Mt Cook stopbank as ‘second flood defence’

Press, 13 April 1978, Page 10

Mt Cook stopbank as ‘second flood defence’

r Timaru reporter The Ministry of Works has almost completed a second stopbank as a “second line of defence” behind the rock-raked stopbank at Black Birch Stream, at Mount Cook. The Ministry is con* cerned about the proximity of the stream to Mount Cook village, and the need to protect both the water supply and the stopbank which is controlling the stream in its channel. “Any serious breakout would have serious consequences because of the siting of the village in the wide Black Birch fan," said the resident engineer for South Canterbury (Mr E. R. L. O’Toole). “Should there be any unforeseen debris fall from the higher mountains, which is possible in a concentrated deluge, a blockage could occur. Any overflow from the first stopbank would be contained bv the second stoobank,” said Mr O’Toole On Boxing Day, 1957, 482 mm of rain fell at

Mount Cook in 24 hours. Last month, from Easter Day to the Tuesday, 401 mm of rain was recorded at Mount Cook. From 7 p.m. on Marco 26 to 8.30 a.m. the next day 169 mm was reg'stered. Over the next 24 hours rainfall amounted to 232 mm. Mr O’Toole said that as the result of rock-raking on the south of the Black Birch fan, the design was based on a calculated flood of 4500 cusecs at a velocity of 17.5 ft a second — a onein- 100-years flood pattern. The December, 1957, flood was approaching that volume and the flood last month was "getting near it.” The main highway was not damaged in last month’s heavy rain, but a recent inspection had shown isolated damage to the armouring at Black Birch, Worryline, Freds e Stream, and Bush Stram. The village was in no danger: the repairs were to ensure that full protection ••as reinstated and

maintained, Mr O’Toole said. In 1957, the Black Birch “ran amukc,” breaking out into a multiple pattern over the entire fan. Snowmelt contributed to the summer run-off from the high catchment and in an area of high-intensity rainfall it could produce an unusual run-off pattern, Mr O’Toole said. Over the last 21 years, the Ministry had adopted a system of control using “rock-raking” to produce single-thread streams in the mountain ’ rivers — one channel as opposed to the multiple channel that used to develop in wide river fans. In this way, said Mr O’Tooole, it had been possible to design bridges and put them in their correct positions within some of the wide river fans. Bigger rocks were segregated from the shingle and interlocked to form armoured stopbanks. This form of protection had proved- effective against many floods, Mr O’Toole said.