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Bioretention Cells Often Result in Cost Savings by Decreasing the Need for Traditional Stormwater Structures, Such As Inlets and Pipes

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Bioretention Cells Often Result in Cost Savings by Decreasing the Need for Traditional Stormwater Structures, Such As Inlets and Pipes BBIIOORREETTEENNTTIIOONN Bioretention is a low-impact development (LID) technique that allows for the effective management of stormwater on-site by using a combination of plants and layers of soil, sand, and mulch to reduce quantity and improve quality of stormwater. How does bioretention work? Stormwater Benefits Bioretention utilizes the chemical, biological, and physical properties of plants, microbes, and soils to remove pollutants 9 Runoff quantity control from stormwater runoff. 9 Runoff quality control 9 Pollutant removal In cases where bioretention is used, stormwater flows over 9 Groundwater recharge impervious surfaces and is conveyed as sheet flow down a vegetated slope, which slows the incoming runoff velocity and Additional Benefits provides initial filtration of particulates from the runoff. The runoff continues to a ponding area where it is filtered by 9 Attractive landscape plants, an organic or mulch layer, and amended soil. Many 9 Wildlife habitat natural and biological processes, such as adsorption, filtration, 9 Cost savings and decomposition, occur during filtration. These processes 9 Educational potential remove pollutants and improve water quality. The filtered runoff is then collected in an underdrain and returned to a conveyance system. Discovery Park Master Plan Discovery Park Master Source: Source: Natural Resources Conservation Service dapted from dapted A Parking lot incorporating bioretention area Notice how the bioretention area helps to make the parking lot an attractive landscape in addition to managing stormwater effectively. Why use bioretention? In addition to providing stormwater management benefits, such as runoff quantity control and pollutant removal, bioretention cells often result in cost savings by decreasing the need for traditional stormwater structures, such as inlets and pipes. Bioretention areas can also improve the aesthetics of areas such as parking lots or curbsides. (See pictures at right). CONTENTS Parking lot lacking bioretention areas Design Components Benefits Not only does this lot have an excessive amount of impervious cover and thus stormwater runoff, it is also Site Considerations Maintenance an unattractive space. Cost Estimate Additional Links DESIGN COMPONENTS Filter strip (optional) The filter strip, or pretreatment area, located between the contributing drainage area and the ponding area, reduces incoming runoff velocity and provides an initial filtration of particulates from the runoff. Grass buffer strips or vegetated swales are commonly used as pretreatment devices. Flow entrance In some cases, space limitations may make it Sound Puget for Manual Guidance Technical LID difficult to incorporate pretreatment areas Source: into the site. In such cases, alternative mechanisms may be used at the flow entrance Planting soil mix to reduce the velocity of runoff as it enters The planting soil mix in the bioretention area the bioretention area. For example, curb cuts acts as a filtration system, in which clay with energy dissipaters, such as landscape particles provide adsorption for hydrocarbons, stone, may be used. Such mechanisms are heavy metals, nutrients, and other pollutants. successful in slowing the velocity of incoming The soil mixture should be approximately 60- runoff but must be cleaned regularly to 75% sand, 25% silt or topsoil, and 10% organic prevent sediment and debris build-up. or leaf compost; clay content should be less than 5%. Depth of the planting soil varies Plant materials depending on the vegetation used, but often Both woody and herbaceous plants in the ranges from 1.5 to 4 feet. ponding area help to remove water through evapotranspiration and remove nutrients and Sand bed (optional) pollutants through plant uptake. Vegetation A 12- to 18-inch sand bed may be placed also serves to stabilize the surrounding soils. between the planting soil and the pea gravel Use of native species is highly recommended. diaphragm to further slow the runoff and help distribute it through the entire basin. The Ponding area sand must have less than 15% silt or clay The ponding area provides for temporary content. storage of stormwater runoff prior to evaporation, infiltration, or uptake, and Pea gravel diaphragm provides additional settling capacity of A pea gravel diaphragm is a layer of gravel particulates. The ponding area is typically located underneath the planting soil that three to four inches deep, with an eight-inch traps sediment and debris, thereby minimizing maximum depth. clogging of the perforated pipe underdrain. It is recommended that a three- to nine-inch Organic or mulch layer pea gravel diaphragm be placed below the The organic or mulch layer is placed on top of amended soil and above the drainage system. the planting soil and provides filtration, adsorption and bonding of heavy metals, as Filter fabric well as an environment conducive to the (In place of pea gravel diaphragm) growth of microorganisms that degrade Many older bioretention designs use geotextile hydrocarbons and organic material. Three filter fabric in place of a pea gravel inches of mature mulch are recommended. diaphragm to filter sediment and minimize clogging of the perforated pipe below. The filter fabric must meet a minimum hydraulic Page 2 conductivity rate of 75 gallons per minute per square foot and must not impede the infiltration rate of the soil medium. The filter What’s the difference between fabric may be positioned under the planting soil mix and along the walls of the rain gardens and bioretention bioretention area to direct flow downward areas? and to reduce underground lateral flow. The Bioretention areas and rain gardens often look similar filter fabric must be placed along the on the surface because they both use plants and ponding sidewalls when installing a facility in a median techniques to filter stormwater runoff. The terms are strip or parking lot landscape island to often used interchangeably; however, there are key prevent lateral flow of water under the differences between the two low-impact development techniques. pavement. Filter fabric 9 Bioretention areas are typically used on sites where there is extensive impervious surface; common (In addition to pea gravel diaphragm) applications include parking lot islands, roadside When a pea gravel diaphragm is incorporated swales, and some residential areas into bioretention design, a permeable filter 9 Rain gardens are used most often in residential or fabric must be placed beneath the pea gravel commercial landscapes where there are smaller drainage areas diaphragm and over the drainage system only 9 Because bioretention areas generally drain more above and two feet to either side of the underdrain. In this case, filter fabric is not surface area, an underdrain helps to prevent ponding by directing excess filtered water to a necessary along sidewalls. stormwater conveyance system; rain gardens receive less runoff and therefore do not require an Drainage system underdrain The drainage system consists of a perforated pipe underdrain in about one foot of pea gravel. It collects runoff that has filtered through the soil layers in the ponding area and returns the treated water to a conveyance system. The pipe itself is typically placed two to five inches from the bottom of the gravel to promote drainage and prevent standing water in the drain. Overflow outlet NRCS An overflow outlet must be provided in order : to convey larger storm flows to the Source downstream drainage system or stabilized Parking Lot Bioretention Area watercourse. Non-erosive velocities (0.5 foot Iowa per second) must be ensured at the outlet point. “Design Components” adapted from the Georgia Stormwater Management Manual Vol. 2; Prince George's County, Maryland, Low-Impact Development Design Strategies Manual; Bioretention.com BENEFITS Runoff Quantity Control plants and soil and is temporarily stored; this Bioretention areas can reduce runoff temporary storage reduces the immediate quantity by temporarily storing runoff volume load on the storm drain system and it water. Instead of being directed to a storm reduces the peak discharge rate. The actual drain system, runoff is directed to a reduction in volume varies depending on the bioretention area where it filters through the site and size of the bioretention area. Page 3 Pollutant Control Bioretention areas are able to significantly reduce pollutants in stormwater runoff when sized, designed, constructed, and maintained in accordance with recommended Our Lady of the Rosary specifications. Bioretention areas can remove Greenville, South Carolina an average of 80% of the total suspended solids, 60% of total phosphorus, 50% of total Our Lady of the Rosary, a nitrogen, and 80% of heavy metals, including local church/school complex, installed cadmium, copper, lead, and zinc. For a new 1.4-acre parking area on their site in 2004. additional information and data on pollutant This increased impervious surface area led to an removal capabilities, see the National increase in stormwater runoff volume that required nd Pollutant Removal Performance Database (2 installation of additional treatment structures. Edition) and the National Stormwater Best Concerns with a dry detention pond included in the Management Practices Database. initial site design led them to use a bioretention area for post-construction water quality control. Additional benefits Bioretention areas can be designed as Because the bioretention area was the first attractive landscape features for areas such to be designed
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