IRRIGATION

Otago Daily Times, Issue 22326, 28 July 1934, Page 11

IRRIGATION

UPPER MANUHERIKIA HUGE DAM AT FALLS WATER FOR OMAKAU BASIN Irrigation farming although older than any other form of highly developed agriculture in the world, is a pursuit with which a very small percentage of farmers in temperate climate are in any way familiar. Otago, of all the provinces of the Dominion, has a special interest in it, and, in fact, contains within its boundaries the only irrigation settlements of any size or importance in the country. So largely has the practice of irrigation bulked in the development of the interior of this province that even coastal landholders and those elsewhere who are favoured with a normal growing rainfall, have lately come to realise not only the great benefits which accrue from irrigation but also the special agricultural skill, additional capital and continued hard work which are' necessary to bring irrigated land into profitable production. The extension of the network of races, tunnels, siphons, and waterways which comprise the irrigation reticulation of Central Otago, and the steady erection of storage dams and reservoirs in some of the wildest •country of the province have for many years been watched with the greatest interest by all sections of the community, both urban and rural, who during the past decade have been given practical and convincing proof of the efficiency of irrigation and the extraordinary fertility and productiveness under irrigation of otherwise arid and barren lands.

A LARGE UNDERTAKING At the present time work is in progress on one of the largest irrigation undertakings the Government has yet launched, and by the commencement of the 1935 irrigation season it is expected that a further 12,000 acres of land in the Oniakau basin, around Chatto Creek and in the Matakanui district, will be brought within the scope of irrigation practice. The scheme is known as _ the Upper Mamtherikia irrigation project, and when completed it will be the most lasting and permanent achievement of the Public Worts Department to date in this sphere of engineering. It is a costly undertaking upon which £158,000 has already been expended, and it is estimated that the total cost will be in the vicinity of £300,000. A Daily Times reporter visited the locality during the week, and was informed by the engineer in charge of the work that it was considered that the project was now more than half completed, the actual proportion finished being 58 per cent. The dam alone will cost £113,000, and the expenditure on the main race, distributories, water tunnels, siphons, and intakes will exceed £150,000. The figures are' high, but any misgivings that ratepayers may have with regard to such a heavy expenditure of public moneys should be counteracted by the strikingly permanent character of the work. Every contingency, even to the possibility of future drainage, has been taken into consideration, and the department looks forward to lower maintenance charges than on any scheme of its kind in Central Otago, The dam site is on the Manuherikia River, just below the falls, and the reticulation of the land to be brought in necessitates the construction of 27 miles of main race and four miles of tunnels and siphons. The land to be brought in is in the semi-arid belt of Central Otago with a low rainfall, less than what is generally regarded a« a normal growing fall. A reference to rainfall will perhaps illustrate more graphically than anything else the need for irrigation in this part of the province. In the irrigated settlement of Central Otago the average annual rainfall ranges from 12 to 20 inches, compared with an average fall for the South Island of 45 inches. Such a comparison wauld seem to be convincing enough, but there is a still worse feature of the position which renders ordinary forming practice almost impossible. The growing period in Central Otago is the driest portion of the year, and the difficulties of landholders may be gauged from the fact that for the whole of the growing season the average rainfall falls sometimes as low as 8.8 inches, "which is considerably b.elow. the average growing fall of 15 or lb inches. The Upper Manuherikia scheme will cover a total area of 12,000 acres, of 9200 acres lie below the main race, and 2800 acres in the Matakanui district, above the race. Of the larger area 8000 acres lie in the wide Omakau basin, and 1200 acres are in the locality of Chatto Creek. The scheme depends on the regulation of the flow of the river and the storage of water for release when flow falls below requirements. A £113.000 DAM The main feature of the work, therefore, is the huge dam structure at the falls, behind which 8000 acre-feet of water will be stored. It should be explained here that an acre-foot of water is the amount of water that would be* required to cover an acre of ground to a depth of one foot, . . „ ( The dam, which is now rapidly approaching completion, will be 110 feet in height, and of the rock-filled type. It will have a maximum base width of 350 feet, and the length of the crest will be 520 feet, and its width 14 feet. The main bulk of the dam is composed of rock-filling, while the whole of the upstream side of the structure is faced with reinforced concrete slabs two feet thick, made up of 44-foot units, placed separately to allow for future settlement. Each unit will be joined to the next by means of waterproof copper strips. The lower edges of the bottom seaps rest firmly on a massive concrete cut-off wall. Between the two bottom rows of slabs there are 160,000 cubic yards of loose rock filling. Another feature of the structure is a huge reinforced concrete hinged block, designed to give flexibility to the face in case of a settlement of the rock filling. All the joints between the slabs ate being filled with a special bitumen compound. , ■ • , ... The dam will have a freeboard of 13 feet above the lip of its spillway, which is up-stream, with a a circular lip 100 feet in diameter, diminishing funnel-wise to a shaft of 17 feet in diameter, connecting with a diversion tunnel of the same diameter, which discharges into the riverbed on the down-stream side of the dam. The diversion tunnel is 10 chains long and fitted with control valves by means o, which the water may be released as reql*After the water has been released through this tunnel it will travel 10 miles downstream, where it will be Picked up at an intake near Blackstone Hill. the main race It is here that the main race commences its 31-mile winding course through the country that will be served by the scheme. Twenty-seven miles of the main waterway is'an earthen race, the balance being made up of reinforced concrete siphons and water tunnels. Excellent progress has been made with the race construction, the average cost of which has worked out _at about £25 per chain. The cost has varied from £7 to £4O per chain, according to the capacity of the race. The largest siphon, which will carry the water across the Lauder Valley will be over a mile long, composed of 42-incli concrete pipes. Another siphon of similar dimensions will cross the wide Manuherikia Valley. In addition, there will be several other siphons of lesser diameter over creek beds and depressions. Altogether, the plans provide for 18,000 feet of siphons and 4000 feet of water tunnels. A considerable length of the main race is now coniplcted, including some of the smaller siphons. Pipes tor all the siphons have been delivered to the various sites, and different gangs arc now busily engaged laying them. All the tunnels have been constructed. The race bivpks have all been constructed with the Utmost care and thoroughness. No risks of breaks have been taken anywhere, so that there will be very little chance of settlers under this scheme suffering in the way irrigates on the original Manuherikia scheme did one season when the main race broke through at Sprmgvalc and caused an almost complete cessation of sunnlies at one of the most critical periods of the year. The earthen banks of the race have been heavily rolled and consolidated, and whenever doubtful country has been encountered recourse ha« been bad to tumie’ling or siphoning. The main work yet remaining to be done is the completion of the falls dam, but

there is still a good deal of excavation required for the construction of distributary races which carry the water from the mam race right to the boxes giving access to the irrigator’s property. Gates and crossings over the races have yet to be completed also. The engineers, however, are confident that the contract will be completed within the estimated time, and that the life-giving streams will flow through the Omakau basin throughout the 1935-30 irrigation season. THE QUALITY OF PERMANENCE One of the big problems of irrigation engineering in the past has been the cost of maintenance and renewals. The replacement of steel piping and flaming Has cost the department a great deal of money and the maintenance of water races has been a heavy item. In the present undertaking the use of steel has been almost entirely eliminated. Concrete siphons and piping have been erected in reinforced concrete, and the cost has been about the same as that of steel installations. Any excess that may exist iu respect of the cost of concrete over that of steel will be more than compensated for by the immense saving in maintenance costs for which only a relatively small allowance need be made. The practice in the past, and a necessary practice, of allowing as much as 5s per acre has greatly added to the cost of administering schemes, but with all-conevete structures and the reduction of replacements to an absolute minimum, the charge on the scheme for maintenance will be greatly decreased. That work of aso much more permanent character has been carried out with comparatively no increase in costs is regarded by the department as highly satisfactory, but there is a further aspect of the work that should appeal to taxpayers. Most of the materials used in the undertaking have been produced or manufactured in New Zealand. In most of the earlier projects heavy importations from overseas were necessary.