Flood protection plan first mooted in 1994 signed off as stopbank erodes

Thursday, 18 July 2019

Rocks to be used as 'groynes' have been stockpiled along the Wairau River ahead of the work starting.

Flood protection along a Marlborough river is going ahead, 25 years after it was first 'envisioned'.

The Marlborough District Council is to spend $250,000 on temporary 'groynes', or rock walls, along the Wairau River, near the intake of the Waihopai River.

Marlborough District Council rivers and drainage manager Geoff Dick in 2018.

The stopbank has been identified as weak, prompting the 'emergency works' to slow the flow of the river and pull water away from the stopbank to help control erosion.

The stopbank was built in 2000, but earlier ideas, in 1994, for stronger flood protection never happened.

**READ MORE:

The Wairau River is at risk of flooding, as the rivers course has changed causing excess erosion.

* Ex-Cyclone Gita floods school, homes in Marlborough's Wairau Valley

* Land bought beside river to bring weakened stopbank forward**

Map showing where the temporary groynes will be built. The red line shows where the flood wall was built in 2000 which is eroding due to the fast flowing Wairau River.

Marlborough District Council rivers and drainage manager Geoff Dick said the course of the river had changed significantly since the stopbank was built, and it had started to actively erode.

The erosion had increased the risk of flooding, which could affect the Southern Valleys irrigation system, Dick said.

The Wairau River stopbank, built in 2000, has been identified as weak.

A report prepared by Dick was presented to an assets and services committee meeting on Thursday. Councillors approved the report and referred it on to full council on August 8 for adoption.

'If we left it [the new work] too long, we couldn't guarantee the stopbank would hold and the water wouldn't spill out like it did in 1983,' Dick said before the meeting.

The Wairau River from Ferry Bridge, Spring Creek, in flood during cyclone Gita in 2018.

'Eventually that could damage the Southern Valleys intake infrastructure, which would mean no water for those Southern Valleys.'

Councillor Geoff Evans said he was 'totally supportive' of the emergency work. 'It's certainly the way to go.'

A total of $250,000 will be spent on emergency works to help stop the stopbank eroding.

The erosion was occurring along the south bank of the Wairau River and the lower 500 metres of the Waihopai River.

A design for permanent flood protection was underway. Until this was completed a series of small temporary rock groynes along the Wairau River would be built to 'halt the erosion and maintain an adequate degree of protection'.

'The purpose of rock groynes is to pull water away from the stopbank,' Dick said.

Dick said rock groynes were not built originally as the river flowed north, but its course had changed so an upgrade was needed.

The permanent upgrade would require a combination of heavy rock to protect the river as it was so fast flowing and powerful. A 'buffer zone of trees' between the river edge and the stopbank would also be required to catch debris.

Four thousand tonnes of rock had been supplied and stockpiled at the site to create the series of small temporary groynes.

Some in-channel river flow alignment work had been completed on the northern river channels.

A total of $4 million had been included in the council's River's Section capital budget over the 10 years. Dick suggested some funding would need to be pulled forward to protect the river against further erosion. This would be proposed in the next annual plan.