The concrete jungle: how hard surfaces are harming our waterways
Australia is one of the most urbanised countries in the world, with 90% of our population living in cities on 0.22% of our nation’s land area.
In our pursuit of the urban lifestyle, we have altered and reshaped our natural surroundings to suit our way of life. We have paved over our soil and fields, giving rise to the construction of roads and buildings. But how have these alterations changed how our ecosystem works?
The key: permeable surfaces, which play a significant role in the water cycle. These surfaces, such as soil and grass, allow rainwater to infiltrate (soak into) the ground to recharge the subsurface water levels.
Hard, impermeable surfaces, such as concrete and asphalt, stop water from penetrating through the soil. Instead, rainwater is being channelled across these hard surfaces into drains and straight into our waterways.
This is bad news for our river systems for several reasons.
The water that would have seeped into the soil is now being collected in our drains. It is then released at a much higher volume and speed than what would naturally occur. These ‘tsunamis’ of stormwater can uproot aquatic plants and erode away creek banks, releasing sediment into the water.
This increases the turbidity of the water (making it murky and muddy) which means there is less sunlight getting through to the aquatic plants beneath the surface, reducing their ability to photosynthesise. This in turn lowers the oxygen levels in the water. You can imagine how this would disrupt life in freshwater habitats!
Another big issue to consider is pollutants. As rainwater runs straight off roads and pours into creeks, it can bring a cocktail of harmful contaminants. These can be pieces of plastic and debris that can harm wildlife and block waterways, or small particulates and nutrients from urban agriculture that can promote toxic algal blooms. These pollutants can cause significant harm to the health of water bodies, and the wildlife and plants that live in and around them.
Impermeable surfaces also reduce the natural ‘base flow’ of urban creeks and streams. Base flow is the flow of water through rivers in periods between rainfall; instead of the waterway being filled up by rain, it is recharged by ground water seeping up through the soil. This is why rivers still flow even when it hasn’t rained in a while.
By paving over permeable soil, we are stopping rainwater from seeping into the ground. Consequently, there is less groundwater to seep into the waterways during dry periods, reducing the base flow and interrupting life within the waterway. This means that between rainfall events, our rivers and streams begin to look shallower and dry up more quickly than what we would expect to happen naturally.
What can we do to help our waterways?
Recently, there has been a great deal of research into creative outdoor designs that can assist in recharging groundwater and reducing runoff.
One example of water-sensitive urban design is multi-storey buildings and car parks, which take up vertical space rather than sprawling across the land.
For those wanting to do their part at home, by replacing paved yards with native grasses and asphalt driveways with ‘softer’ alternatives like gravel or permeable pavement, you can improve stormwater infiltration in your own garden. You can also be mindful of your own impact on stormwater pollution and make sure the water going down your drain at home is clean.
Another great way to help at home is to plant a rain garden in your yard.Rain gardens are garden beds designed to catch stormwater off the street and use natural means (filtering plants, soils and microbes) to treat the stormwater and remove harmful pollutants. The water is then absorbed into the ground and flows into rivers. This recharges the groundwater, increasing base flow and reducing nutrient pollution.
Cities paved in hard surfaces are causing trouble for our waterways and their inhabitants. What we invest now in rectifying these issues can make a huge difference to the urban landscape of tomorrow. With a bit of creativity, our concrete jungle could be green once more!
Kelley Anne Leech
Kelley is an Ecology and Evolutionary Biology graduate with a passion for wildlife, conservation and urban ecology.