How Storm-Water Pits Work

Our not-so-little island nation has a lot of issues to contend with. We don’t get much rainfall. We’re statistically considered the seconds-driest region in the world. You probably think the deserts of Arabia or the drought-stricken regions of Africa beat us out. Not really – the driest section in the world is that way because it’s covered in ice and it’s too cold for liquid precipitation to fall. Yup, it’s the Artic Circle, down by the South Pole. Maybe that’s why we’re Number Two, because we’re closer to that iceberg plantation than any other continent.

New Zealand is lower south than we are, but it’s a much smaller land mass and has a different climatic pattern than we do, so it’s much wetter and greener. Either way, a combination of our low rainfall (the wettest Aussie spaces only get 600mm of rain per year) and large central deserts mean we don’t rank high on the greenery scale. That said, when it does rain, it rains hard. And because most of us live within 50km of the coast, there’s a large risk of flooding. The people that designed urban Aussie spaces were aware of this.

That’s why they designed a comprehensive network of storm drains. During rainy Sydney summers (or the monsoons in some parts of the North), our low sea level and largely sandy soil lead to enhanced flooding. Ocean water contributes too, because it overflows and mixes with the rain, while underground water tables overflow and seep to the surface. This happens in all coastal regions, which is why municipalities have rules about how close to the beach you can build. Unfortunately, these regulations are rarely followed or enforced.

Get the basix right

Still, even in places that are well within construction guidelines, the rainy season can easily overwhelm municipal waterways. That can lead to soil erosion, sinking buildings, and millions in property damage. So as part of Basix building rules, new homes must contain green infrastructure, lower emission features, water tanks, and detention tanks. The latter are storm-water solutions intended to reduce the harm done by excessive rainwater. Because while tanks can collect water for use, they can only hold so much.

Picture a rainy day, and think about all the water that flows into your tank. The top surface of your tank barely stretches a metre across, maybe two. But the entire surface of your property – and the rest of your neighbourhood – all receives the same amount of water. And this rain was enough to fill a tank with a capacity of several thousand litres. Just think how much more rainwater pelted the ground. Plus, there’s run-off from the roofs, streets, and natural water bodies. It adds up to a lot of water.

If all that rain hits the city sewers simultaneously, it’s likely to ‘break the banks’ and spill into streets and houses, crumbling infrastructure, sinking car parks, and collapsing smaller constructions. Detention tanks help by collecting some of that rainwater, and if every home has one, then it collects a LOT of that water. They’re usually connected to the roof, which is their rain supplier, but they also catch ground-level runoff.

Call in the professionals

Storm-water pits are designed by water engineers and built by plumbers. They’ll do careful calculations based on building size and soil type, determining the right size of storm-water tank for your property. They usually have large grates at the top to let water in while keeping larger debris and small animals out, because these could cause a blockage and make the flooding even worse. The system is designed so the inlet is much bigger than the outlet. This means your storm-water pit receives rainwater en masse but releases it more slowly.

As each home’s detention tank gradually releases water into the main storm-pipe, the municipal system can handle it better and won’t be overwhelmed. Gutters and drains sometimes collect dirt, trash, and leaves, and these can enter the storm-water system. They cause blockages, which trigger flooding. But even if they don’t block the system, they pollute the storm-water. This water receives different treatment depending on where it is.

In some places, the storm-water is released directly into natural waterways, dirt and all. In other places, it’s filtered and released into the sewer system, where it joins household and industrial effluent on its way to further treatment. Your role – besides ensuring you have a functioning storm-water pit in your home or building – is to regularly clean out your gutters, especially before the rainy season. You should also avoid littering, because all that garbage you toss around will find its way into storm drains and sewers.

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