Often referred to as on-site detention tanks, OSDs are now compulsory for new or recently-renovated constructions or developments.
Put simply, on-site stormwater detention is a means of detaining the stormwater from your site to ensure all development areas can be safely controlled. If you’re in the construction industry, you have a duty of care to not only understand the use of OSDs, but their effective maintenance, too.
Advancements in construction and therefore a removal of much of our natural landscapes mean there is now nowhere for rainwater to collect. Elsewhere, hard surfaces like pavements, roofs and site drainage mean an increase in the volume and speed of stormwater run-off. Of course, this is something of a cause for concern for land, company and home owners, due to the high risk of flooding.
So, what can you do?
You’ll know the capacity of existing storm water drainage systems can be limited. What this means for you, then, is if you’re in the process of building a new property, OSDs are now a compulsory part of the planning and development.
Manufactured in line with OSD policy design, along with a host of engineered tanks for placement under driveways, parking bays and landscaping, OSDs are designed to dramatically help alleviate the risk of flooding.
How do OSDs work?
Providing temporary storage of storm water run-off, on-site storm water detention ensures the run off rate and volume can be easily controlled to reduce the risk of overload.
While the required size of storage and rate of discharge is location dependant, you can consult your local council along with your hydraulic engineer for specific requirements.
Your Questions: Answered
Got another question about on-site detention tanks? You might find the answer to it here in our FAQ section:
Which properties require the installation of an OSD system?
Are you connecting to or developing a property that connects to a town water supply? If yes, it’s very likely that you’ll need to install an OSD system. Many properties may already have a system like this in place – including commercial, industrial, special use buildings like schools, as well town houses, villas and home units.
Are there any design requirements?
If you’re installing an OSD system, you must first prepare designs that meet the requirements laid out in this guide, alongside the site’s storm water detention policy and current best practice. This can be viewed in the Australian Rainfall and Run-off guidelines of 1987.
Designs of the OSD system must be prepared by an appropriately qualified professional.
Furthermore, the design must also be certified by a chartered structural and hydraulic engineer, with a statement that it has been prepared accordingly.
In terms of design, your OSD system must be equipped to store run-off caused by a storm event of up to 100 year ARI for the site, as well as control the rate of discharge to ensure the system can handle the additional run-off.
Please note that in order to achieve this, the system must be designed with the permissible site discharge in mind, as well as its storage volume.
What are my responsibilities with regards to my OSD system?
Of course, it isn’t just a case of installing an OSD system; there is a certain level of maintenance involved, too. Maintaining your OSD system helps prevent future potential problems, for instance.
Does your OSD system discharge directly into a council-owned drainage system? If so, you must enter into a formal agreement with the corresponding council, which will lay out your ongoing responsibility to the maintenance of the OSD. The agreement must be registered on the property title to ensure it is stored on the title of the land and not lost as the properties change ownership.
What next?
You must first get approval in terms of design; simply submit the following information to your local council in advance of the construction work:
- A verification that your design meets the calculated PSD and SSR, via a certification of design
- A detailed plan and elevation of OSD systems, along with relevant dimensions
- The OSD’s storage location at your proposed development or property
- A detailed design of orifice (which must be a minimum 200mm x 200mm flat stainless steel plate, 3mm in thickness and no fewer than 40mm). This should include all dimensions, the maximum storage discharge rate spec, and the proposed percentage of storm water that could drain to the street or storm water system.
Be aware also that the OSD system must be equipped to store all the run-off for a storm event of up to 100 year ARI, to include the calculated SSR.
The type of storage installed can be above ground, below ground or a combination of the two. In terms of access, it must conform to the current Confined Spaces regulation, as well as feature step irons if the fall is 1200mm or more. Elsewhere; where there is a risk of gas build up or hydrostatic pressure, a release valve should be installed.