Flood in Brisbane, Australia

Permeable pavements aim to protect Australia from flooding

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Engineers from the University of South Australia have proposed a new permeable pavement design to help mitigate the impact of flood disasters in the country.

This year has already seen one of Australia's worst floods on record, in which 23 people died and thousands were left homeless. The damage bill of the disaster is expected to top A$1.5bn (about £850m).

In addition to the impacts of climate change, increasing urban development is contributing to the rise in flood risk. The building of impermeable road and sidewalk surfaces in cities often causes surface water to collect, with nowhere to drain. Overall, the cost of natural disasters is predicted to cost A$33bn (£18.6bn) by 2050.

University of South Australia (UniSA) engineers have proposed a solution, published in an article in the Sustainability journal: permeable pavements.

Permeable pavements are used on many driveways, parking lots and low-traffic roads. Their porous surface allows water and rainfall to run off, directed towards internal reservoirs, where it is collected before being filtrated into the underlying soil.

These pavements usually 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 often made from concrete and asphalt, while stone or aggregate can be used in the base course layer to function as a reservoir and provide filtration. Individual paving blocks with spaces in between them can also be used to construct these paths.

Nonetheless, permeable pavements can only be used in low-traffic areas, as they can eventually become clogged with sediment, requiring more regular maintenance than conventional impermeable paving.

Typical permeable pavement configuration: (a) pavement containing only a granular base course, (b) pavement containing granular base and sub-base courses.

Typical permeable pavement configuration: (a) pavement containing only a granular base course, (b) pavement containing granular base and sub-base courses/Iqbal, A et al., Sustainability

Image credit: Iqbal, A et al., Sustainability

The team of engineers has proposed a new design of permeable pavements, which better suit local rainfall and soil conditions.

They collected data from 107 towns and cities across Australia, designing an optimal permeable pavement system based on a 5 per cent probability of excess rainfall and a storm duration of 30 minutes.

They then built an algorithm to determine the dominant soil types for each locality and their water infiltration rate and proposed a design based on pavements storing 70 per cent of the water in the base course layer, with only 30 per cent 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," said UniSA Professor Mizanur Rahman

The new design is able to curtail urban flooding by up to 50 per cent, the scientists say. However, its success is variable, depending on rainfall intensity, soil type and pavement thickness.

"The pavement needs to be thicker if the rainfall intensity is higher or the soil is less permeable,” he added. “For highly permeable soils, the amount of rainfall is less significant.”

For example, a region like Adelaide, which is characterised by clay-type soils but with low rainfall, would require a permeable pavement with only a minimum base course thickness. At least one-third of Australian towns and cities fall into these category, requiring no more than a 100mm base course layer on most of their road surfaces. However, the north-east of the country has both clay-type 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. The introduction of permeable surfaces on both roads and footpaths, would support this effort, by markedly reducing stormwater loads and mitigating flooding.

In the future, the researchers hope to extend their design to commercial and industrial pavements and continue their work harvesting water using permeable pavements for watering roadside gardens. 

"Our preliminary research shows that the carbon footprint generated in a car park could potentially be neutralised in 15 years by growing trees with harvested water," Rahman said. "Our next step is to improve water quality using permeable pavements."

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