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GISBORNE FLOODS

LAST YEAR'S STORM

ENGINEER'S PAPER

(Special to the "Evening Post.")

AUCKLAND, This Day. The extraordinary flood discharges in streams near Gisborne were the subject of a paper by Mr. O. G. Thornton read at the annual meeting of the New Zealand Institution of Engineers in Auckland. The Iftoods occurred in February last year, and were responsible, in the case of the Kopuawhara disaster, for the loss of 21 lives. ■'■-'■ The rain storm which produced the i ods was of short duration but i reached terrific intensity during several - hours ji the early morning," said Mr. Thornton. "It was characterised by vivid lightning flashes and it would appear that the adjustment of electric "potentials in the atmosphere had some effect in concentrating the widespread coastal rain into areas of special intensity contiguous to the tops of the * . ridges . . . generally somewhat below 1 2000 feet in height but which rise to about 2300 feet at two or three points. Their original forest or scrub covering has been-almost entirely removed and replaced by grass. It is unfortunate that no automatic rain gauges were in operation in any of the catchment areas, and that the peak occurred in the small hours when observers were asleep. The floods in the Kopuawhara, Maraetaha —and especially its tributary the Mangakotutuku—Waimata, and Pakarae were the highest - of which there is any record at the measurement sites. ALL RECORDS EXCEEDED. "The tragic Kopuawhara flood was also well above anything previously known either in Maori legend or pakeha records. This stream is well confined and reasonably regular in character between and through the measuring sites. A staff of qualified engineers was located in the valley on the railway construction works and was able to obtain very definite flood . marks' and to select the best sites for measurements. ! The comparatively small tributaries between the measuring stations were also measured and the progressive increase in flow of the main stream from the upmost to the lowest measuring site was consistent with the additions from these tributaries, f "The tabulated results show exceptionally high velocities in the Kopuawhara for such a large body of water. The scouring effected in such a brief high stage period was fully consistent with such velocities. . . . - The site of No, 4 camp for single m-*n was a flat about 12 feet above ordinary water level, with good soil overlying a tight formation of heavy boulders. It was fringed with a few old willow trees along the bank and dotted with totara trees. One of these trees had been felled before the flood for survey pegs. A section from it was submitted to an officer of the Department of Agriculture. He estimated its age at about 90 years. The whole of this flat was scoured away. As the depth of water ■* over the top of the flat reached only 3 feet it is clear the erosion must have / been largely ,|rqm the side. The velocities tabulated do not appear any higher than wquld be required for such rapid and complete destruction. "The Kopuawhara stream flow at the No. 3 camp measuring point wasi Gauged over a sharp-edged weir a fort-night-after the flood. The total flow was only 22 cusecs as compared with the flood peak of 33,900 cusecs."

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Permanent link to this item

https://paperspast.natlib.govt.nz/newspapers/EP19390221.2.35

Bibliographic details

GISBORNE FLOODS, Evening Post, Volume CXXVII, Issue 43, 21 February 1939

Word Count
541

GISBORNE FLOODS Evening Post, Volume CXXVII, Issue 43, 21 February 1939

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