River breaks through dam
A ‘‘large wall of water’’ up to 15 metres high is surging down the Clarence River and nearby residents are being warned to move to higher ground as soon as possible.
A slip dammed the river about 10 kilometres upstream from the mouth, north of Kaikoura, after Monday morning’s 7.5-magnitude earthquake. The banked-up water burst through the wall of debris about 4.20pm. A helicopter in the area saw the breach and the water beginning to roll downstream.
It would take three to five hours for the deluge to reach the coast, passing alongside about a dozen houses situated along the river.
They have been urged to evacuate for higher ground.
Six kayakers unaccounted for when the surge started have been found safe after abandoning their gear by the river bank. A separate group of 16 rafters on the flooded river were contacted by police. They were upstream from the slip site, away from the danger area.
The river’s water would likely be flowing at 1000 m3/s to 2000 m3/s, Environment Canterbury (Ecan) said.
For comparison, a one-in10 year flood event is a1600 m3/s flow, and a one-in-100 year event was 2500 m3/s.
Ecan had warned people to stay away from the Clarence River and nearby flood plain areas. Ongoing earthquakes meant there were risks of landslides into all North Canterbury rivers.
Former Clarence River Rafting guide Gretchen Dalzell guided tours along the river last season, and said she was receiving updates about the group of rafters.
‘‘They must have left around Thursday or Friday last week,’’ Dalzell said. ‘‘At this time of year we would do five-day trips. We start at Jacks Pass, about a 40-minute drive from Hanmer, and get into the Clarence River from there. ‘‘We would camp on the way down. We’re quite aware it’s a very steep valley, and there are lots of areas you might camp where, in an earthquake, it would be quite treacherous. There’s the potential for lots of rockfall.’’
The river had a grade two rafting classification.
The guides were trained to cope in an emergency. ‘‘But, I mean, how much can you do to prepare for that?’’ - Fairfax NZ