1997: Floodwaters overwhelm Opuha dam; residents evacuated

Surging Canterbury floodwater tore away a third of the massive Opuha dam near Fairlie early yesterday, sending a violent wall of water towards the sea.

The dam section tumbled soon after 1am after heavy machines had been brought in to dig a channel to release floodwater build-up behind the unfinished wall of compacted gravel.

Diggers began clearing a channel about 9pm on Wednesday after it was clear that four days of heavy rain could not be contained in the lake behind the dam.

Muddy water roared through the channel and through a pipe beneath the dam, draining at 50 cubic metres a second, but the structure began to crumble and the huge volume of water in the lake cannoned forward.

The collapse let the water hurtle into the Opuha system, and then, near Raincliff, into the already high Opihi River.

Civil Defence officials declared a state of emergency at 3.15am yesterday and about 200 residents of the riverside settlements of Stratheona and Butlers Crossing, near Pleasant Point, were evacuated.

They were allowed back into their homes about 6am after it was clear the torrent had passed its peak.

On its wild ride through the river system the water punched out the approach to the Skipton bridge over the Opuha, closing State Highway 79 between Geraldine and Fairlie and leaving engineers with a deep gash to repair.

After tearing out the approach the floodwater crossed the highway, veered through paddocks, and re-entered the river system, prompting the Pleasant Point evacuation.

Water levels behind the dam had been building for several days after heavy rain in the catchments of the south and north Opuha rivers, which will feed the planned Lake Opuha behind the dam.

At 3pm on Wednesday the water was 4m from the top of the dam, which had already been pushed 2m higher during the day.

Dam workers were confident then that the pipe beneath the dam would keep the level safe.

Southerly rain, which had filled most South Canterbury rivers during the week, continued throughout the afternoon and into the night, raising the lake level, and fears.

Opuha Dam Company chairman Sir Peter Elworthy drove to the dam site about 8pm on Wednesday after arriving from Wellington.

He stayed until after 11pm, confident the diversion channel would handle the outflow.

He said he had expected, at worst, the channel to deepen and widen, but was not prepared for the devastation he saw yesterday morning.

The dam, shorn away at one end, hung over a brisk flow of muddy water still draining from the Opuha rivers, and wood and rocks were scattered over bent machines and the river bed.

Further downstream, the water wrecked a pump station for the Allandale water supply system, leaving 38 homes without water.

The dam site was insured and consideration of property compensation would be addressed urgently.

Sir Peter said changes to the construction timetable could be decided only after detailed investigation. Engineers would be on the site as soon as possible to assess repairs.

Before the break-out, the dam project, to supply irrigation, power, and form a recreational lake, was about two months ahead of schedule.

The lake was to start filling this winter.

 

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