MODELS MADE FOR TESTING ARAPUNI.

Star (Christchurch), Issue 19239, 28 November 1930, Page 14

MODELS MADE FOR TESTING ARAPUNI.

landscape reduced FOR CLOSE INSPECTION. (Special to the “Star”) WELLINGTON, November 27. Before commencing the remedial measures recommended by Professor Hornell to enable the Arapuni hydroelectric undertaking to resume operations and be developed to its maximum extent, the Hon W. B. Taverner, Minister of Public Works, decided that complete plans and estimates must first be made available. Work has been proceeding on these lines for some time, and one of the most interesting and important features has been the consideration of measures to prevent further erosion of the country below the spillway dam. This will be one of the most expensive parts of the project, but when the beginning is made with the practical work the engineers will have satisfied themselves by elaborate experiments that the immense flow' of water w'hich has to be controlled in that channel will be directed in such a way that no further serious erosion can take place. Working Models Erected. Though there is a considerable amount of engineering literature relating to the flow of water and measures for its control, it is recognised that each particular problem has its own difficulties, and that pure theory is not capable of direct application. Professor Hornell agreed with the opinion of the Public Works engineers that tests ought to be made with scale models before the work of dealing with the erosion below the spillway dam was put actually in hand. Mr Taverner consequently approved of the construction of models of this part of Arapuni country. These have been built in the Department’s Wellington workshops, following data provided by careful surveys of the channel bed and its surroundings. The Minister this morning witnessed some interesting working tests with these models, which are capable of demonstrating the behaviour of overflow from the spillway under the varying conditions which can be imposed by the protective w'orks under consideration. One large model reproduces exactly on the scale of four feet to an inch the channel below the spillway dam through the first and second falls into the deep pool just above the interesting scenic spot popularly known as the Buried Forest. A good supply of water is available, and a controlled flow equal to the known volume of the Waikato at that point, corresponding, of course, to the general scale of the model, was sent over the falls. This reproduced almost exactly the conditions which prevailed prior to the closing down of Arapuni. Contour of Channel. Another model, built to scale, has been arranged to permit of variations in the channel, reproducing the engineering works under consideration, which are being designed to prevent further erosion. Instead of spending thousands on concrete work in the actual channel and possibly having to modify them after they have been tested under stress of the full flow of the river, the engineers are able to vary the contour of the channel and falls and observe the results of the introduction of their proposals. The water supply enables flow conditions to be controlled so as to reproduce the normal volume or flood volume of the Waikato according to scale. There is a total drop of 100 feet from the top of the upper fall to the level of the great pool at the foot of the second fall. The model enables observers accurately to plot the flow of the stream and see its effects, when various baffles are interposed in the channel. The “ cushioning ” effect of pools is being studied, and some promising work has been done in the way of varying contours over the falls and also the shape and depth of the basins beneath. The experiments are being made with the object of designing baffles to slow down the rate of flow between each fall. At the conclusion of his inspection, which included observations of the effect of several variations in the contour of the channel bed, the Minister expressed satisfaction with the experiments, which, he said, would save a good deal of money and enable the work, when once started, to proceed with confidence.