Reclamation.

Any scheme for the improvement of Dunedin Harbour must necessarily embrace the reclamation of land; and as this has an important bearing on the future, not only of the Harbour but of the Port generally, I shall first direct your attention to it.

In all bar-harbours like this there are two antagonistic forces at work. The sea outside is continually closing the entrance by heaping up sand, mud, and shingle, and the rivers or tidal currents from within are as constantly removing the obstructions. When the opposing forces are equal, the width and direction of the channel and the depth of water on the bar are always the same; but should anything occur to disturb the equilibrium, a change immediately takes place. There are no large rivers flowing into Otago Harbour, consequently the channel at the Heads is kept open solely by the tidal currents. Therefore, if we wish to preserve intact the capabilities of the Port, nothing must be done to decrease the force of the tide, and every cubic yard of solid materials deposited between high and low water level is so much towards doing so.

On referring to the Harbour Master's Reports of 1864 and previous years, I find that it was then considered necessary to drag a kind of harrow over the bar, to accelerate its removal by the tide. This proves that the forces engaged in obstructing the channel are naturally very little inferior to those that act in the other direction. For the last few years the channel at the Heads has been deeper than formerly, a result which, I understand, is attributed to the tidal-wave of 1868. Such a formidable scouring agent cannot often be reckoned upon, consequently we must direct our attention to the preservation of the more natural though less powerful one of the daily tides.

Every acre of land reclaimed between high and low water level displaces upwards of 7000 tons of water. From a rough calculation, I find that about 100 acres, averaging about three-fourths of this depth, have already been reclaimed in the Harbour. As page 20 the tide passes over the bar four times in 24 hours, we have thus a daily loss of the scouring power of 2,100,000 tons of water.

Besides the injury done to the Harbour by artificial means, there is another element at work destroying the equilibrium of forces at the Heads—i.e., the silting up caused by alluvial deposits. Before the settlement of the Province, and while the site occupied by Dunedin was all forest and swamp, the quantity of sedimentary matter brought down was comparatively small; but now, when the land is broken up and cultivated, and the heavy traffic is continually grinding the streets into dust and mud, every shower of rain carries tons into the Harbour.

Assuming that a deposit of one foot has taken place on all the tidal banks down to the "White Stone," we have a further displacement of 2,900,000 tons, making a total daily loss of the scouring power of five million tons. I am not aware of any rule by which this can be reduced to width and depth of channel, but when we consider that water flowing at from two to three miles an hour—the speed of the tides near the Heads—will sweep away loose stones 3 inches diameter, it is quite obvious that, without counteracting agencies, there must be a sensible diminution of scouring power.

Against this view of the question, it is urged that as the area reclaimed bears so small a proportion (say 1-500th) to the total area of the Harbour, the effect of the reclamation is practically nil, the channel being only reduced by a five-hundredth part. If the time occupied by the ebb and flow of the tide was shortened in the same proportion, the argument would hold good; but as the greater quantity of water had to flow over the bar in exactly the same time as the lesser, it is quite evident that the current was swifter, or the channel larger than at present. The injury is therefore of a twofold character—first, the scour of the water excluded is entirely lost; and second, the action of what remains is greatly diminished.

I think this will show that whatever relation exists between the tidal area of a harbour like this and the capacity of its entrance, it certainly is not the law of direct proportion; and in calculating on the diminution of scour, we must allow for the greatest possible amount.

Mr. Balfour entered fully into this question in the Report of the Port Chalmers Extension Commission (see "Votes and Proceedings of Provincial Council, Session XIX."); but although he clearly pointed out the pernicious tendency of indiscriminate reclamation throughout the Harbour, his advice seems hitherto to have been disregarded.

There is, however, one part of his scheme with which I disagree. He relies on the construction of training walls or groynes at the Heads, for counteracting the evil. Any works in this situation would be very costly and difficult to construct, and expensive to maintain, while their efficacy is doubtful. On the principle of prevention being better than cure, I would be inclined to lay down as an axiom, that no reclamation whatever should be made without a corresponding amount of excavation in the tidal area.