DUNEDIN CITY WATER SUPPLY.

Otago Witness, Issue 2318, 4 August 1898, Page 54

DUNEDIN CITY WATER SUPPLY.

THE SCHEMES BEFORE THE

COUNCIL,

NO. 111.

In further consideration of the schemes before the City Council bearing on an additional water supply, there are many points worthy of special cote. As regards the gathering grounds, the proposed reservoir on M'Keozie Creek (which i 3 a tributary of the S.lverstrcam) would receive the catchment from 510 acres. The area above the 54Offc draw-eff on the Leitb, a quarter of a mile above the junction of Nicholl's Cieek and 118 ft above the present Ross Creek reservoir, is 3900 acres. If desired, the catchment of 2200 acres from the W&itati could ba utilised. The intake from the Leith and Waitati would tap 6100 acres, or nearly 12 times tbe capacity of theM'Kenzie Creek ground. As regaids Mr Park's \iew as expressed at the meeiisg of the City Council on Wednesday night, it doe 3 cot se«m reasonable to favour the reservoir at M'Kenzie Creek when tbe Waitati supply is ample for the requirements of a much larger population than that of tbe city at the present time. It is quite unlikely that the reservoir at MKenzie Creek cculd augment the water supply to the extent mentioned by Me Park. Mr Biir in his report estimates this supply r qual to the requirements of 14,500 peoplenot 40,000, which Mr Park claims for it. Provision for additional storsga might bring it up to a littlo over 20,000. As regards available rainfall, in their report Messrs Hay and Mirarns aseuma the dryweather daily flow of the creeks at 250 gallons per acre, while Mr Barr baa taken it at 200 ia hia recent report, la their report Messrs Hay and Mirams assume 16in as the available annual rainfall, while Mr Bm- places hit. animate at lOin. Referring to the latter report Mr Mirams, in a report to tbe City Council, says he accepts Mr B.ur's figures as lair and reasonable, and in accordance with hia own cb-ervations at the Ross Creek reservoir. That being so, it follows that in order to make a fair comparison between the two main schemes— those of Mr B-ur, and of Messrs Hay and Mirams— tha calculations referring to the southern grounds m*y be revised, and a much smaller ji.ld accepted a 8 the basis of estimate. la tbe case of the flow of the creeks there ia a drop from 250 to 200 gallons to be deals with— that is to *ay, the figures supplied by Messrs H*y and Mirams require to be reduced by one-fifth to arrive at a " fair and reasonable " estimate. In the case of the annuU rainfall there is the same discrepancy to deal with, tbe difference between tha figures equalling a drop of three-eighths. This point is of importance, for at the meeting of the council Mr Carroll said the yield at the M'Kenzie Creek would be 195,000,000 gallons, but at the tmalier lato estimate by Mr Barr and now recognised by Mr Miram=», the yield would only be 120,000,000 gallons, With the expectation of 195,000,000 gallons the engineers gave it as their opinion that a pumping service would be necessary, in addition to the reservoir. But as the pumping scheme seems cut of the question when other source,? whence an ample gravitation supply might be drawn are available, in considering the merits of the southern and northern sources, respectively, the supply at tho M K«nz ! e Creek has to be reduced to 120,000,000 gallons, which, without augmentation by pump- I icg, is inadequate to the requirements cf (he city. Then again, if the supply of the cily slone be considered apart from the high suburbs and higher parts of the city, then the S;lverBtrGam service is at sufficient elevation. At the council. meeting Mr Carroll said that the Silverstream and the present supply from Hoss'b Creek were sufficient to supply the city proper, while the Whare Flat reservoir would supply the higher level?. This is clearly a misapprehension of the true bearings of the case on the part of Mr Carroll. While it is tme that the proposed reservoir would be considerably higher than the present race, still it must be remembered that the water has to be brought down to that lowc-r level, so that the higher levels will be in exactly the same position as they have been all along. Mr Barr, however, in his Ecbemes makes provision for an effective pressure in the highest part 3of the city. This would be secured by a draw-off at a point 20 chains above tbe junction of Nicholl's Creek, and would ensure the inhabitants of the high levels a constant ueivice. In the consideration of a permanent water supply Bcbeme the suburbs have to be taken into account, as the amalgamation of the city and suburbs for drainage purposes would be facilitated by a common water service.

Mr Ban's proposals to liud a water tupply Eufficient for the requirements of the city by drawing upon the high levels of the L&ilh and Waitati do not meet with the approval of the city surveyor, who presents several Important arguments against the Leith as a source of water supply. Mr Minims says he has never considered the Leith as a permanent source cf supply for the city, and raises the objections that Us proximity to the city renders it moi'9 and more liable to pollution as settlement advances; that there is danger in constructing reservoirs in tha Leitb, which is intensified by our liability to earthquakes ; while the cost of extinguishing riparian rights is an uncertain contingency. As far as the danger to tna inhabitants from constructing additional reservoirs on the Leith is conceroed, we may point out that the embankment of the present Ross Greek reservoir is 70ft, while the reservoir Mr Barr proposes would have an embankment of 41ft for 63,000,000 or 51ft for 94,500,000 gallons. As the height of the embankment has a great deal to <\r> with the safety of a reservoir, Mr Ban's inposal sppcara to offer better grounds for stability than the present iessrvoir in the Leith Valley. And sgain, the outlet pipes in the present reservoir are supported at intervals upon masonry piers constituting a kind of bridge carrying the great weight of the superincumbent bank, while Mr Barr proposes to lay bis outlet pipes through a solid rocky epur, leavirg (he Embankment solid throughout. The present

reservoir is within half a rosle of the paper mill at Wqodhaugh, and the gorge opens straight on to that bnildirg, while immediately below that there are numerous dwellirg hondos. The reservoir proposed by Mr Barr would be two and a-balf miles distant from the paper mill, and for a considerable part of that distance the valley is wide, thereby providing for the spreading of the water, in the unliktly contingency* of the bursting of tbe reservoir, and lessening the danger to the residents in the vicinity of the Leitb. Moreover, the proposed reservoir

ould not be directly in the Leilh, bub in the Mount C'argill Creek; therefore a flood in tbe Leith would not touch the reservoir, Whild we are not ..prepared to advar.ee an opinion against that of an engineer, still, looking at the matter from an outside point of view, it would appear^ that a good deal might le said in favour of the reservoir in Mount C:<rgUl Creelr.

Tbe question cf pollution is a very important one, and, while no one doubts the purity ot the Waitati supply, it must not be overlooked that the bulk of the gathering ground in the ]>Uh is of precisely a similar character to that of the Wuitati." Tbe City Ooiporation have in the Loith between GOO and 700 acres of high-lyirg ground, and there ia aa education endowment of 120 acre?, giving together an area of 50 per csnt. greater than tbe gathcrirg ground of the M'Ketzia Cieek. The whole of the gathering ground of the Leith above the 510' i draw-off is a quarter of a mile above Nioholl's Creek, and of a character wholly unfitted for close settlement. That dmv-eff would be fully a m?le above where the corporation have frequently drawn a supply, the intervening area being freely dotted with dwellings. It needs no arguments to prove that the water collected higher up mm t bs very many degrees purer than the low-level intake that has been drawn upon st times. Mr Birr proposes to convey the water in pipes down the Leith Valley, and that fchould certainly ensure a greater freedom from pollution than the Silverptrtam open race carried through 20 miles of aattled country. Taking all the surroundings into consideration there gesms no grcuads for apprehension on the score of pollution before the water reaches the reservoir. True, it might ba discoloured at times from mechanic&l contamination, but settling in the settling beds would get rid of that to a very great extent. Bat before ar.y water supply can ba quite satisfactory filtration mu3t be resorted to. Filtration to be effectual mast take place immediately before distribution lo the consumers, and therefore muat be below the storage reservoir. Therefore the losa ia he' gut would reduce the pressura below the supply of any but the lower levala of the cifcy if filtration in connection with the preteLt reservoir is resorted to. The higher levols could be supplied with filtered water from the proposed high service from the L*ith and Waitati. It id evident that a satisfactory supply for the city and higher suburbs cannot be" obtained from the Silverstream souice without involving large expenditure to maka provision for pumping. It is said that a mucb. larger catchment area is available in the country surrounding Whare Flat could arrangements be made with private landowners there, but further reports on this aspect of the matter are necessary before it can be considered. While not committing ourselves to any of the schemes propounded by the engineers, from the evidence brought forward it wculd appear thai, ths capacity of the northern fidld greatly exceeds that of the southern, and there is basides tho additional advantage of greater elevation, affording facilities for a supply by gravi*atioa alone. Tha City Council has wisely decided to giva the matter further ccnsideiatioa, and to obtain farther reports before adopting a echeme of water snpply. The somewhat divergent opinions of experts justifies the council in deferring its decision till further information is laid befora it.