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Detention Tanks Explained

Published on: Friday, 24th July, 2015

Detention Tanks Explained Graphic

Your plans are calling for 2000 Litre Stormwater Detention Tank but you’re not sure what that is?

A detention tank (sometimes the term retention tank is incorrectly used) is usually called for when the existing stormwater infrastructure is not adequate.  Basically, a detention tank slows down how fast the rain off your roof gets into the stormwater under the streets.  In newer areas, the stormwater drains have been engineered to allow for the rainwater run-off from the whole street. In some older areas, with the amount 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 of roof space and you have a 5mm downpour, your roof will collect approximately 1000 litres of water.  Usually this 1000 litres would all go rushing into the stormwater system within a couple of minutes, but if the drains are already full to capacity, this can cause a problem.  If you have a detention tank in your property, the water comes off the roof into the detention tank and then drains out slowly (via a orifice outlet) into the stormwater drains. Depending on what has been specified, the water may take hours to get into the drains.

The size of the tank and the size of the orifice outlet 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.

Sometimes a detention tank is a stand-alone tank, but more often it is combined with a retention tank. Put simply, 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 are combining the two you would have perhaps a 3000 Litre tank, use the bottom 2000 Litres for your toilets or garden, and the top 1000 Litres is for detention (as in the picture above).

Our Colorbond 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.

Whether we have been successful in completely confusing you or not, please feel free to drop us a line to discuss these tanks – 1800 804 901 .