Stormwater, gutters and what ends up in the stream

Every river note on this site has been about water in the hills. This one starts closer to home: on the roof, in the gutters and at the grate by the kerb. In towns, that is where a lot of a stream’s water comes from, and where a lot of its problems start.

Two separate pipes

Most New Zealand towns have two separate pipe networks. Wastewater from toilets, showers, sinks and washing machines goes to a treatment plant. Stormwater, the rain that runs off roofs, roads, drives and lawns, goes a different way: usually into streams, estuaries and the sea, often with little or no treatment. A community clean-water group in Auckland, Restore Hibiscus & Bays, puts it plainly: wash-off that goes into an outdoor drain flows directly into local streams, estuaries and eventually the sea.

So the grate at the bottom of the driveway isn’t really a drain in the sense most people imagine. It is the top of a stream.

What rain picks up on the way

Clean rain landing on a roof doesn’t stay clean for long. On its way to the stream it can collect:

• sediment from bare soil, building sites and driveways
• detergent and grime from washing cars on hard surfaces
• paint and building waste washed off brushes, rollers and tools
• oil, fuel and brake dust from vehicles
• garden chemicals and fertiliser, especially when applied before rain
• litter and leaves, which clog drains and add load to streams
• pet waste, which carries bacteria straight into the water

Each of these, in a small amount from one house, seems trivial. Across a suburb, after the first big rain of the season, they show up as cloudy water, algae, poor swimming water and stressed stream life.

Faster water, bigger floods

Hard surfaces also change how fast water arrives. In bush or pasture, much of the rain soaks into the ground and reaches the stream slowly. On roofs, roads and concrete it runs off almost at once. That is why urban streams rise sharply after rain and why their banks erode. It is the same shape as the flood hydrograph in our note on reading river flow data, only steeper.

Slowing it down at home

The Level guide to stormwater control and landscaping, from the building research body BRANZ, lists ways a property can reduce the amount and speed of run-off:

• Collect rainwater in tanks for the garden or toilet flushing.
• Use permeable surfaces such as gravel, grass pavers or open-jointed paving instead of solid concrete for drives and paths.
• Build swales: wide, shallow, planted channels that slow water and let it soak in.
• Plant a rain garden, set slightly below the surrounding ground and filled with plants that like wet feet.
• Keep and plant trees. Level notes that a tree canopy can catch roughly 20–40 percent of rainfall, which then evaporates rather than reaching the ground.

Any of these on one section helps a little. Many of them on a street make a measurable difference to how a stream behaves.

Gutters and downpipes

Gutters and downpipes are the start of the system, and they need a little care:

• Clear gutters of leaves and moss, especially before winter. Blocked gutters overflow against walls and into subfloors.
• Check downpipes connect properly to the stormwater system and aren’t discharging against the foundations.
• Look at the sumps, the small catchpits at the bottom of downpipes and in yards. Clear out silt and leaves so they work as intended.
• Don’t connect stormwater to the wastewater system, or the other way round. Cross-connections overload treatment plants in storms and can send sewage into streams.

The everyday habits

Most of what protects a stream from a house is simple:

• Only rain down the drain. Nothing else goes into an outdoor grate.
• Wash cars on the lawn, or at a car wash that collects its water, rather than on the drive.
• Sweep paths and drives instead of hosing debris into the gutter.
• Store paint, oil and chemicals under cover and take leftovers to a council drop-off.
• Pick up after dogs.
• Keep soil covered during building or landscaping work, so it doesn’t wash away in the first storm.

Painting is a common source of drain pollution, and it gets its own note on washing brushes and rollers without polluting waterways.

Why it matters for the river

Urban streams are often where children first meet fresh water, and many of them still hold native fish and eels. The habitat they need is the same as anywhere: clear water, stable banks, cover and a way to the sea. Our note on native freshwater fish explains more. Every section that slows its run-off and keeps its drains clean makes that habitat a little more likely to survive.