On-Site Stormwater Detention – Storage


Many urban developments, and subdivisions, require stormwater flows from your block to be limited to a maximum flow rate. (See: ‘Onsite Stormwater Dention‘ for the reason why)

Although councils will require the calculations to be done by a qualified hydraulic engineer here is some explanation of the process so you can understand what is required.

Permissible Site Discharge

The ‘Permissible Site Discharge'(PSD), which is a maximum flow rate, will normally be set by your Council based on the block area.

The flow rate is usually controlled by making all the flow pass through a Orifice (a small diameter hole) before it can be discharged.

Designing Storages

Reducing the flow involves the following steps:

  1. Calculating the Peak Discharge from your block. The Local ‘Intensity of Rainfall‘ x Total Impemeable Area (The area of roof and hard paved surface)
  2. Calculating the size of a storage required, the Site Storage Requirement (SSR) to take the rainfall that cannot be discharged until the storm has passed. Your council may require this to be calculated in a particular way and/or using a particular computer program.
  3. Designing the storage to fit on your block.

NB The storage volume is different from rainwater collection volume as the detention storage volume is expected to be empty unless it is raining hard.

Onsite Storage Options

The three options that are available are:

  • Below Ground Tanks. Most expensive option but does have the advantage of being able to located under driveway or garden if you are short of space. May need a pump to empty tank if public stormwater drains are shallow. (Some councils require tank to be emptied by gravity which can make them difficult on some blocks.)

conctrete detention tank

  • Above Ground Tanks These are less expensive than underground tanks but can take up a lot of area so can be a problem for small blocks. Some councils will allow the detention volume to be provided within the same tank as rainwater storage.
  • Basins Basins are the least expensive method of providing storage but require much larger areas than above ground tanks. They are normally a low lying area of the garden which can be flooded for a short period of time. Because of issues such as drowning risk there are often council regulations limiting the depth of storage.


See Drainage for more posts