DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 9

 

BIORETENTION

Version 1.15

 

Note to Reviewers of the Stormwater Design Specifications
 
The Virginia Department of Conservation and Recreation (DCR) has developed an updated set of non-proprietary BMP standards and specifications for use in complying with the Virginia Stormwater Management Law and Regulations.  These standards and specifications were developed with assistance from the Chesapeake Stormwater Network (CSN), Center for Watershed Protection (CWP), Northern Virginia Regional Commission (NVRC), and the Engineers and Surveyors Institute (ESI) of Northern Virginia.  These standards and specifications are based on both the traditional BMPs and Low Impact Development (LID) practices.  The advancements in these standards and specifications are a result of extensive reviews of BMP research studies incorporated into the CWP's National Pollution Removal Performance Database (NPRPD).  In addition, we have borrowed from BMP standards and specifications from other states and research universities in the region. Table 1 describes the overall organization and status of the proposed design specifications under development by DCR.
 
Table 1: Organization and Status of Proposed DCR Stormwater Design Specifications:
Status as of 9/24/2008
#
Practice
Notes
Status 1
1
Rooftop Disconnection
Includes front-yard bioretention
2
2
Filter Strips
Includes grass and conservation filter strips 
2
3
Grass Channels
 
2
4
Soil Compost Amendments
 
3
5
Green Roofs 
 
1
6
Rain Tanks
Includes cisterns
2
7
Permeable Pavement
 
1
8
Infiltration
Includes micro- small scale and conventional infiltration techniques 
2
9
Bioretention
Includes urban bioretention
3
10
Dry Swales
 
2
11
OPEN
 
 
12
Filtering Practices
 
2
13
Constructed Wetlands
Includes wet swales
2
14
Wet Ponds
 
2
15
ED Ponds
 
2
1 Codes: 1= partial draft of design spec; 2 = complete draft of design spec; 
3 = Design specification has undergone one round of external peer review as of 9/24/08
 
Reviewers should be aware that these draft standards and specifications are just the beginning of the process.  Over the coming months, they will be extensively peer-reviewed to develop standards and specifications that can boost performance, increase longevity, reduce the maintenance burden, create attractive amenities, and drive down the unit cost of the treatment provided.
 
Timeline for review and adoption of specifications and Role of the Virginia’s Stormwater BMP Clearinghouse Committee:
 
The CSN will be soliciting input and comment on each standard and specification until the end of 2008 from the research, design and plan review community.  This feedback will ensure that these design standards strike the right balance between prescription and flexibility, and that they work effectively in each physiographic region.  The collective feedback will be presented to the BMP Clearinghouse Committee to help complement their review efforts.  The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR.
 
The revisions to the Virginia Stormwater Management Regulations are not expected to become finalized until late 2009.  The DCR intends that these stormwater BMP standards and specifications will be finalized by the time the regulations become final.
 
The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR, which is vested by the Code of Virginia with the authority to determine what practices are acceptable for use in Virginia to manage stormwater runoff.
 
As with any draft, there are several key caveats, as outlined below:
 
·         Many of the proposed design standards and specifications lack graphics.  Graphics will be produced in the coming months, and some of graphics will be imported from the DCR 1999 Virginia Stormwater Management (SWM) Handbook. The design graphics shown in this current version are meant to be illustrative. Where there are differences between the schematic and the text, the text should be considered the recommended approach.
 
·        There are some inconsistencies in the material specifications for stone, pea gravel and filter fabric between ASTM, VDOT and the DCR 1999 SWM Handbook.  These inconsistencies will be rectified in subsequent versions.
 
·        While the DCR 1999 SWM Handbook was used as the initial foundation for these draft standards and specifications, additional side-by-side comparison will be conducted to ensure continuity.
 
·        Other inconsistencies may exist regarding the specified setbacks from buildings, roads, septic systems, water supply wells and public infrastructure.  These setbacks can be extremely important, and local plan reviewers should provide input to ensure that they strike the appropriate balance between risk aversion and practice feasibility.
 

These practice specifications will be posted in Wikipedia fashion for comment on the Chesapeake Stormwater Network’s web site at http://www.chesapeakestormwater.net, with instructions regarding how to submit comments, answers to remaining questions about the practice, useful graphics, etc.  DCR requests that you provide an email copy of your comments, etc., to Scott Crafton (scott.crafton@dcr.virginia.gov).  The final version will provide appropriate credit and attribution on the sources from which photos, schematics, figures, and text were derived.

 
Thank you for your help in producing the best stormwater design specifications for the Commonwealth.

 

 

DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 9

 

BIORETENTION

Version 1.15

 

 

 

SECTION 1: DESCRIPTION OF PRACTICE

 

Individual bioretention areas serve highly impervious drainage areas less than five 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.  The filter is composed of a sand/soil bed with a surface layer of mulch.  During storms, runoff temporarily ponds 6-12 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 jacket installed along the bottom of the filter bed.

 

Bioretention can also be designed to infiltrate runoff into native soils.  This can be done at sites with highly permeable soils, a low groundwater table, and a low risk of groundwater contamination.  This type of 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 or are eliminated altogether, thereby increasing stormwater infiltration.  Bioretention is also known as a “rain garden” when used on individual residential lots, often without an underdrain.

 

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 bioretention are summarized in Table 1.

 

SECTION 2:  PERFORMANCE CRITERIA

 

Table 1:  Summary of Stormwater Functions Provided by Bioretention Areas

Stormwater Function

Level 1 Design

Level 2 Design

Annual Runoff Reduction

40%

80%

Phosphorus Removal 1

25%

50%

Nitrogen Removal 1

40%

60%

Channel Protection

Moderate.

RRv can be subtracted from CPv

Flood Mitigation

Partial.

Reduced Curve Numbers and Time of Concentration

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 and will be higher than these percentages, as calculated using the Runoff Reduction Spreadsheet Methodology.

Sources: CSN (2008) and CWP (2007).

 

SECTION 3:  PRACTICE APPLICATIONS AND FEASIBILITY

 

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:  Not every open area will be a good candidate for bioretention. Optimally, designers should aim for open areas that are at least 5-10% (3-5%?) of the contributing drainage area.  (NOTE:  The size will vary based on the imperviousness of the CDA.  5-10% works if the CDA is ~100% impervious (based on calcs in section 6 below).  The size is smaller if the IC of the CDA is lower.  Since we will have sizing calcs below, it is probably not necessary to have this general rule here – DCR may want to take this out here, and stick to the equations below.)

 

·        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 bioretention practice discharges (i.e., the bottom elevation needed to tie the underdrain from the bioretention area into the storm drain system).  In general, 4-5 feet of elevation above this invert is needed to create the hydraulic head necessary 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 groundwater table.

 

·        Utilities: Designers should ensure that future tree canopy growth within 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 the Infiltration 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 areas can range from 250 to 200,000 sq. ft. of impervious cover.  Three scales of bioretention are defined in this specification.  These include micro-bioretention (250 to 2500 sf), small-scale bioretention (2500 to 20,000 sf) and bioretention basins (20,000 to 200,000 sf).  Each of these has different design requirements.  The maximum recommended drainage area to a single bioretention cell is five acres. Ideally, the contributing drainage area to a bioretention practice should be as close to 100% impervious as site constraints will allow.

 

·        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.

 

o       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, chlorinated washwater, or other flows.

 

·        Setbacks:  Bioretention areas should not be hydraulically connected to structure foundations or pavement, in order to avoid the risk of seepage.  Setbacks to structures and roads vary based on the scale of bioretention (see Table 2).  At a minimum, small-scale bioretention areas and bioretention basins should be located a minimum horizontal distance of 100 feet from any water supply well, 50 feet from septic systems, and at least 5 feet from down-gradient wet utility lines.  Dry utilities such as gas, electric, cable and telephone lines may cross bioretention areas if they are double-cased.

 

SECTION 4:  COMMUNITY AND ENVIRONMENTAL CONSIDERATIONS

 

Not applicable.  (NOTE:  There are probably some community-related issues and concerns that could be noted here.)

 

SECTION 5:  DESIGN APPLICATIONS AND VARIATIONS

 

The most important design factor to consider when applying bioretention to development sites is the scale at which it will be applied:  micro-bioretention, small scale infiltration or bioretention basins.  Table 2 compares the different design approaches and requirements associated with each scale of bioretention.

 

Table 2:  The Three Design Scales for Bioretention Practices

Design Factor

Micro Bioretention

(Rain Garden) 1

Small-Scale Bioretention 1

Bioretention

Basins

Impervious Area Treated

250 to 2500 sf

2500 to 20,000 sf

20,000 to 200,000 sf

Type of Inflow

Sheetflow or roof leader

Shallow concentrated flow

Concentrated flow

Runoff Reduction Sizing

Minimum 0.1 inches over CDA

Minimum 0.3 inches over CDA

Remaining Tv up to the full Cpv

Minimum Measured Soil Infiltration Rate

0.5 inches/hour

1.0 inches/hour

1.0 inches/hour

Observation Well/ Cleanout Pipes

No

No

Yes

Type of Pretreatment

External (leaf screens, etc)

Filter Strip or Grass Channel

Pretreatment Cell

Recommended Max. Filter Depth

 

Max. 3 foot Depth

 

Max. 5 foot Depth

 

Max. 6 foot Depth (check OSHA requirements)

Media Source

Mixed on site

Obtained from approved vendor

Hydraulic Head

Required

Nominal:

1 to 3 feet

Moderate:

1 to 5 feet

Moderate:

2 to 6 feet

Required Soil

Borings 2

One per practice

Two per practice

One per 500 sq. ft. of proposed bioretention area

Building Setbacks

5 feet down-gradient;

25 feet up-gradient

10 feet down-gradient;

50 feet up-gradient

25 feet down-gradient;

100 feet up-gradient

1 The body of this specification does not address special bioretention areas used in highly urban areas such as expanded tree pits, curb extensions and foundation planters for which customized specifications will be crafted later in 2008.  For information on those variations of Bioretention, see Appendix A of this specification.

2  Soil infiltration tests are only needed when an underdrain will not be used.

 

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.  Typical locations for bioretention include the following:

 

·        Parking lot islands:  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 the depressed island landscaping area.  Curb cuts can also be used for this purpose, but they are more prone to clogging and erosion.

 

·        Parking lot edges:  Parking lots can be graded so that flows reach a curb-less edge or curb cut before reaching catch basins or storm sewer 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 a 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 employed to receive sheet flow from the roadway, a grass channel, or pipe that conveys flow to the bioretention practice.

 

·        Courtyards:  Runoff collected in a storm drain system or roof leaders can be directed to courtyards or other pervious areas on the site.

 

·        Individual residential lots:  Roof leaders can be directed to small bioretention areas, often called “rain gardens,” located in a drainage easement at the front, side, or rear of a home.  For smaller lots, the front yard bioretention corridor design is recommended (see Box 1).

 

·        Unused pervious areas on a site:  Storm flows can be redirected from a storm drainpipe to discharge into a bioretention area (see Box 1).

 

·        Bottom of dry ED pond:  In this configuration, a bioretention cell can be located in the bottom of a dry extended detention pond, after the sediment forebay, in order to boost treatment.

 

·        Retrofitting: A wide range of options are available to retrofit bioretention in urban landscapes, as described in Profile Sheet ST-4 of Schueler et al (2007).

 

 

 

 

 

 

 

Box 1:  Front Yard Bioretention Corridor Design for Small Residential Lots

This special form of bioretention captures rooftop, lawn, and driveway runoff from low- to medium-density residential lots in a slightly depressed area between the home and the street.  The bottom of the bioretention area then connects by an underdrain to the main storm drain pipe located underneath the street.  The concept is to take advantage of the drop from the roof leader to the street storm drainpipe, by creating a 10-foot wide bioretention corridor from roof to the street.  The minimum effective length of the bioretention corridor is 20 feet long. The bioretention corridor is subtly graded to create a shallow 6-12 inch deep ponding area between the roof leader and edge of sidewalk or road.  The ponding area may have a turf or landscape cover, depending on homeowner preference.

 

The bioretention media is approximately 3 feet deep and is over placed a 12-24 inch deep stone reservoir.  A perforated underdrain is placed above the stone reservoir to promote storage and recharge, even on poorly draining soils.  In highly urban settings, the underdrain is directly connected to the major storm drain pipe running underneath the street or in the street right-of- way.  A trench needs to be excavated during construction to connect the underdrain to the street storm drain system, but the construction of the remainder of the front yard bioretention system is deferred until after the lot has been stabilized.  The front yard bioretention design should reduce the risk of homeowner conversion, because it allows a choice of whether they want to cover the area with turf or landscaping.  Front yard bioretention requires regular mowing and/or landscape maintenance to perform effectively.  It should be located in an expanded right-of-way or stormwater easement so that it can be accessed for maintenance in the event that it fails to drain properly.

 

 

SECTION 6:  SIZING AND TESTING GUIDANCE

 

6.1:  Overall Sizing

 

Sizing for bioretention practices is based on three simple equations based on the required Treatment Volume (Tv).

 

For Bioretention Areas with underdrains (Level 1)

 

(1) Tv = 1.0 ft * SA of bioretention area (sq. ft.)

 

For Bioretention Areas without underdrains (Level 2)

 

(2)  Tv = 1.4 ft * SA of bioretention area (sq. ft.)

 

where

Tv = is required Treatment Volume in cubic feet             

SA= surface area of bioretention area in square feet     

 

 

 

(NOTE:  The equations above should be rewritten to solve for SA of bioretention area (i.e. SA = Tv/1.4)  The level 1 and level 2 Tv equations below are correct - you'd first calculate a Tv and then use this number to solve to the bioretention SA.  Since level 2 bioretention areas capture a greater Tv, the size would be larger than a level 1 design.)

 

6.2:  Soil Infiltration Rate Testing

 

In order to determine if an underdrain will be needed, one must measure the infiltration rate of subsoils below the bioretention area.  The infiltration rate of subsoils must exceed 1 in./hr. in order to be allowed to dispense with an underdrain. On-site soil infiltration rate testing procedures are outlined in Appendix A of the Infiltration Specification (No. 8).  A minimum of one test shall be taken per 2500 sq. ft. of planned bioretention surface area.  Soil testing is not necessary for micro-bioretention areas where an underdrain is needed.  The same is true for small-scale bioretention, except that at least one soil boring must be taken to confirm underlying soil properties (e.g., depth to water table, depth to bedrock, active karst, etc.).

 

SECTION 7:  DESIGN CRITERIA   

 

7.1:  Level 1 and 2 Bioretention Design Guidelines

 

The major design goal for bioretention in Virginia 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.  To qualify for Level 2, the bioretention area must meet six of the seven design criteria shown in the right hand column of Table 3.  If soil conditions require an underdrain, bioretention areas can still qualify for the Level 2 design if they contain an underground stone storage layer.

 

Table 3:  Bioretention Design Guidelines

Level 1 Design (RR 40 TP: 25 )

Level 2 Design (RR: 80 TP:  50)

Tv= (1.0)(Rv)(A) / 12

Tv = (1.25)(Rv)(A) / 12

SA of filter bed exceeds 3% of CDA

SA of filter bed exceeds 5% of CDA

Filter media at least 24 inches deep

Filter media at least 36inches deep

One form of accepted pretreatment

Two or more forms of accepted pretreatment

At least 75% plant cover w/ mulch

At least 90% plant cover, including trees.

One cell design

Two cell design

Underdrain needed

Infiltration design or stone storage layer

All Designs: Media mix tested for an acceptable phosphorus index

 

7.2:  Pretreatment

 

Pretreatment of runoff entering bioretention areas is required in order 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:

 

MicroBioretention and Small-Scale Bioretention

 

·        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 Diaphragm (sheet flow): A gravel diaphragm at the end of pavement should run perpendicular to the flow path to promote settling.

 

·        Pea Gravel Flow Spreader (sheet flow): Pea gravel extends along the top of each bank to pretreat lateral runoff moving from the road shoulder to the swale. It calls for a 2 to 4 inch drop from a hard-edged surface into a gravel or stone diaphragm.

 

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 has a storage volume equivalent to at least 15% of the total treatment volume.  It should be designed with a 2:1 length-to-width ratio.  The cell may be constructed from a wooden or stone check dam, or earthen or rock berm.

 

·        Hydrodynamic Structure:  If a pipe inlet is used, a proprietary hydrodynamic structure with demonstrated capability of reducing sediment and hydrocarbons may be used to provide pretreatment.

 

7.3:  Conveyance and Overflow

 

For on-line designs, an overflow structure should always be incorporated to safely convey larger storms through the bioretention area.

 

·        The overflow associated with the 2- and 10-year design storms should be controlled so that velocities are non-erosive at the outlet point and to prevent downstream erosion.

 

·        Common overflow systems within the bioretention practice consist of a yard drain inlet, where the top of the yard drain inlet is placed at the maximum water surface elevation of the bioretention area.  Typically this is 6-12 inches above the surface of the filter bed.

 

·        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 yard inlet.

 

·         The filter bed surface should generally be flat, so the bioretention area fills up like a bathtub.

 

Off-line bioretention designs are often a good option.  In these cases, a flow splitter can be used at the inlet to allow only the Treatment Volume to enter the facility.  This may be achieved with a pipe, weir, or curb opening, sized for the target flow, in conjunction with a bypass channel so that higher flows go around rather than pass over the surface of the filter bed. Using a weir or curb opening prevents clogging and reduces the maintenance frequency.

 

7.4:  Filter Media and Surface Cover

 

There are several key filter media and surface cover factors to take into account:

 

Depth:  The recommended minimum filter bed depth ranges between 30 to 48 inches. However, in constrained applications, pollutant removal benefits may be achieved in filter beds as shallow as 18 to 24 inches.  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 to support the root structure of mature trees.  Use turf, perennials or shrubs instead of trees when landscaping shallower filter beds.

 

General Filter Media Composition:  The recommended bioretention soil mixture is 85-88% sand, 8-12% soil fines, and 3-5 % organic matter.  If nitrogen removal is the goal, it may be advisable to increase the percentage of soil fines.  While media can mixed on-site for micro-bioretention, all other designs should use certified media from an approved vendor that meet the following requirements for phosphorus content, cation exchange capacity (CEC), and media infiltration rate:

 

·        The P-index value should be less than 30.

 

·        Soils with CECs exceeding 10 are preferred for pollutant removal.

 

·        The media should have an infiltration rate of 1-2 inches per hour.

 

Filter Media for Tree Planting Areas:  Within the planting holes for trees, a more organic filter media is recommended.  It should have a ratio of 50% sand, 30% topsoil and 20% acceptable leaf compost.  Vendors supplying this mix should ensure the P-index for topsoil does not exceed 30.

 

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 does not float away.

 

Alternatives to Mulch Cover:  In some situations, designers may consider alternative surface covers such as turf, native groundcover, river stone, or pea gravel.  The decision on what type of surface cover to use should be based on function, cost, and maintenance.  Stone and gravel are not recommended in parking lot applications due to the higher temperatures that result and 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.

 

Underground Storage Layer:  To promote greater runoff reduction for bioretention areas using underdrains, a 12-18 inch deep storage layer may be located below the invert of the underdrain.  The storage layer may consist of 2-3 inch open-graded stone or an approved infiltration module.

 

Observation Wells and Cleanout Pipes:  Bioretention basins should include observation wells and cleanout pipes if their contributing drainage area exceeds 1 acre.  The wells should be tied into any T’s or Y’s in the underdrain system, should extend upwards to be flush with the surface, and should include a vented cap.

 

7.5:  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, and a planting plan that includes the size, stock and sources of plant species to be used.  In addition, the landscaping plan should stipulate the planting sequence, including post nursery care and initial maintenance.  It is highly recommended that a qualified landscape architect prepare the planting plan.

 

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.

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)

Serviceberry

(Amelanchier canadensis)

Pin Oak

(Quercus palustris)

Switchgrass

(Panicum virgatum)

Redbud

(Cercis candensis)

Willow Oak

(Quercus phellos)

Cardinal Flower

(Lobelia cardinalis)

Sweet Pepperbush

(Clethra ainifolia)

Sweetgum

(Liquidambar styraciflua)

Common Three Square

(Scirpus americanus)

Wax Myrtle

(Myrica cerifera)

Black Willow

(Salix nigra)

Sensitive Fern

(Onoclea sensibilis)

Virginia Sweetspire

(Itea virginica)

Grey Birch

(Betula populifolia)

Blue Flag

(Iris versicolor)

Shadbush

(Amelanchier candadensis)

Black Gum

(Nyassa sylvatica)

Woolgrass

(Scirpus cyperninus)

Swamp Azeala

(Azeala viscosum)

Sycamore

(Platanus occidentalis)

Indian Grass

(Sorghastrum nutans)

Button Bush

(Cephalanthus occidentalis)

Green Ash

(Fraxinus pennsylvanica

Marsh Marigold

(Caltha palustris)

Box Elder

(Acer negundo)

Sweetbay Magnolia*

(Magnolia virginiana)

 Joe Pye Weed

(Eupatorium purpureum)

Fringe Tree

(Chionanthus virginicus)

Atlantic White Cedar*

(Charnaecyparis thyoides)

Turk's cap lily

(Lilium superbum)

Black Haw

(Virburnum prunifolium))

Bald Cypress*

(Taxodium distichum)

Bee Balm

(Mornarda didyma)

Indigo Bush

(Amorpha fruticosa)

Grey Dogwood

(Cornus racernosa)

Northern Sea Oats

(Chasmanthium latifolium)

Arrowwood

(Virburum dentatum)

Smooth Alder

(Alnus serrulata))

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 George County, VA

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 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:

 

1.      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).

 

2.   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.

 

3.   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 1-inch caliper and a minimum height of 30 inches.

 

4.   Tree, shrub and herbaceous plants:  This is the traditional landscaping option for bioretention.  It produces the most natural effect and is highly recommended for small-scale bioretention and bioretention basin applications.  The landscaping goal is to simulate the structure and function of native forest plant community.

 

5.   Turf and tree:  This option is a lower maintenance version of Option 4, where turf cover replaces the mulch layer.  Trees are planted within larger mulched islands to prevent damage during mowing operations.

 

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 owner’s capability to maintain the landscape.

 

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. 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 3 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 Specification No. X).

 

·        Designers should remember that filter beds need to be at least 4 feet deep to provide enough soil volume for the root structure of mature trees.

 

·        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 compared to a site that will receive minimum annual maintenance.

 

·        If the bioretention area is to be used for snow storage or will accept snowmelt runoff, it should be planted with salt-tolerant, herbaceous perennials.

 

SECTION 8:  REGIONAL AND CLIMATE ADAPTATIONS

 

11.1:  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 like ponds and wetlands, Level 2 bioretention designs such as 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 of three feet, possess an impermeable bottom liner and an underdrain should work well.  In general, micro-bioretention and small-scale bioretention are preferred over basin bioretention to prevent possible sinkhole formation, although it may be advisable to increase standard setbacks to buildings.

 

11.2:  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 can be 18-24 inches.  It is useful to limit surface ponding to 6to 9inches, and save additional depth by shift to a turf cover rather than mulch.

 

·        The minimum depth to the seasonally high water table can be 1 foot, as long as the bioretention area is equipped with a 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 applications.

 

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.

 

11.3:  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 and be armored with river stone or suitable equivalent.

 

 

 

11.4 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 cannot 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.

 

11.5:  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 9:  TYPICAL GRAPHIC DETAILS

Figure 1.  Bioretention Detail

Figure 2.  Bioretention Section

 

SECTION 10:  MATERIAL SPECIFICATIONS

 

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 less than 30

CECs greater than 10

The media should have an infiltration rate of 1-2 in/hr

Procured from approved filter media vendors

Mulch Layer

Aged, shredded hardwood bark mulch

A 2-3 inch layer on the surface of the filter bed.

Alternative

Surface Cover

A 2-3 inch layer of river stone or pea gravel to suppress weed growth, or turf cover

Topsoil

For Turf Cover

3 inch surface depth of loamy sand or sandy loam texture, with less than 5% clay content, corrected pH to 6-7, and organic matter content of at least 2%

Filter Fabric

Non-woven geotextile fabric with flow rate of >110 gallons/minute/sq. ft. (e.g., Geotex 351 or equivalent) 

Apply to sides only; use on bed ONLY in hotspot or karst areas!

Choking Layer

2-4 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.

 

6-8 inches for underdrain;

12-18 inches for stone storage layer, if needed.

Underdrain

6-inch rigid schedule 40 PVC pipe, with 3/8-inch perforations at 6-inches on center, with each underdrain on a 1-2% slope located no 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 the Ts and Ys.

 

SECTION 11:  CONSTRUCTION SEQUENCE AND INSPECTION

 

11.1:  Construction Sequence

 

Construction Stage E&S 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 on the E&S control plan stating that their maximum excavation depth at the construction stage must be 2 feet less than 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.

 

Bioretention Installation

 

The following is a typical construction sequence to properly install a small-scale bioretention or bioretention basin practice. 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 the entire contributing drainage area has been stabilized with vegetation. The proposed site should be checked for existing utilities prior to any excavation.

 

Step 2:  The designer and the installer should conduct a pre-construction 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, which can produce hydraulically important differences for the proposed bioretention area.  The designer should clearly communicate any project changes needed during the pre-construction meeting.

 

Step 3:  Temporary E&S 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 eroding 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 compact the footprint of the bioretention area.  Contractors should utilize a cell construction approach in larger bioretention basins, whereby the basin is split into 500-1000 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 6-12 inches to promote greater infiltration.

 

Figure 3.  Bioretention Construction Sequence

 

Step 7:  Place the filter fabric on the sides of the bioretention area with a 6-inch overlap on the sides.  Place 6 inches of stone on the bed (for the infiltration chamber) and then lay the perforated pipe.  Pack #57 stone to 3 inches above the top of the underdrain, and then add 3 inches of pea gravel as a filter.

 

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 the 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 the design).

 

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 11.2), and log in the GPS coordinates for each facility in the local maintenance tracking database.

 

11.2:  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 CWP website at

 

http://www.cwp.org/Resource_Library/Center_Docs/SW/pcguidance/Tool6.pdf

 

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

 

·        Drainage areas 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 6-12 hours.

 

·        Ensure that caps are placed on the upstream, 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 inlets and outfalls are stable and non-erosive during storms.

 

The real test for bioretention is its first big storm.  The post-storm inspection should focus on whether the desired sheet flow, shallow concentrated flow, or concentrated flow conditions assumed in the plan are realized in the field.  Also, inspectors should check that the bioretention area’s drains draw down within a minimum 6 hours.  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 12:  OPERATION AND MAINTENANCE

 

12.1:  Maintenance Agreements

 

A legally binding and enforceable maintenance agreement and stormwater easement should be executed between the practice owner and the local program authority.  The agreement should specify the local agency’s right of entry and guarantee of adequate access for inspection, maintenance, and landscaping upkeep for the infiltration practice(s).  The maintenance agreement must contain recommended maintenance tasks and a copy of an annual inspection checklist.  When micro- or small-scale bioretention practices are applied on private residential lots, homeowners will need to be educated regarding their routine maintenance needs.  The existence and purpose of the bioretention area should be noted on the deed of record that is transferable to new owners upon sale.  In addition, the GPS coordinates for all bioretention practices should be provided upon facility acceptance to ensure long-term tracking.

 

12.2:  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 1/2-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 necessary for initial plantings.

 

Watering:  Watering is needed once a week during the first two months, and then as needed during the 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.

 

12.3:  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 CWP website at

 

http://www.cwp.org/Resource_Library/Center_Docs/SW/pcguidance/Tool6.pdf

 

Table 7:  Suggested Spring Maintenance Inspections for Bioretention

Activity

  • Check to see if 90% mulch and 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. 
  • Check for any winter-killed or salt-killed vegetation and replace with hardier species.
  • Note the presence of accumulated sand, sediment and trash in pretreatment cells or filter beds, and remove it.
  • Inspect bioretention side slopes and grass filter strips for evidence of any rill or gully erosion, and repair as needed. 
  • Check bioretention beds 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 immediately.

 

12.4:  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 the surface cover.  A generalized summary of common maintenance tasks and their frequency is provided in Table 8.

 

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

 

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 or underdrain repairs may be needed.  The surface of the filter bed should also be checked for accumulated sediment.  Core aeration or deep tilling may relieve the problem.

 

13:  ADDITIONAL DETAILS AND SCHEMATICS

 

 

                                    Figure 4.  4-inch PVC Cleanout Detail

 

 

 

 

 

 

 

 

 

 

 

Figure 5.  Typical Biofilter Planting Specifications

 

 

 

 

 

 

Figure 6.  Level Spreader Detail

 

            Figure 7.  Typical Off-Line Bioretention Practice Schematic

 

 

14:  REFERENCES

 

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)

 

Low Impact Development Technical Guidance Manual for Puget Sound, WA http://www.psat.wa.gov/Publications/LID_tech_manual05/lid_index.htm

 

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

 

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 OF PRACTICE

 

Special bioretention practices are similar to regular micro-bioretention practices, except they are adapted to fit into “containers” within urban landscapes.  Typically, urban bioretention is installed within a road or its right-of-way, landscaping beds, tree pits and plazas.  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 1. 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 1).  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.

 

 

Figure A-1. Stormwater Planters

 

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 2).  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.

 

Figure A-2. Expanded Tree Pits

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 3).  

 

 

 

 

 

Figure A-3.  Stormwater Curb Extensions

 

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

 

SECTION A-2: PERFORMANCE CRITERIA

 

Table A-1: Summary of Stormwater Functions Provided by Urban Bioretention Areas

Stormwater Function

Level 1 Design

Level 2 Design

Annual Runoff Reduction

40%

NA

Total Phosphorus Removal 1

25%

NA

Total Nitrogen Removal 1

40%

NA

Channel Protection

Moderate. RRv can be subtracted from CPv

Flood Mitigation

Partial. Reduced Curve Numbers and Time of Concentration

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 and will be higher than these percentages, as calculated using the Runoff Reduction Spreadsheet Methodology.

Sources: CSN (2008) and CWP (2007).

 

SECTION A-3:  PRACTICE APPLICATIONS AND FEASIBILITY

 

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 individual units typically serve no more than 2500 sq. ft. of impervious cover.  Multiple units can be installed to maximize treatment area in ultra-urban watersheds.

 

·        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, 3-5 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 telephone and power lines.

SECTIONA-4: COMMUNITY AND ENVIRONMENTAL CONSIDERATIONS

 

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 preventive 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 urban bioretention 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 A-5:  DESIGN APPLICATIONS AND VARIATIONS

 

The advantage of urban bioretention is its ability to accommodate stormwater treatment in a small footprint between the building and streets of the ultra-urban landscape.  The most effective locations for each design variant are described below.

 

·        Stormwater planters generally receive runoff from adjacent rooftop downspouts before they reach the street drainage network.  Planters can be located adjacent to buildings or within a landscaped portion of a plaza, courtyard, riverfront, or streetscape.  While each planter unit treats a small drainage area, they can treat a significant portion of site runoff if they are integrated within a larger streetscaping or urban landscaping project.  Stormwater planters require some degree of waterproofing and/or foundation drains to ensure that they do not negatively impact the structural integrity of the building.

 

·        Extended tree pits are typically located in the street right-of-way, but can also be used as a landscaping element in a plaza, courtyard, or riverfront project.

 

·        Curb extensions are located at grade in the public street right-of-way and intercept runoff generated on nearby impervious surfaces, such as streets and parking lots.

 

SECTION A-6:  SIZING AND TESTING GUIDANCE

 

The required surface area of the urban bioretention box filter is based on the volume of water to be treated and the available storage in the ponding area, computed as shown in Equation 1.  Designers may not be able to treat the entire Treatment Volume (Tv), but it is acceptable to design urban bioretention areas with to accept 20% of the Tv.  The drain time through the filter is based on the volume of water to be treated and the hydraulic properties of the soil media in accordance with Darcy’s Law, computed as shown in Equation 2.  The water quality volume must drain through the filter section in a maximum of 24 hours.  To determine drain time through the filter (Equation 2), assume that the rainfall event has ended and the ponding depth is at maximum elevation prior to the initiation of drawdown.

 

Equation 1.  Filter Surface Area for Vegetative Box Filter

Af  =  Tv/hf

 

where:

Af     = the area of filter (sq. ft.)

Tv    = the treatment volume (cu. ft.)

hf      = the maximum ponding depth (ft.)

 

Equation 2.  Drain Time for Vegetative Box Filter

tf  = (Tv)(df) / [(kf )(0.5hf+df)Af]

 

where:

tf       = drain time through the filter (days)

df      = depth of the filter (ft.)

kf      = coefficient of permeability (ft./day)1

Af     = area of the filter (sq. ft.)

Tv = treatment volume (cu. ft.)

hf      = maximum ponding depth (ft.)

 

1A coefficient of permeability of 3.0 ft./day for the saturated soil media should be used for sizing calculations. 

 

SECTION A-7: DESIGN CRITERIA

 

7.1:  Level 1 and Level 2 Design

 

In general, urban bioretention practices does not qualify for enhanced Level 2 runoff reduction and nutrient removal rates, since they are usually connected to the storm drain system with an underdrain.

 

 

7.2:  General Design Issues for Urban Bioretention

 

Design of urban bioretention should follow the general guidance presented in the main part of this Design Specification (No. 9).  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 practices must be removable to allow maintenance access.

 

·        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 drops.
  • Curb cuts into the bioretention practice.
  • Covered drains that convey flows across sidewalks from the curb or downspouts.
  • Grates or trench drains that capture runoff from the sidewalk or plaza.

The inlet(s) to urban bioretention should be stabilized using VDOT #3 stone, splash block, river stone or other acceptable energy dissipation measures.

 

·        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.
  • A 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.
  • A 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:

  • Sizing curb openings to capture only the water quality volume any bypassing higher flows through the existing gutter.
  • Using landscaping type inlets or standpipes with trash guards as overflow devices.
  • Using a pretreatment chamber with a weir design that limits flow to the filter bed area.

 

7.3:  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.

 

7.4:  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 pit 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 cu. ft. of shared root space.

 

7.5:  Specific Design Issues for Stormwater Curb Extensions 

 

Roadway stability can be a design issue with stormwater curb extensions.  Design standards for 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.

 

7.6:  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-8:  REGIONAL AND CLIMATE DESIGN ADAPTATIONS

 

Not applicable.

 

SECTION A-9: TYPICAL GRAPHIC DETAILS

 

 

Figure A-4.  Stormwater Planter Section

Portland, Oregon has thorough construction details for stormwater curb extensions and expanded tree pits.  These include details for addressing residential utility connections.  See the following web site:

 

http://www.portlandonline.com/bes/index.cfm?c=44213&

 

SECTION A-10:  MATERIAL SPECIFICATIONS 

 

Please consult the main part of this Design Specification (No. 9) for the typical materials 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 VDOT #57 stone (less than 1% passing a #200 sieve).

 

·        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 6 inches of the opposite wall, and may either be centered in the box or offset to one side.

 

·         Be separated from the soil media by geotextile fabric or a 2-3 inch layer of washed VDOT #8 stone or 1/8-3/8 inch pea gravel.

 

SECTION A-11:  CONSTRUCTION SEQUENCE AND INSPECTION

 

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 as outlined in the main part of this Design Specification (No. 9).  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 8-12 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-12: OPERATION AND 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 (No. 9).

 

SECTION A-13: REFERENCES

 

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

 

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