THE WATERWORKS DAM.

Taranaki Daily News, 15 May 1918, Page 6

THE WATERWORKS DAM.

BMGINIiEiR'S REPORT. In his annual report to the New Ply- j mouth. BorougiU Council, the ' engineer (Mr- W. H. Cook) says:— On March 25 the bed of the Mangamahoe River was seriously scoured out immediately from under the tunnel, which suppließ water for power and domestic use, the whole of the storage water being lost in a very short time. At first sight of the damage, it appeared somewihat remarkable that scouring could take place, inasmuch as no floods had been experienced for some time prior to the above date. As quickly as possible the electrical engineer erected a plant, and pumping operations were put in hand to lower the waiter in the scour hole, to enable an examination to 'be made. The inspection disclosed the fact that the bed around the tunnel was scoured out to a depth of about 10£t, and <Jft btslow tflie invert of the tunnel. Further, that about half the thickness of the wall of the dam was resting on top of the tunnel. About 2ft 6in now overhanging had rested upon the bed of the stream, wlhile portions of the wall rested upon cement bags. The tunnel, designed for a different purpose, was called upon to support nearly 34 tons deadweight per linear foot, to which must be added the pressure duo to the head of water when the dam was full, approximately 7501b per square foot. The total weight was considerably over 200 tons on the Icncrlli of the tunr.el under the river. The fmt necessity was to provide as much water as possible for power purposes. A temporary bag-dam was, therefore, built up around the scour hole on the down-stream side, and tlife answered very well. Plans wero put -in hand for repairing the break, and as the old dam had not been taken down to below the tunnel or on to solid ground, I proposed to go to a sound foundation, underpin the tunnel, and practically build the old into a new wall. That it would not be possible to do the work without interference with the water supply was certain. The question naturally arosi as to whether it was wise to save a few hundred pounds in reconstruction and lose possibly more 'by closing down the tramways and lighting or erect an entirely new dam. The council unhesitatingly decided upon the latter, after meeting upon the site to consider ISio master, and there can be no doubt but the decision was a wise one.

I received instructions to draw up plans for a new dam on May 3, and or the 23rd these were submitted for approval, and the work was authorised. The site chosen is some 50ft below tit. old dam, and I decided there was i benefit to be derived in erecting a reir forced structure- A mass concrete ■wall was adopted. At request of the electrical engineer, the wall to be erected was to give 3ft extra Ihead of water. This ha« been allowed for, and pressure due to ISft head of water. Tho bottom thickness is 12ft, top thickness -ft, widened by cornices to 2ft Bin, to form an ample footway, which extends over the by-wash in two 13ft Gin spansThis footbridge and the scour-valve !>latform are the only portions of the dam in which steel reinforcement was necessary. The spHlaway was widened to 2Mt. which amplv covers the calculated flood flow of the iMancamahoe. Tt is intended to allow flood waters from the Waiwakailio to flow into the dam. Th« intake gates to the race should lie down during floods, ae is the usual caae. No {Treat difficulties were met with in the erection, the only anxiety at the commencement 'being that of finding a sound foundation to start the wall from. The sinking had to be taken from 4ft to ftft low# than proposed at first., and at these depths a found, hard, and watertight bottom was found extending for 60ft in l"""tih from the north end to the' centre of the stream Beyond this to the south end of the wall the excavation was taken down to a depth of from Bft to 13ft lower than originally intended. Tho foundation was not improving; if anything, it be&me more sandy. At this depth I resolved no further beneifit could foe obtained from excavating tower. The total length of the wall is 109 ft at the top. 'A 30in cast-iron gate valve is provided to allow of emptying and letting sand out at any time required. A reinforced wall to act aa 'both a retaining wall and to hold b&ck the water in the dam has been erected parallel to the stream on fflie south side. Other works in connection with the supply consisted of breaking away about 20ft of the old dam, cutting growth along the banks which would be sdbmergod at high water, and constructing a new intake to the tunnel. This latter I suggested to the electrical engineer as being a better way to prevent a considerable proportion of the sand entering into the tnnnel. All of Win works are completed with the exception of the intake, and perhaps one other to be decided »n»n —a flood by-wadi to the race. Tho dam lias been filled, and been in use for some time, which has, I believe, coped with 'the present power demands as well a3 the town supply.