SILTING OF HARBOUR

Timaru Herald, Volume CXXV, Issue 18498, 20 February 1930, Page 8

SILTING OF HARBOUR

“LITTORAL DRIFT.” ADDRESS BY MR E. R. ISAAC. Members of the Council of the South Canterbury Chamber of Commerce were last night addressed by Mr E. R. Isaac, a member of the Timaru Harbour Board, on “The Littoral Drift and Dredging.” The meeting was presided over by Mr F. S. Shrimpton, who briefly introduced Mr Isaac.

At the outset, Mr Isaac said that the Timaru Harbour Board had a number of problems to consider in connection with the artificial harbour—range, navigation, littoral drift, dredging, etc. In 1894 the Board were perturbed in regard to the ever-increasing accumulation of shingle, and from that year until 1899 there was a good deal of controversy, on the one side to cart and dredge the shingle, and on the other to put out further protection and continue to trap the shingle. There was not any need for him, he said, to go into detail, as everyone present knew how fortunate it was for South Canterbury that the shingle-shifters were defeated, but more fortunate still that the Board that time had men of vision who were not content with temporary measures to check the shingle, but who fought for a 3000 ft. mole to, practically speaking, finish with the shingle problem for all time, and to make Timaru Harbour the finest artificial harbour south of the line, and a first-class port of call for all steamers. This achievement would probably not have been carried out had it not been for the tenacity of the then chairman, Mr Wm. Evans. At the end of 1906 the Eastern Extension was finished, and the Board's new 1000-ton capacity bucket dredge arrived, and a start was made to dredge the inner harbour to a depth of 30 feet at low water spring tides, and also a channel from the mouth of the harbour to the end of the Eastern Extension. The work of dredging the inner harbour w T as completed in 1913. A channel from the mouth of the harbour along the concrete breakwater, and part of the way down No. 1 wharf also, was dredged, and this had been the means of trapping the silt, and very little dredging had been done inside the harbour since 1913, and the depths had been well maintained.

Mr Isaac went on to say that the channel, from the mouth of the harbour to the end of the Eastern extension, was 2600 feet, with a width of 600 feet, and a mean depth of about 6 feet. This also had been dredged to 30 feet at low water spring tides. There were very few people who seemed to understand the most important reason for this channel. It was considered that it was necessary to get deeper draft for steamers, but that was quite wrong. Just outside the entrance of the harbour was the 22 feet contour, and at the esd of the channel was the 25ft contour. Taking a line running north from the end of the Eastern Extension was the 26 feet contour, and it was the latter which governed the draft that vessels might be loaded down to when leaving the port, for when passing over this line they were exposed to the sea. At high water on ! this line there was a depth of 33 feet, so a vessel leaving port drawing 25 feet, | the maximum draft allowed at this port, would have 8 feet under her when crossing this line, which was quite sufficient, excepting when a very heavy sea was running and on these occasions j the steamer would have to remain in Port. At the 22 feet contour, just out- ! side the mouth of the harbour, at high j water there was a depth of 29 feet, but j-this was in the sheltered area, and a I vessel drawing 25 feet would have no ! difficulty in navigating this line. | Continuing, Mr Isaac said that the : Board spent £17,000 each year on ; dredging, so the question would be ask- | ed, “if the steamers can come and go on practically speaking the same draft as now, why spend this large sum of money each year?” Before answering the question, Mr Isaac said he desired to state that the channel was of considerable value to navigation; in the first place ,if the weather was fine, through having the channel, vessels could go out at half tide, and secondly iff'a cramped area, it was a great assistance having the deeper water under a vessel. But it was certain that if there were no other reasons but these two, the Board would not spend £17,000 a year on this purpose. Therefore, the answer to all this was that the chief function of the channel was, to serve as a trap for silt. The conditions in regard to the littoral drift were such that if a channel had not been dredged, at the present time one could have walked off the end of the Marine Parade on to a beach, which would extend to the end of the Eastern Extension and the harbour would have made a good football ground.

Mr Isaac next dealt with conditions at Oamaru, stating that that port was not affected by the conditions of the littoral drift, and probably would not require the service of a dredge again for the next ten years. At the mouth of the harbour the bottom was of rock and where Timaru could do away with dredging in regard to navigation if it were not for the drift, the Oamaru Board who had very little drift to contend with, in the harbour, would be only to pleased to spend an annual sum in dredging a channel to assist navigation, and would do so were it possible to dredge the rocky bottom at the entrance to the harbour.

The speaker went on to say that there appeared to be a difference of opinion in regard to the annual accumulation of silt in the channel. Taking the annual dredging report for the years 1922-1923 and to 1926-1927, a period of five years, the amount of silt dredged was 1,200,000 tons which did not include new ground or work done in the inner harbour. This averaged out at 240,000 tons a year. Each foot over all the channel represented approximately 60,000 tons of silt, so taking the five years mentioned, it worked out at an annual silting of the i channel of four feet. In 1926 the dredge was laid up, and was out of commission for three and a half years, no dredging being done excepting for two months in 1919, when the Lyttelton dredge was chartered. In 1921-1922 she was again laid up for eighteen months, which in his opinion was a very serious mistake, and on a number of occasions at the Board meetings he had strongly protested. In a little over a year the channel was full, and the chief essential in the Board spending £17,000 a year in dredging a channel trap silt, was lost. Silt always went to shelter, and when the channel was full, it passed over and settled under the shelter of the Eastern Extension. The accumulation of silt there now was about 1,000,000 tons. There seemed to be some difference of opinion in regard to the quantity that came through and over the extension, and he noted that the engineers on one of the Royal Commissions considered it to be one fourth. In his opinion this was far too high. The Eastern Extension was 150 feet at the base, and built up to a height of 38 feet, the depth on the south being 25 feet at low water spring tides, which had been well maintained, and the bottom well swept by the action of the sea. That being so, there was no accumulation of silt. It had been contended that this silting had an equal yearly average since the extension was built, but it could

be proved that that was not so, tor about twelve years after the extension had been finished, a man was reported missing and the harbourmaster and his staff herring-boned all along the Inside for his body, and the depth of 25 feet had been well maintained. In 1925, nine years after the continuous laying up of the dredge, in one part extending for over 100 feet from the extension, the depth was discovered to be only three feet, and then sloping right down to the channel 1000 feet away. This silting up did not at present represent anything serious to the efficiency, excepting possibly in regard to range. Of course, if this should be required at any time for shipping, it would cost the Board about £40.000 to have it dredged. “To-night I am not dealing with our most important problem, the range,'* said Mr Isaac, “but in my opinion this silting has increased the range and if so the neglect in not keeping the dredge going has been serious.” He went on to say that before the accumulation took place, the range coming aerpss the bay was able to spend itself under the extension, but now it was shaped like a saucer, and came back meeting the incoming range in the channel, and the channel lent itself as a lead and the range then travelled through the mouth and into the harbour. The littoral drift came chiefly from the Waitaki river, comprising shingle, mud, sand etc. The light silt and mud went out to sea, but the shingle and heavier material was washed up the coast by the sea action, and disintegration went on the whole time and produced further fine silt ahd sand. On the coast of New Zealand

the current set was always from South jto North, which often reached two knots an hour. There seems to be some difference of opinion in regard to the amount of the littoral drift. In 1899 Mr Maxwell, consulting engineer to the Board, estimated it at 400,000 cubic yards, which was equal to 550,000 tons. The estimated quantity of shingle was about 100.000 tons, leaving a balance of 450,000 tons of silt and sand, so if the speaker’s figures of 240,000 tons accumulation in the channel each year could be accepted, and Mr F. W. Clark’s (resident engineer to the Board), estimate of 135,000 yearly sildng in the Bay, and say 60.000 elsewhere, it would seem that Mr Maxwell’s estimate was fairly correct. He felt sure those present would understand when ‘ they knew that each year 450,000 tons of silt and sand passed between the end of the Eastern Extension and the dashing rocks, that the littoral drift was a big problem. TJie mountains at the back of the Waitaki River consisted to some extent of clay slates, sand and mud stones, find when finely ground up and then aKowed to settle, produced a most tenacious clay. When the dredge was working in the Bay about three years ago, the engineer of the dredge informed him that the bottom was of a most cementftious nature, and was very hard on the wear and tear of the dredge. Dealing with the process of silting in the Bay, Mr Isaac said that the silt and sand were not affected by sea action, but by the currents, and after a long spell of fine weather , it would be noticed that the sand piled up on the beach, and after reaching a level, settled further out in the Bay. As stated before, the deposit was of a very cementitious nature, and what settled out in the Bay remained. The first strong south-easterly that sets in, it would be noticed that first small gutters appeared running up through the accumulation of sand on the beach, and these would get larger until the whole of the top of the beach was washed away. The action of the sea and current was round the Bay from the Dashing Rocks, and beach accumulation and other silt in suspension was J all carried towards the Marine Parade i and to the channel, and it was after these south-easters that the shoaling in parts of the channel became very ! pronounced. It had been suggested that a suction dredge of the type used i at Lyttelton would be more suitable for I work here, but such a type of dredge would be of very little use here. In 1919 the Board chartered the Lyttelton dredge for two months, and she was credited with shifting 3400 tons per day out of the channel, based on 600 ! tons being solid matter in each 1000 ton load, but samples taken from her hopper did not bear this out. The Board's own dredge had not had any trouble in lifting 2000 tons of solid matter a day, and it was very much doubted if the Lyttelton dredge amount were much more. Further, the cost of running their dredge was very much higher than the one in Timaru. A suction dredge with a cutter might meet the requirements here, but it would cost £BOOO to £IO,OOO more a year than the cost at present, and a suction dredge without a cutter would not lift eight hundredweight a day in the Bay. To those who might want some further proof, Oamaru some years ago chartered the Lyttelton dredge, but since then had three times chartered the Timaru dredge. At Dunedin, where a large proportion of the deposit was sand, they already had a large bucket dredge, and their 2000 ton capacity dredge which had just arrived from London being a bucket dredge. Mr Isaac, in conclusion, said that he had made a suggestion at the Board I table abqut three years ago that they should dredge a channel in the Bay, and the resident engineer thought it might do some good. The Board then instructed that it should be carried out, and he was pleased to note that in Mr Clark’s last report on silting in the ; Bay, he suggested that this channel of 1200 feet long 160 feet wide, and 300 J feet from the Marine Parade, should be dredged out again, and the width increased another 160 feet towards the Marine Parade. He had in mind when he suggested this channel that the greater depth would drag the accumulation from the Marine Parade, and while it was empty this proved to be the case, and it was even noticeable as high as the old bathing shed. (Applause.) Mr W. O. Irwin said that it was the opinion of a large number of people that the silt which* was dumped outside the harbour came back again.

Mr Isaac said that he had asked the same question himself many times. He could not answer it, although he had been assured that it did not come back. In reply to Mr W. I. Tait. Mr Isaac said that the last return showed that the sand in the Bay had gone out 44 feet in three years. Mr Tait: "And is still going'out.”

The chairman: “In time we wdll be able to extend the chain of lights to Patiti Point and show posterity where we once had a harbour.” Mr G. D. Virtue asked if the silt which was dredged could be utilised in some protective scheme.

Mr Isaac said that he did not know of any use that it could be put to. Mr Irwin, in moving a hearty vote of thanks to Mr Isaac for his address* said that the subject was a very debatable one, but Mr Isaac had dealt with it very ably. It behoved everyone in South Canterbury to take an interest in harbour matters, and addresses such as that given by Mr Isaac did an immense amount of good. The vote of thanks was carried unanimously.