Benmore Hydra—III Earth Dam Materials Available Near Site

Press, Volume XCVII, Issue 28491, 22 January 1958, Page 16

Benmore Hydra—III Earth Dam Materials Available Near Site

(By a Staff Reporter of “The Press*!

An impressive volume of investigation and preliminary research and design by the Ministry of Works has prepared the way for the £36 million Benmore hydro-electricily scheme. A detailed study of the Waitaki catchment was begun in 1954, and the report compiled from it provided a mass of vital information recorded in 90 pages of text and more than 40 maps and drawings.

Earlier plans envisaged concrete dam structures and several closely adjacent sites received exhaustive investigation. Meantime the technique of earth dam construction advanced very considerably and when the position was later reviewed it was found that materials for a large earth dam were available in ample quantities near a very suitable site. Coneentration on this site led to the present scheme and the preparation of working drawings numbered by the hundred- On the site itself investigation of so types called for the sinking of more than 80 shafts each about six feet by four feet to an average depth of 23ft, as wen as the drilling of more than 150 holes about six inches in diameter, and 20 or more auger 16 inches in diameter. Many* hundreds of samples have beexv taken for testing in the Minis-* try of Works laboratories. Geolecieal Survey A geological survey has been completed by officers of the Department of Scientific and Industrial Research, and the and Survey Department has contributed an aortal survey which has provided, within about two months, topographical information which coold not have been obtained by ground parties in less than two yean. The design team, working in the Wellington design office of the Ministry -jf Works* will provide about 1000 drawings on civil and mechanical engineering aspects while almost as many more prepared by engineers and draughtsmen will cover construction problems and infiormation. Design work is done in close elaboration with the State Hydro-electric Department to ensure that the requirements of electrical, civil and mechanical engineering dovetail. River Silting One of the apparent problems that had to be studied and solved before work could begin was the effect shingle carried down by the river would have on the storage lake behind the Benmore dam. The Waitaki is a steady shingle and silt carrier and frost-riven steam continually feeds down from the high hills and hugs screes on the hillsides along its course and those of its tributaries. The mean flow of the Waitaki at the Benmore site is approximately 12J500 cubic feet a second but in flood the river becomes a raging torrent carrying many times this volume from the moun-

tain catchment areas. Last Christmas the Waitaki flooded at nearly six times its normal flow when the volume of water rushing through the site, sweeping away a high gravel causeway and suspending a Bailey bridge out on its own on piers, was measured at more than 70.000 cusecs. Life of Centuries Fine glacial rock dust is carned in suspension from the alpine glaciers through the lakes to the river and although the total is regarded as great in tonnage, it is now considered small compared with lake or eservoir capacity. The lake that win spread behind the dam will still be a lake 200 years from now, its shingle and silt life being redumed in centuries, the Ministry of Works believes. Long before shingle and silt carried by the Waitaki river has filled the reservoir to any appreciable extent, let alone an alarming degree, Benmore will be long past its budget and operating Life of 60 or 70 years, they say. Long before then. New Zealand, like most countries, might be depending on nuclear power. (To be Continued.)