VIRGINIA DCR STORMWATER
DESIGN SPECIFICATION No. 9 

BIORETENTION

VERSION 1.6
September 30, 2009

SECTION 1: DESCRIPTION

Individual bioretention areas can serve highly impervious drainage areas less than two (2) acres in size. Surface runoff is directed into a shallow landscaped depression that incorporates many of the pollutant removal mechanisms that operate in forested ecosystems. The primary component of a bioretention practice is the filter bed, which has a mixture of sand, soil, and organic material as the filtering media with a surface mulch layer. During storms, runoff temporarily ponds 6 to 12 inches above the mulch layer and then rapidly filters through the bed. Normally, the filtered runoff is collected in an underdrain and returned to the storm drain system. The underdrain consists of a perforated pipe in a gravel layer installed along the bottom of the filter bed. A bioretention facility with an underdrain system is commonly referred to as a Bioretention Filter.

Bioretention can also be designed to infiltrate runoff into native soils. This can be done at sites with permeable soils, a low groundwater table, and a low risk of groundwater contamination. This design features the use of a “partial exfiltration” system that promotes greater groundwater recharge.  Underdrains are only installed beneath a portion of the filter bed, above a stone “sump” layer, or eliminated altogether, thereby increasing stormwater infiltration. A bioretention facility without an underdrain system, or with a storage sump in the bottom is commonly referred to as a Bioretention Basin.

Small scale or Micro-Bioretention used on an individual residential lot is commonly referred to as a “Rain Garden.”.

Bioretention creates a good environment for runoff reduction, filtration, biological uptake, and microbial activity, and provides high pollutant removal. Bioretention can become an attractive landscaping feature with high amenity value and community acceptance. The overall stormwater functions of the bioretention are summarized in Table 1.

SECTION 2:  LEVEL 1 AND 2 DESIGN TABLES

The most important design factor to consider when applying bioretention to development sites is the scale at which it will be applied:  micro-bioretention or bioretention basins:

• Micro-Bioretnetion or Rain Gardens: small, distributed practices designed to treat runoff from small areas, such as individual rooftops, driveways and other on-lot features in single-family detatched residential developments. Inflow is typically sheet flow, or can be concentrated flow with energy dissipation when located at downspouts
• Bioretention Basin: structures treating parking lots and/or commercial rooftops, usually in commercial or institutional areas.  Inflow can be a sheetflow or as concentrated flow.  Bioretention basins may also be distributed throughout a residential subdivision, but located in common area or within drainage easements to treat a combination of roadway and lot runoff.
• Urban Bioretention: structures such as expanded tree pits, curb extensions, and foundation planters located in ultra-urban developed areas such as city streetscapes. Please refer to Bioretention Appendix A for design details for Urban Bioretention.



The major design goal for bioretention is to maximize nutrient removal and runoff reduction. To this end, designers may choose to go with the baseline design (Level 1) or choose an enhanced Level 2 design that maximizes nutrient and runoff reduction. If soil conditions require an underdrain, bioretention areas can still qualify for the Level 2 design if they contain a stone storage layer underneath the invert of the underdrain. 

Both stormwater quality and quantity credit are accounted for in the Runoff Reduction Method (RRM) design spreadsheet. The quality credit represents an annual load reduction as a combination of the annual reduction of runoff volume (40%, 80%, Level 1 and Level 2 respectively) and the reduction in the pollutant event mean concentration (EMC) (25% and 50%, Level 1 & 2).

For computing the quantity reduction for larger storm events, the designer can similarly use the RRM Design Spreadsheet, or as an option, the designer may choose to compute the adjusted curve number associated with the retention storage using the TR-55 Runoff Equations as noted in Table 1. The adjusted curve number is then used to compute the peak discharge for the required design storms.

Tables 2 and 3 outline the Level 1 and 2 design guidelines for the two scales of bioretention design.  

Table 2: Micro-Bioretention (Rain Garden) Design Criteria1
Level 1 Design (RR 40 TP: 25 )	Level 2 Design (RR: 80 TP:  50)
Filter surface area (ft2) = 3%2 of the contributing drainage area (CDA)	Filter surface area (ft2) = 4%2 CDA (can be divided into different cells at downspouts)
Maximum drainage area = 0.5 acres; 25% Impervious Cover (IC)2
One cell design (can be divided into smaller cells at downspout locations) 2
Maximum Ponding Depth = 6 inches
Filter media depth minimum = 18 inches; Recommended maximum = 36 inches	Filter media depth minimum = 24 inches; Recommended maximum = 36 inches
Media = mixed on-site or supplied by vendor	Media = supplied by vendor
All Designs: Media mix tested for an acceptable phosphorus index: P-Index between 10 and 30, OR Between 7 and 21 mg/kg of P in the soil media
Sub-soil testing = not needed if underdrain used; Min infiltration rate > 1.0 inch/hour to remove underdrain requirement	Sub-soil testing = one per practice; Min infiltration rate > 0.5 inches/hour; Min > 1.0 inch/hour to remove underdrain requirement
Underdrain = corrugated HDPE or equivalent	Underdrain = corrugated HDPE or equivalent with minimum 6” stone sump below invert; OR none if soil infiltration requirements are met
Clean-outs = not needed
Inflow = sheetflow or roof leader
Pretreatment = external (leaf screens, grass filter strip, energy dissipater, etc.)	Pretreatment = external + grass strip
Vegetation = turf, herbaceous, or shrubs (min = 1 out of 3)	Vegetation = turf, herbaceous, shrubs, trees (min = 2 out of 4)
Building setbacks = 10’ down-gradient; 25’ up-gradient
1 Please consult Appendix A for further details on Urban Bioretention Practices. 2 Micro Bioretention (Rain Gardens) can be located at individual downspout locations to treat up to 1,000 ft2 of impervious cover (100% IC); and the surface area sized as 5% of the roof area (Level 1) and 6% of the roof area (Level 2), with the remaining Level 1 and Level 2 design criteria remaining as provided in Table 2. If the Rain Garden is located so as to capture multiple rooftops, driveways, and adjacent pervious areas, the sizing rules within Table 2 should apply.

 

Table 3: Bioretention Basin Design Criteria
Level 1 Design (RR 40 TP: 25 )	Level 2 Design (RR: 80 TP:  50)
Sizing (Sec. 5.1): Surface Area (ft2) = (Tv– volume reduced by upstream BMP) /Storage Depth1	Sizing (Sec. 5.1): Surface Area (ft2) = {(1.25)(Tv) – volume reduced by upstream BMP }/Storage Depth1
Maximum Drainage Area = 2.5 acres
Maximum Ponding Depth = 6 to 12 inches2	Maximum Ponding Depth = 6 to 12 inches2
Filter media depth minimum = 24 inches; recommended maximum = 6 feet		Filter media depth minimum = 36 inches; recommended maximum = 6 feet
Media & Surface Cover (Sec. 5.6) = supplied by vendor; tested for acceptable phosphorus index: P-Index between 10 and 30, OR Between 7 and 21 mg/kg of P in the soil media
Sub-soil testing (Sec. 5.2): not needed if underdrain used; Min infiltration rate > 0.5 inch/hour to remove underdrain requirement;	Sub-soil testing (Sec. 5.2): one per 1,000 sf of filter surface; Min infiltration rate > 0.5 inch/hour to remove underdrain requirement
Underdrain (Sec. 5.7) = Schedule 40 PVC with clean-outs	Underdrain & Underground Storage Layer (Sec. 5.7) = Schedule 40 PVC with clean outs, and a minimum 12” stone sump below invert OR none if soil infiltration requirements are met (Sec. 5.2)
Inflow = sheetflow, curb cuts, trench drains, concentrated flow, or equivalent
Geometry (Sec. 5.3): Length of shortest flow path/Overall length = 0.3 OR other design methods to prevent short-circuiting One cell design (not including pretreatment cell)	Geometry (Sec. 5.3): Length of shortest flow path/Overall length = 0.8 OR  other design methods to prevent short-circuiting Two cell design (not including pretreatment cell)
Pretreatment (Sec. 5.4): = pretreatment cell, grass filter strip, gravel/stone diaphragm, gravel/stone flow spreader, or other approved (manufactured) pretreatment structure	Pretreatment (Sec. 5.4) = pretreatment cell + one of the following: grass filter strip, gravel/stone diaphragm, gravel/stone flow spreader, or other approved (manufactured) pretreatment structure
Conveyance & overflow (Sec. 5.5)	Conveyance & overflow (Sec. 5.5)
Planting Plan (Sec. 5.8) = planting template to include turf, herbaceous, shrubs, and/or trees to achieve surface area coverage of at least 75% within 2 years	Planting Plan (Sec. 5.8) = planting template to include turf, herbaceous, shrubs, and/or trees to achieve surface area coverage of at least 90% within 2 years.  If using turf, must combine with other types of vegetation1.
Building setbacks (Sec. 4): 0 to 0.5 Ac  CDA = 10’ down-gradient; 50’ up-gradient 0.5 to 2.5 Ac CDA = 25’ down-gradient; 100’ up-gradient
Deeded maintenance O&M plan (Sec. 7)
1 Storage depth is the sum of the Void Ratio (Vr) of the soil media and gravel layers times their respective depths, plus the surface ponding depth. Refer to Section 5.1 2 Ponding depth of 6 inches is preferred. Ponding depths greater than 6 inches will require a specific planting plan to ensure appropriate plant selection (Bioretention Section 5.8).




SECTION 3: TYPICAL DETAILS

Figures 1 through 4 provide some typical details for several bioretention configurations.



Figure 1: Typical Detail - Micro-Bioretention or Raingardens





Figure 2: Typical Detail – Bioretention Basin Level 1 & Level 2



Figure 3: Typical Detail - Bioretention with Additional Surface Ponding



Figure 4: Typical Detail - Bioretention Basin within the Upper Shelf of ED Pond

SECTION 4:  PHYSICAL FEASIBILITY & DESIGN APPLICATIONS

Bioretention can be applied in most soils or topography since runoff simply percolates through an engineered soil bed and is returned to the stormwater system. Key constraints with bioretention include the following:

• Available Space: Planners and designers can assess the feasibility for utilizing bioretention facilities based on a simple relationship between the contributing drainage area and the corresponding required surface area. The bioretention surface areas will be approximately 3% to 6% of the contributing drainage area depending on imperviousness and the desired bioretention level.
• Site Topography: Bioretention is best applied when the grade of contributing slopes is greater than 1% and less than 5%.
• Available 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 create the hydraulic head needed to drive stormwater through a proposed bioretention filter bed. Less hydraulic head is needed if the underlying soils are permeable enough to dispense with the underdrain.
• Water Table: Bioretention should always be separated from the water table to ensure groundwater does not intersect with the filter bed. Mixing can lead to possible groundwater contamination or practice failure. A separation distance of 2 feet is recommended between the bottom of the excavated bioretention area and the seasonally high ground water table.
• Utilities: Designers should ensure that future tree canopy growth in the bioretention area will not interfere with existing overhead utility lines. Interference with underground utilities should also be avoided, particularly water and sewer lines. Local utility design guidance should be consulted in order to determine the horizontal and vertical clearance required between stormwater infrastructure and other dry and wet utility lines
• Soils: Soil conditions do not constrain the use of bioretention although they determine whether an underdrain is needed. Impermeable soils in Hydrologic Soil Group (HSG) B, C or D usually require an underdrain, whereas HSG A soils generally do not. Designers should verify soil permeability when designing a bioretention practice by using the on-site soil investigation methods provided in Appendix A of Infiltration Design Specification No. 8.
• Contributing Drainage Area:  Bioretention cells work best with smaller drainage areas, where it is easier to achieve flow distribution over the filter bed. Typical drainage area size can range from 0.1 to 2.5 acres and consist of up to 100% impervious cover. Three scales of bioretention are defined in this specification: (1) micro-bioretention or Rain Gardens (up to 0.5 acre contributing drainage area) (2) bioretention basins (up to 2.5 acres of contributing drainage area), and (3) Urban Bioretention (Appendix A) . Each of these has different design requirements (Refer to Tables 2 & 3).  The maximum recommended drainage area to a single bioretention cell is five acres due to limitations on the ability of bioretention to effectively manage large volumes and peak rates of runoff. Ideally, the bioretention facility should be located within the drainage area so as to capture the treatment volume equally from the entire contributing area, and not fill the entire volume from the immediately adjacent area, thereby bypassing the runoff from the more remote portions of the site.
• Hotspot Land Uses:Runoff from hotspot land uses should not be treated with infiltrating bioretention (i.e., without an underdrain). For a list of potential stormwater hotspots, please consult the Infiltration Design Specification No. 8.  An impermeable bottom liner and an underdrain system must be employed when bioretention is used to filter hotspot runoff.
• Floodplains: Bioretention areas should be constructed outside the limits of the ultimate 100 year floodplain.
• No Irrigation or Baseflow. The planned bioretention area should not receive baseflow, irrigation or chlorinated wash-water or other flows.
• Setbacks – Bioretention areas should not be hydraulically connected to structure foundations or pavement to avoid the risk of seepage. Setbacks to structures and roads vary based on the scale of bioretention (see Table 2).  At a minimum, bioretention basins should be located a horizontal distance of 100 feet from any water supply well, and 50 feet from septic systems, and at least 5 feet from down-gradient wet utility lines. Dry utility lines such as gas, electric, cable and telephone may cross bioretention areas if they are double-cased.

Bioretention can be used wherever water can be conveyed to a surface area. Bioretention has been used at commercial, institutional, and residential sites in spaces that are traditionally pervious and landscaped. It should be noted that special care must be taken to provide adequate pre-treatment within the bioretention cell in space constrained high traffic areas. Typical locations for bioretention include the following:

• Parking lot islands: The parking lot grading is designed for sheet flow towards linear landscaping areas and parking islands between rows of spaces. Curb-less edges can be used to convey water into a depressed island landscaping area. Curb cuts can also be used for this purpose, but they are more prone to clogging and erosion.
• Parking lot edge: Small parking lots can be graded so that flows reach a curb-less edge or curb cut before reaching catch basins or storm drain inlets. The turf at the edge of the parking lot functions as a filter strip to provide pre-treatment for the bioretention practice. The depression for bioretention is located in the pervious area adjacent to the parking lot.
• Road medians, roundabouts, interchanges and cul-de-sacs: The road cross-section is designed to slope towards the center median or center island rather than the outer edge, using a curb-less edge.
• Right-of-way or commercial setback: A linear configuration can be used to receive sheet flow from the roadway or a grass channel or pipe may convey flows to the bioretention practice.
• Courtyards: Runoff collected in a storm drain system or roof leaders can be directed courtyards or other pervious areas on site.
• Individual residential lots: Roof leaders can be directed to small bioretention areas, often called “rain gardens” located at the front, side, or rear of a home in a drainage easement. For smaller lots, the front yard bioretention corridor design may be advisable (See Baywide Design Specification No. 1).
• Unused pervious areas on a site: Storm flows can be redirected from a a storm drain pipe to discharge into a bioretention area.
• Dry ED basin: A bioretention cell can be located on an upper shelf of an extended detention basin, after the sediment forebay, in order to boost treatment Depending on the ED basin design, the designer may locate the bioretention cell in the bottom of the basin; however, the design must carefully account for the potentially deeper ponding depths (greater than 6” or 12”) associated with extended detention.
• Retrofitting: A wide range of options are available to retrofit bioretention in the urban landscape, as described in Profile Sheet ST-4 of Schueler et al (2007). 

SECTION 5: DESIGN CRITERIA

5.1. Sizing of Bioretention Practices

Stormwater Quality

Sizing of the surface area (SA) for bioretention practices is based on the computed treatment volume (Tv) of the contributing drainage area and the storage provided in the facility. The required surface area, in square feet, is computed as the treatment volume in cubic feet divided by the equivalent storage depth in feet. The equivalent storage depth is computed as the depth of media, gravel, or surface ponding (in feet) multiplied by the accepted void ratio.

The accepted Void Ratios (Vr) are (see Figure 5):

Bioretention Soil Media Vr = 0.25
    Gravel Vr = 0.40
                                                 Surface Storage Vr = 1.0

The equivalent storage depth for Level 1 with a 6” surface ponding depth is therefore computed as

• (2’ x 0.25) + (1’ x 0.40) + (0.5 x 1.0) = 1.40 ft

 

And the equivalent storage depth for Level 2 with a 6” storage depth is computed as:

• (3’ x 0.25) + (1’ x 0.40) + (0.5 x 1.0) = 1.65 ft



Figure 5: Typical Section with Void Ratios for Volume Computations

Therefore, the Level 1 Bioretention Surface Area (SA) is computed as:

(3)        SA (ft2)  =  {Tv – volume reduced by upstream BMP} / 1.40 ft

And the Level 2 Bioretention SA is computed as:

(4)        SA (ft2) = {(1.25 * Tv) – volume reduced by upstream BMP} / 1.65 ft

Where:
SA= Minimum surface area of bioretention filter (ft2)
Tv = Treatment Volume (ft3) = [(1.0”)(Rv)(A)/12]  

The effective storage depths will vary according to the actual design depths of the soil media and gravel layer.

Stormwater Quantity

In order to accommodate a greater quantity credit for channel and/or flood protection, designers may be able to create additional surface storage by expanding the surface ponding footprint without necessarily increasing the soil media footprint.  In other words, the engineered soil media would only underlay part of the surface area of the bioretention (See Typical Details-Figure 3).   

In this regard, the ponding footprint can be increased as follows to allow for additional storage:

1. 50% surface area increase if the ponding depth is 6 inches or less.
2. 25% surface area increase if the ponding depth is between 6 and 12 inches

These values may be modified as data on the long term permeability of bioretention filters becomes available. 

5.2. 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 as noted in the soil testing requirements for each scale of bioretention Design Tables 2 and 3.The infiltration rate of subsoils must exceed 1.0 inch per hour in order to dispense with the underdrain requirement for raingardens, and 0.5 inch/hour for bioretention basins. On-site soil infiltration rate testing procedures are outlined in Appendix A of the Infiltration Design Specification No. 8.  Soil testing is not needed for Level 1 bioretention areas where an underdrain is used. 

5.3. BMP Geometry

Bioretention basins must be designed with an internal flow path geometry such that the treatment mechanisms provided by the bioretention are not by-passed or short-circuited.  Examples of short-circuiting include inlets or curb cuts that are very close to outlet structures, 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.

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 Figure 6.  In the figure, ratio of the shortest flow path to the overall length is represented as:

            (5)        SFP/L, where:

SFP = length of the shortest flow path
L = length from the most distant inlet to the outlet

For Level 1 designs, the SFP/L ratio must be 0.3 or greater, and the ratio must be 0.8 or greater for Level 2.  In some cases, due to site geometry, some inlets may not be able to meet these ratios; however, the drainage area served by these inlets should constitute no more than 20% of the contributing drainage area. 

Alternately, the designer may incorporate other design features that prevent short-circuiting, including features that help spread and distribute runoff as evenly as possible across the filter surface.  Local reviewers may waive or modify the shortest flow path ratio guideline in cases where:

1. The outlet structure within the bioretention area is raised above the filter surface to the ponding depth elevation; and
2. The filter surface is flat.

With regard to #1 above, field experience has shown that soil media immediately around a raised outlet structure is prone to scouring and erosion, and thus short-circuiting of the treatment mechanism (water can flow straight down through scour holes or sinkholes to the underdrain system) (Hirschman et al., 2009).  Design options should be used to prevent this type of scouring.  One example is shown in Figure 7.

The designer should ensure that incoming flow is spread as evenly as possible across the filter surface to maximize the treatment potential.



Figure 6. Diagram showing shortest flow path as part of BMP geometry.



Figure 7. Typical Detail – Preventing by-pass or short-circuiting around the overflow structure

5.4. Pretreatment

Pretreatment of runoff entering bioretention areas is required to trap coarse sediment particles before they reach the filter bed, which prevents premature clogging. Pretreatment measures shall be designed to evenly spread runoff across the entire width of the bioretention area. Several pretreatment measures are feasible, depending on the scale of the bioretention practice and whether it receives sheet flow, shallow concentrated flow or concentrated flows. The following are appropriate pretreatment options:

Micro Bioretention (Rain Gardens)

• Leaf Screensas part of the gutter system serve to keep the heavy loading of organic debris from accumulating in the bioretention cell.
• Grass Filter Strip (sheet flow) – Applied on residential lots, the lawn area can serve as a grass filter strip adjacent to a rain garden.
• Gravel or Stone Diaphragm (sheet flow or concentrated flow) – A gravel diaphragm at the end of a downspout or other concentrated inflow point should run perpendicular to the flow path to promote settling.

Bioretention Basins 

• Pretreatment Cell (channel flow). Similar to a forebay, this cell is located at piped inlets or curb cuts leading to the bioretention area and consist of an energy dissipater sized for the expected rates of discharge and have 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 earthen or rock berm.  Pretreatment cells do not need underlying engineered soil media, as does the main bioretention cell.
• Grass Filter Strip (sheet flow) – Grass filter strips extend a minimum of 10 feet from edge of pavement to the swale, and a maximum slope of 5%.
• Gravel or Stone Diaphragm (sheet or concentrated flow) at the edge of the pavement or other inflow point should run perpendicular to the flow path to promote settling. Stone must be sized according to the expected rate of discharge.
• Pea Gravel or Stone Flow Spreader (sheet flow) extends along the top of each bank to pretreat lateral runoff from the road shoulder to the swale and involves a two to four inch drop from a hard-edged surface into a gravel or stone diaphragm.
• Innovative or Proprietary Structure: An approved proprietary structure with demonstrated capability of reducing sediment and hydrocarbons may be used to provide treatment. Refer to the Virginia BMP Clearinghouse for information on approved structures.

5.5. Conveyance and Overflow

• 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.
• Off-line bioretention:  Off-line designs are preferred (see Figure 8 for an example). 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.

Another option is to utilize a low flow diversion or flow splitter at the inlet to allow only the treatment volume peak flow rate to enter the facility. This may be achieved with a weir or curb opening sized for the target flow, in combination with a bypass channel (see Figure 3). Using a weir or curb opening helps minimize clogging and reduces the maintenance frequency. (Further guidance on determining the water quality design peak flow rate will be provided.)

5.6. 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. There are several 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:
• 85 to 88 percent sand;
• 8 to 12 percent soil fines; and
• 3 to 5 percent organic matter.

 

• P-Index: The P-Index provides a measure of soil phosphorus content and the risk of that phosphorus moving through the soil media. Phosphorus movement through a soil can be defined by several soil physical properties: texture, structure, total pore space, pore-size 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/cm3.  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, and is expressed in milliequivalents per 100 grams (meq/100g) of soil.  For agricultural purposes, these elements are the basic cations, calcium (Ca+2), magnesium (Mg+2), potassium (K+1) and sodium (Na+1) and the acidic cations, hydrogen (H+1) and aluminum (Al+3). The CEC of soil is determined in part by the amount of clay and/or humus or organic matter

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.



Figure 8. Typical Detail - Off-Line Bioretention

• Infiltration Rate: The bioretention soil media should have an infiltration rate of 1 to 2 inches per hour.
• Depth: The standard minimum filter bed depth ranges between 24 and 36 inches for Level 1 and Level 2, respectively (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 four feet deep to provide enough soil volume for the root structure of mature trees. Use turf, perennials or shrubs instead of trees when landscaping shallower filter beds.
• Filter Media for Tree Planting Areas: Within the planting holes for trees, a more organic filter media is recommended, with a ratio of 50% sand, 30% toposoil and 20% acceptable leaf compost. 
• Mulch: A two to three 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.
• 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 on what 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 temperatures 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.

 

5.7. Underdrain & Underground Storage Layer

Some Level 2 designs will not use an underdrain (where soil infiltration rates meet minimum standards; see Section 5.2 and Section 2 design tables).  For Level 2 designs with an underdrain, an underground storage layer of at 12” should be incorporated below the invert of the underdrain.  The depth of the storage layer will depend on the target treatment and storage volumes needed to meet water quality, channel protection, and/or flood protection criteria.  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 surface, with a vented cap. In addition, cleanout pipes should be provided if the contributing drainage area exceeds 1 acre.

5.8. 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, planting plan, including the size, stock and sources of plant species, and the planting sequence, including post nursery care and initial maintenance. 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.

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 Table 4. Internet links to more detailed bioretention plant lists developed in piedmont and coastal plain communities of the Chesapeake Bay are provided in Table 5.

The planting template refers to the form and combination of native trees, shrubs, and perennial ground covers that maintain the appearance and function of the bioretention area. The five most common bioretention templates are as follows:

• Turf: This option is typically restricted to on-lot micro-bioretention applications such as front yard bioretention. 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 utilizes herbaceous plants and native grasses to create a garden effect with seasonal cover, and may be employed in both micro and small scale bioretention applications. The option is attractive, but requires more maintenance in the form of weeding.
• Perennial garden with shrubs: This landscaping option provides greater vertical form by mixing native shrubs and perennials together in the bioretention area. This option is frequently used when the filter 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 which produces the most natural effect, and is highly recommended for bioretention basin applications. The landscape goal is to simulate the structure and function of native forest plant community.
• Turf and tree: This option is a lower maintenance version of option 4, 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 meadow or roadside vegetated area (for instance, with Joe Pye Weed, New York 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 perimeters.  Erosion control matting can be used in lieu of the conventional mulch layer. 
  

Table 4: Popular Native Plant Materials for Bioretention in the Bay Watershed
Perennials/Herbaceous	Shrubs	Trees
Virginia Wild Rye (Elymus virginicus)	Common Winterberry (Ilex verticillatta)	River Birch (Betula nigra)
Redtop Grass (Agrostis alba)	Inkberry (Ilex glabra)	Red Maple (Acer rubrum)
Swamp Milkweed (Asclepias incarnata)	Sweet Pepperbush (Clethra ainifolia)	Pin Oak (Quercus palustris)
Switchgrass (Panicum virgatum)	Wax Myrtle (Myrica cerifera)	Willow Oak (Quercus phellos)
Cardinal Flower (Lobelia cardinalis)	Virginia Sweetspire (Itea virginica)	Sweetgum (Liquidambar styraciflua)
Common Three Square (Scirpus americanus)	Swamp Azeala (Azeala viscosum)	Black Willow (Salix nigra)
Sensitive Fern (Onoclea sensibilis)	Button Bush (Cephalanthus occidentalis)	Grey Birch (Betula populifolia)
Blue Flag (Iris versicolor)	Black Haw (Virburnum prunifolium))	Black Gum (Nyassa sylvatica)
Woolgrass (Scirpus cyperninus)	Indigo Bush (Amorpha fruticosa)	Sycamore (Platanus occidentalis)
Indian Grass (Sorghastrum nutans)	Arrowwood (Virburum dentatum)	Green Ash (Fraxinus pennsylvanica
Marsh Marigold (Caltha palustris)		Sweetbay Magnolia* (Magnolia virginiana)
Joe Pye Weed (Eupatorium purpureum)		Atlantic White Cedar* (Charnaecyparis thyoides)
Turk's cap lily (Lilium superbum)		Bald Cypress* (Taxodium distichum)
Bee Balm (Mornarda didyma)		Grey Dogwood (Cornus racernosa)
Northern Sea Oats (Chasmanthium latifolium)		Smooth Alder (Alnus serrulata))
		Serviceberry (Amelanchier canadensis)
		Redbud (Cercis candensis)
		Box Elder (Acer negundo)
		Fringe Tree (Chionanthus virginicus)
Note: Please consult bioretention plant lists for more detailed info on inundation, drought and salt tolerance for each species prior to selection. * most applicable to the coastal plain




Table 5 Chesapeake Bay Bioretention Plant Lists

Fairfax County, VA https://166.94.9.135/dpwes/publications/lti/07-03attach3.pdf Prince Georges County, MD http://www.co.pg.md.us/Government/AgencyIndex/DER/ESD/Bioretention/pdf/Plant_list.pdf Suffolk County, VA http://www.suffolk.va.us/citygovt/udo/apdx_c/appendix_c9-2_plant_list.pdf Virginia http://www.ext.vt.edu/pubs/waterquality/426-043/426-043.html Bay Directory of Native Plant Nurseries http://www.montgomerycountymd.gov/Content/DEP/Rainscapes/nurseries.htm Delaware Green Technology Standards and Specifications http://www.dnrec.state.de.us/DNREC2000/Divisions/Soil/Stormwater/New/GT_Stds%20&%20Specs_06-05.pdf

The choice of which planting template to use depends on the scale of bioretention, the context of the site in the urban environment, the filter depth, the desired landscape amenities and the future owners capability to maintain the landscape.  In general, the vegetative goal is to cover up the filter surface with vegetation in a short amount of time.  This means that the herbaceous layer is equally or more important than widely spaced trees and shrubs.  In the past, many bioretention areas in Virginia did not include enough herbaceous plants.

The following additional guidance on developing an effective bioretention landscaping plan is provided:

• Plants should be selected based on a specified zone of hydric tolerance and must be capable of surviving both wet and dry conditions.
• “Wet footed” species should be planted near the center, whereas upland species are better near the edge.
• Woody vegetation should not be located at points of inflow and trees should not be planted directly above underdrains, but be located closer to the perimeter.
• If trees are part of the planting plan, a tree density of approximately one tree per 250 square feet (i.e., 15 feet on-center) is recommended.
• Shrubs and herbaceous vegetation should generally be planted in clusters and at higher densities (10 feet on-center and 1 to 1.5 feet on center, respectively).
• Temporary or supplemental irrigation may be needed for bioretention in order for plant installers to warrant plant material survival.
• Supplemental irrigation by a rain tank system is also recommended (See Stormwater Design Specification No. 6).
• Designers should also remember that planting holes for trees need to be at least 4 feet deep to provide enough soil volume for the root structure of mature trees.  This applies even if the remaining soil media layer is shallower than 4 feet.
• If trees are used, ensure that ground covers planted in the drip line are shade tolerant
• Maintenance is an important factor in selecting plant species. Plant selection differs if the area will be frequently mowed, pruned and weeded, in comparison to a site which will receive minimum annual maintenance.
• If the bioretention area is to be used for snow storage or accept snowmelt runoff, it should be planted with salt-tolerant, herbaceous perennials.

5.9. Bioretention Material Specifications

Table 6 outlines the standard material specifications to construct bioretention areas.




Table 6: Bioretention Material Specifications
Material	Specification	Notes
Filter Media Composition	Filter Media to contain: 85-88% sand 8-12% soil fines 3-5% organic matter in form of leaf compost	Volume of filter media based on 110% of plan volume to account for settling or compaction.
Filter Media Testing	P-Index range 10-30, or Between 7 and 21 mg/kg of P in the soil media CECs greater than 10	Procured from approved filter media vendors
Mulch Layer	Aged, shredded hardwood bark mulch	A 2 to 3 inch layer on the surface of the filter bed.
Alternative Surface Cover	river stone or pea gravel, coir and jute matting, or turf cover	A 2 to 3 inch layer of to suppress weed growth
Top Soil For Turf Cover	Loamy sand or sandy loam texture, with less than 5% clay content, corrected pH 6 to 7, and organic matter content of at least 2%	3 inch surface depth
Geotextile/Liner	Non-woven geotextile fabric w/ flow rate of > 110 gallons/minutes/sq.ft. (e.g., Geotex 351 or equivalent)	Apply to sides and above underdrain only. For hotspots and certain karst sites only, use appropriate liner on bottom.
Choking Layer	2 to 4 inch layer of sand over  a 2 inch layer of choker stone (typically #8 or # 89 washed gravel) over the underdrain stone
Stone Jacket for Underdrain and/or Storage Layer	1 inch stone should be double-washed and clean and free of all fines – VDOT #57 stone	12 inches for underdrain; 12 to 18 inches for stone storage layer, if needed
Underdrains, Cleanouts, and Observation Wells	6 inch rigid schedule 40 PVC pipe, with 3/8” perforations at six inches on center, each underdrain on a 1 or 2% slope located nor more than 20 feet from the next pipe ( or equivalent corrugated HDPE for micro-bioretention)	Perforated pipe under the length of the bioretention cell, and non-perforated pipe as needed to connect with the storm drain system. T’s and Y’s as needed depending on the underdrain configuration. Extend cleanout pipes to the surface with vented caps at Ts and Ys
Plant Materials	One tree per 250 square feet (15 feet on-center, minimum 1 inch caliper). Shrubs, minimum 30” planted minimum 10 feet on-center. Ground cover plugs at 12 to 18 inches o.c., containers at 18 to 24 inches o.c., depending on initial plant size and how large it will grow.	Plant species as per landscaping plan and recommended plant list. In general, plant spacing shall be sufficient to ensure plant material reaches 80% cover in proposed planting areas within a 3-year period. If seed mixes are used, should be from qualified supplier, appropriate for stormwater basin applications, and use native species (unless seeding is to establish maintained turf).




SECTION 6.0  REGIONAL AND SPECIAL CASE DESIGN ADAPTATIONS

6.1 Karst Terrain  

Active karst regions are found in much of the Ridge and Valley province of the Bay watershed, and complicate both development and stormwater design. While bioretention areas produce less deep ponding than conventional stormwater practices (e.g., ponds and wetlands), Level 2 bioretention designs (i.e., infiltration) are not recommended in any area with a moderate or high risk of sinkhole formation (Hyland, 2005). On the other hand, Level 1 designs that meet separation distance requirements (three feet) and possess an impermeable bottom liner and an underdrain should work well. In general, micro-bioretention and bioretention basins with contributing drainage areas not exceeding 20,000 square feet are preferred (compared to bioretention with larger drainage areas) in order to prevent possible sinkhole formation, although it may be advisable to increase standard setbacks to buildings.

6.2 Coastal Plain

The flat terrain, low head and high water table of many coastal plain sites can constrain the application of deeper bioretention areas (particularly Level 2 designs). In these situations, the following design adaptations may be helpful.


• A linear approach to bioretention using multiple cells leading to the ditch system helps conserve head
• The minimum depth of the filter bed may be 18 to 24 inches, and it is useful to limit surface ponding to 6  to 9 inches, and save additional depth by shift to a turf cover rather than mulch. The shallower media depth and the turf cover generally comply with the Dry Swale specification, and therefore will be credited with a slightly lower pollutant removal (See Specification 10 – Dry Swales).
• The minimum depth to the seasonally high water table from the invert of the system can be one foot, as long as the bioretention area is equipped with an large diameter underdrain (e.g., six inches) that is only partially efficient at dewatering the bed
• It is important to maintain at least a 0.5% slope in the underdrain to ensure drainage.
• The underdrain should be tied into the ditch or conveyance system
• The mix of plant species selected should reflect coastal plain plant communities, and should be more wet footed and salt tolerant than typical Piedmont application

While these design criteria permit bioretention to be used on a wider range of coastal plain sites, it is important not to force it into marginal sites. Other stormwater practices, such as wet swales, ditch wetland restoration and smaller linear wetlands, are often preferred alternatives for coastal plain sites.

6.3 Steep Terrain 

Contributing slopes to a bioretention area can be increased to 15% in areas of steep terrain, as long as a two cell design is used to dissipate erosive energy prior to filtering. Designers may also want to terrace a series of bioretention cells to manage runoff across or down a slope. The drop in slope between cells should be limited to a foot, be armored with river stone or suitable equivalent.

6.4 Winter Performance

• Bioretention areas can be used for snow storage as long as an overflow is provided and they are planted with salt-tolerant, non-woody plant species. Tree and shrub locations should not conflict with plowing and piling of snow into storage areas
• Designers may want to evaluate Chesapeake Bay wetland plant species that tolerate slightly brackish water.
• While several studies have shown that bioretention operates effectively in PA and WVA winters, it is a good idea to extend the filter bed and underdrain pipe below the frost line and/or oversize the underdrain by one pipe size to reduce the freezing potential.

6.5 Linear Highway Sites

Bioretention is a preferred practice for constrained highway right of ways when it is designed as a series of individual on-line or off-line cells. In these situations, the final design closely resembles that of dry swales. Salt tolerant species should be selected if the contributing roadway will be salted in the winter. 

 

SECTION 7: BIORETENTION CONSTRUCTION SEQUENCE AND INSPECTION

7.1. Construction Sequence

Construction Stage ESC Controls. 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 ESC plan identifying the maximum excavation depth at the construction stage being at least 1 foot above the post-construction installation, must contain an underdrain, and show the proper procedures for conversion from a temporary practice to a permanent one, including de-watering, cleanouts and stabilization.

7.2 Bioretention Installation

The following is a typical construction sequence to properly install a bioretention basin practice (also see Figure 9). 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. Bioretention may only begin after 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 conduct a preconstruction meeting, and check 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 any project changes needed during the preconstruction meeting.

Step 3. Temporary ESC controls are needed during bioretention installation 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 pretreatment 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 arms with adequate reach so they do not have to work inside the footprint of the bioretention area. Contractors should utilize 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 6 to 12 inches to promote greater infiltration

Step 7. Place the geotextile fabric on the sides of the bioretention area with 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 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.

Step 8. Deliver 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 desired top elevation of the bioretention area is achieved. Wait a few days to check for settlement, and add additional media as needed.

Step 9. Prepare planting holes for any trees and shrubs, install vegetation, and water accordingly. Install any temporary irrigation.

Step 10. Place the surface cover in both cells (mulch, river stone or turf), depending on 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 cut in the matting to install the plants.

Step 11. Install 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 (see Section 9.2), and login the GPS coordinates for each facility in local maintenance tracking database.



Figure 9. Typical Biofilter Construction Sequence

7.3  Construction Inspection

Inspections during construction are needed to ensure that the bioretention practice is built in accordance with these specifications.  Detailed inspection checklists, which include sign-offs by qualified individuals at critical stages of construction, should be used to ensure that the contractor’s interpretation of the plan is consistent with the designer’s intent.  An example construction phase inspection checklist for bioretention areas can be accessed at the Center for Watershed Protection  website at: www.cwp.org/postconstruction.  Go to Tool #6.

Some common pitfalls can be avoided by careful construction supervision that focuses on the following key aspects of bioretention installation.

• Drainage area should be stabilized prior to directing water to the bioretention area.
• Check filter media to confirm that it meets specifications and is added to the correct depth.
• Check elevations such as the invert of the underdrain, inverts for the inflow and outflow points, and the ponding depth provided between the surface of the filter bed and the overflow structure. Make sure the bioretention bed is flat, runoff evenly spreads across it, and the bed drains within 6 to 12 hours.
• Ensure that caps are placed on the upstream (but not the downstream) end of the underdrain.
• Make sure the desired surface cover has been installed and that the planting plan has been followed.
• Inspect pretreatment structures to make sure they are properly installed and working effectively.
• Ensure that the intended amount of water is entering the filter through inlets and that curb inlets and pretreatment devices have the necessary drop from the hard edge surface so that accumulated sediment, debris, and grass do not prevent water from entering the filter.
• Ensure that inlets and outfalls are stable and non-erosive during storms.

The real test for bioretention is after its first big storm. The post-storm inspection should focus on whether the desired sheetflow, shallow concentrated flow or concentrated flow conditions assumed in the plan are realized in the field. Also, inspectors should check that the bioretention areas drains within minimum 6 hour draw down. Minor adjustments are normally needed as a result of this post-storm inspection, such as spot reseeding, gully repair and added armoring at inlets, outfalls and pretreatment devices.

SECTION 8:  BIORETENTION MAINTENANCE

8.1. Maintenance Agreements

Section 4VAC 50-60-124 of the regulations specifies a maintenance agreement to be executed between the owner and the local program.  The section requires a schedule of inspections, compliance procedures if maintenance is neglected, notification of the local program upon transfer of ownership, and right-of-entry for local program personnel.     

For bioretention, the maintenance agreements must contain recommended maintenance tasks and a copy of an annual inspection checklist. When micro-scale bioretention practices are applied on private residential lots, homeowners will need to be educated regarding their routine maintenance needs.  A deed restriction or other mechanism enforceable by the qualifying local program must be in place to help ensure that rain gardens and bioretention filters are maintained and not converted or disturbed, as well as pass the knowledge along to any subsequent owners.  The mechanism should, if possible, grant authority for local agencies to access the property for inspection or corrective action.
In addition, the GPS coordinates for all bioretention practices should be provided upon facility acceptance to ensure long term tracking.

8.2. First Year Maintenance Operations

Successful establishment of bioretention areas requires certain tasks be undertaken in the first year.

• Initial inspections: For the first six months following construction, the site should be inspected at least twice after storm events that exceed a half- inch
• 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: Watering is needed once a week during the first two months, and then as needed during first growing season (Apr – Oct), depending on rainfall
• Remove and replace dead plants. Since up to 10% of plant stock may die off in the first year, construction contracts should include a care and replacement warranty to ensure 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.

8.3. Maintenance Inspections

It is highly recommended that a spring maintenance inspection and cleanup be conducted at each bioretention area. Table 7 presents some of the key maintenance problems to look for; a more detailed annual maintenance inspection checklist for bioretention areas can be accessed at the Center for Watershed Protection  website at: www.cwp.org/postconstruction.  Go to Tool #6.

8.4. Routine and Non-Routine Maintenance Tasks

Maintenance of bioretention areas should be integrated into routine landscaping maintenance tasks. If 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 nature of surface cover. A generalized summary of common maintenance tasks and their frequency is provided in Table 8.

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 using a water source verify that the underdrains are functioning and not clogged, or otherwise in need of repair.
• Remove accumulated sediment and till in 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 to reduce the average concentration of fines in the media bed and promote quicker drawdown times.  Sand wicks can be installed by excavating or augering (using a tree auger or similar) 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.

Table 7: Suggested Spring Maintenance Inspections for Bioretention

Activity

Check to see if  75% to 90% mulch + vegetative cover has been achieved in the bed, and measure depth of remaining mulch

Check for sediment buildup at curb cuts, gravel diaphragms or pavement edges that prevents flow from getting into the bed, and other signs of by-passing

Check for any winter or salt-killed vegetation and replace with hardier species

Note presence of accumulated sand, sediment and trash in pretreatment cell or filter beds and remove.

Inspect bioretention side slopes and grass filter strips for evidence of any rill or gully erosion and repair

Check bioretention bed for evidence of mulch flotation, excessive ponding, dead plants or 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 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.


 

Table 8 Suggested Annual Maintenance Activities for Bioretention
Maintenance Tasks	Frequency
Spring inspection and cleanup	Annual
Add reinforcement planting to maintain desired vegetation density.	As needed
Spot weeding, erosion repair, trash removal, and mulch raking	Twice during growing season
Sediment removal in pretreatment cells and inflow points	Once every two to three years
Mowing of grass filter strips and bioretention turf cover.	At least four times a year
Remove invasive plants using recommended control methods.	As needed
Supplement mulch to maintain a 3 inch layer.	Annual
Replace mulch layer	Every three years
Prune trees and shrubs	Annual
Stabilize contributing drainage area to prevent erosion	When needed

SECTION 9: DESIGN REFERENCES

The following references and resources were used to develop this master specification.

Lake County, OH Bioretention Guidance Manual
http://www2.lakecountyohio.org/smd/Forms.htm

CWP. 2007. National Pollutant Removal Performance Database Version 3.0. Center for Watershed Protection, Ellicott City, MD.

Schueler, T. 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD www.chesapeakestormwater.net

Prince George’s Co., MD Bioretention Manual
http://www.goprincegeorgescounty.com/Government/AgencyIndex/DER/ESD/Bioretention/bioretention.asp?nivel=foldmenu(7)

Maryland Stormwater Design Manual
http://www.mde.state.md.us/Programs/WaterPrograms/SedimentandStormwater/stormwater_design/index.asp

Wisconsin Stormwater Management Technical Standards http://www.dnr.state.wi.us/org/water/wm/nps/stormwater/techstds.htm#Post

Hirschman, D., L. Woodworth and S. Drescher. 2009.  Technical Report: Stormwater BMPs in Virginia’s James River Basin: An Assessment of Field Conditions & Programs. Center for Watershed Protection.  Ellicott City, MD.

Hunt, W.F. III and W.G. Lord. 2006. Bioretention Performance, Design, Construction, and Maintenance. North Carolina Cooperative Extension Service Bulletin. Urban Waterways Series. AG-588-5. North Carolina State University. Raleigh, NC.

Hyland, S. 2005. Analysis of sinkhole susceptibility and karst distribution in the Northern Shenandoah Valley (Virginia); impacts for LID site suitability models. M.S. Thesis. Virginia Polytechnic Institute and State University. Blacksburg, VA.   

Minnesota Stormwater Steering Committee (MSSC). 2005. The Minnesota Stormwater Manual.

Schueler et al  2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection. Ellicott City, MD.

North Shore City  2007. Bioretention Design Guidelines. Sinclair, Knight and Merz. Auckland, New Zealand

In addition, the following individuals provided review and input for this version of the specification. Rick Scafidi (EQR), Bill Hunt (NCSU), Scott Thomas (JCC), Dave Hirschman (CWP) Don Rissmeyer (Randy Greer (DENRC), Doug Biesch (WEG) Stuart Stein (GKY), Tim Schueler (MC), Christie Minami (MD SHA). Special thanks to the staff at WEG for providing the design schematics and details.




APPENDIX A
URBAN BIORETENTION

Stormwater Planters
Expanded Tree Pits
Stormwater Curb Extensions 

VERSION 1.0



SECTION A-1: DESCRIPTION

Urban bioretention practices are similar in function to regular bioretention practices except they are adapted to fit into “containers” within urban landscapes. Typically, urban bioretention is installed within an urban streetscape or city street right-of-way, urban landscaping beds, tree pits and plazas, or other features within an Urban Development Area. Urban bioretention is not intended for large commercial areas, nor should it be used to treat small sub-areas of a large drainage area such as a parking lot. Rather, urban bioretention is intended to be incorporated into small fragmented drainage areas such as shopping or pedestrian plazas within a larger urban development.

Urban bioretention features a hard edge, often with vertical concrete side, as contrasted with the more gentle earthen slopes of regular bioretention. These practices may be open-bottomed to allow some infiltration of runoff into the sub-grade, but they generally are served by an under drain. The typical stormwater functions of an urban bioretention area are described in Table A1. The three major design variants of urban bioretention are described below: 

Stormwater planters (also known as vegetative box filters or foundation planters) take advantage of limited space available for stormwater treatment by placing a soil filter in a container located above ground or at grade in landscaping areas between buildings and roadways (Figure A1). The small footprint of foundation planters is typically contained in a precast or cast-in-place concrete vault.  Other materials may include molded polypropylene cells and precast modular block systems.

Extended tree pits are installed in the sidewalk zone near the street where urban street trees are normally installed. The soil volume for the tree pit is increased and used as a stormwater (see Figure A2). Treatment is increased by using a series on connected tree planting areas together in a row. The surface of the enlarged planting area may be mulch, grates, permeable pavers, or conventional pavement. The large and shared rooting space and reliable water supply increases the growth and survival rates in this otherwise harsh planting environment.

Stormwater curb extensions (also known as parallel bioretention) are installed in the road right-of way either in the sidewalk area or in the road itself. In many cases, curb extensions serve as a traffic calming or street parking control device. The basic design adaptation is to move the raised concrete curb closer to the street or in the street, and then create inlets or curb cuts that divert street runoff into depressed vegetated areas within the expanded right of way (Figure A3).   

Each urban bioretention variant is planted with a mix of trees, shrubs, and grasses as appropriate for its size and landscaping context. 

Table A-1: Summary of Stormwater Functions Provided by Urban Bioretention Areas
Stormwater Function	Level 1 Design	Level 2 Design
Annual Runoff Reduction	40% (for Water Quality credit in the RRM Spreadsheet only) 0% credit for Channel Protection	NA
Total Phosphorus Removal 1	25%	NA
Total Nitrogen Removal 1	40%	NA
Channel Protection	None; or if sized according to Bioretention Basin, follow the Bioretention basin, Level 1 criteria.
Flood Mitigation	None
1 Change in event mean concentration (EMC) through the practice. Actual nutrient mass load removed is the product of the removal rate and the runoff reduction rate. Sources: CWP and CSN (2008) and CWP (2007).

Figure A1 Stormwater Planters

 

Figure A2 Expanded Tree Pits



Figure A3 Stormwater Curb Extensions



SECTION A-2:  DESIGN TABLES

Table A-2: Urban Bioretention Design Criteria

Level 1 Design (RR 40 TP: 25 )

Sizing (Sec. A-5): Surface Area (ft2) = Tv/2 = [((1.0”)(Rv)(A)/12) – volume reduced by upstream BMP]/2

Underdrain (Sec. A.7) = Schedule 40 PVC with clean-outs

Maximum Drainage Area = 2,500 ft2

Maximum Ponding Depth = 6 to 12 inches

Filter media depth minimum = 30 inches; recommended maximum = 48 inches

Media & Surface Cover (Refer to Sec. A-7)

Sub-soil testing (Refer to Sec. A-7)

Inflow = sheetflow, curb cuts, trench drains, roof drains, concentrated flow, or equivalent

Building setbacks (Sec. A-4)

Deeded maintenance O&M plan (Sec. 7)

SECTION A-3:  TYPICAL DETAILS


Ponding depth above 6 inches will require a specific planting plan to ensure appropriate plants (Bioretention Section 5.8).



Figure A4: Typical Detail – Urban Bioretention Cross-Section

Ponding depth above 6 inches will require a specific planting plan to ensure appropriate plants (Bioretention Section 5.8).





Figure A5: Expanded Tree Pit Diagram

Portland, Oregon has thorough construction details for stormwater curb extensions and expanded tree pits. These include details for addressing utility house connections.
http://www.portlandonline.com/bes/index.cfm?c=44213&

 

SECTION A-4:  PHYSICAL FEASIBILITY & DESIGN APPLICATIONS

In general, urban bioretention has the same constraints as regular bioretention, along with a few additional constraints as noted below:

• Contributing Drainage Area: Urban bioretention is classified as a micro-bioretention practice, and therefore limited to 2,500 sq. ft. drainage area to each unit (this is considered a general rule; larger drainage areas may be allowed with sufficient flow controls and other mechanisms to ensure proper function, safety, and community acceptance. The drainage areas in these urban settings is typically considered to be 100% impervious.  While multiple units can be installed to maximize treatment area in ultra-urban watersheds, urban bioretention is not intended to be used as treatment of a large impervious areas (such as parking lots).
• Adequate Drainage: Practice elevations must allow the untreated stormwater runoff to be discharged at the surface of the filter bed and ultimately connect to the local storm drain system.
• Available Head: In general, 3 to 5 feet of elevation difference is needed between the downstream storm drain invert and the inflow point. This is generally not a constraint due to the standard depth of most storm drains systems.
• Setbacks from buildings/roads: If an impermeable liner and an underdrain are used, no setback is needed from the building. Otherwise, the standard 10 foot down-gradient setback applies
• Proximity of Underground Utilities: Urban bioretention frequently competes for space with a variety of utilities. Since they are often located parallel to the road right-of-way, care should be taken to provide utility-specific horizontal and vertical setbacks. However, conflicts with water and sewer laterals (e.g., house connections) may be unavoidable, and the construction sequence must be altered, as necessary, to avoid impact to existing service.
• Overhead Wires: Designers should also check whether future tree canopy heights achieved by urban bioretention will interfere with existing overhead phone and power lines.

Because urban bioretention is installed in a highly urban setting, individual units may be subject to higher public visibility, greater trash loads, pedestrian use, vandalism and even vehicular loads. Therefore, a preventative approach is recommended in their design to address these issues. In addition, designers should clearly recognize the need to perform frequent landscaping maintenance to remove trash, check for clogging, and maintain vegetation. The urban landscaping context may feature naturalized landscaping or a more formal deign.  When urbanbioretention is used in sidewalk areas of high foot traffic, designers should not impede pedestrian movement or create a safety hazard. Designers may also install low fences, grates or other measures to prevent damage from pedestrian short-cutting across the practices.

 

SECTION A5. DESIGN CRITERIA

Urban bioretention practices are similar in function to regular bioretention practices except they are adapted to fit into “containers” within urban landscapes. Therefore, special sizing accommodations are made to allow these practices to fit in very constrained areas where other surface practices may not be feasible.

5.1. Sizing of Urban Bioretention

The required surface area of the Level 1 urban bioretention filter is one half of the treatment volume (Tv); Equation (1) below. This criterion represents a balance between the need to size these structures so as to provide a reasonable alternative in ultra urban settings and the relationship between the surface area size, media permeability, and drawdown requirements. Ideally, urban bioretention facilities are in close proximity to the public or users of the adjacent buildings and/or commercial areas, and thus subjected to increased scrutiny. This provides a theoretical basis for adjusting the clogging factor for the media permeability coefficient (k, ft/day), or an increase in the allowable maximum drawdown time, resulting in the smaller sizing. However, as a result, Level 1 urban bioretention will only count towards water quality credit through the 40% volume reduction and/or the 25% TP pollutant removal. There is no credit given to channel protection due to the reduced surface area and storage volume.

 

Level 1 Urban Bioretention:

(1)        SA (ft2)  =  Tv (ft3) / 2.0 ft

Where
Tv = is required treatment volume in cubic feet
SA= surface area of bioretention area in square feet 

5.2 General Design Criteria for Urban Bioretention

Design of urban bioretention should follow the general guidance presented in the main part of this design specification. The actual geometric design of urban bioretention is usually dictated by other landscape elements such as buildings, sidewalk widths, utility corridors, retaining walls, etc. Designers can divert fractions of the runoff volume from small impervious surfaces into micro-bioretention units that are integrated with the overall landscape design.  Inlets and outlets should be located as far apart as possible. Some additional design guidance that applies to all variations of urban bioretention is presented below:

• The ground surface of the micro-bioretention cell should slope 1% towards the outlet, unless a stormwater planter is used
• At a minimum, the soil media depth should be 30 inches. 
• If large trees and shrubs are to be installed, soil media depths should be a minimum of 4 feet. 
• Each individual urban bioretention unit should be stenciled or otherwise permanently marked to designate it as a stormwater management facility.  The stencil or plaque should indicate its water quality purpose, that it may pond briefly after a storm, and is not to be disturbed except for required maintenance.
• All urban bioretention practices should be designed to fully drain within 24 hours.
• Any grates used above urban bioretention must be removable to allow maintenance access. 
• The inlet(s) to urban bioretention should be stabilized using VDOT #3 stone, splash block, river stone or other acceptable energy dissipation measures. The following forms of inlet stabilization are recommended:
• Downspouts to a stone energy dissipator.
• Sheet flow off of a depressed curb with a 3-inch drop
• Curb cuts into the bioretention area
• Covered drains that convey flows across sidewalks from the curb or downspouts
• Grates or trench drains that capture runoff from the sidewalk or plaza
• Pretreatment options overlap with those of regular bioretention. However, the materials used may be chosen based on their aesthetic qualities in addition to their functional properties. For example, river rock may be used in lieu of rip rap. Other pretreatment options may include a:
• Trash rack between the pre-treatment cell and the main filter bed. This will allow trash to be collected from one location.
• Trash rack across curb cuts. While this trash rack may clog occasionally, it keeps trash in the gutter to be picked up by street sweeping equipment.
• Pre-treatment area above ground or a manhole or grate directly over the pre-treatment area
• Overflows can either be diverted from entering the bioretention cell or dealt with via an overflow inlet. Some methods include:
• Size curb openings to capture only the water quality volume any bypass higher flows through the existing gutter.
• Use landscaping type inlets or standpipes with trash guards as overflow devices.
• Use a pretreatment chamber with a weir design that limits flow to the filter bed area.

5.3 Specific Design Issues for Stormwater Planters 

Since stormwater planters are often located near building foundations, waterproofing by a watertight concrete shell or an impermeable liner is required to prevent seepage.

5.4 Specific Design Issues for Expanded Tree Pits

• The bottom of the soil layer must be a minimum of 4 inches below the root ball of plants to be installed
• Extended tree pits designs sometimes cover portions of the filter media with pervious pavers or cantilevered sidewalks. In these situations, it is important that the filter media is connected beneath the surface so that stormwater and tree roots can share this space
• Tree pit grates over a filter bed media is a possible solution to pedestrian traffic and trash accumulation.
• Low, wrought iron fences can help restrict foot traffic across the tree pit bed and serve as a protective barrier if there is a drop off from the pavement to the micro-bioretention cell
• A removable grate may be used to allow the tree to grow through it but which is capable of supporting H-20 loads
• Each tree needs a minimum of 400 cubic feet of shared root space.

5.5 Specific Design Issues for Stormwater Curb Extensions 

Roadway stability can be a design issue with stormwater curb extensions.Design standards or roadway drainage should be consulted. It may be necessary to provide a barrier to keep water from saturating the road’s sub-base, and demonstrate it is capable of supporting H-20 loads.

5.6 Planting and Landscaping Considerations

The degree of landscaping maintenance that can be provided will determine some of the planting choices for urban bioretention areas. The planting cells can be formal gardens or naturalized landscapes.
In areas where less maintenance will be provided and where trash accumulation in shrubbery or herbaceous plants is a concern, consider a “turf and trees” landscaping model. Spaces for herbaceous flowering plants can be included. This may be attractive at a community entrance location.

Native trees or shrubs are preferred for urban bioretention areas, although some ornamental species may be used.  As with regular bioretention, the selected perennials, shrubs, and trees must be tolerant of salt, drought, and inundation. Additionally, tree species should be those that are known to survive well in the compacted soils and polluted air and water of an urban landscape.

SECTION A-6 URBAN BIORETENTION MATERIAL SPECIFICATIONS 

Please consult the main part of this design specification for the typical materials needed for filter media, stone, mulch and other bioretention features. The unique components for urban bioretention may include the inlet control device, a concrete box or other containing shell, protective grates and an underdrain that daylights to another stormwater practice or connects to the storm drain system. The underdrain should:  

• Consist of slotted pipe greater than or equal to 4 inches in diameter, placed in a layer of washed (less than 1% passing a #200 sieve) VDOT #57 stone. 
• Be a minimum of 2 inches of gravel above and below the pipe. 
• Be laid at a minimum slope of 0.5 %. 
• Extend the length of the box filter from one wall to within six inches of the opposite wall, and may be centered in the box or offset to one side. 
• Be separated from the soil media by non-woven, geotextile fabric or a two to three inch layer of washed VDOT #8 stone or 1/8 to 3/8 inch pea gravel. 
  
  SECTION A-7  URBAN BIORETENTION CONSTRUCTION

The construction sequence and inspection requirements for urban bioretention are generally the same as micro-bioretention practices. Please consult the construction sequence and inspection checklists outlined in the main part of this specification. In cases where urban bioretention is constructed in the road or right of way, the construction sequence may need to be adjusted to account for traffic control, pedestrian access and utility notification.

Urban bioretention areas should only be constructed after the drainage area to the facility is completely stabilized.  The specified growth media should be placed by hand with minimal compaction in order to maintain the porosity of the media.  Spreading should be by hand.  The media should be placed in 8 to 12 inch lifts with no machinery allowed over the media during or after construction.  The media should be overfilled above the proposed surface elevation as needed to allow for natural settlement.  Lifts may be lightly watered to encourage settlement.  After the final lift is placed, the media should be raked to level it, saturated, and allowed to settle for at least one week prior to installation of plant materials.

SECTION A-8  URBAN BIORETENTION MAINTENANCE

Routine operation and maintenance are essential to gain public acceptance of highly visible urban bioretention areas. Weeding, pruning, and trash removal should be done as needed to maintain the aesthetics for community acceptance. During drought conditions, it may be necessary to water the plants as would be the case with any landscaped area.
To ensure proper performance, inspectors should check that stormwater infiltrates properly into the soil within 24 hours after a storm. If excessive water ponding is observed, corrective measures include inspection for soil compaction and underdrain clogging.  Please consult the maintenance inspection checklists and ongoing maintenance tasks as outlined in the main part of this design specification




A-9 DESIGN REFERENCES

The following references and resources were used to develop this additional specification:

Center for Watershed Protection. 2006. Urban Watershed Forestry Manual Part 2: Conserving and Planting Trees at Development Sites. Ellicott City, MD
http://www.cwp.org/forestry/index.htm

City of Portland, Bureau of Environmental Services. (Portland BES). 2004. Portland Stormwater Management Manual. Portland, OR. http://www.portlandonline.com/bes/index.cfm?c=dfbcc

Credit Valley Conservation. 2008. Credit River Stormwater Management Manual. Mississauga, Ontario

Northern Virginia Regional Commission. 2007. Low Impact Development Supplement to the Northern Virginia BMP Handbook. Fairfax, Virginia

Saxton, K.E., W.J. Rawls, J.S. Romberger, and R.I. Papendick. 1986. Estimating generalized soil-water characteristics from texture. Soil Sci. Soc. Am. J. 50(4):1031-1036

Schueler, T., D. Hirschman, M. Novotney and J. Zielinski. 2007. Urban stormwater retrofit practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD

APPENDIX B

A-10. ADDITIONAL DETAILS AND SCHEMATICS FOR REGULAR BIORETENTION