Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

The Pamphlet Collection of Sir Robert Stout: Volume 26

Dredging

Dredging.

The following are some notes with reference to the cost of dredging, and the amount of work that can be done in a given time.

In 1804, six dredges on the Tyne raised 4,180,000 tons, at 1¾d. per ton. In 1866, the cost of conveying the materials about 6 miles, to deep water, was barely l*1/3d. Adding repairs, interest, and depreciation 10 per cent., equal to 1½d., we have a total of 5d. per ton, or 7½d. per cubic yard. In 1871, the cost of dredging on the Clyde, including conveying to Loch Long, a distance of 27 miles, was 3¾d. per cubic yard. Adding 10 per cent, interest and depreciation, equal to 1¾d., we have a total of 5½d. per cubic yard. The total cost of dredging and depositing on the river bank for the same year, including 3½d. for interest and depreciation as above, was 16½d., which leaves a balance of 11d. in favour of carrying the materials to deep water. On the Elbe, dredging costs a little under 4d. a cubic yard, exclusive of interest and depreciation.

page 23

In 1872, one dredge on the Clyde raised 255 cubic feet, equal to 290 tons, per hour, for five consecutive days. A dredge at Carlingford Lough, in Ireland, has raised 4660 tons in one day from an exposed bar, and in easy stuff 600 tons per hour can be dredged. The most powerful dredge in the world is one manufactured by Wingate, of Whiteinch, for the Tyne Commissioners; it can raise 1000 tons per hour from a depth of 35 feet.

The cost of an iron dredger in England ranges from £10,000 to £20,000, according to its capacity.

Mr. Balfour's idea was to dredge a channel 150 feet wide, the contents of which he calculated at 1,248,000 tons. The width of the Suez Canal is 72 feet. Now as it would only be 4½ miles long, I think a channel of this size might accommodate our traffic for some years to come. Assuming such to be the case, the amount of dredging would be about 460,000 cubic yards, or 690,000 tons. Taking the rate for dredging and depositing on land at twice that on the Clyde—say 2s. 9d. per cubic yard the total cost of the work would be £63,250. Of course, this does not include the value of plant, which would cost, according to Mr. Balfour, about £40,000. Under ordinary circumstances, by far the cheapest way of disposing of the material would be to convey it outside the Heads in Hopper barges. But if each yard can be deposited on land in a position that makes it worth 4s. or 5s., there is a large balance on the other side.

Without admitting the desirability of reclamation in any form, it is quite evident that a certain quantity must be done in the neighbourhood of the Docks and Wharves. I would therefore be inclined to apply the dredged materials towards reclaiming land in the most valuable position for business sites. The dredgings from the 72 feet channel would reclaim about 23 acres in the vicinity of the present jetties, allowing 3 acres for street and wharves. The balance, at prices now fetched by land in similar situations, would pay for the cutting of the channel. I do not wish the Committee to infer from this that the whole expenditure can be met in the same manner. Although a profit might be made by widening the channel to 150 feet, and reclaiming 20 acres more, it would not be sufficient to pay for the Docks, Wharves, and training walls that are necessary. I think, however, that I am safe in saying that the largest ship frequenting the Port can be brought up to Dunedin at a nominal expenditure.

Another argument against carrying the dredging to sea is, that it would be necessary to employ a large number of steam barges to keep the dredge working full time. An ordinary barge can be filled in an hour, and as each trip could not be done under six hours, six or seven barges would be constantly required. If the material is deposited on land, common punts, towed by little steam launches, can be employed, and by making them of moderate capacity—say to weigh not more than 20 tons—when full they can be hauled up an incline, and emptied into the ordinary earth wagons. An 8-horse engine could pull up one of these punts in a minute, and 20 punts and 3 steam launches would keep the dredge in constant work. By using lighter punts, an arrangement might be made for running them direct to the embankment, and so save one handling of the stuff.

I believe the high cost of depositing materials on the banks of the Clyde is caused by its being placed in isolated and detached situations where machinery cannot be economically employed for hauling it up. That drawback would not exist at Dunedin, as all the reclamation might be within a radius of 20 chains. One dredge of moderate capacity, with the other appliances above described, would cut a channel 72 feet wide in the bottom, and 21 feet deep at high water, up to Dunedin in 350 working days, and one of the same width, but only 18 feet deep, in 180 days.

Whatever plan of Harbour improvement is adopted, I would recommend that the dredging be first commenced. It is the key to the whole scheme, and the work can be arranged so that the smallest instalment will be beneficial.

The question naturally arises, Will the sides of a channel cut in this manner, and left without protection, not fall in at once? I think not. So far as I am aware, the bottom of the Harbour is composed of tenacious silt and clay, capable of resisting the action of water to a considerable extent, and the sides can be easily sloped off to any inclination required. Besides, as the whole of the cut will be under low water level, it cannot be much exposed to the influence of the waves.