On the Scientific Form of Harbours, as applied to the Port of Melbourne.
|M. of Cantyre,||rise||4|
|Cumberl'd Basin (B. of Fund),||rise||50|
The principal points of contrast in Ireland are Galway and Limerick on the Shannon, the latter running much further inland, page 3 with a higher tide rise by four feet; Wexford and Dublin, the latter greater rise by nine feet; Belfast 9½, Londonderry 7¾, being surpassed by Strangford (14 feet) and Carlingford (17 feet). A reference to the map will demonstrate that the shape of entrance and length of harbour increase the vertical rise of tide. The same facts are observable in England, Scotland, France, America, and throughout the world.
The proof of the advantage of a straight channel is shown by the fact that the tidal wave is deflected in its course by the south-west coast of Ireland (Cape Clear), which presses it over to the east side of St. George's Channel; and the tide rushing in the north channel is shunted off by the Mull of Cantyre, from the Frith of Clyde, into the Solway Frith and southwards. The Land's End also deflects the tidal wave from the south coast of England, and the North Foreland sends it off to the coast of France. From the Solway Frith to the Bristol Channel, and from Calais to Brest, are examples of the effects of two tides meeting; and the prominent headlands of the coast deflecting the course of the tidal wave, causing the immense volume and momentum of the wave to press on these coasts, raising the tides high according to the form of harbour, leaving the opposite sides of the channel with a comparatively small rise. If it was the fact that water would not rise above its level, and that the shape of any harbour or navigable river made no difference, the rise of tides in each port would be the same, taking low water as the accurate sea level and starting point. It is manifest that in all coasts throughout the world the tide will pass by a bad harbour entrance, and fill to overflowing one of an inviting form.
The premises are therefore impregnable that the scientific form of harbour should be a wide entrance and gradually narrowing in width and good length inland, that where the harbour is not at the entrance of a river or estuary the channel should be cut as straight as possible, so that the momentum of the tide should not be neutralised by striking on every bend of the river and increasing the mileage of the tide to travel during the limit of six hours.
Applying these two established principles to the port of Melbourne page 4 the plan of success would be either for the Yarra or New Ship Channel, if the width is to be 400 feet at, or half a mile above, Princes Bridge the entrance from the bay should be three or four times as great; this the Commissioners will perceive provides two vital elements necessary, viz., a safe and rapid outlet for floods and gain a rise of two or three feet each tide as contrasted with a channel of equal width from entrance to head, two channels will be inevitable failure. The examples given apply equally to the channel from the heads to Sandridge. The twisting and shoalings of the channel should be straightened by dredging, and controlled or directed so that the advancing flood tide from the centre of the rip to Sandridge should never lose way by striking on any projecting points. One objection is likely to be urged against a wide entrance to all harbours, viz., that at a given point of the compass the sea would roll in, and unless very deep and capacious, vessels would come into collision and get wrecked by striking the bottom in the hollow of the ground swell while at anchor; few harbours run straight in, if not, provision should be made, unless the difficulties are insurmountable, by cutting out docks on either side of the fairway. This would be in harmony with Mr Cunningham's paper* on harbours, viz., instead of making breakwaters, cut the harbour out of the solid shore. Finally, the foregoing principles are established beyond controversy that combine to make natural or artificial harbours the highest maritime and commercial successes. Two examples in Britain will suffice of success. Liverpool on the Mersey, where two tides or seas meet, estuary and river open as a fairway, the water and shipping impounded in docks on north side of river, accommodation last year, 1877 for six and a half million tons shipping, revenue £700,000. Glasgow on the Clyde, revenue last year, £208.000, and has gained over two feet of a vertical rise in tide since the bends of the river have been cut off and deepened. Two failures in New Zealand last year, viz., Kakanin and Napier, are types of harbours such as entrance to Gippsland Lakes. The page 5 Murray Mouth, Clarence River, Hokitika, Grey Buller, Wangam, Milford, &c., the two first named have works constructed with parallel walls narrowing the entrance, with the intention of deepening it by scouring on the ebb. This plan would have been correct (if they did not require any tidal water) but how few rivers or harbours in the world are independent of the tidal flood, only those of the type of the Mississippi and harbours that are naturally deep at low water. The result of Napier is that the works are a failure (£60,000) and the harbour worse than when in a state of nature. Between the walls it is a little deeper, but has increased the extent of shallow water further from the shore, consequently rendering more certain the destruction of any vessel grounding on the shoal.
An impossibility has been attempted and failure the result; there was both debris within and without the harbours to be removed, and a supposed cheap plan, false in principle, has ended in losing a large sum. The entrance should have been widened and the bar and inner harbour dredged out and removed 10 or 20 miles off and deposited in "many fathoms" deep, where the action of the ocean would never disturb it; they have barred the door on the tidal waters entering, and yet had no permanent supply of any moment from within to equal it. No marine engineer in works such as the foregoing and modifications of the same, can afford now (after four years' test) to ignore the hopper dredger without risk of failure. Had the money wasted on these works been invested in two or three hopper dredgers the work would have been quickly executed, a wide and permanently deep channel gained, their rise of tide increased, and the plant afterwards sold for nearly its original cost.page 6
|Bay of Fundy,||50||feet.|
|Halifax (close to it),||6||feet.|
|Persian Gulf,||5 to 7||feet.|
|Red Sea,||7 to 9||feet.|
|Mediterranean,||3 to 0||feet.|
At the discussion on this paper on the 17th December, 1878,
The President (Mr Mansel) regretted that the paper had not been accompanied by a drawing of the locality, and in part to supply the deficiency, made a rough diagram of Port Philip on the black board, stating that from Port Philip Heads (the narrow entrance to the wide basin of Port Philip) to Melbourne, the distance would be about 35 miles; and that a strong tidal current of about 7 or 8 miles ran through this narrow outer entrance. Mr Jenkins had begun his paper by an observation, "The generally received statement that water never rises above its level, is proved to be entirely devoid of truth." Now, he did not think there was any necessity for that remark, inasmuch as no competent person would apply the hydrostatic proposition of water never rising above its level, to the case of a checked current; any hydraulic ram would at once show the absurdity of that doctrine. In the hydraulic question of water flowing into a gradually contracted basin, the fact that its surface is elevated at the contracted end, is as consistent with experience, and as much a matter of necessity, as the hydrostatic principle that water, wholly at rest, never rises above the general level. When currents, however, exist in any portion of a continuous fluid, these page 7 may give rise to differences of level in other portions; and it is merely a question of circumstances, whether these differences shall be greater or less. This general hydraulic principle Mr Jenkins, very properly, wishes to see applied to the formation of harbours, so as to attain the greatest possible tidal rise, with its consequent advantages of a deeper channel at high water, without dredging, and increased tidal scour to keep that channel open when formed; also, working of dry docks, &c. Mr Jenkins' remarks on this point are well worthy of attention, and also his conclusions as to the superiority of a properly formed dredged, channel, over attempts to form a harbour by flushing and tidal scour. He might mention a piece of information which he had received from a gentleman acquainted with the locality, which seemed to him to have some connection with the hydraulic principle spoken of. To the east of Port Philip, there is a long stretch of bay popularly known as the "Ninety Mile Beach." Off this the water deepens very gradually, somewhere about one fathom per mile, and being exposed to the full fetch of the Pacific at a distance from the land there is always a heavy sea running. Where a ship had to cast anchor, the sea might be quite enough for a small boat to put out, but thence to the shore the waves gradually diminished, so that one could easily step ashore out of the small boat, instead of, as might have been expected, a heavy surge. It would seem that the power involved in the form and movement of waves, running sometimes 30 to 35 feet high, from the form of the bottom and contour of the bay, is in the first place accounted for in an elevation of level near the beach, and, finally, in the generation of outward or lateral currents. With deep water up to the shore, we should doubtless have had a tremendous surf, which no ordinary boat could pass through.
Mr Millar (the Secretary) said that in reference to tidal action he might mention that, on one occasion when crossing the English Channel they left Dover about midnight with what appeared dead low water, and when they got over to Calais, about two hours afterwards, it was high water. Coming back a day after, it appeared to be dead-low water at Calais when they sailed, but high water when page 8 they reached Dover. In both cases it was stormy with a westerly wind, the passage being about two hours. Mr Jenkins gave the rise of tide at Dover as 18¾ feet, and at Calais 19½ feet, so that the vessel must have been carried through a considerable height in the two hours time of passage. Of course there must be a great complication of tides coming down from the German Ocean and coming up from the Atlantic. He mentioned this peculiarity with the view of eliciting information on the subject from members present.
On the motion of the President a cordial vote of thanks was then awarded to Mr Jenkins for his paper.
* See "Trans. Inst. E. S.," Vol. XVIII.