BOROUGH WATERWORKS.

Taranaki Daily News, 15 January 1918, Page 8

BOROUGH WATERWORKS.

KEvY DAM AT INTAKE. The following report on the work at No 2 dam was presented at last night's meeting oi the New Plymouth Borough Council: Work DoneT—Since reporting last upon the above (lie «mk went ahead at a more rapid rate, and to date the whole of tins main wall has been completed to full height, the rail standards set in position, the'top surface plastered, upstream face cement-grouted, the valve rod, pillar and wheel erected, the track from new to old da in commenced, the spillway and supporting column has been erected in the rough, and by-wash walls raised in part The by-wash has been lengthened and spill steps put. in hand, steel reinforcement for beam over spillway prepared and shaped ready for use. The winning of sufficient quantities of gravel has been more difficult, quite five cubic yards having to be removed for each yard of useful gravel. Water Supply—On Saturday evening last the dnm was allowed to' fill to a depth of 12 feet, and from now on J propose to give all possible- head, subject to lowering wht ß finishing work has to be done. Work to Complete.—None of the workremaining to be done is of a very heavy nature. The following are the chief items: Completing ?i.',< : track, putting on hand-railing, finishing down steam face of wall, completing of spillway and by-wash, completing retaining wall and backing and general ckaning up. Intake to Tunnel.—The form of present intake is bad. and I have to suggest to the electrical engineer that opening he made-through the wall of the old dam from the downstream side. By doing this a better entrance will be made, giving greater area of waterway, and doing away with the large amount of frictional resistance of entry. Another benefit will be that of diminished trouble from sand entering the tunnel, which m the past has proved costly to remove and lessens the water supply' to the forebay. At the electrical engineer's request I am preparing flails, showing the proposed alterations. The bag dam will be removed before the final completion, and n portion of tfTe old dam removed to allow a sufficient flow to reach the intake Overflow to race.—Tn constructing the new dam the spillway is raised three feet above previous high water level: the effect is just this extra height of head water, and will also back water along the race to within four chains from thq river intake gales. Thus, al normal times, the water will be at a higher level in the race, taking the form of a wedge, with the intake gates open under normal flow, the depth at three chains from the race tunnel will be three feet. Assuming these conditions and a rise in the Waiwakaiho river of 0 to 8 feet, a not infrequent occurrence, also the Mangamohoe in flood at. the same time, with the extra pressure head water will rise to above the oast wait of tht race and overflow, with possible danger to the supporting banks I have suggested to the electrical engineer that a side weir be constructed to form a. relief flow back to the river, to be put in at a suitable site. Something, of this natnv? will be found necessary, otherwise considerable damage may take place before anyone from the powerhouse could arrive to shut down the gates. Gates.—Those gates are not, easily worked, and at the best are very leaky: the outomatie gate? also !oalc badly. ' I suggested some time back, both in a report and also when conversing with the electrical engineer, that proper gates should be installed, which should easily be worked by hand during emergencies. They could at some expense be electrically operated from the power-house if desired. Shingle trouble.—From time to time shingle and sand accumulates in the bav in front of the intake, and has to be periodically removed. Especially is this necessary during low water in the river. A groin erected for approximately 40 feet, joining to the point of land at 'head of race, would cause the materials to pass beyond the draw of the race, when it may be anticipated the velocity of the river will carry the sand out from Interfering with the intake. To gain the utmost -benefit from the groin the larger boulders below will have to be removed to allow of thn velocity being maintained. I suggest that anothci means could be adopted—to drive a tunnel, commencing at approximately 100 feet below the intake gates and endin" in the deep water under the cliff. It is noticeable that these deep pools have formed at each cliff, and little or no drift shingle has been deposited foi years in such pools. lam confident that this method would solve the problem of ahin/rle deposit. ° Growth on Banks of River.—Bv raising the water 3 feet, it will, of "course submerge a great deal of bracken and vegetable growth, which will it left become decomposed and taint the water supply, also much of it will become detached and b c washed down, coming to rest upon the screens in tho forebay Such growth should be cut dows. Several of tlie foregoing matters nave been mentioned in casual conversation*, and are now brought under your notice for definite consideration. l On the motion of Or. Short, seconded by Cr. Hill, it wag resolved to defer consideration of the report, for a week- in order to allow councillors to inspect the works.