When there is normal flow in a bayou or channel, the detention basin is generally empty, although some have what's called "wet bottoms," which is a permanent lake in order to obtain more storage or for environmental and aesthetic purposes.
Initial Storm Effects
Stormwater will begin to fill the detention basin as it begins to rise in the bayou or channel or surrounding developments drain in to it through storm sewers.
Capturing the Flow of a Heavy Storm
As water continues to rise in the channel or flow in from a development, it fills the detention basin, either from pipes, over the weir, or by spreading out into the excavated area. Stormwater is captured in the detention basin.
Detaining the Flow
Since the systems drain by gravity, the basins will only drain as fast as the channel will let them. The water stays in storage until the channel can gradually remove it. By drawing the water into the detention basin and holding it there, it is not moving downstream and flooding homes and businesses.
Draining Detained Water
As the level of the channel recedes after the peak flow has passed, the channel water level drops and allows the stormwater to drain, but only as fast as the channel can handle it.
Back to Normal Flow
With the water level in the channel normal, the basin is once again empty. It is now ready for the next rainstorm.