WAITAKI DAM

Evening Post, Volume CVI, Issue 11, 13 July 1928, Page 10

WAITAKI DAM

CONTRAST TO ARAPUNI CONTROLLING THE LAKES FLOODS ONLY TEOUBLE Quite a big part will bo played- in the j national hydro-electric scheme by the I Waitaki dam and plant shortly to be commenced. Conditions and surroundinga could hardly form a greater contrast with, those of Arapuni. The country at Kurow is mountainous and rocky, and the Waitaki is subject to heavy floods at certain seasons. Most of the plants in use in New Zealand are relatively high head plants, but the dimensions of the plant at Kurow will be something quite new, as far as the Dominion is concerned. Arapuni, for instance, has a head of 175 ft, while the Waitaki dam will have a head of only 70ft, the lower pressure meaning water passages on a much larger scale. Nothing has been practically done so far, but a good deal of time has been taken up in prospecting for the most suitable site. THE SITE SELECTED. The, first site examined was some distance above Kurow, at a spot above a rock island in the centre of the river. At that spot the river was narrow and swift, and it was found that,-although there was no great distance from water level to shingle, it was necessary to go 60ft down before solid rock bottom could be found. Other sites have been rejected also, but the one ■,chosen is expected to prove thoroughly satisfactory. It is 3£ miles above the Kurow township, where there is only some 15ft of shingle to remove before solid rock is reached. To get v sufficient spillway, an important matter where there are heavy floods,.and to obtain a favourable site for the station, an angle has been put on the dam. The length of the spillway is approximately 1200 ft, and some idea of the volume of the Waitaki in flood may be obtained from the estimate that at maximum flood there will be 12ft of water flowing over the whole of the spillway..'. AN UNCERTAIN BIVER. , The main source of the water is froml Lakes Tekapo, Pukaki, and Ohau, and, though there ate several not inconsiderable streams flowing in at other points, such as the Ahuriri, it is expected that 90 per cent, of the power will be derived from the outflow of th? lakes. For six months the Waitaki) When, working according to schedule, which;is hot always the case, is low, and for six months_ it is high. From May to October it is usually at its lowest, this being the time when everything is frozen hard on the big watershed of the South Island, but a warm rain may always upset calculations. NorJ-west-ters bring the river down bank high. This year there were1 no nor'-westefs until April, quite an unusual state of affairs, but at qther times heavy floods have taken place in December. The variations in flow are perhaps best expressed by the height of the water at the dam. This will vary, from observations taken by, the Hydro-electric Department of the Public Works, from 12ft over the sill maximum to a minimum of 3ft below. It is expected that the dam yrill provide a certain amount of storage, which will be drawn upon in the day peak" period, and will: fill up again at night. At the end of the peak period in the dry season the level will be 3ft,below the crest. More can be drawn off, liut it is not expected that this will be necessary, because the lakes, by controlling dams at tneir outlets, will be nursed, retaining as much as possible of the flood season's water. THE PLANT. . The first; installation will consist of two turbines of 20,000 h.p. each, and with its ultimate development the plant will consist of five such turbines, each of .the three latest turbines to be brought in as the, lakes are harnessed in succession, and the summer flow conserved, for the winter. The difference in height of head as compared with •Arapuni, 70ft asagainst 175 ft, is reflected in the construction of the plant.; The output of, the turbines will be almost' thjg same, but the .reduced head necessitates more water to achieve the same result. The intake gates for each turbino at Kurow will have an area of 700 sq. ft, and the discharge openings from the turbines will bo approximately 14ft in diameter, big enough to contain an' average sitting-Toom, while a small hotel would fit quite comfortably in some of the water passages. The scroll case of each turbine is 15ft high and ,15ft across, while part of the intake passages is 20ft in diameter. Converging walls give the flow acceleration until it reaches the tm> bines, but after passing them the tubes are so built as to retard the velocity until on exit the water flows at only 4ft per second. ERECTION TROUBLES. Constructional difficulties are expected to be encountered only in that part of the spillway crossing the actual river. The rest of. the excavations will be possible "in the dry." The maximum depth at the portion crossing the river is 35ft below water level, and the average depth is only 25ft. It will, however, be necessary to clear away the shingle down to solid bedrock, anU it is in building this portion of the dam that the only trouble, and that only from floods, is anticipated. The Waitaki in full .flood is a fearsome sight. Even at the Glenavy railway bridge, a couple of miles from the mouth, where the river is not confined, it sometimes rises almost to tho bridge level, and, the problem of satisfactorily anchoring coffer dams, which provide the probable means of cleaning out the foundations of the Kurow dam, against such a flood remain to bo faced. Working in the dry season, however, it is hoped to avoid floods such as this. It will mean that the work of laying tho foundations of the river portion of the dam will have to be done as expeditiously as possible, but apart from unexpected floods, tho work will present no trouble. There will not be the slightest difficulty with the foundations for the turbines, generators, of buildings, becauso the ends of the dam run into solid greywacke rock of unusually good quality, and any small interference through sdakage can easily be dealt with by pumping. A SOLID EASE. Measurements, although tho plans are complete, cannot yet be quoted as definite, as much will depend upon the nature of the foundation when uncovered,, but so far the plans provide for a basic width, from hool to toe, of from 90ft to 100 ft. Most of the material for concrete is at hand on the job, of excellent quality. A short spur line will probably be laid from Kurow to convey material and plant. The power station will be on the southern or Kurow side of tho river, and a village for the accommodation of the operating staff will be situated on tho terrace immediately opposite the dam and power station. Already the small township of Kurow, tho gathering place of residents from a big hinterland, but as a rulo lively only on spocial occasions, has begun to prepare for the added population and workmen in tho provision of diversions and hotel accommodation, and no doubt the presence of the dam will permanently add to its importance.