Rock Took Waimakariri Flood Test Well

Press, Volume CVI, Issue 31339, 8 April 1967, Page 14

Rock Took Waimakariri Flood Test Well

Rock protection on groynes and banks stood the test well in the Waimakariri river flood of March 11 and 12, and indications were that standards of construction were adequate, stated a report from the engineering staff received by the North Canterbury Catchment Board at its meeting yesterday.

The flood was gauged at nearly 85,000 cusecs, a flow exceeded only four times in the previous 36 years. Flood-repair costs were estimated at £2500, compared with £18,270 for a similar-sized flood in 1955, and £57,134 for a 100,000-cusec flood in 1957. “This flood was substantial enough to provide the first real test of major works already carried out under the 1960 river improvement scheme,” stated the report, prepared by the chief engineer (Mr E. B. Dalmer), his assistant (Mr J. A. Macdonald), and the design engineer (Mr G. D. Stephen). “No reports were received of any stock losses, and flood damage to river protection works were not extensive, amounting to little more than regular maintenance.” The flood arose from heavy rainfall in the Arthur’s Pass area, where up to 16.83 in of rain fell between March 8 and 11.

“More than half the physical works in the £lm river improvement scheme have

been completed,” the report continued. “The flood provided an opportunity to review the scheme as a whole, and the performance of the various types of work already carried out. “An indication that standards of construction are adequate has been given by the nature and extent of the damage sustained. Since 1961, rock has steadily taken the place of gabion work for groyne construction and control of bank erosion. Piled permeable groynes are being used with some success for river training. “Where erosive forces are greatest, rock has proved to be the most successful and flexible means of providing stable continuous protection, and it offers certain advantages in protecting groyne heads.” The statistical information accompanying the report showed that the minimum amount of “freeboard” on the banks was 3.37 ft.

Asked if a similar report would be made on the Rakaia flood at the same time, Mr Dalmer told the meeting that insufficient data were available. “If we had ordered the flood, it could not have been better,” said Mr L. F. Wright

“It showed how successful the board’s policy with rock protection has been.”

Moving that the report be released for publication, Mr L. W. McCaskill said that the public, who found the money, should be told how the protective works had stood the flood. The board agreed with a suggestion by the engineers that heavy rock, if available, should be stockpiled, Mr Dalmer saying that it would be an advantage to have supplies at points where the rock might be most needed.

The board commended the Ministry of Works for gauging the flood at the Main North road traffic bridge under dangerous conditions at night.