The Okehu Water Supply Scheme.

Wanganui Chronicle, Volume XXXXVII, Issue 11734, 12 September 1902, Page 2

The Okehu Water Supply Scheme.

LETTER FROM MR. MESTAYER.

The following letter from Mr. Msstayer should prove of interest" to all who are devoting attention.to-the important proposal to obtain a water supply, from Okehu:—-

His Worship! the Mayor, Wanganui. Dear Sir; —Re water supply from the Okehu. I beg to acknowledge the receipt of your ■ letter of the 3rd instant, calling; my attention to certain apparent inconsistencies in the discharging capacities of the several, mains referred to in my report and subsequent letters. These inconSist«ncies are purely apparent., and arise entirely from your committee having overlooked the fact that they were trying to compare two things between which a comparison was impossible by mere inspection. Owing to tbe hydraulio grades of the 12-inch and 16----inch mains being quite different, the difference in treir diameters is only one of the factors governing the discharges. You ask if a 12-inch main will carry If million gallons, how is it that a 16-inch mam will only carry 2 millions? The reason is..that the head on the 16-inoh main is 69 feet, and its length 6,912 yards, whilst the head on the 12-inch main is 523 feet, and its length 19,888 yards. If the head and the length were the same in both cases the relative carrying capacity would be as 256 to 144; in other words, if the 16-inch were laid in lieu of the 12-inch, it would deliver nearly 2£ times as much as the latter, but as the actual conditions as to head and length are so different, the discharges will only, be about as 8 to 7. The following are the theoretic discharges -for each section, taken out to the nearest 50,000 gal'ittns per day:—, Head, Works or Upper Section. 18-incb main, 3,100,000 gallons in 24 hours. . 16-inch main, 2,300,000 gallons in 24 hours. Reservoir, or Lower Section. 12-inch main, 1,900,000 gallons in 24 hours. 11-inch main, 1,450,000 gallons in 24 hours. 10-inch main, 1,150,000 gallons in 24 hours. 9-inch main, 890,000 gallons in 24 hours. 7-inch main, 450,000 gallons in 24 liours. From this you can see .that a 16-inch main on the upper section will supply two 10-inch;, one 11-incb atnd one 9-inch; or one ;12-inch and one 7-inch, on the lower section. And that one 18-inch on the upper- section1 will supply two 11-inoh; or one 12-incb and one iO-inch, on the lower section. You a&o say: "If the maximum capacity of the watershed is 2 million gallons, wmy consider an 18----inch at all?" The discussion of an. 18----inch main was not due to-any suggestion of mine,' but in answer to your letter of the 25th ultimo, in which you ask, "In view of "this would 7 OVi recommend an 18----inch up: the (Ravine) Stream; if so, what would he the additional cost, and what would be the advantages (if any) and disadvantages (if any) of doing so?" In reply to this inquiry, I pointed out that the disadvantage would be that in dry seasons the watershed' could not supply all that the 18-inch main couM carry, and that in such times the supply would have to be supplemented -from Virginia, but that if yaur Council did not consider this objection material, an 18-inch main could f be laid, and that then it would be well to lay an 11-inch main to begin with, and duplicate it in the future when necessary. -

When you finst instructed (me to report upon the Okehu; supply i understood that you were desirous of keeping the cotfb down to £50,000, and I endeavoured to devise the best arrangement for utilising the Okehu alone for about this limit. From that standpoint, I consider that my original proposal of a 16-inch to supply two 10-inch mains, one to be Laid to begin with, and duplicated in the future, is better than to (substitute a 12-inch for the first 10-infch, asj the 16-inch main could only supply one 7-inch main in addition to the 12-incb; and the cost of the 12-inch and" 7-inch would be more than that of two 10-inch mains.

If, however, your Council desires to make the fullest use of the Qkehu source, and does not object to a slightly increased cost, or to having to i-upplemenib the supply from the Virginia Lake in dry seasons, then in that case an 18-inch main might with advantage be laid in the first instance for the upper section. If this were done, I consider that the most economical arrangement of mainls in the lower section would be to lay a 11-inch to stait with, and duplicate this in the future. The cost of the 18-inch and 11-inch mains would be approximately the same as that for the 16-inch and 13-inch; but will, of course, be greater than that for a. 16-inoh and a 10-inch. Ferguson's list prices are not of much assistance in determining an exact price, as he only quotes for two thicknesses of metal, Nos. 13 and 14 gauges; and; owing to the great pressures to which these pipes will be subjected, they must be made of stouter material. I had his prices) for 16-inch and 10-inch pipes made of No. 10 gauge, which is about one-eighth of an inch, but have not yet received hit* reply to my request for detailed! prices of thicker plates. In the .absence of this information, the .estimates for 12-inch and 18----inch can only be_ approximate. I trust the explanation abiotve has been clear. The subject of discharges of mains is not an easy one to make clear and distinct to those uinecquainjted with hydraulics, but if there is any point upon which you desire fuller infoumiaticfti please let me know, and I will do my best to supply you with it. Yours faithfully, R. L. MESTAYER,