Southland towns may need to evacuate sooner than originally planned after discovering their stop banks may be at a higher risk of flooding.
The February 2020 Mataura River (pictured) resulted in a state of emergency. Photo: RNZ / Sarah Robson
An evaluation identified capacity concerns with the stop banks on the Mataura catchment.
Environment Southland is urging people in Gore, Wyndham and Mataura to be prepared for flooding events as evacuations could be triggered at a lower river level than previous floods.
Integrated catchment management general manager Paul Hulse said investigations were carried out as part of climate resilience work.
They found further work, including detailed geotechnical investigations and 2D hydraulic modelling, needed to be undertaken urgently to better understand the capacity and integrity of the stop banks, Hulse said.
"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."
They were designed to address flooding issues at the time but more extreme weather events as a result of climate change have put them under pressure, he said.
Some remedial work was carried out in the wake of the February 2020 Mataura River floods, but Hulse said more work was 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."
Calls to evacuate are made by Emergency Management Southland.
"Where we are fortunate, is in being able to predict how quickly flood waters travel and our hydrological team will know up to 12 hours ahead of time, what the situation is likely to be around the catchment," Hulse said.
"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."
The catchment team was gathering more information to establish the best options for protecting these areas in the future, he said.
"We already have a work plan underway for the Gore stop banks, but the Mataura situation is a bit more complicated for a number of reasons, including the proximity of the stop bank to the road.
"We need more data before we can consider what the remediation options here might be."
Environment Southland will host drop-in meetings in Mataura, Gore and Wyndham to discuss the issues with residents, with details expected to be advertised on its website as soon as possible.