1994 flood plan gets hurry-up

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. The nearby stopbank has been identified as weak, prompting 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.

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.

A report prepared by Dick was presented to an assets and services committee meeting yesterday. 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 that the water wouldn’t spill out like it did in 1983,’’ Dick said before the meeting. ‘‘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.’’

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

Design work for permanent flood protection was under way.

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 the rock groynes is to pull water away from the stopbank,’’ Dick said. The rock groynes were not built originally as the river flowed north but with its course changing an upgrade was needed.

The permanent upgrade would require a combination of heavy rock as the river 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 rivers capital budget over 10 years. Dick suggested some funding would need to be pulled forward to protect against further erosion. This would be proposed in the next annual plan.