Meet Your Stormwater Pond
Beautiful neighborhoods, homes around pond, aerial view, Springtime dawn.
Do you know if you have a stormwater pond in your neighborhood? Chances are that you do, but you might call it by another name, perhaps a lagoon, lake, or just the local neighborhood fishing pond. These stormwater pond systems play an important role in protecting not only our community, but also the downstream creeks, rivers, and streams that serve an important role in our South Carolina way of life.
Stormwater Pond Purpose in the Landscape
After a storm event, the rain that falls onto impervious surfaces like roofs, roads, and driveways flows as runoff across the land to storm drains. This stormwater runoff, if unmanaged, can lead to flooding around property, buildings, and streets. As the runoff flows across land, it picks up the contaminants we leave behind (such as fertilizer, auto fluids, trash, and pet waste), carries them to the storm drain, and ultimately to the nearest downstream waterway. Unmanaged stormwater runoff can negatively impact a receiving waterbody, resulting in potential ecological and human health consequences. The Environmental Protection Agency (EPA) has identified unmanaged stormwater as a significant threat to our nation’s waterways.
Stormwater ponds help to manage these potential stormwater impacts in our community. Stormwater ponds, or wet detention basins, are best management practices designed by engineers to provide flood control and help remove pollutants in runoff that would otherwise be carried downstream. In South Carolina, design standards for stormwater ponds regulated ponds include requirement such as:
- Store and release, over a 24-hour period, the first ½ inch of runoff.
- Manage runoff so that the flow and volume of runoff that leaves a development during 2- and 10-year, 24-hour storms is less than or equal to pre-development runoff. A 2-year storm has a 50% probability of occurring in a given year, and a 10-year storm has a 10% probability of occurring in a given year.
In addition, local regulations or other requirements for coastal development may require these ponds to meet additional flood management and water quality standards.
Because of the important role these ponds serve, it is important the community understands the purpose of these ponds in the landscape, their basic function, and how to maintain stormwater ponds to prevent impacts to downstream waterways as well as protect homes.
Design Principles
This schematic of a wet detention pond shows the important features, including permanent pool, storage, and inlet and outlet structures, that comprise these engineered-designed systems.
Photo courtesy the South Carolina Department of Health and Environmental Control.
Water enters the stormwater pond through a variety of means, such as sheetflow across adjacent land, or runoff coming from the upstream storm drainage systems (a collection of yard and street drains, ditches, and underground pipes). These pipe systems collect, concentrate, and direct runoff from the yard or street toward the pond. This collected runoff flows into the pond through one or more inlets. Often, these inlets are concrete, plastic, or even metal pipes, and depending on the location may be submerged below the normal pond water level.
Typically, ponds are created by excavating or damming to form a pond basin. The basin of the pond is composed of a permanent and temporary pool. The permanent pool is designed to hold water year-round, should be between four to six feet in depth, and helps to protect water quality by providing storage for captured sediment, preventing muddy water from being carried downstream. The temporary pool sits above the permanent pool and, following storm events, helps to capture runoff and is designed to release it slowly over at least a 24-hour period.
The temporary pool provides flood management for storms and helps to mitigate pollution. As flow is slowed through the pond, sediment and associated pollutants fall out of suspension and are trapped in the pond’s permanent pool, a process called sedimentation. Because of the pollutant removal processes, following a storm, the water leaving a pond should be cleaner than the water that entered.
The outlet structure may be one of the most important components of a pond as it regulates discharge leaving the system. The elevations on this weir are designed to manage the flow from the pond during different size storm events.
Photo courtesy Chuck Jarman.
Following a storm, water leaves the pond through an outlet. The outlet may look like a horizontal pipe, a box-shaped weir, or a vertical pipe (called a riser).
In some instances, these outlets are helping to direct flow through the pond’s dam. These different types of outlets have been specially engineered for the pond to maintain a specific pond depth and regulate flow. During high-flow conditions following larger storm events, in order to prevent flooding, the flow may be seen leaving the pond through the emergency spillway, which may be shaped like a concrete or grass flume on top of the pond bank.
Once this recently-constructed forebay fills with water, it flows over the rip-rap spillway into the main pond. The forebay, located at Charleston County’s Public Services Building, was installed as a pre-treatment trap for sediment, and associated pollutants, helping protect the main basin of the pond.
Photo courtesy Chuck Jarman.
Other important features of the pond may include a forebay and aquatic bench. A forebay is a small, deep pool located near the inlet of a pond and separated from the main basin by a berm or other type of barrier. Forebays help protect the main pond basin by slowing high-energy flow as it initially enters the pond, and acts as a pre-treatment trap for sediment, preserving storage in the pond’s permanent pool. Forebays may be easier to access and dredge than the pond basin, assisting in pond maintenance.
An aquatic bench, sometimes also referred to as a littoral shelf, is the shallow perimeter of a pond that can be planted with beneficial, native aquatic and wetland plants to help stabilize pond banks and prevent erosion, protect water quality, and support a healthy habitat.
Aquatic benches, also called littoral shelves, can be planted with native, wetland vegetation to provide erosion protection, water quality benefits, and habitat for beneficial wildlife.
Photo courtesy Charleston Aquatic Nurseries.
Our Community Role in Management
Successful pond management is a whole community task. All residents in the neighborhood, not just those that live along the pond, should understand a stormwater pond’s purpose and their individual role in protecting pond function. Maintenance of stormwater pond systems is not only critical to maintaining their benefits to the community, but also on reducing their impact on downstream waterways in the community. For more information on management tips for stormwater pond systems see, HGIC 1881, Stormwater Ponds: Inspection and Maintenance Considerations.
References
- Ellis, K., C Berg, S Drescher, G Hoffmann, B Keppler, M LaRocco, and A Turner. 2014. Low Impact Development in Coastal South Carolina: A Planning and Design Guide. ACE Basin and North Inlet-Winyah Bay National Estuarine Research Reserves, 462 pp.
- Lewitus AJ, Brock LM, Burke MK, DeMattio KA, Wilde SB. 2008. Lagoonal stormwater detention ponds as promoters of harmful algal blooms and eutrophication along the South Carolina coast. Harmful Algae 8(1): 60-65.
- South Carolina Department of Health and Environmental Control (SC DHEC). 2005. South Carolina DHEC Storm Water Management BMP Field Manual.
Originally published 05/20
