A study from NIWA (now Earth Sciences New Zealand) presented to the West Coast Regional Council today says the council’s plan to extend the Havill wall on the north bank of the Waiho will protect the town’s sewerage ponds and other property for another four to seven years.
The bed of the Waiho, which has risen three metres in 15 years and now sits above Franz Josef, is held at bay by massive stopbanks.
But the risk to the tourist town was compounded in 2023 when the wild river suddenly changed course, or avulsed, forging a new channel into the neighbouring Tartare River.
The avulsion destroyed farmland between the two rivers and threatens to back up and undermine the Havill wall.
Mrs Lash says there is more at stake than her council's sewerage plant as the Waiho continues to veer right.
“My biggest concern is if we have a big rain event, we’re going to end up with a flood that will push the Waiho up the Tartare, on into Lake Pratt then run down to Lake Mapourika and on to Ōkārito,” she said.
Lake Pratt had already risen because of the additional water now flowing into the Tartare from the Waiho, she said.
“The risk is not just to our iconic lakes - beside Lake Mapourika we have State Highway 6, the bridge and the power lines going south that could all be wiped out at significant cost - and if it gets to Okarito and the lagoon we’re looking at environmental catastrophe."
The Havill wall was thrown up by the Westland District Council in 2016 after a flood in the Waiho overwhelmed the Franz Josef sewerage plant.
The NIWA study, commissioned by the WCRC to test the feasibility of the $2.8 million stopbank extension, has confirmed earlier predictions that the avulsion is likely to cut back to the downriver end of Havill’s Wall by 2029.
“Once the avulsion reaches the Havill’s Wall stopbank, the stopbank will come under direct attack from the main stem of the Waiho River," NIWA reported.
Extending the stopbank would likely slow the avulsion upriver, giving the councils and property owners four to seven years from now to move their assets, including the sewerage works, school and holiday park.
But it meant investing a further $2.8 million in an asset that would in the short term, come under direct attack from the river, at best be high maintenance and at worst likely have a limited lifespan, the study said.
And the deadline was not fixed.
“If there are more flood events, then the avulsion will progress faster. If there are fewer flood events the avulsion will progress more slowly.”
“The Regional Council likes playing in sandpits - but there are other options, “ she told Local Democracy Reporting.
“The river needs to be forced back towards the south bank - and away from the Tartare and it could be done. They could actually put a ‘dozer in there and start trenching and training it from the middle to go back down the gut."
People who had lived with the river all their lives and knew its ways were suggesting the same thing, Mrs Lash said.
“I laid out the problem and the risks to Minister Shane Jones when he was down here last month, and he has taken the maps and the paperwork to Cabinet.”
The Westland mayor said the New Zealand Transport Agency was also alarmed at the threat to its state highway and bridges as the Waiho avulsion continued.
Mrs Lash said she would be meeting government officials in Wellington within the next fortnight to discuss the options.
- By Lois Williams, Local Democracy Reporter
