Have you ever been in a large parking lot during a rainstorm and observed water standing in the grates? If you had examined the shape of the parking lot, you would have noticed that it was shaped like a bowl with the grate in the bottom. This is a detention basin.
Detention basins are temporary holding areas placed along the stormwater system to protect it from being overwhelmed during times of heavy flow. During the winter, rainfall can fill stormwater pipes quickly, potentially inundating the system. Detention basins are designed to catch some of this water and hold it temporarily (usually no more than a few hours). The water held in these basins drains into the pipe system slowly, allowing the system time to recover.
All detention basins are designed to detain stormwater for a time, allowing it to drain back into the pipe system slowly. Often this is accomplished by fitting a device called an orifice plate over the pipe that drains the basin. The orifice plate has a hole that is much smaller than the pipe it covers, so less water can pass through. When flows are normal, water exits the basin almost as soon as it enters. However, once the flow exceeds the amount able to pass through the orifice, water begins to back up in the basin, holding it until the stormwater system has time to recover. Although standing water may be inconvenient, please allow the system time to work in the manner it was designed.
Types of detention basins include surface basins, parking lot depressions, and large underground pipes. Each basin type has a control structure (inside a manhole or storm grate) containing a flow-restricting device like an orifice plate beneath the lid. Aboveground basins are usually covered with grass or gravel, with the control structure near the highest point of the basin. You can spot parking lot detention basins because of the basin-like depression in the asphalt, with a storm grate at the low point of the depression. The large underground pipe detention basins cannot be seen from above, although they have a manhole lid at both ends for access.
Most detention basins are on private property, with an easement to allow the City access to the basin and control structure. We inspect all basins annually, clean the control structure, and repair as needed. The City is responsible for the maintenance of the control structure, but maintenance of the area around the control structure is the responsibility of the property owner. The lid to the control structure needs to be easily accessible, and aboveground vegetation kept mowed or maintained.
If you have a detention basin on your property, are not sure of the location of the control structure, or have other questions, call Angel Garcia, Stormwater Services, at 503-588-6063, x7732.