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Where Does All the Stormwater Go After the Storm?


Every year around 500 billion litres of stormwater – enough to fill Sydney Harbour – runs from Sydney to the sea. This is an account of its epic journey through the streets.

First – Runoffs

City rain runs off roofs, footpaths, roads and carparks and into pipes and channels, at which point it becomes urban stormwater.

Sydney Water’s 447-kilometre network represents less than 5 per cent of the total metropolitan stormwater infrastructure. The rest fall under local councils.

The Cooks River, upper parts of the Parramatta River and creeks around Rockdale are all channels which were straightened and lined with bricks and concrete to stop erosion, an approach no longer favoured.

Two – Gather Nutrients

From solid land surfaces, stormwater quickly picks up microorganisms and nutrients, like nitrogen and phosphorus, some of which comes from fertiliser, dog droppings and sewer overflows. It is the major source of pollution in Sydney’s waterways.

Along roads and commercial and industrial areas stormwater picks up heavy metals, like zinc, copper and lead in low levels and a certain amount of petroleum and diesel. It also picks up metals associated with the breakdown of the elements of a car, like brake linings and car tyres.

Three – Into Waterways

During the recent big storms, Manly’s waterways were under the pump as they attempted to deal with the immense levels of water hitting Manly Dam, with high tide peaking at 1.9 metres.

The bigger flows that overwhelm the stormwater system, so great they can no longer stay in the pipe, are a major danger during storm events, said Todd Dickinson of Warringah Council.

Four – Filtering

In some areas the water in large stormwater drains will be screened and filtered through Stormwater Quality Improvement Devices (SQIDS), but these only pull out litter and larger matter.

Wetlands are also used as natural filters, stripping pollutants out of stormwater, like the Cup and Saucer Creek along the Cooks River.

Increasingly, councils are installing rain gardens and tree pits in road verges and footpaths to capture litter and remove nutrients and pollutants before stormwater makes its way into pipes and waterways.

Five – Discharge into Rivers and Sea

All of the stormwater systems in the Sydney metropolitan area discharge to one of three receiving waters: to the Parramatta river via Sydney Harbour and out to sea, into Botany Bay and out to sea via the Cooks River, Georges River or Woronora River, or from western Sydney back into the Hawkesbury Nepean River system.

The Illawarra Region drains into other systems.

Sydney Morning Herald

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