 Auckland is New Zealand's largest city, with a population of approximately 401,500 within the city boundary. The city and suburbs cover an area of 60 square kilometres, with many of the suburbs having their own unique character. Auckland is built on a narrow isthmus between two harbours, and is surrounded by extinct volcanoes and picturesque islands. The city is known as the 'City of Sails' due to the number of yachts which sail in the harbours and the adjoining Hauraki Gulf.
The climate is temperate, with warm summers (averaging over 24 degrees) and cooler winters (averaging 16 degrees).
It is estimated that Auckland City’s population will increase dramatically in the next 50 years. Auckland City’s role in managing this increase is to pay particular attention to stormwater to minimise flooding and pollution within the city.
Each year bout 1200mm of rain falls on Auckland city. In a built up urban environment like Auckland we have changed the natural environment, covering much os it with impervious surfaces and nearly all of the rain that falls on the city becomes stormwater – the name we have for rain that has fallen and has begun to flow across a surface which ends up in our aquifers and harbours. At present, around 70 per cent of the rain that falls onto the city has to be disposed of through our stormwater system. Most of the stormwater network is underground – the manhole covers and grates alongside roads lead – but streams and open stormwater channels also play an important role. Stormwater flows downhill through the network and is eventually discharged into the sea either directly or via our streams and rivers, or into the estuaries of the Manukau and Waitemata harbours. In about one third of the city we also rely on volcanic soils to drain away stormwater underground through soakage pits.
Auckland city has devices for treating stormwater to reduce the level of pollutants. These include:
- catch pits with silt traps
- road sweeping
- litter traps, for example in Oakley Creek
- detention ponds, for example in Waiatarua Basin, which hold stormwater for a period, allowing sediments and pollutants to settle, before it is released into the sea
- mechanical filtration systems at Central Park in Penrose
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 Ctiy 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.
Some examples of how property owners can manage stormwater include:
Rain tanks and stormwater planters
Rain tanks and stormwater planters collect run-off from roofs and hold it temporarily during heavy rain, then release it into the stormwater network at a slower rate. This takes the pressure off the network during peak flows.
Grass filter strips
Grass filter strips can be introduced into the paving or driveways to let water soak into the ground.
Swales
Swales look like shallow grass-covered ditches and are designed to collect stormwater, letting it soak into the ground, while trapping sediments and pollutants.
Using pervious surfaces
Solid concrete and paths can be replaced with pervious materials, such as wooden decks, shell or gravel, or with paving slabs with grass or gravel between them.
Auckland City has a number of projects underway, for example at Wesley and Oranga Community Centres, where these techniques can be viewed. Also, the new Landcare Institute building has a variety of stormwater features.
About one-third of Auckland city is built on volcanic soil, which can drain rainwater very efficiently. In these areas, the council is encouraging property owners to develop soakage systems to dispose of stormwater.
Many streams in Auckland pass through private land and are known as private watercourses. It is the responsibility of property owners to look after these streams to prevent flooding, erosion and pollution. Simple maintenance activities to prevent against flooding and erosion include:
- watching for any changes that might be happening to the stream bed or banks
- keeping the stream clear of obstructions
- removing litter and rubbish
- watching out for pollution from upstream
- removing plants that block flow
- keeping the banks planted to avoid erosion.
Most people are unaware that whatever goes down a stormwater drain goes untreated to the nearest body of water. Many people believe that water pollution is caused by “someone else”. However, our everyday actions can cause water pollution ans we each have the power to prevent it. The best way of dealing with stormwater pollution is to prevent pollutants from getting into the stormwater in the first place.
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