What is a Detention Basin?

Mary McMahon
Mary McMahon

A detention basin is a structure which is designed to assist with flood control and water management. Detention basins can take a variety of forms, and can be seen in many regions of the world. They are often constructed during periods of new development, in which the amount of impermeable surfaces in an area is increased, potentially creating a flood risk which needs to be offset with a detention basin.

Man with a drill
Man with a drill

The purpose of a detention basin is to temporarily hold an overflow of water, thereby allowing water to accumulate in a safe, controlled area if unusual amounts of water are present. These structures can be useful for seasonal flood management as well as protection against rare serious storms. They are typically designed to fill and drain with the assistance of gravity, reducing the amount of maintenance required and ensuring that they will work without the need for electrical or mechanical systems.

Dry pond or dry detention basins are designed to be dry most of the time. They consist of recessed areas which may be lined with landscaping, with several inlets for water. As water starts to rise, it enters the inlets, filling the detention basin. Once water levels fall again, the water in the basin is gently pulled out. Often, filtration systems are installed to keep debris out, in which case it is important to periodically inspect the grates, gravel beds, or other filtering methods used to confirm that they are clear.

Retention basins, wet detention basins, or extended detention basins are all terms used for detention basins which stay wet year round. The water level in the detention basin can rise and fall over the course of the year, but some water always remains present. Marshy plants may also be present, creating habitat for animals and adding to flood control, as marshy areas can help contain floodwaters. This type of detention basin can also be used to manage water quality by trapping impurities before water is routed to another location.

These structures can be added to a community for flood control is flooding is an issue of concern, and they may also be constructed whenever new development occurs. In some regions, there are specific laws about how, where, and when detention basins are to be built, with the goal of keeping communities safe and reducing the risk of damage from flooding. In areas undergoing climate shift, building such structures can be especially critical for safety, to allow the community to keep up with changing rainfall levels.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


@Mor - Often it is mismanagement of the land which causes excess flooding rather than the simple fact that people are trying to control the river. We usually know exactly what we're doing wrong, but keep doing it because it's more convenient or cheaper.

For example, people tend to strip trees out from the upper reaches of a river, which allows a lot of water at once to get into the river when it rains, leading to flooding, and pollution.

Detention basin designs which include planting can help with this issue but often they are limited compared to what should be there. Still, they are better than nothing.

My town, in fact, has extensive stormwater detention basins that have been planted with swamp plants and it does a really good job of cleaning rainwater washed down from the city before it reaches the harbor.


@Mor - The thing is, because rivers flood all the time, they leave a lot of silt behind and make for very rich land. It's not like people make the decision to deliberately live in places where flooding can happen because they want to live in difficult circumstances.

Often it just happens because they want to live where the rich soil will allow them to grow better crops. Not to mention being near a river makes irrigation easier.

And then if an area is rich enough it can eventually become a town, and then a city. No one deliberately sets out to need a detention pond system. It just happens that they do in order to achieve other things.


We are really shooting ourselves in the foot the way we build up areas that are prone to flooding and then try to prevent them from doing so artificially.

Rivers are supposed to naturally grow bigger and smaller throughout the year. It's when you try to contain them with storm water detention and so forth that they eventually break out and kill people or damage property.

If people would pay attention to where they are developing areas and build in appropriate places, they wouldn't have to go to the trouble and they wouldn't get caught out whenever there is a bigger than normal storm or some other problem with their systems.

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