CSIRO tests show even 10 'detention' basins cannot 'flood-proof' city

Richmond River High School during the February/March 2022 flood. (Supplied: Rod Evans)

The CSIRO has released the findings of its Richmond River catchment flood mitigation modelling project

The model tests the impact on flooding if up to 10 detention basins were built, a major wetland restored and a new river outlet created

A further $3 million has been committed to assess the technical, environmental, economic and social feasibility of potential interventions

Modelling by the CSIRO shows that even the installation of 10 upstream "detention" basins would not have saved Lismore during the record-breaking floods of 2022.

The organisation has been running scenarios through its super-computer in Canberra to test how flooding would be reduced in northern NSW towns devastated by the 2022 floods.

They include construction of up to 10 "detentions" to hold water in upstream creeks during times of flood, and a canal cut through farmland to create a second outlet for the Richmond River near the coast.

"Right from day one, when they were asking me, 'can I flood-proof the region?' my answer always is, 'nobody can'," CSIRO project lead Jai Vaze said.

The CSIRO tested two 'bundles' of mitigation options against flood data from 2008, 2017 and 2022.

Under February 2022 flood conditions, the scenario that includes 10 detention basins at natural choke points in all river tributaries upstream of Casino and Lismore would reduce flooding in Lismore by 2.07 metres.

A mitigation scenario tested by the CSIRO includes "detention" dams, a second river outlet and restoration of the Tuckean Swamp (Supplied: CSIRO)

That record-breaking flood saw water overtop the Lismore levee at 10.2 metres and reach a height in the CBD of 14.37 metres.

The same mitigation scenario would see flooding reduced in Kyogle by 0.80 metres, in Casino by 0.95 metres, in Coraki by 0.42 metres and in Ballina by 0.73 metres.

Dr Vaze said this meant that even if the full suite of mitigation measures were built, flooding on a par with 2022 would still inundate townships in the catchment, but water levels would be reduced, the duration lessened and the footprint would be less widespread.

Hydrologist Dr Jai Vaze is the lead scientist on the CSIRO project. (ABC News)

"This has always been envisioned by the National Emergency Management Emergency (NEMA) as a pilot project, and the lessons … can become a guiding light for even large international flood mitigation projects," he said.

The $11.4 million project is a hydrodynamic study and does not include the technical, environmental, economic and social