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by Chris Tobin

The Rangitata River protection works remain in a vulnerable state after last month’s devastating flood and there are concerns heavy rain could result in more flooding.

Environment Canterbury (ECan) river engineering manager Leigh Griffiths said the council had communicated the risk to nearby landowners and the public.

“It’s important that people in and around the Rangitata River are aware that the river is vulnerable to outbreaks should river levels rise again,” Mrs Griffiths said.

ECan has identified three key sites needing strengthening in the wake of the flood which cut off State Highway 1 and flooded roads and farmland.

For 10 hours the river reached peak flows of more than 2000 cumecs due to heavy rainfall at the headwaters.

“The key sites are at Arundel above the bridge, at the south branch breakout point and just above the KiwiRail bridge,” she said.

“Once this work is completed we will have more confidence that the river could cope with a moderate flood event.”

At the peak of the emergency recovery work, 11 machines from five companies were operating. This was now down to six.

Mrs Griffiths said the first priority in the recovery work was to stop the outflow of floodwaters from the river on to the floodplain and over roads.

“This has now been completed thanks to a massive effort from machinery operators, contractors and key staff from several organisations including ECan, Timaru District Council and the NZ Transport Agency.”

Mrs Griffiths said the next stage of in-channel work and diversions should be finished by the end of this week.

“The last stage is reinstating banks and the vegetated berms which are crucial in erosion reduction and in providing some limited protection against overflows.

“Hundreds, if not thousands, of established trees were lost and this is something that will take years to recover.”

The repair cost was expected to run into millions of dollars with important in-river work already completed costing about $700,000.

“Beyond this a longer-term plan will be created for possible future work, its time frames and how it will be funded,” Mrs Griffiths said.

“This will require further discussions with the community and the Rangitata River rating district will be consulted on this.”

The Timaru and Ashburton district councils would be approached to discuss funding, along with NZTA, KiwiRail and communication companies.

The causes

Environment Canterbury said different flow patterns and major breakouts from the river caused most of the damage and disruption.

Rangitata south branch: Before the river peaked the main branch of the river broke into the usually dry south branch about 4.5km above SH1. For a time the south branch water flow was greater than in the main branch. It remained large until December 14 when flows were diverted into the main channel.

South branch breakouts: Floodwaters overtopped the south branch 2km upstream of SH1 and caused flooding on SH79, SH1 and in Rangitata village.

Railway line, SH1 flooding from main branch: Waters broke out of the main river just upstream of the railway line, crossed SH1 and flowed into the Rangitata Island area. This stopped traffic. Other breakouts from the main and south branch exacerbated flooding of farmland.

Arundel breakout: A large breakout 2km upstream of Arundel flooded the township and Geraldine-Arundel Rd.