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New modelling from the CSIRO shows that even building 10 upstream detention basins and creating a second outlet for the Richmond River would not have saved Lismore from flooding during the record-breaking 2022 floods. The report, released Tuesday, found that the mitigation measures would have reduced flood levels in Lismore by 2.07 metres during the February 2022 event, but that reduction would not have been enough to stop the Lismore CBD levee from being overtopped.

The $11.4 million Northern Rivers Resilience Initiative, a hydrodynamic study, tested two bundles of mitigation options against flood data from 2008, 2017, and 2022. The measures modelled include up to 10 detention basins at natural choke points in river tributaries upstream of Casino and Lismore, as well as a canal cut through farmland to create a second outlet for the Richmond River near the coast. The scenario also includes restoration of the Tuckean Swamp.

Coverage comparison

The report's findings were covered by ABC Australia and The Guardian, with slightly different emphases. ABC's coverage highlights that the mitigation scenario would reduce flooding in Lismore by 2.07 metres, while noting that even with the measures, floods on a par with 2022 would still inundate townships. The Guardian's report focuses on the fact that building 10 new dams could have reduced flood levels by up to two metres, but not enough to prevent the levee from overspilling.

Both sources report the same core numbers: the 2.07-metre reduction in Lismore reported by ABC aligns with The Guardian's 2.1-metre figure, which rounds the data. Neither outlet disputes the overall conclusion that the measures would reduce but not prevent flooding.

Key claims

The CSIRO modelling shows that the installation of 10 upstream detention basins would not have saved Lismore during the 2022 floods. The mitigation scenario would reduce flooding in Lismore by 2.07 metres, and also reduce flooding in other towns: by 0.80 metres in Kyogle, 0.95 metres in Casino, 0.42 metres in Coraki, and 0.73 metres in Ballina. The scenario includes construction of up to 10 detention basins and a canal cut through farmland to create a second outlet for the Richmond River near the coast.

The CSIRO tested two bundles of mitigation options against flood data from 2008, 2017, and 2022. The project is a hydrodynamic study and does not include technical, environmental, economic, and social feasibility of the mitigation measures.

The 2022 floods led to the deaths of 13 people, left more than 4,000 properties uninhabitable, and damaged another 10,849. The total socioeconomic and structural damage is estimated at $16bn. The flood saw water overtop the Lismore levee at 10.2 metres and reach a height in the CBD of 14.37 metres.

CSIRO hydrologist Dr Jai Vaze, who led the report, said the aim was to reduce the impact of floods. "Right from day one, when they were asking me, 'can I flood-proof the region?' my answer always is, 'nobody can'," he said. "The aim is to reduce the impact of flood." He noted that implementing the measures would likely depend on a business case, detailed architectural proposals, and environmental approvals.

Perspectives

The federal minister for emergency management, Kristy McBain, welcomed the findings, according to The Guardian. NSW Greens upper house member Sue Higginson called for a planned retreat and nature-based solutions, as reported by the same outlet. Neither source provided further details on these statements.

Lismore residents have called for alternative measures, including relocating or establishing a second CBD above the flood level, as reported by The Guardian. The state and federal governments have not committed to delivering the measures modelled in the report, but announced a joint $3m feasibility study.