Stormwater flood management

Photo of light flooding in the Waiarohia Stream, Cafler Park.

Managing stormwater and preventing flooding protects our water quality, harbour and agricultural areas as well as private and public property.

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.

The Hikurangi Flood Management Scheme, north of Whangārei, is drained by the Wairua River. The scheme provides flood protection and flood management to about 5,600 hectares of farmland.

A system of earth banks along the length of the Wairua River help to confine floodwater in the river channel and away from farmland. If the rivers continue to rise beyond a set level, the water spills over the banks through specially constructed spillways into confined areas of land known as pockets.

There are seven pockets which have been designed to accept floodwater flows. Water is held in these pockets until the river level drops sufficiently for pumping stations located within each pocket of land to then pump the water from the flooded areas back into the river.

The Hikurangi Flood Management Scheme Management Plan  and the Flood Riparian and Ox-Bow / Cut-off Channel Management Plan set out the operating and management procedures for the Hikurangi Flood Management Scheme as required by the resource consents.

The Fishery Management Plan is also a condition of the consent and recognises the cultural significance of the traditional tuna fishery to Ngati Hau.

We obtained a new resource consent for the Scheme, with a 35 year term, in May 2010.

Hikurangi Flood Management Scheme Plans and Map

Check our Long Term Plan that describes how we will manage and operate the Scheme:

Long Term Plan