NEW CHANNEL

Otago Daily Times, Issue 24773, 25 November 1941, Page 8

NEW CHANNEL

OTAGO HARBbUR ENTRANCE MOLE TO BE RAISED LOAN PROPOSAL APPROVED Advice that it had approved of the raising of a loan of £24,000 for the purpose of building the Mole at the entrance of the Harbour to full height was given by the Local Government Loans Board to the Otago Harbour Board at its monthly meeting last night, and the engineer, Mr McG. Wilkie, was instructed to proceed with the work at once. Behind the decision of the board lies the story of a long fight against encroaching sand that has been going on since 1884, when, acting on the advice of Sir John Coode, a London consulting engineer, a start was made with the building of a mole to train and concentrate the tidal currents on to the sand bar in order to maintain a navigable channel. During the past few years, however, an entirely new channel, scoured through the sand bank that has gradually swung across the entrance, forcing the present sailing course further and further round towards the shore, has been opened up. ana it is to deepen this new channel that the board has raised loan money once more to continue work on the Mole. Some ships are already using the new seaway, which follows the line of the mole and ru/is straight out to sea. allowing vessels entering and leaving the port to sail through without having to turn sharply round the end of the mole. At and near the end of the mole there is a distinct line of demarcation between the fastmoving ebb stream and the practically dead waUr in the old channel to the northwards, and it has increased the difficulties of navigation, as vessels moving suddenly from still water to a swiftly-ilowing stream tended krbe., swung round in the direction of the current. •'Tnis difficulty has been experienced by masters of ships both leaving and entering the port on either the ebb or the flood tide, but with the removal of the necessity for turning round the end of the Mole, any danger of a sudden swing off the course will be obviated. Sand Encroachment

The building of the Mole between 1884 and 1888 cut a channel through the harbour bar to at least a depth of 30 feet, but it was not in the direction anticipated by Sir John Coode. This was due to the fact that, as the stonework had 'not been built up to high-water mark, the tidal currents were not deflected to the extent that would have been effected, had the original plans been carried out. The channel, however, gradually became deeper, until, in 1914, there was at least a depth of 40 feet, but at the.same time a sand bank rapidly formed parallel to and on the . seaward side of the channel. It gradually increased .in length and width, and steadily swung the channel shoreward, causing a decrease in both width and length. From 1914 onwards it was necessary to alter the line of leading beacons shorewards on several occasions, and although by 1925- the tip of the sandbank became beyond tidal influence and did not appreciably extend lengthwise, it rapidly increased in width, encroaching on the navigable channel. The then Harbour Board at once decided to raise the stonework of the Mole to high-water mark with the object of directing the'tidal currents against the encroaching sand, but it was- not until 1928 that the work had been sufficiently advanced to have any appreciable effect in altering the direction of the ebb current. Every effort was made to accelerate the concentration of the main ebb. stream on the sandbank near the mole end. and for this purpose the stonework was at first confined to a narrow core with the object of achieving the greatest possible length quickly, the intention being to widen the wall later. To some extent this had the desired effect, as the scour began to eat out a new channel, but in 1935 the work on the Mole was stopped when it had been raised to within 300 feet of the outer end. Wisdom of Board's Action It was ;only natural that this should have the effect of lessening the rate of formation of the new channel, and at that time ths 30-foot contour had not broken through the sand bank to deep water off. the seaward side. It became a race between the closing of the old channel and the sufficient deepeninp of the new one. For some time oast vessels have not been able to enter or leave the port on the marked sailing course owing to the encroachment of the sand in the channel, but have had i,o swing inshore. On the other hand, however, the new channel has attained at least a depth of 35 feet in the centre, and while such depth cannot be considered adequate for the needs of the port sufficient depth has been attained to afford proof of the wisdom of the board's action in raising the Mole. The completion of the stonework to high-water mark will, in the opinion of the engineer and the harbour master, hasten the deepening of the channel. It will take about two years to complete the work. In the meantime, wave-borne sand from the south-east is being dropped on the eastern side of the new channel and ,is rapidly building up a bank there. This will have to be guarded against, and the completion of the Mole will greatly assist in stabilising the western contour for some years. As sand accumulates, however, the bank will extend seawards until its tip reaches a point beyond the influence of the ebb tide, and when this happens it is probable that the tip will swing westward across the new channel. There is 70 feel of water hsre, however, and dredging will not probably be required for many years to come.