Green Stormwater Infrastructure
The City of Raleigh recently adopted text changes to the Unified Development Ordinance (UDO) that make it easier for the development community to include Green Infrastructure and Low Impact Development (GI/LID), also known as Green Stormwater Infrastructure (GSI), practices into land development that help reduce environmental impacts to streams and lakes.
What is GSI?
This leading stormwater management practice benefits land development projects by encouraging the use of both natural and constructed landscape features that capture, absorb, and store rainwater. These features remove pollution and reduce impacts from the volume of stormwater that otherwise would go directly into storm drains and local waterways. In addition to improving water quality, GSI allows developers to co-locate site features, such as adding stormwater treatment within building setback; making more efficient use of developed land area; and using landscaping with stormwater management practices.
GSI practices that can be used with land development include:
- Adding cisterns, permeable pavement, green roofs, and rain gardens into building and landscape features; and
- Disconnecting downspouts from storm drains and streets.
Advancing Land Development Practices
Bringing GSI practices to the forefront of land development conserves natural resources and reduces impacts to streams and other water bodies. GSI practices also can make more efficient use of a development site, lower construction costs, and increase property values.
4 Benefits of GSI
With the recent text changes to the City’s UDO, developers can enhance their development plans and make them more “green” by using GSI practices. Here are a few benefits:
- GSI practices can serve multiple purposes in a developer’s required landscape area. This decreases overall landscaping and stormwater management costs and does not require stormwater management to “compete” for a site’s available land area.
- Installing GSI in the right-of-way to treat stormwater runoff coming from streets can make more land area available for other uses, reduce infrastructure costs, and provide more design flexibility.
- There is more flexibility in a site’s development design that make it easier to use GSI practices.
- Developers can choose from more stormwater treatment options in Watershed Protection Overlay Districts rather than using only traditional practices, like wetponds. The new preferred options (including cisterns, permeable pavements, and rain gardens) reduce stormwater runoff volume, velocity, and pollution while lowering stormwater infrastructure costs.
Plantings may be used to meet both stormwater management and landscaping requirements while leaving more space for other uses. This can decrease costs of both requirements and conserve natural resources, like trees and local streams and lakes.
GSI features may be added to the following areas of a development site:
- Parking lots: Parking spaces may be reduced by one parking spot for each tree (12 inches in diameter or larger) that is preserved in the parking lot. This allows developers to spend less on paving costs, generates less stormwater runoff from impervious surfaces, and conserves the amount of trees on a development site. Also, GSI features may be located within parking lot islands if they are part of an approved stormwater management plan for the site. Shrub requirements for perimeter islands and shade trees for interior and median islands may be met using GSI practices.
- Protective yards: More flexibility is provided for selecting trees and shrubs for protective yards and transitional protective yards when using GSI practices that are part of an approved stormwater management plan.
- Drive-thru facilities: Instead of using only evergreen hedges, developers may now use a combination of plants and evergreen hedges within GSI practices that are part of an approved stormwater management plan.
New Stormwater Design Review Requirements
When using GSI practices, submitted stormwater design plans will need to meet these requirements:
- Developers can now use runoff volume matching to meet nitrogen export load requirements if they demonstrate that the amount of stormwater runoff coming off a site after development will be the same or less than before development (Section 20).
- Requirements for new development in the Falls and Swift Creek Watershed Protection Overlay Districts have changed (Section 24 and 25). See section below for more details.
GSI in the Right-of-Way
Developers who install GSI in the existing City right-of-way can be eligible for reimbursement. Reimbursement from the City may pay for the difference in cost between traditional right-of-way infrastructure development and GSI practices. After a project is complete, the City will own and maintain GSI practices located in the right-of-way.
Reimbursement will be determined on a case-by-case basis. To see what reimbursement you may be eligible for, please contact the Stormwater Management Division at 919-996-3940 or RaleighStormwater@raleighnc.gov.
Watershed Protection Overlay District Requirements
Changes and new requirements for the Falls and Swift Creek Watershed Protection Overlay Districts following the UDO text changes are:
Wetponds no longer are the only option to retain or detain stormwater when developing streets and residential lots that have 24 percent impervious surface areas or higher. This change allows developers to use stormwater management options that may be more effective in reducing water pollution and stormwater runoff volume.
Limits on additional impervious surface of a development site in a secondary water supply watershed protection area now include lots or portions of lots that:
- Were built prior to March 1, 1988 or were established outside the subdivision process after this date;
- Are or will be connected to both City water and sewer utilities; and
- Have or will have an impervious surface area of 24 percent or higher.
Additional impervious surface may not be added to a property resulting in “built area” that is more than 30 percent. “Built area” is a new term defined for this text change that represents development area made up of impervious surface (example: concrete, sidewalks, and roofs) and semi-pervious surface (example: semi-permeable pavements/pavers and green roofs) on a given lot.
The volume of stormwater runoff leaving the site after development cannot be more than the amount of runoff on the site before development, based on specified rainfall and pre-development land cover.
Design exceptions may be presented to the City for approval and should show that stormwater runoff from the site will be managed using traditional stormwater rate control requirements based on (but not limited to) the following:
- Low infiltration rates of native soils on the site;
- Shallow depth to seasonally high groundwater table on the site; and
- Shallow depth to bedrock on the site.