Other formats

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


    mail icontwitter iconBlogspot iconrss icon

The Pamphlet Collection of Sir Robert Stout: Volume 26

Mr. Balfour's Plans

Mr. Balfour's Plans.

The Reports reprinted by your Committee contain two distinct schemes of Harbour improvement. The first was designed principally to meet the sanatory requirements of the city, but was also calculated to maintain, and possibly to assist in forming, a deep-water channel on the northern side of the Harbour. The other was intended solely for the improvement of the navigation. In considering these plans, I shall, for convenience, treat them as one scheme.

The principal feature in Mr. Balfour's works is the construction of a longitudinal training wall, commencing at the headland near Portobello, and running up the centre of the Harbour to within half-a-mile of the city. Its object was to lengthen the distance travelled by the tide, and so increase the current and scour. It would also remove the head of the navigation from Dunedin to Portobello, consequently there would be no still water in front of the city to receive and retain sewerage matter and other injurious deposits.

page 21

There can be no doubt that the principle of the scheme is a correct one. It is the application of the immense power contained in the ebb and flow of the tide to the cleansing of the city and deepening the navigable channel. There are, however, several practical objections to this plan. First, it is almost impossible to construct a wall of the section proposed by Mr. Balfour. It is only about 2 feet wide at the top, consequently it could not be constructed from the shore by laying a temporary railway on it, which is the most economical way of forming an embankment. The materials would require to be conveyed on punts, and as the greater portion of the wall runs on banks that are dry at low water, the work would be tedious and expensive in the extreme. He also assumed that the bottom is hard, and takes the average depth below high water at about 4 feet. Now it is well known that the ground on some of the banks is very soft and treacherous, consequently the average depth, including subsidence, cannot be much less than 10 feet. Taking it at this figure, and assuming the bank to be wide enough for a line of rails, I make the total cost of the central training wall to be £120,000, against Mr. Balfour's estimate of £45,000. The other portions of the estimate for the works, under the sanatory scheme, may stand, for although some items are too low, they are balanced by others that have a margin on the opposite side. Another objection to the central training wall is, that it commences at the headland near Portobello, and leaves a deep-water channel round the Quarantine Island. This would cause the wearing away of the adjacent banks, possibly to the detriment of other parts of the Harbour, and would also create a strong cross current at the entrance to the main channel. I think the better plan would be to close the channel between the Quarantine Island and the Peninsula altogether, and begin the wall at the island. By this means, all the tide that enters the Upper Harbour would be concentrated and sent into one channel, in the direction best calculated to scour it out.

In addition to the above, the central training wall is objected to on the ground that, as the current would set strongly round its upper end, it would be almost impossible to construct or maintain it. I think this objection is not a serious one. If the cross channel was not closed up till the last, there could be no scour round the end of the new work, and there would be no difficulty whatever in securing the permanent head by ordinary piling. In all probability, the wall would chiefly be made by running out materials on temporary staging from the nearest points on the Peninsula, so it might be convenient to leave the filling up of the cross channel to the last.

Mr. Balfour himself pointed out three objections to the central training wall. The principal, and in my opinion the only important one, was the severing of the Peninsula from direct communication with Port Chalmers. I think the construction of the railway has to a considerable extent removed it. The passenger trade between Dunedin and. Port Chalmers is independent of water communication, and the only steamer now running is solely for the accommodation of the Peninsula settlers; and as their principal trade is with Dunedin, no serious inconvenience would result from the closing of the cross channel.

With reference to the circular walls proposed to be constructed round the foreshore of the Upper Harbour, I think they are not essential to the scheme. I think the object sought for by them would be obtained by carrying the contralt wall half a mile further on. I may also point out that the railway across Pelichet Bay and the road across Anderson's Bay are so much towards the work. If the circular wall could be dispensed with, a saving of £11,000 would be made on Mr. Balfour's estimate.

Mr. Balfour's scheme for the improvement of the navigation, although not fully described, seems to have been simply that of dredging a channel 150 feet wide and 21 feet deep at high water, from the deep water at Burke's Brewery to Dunedin. Taking the cost at upwards of double the rates on the Clyde, he estimated the work at £100,000, including plant, and states that it would take four years to complete; he calculated that the dredged materials would be deposited in deep water outside the Heads, which, so far as its mere disposal is concerned, is undoubtedly the cheapest way of getting rid of it. As dredging is indispensable and common to all the schemes for Harbour improvement, I shall refer further to it, when considering other works.