Stormwater

Introduction | About stormwater | Solving stormwater problems | Growth and stormwater | What Auckland City is doing | Stormwater projects near you | Stormwater assets | Stormwater guidelines and other documents | Educational information

Growth and stormwater

The community expects stormwater to be managed in a way that allows for more growth, but minimises flooding and continues to protect the environment. That is why we need to find ways to ensure that we retain as many pervious surfaces as possible, and slow down the amount of water entering the stormwater network at any one time.

The more stormwater that can be kept out of the stormwater network and either disposed of by other means or stored temporarily during heavy rain, the less pressure there is on the network and the better it is for our environment.

• The problem of impervious surfaces
• Managing stormwater to allow more intensive use of your land
• How alternative stormwater measures work
• Why alternative stormwater measures are a good idea
• Building consents
• How Auckland City is managing stormwater and growth

The problem of impervious surfaces

The diagram below shows how you may unwittingly increase run-off to your neighbours property resulting in a flooded basement or ponding!

The information relates to the requirements in the Auckland City District Plan (Isthmus Section). Please note that the controls depicted below relate specifically to the district plan and an item defined as a building in this diagram does not necessarily mean that it is defined as a building under the Building Act therefore requiring building consent.

Click on the numbers on the image for details of some common problems. Use the back button on your browser to return to the image.

Managing stormwater to allow more intensive use of your land

• Homeowners
• Developers

The diagram shows some of the alternative examples of how private property owners and developers can manage stormwater on site. Look below to find out how to build them, to help our stormwater system and our streams and beaches cope with growth.

Homeowners

Developers

Developers can also use other alternative stormwater management measures, including those below. For a development of 10-20 dwelling units, the capital cost of the required devices is around $1,000 per unit - a modest figure for the financial gain from covering more of the site.

More detailed technical information about how to build stormwater management measures that allow more intensive development of your property while protecting our natural environment can be found in Auckland City's on-site stormwater management manual.

	A stormwater planter Given the choice of maximising return from a multi-unit development by installing a stormwater planter or using the existing capacity in the local stormwater pipes but having a smaller building coverage, this developer chose the planter. Initially concerned that it would look out of keeping, he is now very happy with it. He chose attractive plants that enhance the appearance of the development as well as helping to avoid local flooding problems.
	An engineered stormwater  wetland Auckland City is committed to sustainable management of the environment as well as its valuable stormwater infrastructure. The many benefits of stormwater treatment wetlands include reducing flood peaks, improving water quality in streams and beaches and restoring some of the city's natural habitat and birdlife.
Swales	Swales look like shallow grass-covered ditches and can do a number of different things. They can convey water to a pipe, stream or soakhole, cleaning it up on the way by trapping sediments and pollutants in it. Or, if so designed, they can let stormwater soak into the ground.
Depression storage and filter strips	Land owners can make the most of land that is not perfectly flat by detaining stormwater in natural or artificial depressions in the ground that delay the arrival of stormwater runoff into pipes or on-site disposal measures. Grassed filter strips can help clean up runoff by trapping sediments and pollutants before the stormwater enters these systems.
Roof and gutter detention	Temporarily storing rain on flat roofs with raised edges and in deep gutters for release into the stormwater network at a slower rate takes the pressure off the network during peak flows. Special design is needed for this, to ensure that roofs can withstand flooding, wide temperature ranges and the extra weight of the temporarily stored water.
Roof gardens or eco-roofs	A roof garden has a cover of soil and vegetation that captures rainfall, some of which is then lost to evaporation and transpiration and the rest of which slowly discharges to the stormwater system. An eco-roof is similar, but with a thinner layer of soil. These need good waterproofing and load-bearing building design and can look extremely attractive, while improving building insulation too.
Detention ponds	Well-designed ponding areas detain stormwater runoff and help attenuate flood peaks by releasing the water after the flows in pipes and streams have passed their peak and can accept more water.
Proprietary devices	The growing seriousness of the stormwater problem has encouraged many private companies to develop devices to help solve problems facing private landowners, developers and councils. If you want to use any of thee commercially available products, contact us to find out our design and approval requirements.

Auckland City has a number of projects underway, for example at Wesley Community Centre and Oranga Community Centre, where some of these techniques can be viewed. The sustainable Landcare Research building at the Tamaki campus of Auckland University also has a variety of stormwater features.

How alternative stormwater measures work

These alternative stormwater measures help restore some of the natural water cycle processes that our city's extensive impervious areas have so badly disrupted:

Auckland City's on-site stormwater management manual and soakage design manual both have detailed design guidance for these measures.

Why alternative stormwater measures are a good idea

In contrast with the conventional approach of discharging stormwater direct to large scale piped systems and then into the environment, on-site devices reflect modern practice for at-source controls that are:

• Good for the land owner: They reduce the problems associated with extensive impervious areas in the city, allowing private landowners more intensive development on their land. Using water from rain tanks for non-potable uses can save you money in the long term too. 
• Good for the infrastructure: They utilise the off-peak capacity of stormwater infrastructure and delay or reduce the need to spend large sums of public money on upgrades or replacement.
• Good for communities: More public awareness of stormwater issues encourages people to protect their local environments and have their say in the city's planning and asset management processes.
• Good for the environment: As more such measures are put in place they will help to reduce erosive flood peaks in streams, retain our streams as wildlife and community assets and reduce pollution of streams and beaches.

Building consents

You may need a building consent for any plumbing, drainage or site works associated with installing on-site stormwater management devices.

Please contact us to check this - but our general advice is that:

• you do not generally need a consent to install a rain tank for watering the garden
• you do need a consent if you want to increase the impervious area on your site, over that which is allowed in the district plan
• you do need a consent and you will need to install a backflow preventer if you want to:
• use rain water on the garden and/or in the toilet and laundry where you do not top up the rain tank with mains supply water
• use gravity or pumping to top up the rain tank with mains supply water for the garden and/or in the toilet and laundry.

Auckland City recommends that you use the public water supply for drinking and for kitchen use and bathroom uses other than flushing the toilet.

How Auckland City is managing stormwater and growth

How we use land in Auckland has a considerable effect on the amount of stormwater. The more we cover the city with buildings, pavements and roads, the greater the peak flow of stormwater becomes.

To control flooding and pollution we must limit these kinds of surfaces. Auckland City produces a district plan that specifies how much of a site can be covered with buildings or paving. In most residential areas impervious surfaces are allowed to cover up to 60 per cent of a site.

Auckland City can produce maps that show impervious surface coverage in the city. These maps help us plan to cater for the stormwater volumes generated around the city.

The population of Auckland is likely to increase by approximately 50 per cent in the next 50 years. All those people will need somewhere to live. The demand for housing will mean more building and more impervious surfaces, which will result in more stormwater having to be disposed of.

There are some areas of the city where growth is being encouraged. These areas have been identified in Auckland City's growth strategy. In these areas, the council is encouraging residents to look at ways to store rainwater temporarily during heavy rain or develop more natural soakage on their properties. In this way, we can have more built-up areas in the city, yet still manage the increased stormwater flows.

Auckland City is processing changes to the district plan that, if adopted, will allow for more building coverage on a building site in specific areas of the city, if alternative methods of stormwater management are used.

Future needs for stormwater services that can meet the demands of both growth and environmental quality will require integrated solutions to roading, property, flooding, pollution, ecosystems, conventional (piped) stormwater assets and innovative (on-site and 'natural') stormwater assets like rain gardens and wetlands.

Auckland City has a comprehensive environmental and asset management process to ensure our natural environment is looked after. While our monitoring tells us that bathing beach water quality meets or exceeds internationally accepted standards, most of the time, it also tells us that after times of heavy rainfall it is sometimes less safe to swim for up to 48 hours after the rain has stopped. Our stormwater needs to be managed within the capacity of the city's existing infrastructure (natural and built) or proposed upgrades to it.

When accommodating growth, any increase in site imperviousness over permitted levels must therefore be mitigated by installing on-site stormwater measures. These on-site measures can help the city maintain or enhance our streams, beaches and aquifers by reducing the frequency and level of peak flows in our stormwater system.

Stormwater is one of the most critical issues associated with growth. Our vision for Auckland City is for urban intensification and city development that provides for:

• liveable communities accommodating growth
• favourable conditions for intensification by focusing infrastructure improvements in growth areas
• coordinating growth with infrastructure improvements
• regulations to protect and enhance the natural environment

The diagram shows the relationships among the agencies charged with strategic management of growth in the region and the planning and works needed to accommodate it:

People in Auckland City cannot simply extend the impervious areas on their site, or they will cause flooding on their own and others' land. Neither can they, the council, or the environment afford to enlarge stormwater pipes to accept this increased runoff.

The least expensive solution for sustainable development is to maximise permeable areas and 'non-asset' solutions (on-site measures) to manage stormwater.

The district plan permits impervious coverage in residential zones to 60 per cent of the site. Auckland City is examining ways of allowing more impervious cover in some areas provided on-site stormwater measures are used to avoid adverse effects on stormwater infrastructure and receiving environments. If successful, this is likely to become a model for more sustainable development in other parts of the city. Find out more by reading plan change 58 in the district plan.