Stormwater Control Measures (SCMs) used in Greensboro are described on this page. For Greensboro's requirements and specifications for SCMs, refer to the Stormwater Manual and NCDWQ Stormwater BMP Manual.What is a SCM?
SCM stands for Stormwater Control Measure. Stormwater 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 SCMs may include such practices as minimizing impervious area for site development, providing vegetative buffers along all streams and waterways, promoting natural infiltration of runoff before it enters a receiving stream, 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.
Stormwater Wet Detention Pond
In Greensboro, the wet detention pond is the most commonly used structural SCM for stormwater quality enhancement. This stormwater SCM 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. Learn how wet detention ponds are inspected.
By building wetlands to treat stormwater, we can try to reproduce 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 SCM 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.
Filtration systems used for stormwater treatment work similar to those that are used in the drinking water purification process. Stormwater filtration systems consist mainly of a pretreatment, or sedimentation area, and the filter area. Runoff first enters the sedimentation area where the runoff velocity is reduced allowing larger pollutant particles to drop out. When the stormwater leaves the sedimentation area, it is spread evenly over the filter bed, where it flows downward through the filter media. As the stormwater flows through the filter, the filtration media trap and absorb pollutants present in the stormwater.
A variety of different filtration media can be used, such as sand, peat, and compost. Filtration systems are beneficial when land space is scarce or expensive, because they can be designed to be placed underground or border the perimeter of a parking lot or other impervious surface.
Bioretention Systems (Rain Gardens)
The bioretention system is a relatively new concept that mimics 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.
Dry Extended Detention Basin
Underground detention facilities are structural SCMs designed to provide temporary storage of stormwater runoff for quantity control purposes. The systems are typically installed beneath parking lots, streets, and parks to maximize property usage and lower development costs. Underground detention system design measures must be taken to trap and store sediments in locations where clean-out and maintenance can be easily performed. Dry extended detention ponds are designed for the water to exit the pond through the principal and emergency spillway.