ELECTRIC POWER

Evening Post, Volume CVII, Issue 53, 6 March 1929, Page 10

ELECTRIC POWER

THE WAIKARE SCHEME

NATURE'S SPADE WORK

THE PROBLEM OF THE LEAKS

(By Our Special Reporter.)

As was mentioned in a previous article, the stage was sot by Nature for a great hydro-electric development at Waikaremoana, but Nature, having laboured mightily with rushing water, earthquake, and huge landslides, gave up the job and passed on elsewhere,' or, more 'likely, is/still working along quietly with details.

But for enormous 1 landslides centuries ago .there would be no Waikaremoana Lake, tho great reservoir 21 square miles in area from which water is drawn for the turbines in; the power station a thousand feet k>w,er i" level. What Nature did not do was to plug up some scores of leaks'at the outlet end of the lake. ,

Dr. Marshall, who, prior to the commencement of. operations at Waikare, prepared a geological report as to the probable origin' of'.th'o Oake, advanced the now fully-accepted theory that many centuries ago, before the coming of the Maori, yet "extremely recently from .a geological standpoint," two landslides of enormous dimensions fell across the deep river valley, probably with,a long interval between, completely blocking the gorge. The country about this spot, lie says, is precisely that which: promotes earth slides of largo dimensions. Hard caps of sandstone, in places at least 700 feet thick, rest upon relatively soft papa (ancient sea mud), which readily develops a greasy surface. Before the slides occurred, he thinks, the valley bottom was certainly 1000: feet below the present lake level, and he assumes that the pre-lake river in one of its meanders, or that one of its tributaries, had cut deeply into the formations to the west and south at the low level of the valley' some two miles below the present out-, let of the lake. The heavy masses o& sandstone' would then have ho support at the lower end of the slope, and under a special.association of extreme conditions would slip down. This vast movement, he considers, wpuld drive the river towards the eastern bank, again undermining the great sandstone capping and preparing the way for a second enormous slide, blocking the old valley with a chaos of jumbled and smashed sandstone ■ blocks, some of them weighing thousands of tons. "With the sliding rock caiiio sufficient papa to make the whole mass moderately watertight, aiid, as the normal rainfall of 60 inches per annum would take about ten years to fill the new lake valley, he says, the huge blockage would within that time obtain considerable consolidation.; '.:"■■■ ■'

En a very few words, Dr. Marshall's conclusions maybe summed up: A prolake river running down a deep valley between great heights of sandstone resting oil papa; an undermining on the west and south-westy leading to a first great.slide; the turning of'tho stream to tho cast and a'second undermining, leading to a lesser,.but still vast,, landslide; then the gradual filling of the lake, to its present level, and finally the overflow through an exceedingly narrow outlet (considering tho area of tho lake), and, in normal times, a greater get-away of water through fissures in the upper layer of tho slip blockage, and their joining up into the Waikaretahcko River, which foams down a thousand feet or so in its first rapid drop to the Whakamarino flat, where the present power-house is built.

The leaks aro the problem. It is to them and their blocking that tho civil engineers of the Public Works Department must address' themselves with great ingenuity. Tho leaks must be stopped before Waikare can be developed to the'full extent of present plans, and so deep is the lake face near the outlet and so jumbled and rugged the sandstone blocks that many thousands must certainly be expended before the work is done. An estimate of £53,000 has been mentioned, but that is not, of course,' anything like an exact estimate. Anyhow, they must bo stopped. The development problems will be dealt with in some detail later, but, very briefly, the position is that the ultimate demands for j water will be so great that excess wini ter rainfall, which now runs away by the surface outlet or leaks away underground, must be conserved to meet summer and autumn requirements. THE LEAKING FACE. Lako Waikaromoana has many delights for those who holiday there—delights which are obviously not well known, otherwise there would be ten visitors- to one at tho < present timewonderful bush,' towering rock faces, a thousand'quiet bays and inlets,, bird life, lake'fishing,: sunshine (but with i some wind), a hundred points of his- | torical interest—for hero To Kooti led j the Imperial'forces, a merry dance. But ' no corner, of the ; much-indented lake shore, not even. ■■, the beautiful Waikareiti and the tiny lake perched on tho top of an island in that lake, is more delightfully interesting than the tossed and broken country immediately below tho lake outlet, Iho upper layer of the slide blockage. It is delightful because of the light bush and fern and gushing lake leakages, interesting in

its slip formation: and tho thought of how on earth are those'scores of leaks, little and big, to be made an end of. Though this curious patch of country is only a few yards below the roadway, extending down the steep hillside for a couple of hundred feet, it is plain that few know of the gushing water and cascades' as the springs gather to form tho tumbling Waikaretaheke stream/for tho bush is dense and untrodden. At tho-time of the writer's visit the lake was still overflowing at the surfaco outlot, but a mere trickle; yet, below the point whero the leaks combine, a fullsized river dashed down flowing 500 cubic feet of water per second.

The peculiar get-away of the water is a very effective regulator of flow, and only on rare occasions does the river fall below 500 cubic feet per second (or cusecs, as the engineers say), though in flood time, after heavy rain over the great forested catchment area about the lake, the flow may rise as high as 2000 cusecs. As in.all hydro-electric schemes it is regularity ■of flow, not great flood flow, that counts.

At the present time only one generator, a giant of 16,000 kilowatts, is in commission; the second is in course, of erection; later, a third of the same capacity will be installed, and ultimately two more power-housea will be erected, one above and the other below the present station, so that the water-will be used three times, in three sets of turbines, of which triple station proposal more later. Each of' the great turbines which drive the generators: has a marvellous swallow, 350 cusecs when all out, 130,000 gallons per minute, a tremendous thirst. As the average flow in the iWaikaretaheke is but 500 cusecs, it is plain that only about a machino and a half can be operated unless the river flow ia controlled. For tho time being thero is ample water, and when the second turbine and generator aro installed there will be sufficient, possibly, for a few years, for a smalllake halfway downfroni the outlet to the powerhouse gives a'fair amount of storage, sufficient to meet heavy demands for brief periods, but as the demand for . power increases still more water must be led down the pipe lines to the power- I house. INTO HUNDREDS OF MILLIONS. So great is the area of the lake, estimated at 21 square miles, that each foot which the level can be raised will give additional storage of perhaps 700 million cubic feet of water, a figure so great that alone.it means not much, but begins to mean something when regarded in relation to the water which roars through the turbines when running at full load. That vast quantity of; water would be swallowed down the turbine throat in roughly three weeks; three turbines would use it in about a week, assuming that they were running at full load all the time. There has been talk of lifting tho level by as much as ten feet, giving enormous additional storage to meet power demands for many years. As a matter of fact, .no power system of the nature of Waikaremoana is normally under constant heavy loading (Mangahao was until Waikarcmoana stepped in, but the demand was unprecedented), and thus winter-excess waters will be banked up in the lake to provide power for the turbines while summer and autumn rainfall is low. But that is when, and not until, the-outlet leaks are dealt with. HOW THE LEAKS MAY BE TACKLED. The first of these leaks spring out about forty feet below normal lake lcvol, and tho lowest ,of them are a couple of hundred feet down. They trickle, burble, or gush heartily from under and between the ' sandstone blocks; somo with just enough flow to keep the ground nicely wet, others issuing from fissures largo enough to crawl into, if they 'were dry. The whole of the leak face is a piled-up-mass of great rocks, so jumbled that it is obvious that no' plugging can be done on that side.

Presumably tho whole thickness of the slip barrier is similarly piled and broken, and' tho leaking water may wind and twist about underground for a 'long way before getting to air again. The scaling must bo done on the lako side and lake bottom, but whether there are merely a few fissures on this side, or whether the water seeps through at a hundred points, is not yet known, nor is ■ there any certainty at all how far down tho weaknesses in the lako wall occur. When the water level falls unusually low some swirls can be seen, indicating the position of fissures, but it is fairly certain that others are much lower o i the face. The engineers have used several hundredweights of various dyes on the lake i'acc and bottom, depositing them at various points and watching for dyed water whore the leaks show out on the hill face, but the results were" not satisfactory. (lowever, the leaks are just one move rtifiiculty which must bo overcome. One of the proposed methods of sealing the weak spots is to drain the lake, either by deepening the surface outlet or by a round-about diversion tunnel, or by a great steel syphon, to the level of tho lowest entrance, and to lay a floor of concrete on the bed of tlie lake; but here • is a job of : outstanding proportions should it- be found that .the lowest entrance:is far down towards the lake bottom. .The fact that the. water issuing through the slip is at about the same temperature as that in the lake, however, suggests that the entrance points are not very deep. Another possibility is to construct a breakwater cutting off the leaky barrier from the main part''of tho lake, and to pump clay into this cairn water to fill up the fissures. Whatever way this strange difficulty is tackled will be an engineering task which will interest engineers far outside New Zealand, for the problem is unique.