Poorly-managed stormwater can damage your property and the wider environment - but, managed properly, it can be good for your garden.
Stormwater is the water that runs off surfaces such as houses, roads, driveways, footpaths. In urban areas, it runs down drains into stormwater pipes or channels and is carried to rivers, lakes or the sea.
Stormwater on private property is the responsibility of the property owner. When it's not properly managed, it can cause flooding, erosion and pollution of waterways. Properly managed, it can help keep your garden lush.
Why does stormwater matter?
In most areas of New Zealand, stormwater is not treated before it is discharged. This means there are several reasons to be concerned about the way stormwater is managed.
Flooding and erosion
If the stormwater pipes in your area are overloaded or blocked, your property and neighbouring properties may be at risk of flooding. Excess stormwater can also cause erosion and a damp home.
With climate change, we are likely to see more long, dry periods followed by sudden downpours which will cause extra strain on the stormwater system.
Managing stormwater is costly. City stormwater networks cost many millions of dollars each year to operate and maintain. Millions more are spent on upgrades and additions to the stormwater network. You pay for this through your rates.
Stormwater isn't pure water. Along the way to its outfall it picks up rubbish, oil, sediment, animal droppings, chemicals, plant nutrients and other contaminants from roads and other paved areas. It can also be contaminated with paint, detergents and anything else people tip down stormwater drains.
These contaminants can do serious harm. High levels of plant nutrients in water can be harmful or fatal to fish and animals. . Increased nutrients can cause phytoplankton blooms, which in turn produce biotoxins that affect shellfish, for example, causing paralytic shellfish poisoning (PSP). For 15 months over 2009/11, health authorities warned against shellfish and seafood gathering along the Bay of Plenty coast because of high levels of PSP which can cause paralysis and respiratory failure.
In addition, stormwater overflowing from overloaded or blocked drains can find its way into the sewerage system. This can result in sewage backflow entering stormwater pipes and being carried to the sea, lakes or rivers, or back onto residential sections.
Benefits of taking action
By taking steps to improve the way stormwater is managed, you'll be:
- helping to keep rivers, lakes and beaches cleaner and safer for swimming, fishing and other recreational activities
- reducing the risk of flooding and erosion on your property and around your neighbourhood
- reducing the need for spending on stormwater infrastructure, leaving money available for other uses or for savings on your rates bills.
Keeping contaminants out
There are several easy ways to reduce the levels of contaminants that get into the stormwater system:
- Pick up litter before it can wash into drains.
- Clean your car on the lawn, without using detergent, or take it to a car wash that recycles water and detergent.
- Sweep up leaves and grass clippings and put them on the garden instead of hosing them down the drain.
- Pick up your dog's droppings.
- Never tip paint, oil or other contaminants down a stormwater drain (or any drain) - contact your landfill about disposing of leftover paint, or give it to someone else.
- Clean paint brushes in a bucket that you can empty in your garden.
- Shovel up soil, cement and other debris when you're building or landscaping. If possible keep it dry and dispose of it at the landfill.
Managing stormwater with landscaping
Appropriate home design and landscaping can reduce the amount of stormwater and contaminants entering the stormwater system.
Keep the amount of concrete and paving to a minimum
Water runs off hard surfaces and ends up in the stormwater system.
- keeping your driveway short by having the garage near the front of the property
- using lawn, decking and semi-porous paving such as stones instead of hard paving or concrete
- using grass or bark mulch strips alongside hard surfaces to absorb runoff
- diverting stormwater from paths and other hard surfaces onto free draining garden areas.
Install a rain garden, pumice wick or swale
A rain garden is an attractive and effective way of slowing stormwater flow. Water is filtered and absorbed as it slowly makes its way through layers of soil and mulch. Moisture-loving plants (like native flaxes and grasses) thrive in rain gardens.
A pumice wick is made up of layers of pumice, newspaper or porous mesh, and dirt. Water directed into the wick slowly filters down and along it - sometimes to a deep gravel-filled hole from which water can be recycled, sometimes to rain garden or swale.
A swale is a wide, gently sloping, vegetated channel that captures water, allowing it to filter into the soil. Choose plants that can cope with dry periods and the ground being saturated. Native plants such as manuka, carex grasses, flaxes and hebes are excellent choices for planting in a swale.
For advice on creating rain gardens, pumice wicks, and swales, ask your local or regional council, or local garden centre.
Note: Pumice wicks, swales, rain gardens or any other work that requires landscaping or earthworks may require a resource consent, or may conflict with local drainage bylaws or policies. Check with your local council before starting work.
A engineer's report may be needed as part of the resource consent process.
When you're carrying out work, ensure soil and sediment remain on your property - you're liable for any damage to neighbouring sites.
Minimise site disturbance
If you're building or renovating, clearing a site or significantly altering the landscape - for example, by levelling or cutting into a slope, or by using large amounts of fill - can change the amount and direction of stormwater runoff, and increase the amount of sediment that gets into the stormwater system.
By leaving land and vegetation as untouched as possible, you're less likely to have problems with flooding and erosion.
Collecting and using rainwater
Rainwater can be collected and used on your garden and - for other household uses such as washing your clothes and flushing the toilet. The more rainwater you collect, store and use, the less there'll be left to run off into the stormwater system. For more detailed information, see Collecting and using rainwater.
From Smarter Homes
- Collecting and using rainwater
- Landforms and waterways
- Outdoor water use
- Planting and landscaping
- Minimising on-site pollution and site impacts
- Reusing greywater
From other sites
Greater Wellington Regional Council's Be the difference website has information about stormwater contamination.
Auckland Council's website has guides to constructing rain gardens, swales and sand filters.
You can download NZ Water and Wastes Association booklet Keep it Clean: Preventing Stormwater Pollution, from the association's website.
Landcare Research has a web page with information and instructions on making a home rain garden. It covers common mistakes and the features that are essential for a rain garden to work properly.