CHANNEL FROM THE UPPER HAR BOUR TO THE OCEAN VIA ANDER SON'S BAY.

Otago Daily Times, Issue 6740, 21 September 1883, Page 4

CHANNEL FROM THE UPPER HAR BOUR TO THE OCEAN VIA ANDER SON'S BAY.

Va'"i TO THE EDITOB." . "'"Sib,—The proposal to cut a channel direct from the Upper Harbour to the Ocean, though ; by? no means a new one, has created a considerable amount of excitement, and may be worthy of consideration; The execution of such a work would present no difficultiesirom an engineering point of view, and were there sufficient data, a pretty close estimate of the cost might be made. But there are many other considerations besides the actual cost which have to be taken note of.

Engineers, in proposing works of public utility, aro, unfortunately, apt to look at tho financial aspect of a project as beneath their notice, or at least that is a subject apart from their profession. Hence .ariso ill-considered schemes, insufficient estimates, and unjemunerative works—triumphs of engineering like the Great Eastern and the Thames Tunnel, but gross financial failures. In Dunedin, the citizens will very shortly havo an example of theirown in the bridge now being constructed over tho new goods-station, a bridge which will actually reduce tho horse-power in Dunedin to less than j one-half of what it is now,'and which, in consequence, will novor be used, at least for heavy traffic. . The project brought forward by Mr A. Smaill appears to ba based upon but very slight data. In tho first place, the difference of level between the high water at Lawyer's Head and in the harbour has hot been ascertained; | in the second, there is nothing to show that the quantities he has entered in his estimate have been obtained from a reliable source; neither do the rates he works on .jppear likely to cover the expenditure. Three shillings per cubic yard for rock excavation 35ft deep, and to a certainty most, if not:all of it, under water, ia rather low. If we take the first item in his estimate, the result, as far as accuracy is concerned,, is not encouraging. Thus,dredginft 100' chains by 150 ft by 27ft I = 990,000 cubic yards, and not, a3 shown in estimate, 1,305,800, which gives a.difference of 310,000 cubic yards.; Again, in his letter dated 17th met, he states, that the difference in the time of high water between Lawyer's Head and Dunedin is'at this time two and a-half hours. This does not appear to be correct. If the time of high water at Port Chalmers be taken at noon, we nave high water at Dunedin at 12.45 p.m., Otago Heads at 11.20 a.m., Taiori River at 11 a.m., the Clutha at 11,38 a.m. The time at Lawyer's Head will bo between 11 and 11.20 a.m.—say, 11 hours 10 mm. Thus the difference would be 1 hour 35 minutes, not 2i hours as stated. / ' ' ] Air Mirams is said to have shown the difference between the high-water marks as 3ft, while two other engineers show no" difference. This is a problem which cannot, be solved by two or tluee: observations, event'"when made by three separate men, and until it is solved no reliable estimate of cost Can be made. If we take the range of the tides at the Otago Heads, in the Dunedin Harbour, and at Lawyer's Head at 6ft Gin, as assumed, for the harbour works and drainage of the Flat, we should have:

At 12.45—High water, 6ft Gin rise at Dunedin; At 12:45-High water, sft 6in rise at Liwyei's Head; ' And At.11.10 a.ni.-High water, 6K 6ln at Lawyer's Head; At 11.10 a.ra.—High rise, sft Bin at Duriedin : showing a difference of lft each way. This iB only theoretical; but it is sure not to be far out, and only actual observation can give the correct difference. There is also the ocean swell to be considered, and the effects of one of the southerly " busters." : If the rise of the tide at Lawyer's and Otago Heads ia similar, and the low water on the: same level at both places—the difference in r the" time of flow being only 10 miv.,: or perhaps a few more minutes, between the, two inlets into the harbour—how can any flow be caused throughout the harbour? If the quantities through each end were equal the currents would meet at Port Chalmers, and on I the:ebb each would return through its own , channel to tho ocean. ~.- " Vi ' "

From rdygh measurements I get..the area of the Upper Harbour at low <yater as 193,653,750 square , feet,- >nd at': high.' water i259,873;759 square feet/ '".The difference, 66,020,009 square foet, between high and low-water area is the estent of .sandbanks ■. uncovered at low water, which may. be taken on an average 2ft ,6in: above low-water level, thus requiring 4ft to fill up.to high-water level. We have then the quantity of water requisite to fill the harbour each spring tide, and which is expected to come in and go out through the 150 ft channel, as—

. ■.;.-.. . .; , , Cubiofcet. Under low-water ar0i..103,653,750x6-5=1,558,749.376 Sandbanks area .. 06,020,009x4 = 264,080,033

Total .. .. .. 1,522,829,471 cubic feet, which, at 34 "93 cubic feet par ton, gives 46,461,766 tons, the whole o£ which has to pass through a channel 150 ft wide by 35 in depth at high water in about six hours ! For the velocity through such a channel, we havo o ■ 1,622,829,471 cubic feet = 13-43 Tcot 150 X 35 X 6.x 3«00 Eer second—B 11 knotß per hour. If Mr Smaiil ad contemplated a lock channel, tne width of 150 ft would have been sufficient; but tv suggest a tidal channel of that size shows that he Has not considered the necessities of his projected work. Tbo width of the channel on_ the lines proposed Bhould not be'less than 750 ft, and have a velocity not exceeding two knots an hour on an average : •'■-'.•: , The proposed width of 150 ft being utterly inadequate to the bringing in of the quantity of wator required duiinq' the time of flood, it follows that the estimate is much below what if; ought to be. In fact three times the amount would bo required ; s*nd this sum, added to that which has already been expended on tlia Upper Harbour, would prove a burden which would utterly sink the com,merco of Punedin. The people of Port Chalmers would be obliged to protect themeelvea and form a harbour of theiv own, thus dividing tho income between the bodies. The only way to succeed in rendering the harbour a fir3t-cla93 one is to proceed with the works as projected by tho Board's engineer and sanctioned by Sir John Coode. Delay in the matter is simply loss of time and money ; no more should be permitted.—l am, &c., M, R, DWTCASJ. ■ Caversbam, September 10.