ABOUT THE TOWN

Chronicle (Levin), 19 October 1946, Page 3

ABOUT THE TOWN

SHANNON SIDE-LIGHTS IBy T. S. D.) Perhaps 'the greatest benefactor to the prosperity of Shannon next to the flax industry was the Mangahao hydro'-electric scheme in the early 'tw'enties. There hundreds of men were emplqyed building the giant dams an'd blasting through solid rock to make the huge tupnels which carry the water through to the power house at Mangaore. Few people in the district have not availed themselves of the most educational trip to the hydro works. Travelling past green farms along the East Road for about three miles one reaches the township of Mangaore, where the main power house building is situated. It is one of the head centres of electrical supply for the North Island. Transmission lines radiate north, south, east and west, carrying the all important power beyond Wanganui, the Wairarapa and Wellington, ' to milk the cows on the farms, to drive the machinery in the factories and to haul the trams' through the streets of the cities. There, where Mangatangi's weeping waters meet Mangaore's murmuring stream, man has ' built his steel and concrete structure, and placed in it those marvels of engineering skill — the turbines, the generators, the transformers and all the wonderfui- machinery that converts the power qf falling water into the beneficient electrical energy that is such a wonderfui servant to mankind. Oue of the main features in the conception of the Mangahao scheme was the conservation of water from an area of 4.0 square miles in Ihe upper reaches of the Mangahao River, at an altitqde of 1073 feet above 'sea level, This. water-shed, Teaching to the heart of the Tararua Range,-is one of the consistently wet afeas of New Zealand. The present scheme, by the introduction of pressure tunnels and the central regulating and storage basin at Arapeti, formed.by the Tokomaru dam, gives a d.irect route from the fiver to the low levels of the plain. The r.ai.nfall at; Mangahao averages about "100 incliCs per aiinum, but, as comparatively low river gaugings have been recorded during dry' summerS, aniple storage in the Mangahao Vailey was essential ,-tO"4he--success-nf— the scheme, and two dams were therefore provided. The upper one, 15 miles from Shannon, has a Capacity of 167,500,000 cubic feet, andvthe lower one 65,800,000 cubio.feet. • From the lower Mangahao dam the water is conveyed through a tunnel . 8i : chains ''tong and ' seven feet -iii,- ihtarns4 |4fameter» to Arapeti, Where thei tokomaru ;Str:eam and its branches supplement. the storage of the Tokomaru dam, which has an effective capacity of 44,600,000 cubic feet. A secpnd tunnel, 105 chains long and eight feet in internal diameter, connects this central basin with the surge chamber, situated at the head of the pipe lines, which have a fall to the power house:. of 890 feet in 55 chains. Each of the tunnels referred to is circular in section and lined throughout with a minimum thickness of six inches of concrete, while the intake of each is protected by a reinforced grillage to prevent the drawing in of timber, etc. Near the tunnel inlet at the Mangahao River a gate shaft and sluice gate is provided, while at the gate shaft in the second tunnel at the Arapeti end there are twin sluice gates and screens. All sluice gates in the development are motor qperated, and provide a complete system of control, both for normal working and for any exceptional circumstances that may arise. The lower Mangahao dam contains 21,850' cubic tyard's of concrete and consists partfy of a main gravity section dam in the gorge, with two automatic crest regulating gates, and partly df a concrete extension corewall on the same alignment through an old river bed deposit of, very tightly packed shingle on the right bank. This old bed forms the base of an earthfilled dam, rock pitched on the upstream face, of which the aforementioned corewall is an essential feature. The embankment of 12,000 cubic yards is protected on the gorge side by the wing walls. Excellent rock fonnations were obtained, both for the main dam and corewall. The geological history of the Mangahao River, with particular reference to the interesting formation at the dams, has been described in contributed papers to the Journal of Science and Technology. To divert the water of the river during the time of construction, a bypass tunnel, eight feet in diameter. was driven through the rock on the left bank. This tunnel, seven chains long, was provided with a gatesliaft and sluice gate, which form a permanent feature oi' the construction. On the gravity section of the dam the automatic tilting gates are so constructed as to retain the water at practically a constant level, whether there be a fiood in the river or not. These gates, when in action during a flood, form an unusually interesting and fascinating sight, and give one an idea of some of the difficulties the engineers had to contend with in harnessing a river •such as the Mangahao, which has

a flood flow of 112,000 gallons per second. Next week it is our hope to deal with the upper Mangahao and Tokomaru dams and the surge chamber. As we are working under difliculties with old reports and papers used during the construction of the scheme, any further or more modern information would be greatly appreciateck