wet or saturated conditions (compaction can reach depths of 2-3 feet during wet conditions and mitigation is likely to not be possible). Excavation should be performed by machinery operating adjacent to the bioretention BMP, and no heavy equipment with narrow tracks, narrow tires, or large lugged, high pressure tires should be allowed on the bottom of the bioretention BMP. If machinery must operate in the bioretention area for excavation, use light weight, low ground-contact pressure equipment and rip the base at completion to refracture soil to a minimum of 12 inches. If machinery operates in the BMP footprint, subgrade infiltration rates must be field tested and compared to initial Ksat tests obtained during design. Failure to meet or exceed the initial Ksat tests will require revised engineering designs to verify achievement of Runoff Treatment and Flow Control benefits that were estimated in the Stormwater Site Plan.

Prior to placement of the bioretention soil mix, the finished subgrade shall:

Sidewalls of the BMP, beneath the surface of the bioretention soil mix, can be vertical if soil stability is adequate. Exposed sidewalls of the completed bioretention area with bioretention soil mix in place should be no steeper than 3H:1V. The bottom of the BMP should be flat.

Soil Placement

On-site soil mixing or placement shall not be performed if bioretention soil mix or subgrade soil is saturated. The bioretention soil mix should be placed and graded by machinery operating adjacent to the bioretention BMP. If machinery must operate in the bioretention cell for soil placement, use light weight equipment with low ground-contact pressure. If machinery operates in the BMP footprint, subgrade infiltration rates must be field tested and compared to initial Ksat tests obtained during design. Failure to meet or exceed the initial Ksat tests will require revised engineering designs to verify achievement of Runoff Treatment and Flow Control benefits that were estimated in the Stormwater Site Plan.

The soil mixture shall be placed in horizontal layers not to exceed 6 inches per lift for the entire area of the bioretention BMP.

Compact the bioretention soil mix to a relative compaction of 85 percent of modified maximum dry density (ASTM D 1557). Compaction can be achieved by boot packing (simply walking over all areas of each lift), and then apply 0.2 inches (0.5 cm) of water per 1 inch (2.5 cm) of bioretention soil mix depth. Water for settling should be applied by spraying or sprinkling.

Temporary Erosion and Sediment Control (TESC)

Controlling erosion and sediment are most difficult during clearing, grading, and construction; accordingly, minimizing site disturbance to the greatest extent practicable is the most effective sediment management. During construction:

Every effort during design, construction sequencing and construction should be made to prevent sediment from entering bioretention BMPs. However, bioretention areas are often distributed throughout the project area and can present unique challenges during construction. See the Low Impact Development Technical Guidance Manual for Puget Sound (Hinman and Wulkan, 2012) for guidelines if no other options exist and runoff during construction must be directed through the bioretention BMPs.

Erosion and sediment control practices must be inspected and maintained on a regular basis.


If using the default bioretention soil mix, pre-placement laboratory analysis for saturated hydraulic conductivity of the bioretention soil mix is not required. Verification of the mineral aggregate gradation, compliance with the compost specifications, and the mix ratio must be provided.

If using a custom bioretention soil mix, verification of compliance with the minimum design criteria cited above for such custom mixes must be provided. This will require laboratory testing of the material that will be used in the installation. Testing shall be performed by a Seal of Testing Assurance, AASHTO, ASTM or other standards organization accredited laboratory with current and maintained certification. Samples for testing must be supplied from the bioretention soil mix that will be placed in the bioretention areas.

If testing infiltration rates is necessary for post-construction verification, use the Pilot Infiltration Test (PIT) method or a double ring infiltrometer test (or other small-scale testing allowed by the local government with jurisdiction). If using the PIT method, do not excavate the bioretention soil mix (conduct the test at the elevation of the finished bioretention soil mix), use a maximum of 6 inch ponding depth and conduct the test before plants are installed.


Bioretention areas require annual plant, soil, and mulch layer maintenance to ensure optimum infiltration, storage, and pollutant removal capabilities. In general, bioretention maintenance requirements are typical landscape care procedures and include:

Refer to Appendix V-A: BMP Maintenance Tables for additional maintenance guidelines.