5.4.6
5.4.6.1 BMP Description
Bioretention areas (BMP T7.30 in Volume V, Chapter 5 of the Ecology Manual) are engineered, shallow, landscaped depressions, with a designed soil mix and plants adapted to the local climate and soil moisture conditions that receive stormwater from a contributing area.
The term, bioretention, is used to describe various designs using soil and plant complexes to manage stormwater, and can include:
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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.
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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.
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Bioretention Planters and Planter Boxes – Designed 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.
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.
Stormwater planters in the right of way require urban design and tailoring to street typology and context. NACTO Urban Street Stormwater Guide provides guidance for designing roadside stormwater planters.
See Typical Bioretention, Typical Bioretention w/Underdrain, Typical Bioretention w/Liner (Not LID), and Example of a Bioretention Planter figures in Volume V, Chapter 5 of the Ecology Manual for examples of various types of bioretention configurations.
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 Vol II–5.4.30 Manufactured Treatment Devices as BMPs for more information on manufactured treatment devices.
5.4.6.2 Performance Mechanism
Bioretention provides effective removal of many stormwater pollutants by passing stormwater through a soil profile that meets specified characteristics. Bioretention can also reduce stormwater runoff quantity and surface runoff flow rates significantly where the exfiltrate from the design soil is allowed to infiltrate into the surrounding native soils. Bioretention can be used as a primary or supplemental detention/retention system. Where the native soils have low infiltration rates, underdrain systems can be installed, and the BMP used to filter pollutants and detain flows. However, designs utilizing underdrains provide less flow control benefits.
5.4.6.3 Applications and Limitations
For projects electing to use List #2 of MR #5 (Minimum Requirement #5: On-site Stormwater Management), bioretention BMPs shall be used to the extent feasible for runoff from roofs and other hard surfaces unless a higher priority BMP is feasible. Small projects that are only required to meet MR #1–5 may also use bioretention BMPs in place of rain gardens.
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 generation whenever possible. Distributing bioretention cells throughout the project site (i.e., along roadways, in open space areas, on private properties, etc.) can significantly help achieve this goal.
Bioretention BMPs can but are not required to fully meet the MR #6: Runoff Treatment standard to treat 91% of the stormwater runoff file from pollution-generating surfaces (Vol I–4.2.6 Minimum Requirement #6: Runoff Treatment). They can be sized to fully or partially meet the standard and can be paired with other Runoff Treatment BMPs as needed to fully satisfy the standard.
Bioretention BMPs that infiltrate into the ground can also serve a significant flow reduction function. They can but are not required to fully meet the flow control duration standard of MR #7: Flow Control.
Bioretention constructed with imported composted material shall not be used within 0.25 mile of phosphorus-sensitive water bodies if the underlying native soil does not meet the site suitability criteria for treatment in Volume V, Section 5.6 of the Ecology Manual. Preliminary monitoring indicates that new bioretention BMPs can add phosphorus to stormwater. Therefore, they shall 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:
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Individual lots for rooftop, driveway, and other on-lot impervious surface.
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Shared facilities located in common areas for individual lots.
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Areas within loop roads or cul-de-sacs.
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Landscaped parking lot islands.
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Within rights of way along roads (often linear bioretention swales and cells).
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Common landscaped areas in apartment complexes or other multifamily housing designs.
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Planters on building roofs, patios, and as part of streetscapes.
See Table II-5.10 for a summary of which minimum requirements bioretention BMPs can be used to fully or partially satisfy.
| BMP | MR #7: Flow Control | ||||||
|---|---|---|---|---|---|---|---|
|
List |
LID Performance Standard |
Basic |
Enhanced |
Oil Control |
Phosphorus |
||
|
Bioretention |
X |
X |
X |
X |
|
X | |
5.4.6.4 Site Considerations
For bioretention BMPs utilizing infiltration into the native soils, see the infiltration feasibility criteria provided in Vol II–5.3.2 Determine Infiltration Feasibility. See also the geometry requirements provided in BMP T7.30 in Volume V, Chapter 5 of the Ecology Manual to determine which types of bioretention fit best within the available space on site.
5.4.6.5 Design Information
Pre-settling is required when the impervious area contributing to a single flow entrance is equal to or greater than 5,000 square feet. For impervious contributing areas between 5,000 and 10,000 square feet, the first 2 to 3 feet of the upstream bioretention cell (at the flow entrance) shall be designated as the pre-settling zone. This bottom area of the cell shall be constructed of cobbles, concrete open celled paving grids, roughened concrete pad, or similar material to collect sediment and streamline O&M activities. Alternatively, a catch basin with a minimum 2‑foot sump may be used as the pre-settling zone. Wherever a pipe daylights into the bioretention cell, provide energy dissipation within the cell.
For impervious contributing areas greater than 10,000 square feet, pre-settling requirements will be determined on a case-by-case basis and design of pre-settling BMPs shall be reviewed and approved by the director.
See BMP T7.30 in Volume V, Chapter 5 of the Ecology Manual and the LID Technical Guidance Manual (Hinman and Wulkan 2012) for detailed design guidance and criteria. County-specific design criteria include the following:
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Plantings
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Invasive species and noxious weed control will be required as typical with all planted landscape areas.
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Trees are not allowed in the bottom of a linear bioretention cell. Trees may be allowed on unlined side slopes on a case-by-case basis.
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Trees shall be located a minimum horizontal offset of 5 feet or one half the mature root zone (whichever is greater) from underdrains. Trees shall have a minimum 3‑foot vertical offset from underdrains.
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A recommended bioretention plant list is included in Appendix B of the Kitsap County Low Impact Guidance Manual (Kitsap Home Builders 2009).
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See BMP T7.30 in Volume V, Chapter 5 of the Ecology Manual for guidance on Runoff Model Representation.
5.4.6.6 Minimum Construction Requirements
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 tool. During construction:
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Bioretention BMPs shall not be used as sediment control BMPs and all drainage shall 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, refer to the other recommendations provided in this section.
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Construction of bioretention BMPs shall not begin until all contributing drainage areas are stabilized according to erosion and sediment control BMPs and to the satisfaction of the engineer.
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If the design includes curb and gutter, the curb cuts and inlets shall 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 shall 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 LID Technical Guidance Manual (Hinman and Wulkan 2012) for guidelines if no other options exist and runoff during construction must be directed through the bioretention BMPs.
Note that the LID Technical Guidance Manual is for additional informational purposes only. The engineer shall follow the guidance within this manual if there are any discrepancies between this manual and the LID Technical Guidance Manual.
Erosion and sediment control practices shall be inspected and maintained on a regular basis.
5.4.6.7 Operations and Maintenance Requirements
Refer to Volume V, Appendix V‑A of the Ecology Manual for specific maintenance activities and schedules for bioretention BMPs. See also the general infiltration BMP maintenance guidelines in that appendix.