Waikaremoana Lake Leaks Problem Solved

Northern Advocate, 18 April 1949, Page 2

Waikaremoana Lake Leaks Problem Solved

WELLINGTON, Mon. (Sp.) .—The problem of sealing leaks through the hillside beneath the outiall of Lake Waikai emoana has been solved, and when the work is completed the storage capacity of this immense natural reservoir ot h\clio-clccti ic power will be enormously increased.

Lake Waikaremoana has an area ot' til square miles, and is 846 ft deep. Ih<? utilisation of its water-power resources lias always been limited because over 400 cubic feet of water per second escaped through the jumbled •train of an ancient landslide which fell across a valley, forming tin natural dam which created the lake The ideal of the hydro-electric engineers has been to secure complete ■ o’W-ol over the lake level up to th outflow point, 2001 ft above sea level, and thus conserve the rainfall of the watershed for use when power is most needed. •"■’AAF r > <>*T However, even at low levels, the leaks permitted the escape of about 300 cusecs, sufficient to provide all the water needed in the two generating stations at Tuai and Puripaua. A third station at Kaitawa, _ nearer the lake outfall is now in operation. The Works Department constructed a low-level tunnel running well out into the deep lake, which enables Kaitawa and the other two generating stations to draw off an additional 40 feet of water. Previous to the construction of the tunnel and the third station at Kaitawa the Tuai and Puripaua stations depended on the leak water. Now, however, the leaks arc superfluous and are being sealed off. MORE THAN ONE PLAN Information made available by the Minister of Works Mr Semple) shows that more than one scheme was subgested to deal with the subterranean leaks. At one stage, an embitious plan was considered, involving the bifliding of a causeway at the outfall which, it was hoped, would cover all the points of water leakage. Had it been possible to lower the water level abnormally by using the tunnel tunning into the bed of the lake, the leaks could have been found and more easily sealed.

But the engineers dealing w:th the leakage problem could not be helped in this way because Waikarcmoana’s storage, even under existing conditions, had to be maintained, and every inch of rainfall conserved for u; - in the season of heaviest demand. Those concerned in the leakage problem were therefore unable to handle it under the comparatively easv conditions “in *■- -- "y DIVERS USED The leaks were charted some years ago, and for most cases divers nae bo used. . They ascertained the velocity ot the flow of water through these natural vents by means of meters. Many of the larger leaks could be detected by the collection of driftwood and old trees sucked into the orifice. Others disclosed their presence by r slight vortex in the sand of the lake bottom. The draw off for power purposes brought the lake down to an unusuallv low level about a year ago, and a start was made to seal some of the leaks using a method devised M" r Turner (assistant engineer-in-chie£) end his staff. Mr Turner is in charge of hydro-electric, development. / PROVED EFFICIENT’ The plan which was adopted • has already proved itself by reducing the leakage from 400 cusecs to 300 cusecs. It is called by the engineers “the re-

verse filter method." The first job was to remove debris from the vents. Big logs were found jammed in the vents. They were similar to those found in the excavations for the Kaitawa power station lower down the valley—indigenous timber similar to that still growing in the VVaikarcnioana watershed As it is necessary to maintain the lake level at the highest possible point, many leaks are well below water-level, therefore clearing work had to be done by divers. A leak having been cleaned of all loose material, the next process is to pack in the largest boulders it will take Then come a succession of smaller stones, and the velocity of the leak commences to abate. BLANKET OF CLAY At that stage, sand is dropped into the interstices, and the flow iurther reduced so that finally the leak can bo completely sealed by a top blanket of impervious cki>. >va.en 1 a protective coating of boulciers. The big job is not complete, but up to date the scaling system has used some GOO yarc quantity of crushed rock of smallci dimensions, 400 cubic yards of sand and 900 cubic yards of clay. Timber taken from the leaks hi the lake bed is estimated to amount to about 240 cords GOOD PROGRESS "The progress made in sealing the leaks at the Waikaremoana outfall,” commented Mr Semple, "indicates that within a yea’ v- mi >- (solved. The engineers responsible are to bo congratulated on the patience and skill with which they hnvcevolved , a method of enabling about 45 lcct of the depth of Lake Waikaremoana i ’ i utilised for the generation of hydroelectric power. "Its economic advantage to vho Dominion is important, and it represents another stage of progress in the fullest utilisation of our great resources of power." «