Flooding risk after earthquake damages Marlborough stopbanks

Professor Russell Green from Virginia Tech and Associate Professor Adda Zekkos from the University of Michigan use laser ...
SCOTT HAMMOND/FAIRFAX NZ

Professor Russell Green from Virginia Tech and Associate Professor Adda Zekkos from the University of Michigan use laser technology to get a 3D-image of earthquake damage.

Hundreds of Marlborough homes are at risk of flooding in heavy rain as stopbank repairs following last month's earthquake are yet to get underway.

Earthquake damage to the region's 180-kilometre stopbank network has been identified near the settlement of Spring Creek, north of Blenheim.

Marlborough District Council rivers and drainage engineer Geoff Dick said work would begin at Spring Creek early next year after a geotechnical report was completed. 

University of Auckland senior lecturer Dr Liam Wotherspoon sends vibrations into the soil beneath the stopbank at the ...
SCOTT HAMMOND/FAIRFAX NZ

University of Auckland senior lecturer Dr Liam Wotherspoon sends vibrations into the soil beneath the stopbank at the Wairau Diversion to see how firm it is.

"We've got to take it half down. We really need fine weather," he said. 

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Residents were left confused after an evacuation warning was mistakenly issued in Spring Creek after heavy rain last week. 

Cracks on the road on top of the Wairau Diversion stopbank.
SUPPLIED/MDC

Cracks on the road on top of the Wairau Diversion stopbank.

About 1275 people live in the Spring Creek and nearby Grovetown areas, according to the 2013 census. 

Marlborough District Council chief executive Mark Wheeler said the stopbank at Spring Creek could still contain 2500 cubic metres of water per second, but people were encouraged to self-evacuate if the water level rose about 2000 cubic metres.

"It's giving people a buffer," he said. 

The Wairau River contained only 68 cubic metres of water per second on Tuesday morning.

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A leaflet was going out to residents in the Grovetown, Spring Creek and Wairau Diversion areas telling them about what to do in the event of heavy rainfall.

They were urged to listen for emergency broadcasts on Brian FM, More FM, The Breeze and RadioLive, and to monitor the council's social media posts and flood watch page, hydro.marlborough.govt.nz/floodwatch/

They were also encouraged to have a personal evacuation plan including a "grab bag" with essential items, and to keep their insurance policy numbers handy. 

A council spokesman said the public would be asked how they would prefer to be told to evacuate at a public meeting on December 15, at 7.30pm in the Spring Creek Hall. 

The school at Spring Creek already had a phone tree, where residents were given a list of neighbours to phone, so possibly the council could use that if necessary to alert people.

Dick said the council had only carried out visual inspections of stopbank damage.

Geotechnical reports would be carried out on the stopbanks, and repairs would be carried out in the New Year. 

As soon as Spring Creek and Grovetown people got onto State Highway 1, they would be out of the immediate risk zone, Dick said. 

The council also intended to get an aerial survey done to get a clearer picture of damage around the region. A fuller picture of how the quake had affected stopbanks would be available by the next assets and services committee meeting in February, Dick said. 

According to the council leaflet repairs to affected stopbanks around the region were likely to take up to six months. 

During the heavy rain after the earthquake the Wairau River reached 1200 or 1300 cubic metres, Dick said. The last time the river reached 2500 cubic metres was in 2012. 

The last serious flood in Marlborough took place in 1983, but people should not get too complacent, Dick said. 

"Stopbanks are not completely infallible." 

The flood waters passed over 4350 hectares of farmland and urban land during the 1983 flood, and insurance payouts were reported to reach $2.3 million. 

The national Quake CoRE research team were out testing damage to the badly-cracked stopbank at the Wairau Diversion on Tuesday.

The stopbanks were more likely to move if they were situated on wet, swampy ground, Dick said. 

University of Auckland civil and environmental engineering senior lecturer Dr Liam Wotherspoon said the Quake CoRE team was onsite with the American Geotechnical Extreme Events Reconnaissance team, testing the firmness of the soil beneath the damaged stopbanks. 

By putting sensors on the surface and sending vibrations down into the earth they could test how stable the soil was, up to 30 metres down.

The results would help when it came to repairing the stopbank and building future constructions, Wotherspoon said. 

Over the summer more work would be done, and would link in with what local engineering firms and the council were doing. 

The stopbank was weakened but it was hard to say by how much, he said. 

The road along the Wairau Diversion stopbank was closed as it was impossible to drive across. 

Dick said the Opawa River had been affected and there would be research done in the future as to how the channels had been narrowed. 

New Zealand Institute for Plant and Food Research scientist Rob Agnew said there was a 50 per cent chance of Marlborough having an "average" rainfall during December and January, and only a 25 per cent chance of rainfall above average. 

The average was 48 millilitres for December, and 44ml for January. 

However rainfall during those months tended to be "all over the place", Agnew said. 

 - The Marlborough Express

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