SHOALS IN LOWER HARBOR

Evening Star, Issue 16033, 9 February 1916, Page 3

SHOALS IN LOWER HARBOR

DREDGING TO BE DONE TO OBTAIN 26 FEET. COST, £3,750. The belated discovery that shoals in the Lower Harbor have hindered if not endangered the navigation of large vessels was the subject of a special meeting of the Otago Harbor Board last night, when reports from the harbor master and engineer were received; the former stating what was necessary to be done to enable vessels drawing 28ft to navigate the Lower Harbor at all states of the tide, the latter submitting a long report on the cost of such work, which he estimated to be £7,450. and the annual cost of maintenance £2,050. It was resolved to take the matter in committee. The report of the engineer (summarised) was as under: —Navigation at All Stales of Tide.— . As this question opens up an alteration in the policy of the board, an examination is necessary of what it involves. The channel has gradually become deeper until, in 1908, this ceased to be a tidal port for all but a few large vessels. Since that tmie the depth has further increased, and by 1915 the navigable depth at. the bend was 31ft at low water. To obtain advantage of this increase, it became necessary to consider the further deepening of Deborah Bay and the provision at Port Chalmers of deeper berthage accommodation. The Railway Department was duly approached and asked to widen the George street pier, the hoard undertaking to carry out the necessary dredging at the pier and approaches to provide for the reception of 30ft draught vessels. Dredging was at the same time recommended in the channel off Deborah Bay and at Port Chalmers, with a view ultimately to obtaining a navigable depth of 26ft. ‘low water, for°tha easy passage of vessels of 50ft draught vrhen the necessary widening of the pier was accomplished. Given a clear navigable depth of. 26ft as the minimum in the channel (at Deborah Bay), and the provision of the necessary berthage depths, a vessel of oOft draught could, under ordinary tidal conditions, use the channel for two hours before and after the time of high water. As the depth of 26ft is ample for the majority of vessels trading, it becomes necessary to ascertain whether the board will be justified in opening the port to vessels of greater draught at all stales of the ride. In order to guide the board in coining to a decision, a return was prepared of oversea vessels and cargo for the years 1914 and 1915, which showed that in 1914 92.9 per cent, of oversea carcro landed and shipped was carried hv 91 vessels, drawing 24ft and under (average draught 18ft oin), and in 1915, 92.6 percent, was carried by similar vessels (average draught 18ft 6in). The deepest draught in 1914 was outwards 2ft Sin, and in 1915 25ft 9in outwards. The deepest draught inwards in 1914 was 25ft Tin. and in 1915 25ct 3in, the capacity of the channel at high water being 30ft. Largo vessels outward bound prefer to leave ports about high water (mandatory with some companies), and the deepestdraught vessels inwards in 1914 and 1915 coulcf have used the channel and berthed at the port, save for lj hours before or after low tide. The influence of tidal conditions on vessels trading during these years has thus been small, and it is appar--ent that with the advent of vessels drawing 2Sft (which tire widening of the George street pier will shortly allow of), there would be considerable margin for these vessels on an ordinary tide. "With 26ft low water in the channel (soon to be available), the port will bo non-tidal for all vessels less than 26ft draught, while those of greater draught can work the port with ease at other times. The question of providing low-water depths for the small percentage of deep-draught vessels is au important one with all harbor authorities where expensive works and dredging with recurring maintenance charges have to be faced, and instances mar he quoted as showing that such pn-ovisioiT is not usually considered necessary. —Dredging and Maintenance. —

The widening of the George street pier on the east side will furnish a berth for vessels of 30ft draught. A little dredging in the approach to the wharf and at Deborah Bay will be all that is necessary to be done to navigate such vessels to or from the pier at high water. If it is required that the depths at the wharf approaches and in the channels shall be increased so as to give access at low water or any particular state of the tide extensive dredging operations must be undertaken. Assuming a width nf channel of 350 ft to allow of safe navigation, the estimated cost of dredging to various depths and tho estimated , annual maintenance costs would be as follows (estimates based on a rate of 6d per hopper cubic yard) Approach to pier, Koputni Bay.

L.W.O.S.T. Estimated Annual to cost. Maintenance. 26ft £2,300 £250 28ft 4,200 350 30ft 6.000 500 32ft 7,850 700 34ft 10,400 800 Channel off Deborah Bav. 26ft £550 '£4oo 28ft 2.050 500 30ft 4,550 650 32ft 7,550 950 34ft 10,750 1,300 Bend, Harrington Point. 26ft £9OO £6OO 28ft 1,200 1,200 30ft 1,500 1,500 32ft 1.700 1.700 34ft 2.000 2,000 Summation of above estimates of expenditure in Lower Harbor channel and approach to pier, Port Chalmers : e si) . • o .IT £b C-I H _ c 'Zy B S d-g, 5? Is® M -| 2 is 1-3 S C-q ° g 26ft 32ft £3,750 £1,250 28ft 34ft 7.450 2,050 30ft 36ft 12,050 2,650 32ft 38ft 17.100 3.350 34ft 40ft 25,150 4JOO These figures show- that to attain a deptli of 26ft at low water at.Koputai Bay and in the channel {3soft minimum width) about five months’ dredging would be required, costing £3,750, and about two months a year thereafter, at a cost of £1,250 for maintenance dredging. The harbor would then meet all demands of the existing trade. To attain 28ft at lowwater would require a further estimated expenditure of £3,700, while the maintenance charge would be increased to £2,050, .In my opinion the board should not seek in the meantime to go beyond the 23ft low water, as such a depth will, meet all reasonable demands of shipping, while in the case of large and deep draught vessels they will, in order to berth and depart in safety from the Port Chalmers wharves, wait for high water and slack tide, and which will give them an ample depth up to 34ft in the waterways. —Safe Width and Deptli.—

In all commercial harbors frequented by large vessels it is usual to mark the limits of the navigable waters where such are not naturally defined by buoys or beacons placed in 18ft to 20ft of water, and to rely on auxiliary marks or the knowledge of the pilots for guiding vessels in depths suited to their draught. Tire main channel, Lower Harbor, is defined as all waters 18ft and over an depth lying between the black and red beacons, its greatest width between the beacons being 1,200 ft and least width 600 ft. Whore the circumstances allow of it, the channel as made wide enough for vessels to pass each other in safety. In difficult situations, such as where dependence ie placed upon tidal scour for 'the maintenance of depths, the width may only allow one vessel to occupy the channel. The width of the old channel at the bend illustrates this point. For I,oooft of its course it maintained a width of 250 ft between the 18ft contours and a width of 150 ft between the 24ft contours. While there was room for a large jesgel to- -proceed,

not sufficient for another to pass J a.t the same time. To have widened the old channel by dredging would have interfered with natural conditions, and the tendency of the channel by siltation to revert to its former state would have existed. It is advisable, therefore, in the Lower Harbor, to make and keep the channels of such width only as will allow for the safe navigation of ordinary-sized vessels in Passing each other, and for the largest vessels, such as vessels exceeding 400 ft in length, one at a time. It is considered that a width of 350 ft. of a. depth not less than the limitng depth of the port, will amply meet the requirements for safe navigation of this port for many years to come. —The Summary.— To sum up, my recommendations would be as follows:—(a) That the channel and wharf approach at Port Chalmers b© dredged to a minimum of 28ft 1.w.0.5.t. Estimated cost, £7,450; annual maintenance dredging, £2,050. (b) That the minimum width of channel at the bottom carrying 28ft shall be 350 ft, a width than can he navigated safely, using ordinary care and attention. THE BOARD’S DECISION. The board decided:—“ That the approach and fair-way from the sea to the .Port Chalmers wharves be immediately dredged to a depth of 26ft at low water and a width of 350 ft; and that the question of dredging to a depth of 28fb at low water bo considered when- the railway wharves at Port Chalmers are reconstructed.”