The unprecedented flooding in New South Wales in 2022 killed 13 people across the state , left 4,000 properties uninhabitable and damaged a further 10,900 homes. Lismore was especially hard hit due to its location on the Wilson River floodplain.
In response, Australia’s national science agency CSIRO has developed a high-resolution computer model to support flood mitigation research. In a recent report, the agency outlined how building ten large “detention” dams – could reduce flooding in the Richmond and Wilson River valleys.
Governments have committed several million dollars to a detailed study that will look at the economic, environmental and structural aspects of the proposed dams.
However, there are many reasons authorities should be cautious before committing financial and political resources to these schemes.
How would it work?
The modelled dams would hold water in upstream creeks during flooding, reducing flood heights in Lismore by about two metres and providing smaller benefits to several other towns.
These dams would only partially reduce damage from future floods on the scale of 2022 . If built, Australians would have to pay for the costs of the dams – likely billions of dollars – plus future flood damage.
Major cost blowouts are typical of large water engineering projects in Australia and around the world. A 2019 Australian study found pre-construction estimates are systematically biased on the low side. Using dams for flood protection can also face significant operational and legal challenges .
Other options deserve more detailed investigation. The Australian National University , Richmond Landcare Network and Southern Cross University are investigating “nature-based” solutions . These can mitigate future floods by retaining water upstream, reducing the extent of flooding and enhancing habitat and water quality.
Revegetation and ‘softer’ river engineering
ANU research suggests revegetation of the upper slopes of hills, wetlands and riparian zones – the area between land and waterways – lower flood heights and reduce their impacts. Small upstream floodplains known as “pinch points” may have the similar effects. Strategic revegetation works by detaining water upstream.
These approaches aim to work with nature rather than against it. With increasing international evidence that they work , they deserve more research to determine their effectiveness in specific catchments in Australia.
This kind of revegetation is already underway in the region , and could be combined with “softer” river engineering.
What about channels or levees?
One option worth further investigation is a diversion channel to shift floodwaters around Lismore. This would require re-engineering the Wilson River floodplain west of its current course and constructing levees from excavated material.
The diversion channel was first proposed by the Italian businessman, Floriano Volpato , a café proprietor, community leader and founder of Lismore’s Continental Club. After the damaging 1974 floods, he paid two Italian flood engineers from the Po Valley to visit and design a flood mitigation scheme .
The diversion channel proposal was rejected by the council in 1981. Instead, a levee able to protect against a one-in-ten-year flood was built around the central business district. Since being completed in 2005, the three larger floods have exceeded the levee design height of 10.6 metres.
Levees can and do work. They’re successfully used around the world , including in major rivers such as the Rhine . About 100 kilometres south of Lismore, the Grafton levee has protected the city from Clarence River floods for decades.
There are concerns over a ” levee paradox ” – levees leading to community complacency about flood risks. However, many well designed, well built and maintained
levees have successfully restrained floods, stopping major damage and disruption.
Transformation of the Northern Rivers
In anticipation of future flooding, Australia could combine traditional structural and flood mitigation measures, such as dams and levees, with nature-inspired designs.
It is important people do not lose faith in our collective capacity to plan and prepare for future damaging floods. CSIRO’s state-of-the-art hydrodynamic model is an impressive piece of scientific infrastructure that can underpin future flood planning.
Strategic revegetation and a re-engineered Wilson River floodplain could make Lismore Australia’s first ” sponge city “, where natural and engineered systems absorb rainfall and prevent flooding.
This would demonstrate Australia is serious about climate adaptation and disaster resilience in ways that build social harmony.
