University of South Australia (UniSA) engineers have proposed one flood control measure that recommends designing permeable pavements to specifically suit local rainfall and soil conditions and reduce flood impacts.
Australia has experienced one of the worst flood disasters on record in the eastern states this year, with 23 people killed, thousands left homeless, and a damage bill expected to top $1.5 billion.
Permeable pavements are used on many driveways, carparks and roads (excluding main arterial roads and motorways) and typically consist of permeable pavers laid on an upper bedding layer of between 2-6 millimetres of gravel under which lies a base course layer above natural soil. They are designed to allow rainfall to infiltrate through their surface, urban flooding by up to 50 percent. However, their success is variable, depending on rainfall intensity, soil type, and pavement thickness.
In a study published in the journal Sustainability, UniSA engineers collected data from 107 towns and cities across Australia. They designed an optimal permeable pavement system based on a five percent probability of excess rainfall and a storm duration of 30 minutes.
The team built an algorithm to determine the dominant soil types (clay, silt, sand, or gravel) for each locality, which infiltrate water at different rates. Sand and gravel are highly permeable, for instance, whereas clay soil has a low permeability.
UniSA Professor in Geotechnical Engineering, Mizanur Rahman, says the design proposal is based on pavements storing 70 percent of the water in the base course layer, with only 30 percent released as stormwater runoff.
“Our study shows that this is possible if the base course layer in permeable pavements is suitable for local conditions, taking into account the soil type and rainfall intensity,” says Rahman. “The pavement needs to be thicker if the rainfall intensity is higher or the soil is less permeable. For highly permeable soils, the amount of rainfall is less significant.
At least one third of Australian towns and cities fall in low to moderate rainfall areas, requiring no more than a 100mm base course layer on most of their road surfaces. However, the north-east of the country has both clayey soils and intense rainfall, requiring much thicker permeable pavements to reduce the stormwater runoff.
Many councils across Australia are already installing permeable footpaths, significantly reducing stormwater runoff to the roads, as well as storing water to support roadside watering of trees.
“We are hoping to extend our design to commercial and industrial pavements, and to continue our work harvesting water using permeable pavements for watering roadside gardens,” says Rahman. “Our preliminary research shows that the carbon footprint generated in a car park could potentially be neutralized in 15 years by growing trees with harvested water. Our next step is to improve water quality using permeable pavements.”
