UNDERSTANDING DETENTION/RETENTION PONDS AND OTHER STORMWATER CONTROL STRUCTURES
Stormwater ponds and other structures are important features of best management practices (BMPs) for stormwater management. They are used to prevent flooding and excessive downstream erosion. They can also be used to recharge the local groundwater aquifers. Maintaining these ponds can be difficult when there is excessive stormwater pollution. The GCWA works with detention pond and lake owners in the Gills Creek Watershed to help address issues of stormwater pollution.
Wet Detention Basin
Stormwater ponds are extremely important components of a community's drainage system. They are designed to provide two critical services:
- prevent flooding by suppressing surges of stormwater runoff that wash from lawns, buildings, and paved surfaces
- protect water quality by holding water long so that sediment and attached and other pollutants settle to the bottom of the pond before discharge to nearby rivers or beaches.
The primary purpose of all stormwater ponds is to manage stormwater runoff generated by impervious surfaces such as rooftops and pavement. These ponds are not designed to be recreational ponds for fishing or boating, and they are not permitted solely for beautification of the landscape. They are engineered devices, intended to moderate flood surges and reduce stormwater pollution. As with other engineered devices, stormwater ponds require maintenance to prevent them from falling into disrepair. That being said, stormwater ponds that are well maintained may provide additional benefits beyond simply managing stormwater, such as fishing, boating, and improved property values. Follow the links below to find solutions to the maintenance challenges you are experiencing.
The primary function of a detention basin is to control the quantity of stormwater runoff. Most stormwater management policies require that the postdevelopment peak flow rates be reduced to predevelopment peak flow rates for one or more specified design return periods such as 2, 10, and 25 years. Peak flow reduction is achieved by routing the postdevelopment runoff through a detention basin, that is by detaining the runoff temporarily in a basin.
Stormwater management BMPs are control measures taken to mitigate changes to both quantity and quality of urban runoff caused through changes to land use. Generally BMPs focus on water quality problems caused by increased impervious surfaces from land development. BMPs are designed to reduce stormwater volume, peak flows, and/or nonpoint source pollution through evapotranspiration, infiltration, detention, and filtration or biological and chemical actions.
Stormwater BMPs can be classified as "structural" (i.e., devices installed or constructed on a site) or "non-structural" (rain gardens
or modified landcaping practices
). There are a variety of BMPs available; selection typically depends on site characteristics and pollutant removal objectives.
How do stormwater ponds work?
Stormwater ponds detain stormwater in order to prevent flooding and minimize erosion. They do so by collecting runoff in a basin of a predetermined volume. The basin is designed to fill with water during the storm and discharge the water through an outlet structure which releases water at a rate similar to the rate of flow before the watershed was developed. To capture runoff, each stormwater pond must have an empty space that can fill with water during the storm. The empty space from the normal water level to the top of the bank slope is known as the temporary storage capacity.
Water Quality Protection:
Stormwater ponds are designed to permanently retain water in the basin. This volume of water is known as the permanent pool or treatment pool. In South Carolina, the treatment pool must be the same volume as if 1 inch of rainfall were to fall on the watershed that drains to the pond. The treatment pool is designed to slow the water down and hold it long enough to allow gravity to pull sediments out of the water column and allow sunlight and biochemical processes to break down pollutants before they are released to rivers and beaches. In most cases the treatment pool has an average depth of 4 to 6 feet, which has been shown to limit submerged vegetation and provide the necessary treatment and sediment capture.
New construction must take into account requirements for stormwater management, but the effectiveness of post-construction stormwater control best management practices (BMPs) depends upon regular inspections of the control measures. Generally, BMP inspection and maintenance falls into two categories: expected routine maintenance and non-routine (repair) maintenance. GCWA encourages current property owners to monitor the stormwater management structures on their properties where they exist and to consider inplementing or upgrading where necessary. We also encourage individuals to observe and report problems with detention/retention basins and other structures.
Providing a comprehensive understanding of stormwater detention structures is beyond the scope of this web page, but Gills Creek Watershed Association has developed a relatively easy way to determine if an existing structure is working in the way it should be. Stormwater Detention Pond Inspection Checklist
Mr Franklin Buie, a dedicated GCWA volunteer, has developed a much more comprehensive assessment instrument: Stormwater Detention Pond Observations and Form.pdf
and also a more detailed document, Retention Compound Evaluation for Laymen
, describing detention structure design, implementation and maintenance.
If you see problems, please report them to GCWA at 803-727-8326.
For more information about stormwater detention pond design, see the following web sites:
Clemson Cooperative Extension Service Stormwater Pond Design, Construction, and Sedimentation
A good general description of construction, function and maintenance.
Clemson Cooperative Extension Service Managing Stormwater Ponds
Information for property owners with stormwater structures on flood recovery and pond help, maintenance and problem solving.
Environmental Protection Agency (EPA) Guidance Manual for Developing Best Management Practices (BMPs)
- green infrastructure information including bioretention facilities, rain gardens, vegetated rooftops, rain barrels, and permeable pavements.
North Carolina DOT Stormwater Control Devices document
which describes and illustrates 16 types of control strucures.
Any mention of commercial products is for information only; it does not imply recommendation or endorsement by Gills Creek Watershed Association.
Gills Creek Watershed Association would like to acknowledge the tireless efforts of Mr Frank Buie in bringing to light the importance of correctly designed, implemented and maintained detention structures to reduce runoff and sedimentation in the watershed.