The existing stormwater system was constructed decades ago and therefore was not designed to cater for the amount of runoff generated from increased recent development. As a result, the existing drainage infrastructure cannot cater for the increase in flows from development which can cause local flooding issues for downstream properties.
On-site detention is an effective method of minimising the impact of stormwater runoff on the existing drainage system which may be caused by re-development of your site. An experienced engineer can implement on-site detention into your development using different systems that can control the runoff from your property.
How would I know if on-site detention is required for my development?
Many Councils are requiring on-site detention to be included as part of the stormwater management for redevelopments and on-site detention requirements are generally determined and enforced by your local council who have specific development controls that apply to the on-site detention design and guidelines that determine the types of development that do and do not require on-site detention.
As a general rule of thumb, all new developments require on-site detention. Exceptions to this general rule may be (but not limited to):
- Minor alterations and additions
- Flood affected sites
- Sites where drainage is near significant water body; example creek, channel, harbour, river.
How is on-site detention sized and what options are available?
Various methods and analysis can be undertaken to determine the on-site detention design. The objective is to limit the discharge (controlled) to a minor storm event such as the 1 in 5 year storm and store the volume for the 1 in 20 year or 1 in 100 year storm depending on the specific council guidelines for your area.
Analysis of the design can be undertaken through modelling software and/or hydraulic calculations of rainfall runoff methods. It is advised that you engage an experienced engineer to model the most cost-effective solution for your development.
There are different on-site detention system options available which are dependent on the council. Each on-site detention system has their own advantages and cost implications for your development. These options are:
- Below ground
- Concrete pit/tank
- Rainwater tank
- Above ground
- Rainwater tank
If I require on-site detention, what are the next steps?
It is important that you engage a suitably qualified civil/stormwater engineer who can explain the various options that are available for your development. The civil engineer will be able to prepare a Stormwater Concept Plan/Report which would need to be submitted with any Development Application or Complying Development Submission.
Donovan Associates have suitably qualified engineers with 40 years of experience in civil/stormwater design and on-site detention design. Our engineers will communicate with council and will liaise with the property owner, architects, developers and other stakeholders in the early stages of the development to ensure that the requirements and potential on-site detention design is addressed and optimised for your development.