PROBLEMS MET

Ashburton Guardian, Volume 61, Issue 18, 1 November 1940, Page 8

PROBLEMS MET

IRRIGATION IM* THE COUNTY. PAINSTAKING EXPERTSTENTS. . > (Special to the “ Guardian ”) CHRISTCHURCH, This Day. ; “The structures for irrigation works generally take as long to design as they do to build,” said Mr T. G. Beck, district engineer to the Public “Works Department, in an address last evening to the Christchurch branch of the New Zealand branch of the New Zealand Institute of Engineers. Mr Beck produced amplo evidence in support of this assertion in photographic slides of the exhaustive and detailed designs and plans prepared by his staff, and in his address lie dealt with many of the difficult and unexpected problems which the department has met and overcome. •-Mr Beck told his audience of painstaking research among the accumulated records of countries which have had long experience of irrigation, of equally painstaking experiments with models to * ascertain the effect of proposed structures, of the application of mass production and standardisation to the schemes being developed in New Zealand, of the difficulties which have arisen since the war and which have necessitated changes of plans and the use of substitute materials and methods.

“In our new irrigation schemes i:i Canterbury we have tried to keep abreast of modern developments of technique, not only because of their greater efficiency, but because of their ultimate economy in maintenance,” said Mr Beck, who said that usage all over the world tended to reinforced concrete structures of more arid more radical and complicated forms, and also to the use of pre-cast work and temperature curing of concrete. .Boxing for the construction of these reinforced concrete structures tended to become more and more permanent in design, and hardwoods and sheet-iron linings were frequently adopted where the boxing could be used repeatedly. Standardisation of these materials enabled savings to be made in the employment of skilled labour and savings in time as wfell. Safeguards for Emergency. Steel reinforcing cages with spotwelted joints were little used in New Zealand so far, but as soft binding Wire was becoming more and more difficult to obtain, Mr Beck forecast the increasing use of this method of assembling reinforcement cakes, especially as electric power was plentiful aiid well distributed. After describing tlio various kinds of intake structures used in Canterbury, Mr Beck went on to deal with the ivide variety of safeguards and protective measures incorporated in the irrigation races. “By-waslies” bad to be provided at various points to ensure that if by some human error or natural mishap t6o much water was admitted to a race it was not allowed to discharge over farm land which might be damaged. Provision was usually made for such excess water to discharge at convenient points into river-beds. Both, weirs and pressure pipes were used, but the former were preferable as’ tlio latter was subject to blockage. “Farmer# have a habit of dropping their gorse clippings into the races, arid the nor’-wester could easily lift a haystack into one,” said Mr Beck. These safety devices were costly but essential. A combined check gate and siphonic spillway alone might cost £SOOO. In the Rangitata diversion race there was ample provision to carry the whole of the capacity of the race away safely into rivers in the event of an emergency. Nearly all these safeguards were mechanically operated and could be adapted to remote control from a single point. This would enable the flow of the race to be brought to a standstill in half an hour. Weir for the Ranfgitata. v Drops in 'the level of a race all meant the building of structures to absorb the head of water. It was estimated that a third of the cost of irrigation construction was in structures, and a third of those structures were for drops. An interesting development was the lining of falls and chutes with large cobbles embedded partly in concrete. At the intake to the Rangitata diversion race it was found necessary to build a weir across the shingle bed of the river. Experiments with models showed £hat two or three rows of tuncated tetrahedral concrete blocks, each seven feet high and weighing eight tons, placed across the river, would achieve the object of raising the level at the intake by about 18 inches. This kind of weir had its greatest effect in altering the head when the river was at its lowest flow. The almost incredible destructive power of large volumes of water passing at pressure through concrete structures: was mentioned by Mr Beck as another problem of the engineers. In spite of sahd and silt traps, the water retained small particles of abrasive material which would wear through concrete in a surprisingly short time. “Surges” in the level of water in an irrigation race, similar in some respects to the ripples of the running water of a gutter, also had to ho provided against. Water driving at,pressure through siphons, too, could set up destructive “buzzing,” or oven “chugging” vibrations, and where this was likely to happen especially strong steel reinforcement had to he employed.