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Stormwater Attenuation

Image for the Stormwater Attenuation page.
 
This page contains information and guidance notes about stormwater attenuation requirements for building projects.
Updated: 2/10/2013 4:23 p.m.

Attenuation relates to 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.

The impact is that 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.

What you can do to help

If you are undertaking a building project, you will need to consider whether attenuation is required. The decision flow chart will help you to determine this.

If attenuation is required, consult the sizing chart which will give you guidance on the level of attenuation required and help you to determine the volume of storage needed.

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 do have the option of instructing 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.

Follow the links below to view the attenuation tools.

Attenuation Decision Flow Chart [10kb]

Attenuation Sizing Chart [153kb]

Resources

Disclaimer/Copyright

Whangarei District Council owns this website and the information, images and photographs in it are subject to copyright. No portion may be copied or republished without prior permission of Whangarei District Council. We have made every reasonable effort to provide accurate and reliable information. The use of any information is at the website visitor’s own risk and discretion.