THE HARBOR WORKS.

Oamaru Mail, Volume IV, Issue 1172, 19 January 1880, Page 2

THE HARBOR WORKS.

The Engineer to the Oamaru Harbor Board attended at the meeting held today, and submitted plans for a complete harbor scheme, accompanied by the following report: — Dunedin, January 17, 1880. The Chairman, Oamaru Harbor Board.

Sir, —Herewith I have the honor to submit for the consideration of the Board a general plan of the harbor, showing the Breakwater completed, the north wall, starting from the line of Itchen-street and running eastwards for a distance of 1800 feet, the proposed reclamation from the the inner side of the foreshore to the western side of the Normanby Wharf, with three new jetties projecting into the basin, at right angles to the proposed line of reclamation. This plan embraces what I consider to be a complete harbor scheme for the port'.pf Oamaru, and the design should be carried out as follows : 1. The Breakwater should be completed to the full length as shown on plan. 2. The north wall should be constructed... 3.' The' reclamation with a rubble proiection fence should be proceeded with. 4. The "wharves or jetties and dredging approaches thereto should be gradually carried out to meet the re- - quiremeuts of the port. COMPLETION OF BREAKWATER;. .

I have so frequently expressed my opinion that the construction of the breakwater to the full extent contemplated should be considered as the most important work at present to be pushed on vigorously that i need scarcely ask the Board again to bear this in mind. You are aware that the present contract provides for an extension of 700 lineal feet, and a solid wharf inside 814 feet long. I am strongly inclined to recommend that the money proposed to be expended on this wharf should be devoted entirely to extending the breakwater. The contract value of the wharf, including cement, is in round numbers L 22,000, and this sum applied to extension of the breakwater would add another 400 fgpt to its length, thus completing it in accordance with the general plan now submitted. Last time this subject was under discussion by the Board, a doubt was raised as to the stability of the breakwater as. it advanced seawards, and became more exposed to the full force of the heavy southeast seas, without the solid wharf to back it up. There is no doubt that the wharf contracted for and intended to be carried out would add immense additional strength to the breakwater. There is, however, at' the present moment 510 lineal feet of breakwater standing alone in advance of the end of wharf, and this length has' been exposed to many heavy storms without showing the slightest signs of -weakness. Since the use of rubble pockets in the centre of the wall under low water line was abandoned, and three rows of solid concrete blocks substituted for the foundation, the Breakwater re[quired no artificial support from any I structure built inside, and I am of

opinion that the design as now being carried out is capable of resisting successfully the heaviest waves that roll into the bay at Oamaru. If the Board agrees with me in this opinion, it will be necessary to grant authority to cancel the wharf contract, as provided for by a special clause in the original contract with Messrs.' Miller 'and Smillie, and substitute a sufficient length ; of the breakwater to cover the value of the wharf deducted from the contract. ; NORTH WAIL. This wall is shown starting on a line with Itchen-street from the beach at the south side of the Lagoon, so as to allow the flood waters and sewage to be discharged from the creek through the shingle beach, and: carried to sea clear of the liarbor altogether. I have prepared four different designs for this wall, with 'the object of showing the comparative cost of walls constructed with different materials and of different cross-section specially adapted to kind of material proposed to be used. No. l is a solid concrete wall 24 feet wide, vertical on both sides, and constructed, exactly similar to the present breakwater (with the exception of being one-third less width); estimated cost L4115s 2d per lineal foot, equal to a total of L 75,165 for 1800 lineal feet. No. 2 is a wall 24 feet wide,-vertical on both sides, constructed similar to the Norraanby Wharf, with concrete facing and rubble hearting ; estimated cost L 24 per lineal foot, equal to a total of L 43,200 for 1800 lineal feet.

No. 3 is a.wall 24 feet wide on the top, vertical on the north or seaward side, and sloped at an angle of 1 to 1 on the inner or harbor side. This wall is constructed with concrete facing ori the sea side, similar to design No. 2, but backed with rubble on the inner side ; estimated cost Ll9 4s 6d per lineal foot, equal to a total of L 35,055 for 1800 lineal feet. No. 4is a wall 24 feet wide on the top sloped on both sides at angles cf 1 to 1, and is constructed entirely of rubble ; estimated cost Ll3 13s per lineal foot, equal to a total of L 24,570 for 1800 lineal feet.

The choice of designs for the north wall lies between Nos. 3 and 4, and, as you will observe, there is a difference in cost in favor of No. 4 of about L 10,500 on the total length Of the wall. . The question whether a rubble wall of this description can be constructed with safety in the position shown on plan entirely depends on the nature of the stone that can be procured from the Cape quarries. From our past experience, the quarries opened have not produced stone of such large dimensions and in sufficient quantity as would be required for the construction of the seaward side of the mole. The bulk of the outer slope would require to be protected with stones weighing from 1 to 3 tons to make it perfectly safe from easterly seas that would come round the end of'the breakwater and run along the face of the northern wall with considerable force.

A smooth vertical wall would present very little resistance to the action of the waves running along obliquely; but a sloping surface composed of rubble pitching, however, well laid, would present considerable resistance, and unless the stones were' large and carefully arranged, a certain amount of damage would occur after every heavy storm, and consequently the annual expense for repairs would probadly amount to more than the capital value of the difference in first cost between design No. 3 and 4. If it is decided to adopt and carry out design No. 4, because it is the cheapest, it will be necessary to carry on at the same time a portion, at least, of the proposed reclamation and protective walling in front of the beach. During the process of quarrying the stone that would be required to construct the mole 1800 feet long a very large proportion of clay and " smalls" would be turned over, and must be removed from the quarry face. Therefore, to dispose of this surplus material to the best advantage the reclamation I refer to should be carried out at the same time. Were it not for the difference in cost I should certainly recommend design No. 2, with .the vertical concrete face seawards for the northern wall, and when the Board has decided to proceed with the work I will suggest that alternative tenders be invited for designs Nos. 3 and 4, and the final decision arrived at when the contractor opens up the quarry on a large scale. RECLAMATION. The area of land colored yellow on plan starting from the inner side of the northern wall running in front of the beach, arid round the foreshore in front of the breakwater railway line to the western side of the Normanby Wharf, is proposed to be reclaimed with a rubble facing sloped 1 to 1 to protect it from being washed away. It is proposed to commence the reclamation from the north wall (when the latter is sufficiently advanced seawards to afford the necessary protection), and work gradually over towards the corner at the old landing place. The quantity of ground that may be reclaimed from time to time should be regulated by the commercial requirements of the port, and to a certain extent on the surplus quantity of the material turned out of the quarries in procuring larger rubble for the northern wall. Any estimates at present of the cost of this would only tend to mislead the Board, as the, area shown on the plan will not be required for many years to come. WHARVES. The construction of wharves inside the harbor when the basin is enclosed by the breakwater and northern wall will, I presume, also' be governed hy the shipping trade of the port. The wharves shown on plan'projecting into the basin from the reclamation are intended for double wharves, constructed of timber, and are estimated to cost L 29 per lineal foot.

The single wharves that can be constructed running along in front of the reclamation from the western side of the No'rmanby Wharf rqund. to the northern wall, and thence along the whole of its length on the inner side, are estimated to cost Ll2 17s" 6d per lineal foot. The length of these wharves that will require to be constructed at any particular time must of course depend upon the develop.ment of the trade at the port of Oamaru ; but it may be interesting to the Board to know that, when the shipping trade requires it, a total of 6000 lineal feet of wharfage can be provided within the Oamaru harbor when the basin is enclosed. DREDGING. I have already reported to the Board that a series of borings . have been taken under Mr. Forrester's supervision at several points, extending over a large area of the harbar, with the object of ascertaining the real nature of the bottom and the possibility or otherwise of increasing the depth of water by dredging. From the notes taken at the time and samples of the material obtained from the bore holes, I have plotted a section of each I ore, showing the strata passed through. Contrary to general expectation, the material of which the bottom is composed can easily be removed to any required depth by the application of proper dredging machinery. But the actual work of dredging deep water channels or any considerable area of the enclosed harbor can only be proceeded with as still water is obtained by the construction of the breakwater and the north wall.

My object some time ago in recommending the Board to purchase a small side dredge was to remove the sand that has accumulated in front of the Macandrew Wharf, work round the front of No. 3 and Iformanby wharves, removing the sand that lies on the natural bottom, and

thereby obtain an extra depth of water of 3 or 4 feet, which would admit of a larger class of ships to be discharged and loaded at the wharves without touching the bottom. Could this much even bo accomplished at present, it would be a step in the right direction; but the dredging machinery required to excavate approaches and permanent channels to the proposed deepwater wharves and jetties intended to be constructed inside the north wall and in front of the beach, would be on a much larger, scale and of a more expensive character. The dredging plant suitable for Oamaru would be a moderate sized Hopper dredge, or a stationary dredge with a couple of small steam Hopper barges that would convey all the stuff out to sea. It is clearly demonstrated by recent experience that it is a costly mis-take-to attempt-to reclaim land with material obtained from dredging, provided other material can be got on dry land at any reasonable distance from the area to be reclaimed. The general rule would not hold good when only a small quantity of material had to be removed, such as dredging alonside of wharves, because the plant employed should bo in proportion to the work to be done. The total cost of the works shown on the accompanying plan, and referred to in this report for completing the harbor works at Oamaru, need not be taken into consideration by the Board at the present time, inasmuch as a large proportion of the scheme cannot be carried out and will not be required for many years to come. The actual cost of every lineal foot of breakwater, protective wall, or wharf and cubic yard of dredging or reclamation, can now be reduced to a certainty in connection with the Oamaru harbor works, and the question of total cost is decided by the total quantity of work proposed to be carried out at any particular time. • I have, &c., John M'Gregor, Engineer. After a little discussion, it was resolved that the further discussion of the matter should be postponed until the next ordinary meeting, the Secretary to give special notice to members of the intention to consider the matter,- and the Engineer to be asked to be present.