Likely flood areas to be studied

Press, 1 October 1983, Page 2

Likely flood areas to be studied

Staff of the North Canterbury Catchment Board will take a close look at some Canterbury rivers to see how they would cope with rainfall similar to that which caused the July floods in the Nelson and Marlborough areas. The board’s operations committee heard a report from the rivers and drainage engineer, Mr R. E. Reid and the design engineer, Mr A. J. Boyle, who had visited the Marlborough and Nelson catchment boards to observe the performance of river-protection works during the floods. During the worst part of the rainfall, 400 mm fell in 72 hours in the region, which caused most of the flooding, they said. They noted the extent of the ?57 million flood relief scheme on the Wairau River in Marlborough, and pointed out the comparatively low

repair cost of $1 million. Staff would look at preparing a “contingency plan” involving emergency resources for flood control on the Waimakariri River in the event of such a high rainfall, said the operations manager, Mr B. P. Dwyer.

Members of the committee pointed out that communications, access, emergency sandbags and rock, and finance for flood damages should be looked into.

Protection works on the Ashley River and the Selwyn River should also be scrutinised. The committee’s chairman, Mr R. B. Johnson, said that a hypothetical flood exercise should be carried out, in conjunction with Civil Defence.

He suggested staff approach metereological experts to see if such a high rainfall could occur in the Main Divide, as it was very

different from the region at the top of the South Island. Judging from figures given in the report, the board’s chairman, Mr M. J. O. Dixon, said that such a rainfall would bring 100,000 more cumecs of water down the Waimakariri than occurred in the floods of December 1957.

“If we get that amount of rain, heaven knows what would happen to Christchurch,” he said. The report said that the Marlborough Catchment Board would have had to have 500 years of rainfall data to predict the return of similar conditions.

“Whether, in the light of the increasing value of Christchurch, a 100-year return period for our data is enough, I do not know,” Mr Dixon said. “We would have to rethink our original policy decision,” he said.