Some years ago a friend and I organised to take our kids snorkelling and then have a sleepover one summer weekend. It had been raining the day before but it was a fine sunny afternoon. The snorkel was enjoyed by all and we had a fun evening before settling everyone to bed. All was well until the first, then second, third and fourth child all woke up with gastro. We soon realized the error of my poor planning and as the night wore on, we talked about the ignored perils of swimming in Nerm/Port Phillip Bay after heavy rainfall. But why does this happen?
Stormwater describes the runoff that gets channelled from our urban areas into drains, flowing into rivers and waterways, and eventually into bays and oceans. When rain falls onto the impervious surfaces of cities and suburban areas, it runs into gutters and street drains, merging underground in a network of stormwater pipes, and being released into creeks and rivers. As the water flows, it picks up anything on these surfaces and washes it into the waterways – litter, pollutants, debris, chemicals, dog poo and more. What impacts does it have when this unintended stuff reaches the river?
Litter has a clear and visible impact on our landscape. After the floods of late 2022, the trees lining the Wirribi Yalluk/Werribee (and most) rivers were festooned with tattered plastic for many months, showing the reach of the high water mark. Food and drink containers, perhaps the easiest items to prevent reaching our waterways, float downstream and gather in eddies along the banks. Smaller pieces of plastic can resemble food to a curious fish or bird.  Fishing line and litter, a near-invisible and more dangerous problem, can entangle swimming and diving animals, including our iconic platypus. Rubber bands, elastic hair ties and the plastic rings from jars can also be a hazard to our water creatures. Live trapping surveys of platypus have found up to 40% of animals show signs of previous or current entanglement with litter.
Our streets and roads carry pollutants from vehicles including oil, heavy metals and rubber particles from tyres. Detergents used in cleaning cars and buildings can be washed into drains. Sewage pipes, especially older infrastructure, can leak or be inundated with groundwater in times of high rainfall. Chemicals from agriculture, industry or home gardens may accumulate into toxic levels in the water, and detergents have an especially devastating impacts on frogs and their sensitive skins. Fertilisers, weedkillers and insecticides continue their effect once they are in the water, adding excess nutrients which can lead to algae blooms, impacting on aquatic plants or wiping out waterbugs populations. Some of the waterbugs most sensitive to pollution in the water are the types platypus rely on for their food source.
Along the bay, plumes of brown water can be seen spreading from the mouths of waterways after rain events. Brown, cloudy water is associated with floodwaters and many fast-flowing rivers. But what makes it this colour and is it a problem? Turbidity describes this issue, and it’s caused by sediment particles getting carried along. Essentially, muddy waters.
Our country was once covered in a rich tapestry of groundcovers, mosses, lichens, wildflowers, grasses, shrubs and trees. The surface of the land, having been carefully managed and cultivated for countless generations by Traditional Custodians, was soft and porous. When rain fell, it was absorbed into the topsoil and moved slowly downward, replenishing the water table (the layer of water that sits upon the bedrock of the ground) and aquifers. The multitude of plants protected the precious and fertile layer of topsoil from being washed away by downpours, and slowed evaporation of water back into the atmosphere when the sun came out again.
The advent of European farming, with droves of hard-hoofed, hungry livestock, led to the disappearance of much of these diverse plant communities, the staple food, medicine and materials source for First Peoples. The ground became hard and compacted, meaning that when the rain falls, it runs off rapidly, and carries away any loose particles of topsoil. Quicker flows of water also mean greater volumes of water moving faster along creeks and rivers, which can cause erosion of banks and more dirt being swept along. Broad scale clearing of land surfaces, for example with housing developments, results in dust being lifted into the air and earth being piled up and moved around. Sediment can find its way out into the bay, swirling out of the estuaries and settling to the bottom as the water energy reduces, often smothering water plants or seagrass meadows, and their inhabitants, where it lands.
Recent news articles have shown hardy swimmers in Nerm/Port Phillip Bay emerging covered in brown goo. This appears to be a combination of marine debris such as decomposed seaweeds, turbidity, and more unpleasant substances, all binding together into a suspended slick in the upper water surface. Nasty bacteria, which can make us sick through a range of infections, can thrive in these conditions and are best avoided, or at least precautions taken. Port Phillip Bay water quality (theage.com.au).The heavy rainfalls churning down the rivers as the catchments drain, and disruption to the usual ‘flushing’ of the bay with increased volumes of stormwater, means that the saltwater can take a few days to circulate out stormwater to deeper marine areas, and return to its usual clear, attractive state. Is it safe to swim in? The Environmental Protection Agency (EPA) Beach Report for Nerm/Port Phillip Bay can help you decide, along with your own observations of how the beach looks on any given day. Beach Report | Environment Protection Authority Victoria (epa.vic.gov.au)
What is done to manage stormwater and reduce the effects of human activities on our water systems? In Melbourne, the Royal Park Stormwater Harvesting Project, managed by Melbourne City Council and Melbourne Water, collects and stores rainwater, where it is diverted into storage and a constructed wetland. The wetland acts both as a filtration system to clean the water, and a biodiversity haven for local species. Excess water can then be used for irrigating the park and surrounding streetscapes, or fed into Moonee Ponds Creek. Royal Park Stormwater Harvesting Project | Clearwater – training and events on sustainable urban water management (clearwatervic.com.au)
Victoria University is researching ways to utilise stormwater in the western suburbs around Werribee, including harvesting, treatment and usage as an alternative to freshwater supplies. Melbourne Water maintains numerous wetlands in the Wirribi Yalluk/Werribee River catchment, some of which have been modified and are maintained to retain and filter stormwater runoff. The array of plants in these ecosystems provide habitat for frogs, birds and other native creatures.
Green roofs are becoming more common, where suitable buildings can install a layer of substrate on the roof where indigenous grasses, wildflowers and plants can grow. These designs retain rainfall, have biodiversity benefits for insects such as pollinators, can be fire retardant, and possess insulating properties, reducing heating and cooling energy demands.
Rain gardens incorporate planting and layout designs for home gardeners that use passive methods of collecting and irrigation, encourage indigenous plants, provide habitat values for wildlife and reduce the impacts of erosion from heavy rain when it does occur. By slowing down stormwater when it falls, and allowing it to gradually enter our creeks and rivers in a filtered state, it protects our waterways and gives us more resilient gardens, better at storing water on the land during drier times.
Our weather is renowned for its extremes, and these are likely to become more pronounced with the impacts of climate change. By anticipating for and planning our cities in smarter ways to deal with stormwater, we can ensure that our rivers and all their inhabitants receive the quality of water they need to thrive.