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retention vs detention pond

Retention Ponds (wet) hold water for a period of time. Detention Ponds (dry) do not hold water.

Retention ponds are built to control water runoff and to help limit flooding. Without a retention pond, where would the water go? It would likely pour into the local sewer systems, potentially filling them up and overflowing them. Instead of flooding, the water can flow into the lower elevation retention pond. Retention ponds are just one type of water retention basin. They are named so because they are deigned to hold permanent pools of water into which storm water runoff is directed. Runoff from each rain event is detained and treated in the pond until it is displaced by runoff from the next storm. They usually also have an overflow pipe so that the water level doesn’t get too high. By capturing and retaining runoff during storm events, wet retention ponds control both storm water quantity and quality. The ponds help so that the city doesn’t have to go through a massive storm sewer system upgrade (sometimes it makes more financial sense to put in a retention pond instead of digging up and upgrading a storm sewer system) and it helps the environment as well. The storm water is given the opportunity to settle out solids and phosphorus with the end result of cleaner storm water leaving our community.

There’s another version called a “detention pond” or “detention basin”. Detention ponds are slightly different in that they simply detain water for a short period of time. It is designed to capture and slowly release runoff water for a period of 72 hours or less after a precipitation event. They can remain dry most of the time and simply hold water overflow during rainy periods. Dry ponds do not treat the storm water and are typically constructed in areas where flood control is the greatest concern. Dry detention ponds typically are not a haven for mosquitos– Except in cases where the dry ponds are improperly designed or improperly maintained and do not drain within 72 hours after a precipitation event. Maintenance is a must to ward off snakes, rodents and mosquitos.