PORT EXTENSIONS AT LYTTELTON

Press, Volume LXXXIX, Issue 27165, 8 October 1953, Page 13

PORT EXTENSIONS AT LYTTELTON

Interim Survey Of Investigations REPORT LIKELY IN SIX MONTHS A. final report on investigations about the enlargement of harbour accommodation and the reconstruction of the port is expected to be placed before the LytteltonHarbour Board within a year, probably at the end of about six months. This was reported to a meeting of the board yesterday by the engineer -in-chief (Mr J. A. Cashin), who tabled an interim survey of nearly 6000 words on the progress of investigations made up to date.

Mr Cashin’s report had been marked on the agenda by the chairman (Mr J. Halligan) to be taken in committee. This procedure was opposed by Mr W. P. Glue, on whose motion the board decided to allow the report and discussion on it to be made public.

Having in mind that the existing harbour was constructed between 1862 and 1876 to accommodate and protect the small sailing vesels of that day, few of which exceeded 1000 tons register, it appeared to me that to readopt the lay-out of almost 100 years ago might not necessarily be the best and cheapest for modern steam and motor-vessels,” said Mr Cashin. “I should not like these remarks to be construed as criticism of the lay-out referred to; it is of the utmost importance that works of this magnitude provide for the safety of the vessels using them, and as the 1862 lay-out is found to be effective even today, safety would be assured by adopting a similar one for future extensions.

‘The development of the technique of investigation by hydraulic models, however, has made available a method which enables proposals to be studied, modified if necessary and adopted, which without such investigations might be considered bold, even to the point of risk,” Mr Cashin said. “Thus very considerable savings in materials and labour and consequently in cost can often be effected by studying all proposed designs by the most up-to-date methods available.

“For these reasons, an approach was made to Sir Claude Inglis, Director of Hydraulics Research, Department of Scientific and Industrial Research, England, with a view to having investigations made upon the proposed extensions. After some correspondence about the nature of the problems, an agreement was signed in August, 1951.”

Two Models Constructed Mr Cashin said that two models had been constructed—one 80ft long by 80ft wide, covering an area of the harbour between Church Bay on the west and Gollans Bay on the east, for wave studies; and another 80ft long by 40ft wide, covering the whole of the ,port from beyond the heads to the upper bays, for studying sieches (ranging), siltation, tidal flow, and currents. “It is necessary that the greatest care be exercised in collecting data for the construction of a model,” said Mr Cashin. “Similarity with the prototype must be achieved, not only in outline and depths, but also in the tidal form and wave phenomena, including heights, periods, wave-lengths, velocities and directions. In a word, models must demonstrate that they can reproduce cause and effect from the past before they can be relied upon to forecast the future.” It has been necessary first to attempt to find the reasons for the fact that deposit occurred only where dredging was carried out, he said. The nature of the harbour bed was peculiar and perhaps unique. Many sea inlets had a maze of mudbanks and unstable submerged channels which made access to the port difficult and sometimes hazardous. At Lyttelton, however, there were no such obstacles to navigation, the bottom being almost flat from edge to edge. Longitudinally it sloped toward the sea with a small uniform slope of about one in a thousand, or one-sixteenth of an inqh in five feet. It was unusual for a harbour, five miles from the sea in a very sheltered inlet, to require breakwaters at all; and they were necessary because of the .effects of the extrmely flat bed.

“Commonly a sea inlet is deeper along the middle, shallowing to the sides, often with a fringe of beaches,” said Mr Cashin. “Waves running in from the sea have their flanks refracted, or turned so that they break upon the beaches, thus having their energy continuously dissipated as they progress.

“Waves Almost Unchecked” “In Port Lyttelton, however, as there is no such shallowing from middle to sides, the waves roll up almost unchecked until they reach Purau Bay, where some part Of the southern flank is refracted to run up the bay. Nevertheless, waves may still have a height of 10 feet at the eastern breakwater. “Lyttelton is fortunate in being a sheltered inlet, and this has been appreciated from the earliest times by whalers and shipmasters generally, because of its shelter from south-west and south-east winds,” Mr Cashin said. “However, a rather surprising sea is raised by winds blowing over Chatteris Bay,' in which direction the ‘fetch’ is greatest. ‘Fetch’ is the term used for the free distance over which a wind can blow to generate waves, and it has been established that the height of the wave can be limited by the ‘fetch.’ “A considerable amount of investigation in this field has been carried out within the 'last few years by the United States Beach Erosion Board, and if we use their latest data, the possible height of wave reaching the entrance to the present moles is about 4ft from trough to crest, with a wavelength from crest to crest of about 45ft. Our own observations over recent years have not so far recorded waves as high as this. “The present inner harbour, built for the tiny vessels of a century ago, had perforce to provide protection in the form of the eastern mole, but a 4ft broadside wave will have little effect upon modern vessels. ‘Fetches’ at a number of New Zealand ports exceed the maximum at Lyttelton, and inquiries made at Auckland, Wellington, Napier, and New Plymouth show that waves of at least this height cause no difficulties,” Mr Cashin said. Discussion by Members

In reply to a question by Mr F. W. Freeman, Mr Cashin said the final report was expected to be ready in about six months’ time. “There is every indication that the scheme will be very satisfactory,” he said. “In a year from now it should be right, then?” asked Mr Freeman. Mr Cashin: Yes. “This will give an indication to the public of the care the board is taking before spending a large amount of money in the enlargement of the port,” said Mr G. Manning. “This board is not going to spend millions of pounds until it is sure of its ground and is satisfied that the conditions are worth-

while.” “I agree,” said Mr E. C. Bathurst. “It is showing us that full investigations from an engineering point of view on reconstructing the harbour are being made. When the engineering aspects are concluded, the board then can consider the financial aspect.” Mr Glue congratulated the engineer on the report. “It looks as if the waves act like a grader on the bottom of the harbour,” he said. “They fill in the hollows and remove the bumps. It does not look as if we can dredge the harbour much more.” “I remember engineers telling us we would be wise to spend £ 12,000 on paper before beginning work,” said Mr W. S. Mac Gibbon. “They were very wise; and we were wise to take thfeir advice.” “I think the whole report shows what'l said before,” said Mr P. Mowat. “We are not in a position to do anything at the moment. We cannot say what it is going to cost. We must wait another six months for the result of tests before we decide anything. I am very pleased that this matter has been taken in open board—the public will now know where we stand.”