Stormwater attenuation minimises the effects of increased runoff from small sites in Christchurch.

Why do we need to reduce stormwater runoff

On undeveloped land, a large proportion of rain soaks into the ground and either flows slowly through the upper soil layers into streams or seeps down into groundwater, as illustrated in the diagram below. Development results in increased impervious (hard) surfaces that have a number of negative effects on stormwater:

shows difference between natural vegetation and housing or roads

Before development                After development

  • The volume of runoff is increased as less water soaks into the ground (think of what happens to rain on a roof as opposed to a grass surface)
  • The speed at which runoff reaches a stream increases, resulting in changed flows in the stream
  • There are increased flows in streams which increase erosion and affect habitats
  • Flooding becomes more common due to the increased volume of runoff and the speed with which it reaches pipes and waterways
  • Pollutants are more easily washed off from hard surfaces and affect stream water quality

Even small sites can have a negative effect on stormwater, and when this is combined with hundreds of other small sites the effect can be significant. It is therefore important to mitigate these effects to help clean up Christchurch’s streams and to reduce flooding. The measures described can also be used by anyone wishing to develop their property in a more sustainable way. Reducing runoff helps to recreate the way that rain behaves on undeveloped land. This can be achieved, in order of preference, by:

  1. Reducing the amount of runoff generated within the site by minimising the impervious surfaces;
  2. Increasing the amount of water soaking into soil, such as through soakage systems, permeable pavement, or rain gardens; or
  3. Holding back as much of the runoff as possible using a stormwater storage system and releasing it slowly.

Acceptable solutions follow for minimising the effects of increased runoff from small sites in Christchurch.

Situations covered by these solutions

These solutions are applicable for development on small (less than 1000 m²) residential or commercial sites. However, not all development on small sites requires runoff to be reduced. The criteria below are used by Council to determine whether or not a site requires mitigation.

Hill sites (>5° slope) All hill sites are required to install tanks or other suitable mitigation devices when new development (or intensification) takes place.
Flat, urban areas

Mitigation is required only if:

  1. The additional impervious area added is greater than 150 m²; and
  2. The resultant impervious area covers more than 70% of the total site area.

Developers of existing sites where the coverage is already greater than 70% of the total site area will need to discuss attenuation requirements with the Council.