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The Pamphlet Collection of Sir Robert Stout: Volume 26

Digest of Engineers' Reports

Digest of Engineers' Reports.

It would be as uncourteous to these gentlemen as it would be altogether beside the duties of this Committee to enter upon anything like a criticism of these reports, or to pronounce judgment upon them in any way; but it may not altogether be out of place, and may help the public the better to arrive at something like sound and fixed conclusions on this important subject, if the various points in which these Reports agree, and in which they differ, are clearly and concisely stated.

First.—There is but one opinion amongst professional men as to the desirability and feasibility of improving the Upper Harbour, so as to enable all vessels entering the Heads to come up to Dunedin.

Mr. M'Gregor says :—"I believe dredging plant was never applied to an easier piece of work than deepening the Harbour of Dunedin, and I am of opinion that the largest vessels that enter Otago Heads can be page 4 brought up and berthed alongside the Dunedin wharves within two years from the date of commencing dredging operations; and if the scheme I have suggested were carried out, Dunedin would possess one of the finest commercial harbours in New Zealand."

Mr. Barr says :—"The three most important portions of these Public Works, viz., Harbour Improvement, Reclamation, and Sewage should be carried out as a whole. With reference specially to the latter, this subject must be decided one way or another within a few years, unless the health of the inhabitants is to suffer to a probably alarming extent."

Mr. Blair enters at length into an explanation of the forces now in operation which are slowly but surely deteriorating our Harbour, and which are to be counteracted in a great measure by improving the Upper Harbour. He states generally :—"There can be no doubt that the principle of Mr. Balfour's scheme is a correct one." And again, "I think, however, that I am safe in saying that the largest ship frequenting the port can be brought up to Dunedin at a nominal expenditure."

Mr. Thomson writes on the matter as taken for granted, and says :—"The limit of depth at Dunedin will be governed by the depth of water on the bar, and this will allow vessels drawing 21 feet at neaps, and 22 feet at springs, to safely come up to Dunedin."

Second.—There is absolute unanimity amongst the Engineers as to dredging, on a more or less extensive scale, constituting the main feature in any scheme of Harbour Improvement. Whatever differences of opinion may exist as to the best means of keeping the deep water channel open after it has been dug out, or as to the best description of machinery to be employed for the purpose of dredging, or the most economical mode of disposing of the stuff excavated, there is only one opinion—that by dredging operations first of all must the much-desired deep water channel be formed.

Mr. Blair states that "Dredging is indispensable and common to all the schemes for Harbour Improvement." Again, "Whatever plan of Harbour Improvement is adopted, I would recommend that the dredging be first commenced. It is the key to the whole scheme, and the work can be arranged so that the smallest instalment will be beneficial."

A perusal of the Reports of the other Engineers will show that in this their views coincide. Mr. Blair estimates the cost of dredging a channel the same width as the Suez Canal, 72 feet, which he thinks might accommodate our traffic for some years, by 21 feet deep, at £63,250, taking the cost at double the Clyde rate, exclusive of plant, which Mr. Balfour reckons at £40,000. And "the dredged stuff would reclaim land sufficient to pay for the cost of cutting the channel." "One dredge could accomplish the work in 350 working days."

Mr. Thomson estimates the cost of dredging a channel from 100 to 130 yards wide, and 21 feet deep, with a dock for vessels to lay in 23 feet deep, at about £65,784, including plant, but excluding wharves.

Mr. Barr estimates the cost of dredging a channel 150 feet wide by 21 feet deep, and a channel along a circular wall in front of Dunedin, at £65,610 exclusive of plant, which he reckons at £22,500, and the time he puts down at five years.

Mr. M'Gregor, who seems to have had the most recent information as to dredgers, and who describes a new "Combined Screw Hopper Dredge" which is being largely employed by the Canadian Government after full trial, and which is capable of making the voyage to New Zealand under steam and canvas, reckons the cost of dredging a channel 250 feet wide, by 21 feet deep, with a large deep water area in front of Dunedin, at £50,000, and the page 5 cost of two dredgers at £25,000, one of which could however afterwards be sold, making the cost in all about £62,500, and the time as stated already, two years.

It will be at once seen that there is a remarkable agreement as to cost in these various estimates, and the Committee have no hesitation in saying that, making allowance for differences of methods, sizes of channels, &c., there can be no doubt that most reliable data as to the probable limit of cost of this portion of the work have been obtained, and that it may safely be put down at not more than £65,000.

Third.—All agree that as little reclamation as possible should take place in the Upper Harbour. Mr. Thomson states that, since he reported to the Government in 1859 and 1868, greater reclamation has been proceeded with than he advised. He lays it down as one of the principles which he recommends, "that as little reclamation as possible should be gone on with and he winds up his Report with this statement—

"To extensive reclamation I am also opposed. 1st. As regards the town of Dunedin. That such would obstruct properly-inclined sewerage, and thus create pest centres dangerous to the health of the inhabitants. And 2nd. That, as regards the Harbour itself, to diminish the water flowing up and down the Harbour (the effect of reclamation) is to diminish the water on your bar, and thus deteriorate the Harbour as a shipping port."

Mr. Blair says :—"On the principle of prevention being better than cure, I would be inclined to lay down as an axiom that no reclamation whatever should be made without a corresponding amount of excavation in the tidal area." And further on—"Without admitting the desirability of reclamation in any form, it is quite evident that a certain quantity must be done in the neighbourhood of the Docks and Wharves. I would, therefore, be inclined to apply the dredged materials towards reclaiming land in the most valuable position for building sites."

Mr. M'Gregor proposes to carry the dredged stuff to sea, and not to reclaim any land with it, and makes the following important suggestion :—"It is part of my scheme for improving the Upper Harbour to dredge off the tops from all shoal banks that project above the level of low water. My object in doing this is to increase the tidal capacity of the Upper Harbour, and thereby increase the mechanical power for permanently maintaining it. Another advantage would be, that by removing the tops of the banks there would be no danger of deposit being washed into the deep water channel. Nothing could get into the channel but the deposit from the hill sides surrounding the Upper Harbour, and a large proportion of this even would be intercepted," &c. He also says:—"I would, however, on no account recommend that any wharves or piers of any kind should be projected beyond the present end of Rattray street Jetty. All projections beyond this I should remove."

Mr. Barr also, all through his Report, refers to the reclamation as a part of the work which should only be canned out so far as is absolutely necessary. The Committee, therefore, are fully satisfied that the "axiom" laid down by Mr. Blair is indisputably the only safe principle to be guided by as regards reclamation, and the only differences amongst the Engineers on this matter are in relation to the economical utilising of the dredged stuff; and it is fair to assume that if the taking the stuff to sea, as recommended by Mr M'Gregor, were proved to be cheapest, all would agree in reclaiming only so much as is involved in the construction of the quays, wharves, and basins that may be determined upon.

Fourth.—These Reports all show clearly that any comprehensive scheme of Harbour Improvement must embrace and fix the principles on which the page 6 future scheme of sewerage for the City shall be carried out, discarding (as they all do) any idea of carrying the sewerage in any other direction than the natural outlet—the Harbour.

Mr. Thomson says :—"In connection with this subject, it would be an oversight in me were I not also to notice the sewerage of the town of Dunedin, as the works are intimately connected. Various schemes have been propounded for this, some Engineers wishing to carry it to the Ocean Beach; others, as far as the Taieri Plain—[See Swyer's Report.] These would involve sewers and pumping machinery such as only a wealthy city like London, for instance, could afford. I note, too, that one important fact has been lost sight of by all the parties reporting on this—viz., that the Harbour being salt water, it acts as a disinfectant; and this renders sewerage innocuous. The only objection to the sewerage going into it is, therefore, merely its bulk or quantity, in as far as its solids are concerned. This, for many years, must be an immaterial consideration."

Mr. Barr's opinion on this has already been quoted under the first head, and Mr. Blair and Mr. M'Gregor both propose making provision for it going into the Harbour. There are, however, two modes of dealing with it contemplated by these gentlemen, Messrs. Thomson and M'Gregor agreeing in their mode, and Mr. Blair and Mr. Barr generally in theirs. Mr. Thomson would "carry the outlet of the sewerage into the middle of the stream below Pelichet Bay, where in future years covered reservoirs would be constructed to pen up the liquid till the tide ebbed. Thus, by this measure, none of the offensive matter would flow up the Harbour in front of the Town. This arrangement would be far preferable to having the sewerage carried backwards and forwards by the flow and ebb in front of the Town, however rapid these might be." Mr. M'Gregor says—" I would suggest that the deep water wharf (in front of the City) should be constructed of stone and concrete in a substantial manner, and in the centre of the wall I would form a main intercepting sewer capable of receiving the whole of the surface drainage of the City. The sewer should have a fall from the centre north and south; and at each end, at some convenient spot near Pelichet Bay and Anderson's Bay, silt basins would be constructed, so as to receive the deposit from the sewer, and nothing but water discharged into the Harbour. This plan would remove all the objections brought forward regarding the danger of filling up the dredged channel from the surface deposit now discharged into the Harbour. Moreover, it would keep the future cost of maintaining the permanent depth down to a minimum."

On the other hand Mr. Blair would depend largely upon the artificial current created by the central training wall to sweep the shores of the upper part of the Harbour of the sewerage of the City. He also suggests an idea to increase the scour, viz.—"To make the Railway embankment at Pelichet Bay watertight, and to run another of the same kind (to be utilised as a road or railway) from Jetty Street to Burns's Point on the Peninsula, and to use the area thus enclosed for holding the tide till near low water, when it would be allowed to escape into the channel. This method of scouring is extensively used in tidal basins throughout Europe."

Mr. Barr, while also advocating an artificial tidal current past the City, says—' At this stage, however, I am more in a position to urge what not to do with the refuse than what to do with it, which, after the expenditure of much money and years of experiment in Britain, is just the position that scores of Municipal Corporations find themselves in with regard to their sewerage. The only general and indisputable conclusion that has been come to is not to discharge it in its unpurified state into any Harbour or River, and that is the only extent to which I would feel justified in giving page 7 a decided opinion at present with regard to our own case." He then proceeds "to throw out a few hints" as to how the proposed improvements may be made to fit into the system of sewerage disposal by a system of intermittent filtration and irrigation, which has been found most successful in the older towns of Britain; to be carried out on an area of about ten acres partially reclaimed near Anderson's Bay, and covered to a depth of three feet with sand from the Ocean Beach. For the details of this, however, we must refer the reader to his own Report.