Stormwater Control Measures (SCMs) are designed to remove pollutants from urban runoff, improve water quality and control quantity before the water reaches our streams and drinking water supply reservoirs.
Stormwater SCMs offer both "non-structural" and "structural" approaches to water quality protection. Non-structural measures include such practices as minimizing impervious area for site development, providing vegetative buffers along all streams and waterways, promoting natural infiltration of runoff, pollution prevention practices such as regular sweeping of parking lots and public environmental outreach programs.
Structural SCMs are permanent devices, which are designed, constructed and maintained to remove pollutants from runoff. While it is important to note that structural SCMs are only one part of a comprehensive watershed management plan, they play a critical role in protecting water quality in our receiving streams and lakes by removing or filtering out pollutants in runoff. Without these constructed devices, pollutants in urban runoff would directly enter the closest stream or lake, possibly impair downstream water quality or aquatic life and also degrade the quality of our drinking water reservoirs.
Many different kinds of SCMs can be installed, such as stormwater wetlands, bioretention cells, infiltration basins, dry detention areas or wet detention ponds.
Dry Detention Basin
Dry detention basins are primarily used to manage peak flow from new development projects by reducing peak runoff for the one-year storm to pre-development levels. Dry ponds are designed for the water to exit the pond through the principal and emergency spillway. Nutrient removal is limited.
Wet Detention Basin
A wet detention pond improves stormwater quality by detaining stormwater runoff for an extended period of time to allow pollutants that are suspended in the runoff to settle out. As runoff enters the pond, its velocity is dramatically reduced, allowing suspended pollutants to begin settling.
Many pollutant particles found in stormwater runoff are very small and, because smaller particles settle slower than larger particles, the pond is designed to provide adequate detention time so smaller particles have a chance to settle out.
The components of the wet detention pond that help increase the pond’s pollutant removal efficiency are the permanent pool, temporary pool and forebay. The permanent pool prevents particles that have settled to the pond bottom from re-suspending when runoff flows into the pond. The temporary pool is storage above the permanent pool which is utilized to control runoff during a storm event. To increase the detention time of the runoff, the temporary pool is slowly released. A separate smaller pond, called a forebay, is placed upstream of the main pond to trap a majority of the suspended solids in the runoff before it enters the main pond.
Constructed Wetland
Constructed wetlands provide the superior pollutant removal capability of natural wetlands. Wetlands remove pollutants primarily through physical filtration and settling and by biological processes of wetland plants. This Stormwater Control Measure is somewhat similar to wet detention ponds in that they both have a permanent pool and a temporary pool. Generally, stormwater wetlands have a shallower permanent pool than wet detention ponds so that wetland plant species can thrive in the basin. Runoff that is captured by the wetland area first enters a micropool or forebay, which is a relatively deep pool that promotes initial settling of larger pollutant particles. The stormwater slowly flows through the shallow areas of the wetland where the wetland plants filter suspended pollutants and reduce nutrient pollution through uptake.
Bioretention Area (Rain Garden)
The bioretention system mimic forest ecosystems to enhance stormwater quality. A bioretention system consists of a depression in the ground filled with soil media mixture, mulch and plantings and is designed to appear as a landscaped area, giving this SCM a very natural and appealing image. The manner in which runoff flows through the bioretention system is very similar, on a smaller scale, to watering potted plants.
Stormwater runoff enters the bioretention area and is temporarily stored in a shallow pond on top of the mulch layer. The ponded water then slowly filters downward through the soil media mixture and is absorbed through the plantings. As the excess water filters through the system, it is collected by an under drain pipe and discharged to a storm conveyance system.
Infiltration Trench
An Infiltration Trench is a “leaky” pipe in a stone filled trench with a level bottom. An Infiltration Trench may be used as part of a larger storm sewer system, such as a relatively flat section of storm sewer or it may serve as a portion of a stormwater system for a small area, such as a portion of a roof or a single catch basin. In all cases, an Infiltration Trench should be designed with a positive overflow.
Cistern (above or below ground tanks)
Pictured: Above-ground cisterns collect water from downspouts; single rain barrel under downspout; underground tanks collect runoff and disperse water slowly with a pump system.
Cisterns are storage tanks, located either above or below ground, that hold rainwater for beneficial reuse. Rainwater may be collected from rooftops or other impervious surfaces and conveyed to cisterns for storage. Stored water may drain by gravity or be pumped to its ultimate end use. Collected rainwater may be used for irrigation.
SCM Regulations
(A) Stormwater control measures capturing stormwater runoff from multiple lots shall be located in a common area of the development and shall be maintained by a property owners' or homeowners' association.
[Amended on 10/21/2019 by OA-01-19 .]
(B) Easements.
- The developer must record a document or documents showing a permanent easement for each of the following purposes: drainage, all stormwater control measures, access by Wake County for necessary inspections and enforcement and maintenance access by the private entity (property owners' or homeowners' association) responsible for maintenance.
- The stormwater control measures shall be shown and labeled within the easement. The entire footprint of the stormwater control measure system must be included in the access and maintenance easement
- The maintenance easement shall be ten or more feet in width, not including lateral or inclined slopes that exceed 3:1 (horizontal to vertical) around the stormwater control measure to provide sufficient room to complete maintenance tasks. The stormwater control measure system may include, but is not limited to: forebay, riser structure, the stormwater control measure device, dam embankment, outlet and emergency spillway.
- The access easement shall extend to the nearest public right-of-way or public easement.
(C) Stormwater control measures shall be set back from all wastewater system components in accordance with the applicable provisions of the Regulations Governing Wastewater Treatment and Dispersal Systems in Wake County.
Article 9-31 Construction and Maintenance of SCMs
9-30-1 Party Responsible for Completion of Improvements.
The developer is responsible for completing all stormwater improvements in accordance with the requirements of this Article and other applicable ordinances and laws.
9-31 Assurance that Improvements will be Completed.
9-31-1 Performance Guarantee. The county may not approve a record plat, or in the case of single-lot development not requiring a record plat may not issue a building permit, until those stormwater improvements required of the developer have been completed or a performance guarantee has been provided. Such performance guarantees must comply with the performance guarantee provisions of Sec. 8-22.
9-31-2 As-Built Plans. Upon completion of required improvements, the developer or the developer's representative must submit as-built plans prepared by a licensed surveyor of required stormwater improvements to the Wake County Department of Environmental Services. These plans must indicate whether stormwater improvements were constructed in accordance with the county approved stormwater plan.
9-32 Assurance that Improvements will be Maintained.
9-32-1 Maintenance Required. All stormwater improvements must be maintained so they will continue to serve their intended functions.
9-32-2 Parties Responsible for Maintenance of Improvements.
(A) The developer must maintain stormwater improvements until accepted by a property owners' association or lot owner via a Stormwater Agreement. The developer must disclose which party will be responsible for continued maintenance on the record plat and on the stormwater management plan.
(B) Before improvements are accepted for maintenance by the property owners' association or lot owner, the developer or the developer's engineer or other representative, as authorized by Statute, must certify to the property owners' association or lot owner and to the county that improvements are complete and functioning as designed.
9-32-3 Maintenance Plan.
(A) The developer must record, and reference on the record plat, a maintenance plan that instructs the property owners' association or lot owner about the annual maintenance tasks and associated costs for at least a 20-year period.
(B) It will be the responsibility of the property owners' association or lot owner to update the maintenance plan at least every ten years.
9-32-4 Maintenance Agreement.
(A) The developer must record, and reference on the record plat, a maintenance agreement, or restrictive covenant that sets forth the property owners' association's or lot owner's continuing responsibilities for maintenance, including specifying how cost will be apportioned among lot owners served.
(B) The maintenance agreement must provide that the association and its individual members are jointly and severably liable for maintenance.
9-32-5 Annual Maintenance Inspection and Report.
(A) The person or entity responsible for maintenance of any structural and non-structural stormwater control measures installed pursuant to this ordinance shall submit an annual inspection report from one of the following persons providing services only in their area of competence: a qualified registered North Carolina professional engineer, surveyor, landscape architect, soil scientist, aquatic biologist, or person certified by the North Carolina Cooperative Extension Service for stormwater treatment practice inspection and maintenance or other certification as approved by Wake County.
(B) Annual inspection reports are due by June 30 of each year. The first annual report is due by June 30 following one year after approval of the as-built plan. For structural stormwater control measures located on properties subject to property owners' association agreements, the property owners' association is responsible for collecting and submitting information on all individual lot structural stormwater conveyance measures installed pursuant to this ordinance on an annual basis.
(C) The inspection report shall contain all of the following: The name and address of the parcel owner, the name and address of the party responsible for maintenance of the stormwater control measure, the name of the subdivision or development, the recorded book and page number of the lot of each structural and non-structural stormwater control measure; a statement regarding whether or not inspected structural and non-structural stormwater control measures are performing properly and are in compliance with the terms and conditions of the approved maintenance agreement required by this ordinance. The report shall include any noted deficiencies, needed maintenance and recommended corrective action. The report shall include the name, contact information, original signature, and seal (if applicable) of the qualified person conducting the inspection.
(D) All reports shall be on a form provided by or approved by Wake County.
9-32-6 Documents Required Before Plat Approval or Building Permit. All maintenance documents required by this Article must be submitted to the Subdivision Administrator or Environmental Services Stormwater Engineer before record plat approval, and such documents must be referenced on the record plat, or, in the case of single-lot developments not requiring record plats, documentation must be submitted to the Zoning Administrator or Environmental Services Stormwater Engineer before building permit issuance.