Attenuation is the storage of excess stormwater during the peak of a storm, followed by controlled release of the stored water. The rate of release is designed to be lower than the rate of runoff from a site prior to development, effectively mimicking the natural flows from the site.
Why attenuation is necessary
The construction of hard surfaces (roofs, patios, driveways etc) on areas of land that were previously undeveloped, changes the way water runs off the property. It removes natural soakage and vegetation that would have previously slowed the run off.
This means the volume of runoff increases and the stormwater network (pipes, drains and streams) reaches capacity far more often and much more quickly.
The effects of increased runoff
This additional flow can have a number of effects on the stormwater system, including:
- increased water levels causing surface flows and possible flooding
- potential for some of this water to enter the wastewater system and increase the risk of sewage discharges to the environment
- increased velocities in the streams causing erosion to banks and environmental damage
- increased stress on the piped network leading to it failing earlier, requiring increased levels of maintenance.
Decision flow chart
If you are undertaking a building project, you will need to consider whether attenuation is required. The decision flow chart will help you work this out.
Attenuation Decision Flow Chart(PDF, 858KB)
Attenuation sizing chart
If attenuation is required, use the sizing chart for guidance on the level of attenuation required and the volume of storage needed.
Stormwater Attenuation Sizing Chart(PDF, 242KB)
We have developed these tools to help you to avoid the additional cost of appointing consulting engineers specifically for this purpose. The charts are a conservative estimate and you can engage an engineer to undertake specific engineering design for your project if you wish.
There are a number of ways of attenuating flow, including storage tanks, underground storage and ponds. Depending on your site constraints one or more of these may be suitable.