Onsite Detention – A Beginner’s Guide

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If you’re planning a reasonable renovation to your home that involves changes to your roof or the lawns/pavements surrounding your house, there’s a good chance you’ll encounter the words onsite detention. No, it’s not some form of Council imprisonment!  It’s just a clever way to deal with all the water that falls on your property when it rains.  Let’s explore…

What is Onsite Detention (OSD)?

Many Councils are requiring on-site detention to be included as part of the stormwater management for redevelopments in Sydney. But why is this?  Doesn’t water just run down hill?  Yes, but that’s half the problem.  Existing Council stormwater infrastructure is old and was not designed to cater for the amount of recent development, which has resulted in higher stormwater flows off site. The increase in flow rates have been generated due to additional impervious areas (i.e. roof, driveways, hardstands, etc), where once these would previously have just naturally infiltrated into the ground.  Furthermore, the last two decades have seen increased frequency and intensity of rainfall events, which – again – deliver higher storm flow rates than the old infrastructure was designed to accommodate.  The runoff from the impervious areas are generally collected via pits and pipes and discharged directly into the Council System.

In order to provide relief to the downstream Council drainage infrastructure, Councils require that the runoff is stored temporarily on site and released at a controlled rate that the downstream stormwater system can adequately handle. This is called on-site detention.

Will Council require a stormwater management concept plan showing the OSD system for DA?

Depending on the scale of the development, it is likely that Council will require a stormwater management concept plan for the Development Application. Where the redevelopment triggers the need for OSD to be incorporated, a suitably qualified hydraulic/civil engineer is usually commissioned to undertake the design.

Is my proposed re-development exempt from OSD?

Each Council has its own list of requirements that trigger the need to provide an OSD system and it is recommended that a hydraulic engineer or Council engineer be contacted to confirm these requirements.

The following scenarios may result in a development being exempt from OSD:

  1. If the redevelopment is relatively minor and only increases the impervious area by a small amount, then Council may accept this and will not impose OSD;
  2. If the property is subject to flooding; and
  3. If the property drains to a location that does not affect Council’s infrastructure (i.e. harbour, rivers etc).

How is it sized?

The volume of the OSD system is determined by each Council setting out their own requirements but generally, as a rule of thumb, the OSD system is sized to cater for the additional runoff generated from the development, compared to the existing site conditions. Some Councils may permit the OSD volume to be reduced if rainwater storage tanks are integrated into the stormwater system.  Hydraulic engineers then calculate the areas involved and determine the volume of storage required, and how to direct all of the stormwater to the tank.

How can this be provided on-site?

OSD can be stored on site via two methods, chiefly either above ground storage or below ground storage. Above ground storage can be achieved by providing ponding in landscaped basins or within car parks. Basins need to be designed to ensure that there is no danger to end users and buildings when ponding occurs. Due to site constraints, such as availability of space and the topography of the land, these systems may not be achievable for many residential developments. Therefore, below ground tanks are more commonly used to detain water on site. It is critical that below ground tanks are suitably designed to permit maintenance and are provided with appropriate overflow provisions should the system encounter a blockage.

How can Partridge help?

Partridge Hydraulic Services has significant experience in preparing stormwater management plans for both Development Applications and Construction Certificate Applications. We have worked with many property owners and architects detailing OSD on residential, commercial and industrial developments. We actively engage with Council and then work closely with the architect to ensure the stormwater design meets Council’s requirements, whilst also remaining faithful to the original design vision of the development.

What happens after your stormwater management concept plan and development application is approved? We’ll explore that in our next piece.  Stay tuned…


Just say Your opinion.

  • Adrian Perth

    5 years ago

    By reading this complete article with detailed information. I agree that “Some Councils may permit the OSD volume to be reduced if rainwater storage tanks are integrated into the stormwater system.” Thanks for sharing us. Much appreciated.