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Public Works - Detention vs Retention Ponds vs Wetlands
A retention pond is designed to hold a specific amount of water indefinitely. Usually the pond is designed to have drainage leading to another location when the water level gets above the pond capacity, but still maintains a certain capacity.
A water detention pond, by definition, detains water. When an area is paved, or covered with a building, water runs off the property much faster than when it is in a natural state. The total amount of discharge is the same, but the discharge happens over a shorter amount of time. A hydrologist will design a water detention pond to temporarily detain the water and keep the runoff to the desired rate. When the rain ends, though, the water detention pond will be empty shortly afterwards.
A water retention pond, on the other hand, retains water all the time. The pond level may go up and down, but ordinarily the pond has some water in it. So, if the pond is typically empty except during and shortly after rain or other precipitation, it is a detention pond. If the pond always has water in it, then it is a retention pond.
Ponds exist in order to help control potential flooding from storm water runoff and to help improve the water quality that leads into streams. Homeowners' associations and business owners are responsible for maintaining their detention basins. Detention basins require maintenance to ensure that they function properly. Some subdivisions have established Special Assessment Districts also known as (SAD) to fund the maintenance of a detention basin, pumps (as required), and drain system. These funds are assessed to the property taxes each year of every home/property owner within the subdivision for the sole purpose of maintaining the detention system that regulates the storm water for that subdivision. Ponds and systems funded by an SAD are normally maintained through your local municipality.
A wetland is an area of land that is saturated with water for at least part of the year and contains plants and animals that are adapted to live in these wet conditions. A wetland is also known as a swamp, bog, or marsh. You may even have some wetlands in your area.
Frequently Asked Questions
What is a Detention Basin?
Detention basins are engineered structures that are used primarily to control downstream flooding. They also have a secondary function to improve storm water quality.
What are the different types of detention ponds?
· Dry Detention Basins: Dry detention basins are typically dry depressions in the ground surface, except after a major rain storm when they temporarily fill with storm water.
· Wet Detention Basins: Wet detention basins typically have a permanent pool of water and more wetland plant life. The permanent pool of water allows pollutants such as sediments to settle to the bottom of the basin.
· Storm water Marsh Basins: Storm water marsh basins are similar to wet detention basins, but contain more wetland plants such as cattails bulrush and sedges.
How are detention basins maintained?
Detention basins require regular inspection and maintenance to ensure that they are functioning properly to protect private property and improve water quality. At a minimum, the Homeowners' Association or business owner should conduct an annual inspection and an inspection after major storms.
How should detention basins be inspected?
· Inspect inlet and outlet pipes. Keeping these pipes clear of debris and sediment will help ensure proper pond function.
· Inspect for litter and debris: Twice each year (spring and fall) and after major storms, check for debris in the pond, on the banks, and around inlets/outlets.
· Examine side slopes for erosion: Twice each year and after a major storm, check for gullies or sloughing of the banks and other disturbances from animals or vehicles. If erosion is occurring, contact your local Department of Public Works for repair recommendations.
· Inspect vegetation: In the spring and fall, inspect the vegetation on the banks and in the basin. Living vegetation greatly improves the water quality by filtering out pollutants such as fertilizers, pesticides, oils and grease from the storm water. Late fall is a good time to cut down cattails. Cut cattails should be disposed of with other compost materials. Repair bare spots along banks with grass seed, native plant seed or wildflowers.
· Mowing: The amount of mowing required depends on the type of detention basin and the desired appearance. Typically, basins with turf grass only need to be mowed two or three times a year. Basins with native grasses and wildflower plantings should be mowed only once a year in the late fall or early spring.
· Maintain pump stations (if present): If your pond has a pump station, only a licensed electrician or company that provided the pump system should conduct any maintenance work.
· Keep records of all inspections including the date, name of inspector, what was observed, and maintenance activities performed. Good records will help you make adjustments to the maintenance program as needed.