THE RIVER ENTRANCE.

Gisborne Times, Volume XXVII, Issue 2630, 12 October 1909, Page 5

THE RIVER ENTRANCE.

REMEDYING THE RANGE

MR MARCH ANT’S SUGGESTIONS

A special meeting of the Gisborne Harbor Board was held yesterday aiternoon for the purpose of receiving the report of Mr F. W. Marchant, M.I. C.E., with regard to the river entrance. Mr John Townley, Chairman of the Board, presided, and other members present were : The Hon. Capt. Tucker, Messrs F. Harris, W. D. Lysnar, G. J. Parker, and J. B. Kells. Mr Marchant was also in attendance. The report, which is the second made by Mr Marchant, reads as follows: “As instructed, I have again looked into the question of providing a safer entrance to the trained channel of the Turanganui River. “In my report of July, 1906, I dealt with the physical conditions of the harbor works in relation to the movements of the sea and river, which need not now be repeated. I then suggested as a proper remedy for the defects of the situation an extension of the breakwater, constructed with papa rubble covered with concrete blocks.

“On again considering the matter and discussing it with experienced local men, I make the following proposals: — “The breakwater should be extended not less than 300 feet. The extension should not be put on in the same direction as the existing work, but should be canted seawards, so as to afford an entrance 350 feet wide under the lee of the extension before entering the trained channel. The present channel is only 250 feet wide between the walls, and this is quite insufficient for safe navigation when vessels are entering tho channel and exposed to the cross sea which usually runs here. 1 should have preferred to put on an extension of 600 feet, giving an entrance width of 450 feet, vvhich -would have made a vast improvement to the port, but understanding that the Board desires to keep the expenditure on this work down to the lowest possible amount, I recommend that, to begin with, the length of 300 feet should be constructed as a minimum that would be of any real value. It is quite unnecessary to construct the extension of anything approximating to the class of structure constituting the existing breakwater. Proof of this may be seen in the design of the groyne, which, though on the lee side of the channel, is exposed for a length of 200 feet to the unbroken force of the heaviest storm waves without any protection at all from the breakwater.

“I propose to build the extension from a timber staging, using papa rubble covered with random concrete blockwork. The concrete blocks would be 15 tons in weight, and carried up to 3ft above high water mark. This work would be of superabundant stability, and would, by reason of the random block-work method of construction, break up the waves in a manner that no other could approximate to. “Various proposals have been made from time to time for some inexpensive sheltering works on the weather side of this entrance, but I am unable to give them my support. Any timber work would soon be destroyed _ by the teredo, and when knocked to pieces by a storm some time or other, the channel would be strewn with the wreckage of it.

“I estimate the cost of the works proposed at about £36 per foot, built in an average low water depth of 19 feet. It would be necessary to obtain a good crane and some other plant and suxidries for the work, the cost of which would be about £2500. For an extension of 300 feet, the cost then would be £IO,SOO for the work and staging, plus £2500 for plant, or a grand total of £13.300.

“Judging from a perusal of the, reports made iu past years in connection with these works generally, it was thought and hoped by some that this channel would maintain its depth by natural scour, and that the inevitable shoal about the entrance would also be kept down by the same means. It is evident to me that nothing except dredging will enable a good depth of water to be maintained in it. This has no bearing on the question under consideration, except this, that I desire to anticipate the criticism that such an extension as I propose will cause a sand shoal to form under its lee. A shoal will to some extent be formed, and must be kept down by dredging, as has to be done at other places. There is, however, but a small quantity of littoral drift here, and the expense of removing this shoal will be very little. The Chairman read the report, and remarked that it was what they had expected, demonstrating as it did the fact that they would have to extend the breakwater. The report, however, did not give any particulars as to how this was to bo done. Mr Marchant: I can explain that, of course. This, after careful consideration, is the best scheme you can suggest ?” asked the Chairman. Mr Marchant: Yes. Mr Lysnar: What effect would reducing the groyne one side have? Mr Marchant: The whole difficulty is that the groyne projects past the covering breakwater. Mr Lysnar: Then the same^ object could be achieved by taking 250 feet off the groyne ? Mr Marchant: It would want 300 feet off.

In reply to further Questions from Mr Lysnar, Mr Marchant said that the cost of cutting down the groyne would be about £ISOO or possibly a little more. Mr Kells: Would you have to remove it entirely?—Yes, it would have t-o be made quite level or you would have a reef of debris that would bo very dangerous. Mr Marchant continued, in reply to Mr Lvsnar, that he would not like to say that the taking away of part of the groyne would entirely do away with any danger. The entrance had been built too narrow originally. The reduction of the groyne, however, would do away with a good deal of the “rip” that made the place so unsafe at present. If the groyne were reduced, the entrance would have to be dredged regularly. Mr Lysnar: Would this increase the drift of sand. Capt. Tucker: We hardly want to ask Mr Marchant questions that any reasonable man can answer for himself. The groyne was put there to prevent drift, so it stands to reason that if it were shortened the drift would increase. Mr Lysnar: Is there any danger of the sand creeping round the end of the groyne at present? Mr Marchant: Of course there is such a tremendous commotion with the seas rebounding off the groyne that nothing has a chance to settle. If the groyne were shortened the Board would have to do more dredging. He would not advise them, however, to out the groyne. Extension was the proper thing, but if the expense rendered this impossible, then lie would advise the other as an alternative. By Mr Harris: He looked npou tho “rip” coming up against the groyne as dangerous to life.

Mr Marchant added that for carrying out his scheme rubble could be got from Pa Hill, which would cost them 6s or. 8s a yard, and as far as concrete was concerned they could use the shingl® the Borough Council was getting. A quarry crane would bo required, as tho present one would not bo nearly power* ful enough to lift the concrete blocks. There was no doubt that the entrance should not be left as it was. It was for the Board' to say whether they would extend the breakwater or cut the groyne. The former was the better method..

Mr Lysnar suggested that Mr Marchant should be asked to prepare plans for the extension scheme arid its alter-. native. / it ¥ The Chairman remarked that the tension would be a permanent inent, whereas the other expedient would only be temporary and would.-, moreover, be retrogressive. Captain Tucker said that if it would be an}’ trouble for Mr Marchant to prepare a report on the removal of part of the groyne he would rather ho did not do so. In the speaker’s opinion the extension scheme was the only possible thing, though it would be more expensive than the alternative. The question of danger to life, however, should bo considered. If the launch overturned at any time they would not get off with.a loss much leas than 50 lives. Mr Lysnar: It may not be possible to undertake the extension. Capt. Tucker: It must be, where life is concerned ! Mr Lysnar: There is no reason why we should not have alternative plans. Eventually it was decided to instruct Mr Marchant to prepare plans and specifications of the proposed extension and a report as to the best method of the removal of 300 feet of the groyne for submission to the Marine Department.