BOROUGH WATER SUPPLY.

Wanganui Chronicle, Volume LXXVI, Issue 18438, 23 March 1922, Page 9

BOROUGH WATER SUPPLY.

COUNCIL DECIDES ITS POLICY, OKEHU SCHEME TO BE DEVELOPED. Following on the visit to the Okehu Water Works on Monday, the Borough Council last evening considered its proposals for the future provision of affi adequate supply for the town and suburbs. The Council decided to proceed with the development of the Okehu Water Works and the engineer’s report on a scheme which would provide water for Wanganui for a 20 years period and a population of 45,000 was adopted. THE ENGINEER’S REPORT. Mr Staveiey’s report was as follows: In accordance with your instructions I submit herewith a schedule of works for this loan; but, before doing so, it is necessary to add to the information already in your hands a few remarks on the extent of the work to be undertaken.

In my report of May 11th, 1920, I proposed certain additions to the supply, sufficient for at least ten, and possibly fifteen years, and gave reasons, namely high cost of materials etc., for adopting this comparatively short period' at that time. During the last few months however, there has been some reduction in costs, and it is, I think, the general opinion that the present proposals should be enlarged, so as to provide if possible for the next twenty years; that is to say for a population of about 45,000 consuming at maximum demand 31 million gallons or 80 gallons per head per day. (I have adopted 80 instead of 90 gallons as a maximum, on further consideration of all the circumstances.) You will have noted that I stated in the above report that the maximum capacity of the Okehu scheme with increased storage was 23 million gallons; basing this on the capacity of the 161 n pipe from the dam to the Rangitatau break-pressure tank. If however, this 16in. as well as the 12in. pipe, were duplicated, (which would be a decided advantage in assisting continuity of supply) the capacity of the scheme could of course be increased. This duplication would be, owing to enlargement of tunnels, bridges, and cuttings a fairly expensive matter, but as the lining of the tunnels should be done in any case, one complete job can be made. The size of the second pipe will be 12in. and the combined delivery 33 millions. The cost of this section of 4 miles will be approximately, including tuunel-liuing, £28,000.

Okehu Dam and Storage Required. —lt is advisable at this stage to indicate the ultimate capacity of the Okehu and Mangahoropito watersheds in relation to the average daily demand, assuming that the storage will be increased to provide for the maximum demand. The mean rainfall on this area, observed during the past 17 years, but based on that of the three driest consecutive years, as is the accepted practice, is 47.62 inches. This compares fairly closely with that assumed by Mr Mestayer from the information then in his possession, namely 45.42 inches; which he reduced to 41.75 inches to be on the safe side, as his data were insufficient. Having now a further series of observations taken on the water-shed itself, we can safely work oh his higher figure, and still have a margin of safety. An annual fall of 453 inches, after making the necessary allowances for loss by absorption, evaporation, and floods, would supply an average of two million gallons per day; which is, so far as can be ascertained in the absence of a recording meter on the trunk main, more than twice the average demand. That is to say the Okehu scheme can be made to supply twice the present population, say 42,000 people, which is estimated to be attained in or about 1940. Thus the period provided for will be from present date 18 years, which with proper regulation will almost certainly be extended to 20 years or over. It is next necessary to consider the storage required to maintain the supply, 33 millions daily, during maximum demand, which will occur probably for a week or two at a time over the three summer months.

The dry-weather stream-flow being 13 millions, if there were a continuous drought of three months duration the whole of the extra water required, amounting to 180 millions, would have to be provided by storage. This is, however, such a remote contingency that it need not, and probably could not be fully provided for, I consider it sufficient to allow for a drought or one-third of this period, or 60 million gallons. We find that the raintail for the three months of the driest summer of the 17 year period observed is 6 J inches, which would yield after deduction for evaporation etc., 157 millions; or, over and above the stream-flow, 22 millions, leaving the balance, of 38 millions to be provided by storage.

Of this amount, 15 millions will be impounded by the Okehu dam when fully raised, and there will be 5 millions at Westmere. The remaining 18 millions will have to be stored behind an auxiliary dam or dams in either the Mangahoropito or Okehu valleys, the former for preference if suitable foundations can be obtained. Borings have been started to enable this point to be settled.

The estimate for this dam should not exceed £15,000. If it does so, further extensions at the Westmere reservoir should be carefully considered as an (complete or partial) alternative; storage at this point being of more value than at Okehu, but probably more expensive. The amount mentioned is included in the loan schedule under the heading “Auxiliary Storage,” and it is not advisable to make it more specific at present. Main Duplication, first section.— As already explained, the 4 miles of 16in. main will have to be supplemented by a 12in., the cost of this

section being £28,000. Second Section. —This comprises 11 1 miles of existing 12111. pipe discharging 1.6 million gallons per day. To bring its capacity up to 33 millions requires a 13in. pipe. This is an odd size and introduces complications in manufacture and re valves etc., which it is desirable to avoid. The position is that two 12 inch, pipes give 3i millions, and one 12in and one loin, give 3} millions. It would be possible by shifting the break-pressure lank back to a higher point on the pipe-line as originally designed, to Increase the head on this pipe so as to make the two pipes deliver the required 33 millions. As this would Increase the already very high pressure on part of this pipe, I would not recommend that it be done until absolutely necessary; but meanwhile the duplication could be made in 12 in. pipe which will carry on to very nearly the time specified without alteration. The cost of this section will be £45,000. Third Section. —This is the 15in. main from Westmere to town, effective capacity 2,660,000 gallons. The duplication of this pipe was urged from a fire-prevention aspect by Captain Hugo, N.Z. Inspector of Fire Brigades, in a report in 1918. and is for other reasons a matter of great importance. The position is that if for repairs or other reasons it is necessary to shut off this pipe between Virginia Lake and 'Westmere, the town is fed only by the low pressure from the lake, it may be for several days. But if the pipe should require repair between. Virgina Lake and Parsons Street, the town would be without water at all, except what could be got through a three-inch circuit. This was one of the principal reasons for my advocacy of the lake pumping scheme in my former report, since that scheme, with the half-mile of 12in. mentioned in connection with it, would have avoided the duplication of the 15in. tor full distance of 23 miles from Westmere to Parsons Street, besides supplementing the supply at any time of shortage. For the scheme now proposed it should be duplicated in 15in., at a cost of £12,500.

Westmere Reservoir.-—The present reservoir has a capacity of 23 million gallons, and it is proposed to enlarge this to 5 million gallons. A balance of £6,000 over the funds available is necessary for this purpose. At the same time, further enlargement will be considered as an alternative to auxiliary head-works storage as mentioned above.

High-Level Scheme.—The best way to provide for Durie anti Bastia Hills, is as already reported, by pumping from the present system into a water-tower on the highest point. A sum ot £11,500 is provided tor this purpose. Filters at Westmere.—The cost of these is not yet definitely ascertained, but may be assumed for present purposes at £lO,OOO. Replacement and Extension of Town Mains Etc.—Several of the older mains are badly in need of replacement owing to incrustation; extensions are required both in the town and suburbs; and it is very desirable that the fire-hydrants be converted to the standard ball-pat-tern in. place of the obsolete type now In use. The sum allowd for these purposes is £7OOO. Conclusion. —A schedule of the works dutlined above is attached, alto an alternative schedule showing works required fcr a 12 year scheme. It will be seen from the foregoing that the 33 million gallon scheme is the utmost that can be got from Okehu, and it is right to add that the late Mr Mestayer considered it capable of only 23 millions scheme. I do not think, however, that he was fully aware of the possibilities of storage in the Mangahoropito valley, and, as mentioned above, the rainfall Is heavier than he anticipated. The expenditure required is no doubt considerable, but it must be remembered that this towu requires a very large quantity of water per head. That this quantity could be reduced by metering is of course obvious, and it is a question of policy whether this should be done. In any case the extension of the water-supply is a matter of the greatest urgency, and the twelve year period seems all too short for future provision under the conditions now prevailing.

SCHEDULE OF WORKS FOR WATER SUPPLY.

(A) —18-20 year period. 33 million gallons scheme:

After discussion, our report of which is held over owing to pressure on our space, a motion to the following effect, moved by Cr. Burgess and seconded by Cr. Luxford, was carried. That a proposal should be placed before the ratepayers:—To raise the Okehu dam by 17 or 18 feet, to provide auxiliary storage at Okehu, to add to the present tank at Westmere by increasing it to 10 million gallons, to duplicate the present main, to carry out a high level scheme by a water tower on Bastia Hill, to provide a filtration plant, to replace mains and hydrants, a total cost of about £169,00'0, the engineer to prepare estimates for a loan to be placed before the ratepayers. Cr. Burgess said steps should be taken at once to increase the reserve supply at Okehu. The Council should not consider putting in a larger duplication main than 12in.

1. Storage (a) Raising dam 9000 (b) Auxiliary storage 15000 (c) Enlarging reservoir (bal.) .. 6000 2. Duplication of trunk mains (full length) . . 8*5 5 0 0 3. High level scheme (Durie & Bastia Hills) 11500 4. Filtration plant . . . . 10000 5. Replacing & extensions of mains & hydrants . . 7000 144000 Engineering & legal costs & contingencies 10% 14400 158400 Say for loan purposes £160000 (B)—10-12 year period. 2i million gallons scheme. 1. Storage (a) etc as above 15000 2. Main duplication (of 2nd & 3rd sections only, in 12in. and 15in) . . . . 57500 3, 4, and 5 as above . . . . 28500 101000 Engineering and legal costs and contingencies .... , 10100 111100 Say for loan purposes £112000 Second section increased from 1 0 in. to 12in. to enable scheme (a) to be completed later. LOAN TO BE ASKED FOR.