by Shane Sody

An innovative system is being used to keep the lake in Rymill Park / Murlawirrapurka (Park 14) free from blue-green algae.

The City of Adelaide has installed a “rain garden” which, despite its name, doesn’t actually need rain to do its job, cleaning the lake’s water 24 hours a day.

a timber shell hides the tank on the northern side of the lake

raingarden from centre looking west to tank.jpg

the rain garden, soaking up and cleaning the lake water

One of the challenges of Rymill Park’s shallow ornamental lake is that plants can’t be grown in the lake because of the artificial lining on the lake bed.

In other lakes, in creeks and in wetlands water-loving plants play a vital role in filtering water and removing sediments and nutrients.

In Rymill Park/Murlawirrapurka (Park 14) such water-loving plants are doing the same job but the plants aren’t in the lake. Instead, they’ve been planted nearby in a “raingarden” on the northern side of the lake, where in an engineered way, they can fulfill the role of natural filters.

Robin Allison from “Design Flow” explains in a City of Adelaide video, that:

“We draw water from the lake and pump it to a tank behind the timber shell and every hour and a half or so the tank releases a pulse of water over the rain garden surface and then the water percolates down through the sandy media of the rain garden. The rain gardens act like the kidneys of the system. So we're trying to filter out the sediments and the nutrients that that are in the water to try and reduce the incidence of algae and other water quality issues.

*Robin Allison from “Design Flow”. Pic: City of Adelaide*

“About every hour and a half, that the tank fills, and then the pulse of water over the surface of the rain garden floods the rain garden to about 50 millimetres deep over the whole system. And then, over the next 20 minutes, that water percolates down; the rain garden then dries, and then it's ready for another dose. And then in about another hour after that. Yeah. The plants are designed to cope with that wetting and drying cycle and they're really important in the filtration of the water system.

“Their roots in particular support biofilms or the slimes that grow on the on the roots. And that's really good at soaking up the nutrients carried in the water.

“Obviously the lake's very popular for ducks. Ducks bring in a lot of nutrients. And so it's a constant battle of reducing the nutrient load in the lake because the higher the nutrient load, the more risk of, of the algae.”