5.4.

6 Bioretention
Description: Bioretention areas are vegetated, shallow
surface depressions that use the interaction of
plants, soil, and microorganisms to store, treat, and
reduce runoff volume, and to reduce the flow rate
of stormwater runoff. Bioretention areas are generally
flat and include engineered or modified soils that
allow drainage of stormwater through soils. During
storms, runoff temporarily ponds 6 to 12 inches
above the mulch layer and then rapidly filters
through the bed.

Figure 1: Bioretention capture water from parking

lot in Powell, TN (Source: The SMART Center).

Key Design Criteria: Advantages:

• Min. filter media depth: 18-24 inches. • Reduces runoff volume, peak discharge rate,

5.4.6 – Bioretention
Recommended maximum: 36 inches TSS, pollutant, and runoff temperature.
• Ponding depth: 6-12 inches • Creates habitat and improves aesthetic.
• Length of shortest flow path/length: 0.3 • Flexible dimensions to fit conditions
• Excellent retrofit capability.
Site Constraints:
• Depth: 2 feet to water table / bedrock Disadvantages:
• Steep slopes: <20% or terraced to slow flow. • Built on areas that are generally level (or
• Media permeability: ≤ 0.5in/h or needs graded level).
underdrain • Steep slopes may require larger footprint to
• Hotspots: Needs to use impermeable create level grading.
bottom liner and an underdrain system. • Vegetation and soils must be protected from
• Min. distance requirement from: damage and compaction.
Water supply wells: 100 feet • Maintenance is required to maintain both
Surface water: 30 feet performance and aesthetics.

Maintenance: Design Checklist:

• Monitor sediment accumulation and • Determine whether an Infiltration or
remove as necessary filtration Design is best for the site based on
• Inspect channel and repair any eroding permeability or other site limitations.
surfaces or vegetation • Check bioretention sizing guidance.
• Ensure vegetation is well established • Design bioretention in accordance with
• Remove debris from any inlet and outlet design criteria and typical details.
structures. • Submit plans for review.

Tennessee Permanent Stormwater Management and Design Guidance Manual 141

 n
1. Design

1.1 Suggested Applications and Scale

Bioretention can be applied in most soils or topography, since runoff simply percolates through an
engineered soil bed. Consider locating bioretention areas in places that are generally “not used” such as
traffic islands; between parked cars in parking lots; along edges of public playgrounds, school yards, and
plazas; in courtyards; and in place of traditional landscape planting areas. The following site-specific
conditions should be considered:
• Select location to prevent vegetation damage and soil compaction from pedestrian traffic or
unintended vehicle compaction. Ideal locations are often located to the side or downhill of high
vehicle or pedestrian traffic areas. If necessary, provide for pedestrian passage and maintenance
access.
• Locate bioretention areas:
− Close to the source of runoff.
− To capture runoff from impervious areas and highly compacted pervious areas such as athletic
fields and lawns.
− To capture smaller drainage areas. If necessary, use several connected bioretention areas to
address larger areas.
The most important design factor to consider when applying
bioretention to development sites is the scale at which it
will be applied, as follows:
5.4.6 – Bioretention

• Micro-Bioretention or Rain Gardens. These are

small, distributed practices designed to treat runoff
from small areas, such as individual rooftops,
driveways and other on-lot features in single-
family detached residential developments. Inflow is
typically sheet flow, or can be concentrated flow with
energy dissipation, when located at downspouts.
• Bioretention Basins. These are structures treating
parking lots and/or commercial rooftops, usually
in commercial or institutional areas. Inflow can be
either sheetflow or concentrated flow. Bioretention
basins may also be distributed throughout a
residential subdivision, but ideally they should be
located in common areas or within drainage
easements, to treat a combination of roadway and
lot runoff.
• Urban Bioretention. These are structures such as
expanded tree pits, curb extensions, and foundation
planters located in ultra-urban developed areas
such as city streetscapes. Figures 2 & 3: roof leaders are directly
connected to the bed. Bioretention that
manages runoff from a parking lot
(Source: The SMART Center).

142 Tennessee Permanent Stormwater Management and Design Guidance Manual

1.2 Major Design Elements
Table 1: Bioretention major design elements.

Contributing Drainage Bioretention cells work best with smaller CDAs, where it is easier to
Area (CDA) achieve flow distribution over the filter bed. Typical drainage area size
for traditional Bioretention areas can range from 0.1 to 2.5 acres and
consist of up to 100% impervious cover. Drainage areas to smaller
bioretention practices (Urban Bioretention, Residential Rain Gardens)
typically range from 0.5 acre to 1.0.

Slopes Bioretention can be used for sites with a variety of topographic conditions,
but is best applied when the grade of the area immediately adjacent to the
bioretention practice (within approximately 15 to 20 feet) is greater than
1% and less than 5%. For sites with steep grades, Bioretention should be
split into multiple cells with adequate conveyance between the cells to take
advantage of relatively flat and/or areas in cut sections (rather than fill).

Soils Soil conditions do not typically constrain the use of bioretention, although
they do determine whether an underdrain is needed. Underdrains are
needed if the measured permeability of the underlying soils is less than
0.5 inches per hour. When designing Bioretention practices without
underdrains and with drainage areas greater than 0.5 acre, designers
should verify soil permeability by using the on-site soil investigation
methods provided in Appendix A of the manual (Infiltration and Soil

5.4.6 – Bioretention
Texture Testing Methods).

Depth to Water Table Bioretention should always be separated from the water table to ensure
that groundwater does not intersect the filter bed. Mixing can lead to
possible groundwater contamination or failure of the bioretention facility.
A separation distance of 2 feet is required between the bottom of the
excavated Bioretention area and the seasonally high ground water table.

Available Hydraulic Head Bioretention is fundamentally constrained by the invert elevation of

the existing conveyance system to which the practice discharges (i.e.,
the bottom elevation needed to tie the underdrain from the
Bioretention area into the storm drain system). In general, 4 to 5 feet of
elevation above this invert is needed to accommodate the required
ponding and filter media depths. For infiltration designs, the available
head is less important.

Floodplains Bioretention areas should be constructed outside the limits of the 100-
year floodplain unless it is approved by local program, but must meet
minimum distances identified in site constraints.

Utilities and Setbacks Interference with underground utilities should be avoided whenever
possible, particularly water and sewer lines. Approval from the
applicable utility company or agency is required if utility lines will run
below or through the Bioretention area. Conflicts with water and sewer
laterals (e.g., house connections) may be unavoidable, and the
construction sequence must be altered, as necessary, to avoid impacts
to existing service.
Additionally, designers should ensure that future tree canopy growth in
the bioretention area will not interfere with existing overhead utility
lines.

Tennessee Permanent Stormwater Management and Design Guidance Manual 143

 Setbacks To avoid the risk of seepage, do not allow bioretention areas to be
hydraulically connected to structure foundations or pavement. Setbacks
to structures and roads vary, based on the scale of the bioretention
design. At a minimum, bioretention basins should be located a horizontal
distance of 100 feet from any water supply well (50 feet if the bioretention
is lined), 50 feet from septic systems (25 feet if the bioretention is lined),
and at least 5 feet from down-gradient wet utility lines. Dry utility lines
such as gas, electric, cable and telephone may cross under bioretention
areas if they are double-cased.

No Irrigation or Baseflow The planned bioretention area should not receive baseflow, irrigation
water, chlorinated wash-water or other such non-stormwater flows
that are not stormwater runoff.

Hotspot Land Uses Runoff from hotspot land uses should not be treated with infiltrating
bioretention (i.e., constructed without an underdrain) unless
pretreatment has been provided. An impermeable bottom liner and an
underdrain system must be employed when bioretention is used to
receive and treat hotspot runoff.

1.3 Design Criteria

5.4.6 – Bioretention

Figure 4: Flow diagram.

1.3.1 Pre‐treatment
Pre-treatment of runoff entering bioretention areas is necessary to trap coarse sediment particles before
they reach and prematurely clog the filter bed. Pre-treatment measures must be designed to evenly
spread runoff across the entire width of the bioretention area. Several pre-treatment measures are
feasible, depending on the scale of the bioretention practice and whether it receives sheet flow, shallow
concentrated flow or deeper concentrated flows. The following are appropriate pretreatment options:
• Pre‐treatment Cells (channel flow): Similar to a forebay, this cell is located at piped inlets or
curb cuts leading to the bioretention area and consists of an energy dissipater sized for the
expected rates of discharge. It has a storage volume equivalent to at least 15% of the total
Treatment Volume (inclusive) with a 2:1 length-to-width ratio. The cell may be formed by a
wooden or stone check dam or an earthen or rock berm. Pretreatment cells do not need
underlying engineered soil media, in contrast to the main bioretention cell.
• Grass Filter Strips (sheet flow): Grass filter strips extend from the edge of pavement to the
bottom of the bioretention basin at a 5:1 slope or flatter. Alternatively, provide a combined 5
feet of grass filter strip at a maximum 5% (20:1) slope and 3:1 or flatter side slopes on the
bioretention basin (see Figure 6).
• Gravel or Stone Diaphragms (sheet flow): A gravel diaphragm located at the edge of the pavement
should be oriented perpendicular to the flow path to pre-treat lateral runoff, with a 2 to 4 inch
drop. The stone must be sized according to the expected rate of discharge. (See Figure 6)
• Gravel or Stone Flow Spreaders (concentrated flow). The gravel flow spreader is located at curb
cuts, downspouts, or other concentrated inflow points, and should have a 2 to 4 inch elevation

144 Tennessee Permanent Stormwater Management and Design Guidance Manual

 drop from a hard-edged surface into a gravel or stone diaphragm. The gravel should extend the
entire width of the opening and create a level stone weir at the bottom or treatment elevation
of the basin (see Figure 7).
• Innovative or Proprietary Structure: An approved proprietary structure with demonstrated
capability of reducing sediment and hydrocarbons may be used to provide pre-treatment.

5.4.6 – Bioretention
Figure 5: Pretreatment option – grass filter for sheet flow (Source: VADCR).

Figure 6: Pretreatment option – gravel diaphragm for sheet flow (Source: VADCR).

Tennessee Permanent Stormwater Management and Design Guidance Manual 145

5.4.6 – Bioretention

Figure 7: Pretreatment option – gravel flow spreader for concentrated flow (Source: VADCR).

1.3.2 Entrance/Flow conditions

Captured runoff may enter a bioretention area in one of three ways:
a) Through dispersed surface flow such as along a depressed curb, lawn area, or edge of pavement
as shown in Figures 8 and 9. Careful grading is essential to prevent concentrated flow points
and potential erosion. For bioretention adjacent to existing impervious pavement, such as in a
retrofit installation or modification to an existing site, it is recommended that the adjacent
pavement be milled and repaved/replaced to provide a uniform edge and dispersed sheet flow
into the bioretention area.

Figure 8: bioretention to capture sheet flow Figure 9: bioretention to capture runoff

from neighborhood in Knoxville TN from a parking lot in Powell TN
(Source: The SMART Center). (Source: The SMART Center).

146 Tennessee Permanent Stormwater Management and Design Guidance Manual

 b) Through a concentrated discharge location such as a trench drain, outlet pipe, or curb cut.
Bioretention soils and mulch are highly erosive. Entrance velocities should not exceed 1 fps
unless designed with entrance measures to prevent erosion, such as:
• Cobble splash blocks
• Small level spreaders
• Turf reinforcement materials
Supporting entrance velocity calculations are required for all concentrated surface discharges into
bioretention areas.
c) Via a direct connection (such as a pipe) into the underlying stone storage bed. This is a good
option for “clean” runoff discharging at high velocities. For example, a roof leader may be
connected directly to a stone storage bed (see Figure 10).

5.4.6 – Bioretention
Figure 10: Roof leaders can convey high-velocity flows from the roof directly
into the stone bed to prevent erosive conditions (Source: CHCRPC).

1.3.3 Filter Media and Surface Cover

The filter media and surface cover are the two most important elements of a bioretention facility in terms
of long-term performance. The following are key factors to consider in determining an acceptable soil
media mixture.
• General Filter Media Composition. The recommended bioretention soil mixture is generally
classified as a loamy sand on the USDA Texture Triangle, with the following composition:
− 85% to 88% sand;
− 8% to 12% soil fines; and
− 3% to 5% organic matter.
It may be advisable to start with an open-graded coarse sand material and proportionately mix in topsoil
that will likely contain anywhere from 30% to 50% soil fines (sandy loam, loamy sand) to achieve the
desired ratio of sand and fines. An additional 3% to 5% organic matter can then be added. (The exact
composition of organic matter and topsoil material will vary, making particle size distribution and recipe
for the total soil media mixture difficult to define in advance of evaluating the available material.)

Tennessee Permanent Stormwater Management and Design Guidance Manual 147

 • P‐Index. The P-Index provides a measure of soil phosphorus content and the risk of that
phosphorus moving through the soil media. The risk of phosphorus movement through a soil is
influenced by several soil physical properties: texture, structure, total pore space, pore-size,
pore distribution, and organic matter. A soil with a lot of fines will hold phosphorus while also
limiting the movement of water. A soil that is sandy will have a high permeability, and will
therefore be less likely to hold phosphorus within the soil matrix. A primary factor in interpreting
the desired P-Index of a soil is the bulk density. Saxton et. al. (1986) estimated generalized bulk
densities and soil-water characteristics from soil texture. The expected bulk density of the loamy
sand soil composition described above should be in the range of 1.6 to 1.7 g/cu. cm. Therefore,
the recommended range for bioretention soil P-index of between 10 and 30 corresponds to a
phosphorus content range (mg of P to kg of soil) within the soil media of 7 mg/kg to 23 mg/kg.
• Cation Exchange Capacity (CEC).The CEC of a soil refers to the total amount of positively charged
elements that a soil can hold; it is expressed in milliequivalents per 100 grams (meq/100g) of
soil. For agricultural purposes, these elements are the basic cations of calcium (Ca+2),
magnesium (Mg+2), potassium (K+1) and sodium (Na+1) and the acidic cations of hydrogen
(H+1) and aluminum (Al+3). The CEC of the soil is determined in part by the amount of clay
and/or humus or organic matter present. Soils with CECs exceeding 10 are preferred for pollutant
removal. Increasing the organic matter content of any soil will help to increase the CEC, since it
also holds cations like the clays.
• Infiltration Rate. The bioretention soil media should have a minimum infiltration rate of 0.5
inches per hour (a proper soil mix will have an initial infiltration rate that is significantly higher).
• Depth. The standard minimum filter should be 24 inches for grass cells and 36 inches for shrub
5.4.6 – Bioretention

cells, and 18 to 24 inches for rain gardens or micro-bioretention. If trees are included in the
bioretention planting plan, tree planting holes in the filter bed must be at least 4 feet deep to
provide enough soil volume for the root structure of mature trees. Use turf, perennials or shrubs
instead of trees to landscape shallower filter beds.
• Filter Media for Tree Planting Areas. A more organic filter media is recommended within the
planting holes for trees, with ratio of 50% sand, 30% top soil and 20% acceptable leaf compost.
• Mulch. A 2 to 3 inch layer of mulch on the surface of the filter bed enhances plant survival,
suppresses weed growth, and pre-treats runoff before it reaches the filter media. Shredded,
aged hardwood bark mulch makes a very good surface cover, as it retains a significant amount
of nitrogen and typically will not float away. The use of woodchips, which may “float” should
be prohibited.
• Alternative to Mulch Cover. In some situations, designers may consider alternative surface
covers such as turf, native groundcover, erosion control matting (coir or jute matting), river
stone, or pea gravel. The decision regarding the type of surface cover to use should be based
on function, cost and maintenance. Stone or gravel are not recommended in parking lot
applications, since they increase soil temperature and have low water holding capacity.
• Media for Turf Cover. One adaptation is to design the filter media primarily as a sand filter with
organic content only at the top. Leaf compost tilled into the top layers will provide organic
content for the vegetative cover. If grass is the only vegetation, the ratio of compost may be reduced.

1.3.4 Soil Infiltration Rate Testing

In order to determine if an underdrain will be needed, one must measure the infiltration rate of subsoils
at the invert elevation of the bioretention area. The infiltration rate of subsoils must exceed 0.5 inch per
hour in order to dispense with the underdrain requirement. Soil testing is not needed where an
underdrain is used.

148 Tennessee Permanent Stormwater Management and Design Guidance Manual

1.3.5 Underdrain and Underground Storage Layer
The depth of the storage layer will depend on the target treatment and storage volumes needed to meet
water quality. However, the bottom of the storage layer must be at least 2 feet above the seasonally high
water table. The storage layer should consist of clean, washed #57 stone or an approved infiltration
module.
All bioretention basins should include observation wells. The observation wells should be tied into any
T’s or Y’s in the underdrain system, and should extend upwards to be flush with the surface, with a vented
cap. In addition, cleanout pipes should be provided if the contributing drainage area exceeds 1 acre.
1.3.5.1 Internal Water Storage Zones (IWS)
An Internal Water Storage Zone (IWS) can be created by the addition of an elbow in the underdrain piping
at a 90º angle vertically perpendicular to the horizontal underdrain, either in retrofit conditions or in new
installations. This up-turned elbow on underdrains can force water to remain longer in the bottom of the
cell, creating a saturated internal water storage zone (IWS). If this zone remains saturated long enough,
anaerobic conditions are created, promoting denitrification and increased N removal (Passeport et al., 2009).
There are several benefits to using upturned elbows and IWS. The IWS works for both pollutant and peak
flow reduction as anaerobic conditions can be created to increase nitrogen removal. It also allows more
water to infiltrate into the surrounding soils. If an upturned elbow is installed correctly in sufficiently
permeable soils, it may only rarely generate outflows. The use of upturned elbows and an IWS is especially
beneficial in the areas where surrounding sandy soils can be ideal for infiltration, reducing outflows and
surface water runoff. There can be a thermal benefit to IWS use as water is pulled from the coolest zone
at the bottom of the cell. This is especially beneficial for temperature reductions in trout waters. Finally,
there is often a cost benefit for using upturned elbows, both for new installations and retrofits. In new

5.4.6 – Bioretention
installations, there is a cost-savings associated with installation since the invert of the outlet is not as
deep. Often with IWS there can be less trenching and fewer materials associated with using it. In retrofits,
upturned elbows can be cheaply added to existing Bioretention cells where increased N & P removal rates
are needed. Additionally, cells with IWS can be added as retrofits even in areas with restricted outlet depth.
In order for an internal water storage zone to work correctly, the underlying soils must have some
permeability. In general, if the underlying soils are Group A or B soils with a low clay content, then the
IWS will be effective. If soils are too compacted, water will not infiltrate and may stagnate in the lower
portion, causing problems for the BMP. Media depth above the bottom gravel and underdrain layer must
be at least 3 feet. The top of IWS should be separated from the outlet and bowl surface by at least 12,
but ideally 18 inches (see Figure 11).

Figure 11: Bioretention cell showing IWS zones (Source: NCDENR).

Tennessee Permanent Stormwater Management and Design Guidance Manual 149

 1.3.6 Overflow/Conveyance
For On‐line bioretention:
An overflow structure should always be incorporated into on-line designs to safely convey larger storms
through the bioretention area. The following criteria apply to overflow structures:
• The overflow associated with the 2 and 10 year design storms should be controlled so that velocities
are non-erosive at the outlet point (i.e., to prevent downstream erosion).
• Common overflow systems within bioretention practices consist of an inlet structure, where
the top of the structure is placed at the maximum water surface elevation of the bioretention area,
which is typically 6 to 12 inches above the surface of the filter bed (6 inches is the preferred
ponding depth).
• The overflow capture device (typically a yard inlet) should be scaled to the application – this
may be a landscape grate inlet or a commercial-type structure.
• The filter bed surface should generally be flat so the bioretention area fills up like a bathtub.
5.4.6 – Bioretention

Figure 12: Bioretention units can be designed using an overflow device so that water
in excess of the treatment volume overflows to a filter strip. This example shows
a filter strip, though it is not required for every design (Source: NCDENR).

150 Tennessee Permanent Stormwater Management and Design Guidance Manual

Off‐line bioretention:
Off-line designs are preferred (see Figure 13). One common approach is to create an alternate flow path
at the inflow point into the structure such that when the maximum ponding depth is reached, the
incoming flow is diverted past the facility. In this case, the higher flows do not pass over the filter bed
and through the facility, and additional flow is able to enter as the ponding water filtrates through the
soil media.

5.4.6 – Bioretention

Figure 13: Typical Details for Off-Line Bioretention (Source: VADCR).

Tennessee Permanent Stormwater Management and Design Guidance Manual 151

 1.3.7 Freeboard
It is recommended that bioretention areas include a minimum of 6 inches of freeboard above the overflow
route.

1.3.8 Bioretention Planting Plans

A landscaping plan must be provided for each bioretention area. Minimum plan elements shall include
the proposed bioretention template to be used, delineation of planting areas, the planting plan, including
the size, the list of planting stock, sources of plant species, and the planting sequence, including post-
nursery care and initial maintenance requirements. It is highly recommended that the planting plan be
prepared by a qualified landscape architect, in order to tailor the planting plan to the site-specific conditions.
Tennessee native plant species are preferred over non-native species, but some ornamental species may
be used for landscaping effect if they are not aggressive or invasive. Some popular native species that
work well in bioretention areas and are commercially available can be found in Appendix D. The six most
common bioretention templates are as follows:
• Turf. This option is typically restricted to on-lot micro-bioretention applications, such as a front
yard rain garden. Grass species should be selected that have dense cover, are relatively slow
growing, and require the least mowing and chemical inputs (e.g., fine fescue, tall fescue).
• Perennial garden. This option uses herbaceous plants and native grasses to create a garden
effect with seasonal cover. It may be employed in both micro-scale and small scale bioretention
applications. This option is attractive, but it requires more maintenance in the form of weeding.
• Perennial garden with shrubs. This option provides greater vertical form by mixing native shrubs
and perennials together in the bioretention area. This option is frequently used when the filter
5.4.6 – Bioretention

bed is too shallow to support tree roots. Shrubs should have a minimum height of 30 inches.
• Tree, shrub and herbaceous plants. This is the traditional landscaping option for bioretention.
It produces the most natural effect, and it is highly recommended for bioretention basin
applications. The landscape goal is to simulate the structure and function of a native forest plant
community.
• Turf and tree. This option is a lower maintenance version of the tree-shrub-herbaceous option
where the mulch layer is replaced by turf cover. Trees are planted within larger mulched islands
to prevent damage during mowing operations.
• Herbaceous meadow. This is another lower maintenance approach that focuses on the herbaceous
layer and may resemble a wildflower with Joe Pye Weed, Ironweed, sedges, grasses, etc.). The
goal is to establish a more natural look that may be appropriate if the facility is located in a
lower maintenance area (e.g., further from buildings and parking lots). Shrubs and trees may
be incorporated around the perimeter. Erosion control matting can be used in lieu of the
conventional mulch layer.

152 Tennessee Permanent Stormwater Management and Design Guidance Manual

1.3.9 Material Specifications
Table 2: Bioretention material specifications.

Material Specification Notes

Filter Media Filter media to contain: The volume of filter media based on 110% of
Composition • 85%-88% sand the plan volume, to account for settling or
• 8%-12% soil fines compaction.
• 3%-5% organic matter in the form
of leaf compost
Filter Media P-Index range = 10-30, OR between 7 The media must be procured from approved
Testing and 21 mg/kg of P in the soil media. filter media vendors.
CECs greater than 10

Mulch Layer Use aged, shredded hardwood bark Lay a 2 to 3 inch layer on the surface of the
mulch filter bed.
Alternative Use river stone or pea gravel, coir and Lay a 2 to 3 inch layer of cover to suppress
Surface Cover jute matting, or turf cover. weed growth.
Top Soil for Loamy sand or sandy loam texture, with 3 inch surface depth.
Turf Cover less than 5% clay content, pH corrected
to between 6 and 7, and an organic
matter content of at least 2%

5.4.6 – Bioretention
Geotextile/Liner Use a non-woven geotextile fabric with a Apply only to the sides and above the underdrain.
flow rate of > 110 gal./min./sq. ft. For hotspots and certain karst sites only, use an
appropriate liner on bottom.
Choking Layer Lay a 2 to 4 inch layer of sand over a 2 inch layer of choker stone (typically #8 or #89
washed gravel), which is laid over the underdrain stone

Stone Jacket for 1 inch stone should be double-washed 12 inches for the underdrain; 12 to 18 inches
Underdrain and/or and clean and free of all fines (e.g., TDOT for the stone storage layer, if needed
Storage Layer #57 stone).
Underdrains, Use 6 inch rigid schedule 40 PVC pipe (or Lay the perforated pipe under the length of the
Cleanouts, and equivalent corrugated HDPE for micro- bioretention cell, and install non-perforated pipe
Observation bioretention), with 3/8-inch perforations as needed to connect with the storm drain system.
Wells at 6 inches on center; position each Install T’s and Y’s as needed, depending on the
underdrain on a 1% or 2% slope located underdrain configuration. Extend cleanout pipes
no more than 20 feet from the next pipe. to the surface with vented caps at the Ts and Ys.
Use elbow outlet pipe to provide Internal
Water Storage (IWS).

Plant Materials • Plant one tree per 250 square feet (15 Establish plant materials as specified in the
feet on-center, minimum 1 inch caliper). landscaping plan and the recommended plant
• Shrubs a minimum of 30 inches high list. In general, plant spacing must be sufficient
planted a minimum of 10 feet on- to ensure the plant material achieves 80% cover
center. in the proposed planting areas within a 3-year
• Plant ground cover plugs at 12 to 18 period. If seed mixes are used, they should be
inches on-center; Plant container- from a qualified supplier, should be appropriate
grown plants at 18 to 24 inches on- for stormwater basin applications, and should
center, depending on the initial plant consist of native species (unless the seeding is
size and how large it will grow. to establish maintained turf).

Tennessee Permanent Stormwater Management and Design Guidance Manual 153

 1.3.10 Overview
Table 3: Bioretention design criteria.

Micro‐bioretention (rain garden). Max Bioretention filter & basin area. Max CDA:
CDA: 0.5 acres, 25% impervious cover. 2.5 acres.

Sizing Filter surface area (sq. ft.) = 3% of the Surface Area (sq. ft.) = Total runoff volume – the
contributing drainage area (CDA). volume reduced by an upstream SCM(s)/Storage
Depth1
Maximum 6 inches 6 - 12 inches 2
Ponding Depth

Filter Media Min: 18 inches; max: 24 inches Min: 24 inches for grass; 36 inches for shrubs;
Depth max: 6 feet

Media / surface Mixed on-site or supplied by vendor. Media mix tested for an acceptable phosphorus index
cover (P-Index) of between 10 and 30, OR Between 7 and 21 mg/kg of P in the soil media

Sub‐soil testing Not needed if an underdrain is used; Not needed if an underdrain used; Min
Min infiltration rate > 0.5 inch/hour infiltration rate ≥ 2 inch/hour in order
in order to remove the underdrain to remove the underdrain requirement.
requirement.
5.4.6 – Bioretention

Underdrain Corrugated HDPE or equivalent Schedule 40 PVC with clean-outs

Clean‐outs Not needed Needed
Inflow Sheet flow or roof leader Sheet flow, curb cuts, trench drains, or
concentrated flow.

Geometry Length of shortest flow path/Overall length = 0.3

Pretreatment External (leaf screens, grass filter strip, A pretreatment cell, grass filter strip, gravel
energy dissipater, etc.) diaphragm, gravel flow spreader, or another
approved (manufactured) pre-treatment
structure.
Building 10 feet down-gradient; 25 feet up- 0 to 0.5 acre CDA = 10 feet if down-gradient
setbacks 3 gradient from building from building; 50 feet if up-gradient. 0.5 to 2.5
acre CDA = 25 feet if down-gradient from
building; 100 feet if up-gradient.
Planting Plan A planting template to include turf, herbaceous
vegetation, shrubs, and/or trees to achieve
surface area coverage of at least 80% within
3 years.

1. Storage depth is the sum of the Void Ratio (Vr) of the soil media and gravel layers multiplied by their respective depths,
plus the surface ponding depth. .
2. A ponding depth of 6 inches is preferred. Ponding depths greater than 6 inches will require a specific planting plan to
ensure appropriate plant selection.
3. These are recommendations for simple building foundations. If an in-ground basement or other special conditions exist,
the design should be reviewed by a licensed engineer. Also, a special footing or drainage design may be used to justify
a reduction of the setbacks noted above.

154 Tennessee Permanent Stormwater Management and Design Guidance Manual

 6”-12” MAX. PONDING DEPTH

Vr = 0.25

Vr = 0.4

Figure 14: Typical bioretention section with void ratios for volume computations
(Source: CHCRPC).

1.4 Calculations
1.4.1 Practice Dimensions using TNRRAT
Sizing the practice dimension can be done using the Tennessee Stormwater Runoff Reduction Assessment
Tool (TNRRAT). The tool allows users to iteratively size their SCM(s) to meet the goal of 1-inch runoff

5.4.6 – Bioretention
reduction and 80% pollutant removal. The inputs needed for the tool are:
− Location
− Area per management types and surface management types (such as impervious, bare soil,
good forest, bioretention, etc)
− Soil texture
− Depth surface to restrictive layer
− Type of layer materials
− Depth of layer materials

1.4.2 Practice Dimensions using other method

Although using the TNRRAT is recommended, designer and engineers are welcomed to use other methods
to size bioretention and other SCMs.
1.4.2.1Runoff Volume
Use standard engineering methods to calculate runoff volume.
1.4.2.2 Surface Area
The size and surface area of a bioretention system may
be a function of the drainage area that will discharge to Inputs needed for calculation:
the bioretention system. It is important not to concentrate
too much flow in one location. A basic rule-of-thumb is − Location
to design a bioretention system with a surface area that − Size of CDA
is a ratio of the impervious and compacted pervious areas − Cover type
draining to it. A 1:10 ratio of surface area to impervious − Approximation of practice
drainage area base on design rainfall depth can be used surface area and practice depth
to estimate a bioretention area. − Void ratio values

Tennessee Permanent Stormwater Management and Design Guidance Manual 155

 1.4.2.3 Design Depth
With an estimate of the required bioretention area and runoff volume, the designer can estimate the
depth of water, soil, and stone storage.
The recommended depths for surface water storage, soil storage, and stone storage are:
• Surface Water Storage Depth:
− 6 inches maximum in high-use areas (along streets, at schools, in public landscapes, etc.)
− 12 inches in less used areas (away from frequent public access)
• Bioretention Soil Depth: Between 12 and 36 inches
• Gravel storage Depth: Between 12 and 36 inches
Void ratios (Vr) are generally:
• 0.20 for bioretention soils
• 0.40 for clean-washed aggregate such as AASHTO No.3
• 0.85 to 0.95 for manufactured storage units depending on manufacturer
Storage depth:
[Soil depth x Soil media Vr] + [Gravel depth x Gravel Vr] + [Surface storage depth x surface storage]
(Equation 1)
Example: (2 ft. x 0.25) + (1 ft. x 0.40) + (0.5 x 1.0) = 1.40 ft.

1.4.2.4 Practice Volume

Storage Volume (ft3) = Surface Water Volume + Soil Storage Volume + Stone Storage Volume
5.4.6 – Bioretention

(Equation 2)
Where:
− Surface Water Volume: Available surface water storage between soil
surface and overflow structure (always equal to or less than 12 inches). Storage volume has
The designer should consider the bed side slopes when estimating to be ≥ runoff
volume. Surface water volume = Surface water area (ft ) x Surface
2 volume
water depth (ft) x Void Ratio
− Soil Storage Volume (ft3) = Soil Area (ft2) x Soil depth (ft) below overflow x Void ratio
− Gravel Storage Volume (ft3) = Gravel area (ft2) x Gravel Depth (ft) below overflow x Void Ratio
1.4.3 Design Geometry
Bioretention basins must be designed with internal flow path geometry such that the treatment
mechanisms provided by the bioretention are not bypassed or short-circuited. Examples of short-circuiting
include inlets or curb cuts that are very close to outlet structures (see Figure 15) or incoming flow that is
diverted immediately to the underdrain through stone layers. Short-circuiting can be particularly
problematic when there are multiple curb cuts or inlets.

Figure 15: Short circuiting in bioretention (Source: VADCR).

156 Tennessee Permanent Stormwater Management and Design Guidance Manual

In order for these bioretention areas to have an acceptable internal geometry, the “travel time” from
each inlet to the outlet should be maximized, and incoming flow must be distributed as evenly as possible
across the filter surface area. One important characteristic is the length of the shortest flow path
compared to the overall length, as shown in Figures 16 and 17. In this figure, the ratio of the shortest
flow path to the overall length is represented as:
Ratio of Shortest Flow Path to Overall Length:
SFP / L
Where:
SFP = length of the shortest flow path
L = length from the most distant inlet to the outlet

5.4.6 – Bioretention
Figure 16: Flow path ratio.

Figure 17: Typical Detail of how to prevent bypass or short-circuiting around

the overflow structure (Source: VADCR).

Tennessee Permanent Stormwater Management and Design Guidance Manual 157

 1.5 Typical Details

Figure 18: Typical section with underdrain (Source: VADCR).

5.4.6 – Bioretention

Figure 19: Typical section with underdrain (Source: VADCR).

158 Tennessee Permanent Stormwater Management and Design Guidance Manual

 BIORETENTION OR
OTHER TREATMENT
PRACTICE

INFILTRATION AREA

Figure 20: Residential Rooftop Treatment – Plan View (Source: WVDEP).

5.4.6 – Bioretention

Figure 21: Residential Rooftop Disconnection to downstream raingarden –

Bioretention without an underdrain (Source: VADCR).

Tennessee Permanent Stormwater Management and Design Guidance Manual 159

 Figure 22: Bioretention with an underdrain (Source: VADCR).
5.4.6 – Bioretention

160 Tennessee Permanent Stormwater Management and Design Guidance Manual

 5.4.6 – Bioretention

Figure 23: Typical Detail of bioretention with additional surface ponding (Source: VADCR).

Tennessee Permanent Stormwater Management and Design Guidance Manual 161

5.4.6 – Bioretention

Figure 24: Typical Detail of bioretention with the upper shelf of an extended
detention storage (Source: VADCR).

162 Tennessee Permanent Stormwater Management and Design Guidance Manual

1.6 Variation – Bioretention Cells on Steep Slopes

Figure 25: Bioretention suitable for use on slopes 10-20% (Source: NCDENR).

Figure 25 depicts a bioretention terrace that can be used in sloping terrain (for 10-20% slopes). An impermeable
or very low permeability geomembrane must be used against the gabions or similar retaining structure to
prevent flow from leaving the treatment unit through that surface. An underdrain could be placed at the low
point of the filter if the native soil that the unit is built against will not provide adequate infiltration capacity.

n
2. Construction
The following is a typical construction sequence to properly install a bioretention, although steps may be

5.4.6 – Bioretention
modified to reflect different site conditions. Grass channels should be installed at a time of year that is
best to establish turf cover without irrigation.
2.1 Pre‐Construction
Micro-bioretention and small-scale bioretention areas should be fully protected by silt fence or construction
fencing, particularly if they will rely on infiltration (i.e., have no underdrains). Ideally, bioretention should
remain outside the limit of disturbance during construction to prevent soil compaction by heavy equipment.
Bioretention basin locations may be used as small sediment traps or basins during construction. However,
these must be accompanied by notes and graphic details on the EPSC plan specifying that (1) the maximum
excavation depth at the construction stage must be at least 1 foot above the post-construction installation,
and (2) the facility must contain an underdrain. The plan must also show the proper procedures for converting
the temporary sediment control practice to a permanent bioretention facility, including dewatering, cleanout
and stabilization.
2.2 Construction
The following is a typical construction sequence to properly install a bioretention measure. The construction
sequence for micro-bioretention is more simplified. These steps may be modified to reflect different
bioretention applications or expected site conditions:
• Step 1. Construction of the bioretention area may only begin after the entire contributing drainage
area has been stabilized with vegetation. It may be necessary to block certain curb or other
inlets while the bioretention area is being constructed. The proposed site should be checked
for existing utilities prior to any excavation.
• Step 2. The designer and the installer should have a preconstruction meeting, checking the
boundaries of the contributing drainage area and the actual inlet elevations to ensure they
conform to original design. Since other contractors may be responsible for constructing portions
of the site, it is quite common to find subtle differences in site grading, drainage and paving
elevations that can produce hydraulically important differences for the proposed bioretention
area. The designer should clearly communicate, in writing, any project changes determined during
the preconstruction meeting to the installer and the plan review/inspection authority.

Tennessee Permanent Stormwater Management and Design Guidance Manual 163

 • Step 3. Temporary E&S controls are needed during construction of the bioretention area to
divert stormwater away from the bioretention area until it is completed. Special protection
measures such as erosion control fabrics may be needed to protect vulnerable side slopes from
erosion during the construction process.
• Step 4. Any pre-treatment cells should be excavated first and then sealed to trap sediments.
• Step 5. Excavators or backhoes should work from the sides to excavate the bioretention area to
its appropriate design depth and dimensions. Excavating equipment should have scoops with
adequate reach so they do not have to sit inside the footprint of the bioretention area.
Contractors should use a cell construction approach in larger bioretention basins, whereby the
basin is split into 500 to 1,000 sq. ft. temporary cells with a 10-15 foot earth bridge in between,
so that cells can be excavated from the side.
• Step 6. It may be necessary to rip the bottom soils to a depth of 8 to 12 inches to promote greater
infiltration.
• Step 7. Place geotextile fabric on the sides of the bioretention area with a 6-inch overlap on the
sides. If a stone storage layer will be used, place the appropriate depth of #57 stone on the
bottom, install the perforated underdrain pipe, pack #57 stone to 3 inches above the underdrain
pipe, and add approximately 3 inches of choker stone/pea gravel as a filter between the
underdrain and the soil media layer. If no stone storage layer is used, start with 6 inches of #57
stone on the bottom, and proceed with the layering as described above. If IWS is desired, use
6” solid pipe for the up-turned elbow.
• Step 8. Deliver the soil media from an approved vendor, and store it on an adjacent impervious
area or plastic sheeting. Apply the media in 12-inch lifts until the desired top elevation of the
5.4.6 – Bioretention

bioretention area is achieved. Wait a few days to check for settlement, and add additional media,
as needed, to achieve the design elevation.
• Step 9. Prepare planting holes for any trees and shrubs, install the vegetation, and water
accordingly. Install any temporary irrigation.
• Step 10. Place the surface cover in both cells (mulch, river stone or turf), depending on the
design. If coir or jute matting will be used in lieu of mulch, the matting will need to be installed
prior to planting (Step 9), and holes or slits will have to be cut in the matting to install the plants.
• Step 11. Install the plant materials as shown in the landscaping plan, and water them during
weeks of no rain for the first two months.
• Step 12. Conduct the final construction inspection. Then log the GPS coordinates for each
bioretention facility and submit them for entry into the local maintenance tracking database.

164 Tennessee Permanent Stormwater Management and Design Guidance Manual

 Figure 26: Typical bioretention construction sequence (Source:VADCR).

5.4.6 – Bioretention
n
3. Maintenance
3.1 Agreements
Examples of maintenance documents can be found in Appendix F. They may include the execution and
recording of an Inspection and Maintenance Agreement or a Declaration of Restrictions and Covenants,
and the development of a Long Term Maintenance Plan (LTMP) by the design engineer. The LTMP contains
a description of the stormwater system components and information on the required inspection and
maintenance activities.
3.2 First Year Maintenance Operations
Successful establishment of bioretention areas requires that the following tasks be undertaken in the
first year following installation:
• Initial inspections. For the first 6 months following construction, the site should be inspected
at least twice after storm events that exceed 0.5 inch of rainfall.
• Spot reseeding. Inspectors should look for bare or eroding areas in the contributing drainage
area or around the bioretention area, and make sure they are immediately stabilized with grass
cover.
• Fertilization. One-time, spot fertilization may be needed for initial plantings.
• Watering. Depending on rainfall, watering may be necessary once a week during the first 2 months,
and then as needed during first growing season (April-October), depending on rainfall.
• Remove and replace dead plants. Since up to 10% of the plant stock may die off in the first year,
construction contracts should include a care and replacement warranty to ensure that vegetation
is properly established and survives during the first growing season following construction. The
typical thresholds below which replacement is required are 85% survival of plant material and
100% survival of trees.

Tennessee Permanent Stormwater Management and Design Guidance Manual 165

 3.3 Maintenance Inspections
It is highly recommended that a spring maintenance inspection and cleanup be conducted at each
bioretention area. The following is a list of some of the key maintenance problems to look for:
• Check to see if 75% to 90% cover (mulch plus vegetative cover) has been achieved in the bed,
and measure the depth of the remaining mulch.
• Check for sediment buildup at curb cuts, gravel diaphragms or pavement edges that prevents
flow from getting into the bed, and check for other signs of bypassing.
• Check for any winter- or salt-killed vegetation, and replace it with hardier species.
• Note presence of accumulated sand, sediment and trash in the pre-treatment cell or filter beds,
and remove it.
• Inspect bioretention side slopes and grass filter strips for evidence of any rill or gully erosion,
and repair it.
• Check the bioretention bed for evidence of mulch flotation, excessive ponding, dead plants or
concentrated flows, and take appropriate remedial action.
• Check inflow points for clogging, and remove any sediment.
• Look for any bare soil or sediment sources in the contributing drainage area, and stabilize them
immediately.
• Check for clogged or slow-draining soil media, a crust formed on the top layer, inappropriate soil
media, or other causes of insufficient filtering time, and restore proper filtration characteristics.
3.4 Routine and Non‐Routine Maintenance Tasks
Maintenance of bioretention areas should be integrated into routine landscape maintenance tasks. If
5.4.6 – Bioretention

landscaping contractors will be expected to perform maintenance, their contracts should contain specifics
on unique bioretention landscaping needs, such as maintaining elevation differences needed for ponding,
proper mulching, sediment and trash removal, and limited use of fertilizers and pesticides. A customized
maintenance schedule must be prepared for each bioretention facility, since the maintenance tasks will
differ depending on the scale of bioretention, the landscaping template chosen, and the type of surface
cover. A generalized summary of common maintenance tasks and their frequency is provided in Table 4.
The most common non-routine maintenance problem involves standing water. If water remains on the
surface for more than 48 hours after a storm, adjustments to the grading may be needed or underdrain
repairs may be needed. The surface of the filter bed should also be checked for accumulated sediment
or a fine crust that builds up after the first several storm events. There are several methods that can be
used to rehabilitate the filter (try the easiest things first, as listed below):
• Open the underdrain observation well or cleanout and pour in water to verify that the underdrains
are functioning and not clogged or otherwise in need of repair. The purpose of this check is to
see if there is standing water all the way down through the soil. If there is standing water on
top, but not in the underdrain, then there is a clogged soil layer. If the underdrain and stand
pipe indicates standing water, then the underdrain must be clogged and will need to be snaked.
• Remove accumulated sediment and till 2 to 3 inches of sand into the upper 8 to 12 inches of soil.
• Install sand wicks from 3 inches below the surface to the underdrain layer. This reduces the
average concentration of fines in the media bed and promotes quicker drawdown times. Sand
wicks can be installed by excavating or augering (using a tree auger or similar tool) down to the
gravel storage zone to create vertical columns which are then filled with a clean open-graded
coarse sand material (ASTM C-33 concrete sand or similar approved sand mix for bioretention
media). A sufficient number of wick drains of sufficient dimension should be installed to meet
the design dewatering time for the facility.
• Remove and replace some or all of the soil media.

166 Tennessee Permanent Stormwater Management and Design Guidance Manual

 Table 4: Suggested Annual Maintenance Activities for Bioretention.

Maintenance Tasks Frequency

• Mowing of grass filter strips and bioretention turf cover At least 4 times a year

• Spot weeding, erosion repair, trash removal, and mulch raking Twice during growing season

• Add reinforcement planting to maintain the desired vegetation density As needed

• Remove invasive plants using recommended control methods
• Stabilize the contributing drainage area to prevent erosion

• Spring inspection and cleanup Annually

• Supplement mulch to maintain a 3 inch layer
• Prune trees and shrubs

• Remove sediment in pre-treatment cells and inflow points Once every 2 to 3 years

• Replace the mulch layer Every 3 years

REFERENCES

5.4.6 – Bioretention
Chattanooga Hamilton County Regional Planning Commission (CHCRPC). “Bioretention.” Rainwater
Management Guide. Chattanooga: 2012.
Metro Water Service, Metropolitan Government of Nashville and Davidson County. “Bioretention.”
Volume 5 Low Impact Development Stormwater Management Manual. 2012.
North Carolina Department of Environmental and Natural Resources (NCDENR). “12. Bioretention.”
NCDENR Stormwater BMP Manual. 2012.
Passeport, Elodie et al. “Field Study of the Ability of Two Grassed Bioretention Cells to Reduce
Storm-Water Runoff Pollution.” Journal of Irrigation and Drainage Engineering 135.4 (2009): 505–
510. Web. 19 Dec. 2014.
Saxton, K. E. et al. “Estimating Generalized Soil-Water Characteristics from Texture1.” Soil Science
Society of America Journal 50.4 (1986): 1031.
VADCR. “Virginia DEQ Stormwater Specification No. 9: Bioretention.” Virginia Stormwater
Management Handbook. 1.9 ed. 2011.
West Virginia Department of Environmental Protection (WVDEP). “Bioretention.” West Virginia
Stormwater Management and Design Guidance Manual. 2012.

Tennessee Permanent Stormwater Management and Design Guidance Manual 167

5.4.6 – Bioretention

168 Tennessee Permanent Stormwater Management and Design Guidance Manual