LOWER HARBOUR.

Otago Witness, Issue 3085, 30 April 1913, Page 34

LOWER HARBOUR.

ENGINEER’S REPORT ON IMPROVEMENTS. The engineer (Mr J. Blair Mason) submitted the following report to last week’s meeting of the Harbour Board : —ln accordance with the resolution of the Board of September 27th, 1912, that a report should be furnished with respect to the harbour being utilised as a sub-naval base, ais suggested in the motion of Mr Scollay, I have now the honour to submit a plan, showing lines of improvement in the Lower Harbour which would, when carried out, give facilities for the docking and berthing of vessels up to 1000 feet in length, and render the port adequate for the reception and refitting of any class of naval or mercantile vessel. The requirements to this end would include adequate width and depth of channel from the entrance to the berthage, a sufficiency of water at the berthage to allow free movement of vessels at all states of the tide for vessels up to 1000 feet long, and provision for berthage and dockng such vessels. Width and Depth of Channel. —It is satisfactory to be able to report that the main channel from the entrance to Deborah Bay has now a minimum depth of 31 feet at low water, with a navigable width of 700 feet at the bend south of Harrington Point. The depth in the entrance channel north of Taiarca Head is 37 feet at low water, and the narrowest part off the mole light is 700 feet wide with a depth of 50 feet. From the bend .at Harrington Point to Pulling Point the channel averages a depth of 40 feet with a general width of 1200 feet. Thence to Deborah Bay, where the water spreads out fan shape, the depth decreases gradually to 24 feet low water, deepening again until opposite Port Chalmers wharves, when the depth is 40 feet. With respect to the maintenance of the channel, one has no reason to doubt but that, provided the tidal capacity of the harbour is maintained, channel depths and widths will, with 2 )el 'h a P s periodic fluctuations, be maintained without resort to artificial deepening by dredging. There is evidence that the limit of improvement at the bend, due to the construction of the training wall, has not yet been reached; the channel there is still increasing in width and depth. It is anticipated, however, that to maintain the full depth of 30ft low water it will be necessary from time to time to dredge the eastern end of the cross channel, where a tendency to shoal exists; but such, up to the present, has not been extensive or costly. It is estimated that the expenditure of £IOOO yearly will meet the cost of maintenance dredging in the channel from the sea to Deborah Bay. Training Walls, Deepening Channel (Deborah Bay to Port Chalmers), and Provision of Swinging Basins, etc.—The work necessary to deepen and widen the channel from Deborah Bay to Port Chalmers and provide the works necessary for the maintenance of the same as shown on the plan would comprise the construction of a rubble wall extending 1600 ft in a curved direction from Acheron Head towards Rocky Point, a similar wall 2400 ft long along the edge of the bank opposite Koputai Bay, the removal of the sandbank opposite Deborah Bay to a depth of 20ft low water, also the sandbank in Deborah Bay and the deepening of the bay itself to an average depth of 15ft low water. It would also comprise the deepening of the main channel opposite Deborah Bay to 31ft low water, with a width of IOOOIt, and the deepening to 36ft low water of an area of 98 acres in the vicinity of the proposed dock and wharves as shown on the plan. Reclamation, Wharves, and Docks.—lt is essential for the requirements of a refitting port and for shipping industries that areas of flat land along the water front be provided, and as there are no such lands at Port Chalmers it becomes necessary to consider the space that can be furnished by reclamation along the foreshore, and how far reclamation can be carried without prejudicially affecting the tidal scour. After careful analysis I have concluded that the carrying out of the removal of banks and dredging at Deborah Bay and vicinity will balance the loss of tidal capacity brought about by the reclamation of the area of 41 acres at Carey’s Bay as shown on plan. This area is admirably situated with respect to the main channel, and would make an excellent dock and slip site! The frontage will afford 2400 feet of deep water quayage, which from its position would not be liable to siltation. The reclamation of the area can be so arranged as to leave water spaces for a dock capable of taking vessels up to 1000 feet in length, am] a slipway for vessels up to 700 feet. The portions shown in yellow ’on the harbour print at the entrance to Koputai Bay might be reserved for storage and supply sites, while those along the north-west foreshore of Carey’s Bay could be availed of for workshops and factories. For the dredging operations entailed in the scheme it would be necessary to provide dredging plant at an estimated cost of £40,000, but as such plant would subsequently be useful, and ultimately required in other works in the upper harbour, it is calculated that one-fourth of the cost (viz., £10,000) need be charged to it. With respect to maintain - ing the depths of the portions dredged, it is anticipated that the works shown will ensure a self-cleansing velocity in the main channel, while the removal of 20 feet of the bank in the east side of the bend at Deborah Bay will furnish a settling ground for any material derived from the channel. Those portions coloured black in the plan may be expected to shoal slowly,

but being outside the slip channel proper it can be economically and quickly deepened without dredging operations being interrupted by the general traffic. The estimated cost for such dredging, taken as a yearly charge, is £ISOO per annum. With respect to the proposal to reclaim 150 acres in the half-tide bank, I am of opinion that if the necessity arose for the reclamation of a larger area than embraced in the scheme outlined it would be better to reclaim the strip of 44 acres coloured brown on plan, being the northern foreshore of Deborah Bay. The water front of this area would extend 30G0ft and carry from 30ft to 15ft alongside without being subject to undue siltation, which would be the case at the former site. The estimated cost of walling and reclaiming 150 acres of the half-tide bank is £38,000, while that of the area in Deborah Bav as described is estimated at £28,000. Estimated Cost. —The following statement gives the estimated cost of the various works .and operations as set forth: Training wall, Acheron Head, £4600; training wall along bank opposite Koputai Bay, £5300; walling and reclamation of 41 acres for dockyards, shipyards, berthing, and storage sites, £64,000; dredging swinging and berthing area to 36ft low water (98 acres), £50,500; dredging channel Deborah Bav to swinging area to 31ft and 1000 ft wide (108 acres), £22,800; dredging approach to shipyard sites, Deborah Bay, to 16ft (21 acres), £5200; dredging portion of Deborah Bay reserved for defence purposes (74 acres), £15,700; dredging bank on east side of main channel opposite Deborah Bay to 20ft low water (86 acres), £34,800; one-fourth capital cost of dredging plant, £IO,OOO; — total estimated expenditure, £192,900; if 44 acres additional are reclaimed in Deborah Bay, £28,000, which gives a total cost of £220,900. Total annual cost of maintenance dredging, £2500. It is estimated that the works in the scheme as set forth can be completed in six years. Annual Charges of Item. —The cost of maintenance dredging can be provided out of ordinary revenue. The annual interest charges of 5 per cent, on .a total estimated expenditure of £192,900 would amount to £9645, a sum which the board with its existing powers and in its present position can only raise by an increase in tire dues on goods and shipping. But although the board may not deem it expedient to early proceed with works of such magnitude, yet I am strongly of opinion that a scheme of harbour improvement at Port Chalmers on the lines herein suggested or analogous thereto should be adopted by the board. Then all works, extensions of wharves, reclamations, and excavations. which may be undertaken from time to time as circumstances may demand will bo on definitely decided lines, and thus largely prevent the construction of haphazard and temporary works, or works which, with time, may require to be undone. Mr Scollay said this report embodied a practical seneme for making Otago Harbour absolutely the leading port of the dominion. The point he wished to emphasise was this scheme, if carried out, would make Otago not only accessible at all states of the tide to vessels up to 1000 ft in length, but it would also provide adenuate deep-water berthage and first-class docking facilities for such vessels. A vessel with a draught of 30ft could enter or leave the harbour at low water, and a vessel with a draught of 36ft could enter the harbour at high water, and could move about freely in the vicinity of berthage and dock entrance at low water. Bites would be available adjacent to the dock and slip at Carey’s Bay for extensive a;.a up-to-date repair shops of all classes, and shipyard sites of 44 acres in extent would be available at Deborah Bay, with a 3000 it water frontage, ranging ui depth from 30ft to 15ft. He moved that the report be received, and referred to the favourable consideration of the incoming board as a uractical harbour improvement scheme which will, when completed, not mly render Otago Harbour fully accessible to the largest vessels for many years io corvr, but easily make it the best harbour in 1 . e dominion. . Mr Dickson seconded the motion, which was carried without discussion.