Flooding & Floodplains
Stormwater Detention: How it Works
A Lake With No Water
Stormwater detention basins are a place to store potentially damaging floodwaters temporarily until the channels can safely carry the water away. As flat as Harris County is, most of our stormwater storage has to be excavated. The Flood Control District uses stormwater detention extensively to reduce the risk of flooding throughout the county. Flood Control District detention basins are typically large regional facilities, which may be several hundred acres in size. New developments often use stormwater detention to offset or mitigate the negative effect development may have on flooding (due to covering up soil with buildings and concrete and speeding up the rate water runs off an area). When full, detention basins often resemble lakes. When dry, detention basins are large excavated open space areas. Some systems are being designed to have a permanent water level, or small lake, in the bottom of the basin, and the flood storage is provided above the normal surface of the lake.
Detention Basin? Retention Basin? Which One Is It?
The short of it is this:
DETENTION is the temporary, short-term storage of excess stormwater.
RETENTION implies that stormwater is stored indefinitely
Detention basins are engineered, constructed and utilized extensively in Harris County (retention basins are not). You will always hear the District call them detention basins. The Harris County Flood Control District owns approximately 70 large regional detention basin sites throughout Harris County, and there are hundreds of smaller developer-built basins. Countywide, these basins hold billions of gallons of stormwater during heavy rainstorms.
How the Water Gets In
Stormwater detention basins are specifically designed and engineered to receive and temporarily hold large amounts of stormwater. When detention basins are used for regional flood damage reduction projects, they are sometimes designed with a weir structure. The weir serves as the designated location where stormwater rising in the channel can spill into the detention basin. Other detention basins have no weir and are simply open to a channel. In this circumstance, stormwater fills the basin as it rises in the channel.
When a stormwater detention basin is used to mitigate development projects, it may not necessarily be adjacent to a channel. Under this condition, the detention basin will typically be filled by a storm sewer pipe or sheet flow draining into it from the surrounding development. The basic concept here is "big pipe in - little pipe out" to provide storage of the excess runoff produced by development. Also note, that if a single development is part of a larger planned development, the detention site may not be located on each property but is centralized into one larger facility in another nearby location.
How the Water Gets Out
Stormwater detention basins in the county are typically designed so that they drain by gravity, as opposed to having pump systems drain them. Because this is the case, the depth of the detention basin and location and depth of its outfall or drain is dictated by the depth of the receiving channel. The rate at which stormwater drains out of a detention basin is influenced by several factors, one of which is the stormwater level in the receiving channel. Typically, as stormwater levels recede in the receiving channel, stormwater will drain out of the detention basin accordingly.
How Engineers Design a Detention Basin
Engineering calculations are made according to specified criteria to determine the volume of stormwater that should be captured in the detention basin. That volume of stormwater, which is usually measured in acre-feet, translates to a width, length and depth of a detention basin. The amount of time stormwater stays in a gravity drained detention basin depends on several factors including stormwater levels in the receiving channel and how much of the basin's capacity was filled with stormwater. In Harris County, this time is usually measured in hours, not days.
How the Stormwater Detention Storage Process Works
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.