If there were any doubts about the growing impact of stormwater runoff on Adelaide’s suburbs, Mitcham Council’s Tim Johnson has laid them to rest with one simple statistic.
“The 400 megalitres of rainfall runoff from footpaths in Mitcham each year is enough to fill the Urrbrae Wetlands 20 times over,” the arborist and recent UniSA graduate reveals.
Not only does stormwater create localised flooding in many Adelaide suburbs, it transfers pollutants from the urban environment into local streams, rivers and coastal waterways.
Collective research by Dr Johnson, the council’s sustainable infrastructure engineer, and UniSA PhD student Harsha Sapdhare, is helping to both mitigate serious stormwater issues and also provide a passive watering option for the city’s street trees.
Dr Johnson, who completed his thesis in 2017 in UniSA’s School of Natural and Built Environments, is working on a project to install thousands of square metres of permeable footpaths in Mitcham to prevent flooding and also damage caused by protruding tree roots.
Initial trials showed that permeable paving allows rain to run into the underlying soil instead of flowing into the gutters and drains, improving both the soil and reducing stormwater runoff.
And contrary to many civil engineering fears that Adelaide’s reactive clay soils – which shrink or swell with moisture change – are not suitable for permeable paving, the results show otherwise, Dr Johnson says.
“Permeable paving lets the water in but because it’s porous, the soil surface directly under the pavement’s gravel base dries out pretty quickly so tree roots grow more deeply and you get fewer problems with damaged footpaths, unlike traditional impermeable concrete block paving,” Dr Johnson says. “The environment is healthier for root growth and you reduce localised flooding so it is a win-win situation.”
His work is being supported by PhD candidate Harsha Sapdhare, the first researcher to look at the best methods for harvesting stormwater via kerbside inlets and leaky wells installed in Mitcham and surrounding suburbs.
The inlets and wells, invented by local resident David Lawry OAM, capture stormwater runoff and move polluted nutrient-rich water (containing nitrogen and phosphorus) from the gutter to passively irrigate street trees.
The end result is healthier trees, more liveable urban environments, reduced localised flooding and less polluted stormwater entering waterways and sea beds.
Harsha’s research is helping authorities determine which filters are the most cost effective in removing pollutants and providing the best water quality.
Between 50 and 150 litres of stormwater can be collected via the inlets and leaky wells, delivering thousands of litres of water into the soil each year.
“In times of drought, street trees often become stressed due to lack of water, many of them dying through harsh summers. Because water is a scarce (and expensive) resource, most councils only water young trees until they are established.”
Preliminary results for the permeable paving and kerbside inlet experiments at Mitcham are promising and it is expected that similar designs will soon be rolled out in other councils across Adelaide.
Dr Johnson’s PhD Trees, stormwater, soil and civil infrastructure: synergies towards sustainable urban design was completed in mid-2017.