OUR WATER SUPPLY.

Auckland Star, Volume XXIX, Issue 68, 22 March 1898, Page 3

OUR WATER SUPPLY.

SUPPLEMENTARY REPORT.

MrYY. Anderson, City Council Engineer, and Mr James Stewart, C.E., have forwarded a supplementary report with regard to the question of obtaining the best water supply tor Auckland : —

,!, Wtl; reference to the very pronounced attitude taken by a portion of the public press la respect to several points in our report of the 14th ultimo, we think it proper to make the following observations for your consideration in the event of it beinoconsidered advisable to acquire more explicit data than we are at present in possession of, more especially with regard to Waitakerei.

_ "Our object in conducting the previous inquiry was to gather data, and to place before the Council a statement of observations, as concise as possible, in view of the wide range of ground to be covered, and towards this end we are in duty bound to curtail time and expense by making use of our experience in estimating the value of elements relating to the inquiry where such estimation, in our opinion, afforded sufficient margin, and would save the expense of detail surveys and measurements. For instance, to accurately measure the present outflow from the Takapuna Lake would take much time, and cause considerable expense. But to those with experience in measuring flowing water it is remarkably easy to estimate within narrow limits of error the smaller streams, and where, asm this case, the required amount is five millions, and the estimated total yield of all the visible points of natural outflow certainly under one quarter of a million, we considered it would be a waste of time and money to accurately determine the discharge. "In like manner, although the old surveys and gaugings _of the Nihotapu, made by the late Provincial Government, appear to have been lost, it is remembered that the minimum flow was about one million gallons per day, and the height at the bridge about 800 ft. These data would be ot very small service on which to found even an approximate estimate of the value of this source of supply were it not for the possibility of comparison with other wellknown gathering grounds and the respective cost of storage. When the Nihotapu was explored, the extent of the proposed water supply was one million gallons per day; now it is live millions, and considering the main circumstances affecting the case, and applying recognised rules of practice, we find that one thousand millions must be stored to be safe. It is not a question of sufficiency of gathering ground and rainfall. There is, no doubt, an available area and a rainfall far beyond what would furnish a supply on the proposed scale.

" It is the possibility of storing the water at a reasonable cost that forms the ground of our objection. It is here, we repeat, that we must avail ourselves of experience, and when we see that all our.valleys are more or less narrow gorges, having heavy longitudinal inclinations, we know that that is a country Avhere the most expensive dams will give the least useful result. To store one thousand million gallons there would he very costly compared with the most expensive reservoirs we knew of. And if at greater cost the storage requisite for the present scheme can be found it is open to grave doubt if, at any expenditure within commercial possibilities, a second instalment of an equal water supply could be effected at the necessary level in the Waitakerei district.

" It was in respect of all this that we advised your Committee that Aye could not commend the costly process of a complete engineering surA'ey of this scheme. It is possible, hoAvever, that the public Avill not be satisfied Avithout an accurate investigation of the whole possibilities of a gravitation supply, present and future. It is in vieAV of this contingency that we noAv beg to lay before you Avhat such a procedure would invoh^e. , " 1. It Avould be necessary to determine, by absolute levels, the points on all the available streams, including the Waitakerei, which would command the Avorks, having in view the fall iv pipe track, clear Avater basins and niters.

"2. Those available streams should then be examined as to the best possible methods of being utilised in the scheme, and the length and position of all connecting channels and tunnels. "3. All the available streams should be gauged, the daily readings extending over one year at least, and noting the periods of extreme discolouration of the water and the maximum aud minimum How available for storage purposes. "4. All possible sites for dams should be examined by longitudinal and crosssection levelling and contouring, so as to learn the absolute storage capacity of each site. "5. The sites of the possible dams should be thoroughly examined by borings or sinkings, and data collected as to the best means of forming such dams, whether by earthern embankments and puddle walls, or by masonry, and the respective cost of each. Also cost of masonry in waste weirs, valve towers, outlet and cleansing tunnels, by-wash, etc., etc. " 6. The line of pipes and aqueduct from the lowest reservoir to the open country, where it is most likely the filters and clear Avater basins would be placed, should then be studied, so as to determine its most efficient form, the extent of contour, cut and cover, tunnels and inverted syphons. " The above work, lying almost wholly in steep and very broken bush country, would necessarily be expensive, a3 the general points and features would have to be determined with the same accuracy as if for working drawings. "The time required cannot be closely estimated, as this would depend on the number of field parties who could be closely supervised, and kept at profitable Avork. Rouo-hly estimated, it might run into five or s?x months, and the expense about £1,200 for field Avork and expenses alone, besides the plotting and reduction of woik in the office. , " As stated in our previous report, we do not think the prospects of this scheme Avarrant the expenditure; but, nevertheless, Aye may be Avrong in this view, and it maybe held that the possession of data such as above set forth ought to be effected at any reasonable cost. It the investigation be entered on it must be carried through in a systematic and thorough manner, so that any intelligent person may at once grasp the Avhole position and capabilities of the scheme."