PORT OF OTAGO

Otago Daily Times, Issue 27042, 29 March 1949, Page 6

PORT OF OTAGO

Sir,—Mr Thompson seems definitely to neglect the Lower Harbour to the advantage of Victoria channel. Will he inform us how many times during the last 20 years, excepting' the Deborah Bay bend, that the dredge has been in the Lower Harbour? The harbour entrance cannot be considered Lower Harbour, as such dredging has been imperative through engineers’ tinkering policy of an end to a means for the time being of keeping down expenses Forget the Lower Harbour, blow away the islands, widen and deepen Victoria channel irrespective of cost, is Mr Thompson's idea of Otago's progress. He does not mention the antiquated and costly methods of dredging and carrying the spoil to sea, but has been an advocate of such methods because Mr Wilkie had it in his report. I definitely say again, even if the islands are blown away and the Victoira channel widened and deepened the Ship Owners’ Committee will never be induced to alter its opinion of a safe working length for deep-draught oversea steamers, which are different from light draft oil tankers. Will Mr Thompson explain why, on Friday, the Scottish Prince, drawing 19 feet aft, when leaving Birch street wharf (31 feet at high water), stirred up black, smelly mud containing living vermin which the seagulls enjoyed? Yet she had 12 feet under her bottom. It is lucky the Lower Harbour up to date has naturally kept itself open, but not for much longer, and when the full length of over 20 miles has to be maintenance dredged Mr Thompson and the public will receive a headache as to costs. He talks of Mr Wilkie's monumental report, which, let me inform the public, is very contradictory. First, it says the removal of the Spit and the erection of a mole on the South Head, as suggested by Sir John Goode, Is the best permanent recommendation, but it would be costly. Secondly, meantime to save expenditure, raise present mole construction, and leave it to nature to do the rest. If this report is all Mr Thompson claims, why all the shoaling at the entrance, and so many bends in the Lower Harbour and such heavy costs of dredging and mole destruction under his stewardship? Tinkering, as Mr Thompson mentioned in previous correspondence, will in the end cost considerably more than the permanent schemes and further will not give the results

Mr Thompson speaks of the action of laymen. He will doubtless remember 20 years ago that laymen, of whom I was one, recalled the engineer from Great Britain on account of lack of vision. The raising of the groynes on the Kaik is another engineering mistake, as was proved by the late Mr Mason, who placed the groynes there and moored a Union Steam Ship Company steamer and worked her propeller to expound his theory, which was resultless. The, bend from Harington Point can only be rectified and straightened by removing the Spit, the creation of which was another engineers’ mistake. Such removal will alter and straighten the whole channel and create a wider scour of open water on the narrow bar entrance. Of course, with south head mole construction, otherwise as at present, more sand will come in on the bar on the flood tide, which to-day has a greater scour effect than ebb tide, hence the continuous future trouble at the entrance necessitating dredging and shifting time to time of leading lights.

Economy for the time being is not a progressive policy and can be overcome to the satisfaction of the heads as the constitution of the board is sometimes altered by the election and new members are not conversant with past policy manoeuvres. If the Upper Harbour had been reclaimed years ago from Lower Harbour spoil, Dunedin would have been one of the leading manufacturing and progressive cities in the Dominion with an up-to-date port at Port Chalmers in-‘ stead of in fact drifting to a sixth-rate port under Mr Thompson’s policy of having a deep Upper Harbour, no entrance and no lower navigable harbour.—l am, etc.. Thos. Anderson.

Sir,—ln a previous letter I referred to the crass stupidity of attempting with the Otakou’s makeshift suction equipment permanently to deepen beyond the scoured depth the entrance channel across the seaward sandbank at the port's entrance where the least scoured depth is now 34ft at low water. No engineer has ever recommended or would recommend such a foolish proposal, which has obviously emanated from laymen who are completely ignorant of the first principles of hydraulics, which uncompromisingly dictate that from the outset such a proposal is doomed to complete and utter failure. Consequently any attempt to do so would be a wilful and wanton waste of valuable dredging time and public money. The following extracts from the engineers’ reports amply confirm this view: A comhined report on the Mole, February 17, 1947, compiled by the Harbour Board’s consulting engineer, Mr Wilkie, resident engineer, Mr Swanson, harbour master. Captain Macdonald, and the superintendent mechanical engineer, Mr Coxheaa, made no reference whatsoever to development dredging at the port’s entrance and referred only to maintenance dredging there in connection with the inevitable westward movement of the tip of the sandbank which, under existing conditions, will once again in due course force the entrance channel to the westward.

In their very valuable report this galaxy of talent said: (a) "As an indication of the quantity of wave-borne sand which might have to be dredged, it can be stated that from March. 1942, to December 1946, the eastern bank increased by some 945,000 cubic yards of sand or at the rate of 250,000 cubic yards a year. Figures taken out for various periods since 1925 indicate that sand accumulates on this bank at the rate of between 200,000 and 300.000 cubic yards a year. Since dredge Otakou can operate only in a fairly calm sea with its present suction equipment, it is highly improbable that it could cope with this amount of sand even if the whole of its efforts were concentrated on the entrance." (b) “ Probably the westward movement of the. tip of the bank will be retarded by suction dredging, and we recommend that a new suction hose be put on order so that dredge Otakou could be utilised for this work if found necessary.” It is important to note. (1) That the recommendation to purchase the suction hose was purely precautionary; (2) that the report recommended maintenance dredging only at the port’s entrance; (3) the report clearly indicated that only intermittent dredging by the Otakpu, with its makeshift suction gear, is possible there; and (4) that,the best that can be expected is only to retard the inevitable westward movement of the tip of the sand bank. In other words, tne feeble efforts of the Otakou, with its makeshift suction gear, would be a repetition of past failures to halt the westward movement of the tip of the sandbank. Mr Wilkie in his monumental report, May 7. 1947, on the harbour entrance, said; (c) "I am confident that if the present Mole is built up for its full length without delay a channel at least 33 feet deep at low water can be maintained with the aid of a little dredging, although as the sandbank grows the navigable channel will be forced to the westward, necessitating alterations to the sailing course from time to time”; and (d) “when the sandbank has grown beyond the influence of the outgoing tide and begins to swing westward to such an extent as to impede navigation consideration should be given to the possibility of maintaining adequate depths by means of a suction dredge designed to operate a seaway rather than by an extension of the Mole.” X desire to draw pointed attention to the fact that Mr Wilkie in his report also made no recommendation, either directly or by inference, that the Otakou, with its makeshift suction gear, should undertake development dredging on the seaward sandbank at the port’s entrance. His recommendation was based on his opinion that the port’s entrance channel can be maintained at a low-water depth of 33 feet with the aid of a little maintenance dredging, and it would be most improper and damaging to his professional reputation if anyone dares to impute that Mr Wilkie meant or even inferred that a permanent increase beyond the scoured depth at the port’s entrance can be obtained by the Otakou undertaking development dredging there, which is indubitably a rank impossibility.—l am etc.. Wetherstones. Richard S. Thompson. [Because of the nature of the questions raised in this correspondence, we have permitted the disputants very considerable space. We cannot continue to do so, and must ask them to be brief, otherwise the correspondence will be summarily closed.—Ed. ODT.I