Engineers have concerns about the capacity of flood banks in Southland’s Mataura River catchment.
Environment Southland manager of integrated catchment management Paul Hulse said flood protection assessments being done as part of climate resilience works have identified the need for detailed geotechnical investigations and 2D hydraulic modelling to calculate the capacity and integrity of the stop banks.
Hulse said this means urban communities in Gore, Wyndham and Mataura may need to be evacuated at a lower river flood level than previously, until there is a better understanding of stop bank capacity.
“We know that most of our stop banks are at least 30 years old and they have some design limitations that may reduce their ability to hold large volumes of water or sustained volumes of water, putting the community at increased risk of flooding,” Hulse said.
The stop banks were designed to address previous flooding issues but could be tested by the effects of increased flood events and high river flows associated with climate change.
The February 2020 floods on the Mataura River also highlighted the issue, and while some remedial work was carried out, more is needed.
“We now know that the frequency of flooding events is likely to increase and we will be working closely with Emergency Management Southland to take a precautionary approach to keeping our communities safe,” he said.
Flood events in the catchment can be predicted up to 12 hours ahead of time.
“We also know from experience that we can quickly and effectively evacuate these communities, having done it most recently in 2020 within a couple of hours.”
Geotechnical data and hydraulic modelling is being compiled to help establish the best options to protect these areas in the future, while a work plan is underway for the Gore stop banks.
