Detention Tanks

Detention Tank Solutions

Reo SacEco SacLiquidity VLSAquaCladWaterplex
Request a Quote

Demystifying Rainwater and Stormwater Retention and Detention Tanks

Are you building a new home and wondering why you have to install a detention tank? And are you mystified as to why you would capture rainwater and then “let it go” without putting it to good use?

And are you wondering what the difference is between stormwater detention and rainwater retention?

Here are the key facts you need to know about stormwater detention tanks along with the key considerations to be taken into account when selecting the right solution to meet stormwater detention objectives.

Why Do We need Detention Tanks?

As urban areas continue to be developed and re-developed, the amount of “hard surface” land in our cities and urban environments continues to increase. Hard surfaces is a general term used to describe non-porous services that don’t absorb water and that replace previously porous Urban Sprawlsurfaces – such as soil and grass. Examples of hard surfaces are roofs, concrete paths, drive ways and paved areas. A significant impact of this urban development is the increase of stormwater run-off that is no longer being absorbed into the land. The downstream effect is that our city’s storm water system and water ways are not able to cope with the increased flows of rainwater that previously were absorbed in the ground.

What is the Difference between Stormwater and Rainwater?

Often the terms rainwater and stormwater are used interchangeably. But they are different.

Rainwater is most commonly used to describe rain that falls on roofs and can be directed to water tanks and stored for re-use. Stormwater is used to describe water that falls on hardstand surfaces other than roof areas. The reason for the distinction is that there are many more potential contaminants in stormwater than in rainwater. Think of the footpath outside your house or your driveway; they may have oil drops, pet droppings, general rubbish or other contaminants. While there may be dust and debris on house roofs, the debris is likely to be organic and extensive research has proven that the quality of rainwater harvested from a roof and stored in water tanks is excellent (you can see a research article here).

Detention tanks vs Retention Tanks

A retention tank is a tank that collects and stores rainwater over time. That water can then be used as and when required. An example of a retention tank is a rainwater tank that captures water from a roof that is then used when required to water a garden.

Stormwater Management Utilising Detention Tanks

This schematic shows the attempt to match stormwater flows using a combination of porous surfaces and detention tanks

A detention tank is a tank that captures rainwater and temporarily “detains” that water but does not store that water over time. While the same amount of water will enter the stormwater system, detention tanks slow down the rate of water entering the system to compensate for the increased flow sue to hard surfaces. A detention tank will have an outlet that is always open so that water can always escape. The rate at which the water will escape from the tank is a function of the diameter of the outlet (usually called an “orifice” in a detention tank) and the capacity and dimensions of the tank. The size of the orifice required to allow water to escape is determined by taking into account the force of the water in the tank primarily determined by the tank dimensions and height.

Retention tanks and detention tanks can be combined provided the source of the water is suitable for retention and reuse (roof vs other non-porous surfaces). The combination of both types can be achieved in a number of ways, but most commonly by installing the detention tank orifice at a height where, once empty, it has achieved the detention tank capacity required and leaving the water below that orifice level as the rainwater storage. 

Waterplex Orifice Plate in Steel Detention Tank

In some cases, councils may allow detention tanks to be “interchangeable” with retention tanks. For example, it may be possible to get a concession from a local council to replace a detention tank with a retention tank. The benefit to the property owner is that captured rainwater is not released straight away. It can be used when required. In order for a council to allow a concession, it is likely they will require a tank size larger than the minimum detention size specified and / or require some certainty of continued use of the water such as reticulation into a property (eg toilets or laundry). That’s because councils was some certainty that each time it rains there will be some available detention capacity – and a water tank will not provide detention capacity if it is already full at the point it starts to rain.

Regulation of Detention Tanks

Local councils and state governments have responded to the issue of managing increased stormwater run-off by regulating the installation of detention tanks in developments that increase hard stand at the expense of porous surfaces (such as grass). The regulation of detention tanks may also extend to the retro-specification for re-developments of land where there is existing hard stand.

Detention Tank with installed orifice plate and overflowLocal councils will usually specify the size of the detention tank required and provide the basis for the calculation of the size of the orifice. The councils also specify the flow rate of water to be released from the detention tank. They may also specify a % of roof or ground area that needs to be used to harvest rainwater for the detention tank.

A detention tank captures a pre-determined amount of stormwater and releases it slowly to the stormwater system via a pre-determined outlet size that is determined by the flow rate specified by the council. The outlet size is normally achieved by the use of specified outlet size (often achieve with an “orifice plate” with a predetermined hole size) that is set in place and is unable to be varied or closed.

A detention tank is designed so that it will always empty the required amount of water at the required rate.

By definition, a detention tank slows the rate of water entering the stormwater system. That means during significant rain events it is likely that the tank will be filling faster than the water is being allowed to escape. All detention tanks must be designed to have an overflow (that allows the specified capacity to be reached) at least equal to the inflowing water. Once full and overflowing, a detention tank will not slow the rate of storm water entering the water ways.

Potable Tank liner in bessa block rainwater tank (for internal reticulation) overflowing into a stormwater detention tank with orifice outlet

Detention Tank Solutions

The issue for all building professionals and owners is where to install the detention tank and what type of detention tank to install.

A 10,000 litre detention tank will measure at least 10 cubic metres and weigh at least 10 tonnes when full. So the selection of the type of tank and the installation of that tank is an important decision in the design and specification process.

Detention tanks can take many different forms which include (but are not limited to) the following:

The factors to be considered when selecting the type of detention tank include:

  • The specified capacity and size of orifice
  • Space and aesthetic considerations
  • Budget considerations
  • Ease of installation and maintenance

Stormwater detention systems should only be designed by qualified engineers. However, product & solution suppliers have invaluable experience in executing plans and should be contacted and involved in the specification and building process to ensure that the specified products will produce the desired outcome.

Detention Tank Considerations

In additional to choosing the right tank and the best location, there are a number of other considerations to be taken into account when selecting the right type and location of detention tank for your site:

  1. Make sure the inlet, outlet and overflows are able to be inspected and,if necessary, cleared of debris at any time. You should also be aware that councils may choose to inspect the tank and the orifice plate at any time.
  2. Ensure a silt trap / debris screen or similar is installed in the tank to ensure that the orifice is not blocked thus preventing the tank from performing its function
  3. Ensure that screens (such as rain heads with mesh) are used to minimise the ingress of debris to the tank
  4. The size of the orifice can sometimes be a non-standard diameter. For example the specified orifice diameter might be 56mm for which there is no standard tank fittings. The way to achieve these non-standard diameters is to use an “orifice plate” in which the correct diameter hole can be drilled. Orifice plates can be incorporated in detention tanks in a number of different ways depending on the type and location of the tank and orifice outlet.

If you have any questions regarding detention tanks and the various solutions available, contact the team at Waterplex on 1300 72 66 70.

Request a Quote