Flood scheme cost benefits ‘significant’

The proposed ring embankment for a $26 million Westport flood scheme will not provide uniform flood protection across the town.

But a cost analysis projects about $212 million in savings in a future flood.

A meeting of the Westport Joint Rating District this week received the options proposed under the $26 million business case, which will go to the Government at the end of the month.

Two ring bank options estimated at $16.9 million and $18.9 million, plus a range of other mitigation measures, have been assessed using Niwa’s "riskscape" model for the business case for the proposed scheme.

It included confirmed "significant cost benefits" for what would be a $25 million investment, a report said.

Niwa’s report concluded that under an ARI100/RCP6 flooding scenario, about $212 million of damages would be estimated to occur to Westport buildings.

"The work of Niwa thereby confirms significant cost benefits will arise from the investment of $25 million in the proposed Westport flood risk mitigation scheme."

ARI is the average time period between floods of a certain size, while RCP is the modelling used to predict climate change effects. Both criteria have been used in the modelling for the proposed scheme.

Consultant for the Westport scheme business case John Hutchings said the cost benefit analysis for the proposed scheme was based on a $212 million saving in damage to buildings in the worst disaster scenario, using the Niwa modelling.

The cost benefit from the proposed $26 million scheme "on the face of it stacks well", he said.

The total cost of $26 million incorporates a $1 million contingency at this stage.

Mr Hutchings said the value of the investment would be significantly better than having nothing to protect Westport, as had been the case last year.

The July 2021 flood is estimated to have caused $100 million in damage to the town.

While the Buller River side of the stopbank scheme had to be designed to accommodate climate change effects in the next 100 years on top of a one-in-100-year flood, the Orowaiti River side of the scheme with lower stopbanks would simply accommodate a one-in-100-year flood.

There was worry it would not meet community expectations, including its exclusion of the Snodgrass Rd residential area near the Orowaiti River.

Buller Mayor Jamie Cleine said he was worried the town was being asked to accept a lower level of protection which, for $650,000 extra, could be fixed.

"I understand the less dangerous nature of water from the Orowaiti in terms of life, but not property damage. One of the arguments [for the scheme] was for property protection.

"If we accept this lower level then we haven’t done our best."

Mr Hutchings said priority was given to the "most dangerous aspect" — the Buller River.

By Brendon McMahon

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