Arapuni Problem

Sun (Auckland), Volume IV, Issue 1073, 10 September 1930, Page 10

Arapuni Problem

VALUE OF LAKE TAUPO “Huge, Natural Regulator” « DEALING WITH SURPLUS WATER A suggestion that immunity from further trouble at Arapuni be secured by utilising Lake Taupo as "a huge, natural regulator” for the flow in the Waikato River is put forward by a correspondent who signs himself “Taupo. In the course of the following interesting letter he discusses, also, an alternative method of dealing with surplus water by the establishment of a “water cushion” below the dam.

The* subject is almost thread-bare, hut still the question is constantly heard: Will Arapuni ever start again? writes “Taupo.” Anyone who considers the cost already incurred, the comparatively small additional cost necessary to ensure future security, and the relatively small final cost per kilowatt, must answer that question with a vigorous affirmative The problem is how to obtain immunity from further trouble. Various expedients have been suggested in the pliblic Press —some sane and some the reverse. So far, no one appears to have recognised that, in Lake Taupo, there is already provided a huge natural regulator for the flow' in the Waikato River. It would be of local benefit to lower Lake Taupo five or six feet, as it would render fit for close settlement some extensive and fertile flats on the lake near Tokaanu, which, at present, owing to danger from floods, are of no practical The regulation of the discharge j from the lake would also greatly j mitigate the flooding of the lower ; Waikato from Huntly downward, and alijo render practicable the unwatering : of the very extensive areas of poten- ; tially rich swamps in that region. DEEPENING OF RAPIDS The fall In the Waikato River over the first four miles from the lake is 14 feet, so that the undertaking to lower the general level of the lake by six feet would only involve the deepening of various rapids over the first three miles or so, and would not be unreasonably costly. A weir at some suitable rapid would have to be installed to control or even, on occasions, altogether stop the discharge. The area of the lake is 230 square miles. The normal flow from the lake is about 6,000 cubic feet a second. To raise the lake by one foot all over would normally take 12 days, or. in times of heavy flood, six days. This gives some idea of the facilities that would thus be vreated for How regulation. Were the take level lowered six feet, and all downward flow stopped, it would take six days’ heavy flood to bring the lake up to its present level. By the establishment of the weir, with mpvable gates, just sufficient water to work the turbines at Arapuni, and for other purposes, could be let through. There is certainly an extensive drainage area between the lake and Arapuni. but owing to its narrowness and to the porous nature of the soil, as well as too much of it being under heavy forest, the streams draining this area are small and, under normal circumstances, have no appreciable influence on the volume of the Waikato. The existing diversion tunnel at the dam is capable of discharging up to 18,000 cubic feet a second, even without head; that is when ,the Arapuni Lake is empty, while under a full head it could discharge practically three times as much. Therefore, any surplus water arriving at the dam could be dealt with by sufficiently opening the gate of the diversion tunnel. The diversion tunnel under full head could carry a volume of water equal to twice the highest recorded flood. ANOTHER METHOD Another quite practicable method of dealing with surplus water at the dam is to establish a water cushion, say 200 feet .square, 20 feet in depth and concrete-lined throughout, just below the dam. Then the dam could he pierced 20 feet below high-water level by sufficient sluices and gates to deal with all possible surplus water. With this water cushion, and with an additional sluice gate near the base of the dam, it Would be practicable to All up the diversion tunnel-when the usefulness of the present gate became doubtful. The water cushion would be competent to deal with the surge from this lower sluice gate. With the flow at Lake Taupo regulated by the weir, and with all surplus water dealt with by sluices at the dam, the necessity for the spillway above the power house would no* longer exist; and thus the unknown but enormous, cost of concreting the overflow channel from the spillway to the waterfall above the buried forest would be obviated. The present structure containing the openings at the spillway could then be built up in concrete as a solid roadway or bridge, and the existing transformer station be shifted and reerected on the eastern side of the spillway, instead of, as at present, on the western side between the power house and the spillway.

There is a strong probability that the excavations for the power house and ' the penstock tunnel helped still further to weaken the already weakest part of the ridge between the old and new channels. Thefextra weight of the transformer station the weight of water in the new channel, and even the pull of the suspension bridge, the vibrating of the turbines and various geographical causes were all contributing factors ; to the cracking that has occurred. The removal of the transformer station and the lowering of the ridge down to the level of the penstock gates, or even lower, would certainly lessen, and probably entirely obviate, further trouble. But, to render security double sure, the new channel would have to,be concreted from the spillway upwards, above tlio furthest sign of cracking. , This surface concreting with pressure ; grouting, where expedient, would ensure freedom from further leakage. The regulation of the flow of the river, both at Lake Taupo and at the dam so that there shall not be any overflow at the spillway, the removal of the transformer station and the lowering of the ridge, would be all that is necessary to secure future freedom from trouble; and the cost of these works would be less than the one simple item of concreting the waterfall. The average cost of power that can be develgped at Arapuni, even at an expenditure of £3,000,000, is only £2 j a kilowatt, whereas, at Lake Coleridge. it is £3O: at Mangahao £56, and at Horahora £3O. The expenditure to date- at Arapuni is approximately £2.500,000. so that, even if half a million additional is spent, the scheme is still a financial proposition: that is provided all the power developed can be utilised, and this is only a question of time. Tinbenefits derivable from the regulation of the flow from Lake Taupo would be most evident at Arapuni. But the prevention of flooding in the low-lying flats of Lake Taupo .and along the sw amps about the Waikato are also strong arguments in favour of such treatment. When Waikaremoana, Mangahao and Arapuni are finally linked up, there will not be the absolute dependence for power from Arapuni as there is at present. In J'act. after these sources are all combined, it would become quite practicable to shut any one of them, even Arapuni itself, when inspections or repairs became necessary. Aratiatia would probably have provided, especially in the early stages, a much more economical scheme than Arapuni, but such an enormous expenditure has already taken place at Arapuni that the abandonment of that scheme and the undertaking of some alternative is now unthinkable.