What is a detention tank?
An On-site Stormwater Detention Tank (OSD) (sometimes the term “retention tank” is incorrectly used) is usually called for when the existing stormwater infrastructure is inadequate. A detention tank slows down how fast the rain off your roof gets into the stormwater under the streets and is intended to reduce the peak flow from a site.
In newer areas, the stormwater drains have been engineered to allow rainwater run-off from the whole street. In some older areas, the number of dwellings per street rising, it places extra pressure on infrastructure, and councils often require detention systems to alleviate this.
For example, if you have a 200m2 roof space and a 5mm downpour, your roof will collect approximately 1,000 litres of water. Usually, these 1,000 litres would all go rushing into the stormwater system within a couple of minutes, but if the drains are already full, this can cause a problem. If you have a detention tank on your property, the water comes off the roof into the detention tank and then drains out slowly (via an orifice outlet) into the stormwater drains. Depending on what has been specified, the water may take hours to get into the drains.
The tank’s size and the orifice outlet’s size are engineered to allow a particular flow rate to suit the requirements of the area you are building in. A stormwater detention tank will usually be empty except during periods of rainfall and immediately after.
Detention Tank vs Retention Tank
Sometimes a detention tank is a stand-alone tank, but it is often combined with a retention tank. A retention tank is simply a tank for storing water for a rainy day (hang on, perhaps a sunny day would match better), in contrast to a detention tank. If you combine the two, you would have perhaps a 3,000 Litre tank, use the bottom 2,000 Litres for your toilets or garden, and the top 1,000 litres for detention (as in the picture below).
Our Colorbond water tanks are a perfect match for a stormwater detention system, as it is easy to customise the tank size and the outlet sizes to suit. Other types of tanks can also be used for detention systems, including underground tanks.