ARAPUNI GATES.

Auckland Star, Volume LX, Issue 227, 25 September 1929, Page 7

ARAPUNI GATES.

DIVERSION TUNNEL USED

EROSION BELOW SPILLWAY.

LOWERING OF WATER.

QUESTIONS FOR ENGINEERS,

(By Telegraph.—Special to "Star.")

AEAPUNI, this day.

The volume of water that has been flowing over the spillway at Arapuni was checked yesterday, when the diversion tunnel gates were reopened. The accomplishment was made safe as the result of the strengthening of the steel lining below the gates in the tunnel, and the use of lifting gear of simple design.

During the day the gates were opened from one to four feet, wlien (5000 cusecs, or three-quarters of the normal (low of the Waikato River, were released. A tremendous force of water was let loose as the outcome of the operations, and after it had spurted out for a distance of IS yards, it became a raging torrent of extreme beauty. Once again the mighty Waikato had justification for rebelling against the interference of man, and made full play of her fury. Huge Water Pressure. Long before the gates were opened to the extent deemed advisable 50,01)0 or 60,000 horsepower was represented by the awe-inspiring current beneath the gates. Within a few hours of the lifting of the gates the volume of the water passing over the spillway was appreciably reduced, and some indication of the seriousness of recent erosion given. Actually the total amount of water discharged by way of the spillway, the penstock tunnels, which operate the powerhouse deep down in Arapuni gorge, and the diversion tunnel, was 2000 cusecs greater than the inflow into the lake, and this must always be the case in releasing a body of impounded water. The change caused the fall below- the spillway to lose much of its old-time "devil," and enabled an impression to be gained as to the formation of the I 'bed of the channel that the river has scoured out since the river was diverted. It was apparent that the third and. softest stratum had 1 dipped, that the second seam, a tough, unjointed body, was thicker, and that the midway step of the falls was being carried on its surface without new signs of erosion or undermining. Position More Promising. For six weeks there has been no serious fall of earth below the spillway, and from the manner in which the eroded channel has maintained itself, the position is much more promising than it first appeared. Concern is still felt, however, as it is doubtful as to whether it will be possible to keep the diversion tunnel open for any length of time. As yet it has not been determined exactly what will be done to strengthen the new channel bed to prevent the water eating back to the spillway, but, in the meantime, a road is being cut down to it to facilitate the transportation of materials that will be necessary for any remedial work. In addition a wire rope to support a pipe to carry compressed air has been stretched over the new gorge at the fall, and on the west side a cutting is being put through a ridge to give access to vast deposits of sand which is highly suitable for concreting. It is impossible for the engineers to definitely state their intentions till the water has been stopped from runnng over the spillway, and the rock and channel thoroughly investigated. Arrangements have been made for a large supply ot metal and the probabilities are that a commencement will soon be made with the putting down of a sloping concrete apron in the channel. By this means the erosive effect of a fall will be eliminated. However, if the diversion tunnel is able to meet the situation in a satisfactory manner, little time will be lost in the concreting work. The diversion tunnel is the key to the whole situation. The engineers are expected to say to-day if the lining in the tunnel can be trusted to withstand the months of friction which the unwatering of the spillway and the concreting of the. falls would involve. The gates are to be closed once more, and after an hour has elapsed, Mr. T. Eabone will make an inspection of the tunnel. No doubt is held by the engineers, however, that the test will prove satisfactory. Prevention of Flooding. The second generator at Arapuni is now in commission and is running interlocked electrically with the No. 1 unit. This means that both the sets are on half-load. The additional generating capacity of the power station will assist to relieve the demand for current made on the Horahora plant, and eliminate subsidiary sources of supply. It is calculated that the third unit will be available by Christinas. No difficulty is entertained as to keeping the powerstation in operation during the concreting of the spillway falls, but control of the river flow must be exact so that driving energy for Horahora is not retricted, or the river is not flooded. The discharge through the diversion tunnel can be easily regulated to accommodate all the" surplus water without the level of the headrace falling below the requirements of the penstocks.