On 24 May there was a panel and discussion on Climate Risk: Can we safeguard our community. On the panel were experts in their field such as a civil engineer, Landcare worker and an environment lawyer.

While most of the night was spent discussing engineering solutions and how they may work on a broader scale such as can they be built to last a long time in an ever-changing climate, rather than specific Lismore Flood Mitigation solutions, the Dunoon Dam was one solution that was discussed in more detail.

This story is not concerned with the long term water strategies for the Northern Rivers but asking the question would it help in mitigating flood levels in future flood events.

Duncan Thomson, an environmental engineer from GeoLink, attended the meeting and then got in touch with the Lismore App to make the following observations.

"The Dunoon Dam would not significantly reduce flooding in Lismore, regardless of how it was designed. This is clear from a quick comparison of the relative catchment areas. That is, the area of upstream land from which flood flows originate."

"The combined area of the Leycester Creek and Wilsons River catchments upstream of Lismore is 1,400 km². The catchment area of the Dunoon Dam would only be 50 km² within this broader area (only 3.6% of the catchment upstream of Lismore). Even if it was possible for the Dunoon Dam to catch most of the runoff from 3.6% of the catchment, this would have an insignificant effect on flood flows reaching Lismore."

"People may be aware that the Wivenhoe Dam in the Brisbane River catchment can be managed to reduce downstream flooding. This is a completely different situation. The catchment area of the Wivenhoe Dam is more than 50% of the overall Brisbane River catchment. Therefore, it is possible to significantly reduce flood flows by capturing and detaining runoff in the Wivenhoe Dam. Importantly, the Wivenhoe Dam was strategically located and designed for the dual purpose of water supply and flood mitigation."

While the Dunoon Dam looks to be not a viable option for flood mitigation, the Wivenhoe Dam example does raise the question as to whether retention basins strategically built in the catchment areas of Leycester Creek and Wilsons River can be used for the dual purpose of flood mitigation and water storage.

Duncan said he has seen retention basins work on a smaller scale in new subdivisions and new developments so there was no reason they could not work on a larger scale.

"The Wilsons catchment would be tricky due to its size so to be effective it would probably need multiple retention basins to cover a large proportion of the catchment."

Of course, this would not be the only possible solution but could it be part of a number of options to control and lower the impact of floods in Lismore, Coraki and Woodburn.

"Most things can be done but is it right?," Duncan said, "The CSIRO is the right way to go to simulate the water flow through the catchment using different computer modelling."

"While the computer models are not perfect they are pretty good. They can assimilate historic floods, the last being the 2017 flood which displayed a good representation of how the flood occurred. It gives you confidence that the modelling is doing the right job."

Due to the size and scale of the catchment area, Duncan believes that we need more community input to put all available options on the table.

"The last thing we want is to miss a great idea," Duncan said, "We need to be open to big ideas and carefully assess potential options, but now is not the time to be distracted by red herrings. As a local engineer with flood management experience, I consider it irresponsible to suggest that the Dunoon Dam could be part of the flood solution."

Duncan Thomson, Ballina resident & Lismore business owner