Cracked Marlborough stopbanks to be rebuilt 'bigger and beefier'

The trigger point for evacuating Spring Creek has changed as council revealed the findings from drilling into the cracked stopbanks that protect the small Marlborough settlement.

A meeting at the Spring Creek Community Hall was held on Wednesday night to update the community on the state of the stopbanks previously said to be at "catastrophic risk of failure".

Three months earlier, Marlborough District Council rivers and drainage engineering manager Andy White had told the community the council needed to find $8.7 million to fix the stopbanks, and there could be difficulty sourcing enough rock for the work.

The stopbanks were critical for protecting the township of about 600 people. One of the stopbanks started showing signs of cracking after the 2016 Kaikōura earthquake. Cracks started to appear in the lower section of the other one during the July 2021 floods.

A geotechnical assessment, which involved drilling boreholes and test pits into the Peninsula Rd stopbanks, had since been completed to understand why they cracked.

"My concern at the time was that this could be the start of the worst-case scenario," White said on Wednesday.

"If we have a berm loss, we've got a much bigger issue to deal with. I can tell you now, we don't have that problem."

White said core samples taken from the geotechnical drilling now gave the council "confidence" about what the ground was made up of.

"This is really important, because if we're going to raise that stopbank, we don't want to be putting it on crap material, excuse my language.

"We don't want that to fall apart once we start loading it up."

The report that came from the assessment found that in the 2016 earthquake, two "direct faults" had occurred in the middle of the bank.

"That's what we call lateral spread."

Then during the July 2021 flood event, water had come through those cracks.

When the water then retreated, it sucked out some of the fine material, he said.

"That's not uncommon, it's like liquefaction. The result of that is cracking through the bank."

White said it was "structural" and did not compromise the berm itself.

The biggest risk was if there was another earthquake.

"It wouldn't hold up in another earthquake, and therefore we can't build on top of it — we need to rebuild the whole thing."

White said it was hard to know how the stopbank would behave in another flood, but he believed it was likely to hold up.

"But as we get more and more draw down, you're going to get more cavities, and that bank will start to fall apart as the water recedes.

"We are looking at a problem that was scoped for in the original proposal for council. It is a problem we can fix, and it is a problem we have got the money to fix."

Earlier this week, it was confirmed by Regional Development Minister Shane Jones the council would receive $3.6m from the Government to help with the project. A further $2.8m was also announced for flood protection on the Wairau River.

White said he and mayor Nadine Taylor had been working hard behind the scenes to make sure that money, first announced as part of Budget 2024, came through as a grant, rather than a loan.

Through its long-term plan, the council had also approved $8.7m for the project.

"We can reallocate some of that money to other projects that we've got within the flood protection schemes."

The trigger point for an evacuation had also been changed. It used to be when the river reached 3500m³ per second.

"What we're thinking now is raising that to 4000m³ and rising."

That was a 1-in-20-year flood event level.

Spring Creek resident Ruth Stagg questioned the council's modelling from an evacuation earlier this year for households from the river to March St in Spring Creek.

White said that a forecast for the water level to reach 3500m³ had triggered that evacuation, with modelling suggesting the water would reach to March St if the stopbank breached.

However, with the evacuation trigger level rising to 4000m³, the size of the evacuation area would change as there was a risk of more water spilling through the town.

Stagg said after the meeting the update was largely positive, but said it was a shame it could not get fixed sooner.

The repair, which would see the stopbanks build back "bigger and beefier", would take more than two years.

White also showed the meeting how they planned to repair the stopbanks. Concept designs were under way and White said he was confident they would be able to get enough rock despite earlier concerns.

Some enabling works had started, but the major river works would start in spring next year following more detailed design.

At least 35,000 tonnes of rock would be needed for the job.

LDR is local body journalism co-funded by RNZ and NZ On Air

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