Using Green Stormwater BMPs in Urban Areas

Goals

The goal of using green stormwater BMPs is to replicate hydrologic cycle elements that have been lost in urban areas to meet stormwater management objectives. Interest is growing for using these BMPs for retrofits and for controlling combined sewer overflows (CSOs). By constructing small-scale, distributed stormwater management devices at individual sites, these stormwater BMPs have the capability to meet multiple stormwater management objectives, in a way that conventional approaches do not. Stormwater BMPs use unit processes of the hydrologic cycle, such as infiltration, detention, and evapotranspiration to meet these objectives. The ability to use multiple hydrologic and hydraulic processes allows the controls to be combined into a "treatment train" to meet targeted stormwater management objectives.

Objectives

While conventional end-of-pipe devices, such as detention systems, are typically designed to meet the single stormwater management objective of peak discharge attenuation, stormwater BMPs are designed to treat rain where it falls to address three criteria that are critical to managing urban stormwater runoff and CSOs.

  • Volume: Reduce or delay the volume of stormwater that enters the sewer system.
  • Peak Discharge: Reduce the maximum flow rate into the combined system by decreasing the stormwater volume and lengthening the duration of discharge. This inherently lowers the frequency of combined sewer overflows.
  • Water Quality: Improve water quality through volume reduction, filtering, and biological and chemical processes.

In addition to satisfying the three urban stormwater control requirements, stormwater BMPs also fulfill several other criteria.

  • Multiple Objectives: The BMP should not be one-dimensional (i.e., satisfy only a single stormwater management objective) but must at least partially meet more than one of the stormwater or other urban objectives.
  • Hydrologic Cycle Elements: The BMP's constituent processes should be identifiable as elements of the natural hydrologic cycle. The function of the BMP should be able to be described and quantified entirely in terms of those processes.
  • Treatment Train: The BMP should be able to be combined with other BMPs to form a stormwater treatment train. The development of a treatment train allows for enhanced water quality protections.
  • Site Integration: The BMP should be able to be integrated into other functional components of the site, such as the landscaping and infrastructure, without impeding or significantly affecting their function. Land that the BMP occupies should be able to serve purposes in addition to stormwater management, such as open space or parking.
  • Design Storm: The BMP should satisfy stormwater management objectives for small, frequently-occurring storms in order to reduce the incidence of CSOs and mitigate urban stormwater pollution.
  • Source Control: The BMP should be able to meet the stormwater management objectives for relatively small quantities of stormwater at or near the source, rather than at a centralized collection point. The BMP should be able to be used in different locations at a site, if necessary, to ensure distributed control of stormwater.

Siting

Stormwater BMPs treat stormwater near the source, and therefore require a significantly smaller footprint (and overall storage volume) than conventional controls. This small scale allows them to be integrated into many areas of a site (e.g., on a roof, in a parking lot, or in landscaped green space). When designed properly, stormwater BMPs blend in with the landscape and infrastructure; in some cases, such as a permeable parking lot, the BMP is the infrastructure. They often serve additional purposes besides stormwater management, such as beautification and recreational amenities. This approach to land use is well suited to the space constraints that exist in ultra-urban areas.

Stormwater BMPs can adapt to the physical constraints of the site, such as available open space, degree of soil compaction, and the vertical location of sewers, utilities, and the groundwater table. For instance, the depth of ponding in a swale can be reduced if the groundwater table is high, or if sewers are near the ground surface. With conventional controls, urban site constraints may severely limit the degree to which stormwater objectives can be met.

The adaptability of stormwater BMPs provides a great deal of flexibility with design and implementation, but land use will also define the applicability of individual BMPs for a site. However, from both a functional and maintenance point of view, some land uses are better suited for a particular BMP than others. For instance, green roofs are used frequently in dense, urban applications but rarely for single family residential settings. Other BMPs, such as permeable pavements, may have more universal applications.

Siting, therefore, is based upon satisfying the hydrologic or water quality objectives within the context of a particular land use. The following tables provide the stormwater BMPs most appropriate for urban use and their ability to satisfy the performance objectives of urban stormwater management and the hydrologic elements utilized by each BMP.

Stormwater BMP Land Use and Performance Objective Selection Matrix
Practices Criteria
Land Use Performance Objective
Schools Com./ Indust. SF Res. MF Res. Govt. Parks/ Open Space Volume Peak Flow Water Quality
Downspout Disconnection
Filter Strips
Infiltration Practices
Pocket Wetlands
Porous Pavement
Rain Barrels/ Cisterns
Rain Gardens
Soil Amendments
Tree Box Filters
Vegetated Roofs
Vegetated Swales
-  Well suited for land use applications or high effectiveness at meeting
   performance objective.
-  Average suitability for land use applications or average effectiveness
   at meeting performance objective.
-  Below average suitability (not commonly used) or low effectiveness at meeting performance objective.
Blank  -  Not applicable for land use.

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Use of Hydrologic Cycle Elements by BMPs
BMP Infiltration ETa Intb Conveyance Detention Retention Reuse*
Downspout Disconnection
Filter Strips
Infiltration Practices
Pocket Wetlands
Porous Pavement
Rain Barrel/ Cisterns
Rain Gardens
Soil Amendments
Tree Box Filters
Vegetated Roofs
Vegetated Swales
a Evapotranspiration
b Interception
* Collected water can be used for landscaping, non-potable building uses (e.g., toilets), or as raw water to be treated for drinking.
Key: : High Reliance     : Medium Reliance     : Low Reliance     Blank: N/A
Rankings are qualitative. "High reliance" means that the process is integral to the BMP's ability to meet stormwater management objectives, and that the BMP uses the process to its full potential in the urban environment. "Medium reliance" was assigned when a process is a secondary component of the BMP's operation, or when the BMP does not use the process to its full potential. "Low reliance" means that the process only marginally contributes to the BMP's ability to meet stormwater management objectives.

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Applications

Because they prevent, intercept, and/or treat stormwater near the source, stormwater BMPs can be customized to meet the stormwater management objectives of a specific site, at the site. Many CSO Long Term Control Plans (LTCPs) have proposed constructing massive storage/conveyance tunnels to intercept the overflows that would otherwise be released untreated into local water bodies. After the storm ends, the tunnels will gradually release the intercepted stormwater/wastewater to wastewater treatment plants (WWTPs). Stormwater BMPs can be used in conjunction with this approach, potentially reducing the infrastructure requirements of centralized facilities.

First, stormwater BMPs may collectively reduce the required tunnel storage volume by retaining and detaining stormwater at the source. Secondly, once LTCPs are implemented, all stormwater runoff from the CSO area will eventually pass through the WWTP, requiring the facility to process a variety of non-point source urban pollutants as well as trash and sediment. Stormwater BMPs that address water quality will augment the treatment provided at the WWTP and may improve biosolids quality by controlling urban pollutants at the source. A complementary system of stormwater BMPs will prevent stormwater and pollutants from ever entering the collection system.


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