BMP T7.30: Bioretention

Purpose

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.

Description

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:

• Bioretention cells: Shallow depressions with a designed planting soil mix and a variety of plant material, including trees, shrubs, grasses, and/or other herbaceous plants. Bioretention cells may or may not have an underdrain and are not designed as a conveyance system.

• Bioretention swales: Incorporate the same design features as bioretention cells; however, bioretention swales are designed as part of a system that can convey stormwater when maximum ponding depth is exceeded. Bioretention swales have relatively gentle side slopes and ponding depths that are typically 6 to 12 inches.

• Bioretention planters and planter boxes: Bioretention soil mix and a variety of plant material including trees, shrubs, grasses, and/or other herbaceous plants within a vertical walled container usually constructed from formed concrete, but could include other materials. Planter boxes are completely impervious and include a bottom (must include an underdrain). Planters have an open bottom and allow infiltration to the subgrade. These designs are often used in ultra-urban settings.

  Stormwater planters in the ROW require urban design and tailoring it to street typology and context. NACTO Urban Street Stormwater Guide provides guidance for designing roadside stormwater planters. https://nacto.org/publication/urban-street-stormwater-guide/

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

pdf download

Figure V-5.13: Typical Bioretention w/Underdrain

pdf download

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

pdf download

Figure V-5.15: Example of a Bioretention Planter

pdf download

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:

• Individual lots for rooftop, driveway, and other on-lot impervious surfaces.

• Shared facilities located in common areas for individual lots.

• Areas within loop roads or cul-de-sacs.

• Landscaped parking lot islands.

• Within right-of-ways along roads (often linear bioretention swales or cells).

• Common landscaped areas in apartment complexes or other multifamily housing designs.

• Planters on building roofs, patios, and as part of streetscapes.

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:

• Citation of any of the following infeasibility criteria must be based on an evaluation of site-specific conditions and a written recommendation from an appropriate licensed professional (e.g., engineer, geologist, hydrogeologist):

  • Where professional geotechnical evaluation recommends infiltration not be used due to reasonable concerns about erosion, slope failure, or down gradient flooding.

  • Within an area whose ground water drains into an erosion hazard, or landslide hazard area.

  • Where the only area available for siting would threaten the safety or reliability of pre-existing underground utilities, pre-existing underground storage tanks, pre-existing structures, or pre-existing road or parking lot surfaces.

  • Where the only area available for siting does not allow for a safe overflow pathway to the municipal separate storm sewer system or private storm sewer system.

  • Where there is a lack of usable space for bioretention BMPs at re-development sites, or where there is insufficient space within the existing public right-of-way on public road projects.

  • Where infiltrating water would threaten existing below grade basements.

  • Where infiltrating water would threaten shoreline structures such as bulkheads.

• The following infeasibility criteria are based on conditions such as topography and distances to predetermined boundaries. Citation of the following criteria do not need site-specific written recommendations from a licensed professional, although some may require professional services to determine:

  • Within setbacks from structures as established by the local government with jurisdiction.

  • Where they are not compatible with the surrounding drainage system as determined by the local government with jurisdiction (e.g., project drains to an existing stormwater collection system whose elevation or location precludes connection to a properly functioning bioretention BMP).

  • Where land for bioretention is within area designated as an erosion hazard or landslide hazard.

  • Where the site cannot be reasonably designed to locate bioretention BMPs on slopes less than 8%.

  • Within 50 feet from the top of slopes that are greater than 20% and over 10 feet of vertical relief.

  • For properties with known soil or ground water contamination (typically federal Superfund sites or state cleanup sites under the Model Toxics Control Act (MTCA)):

    • Within 100 feet of an area known to have deep soil contamination;

    • Where ground water modeling indicates infiltration will likely increase or change the direction of the migration of pollutants in the ground water;

    • Wherever surface soils have been found to be contaminated unless those soils are removed within 10 horizontal feet from the infiltration area;

    • Any area where these BMPs are prohibited by an approved cleanup plan under the state Model Toxics Control Act or Federal Superfund Law, or an environmental covenant under Chapter 64.70 RCW.

  • Within 100 feet of a closed or active landfill.

  • Within 100 feet of a drinking water well, or a spring used for drinking water supply.

  • Within 10 feet of small on-site sewage disposal drainfield, including reserve areas, and grey water reuse systems. For setbacks from a “large on-site sewage disposal system”, see Chapter 246-272B WAC.

  • Within 10 feet of an underground storage tank and connecting underground pipes when the capacity of the tank and pipe system is 1100 gallons or less. (As used in these criteria, an underground storage tank means any tank used to store petroleum products, chemicals, or liquid hazardous wastes of which 10% or more of the storage volume (including volume in the connecting piping system) is beneath the ground surface.

  • Within 100 feet of an underground storage tank and connecting underground pipes when the capacity of the tank and pipe system is greater than 1100 gallons.

  • Where the minimum vertical separation of 1 foot to the seasonal high water table, bedrock, or other impervious layer would not be achieved below bioretention that would serve a drainage area that is less than:

    1. 5,000 sq. ft. of pollution-generating impervious surface, and

    2. 10,000 sq. ft. of impervious surface, and

    3. three-quarter (3/4) acres of pervious surface.

  • Where the minimum vertical separation of 3 feet to the seasonal high water table, bedrock, or other impervious layer would not be achieved below bioretention that would serve a drainage area that meets or exceeds:

    1. 5,000 sq. ft. of pollution-generating impervious surface, or

    2. 10,000 sq. ft. of impervious surface, or

    3. three-quarter (3/4) acres of pervious surface.

    AND

    cannot reasonably be broken down into amounts smaller than those listed in 1-3 (above).

  • Where the field testing indicates potential bioretention sites have a measured (a.k.a., initial) native soil saturated hydraulic conductivity less than 0.30 inches per hour.

    If the measured native soil infiltration rate is less than 0.30 in/hour, bioretention should not be used to meet the The List Approach of I-3.4.5 MR5: On-Site Stormwater Management. In these slow draining soils, a bioretention BMP with an underdrain may be used to treat pollution-generating surfaces to help meet I-3.4.6 MR6: Runoff Treatment. If the underdrain is elevated within a base course of gravel, the bioretention BMP will also provide some modest flow reduction benefit that will help achieve the LID Performance Standard within I-3.4.5 MR5: On-Site Stormwater Management and/or the Flow Control Performance Standard within I-3.4.7 MR7: Flow Control.

• A local government may designate geographic boundaries within which bioretention BMPs may be designated as infeasible due to year-round, seasonal or periodic high groundwater conditions, or due to inadequate infiltration rates. Designations must be based upon a pre-ponderance of field data, collected within the area of concern, that indicate a high likelihood of failure to achieve the minimum ground water clearance or infiltration rates identified in the above infeasibility criteria. The local government must develop a technical report and make it available upon request to Ecology. The report must be authored by (a) professional(s) with appropriate expertise (e.g., registered engineer, geologist, hydrogeologist, or certified soil scientist), and document the location and the pertinent values/observations of data that were used to recommend the designation and boundaries for the geographic area. The types of pertinent data include, but are not limited to:

  • Standing water heights or evidence of recent saturated conditions in observation wells, test pits, test holes, and well logs.

  • Observations of areal extent and time of surface ponding, including local government or professional observations of high water tables, frequent or long durations of standing water, springs, wetlands, and/or frequent flooding.

  • Results of infiltration tests

• In addition, a local government can map areas that meet a specific infeasibility criterion listed above provided they have an adequate data basis. Criteria that are most amenable to mapping are:

  • Where land for bioretention is within an area designated by the local government as an erosion hazard, or landslide hazard

  • Within 50 feet from the top of slopes that are greater than 20% and over 10 feet vertical relief

  • Within 100 feet of a closed or active landfill

Design Criteria

General Design Criteria

• Utility conflicts: Consult local jurisdiction requirements for horizontal and vertical separation required for publicly-owned utilities, such as water and sewer. Consult the appropriate franchise utility owners for separation requirements from their utilities, which may include communications and gas. When separation requirements cannot be met, designs should include appropriate mitigation measures, such as impermeable liners over the utility, sleeving utilities, fixing known leaky joints or cracked conduits, and/or adding an underdrain to the bioretention.

• Transportation safety: The design configuration and selected plant types should provide adequate sight distances, clear zones, and appropriate setbacks for roadway applications in accordance with local jurisdiction requirements.

• Ponding depth and surface water draw-down: Flow Control needs, as well as location in the development, and mosquito breeding cycles will determine draw-down timing. For example, front yards and entrances to residential or commercial developments may require rapid surface dewatering for aesthetics. In no case shall draw down time exceed 48 hours.

• Impacts of surrounding activities: Human activity influences the location of the BMP in the development. For example, locate bioretention BMPs away from traveled areas on individual lots to prevent soil compaction and damage to vegetation or provide elevated or bermed pathways in areas where foot traffic is inevitable. Provide barriers, such as wheel stops, to restrict vehicle access in roadside applications.

• Visual buffering: Bioretention BMPs can be used to buffer structures from roads, enhance privacy among residences, and for an aesthetic site feature.

• Site growing characteristics and plant selection: Appropriate plants should be selected for sun exposure, soil moisture, and adjacent plant communities. Native species or hardy cultivars are recommended and can flourish in the properly designed and placed bioretention soil mix with no nutrient or pesticide inputs and 2-3 years irrigation for establishment. Invasive species and noxious weed control will be required as typical with all planted landscape areas.

• Project submission requirements: Submit the results of infiltration (Ksat) testing and ground water elevation testing (or other documentation and justification for the rates and hydraulic restriction layer clearances) with the Stormwater Site Plan as justification for the feasibility decision regarding bioretention and as justification for assumptions made in the runoff modeling.

• Legal documentation to track bioretention obligations: Where drainage plan submittals include assumptions with regard to size and location of bioretention BMPs, approval of the plat, short-plat, or building permit should identify the bioretention obligation of each lot; and the appropriate lots should have deed requirements for construction and maintenance of those BMPs

• Much of the design criteria within this BMP originated from the Low Impact Development Technical Guidance Manual for Puget Sound (Hinman and Wulkan, 2012). Refer to that document for additional explanations and background.

  Note that the Low Impact Development Technical Guidance Manual for Puget Sound (Hinman and Wulkan, 2012) is for additional information purposes only. You must follow the guidance within this manual if there are any discrepancies between this manual and the Low Impact Development Technical Guidance Manual for Puget Sound (Hinman and Wulkan, 2012).

• Geotechnical analysis is an important first step to develop an initial assessment of the variability of site soils, infiltration characteristics and the necessary frequency and depth of infiltration tests. See V-5.2 Infiltration BMP Design Steps.

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.

• Small bioretention cells (bioretention BMPs made up of one or multiple cells that receive water from 1 or 2 individual lots or < 1/4 acre of pavement or other impervious surface) have the following options for determining the native soil infiltration rate:

  1. Small-scale pilot infiltration test (PIT) as described in V-5.4 Determining the Design Infiltration Rate of the Native Soils.

  2. If the site is underlain with soils not consolidated by glacial advance (e.g., recessional outwash soils), then the designer may use the grain size analysis method described in V-5.4 Determining the Design Infiltration Rate of the Native Soils based on the layer(s) identified in results of one soil test pit or boring.

• Large bioretention cells (bioretention BMPs made up of one or multiple cells that receive water from several lots or 1/4 acre or more of pavement or other impervious surface) have the following options for determining the native soil infiltration rate:

  1. Multiple small-scale or one large-scale PIT. If using the small-scale test, measurements should be taken at several locations within the area of interest.

  2. If the site is underlain with soils not consolidated by glacial advance (e.g., recessional outwash soils), then the designer may use the grain size analysis method described in V-5.4 Determining the Design Infiltration Rate of the Native Soils. Use the grain size analysis method based on more than one soil test pit or boring. The more test pits/borings used, and the more evidence of consistency in the soils, the less of a correction factor may be used.

• Bioretention swales have the following options for determining the native soil infiltration rate:

  1. Approximately 1 small-scale PIT per 200 feet of swale, and within each length of road with significant differences in subsurface characteristics.

  2. If the site is underlain with soils not consolidated by glacial advance (e.g., recessional outwash soils), then the designer may use the grain size analysis method described in V-5.4 Determining the Design Infiltration Rate of the Native Soils. Approximately 1 soil test pit/boring per 200 feet of swale and within each length of road with significant differences in subsurface characteristics.

• On a single, smaller commercial property, one bioretention BMP will likely be appropriate. In that case, a small-scale PIT – or an alternative small scale test specified by the local government - should be performed at the proposed bioretention location. Tests at more than one site could reveal the advantages of one location over another.

• On larger commercial sites, a small-scale PIT every 5,000 sq. ft. is advisable. If soil characteristics across the site are consistent, a geotechnical professional may recommend a reduction in the number of tests.

• On multi-lot residential developments, multiple bioretention BMPs, or a BMP stretching over multiple properties are appropriate. In most cases, it is necessary to perform small-scale PITs, or other small-scale tests as allowed by the local jurisdiction. A test is advisable at each potential bioretention site. Long, narrow bioretention BMPs, such as one following the road right-of-way, should have a test location at least every 200 lineal feet, and within each length of road with significant differences in subsurface characteristics.

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:

• The overlying bioretention soil mix provides excellent protection for the underlying native soil from sedimentation. Accordingly, when using The Simplified Approach to Calculating the Design Infiltration Rate of the Native Soils as described in V-5.4 Determining the Design Infiltration Rate of the Native Soils, the correction factor for the sub-grade soil does not have to take into consideration the extent of influent control and clogging over time. The correction factor to be applied to in-situ, small-scale infiltration test results for bioretention sites is determined by the site variability and number of locations tested as well as the method used to determine initial K_sat. Using Table V-5.1: Correction Factors to be Used With In-Situ Saturated Hydraulic Conductivity Measurements to Estimate Design Rates, the correction factor for bioretention design is revised based on this guidance as:

  Total Correction Factor, CFT = CFv x CFt

• Tests should be located and be at an adequate frequency capable of producing a soil profile characterization that fully represents the infiltration capability where the bioretention areas are to be located. The partial correction factor CF_V depends on the level of uncertainty that variable subsurface conditions justify. If a pilot infiltration test is conducted for all bioretention areas or the range of uncertainty is low (for example, conditions are known to be uniform through previous exploration and site geological factors) one pilot infiltration test may be adequate to justify a CF_V of one. If the level of uncertainty is high, a CF_V near the low end of the range may be appropriate. Two example scenarios where low CF_Vs may be appropriate include:

  • Site conditions are highly variable due to a deposit of ancient landslide debris, or buried stream channels. In these cases, even with many explorations and several pilot infiltration tests, the level of uncertainty may still be high.

  • Conditions are variable, but few explorations and only one pilot infiltration test is conducted. That is, the number of explorations and tests conducted do not match the degree of site variability anticipated.

Determining the Bioretention Soil Mix Design Infiltration Rate

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

   • If using Ecology's default bioretention soil mix (detailed below), the initial K_sat 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 K_sat value into the continuous modeling software.

2. After determining the initial K_sat, 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 K_sat 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 K_sat safety factor.

3. The continuous runoff model has a field for entering K_sat 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 (K_sat). 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:

• Coefficient of Uniformity (Cu = D60/D10) equal to or greater than 4, and

• Coefficient of Curve (Cc = (D30)2/D60 x D10) greater than or equal to 1 and less than or equal to 3.

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.

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 to aggregate ratio: 60-65 percent mineral aggregate, 35 – 40 percent compost by volume.

• Organic matter content: 5 – 8 percent by weight.

• Cation Exchange Capacity (CEC) must be > 5 milliequivalents/100 g dry soil Note: Soil mixes meeting the above specifications do not have to be tested for CEC. They will readily meet the minimum CEC.

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.

• Meets the definition of “composted material” in WAC 173-350-100 and complies with testing parameters and other standards in WAC 173-350-220.

• Produced at a composting facility that is permitted by the jurisdictional health authority. Permitted compost facilities in Washington are included in a spreadsheet titled Washington composting facilities and material types – 2017 at the following web address:

  https://ecology.wa.gov/Waste-Toxics/Reducing-recycling-waste/Organic-materials/Managing-organics-compost

• The compost product must originate a minimum of 65 percent by volume from recycled plant waste comprised of ”yard debris,” “crop residues,” and “bulking agents” as those terms are defined in WAC 173-350-100. A maximum of 35 percent by volume of “post-consumer food waste” as defined in WAC 173-350-100, but not including biosolids or manure, may be substituted for recycled plant waste.

• Stable (low oxygen use and CO₂ generation) and mature (capable of supporting plant growth) by tests shown below. This is critical to plant success in bioretention soil mixes.

• Moisture content range: no visible free water or dust produced when handling the material.

• Tested in accordance with the U.S. Composting Council “Test Method for the Examination of Compost and Composting” (TMECC), as established in the Composting Council’s “Seal of Testing Assurance” (STA) program. Most Washington compost facilities now use these tests.

• Screened to the following size gradations for Fine Compost when tested in accordance with TMECC test method 02.02-B, Sample Sieving for Aggregate Size Classification.”

  Fine Compost shall meet the following gradation by dry weight

  Minimum percent passing 2”: 100%

  Minimum percent passing 1”: 99%

  Minimum percent passing 5/8”: 90%

  Minimum percent passing ¼”: 75%

• pH between 6.0 and 8.5 (TMECC 04.11-A). “Physical contaminants” (as defined in WAC 173-350-100) content less that 1% by weight (TMECC 03.08-A) total, not to exceed 0.25 percent film plastic by dry weight.

• Minimum organic matter content of 40% (TMECC 05.07-A “Loss on Ignition)

• Soluble salt content less than 4.0 dS/m (mmhos/cm) (TMECC 04.10-A “Electrical Conductivity, 1:5 Slurry Method, Mass Basis”)

• Maturity indicators from a cucumber bioassay (TMECC 05.05-A “Seedling Emergence and Relative Growth ) must be greater than 80%for both emergence and vigor”)

• Stability of 7 mg CO2-C/g OM/day or below (TMECC 05.08-B “Carbon Dioxide Evolution Rate”)

• Carbon to nitrogen ratio (TMECC 05.02A “Carbon to Nitrogen Ratio” which uses 04.01 “Organic Carbon” and 04.02D “Total Nitrogen by Oxidation”) of less than 25:1. The C:N ratio may be up to 35:1 for plantings composed entirely of Puget Sound Lowland native species and up to 40:1 for coarse compost to be used as a surface mulch (not in a soil mix).

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:

• CEC ≥ 5 meq/100 grams of dry soil; USEPA 9081

• pH between 5.5 and 7.0

• 5 - 8 percent organic matter content before and after the saturated hydraulic conductivity test; ASTM D2974 (Standard Test Method for Moisture, Ash, and Organic Matter of Peat and Other Organic Soils)

• 2-5 percent fines passing the 200 sieve; TMECC 04.11-A

• Measured (Initial) saturated hydraulic conductivity (K_sat) of less than 12 inches per hour; ASTM D 2434 (Standard Test Method for Permeability of Granular Soils (Constant Head)) at 85% compaction per ASTM D 1557 (Standard Test Method s for Laboratory Compaction Characteristics of Soil Using Modified Effort). Also, use Recommended Modifications to ASTM D 2434 When Measuring Hydraulic Conductivity for Bioretention Soil Mixes (as detailed above).

• Design (long-term) saturated hydraulic conductivity of more than 1 inch per hour. Note: Design saturated hydraulic conductivity is determined by applying the appropriate infiltration correction factors as explained above under Determining the Bioretention Soil Mix Design Infiltration Rate.

• If compost is used in creating the custom bioretention soil mix, it must meet all of the specifications listed above in Compost for Default BSM, except for the gradation specification. An alternative gradation specification must indicate the minimum percent passing for a range of similar particle sizes.

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:

• Dispersed, low velocity flow across a landscape area: Landscape areas and vegetated buffer strips slow incoming flows and provide an initial settling of particulates and are the preferred method of delivering flows to bioretention. Dispersed flow may not be possible given space limitations or if the BMP is controlling roadway or parking lot flows where curbs are mandatory.

• Dispersed or sheet flow across pavement or gravel and past wheel stops for parking areas.

• Curb cuts for roadside, driveway or parking lot areas: Curb cuts should include a rock pad, concrete or other erosion protection material in the channel entrance to dissipate energy. Minimum curb cut width should be 12 inches; however, 18 inches is recommended. The designer should calculate the size and choose the style of curb cut that is appropriate for the site conditions and runoff expectations. Avoid the use of angular rock or quarry spalls and instead use round (river) rock if needed. Removing sediment from angular rock is difficult. The flow entrance should slope steeply (at least 1:1) from the curb line to the bioretention, dropping at least 3", and provide an area for settling and periodic removal of sediment and coarse material before flow dissipates to the remainder of the bioretention area.

  Curb cuts used for bioretention areas in high use parking lots or roadways require an increased level of maintenance due to high coarse particulates and trash accumulation in the flow entrance and associated bypass of flows. The following are methods recommended for areas where heavy trash and coarse particulates are anticipated:

  • Curb cut width: 18 inches.

  • At a minimum the flow entrance should drop 2 to 3 inches from the gutter line into the bioretention area and provide an area for settling and periodic removal of debris.

  • Anticipate relatively more frequent inspection and maintenance for areas with large impervious areas, high traffic loads and larger debris loads.

  • Catch basins or forebays may be necessary at the flow entrance to adequately capture debris and sediment load from large contributing areas and high use areas. Piped flow entrance in this setting can easily clog and catch basins with regular maintenance are necessary to capture coarse and fine debris and sediment.

• Pipe flow entrance: Piped entrances should include rock or other erosion protection material in the channel entrance to dissipate energy and disperse flow.

• Catch basin: In some locations where road sanding or higher than usual sediment inputs are anticipated, catch basins can be used to settle sediment and release water to the bioretention area through a grate for filtering coarse material.

• Trench drains: Trench drains can be used to cross sidewalks or driveways where a deeper pipe conveyance creates elevation problems. Trench drains tend to clog and may require additional maintenance.

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:

• Maximum planted side slope if total cell depth is greater than 3 feet: 3H:1V. If steeper side slopes are necessary rockeries, concrete walls or soil wraps may be effective design options. Local jurisdictions may require bike and/or pedestrian safety features, such as railings or curbs with curb cuts, when steep side slopes are adjacent to sidewalks, walkways, or bike lanes.

• Minimum bottom width for bioretention swales: 2 feet recommended and 1 foot minimum. Carefully consider flow depths and velocities, flow velocity control (check dams) and appropriate vegetation or rock mulch to prevent erosion and channelization at bottom widths less than 2 feet.

• Bioretention areas should have a minimum shoulder of 12 inches (30.5 cm) between the road edge and beginning of the bioretention side slope where flush curbs are used. Compaction effort for the shoulder should 90 percent proctor.

Ponding Area

Ponding depth recommendations:

• Maximum ponding depth: 12 inches (30.5 cm).

• Surface pool drawdown time: 24 hours

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 K_sat 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:

• the invert of the underdrain must be elevated 6 inches above the bottom of the aggregate bedding layer. A larger distance between the underdrain and bottom of the bedding layer is desirable, but cannot be used to trigger infeasibility due to inadequate vertical separation to the seasonal high water table, bedrock, or other impermeable layer.

• the distance between the bottom of the bioretention soil mix and the crown of the underdrain pipe must be not less than 6 but not more than 12 inches;

• the aggregate bedding layer must run the full length and the full width of the bottom of the bioretention BMP;

• the BMP must not be underlain by a low permeability liner that prevents infiltration into the native soil.

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:

• Located near sensitive infrastructure (e.g., unsealed basements) and potential for flooding is likely.

• Used for filtering storm flows from gas stations or other pollutant hotspots (requires impermeable liner).

• Located above native soils with infiltration rates that are not adequate to meet maximum pool and system dewater rates, or are below a minimum rate allowed by the local government.

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:

• Minimum pipe diameter: 4 inches (pipe diameter will depend on hydraulic capacity required, 4 to 8 inches is common).

• Slotted subsurface drain PVC per ASTM D1785 SCH 40.

• Slots should be cut perpendicular to the long axis of the pipe and be 0.04 to 0.069 inches by 1 inch long and be spaced 0.25 inches apart (spaced longitudinally). Slots should be arranged in four rows spaced on 45-degree centers and cover ½ of the circumference of the pipe. See Underdrain Aggregate Filter and Bedding Layer (below) for aggregate gradation appropriate for this slot size.

• Underdrains should be sloped at a minimum of 0.5 percent unless otherwise specified by an engineer.

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.

• Place the underdrain pipe on a bed of the Type 26 aggregate with a minimum thickness of 6 inches and cover with Type 26 aggregate to provide a 1-foot minimum depth around the top and sides of the slotted pipe. See the Low Impact Development Technical Guidance Manual for Puget Sound (Hinman and Wulkan, 2012).

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:

• Clay (bentonite) liners are low permeability liners. Where clay liners are used underdrain systems are necessary. See V-1.3.3 Low Permeability Liners for guidelines.

• Geomembrane liners completely block infiltration to subgrade soils and are used for ground water protection when bioretention BMPs are installed to filter storm flows from pollutant hotspots or on sidewalls of bioretention areas to restrict lateral flows to roadbeds or other sensitive infrastructure. Where geomembrane liners are used to line the entire BMP, underdrain systems are necessary. See V-1.3.3 Low Permeability Liners for guidelines.

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:

https://www.seattle.gov/util/cs/groups/public/@spu/@engineering/documents/webcontent/1_079167.pdf

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:

• Medium compost in the bottom of the BMP (compost is less likely to float during cell inundation). Compost shall not include biosolids or manures.

• Shredded or chipped hardwood or softwood on side slopes above ponding elevation and rim area. Arborist mulch is mostly woody trimmings from trees and shrubs and is a good source of mulch material. Wood chip operations are a good source for mulch material that has more control of size distribution and consistency. Do not use shredded construction wood debris or any shredded wood to which preservatives have been added.

• Free of weed seeds, soil, roots and other material that is not bole or branch wood and bark.

• A maximum of 2 to 3 inches thick.

Mulch shall not be:

• Grass clippings (decomposing grass clippings are a source of nitrogen and are not recommended for mulch in bioretention areas).

• Pure bark (bark is essentially sterile and inhibits plant establishment).

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).

Modeling:

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

Excavation

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 K_sat tests obtained during design. Failure to meet or exceed the initial K_sat 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:

• Be scarified to a minimum depth of 3 inches.

• Have any sediment deposited from construction runoff removed. To remove all introduced sediment, subgrade soil should be removed to a depth of 3-6 inches and replaced with bioretention soil mix.

• Be inspected by the responsible engineer to verify required subgrade condition.

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 K_sat tests obtained during design. Failure to meet or exceed the initial K_sat 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:

• Bioretention BMPs should not be used as sediment control BMPs, and all drainage should be directed away from bioretention BMPs after initial rough grading. Flow can be directed away from the BMP with temporary diversion swales or other approved protection. If introduction of construction runoff cannot be avoided see below for guidelines.

• Construction on bioretention BMPs should not begin until all contributing drainage areas are stabilized according to erosion and sediment control BMPs and to the satisfaction of the engineer.

• If the design includes curb and gutter, the curb cuts and inlets should be blocked until bioretention soil mix and mulch have been placed and planting completed (when possible), and dispersion pads are in place.

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.

Verification

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.

Maintenance

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:

• Watering: Plants should be selected to be drought tolerant and not require watering after establishment (2 to 3 years). Watering may be required during prolonged dry periods after plants are established.

• Erosion control: Inspect flow entrances, ponding area, and surface overflow areas periodically, and replace soil, plant material, and/or mulch layer in areas if erosion has occurred. Properly designed BMPs with appropriate flow velocities should not have erosion problems except perhaps in extreme events. If erosion problems occur, the following should be reassessed: (1) flow volumes from contributing areas and bioretention cell sizing; (2) flow velocities and gradients within the cell; and (3) flow dissipation and erosion protection strategies in the pretreatment area and flow entrance. If sediment is deposited in the bioretention area, immediately determine the source within the contributing area, stabilize, and remove excess surface deposits.

• Sediment removal: Follow the maintenance plan schedule for visual inspection and remove sediment if the volume of the ponding area has been compromised.

• Plant material: Depending on aesthetic requirements, occasional pruning and removing dead plant material may be necessary. Replace all dead plants and if specific plants have a high mortality rate, assess the cause and replace with appropriate species. Periodic weeding is necessary until plants are established.

• Weeding: Invasive or nuisance plants should be removed regularly and not allowed to accumulate and exclude planted species. At a minimum, schedule weeding with inspections to coincide with important horticultural cycles (e.g., prior to major weed varieties dispersing seeds). Weeding should be done manually and without herbicide applications. The weeding schedule should become less frequent if the appropriate plant species and planting density are used and the selected plants grow to capture the site and exclude undesirable weeds.

• Nutrient and pesticides: The soil mix and plants are selected for optimum fertility, plant establishment, and growth. Nutrient and pesticide inputs should not be required and may degrade the pollutant processing capability of the bioretention area, as well as contribute pollutant loads to receiving waters. By design, bioretention BMPs are located in areas where phosphorous and nitrogen levels may be elevated and these should not be limiting nutrients. If in question, have soil analyzed for fertility.

• Mulch: Replace mulch annually in bioretention BMPs where heavy metal deposition is high (e.g., contributing areas that include gas stations, ports and roads with high traffic loads). In residential settings or other areas where metals or other pollutant loads are not anticipated to be high, replace or add mulch as needed (likely 3 to 5 years) to maintain a 2 to 3 inch depth.

• Soil: Soil mixes for bioretention BMPs are designed to maintain long-term fertility and pollutant processing capability. Estimates from metal attenuation research suggest that metal accumulation should not present an environmental concern for at least 20 years in bioretention systems, but this will vary according to pollutant load. Replacing mulch media in bioretention BMPs where heavy metal deposition is likely provides an additional level of protection for prolonged performance. If in question, have soil analyzed for fertility and pollutant levels.

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