The Pamphlet Collection of Sir Robert Stout: Volume 26
Mr. J. T. Thomson's Report
Mr. J. T. Thomson's Report.
15th October, 1873.
Messrs. Tewsley and Davie.
Gentlemen—I beg to acknowledge the receipt of your letter dated 13th inst., requesting me to furnish you with an outline plan, such as I would consider fit for the improvement of Dunedin Upper Harbour; and, in reply, I beg to forward to you a tracing herewith. On considering the subject at this date, I find that I had reported on the Harbour, in letters dated 27th December, 1859, and 31st March, 1868, and in both periods I advised adherence to the same principle, though in the upper section fronting the town, as reclamation has since proceeded across my intended dock and quay, the plan must now be so far modified. In the tracing herewith, therefore, I have drawn out a new scheme suited to the present circumstances of town extension. I have also shown the original scheme in faint blue lines, from whence the artificial channel or canal followed the same direction down to the Fork as now delineated. It would appear also that the reclamation recommended by me did not exceed 208 acres, the large reclamation of 402 acres having been laid out at the instance of the late Captain Cargill, who was at that time Superintendent of the Province. The black lines show the reclamation proposed by Mr. Swyer, amounting to 402 acres, and the plans of the late Mr. Balfour being in your hands, I need say nothing further regarding them.
The principles that I have always recommended in dealing with the Harbour are to interfere with it as little as possible, for to do this is to create other tidal actions that we may not anticipate, and which may prove detrimental, and to go on with as little reclamation as possible; further, that there should be no obstructions placed against the easy influx and reflux of the tide. There are at present two channels used by vessels coming up to Dunedin—viz., the North and the South—the latter being the deeper and wider, and which at the Fork is approached by what is called the cross channel. My advice has been, and is now, to retain the deeper and wider channel intact, and for its present use—viz., the coasting trade, and with which the improvement of the north channel will in no way interfere. Vested interests in property, and in the villages on the Peninsula, which will have large future expansion, will also thus be respected.
So much am I against any real or apparent obstruction to the tidal currents, that I would restrict the height of the training walls to that only which may be necessary to guard the artificial channel from having the dug-out mud coming back upon it.
Commencing at Dunedin, there will be 3,800 feet half-tide wall, then 12,000 feet low-tide, then 2,500 feet high-tide, and 13,000 feet of three-quarters tide wall. As there are only 17 feet on the bar below Port Chalmers, this limit of depth will govern the artificial channel in the Upper Harbour. I have therefore estimated the excavation to that depth. This will allow vessels drawing 21 feet at neaps and 22 feet at springs to safely come up to Dunedin. The upper dock for this purpose would have to be excavated to a depth of 23 feet. I would suggest that Dock No. 1 would suffice for many years to come, after which Nos. 2, 3, and 4 would follow in succession as the settlement increased.page break page 13
The cost of the works will be as follows (not including Wharves):—
I may state that in my estimate I have taken Mr. Balfour's highest price for rubble—viz., that for his mud bank walls, ¾; his deep water wall being at the rate of only ½ per cubic yard. The dredging I estimate to be done with the travelling mud shoot which I designed and had in very successful operation when I was formerly in charge of the works, and which Mr. Barr, my successor, found only to cost 31-10d per cubic yard (see Report of 31st March, 1869). I would use this in all the first excavations, as to send the mud out to sea in Hopper barges, as recommended by some Engineers, would cost at least 3/ per cubic yard, that is, eight times the present estimate. Ultimately, no doubt, it would be advisable to use mud barges for the annual clearing of the channel, but at first the backing of the training walls by material would be beneficial rather than otherwise, and would in no way affect detrimentally so wide an expanse as Otago Harbour presents.
The works should commence at the head of the Harbour, proceeding downwards. By this means no difficulties would occur from having to deal with cross currents; very formidable affairs if attempted to be diverted in such a body of water as the volume of the Upper Harbour. No scheme, therefore, should be attempted that is a mere experiment, and which involves measures of which we have had no previous example.
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 that one important fact has been lost sight of by all the persons reporting on this—viz., that the Harbour being salt water, it acts as a disinfectant, and thus 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 will be an immaterial consideration. I have, therefore, shown the outlet, of the sewerage to be carried 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.
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.
Your obedient servant,
J. T. Thomson, C.E.