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The rain water that flows over the ground is called runoff or stormwater. Stormwater runs into harbours, lakes and the sea. It can cause flooding if stormwater drains and pipes are not looked after.

What happens to rainwater?

Rainwater, or snow melt, either:

  • soaks into the ground to become groundwater
  • evaporates
  • or flows over the surface of the land. 

In cities and towns

In cities and towns there are lots of surfaces like roads, concrete and buildings which water can not soak through. This means that rain water will flow over these surfaces rather than soak into the ground. This run-off is called stormwater.

Natural areas

Soil, bush and grassed surfaces let water soak through them so that it can enter groundwater and aquifers. These natural surfaces do not flood as easily.

Run-off or stormwater

Run-off or stormwater is the water that does not soak into the ground. Run-off flows overland into our rivers, streams and beaches. In towns and cities there are a lot of solid surfaces and therefore a lot of stormwater. This stormwater needs to be managed. Most stormwater is directed into the stormwater system of drains and pipes, or flows overland as run-off.

Why have a stormwater system?

Large amounts of stormwater can cause flooding and erosion and less water will soak through the ground so there will be less groundwater. Stormwater flows down our driveways, footpaths, roads and carparks. If there were no gutters, drains and pipes for stormwater to run into flooding would occur. Stormwater drains and pipes carry the water to our streams, rivers, lakes, and harbours.

Unlike wastewater, stormwater does not get treated.

What goes down the drain?

Stormwater picks up pollutants including litter, animal pooh, dust, plant materials, petrol, oil, lead, and other metals or materials left behind on city roads.

This water then travels through a system of underground pipes and into the local stream, river, or sea where it can harm plants and animals and affect the health of people using these waterways for swimming or food gathering.

Reducing pollution of stormwater 

Only water should go down the drain. 

Here are some tips to reduce stormwater pollution and overflows.

  1. Wash your car on the lawn.
  2. Use cleaning products carefully and follow the instructions on the label.
  3. Pour any left-over cleaning water down the laundry sink, or on the garden, not down the gutter.
  4. If using water based paints, clean brushes in the laundry - don’t wash paint down a stormwater drain. Save any left over paint.
  5. Take toxic chemicals to a hazardous waste facility. Do not put these chemicals down the stormwater drain or into the wastewater system.
  6. Use less water and have a rainwater tank to collect water for the garden.
  7. Clear leaves and other rubbish away from stormwater drains.

Ready for a quiz?



Audio Māori keywords: 

How is stormwater managed in your area? Do you know where your nearest stormwater drains are?

Stormwater is managed through a system of gutters, drains and pipes. Where does this water end up? Image: Wellington City Council.

Stormwater pipes are buried under ground. How do you think the stormwater system is managed? Image: Wellington Water.

Contaminated stormwater enters an open stormwater drain. Image: Pete Pattinson, NIWA.

Stormwater is not treated and ends up in our waterways. What can you do to reduce pollution of stormwater? Image: LEARNZ.