ON THE ORIGIN AND REMOVAL OF HARBOUR BARS.

Temuka Leader, Issue 186, 15 October 1879, Page 2

ON THE ORIGIN AND REMOVAL OF HARBOUR BARS.

Bt J. Aiton, C.E.

(From “ The Saturday Advertiser.”) When we consider that natural causes tend in a measure to influence the losa--tion of townships and commercial centres on or near the banks of rivers, bays, and estuaries; and likewise that natural causes tend to originate and build up the bars and sand banks at the mouths of these inland tidal reaches, we fail to be surprised at the universel interest evoked, or at the amount of attention deyeted by engineers and others to the question of the effectual removal of these formidable obstructions to maritime navigatian. Whether viewed in the light of an jimet to the influence and action of currents causing tidal scour, or , simply as an antagonistic element thwarting the combined action of w md- wjuvcs and ground swell, an artificial ombadking must form the nucleus ..( <#■ operations to counteract mU-r-1 !a>\s; and it being shown that atiitvid-deposit rsaady takes

place where and when conflicting forces are truly balanced, and stillness and tranquility attained, it stands to reason that when this balance of power is disturbed locally, and the strength and velocity of the outflowing current prolonged seaward, the deposit of former times will gradually be shifted or eut'rely - .dispersed. In Dublin Say,-unto which the rivers Liffey and Dodder' flow; there are extensive sand banks laid bare at low water, and the channel cut b> the scour from the rivers was'" used in early times by /■ vessels frequintingv the port. ily and Great South' Wall' was erected to and,pfbtect this channel from the influence of southerly winds and the encroachment of sand. Some years later -the Clontarf shore was connected by an . embankment to shelter the harbour on the north side likewise from the combined action of wind and sand. The bar proper stretched from the north side of the Bay and across the entrance to the harbour in hook form ; while the deepest channel for navigation was round the -end of the bank, the track over the bar in a direct line seaward had only a depth of from 5 to 6 feet at low water of springtides. From the Clontarf shore towards Poolbeg the; wall was extended 2000 feet, three-forths of the length the tide overflowed at half-flood, and the remainder was carried up to the level of springtides. The result lias been that the tidal scour, -induced by the embankment, has deepenecT the channel across the bar to 16 feet at'Tow water of spring tides, and yet better results are anticipated from additional waking proposed to he put down.

It may be as well to state here that many eminent engineers have from time to time been consulted relalive to an improvement of the entrance to the port of Dublin ; amongst others. Sir Thomas Hyde Page, and Captain Bligh werccalied in, who both recommended nearly identical schemes, namely, the laying down of a tidal wall on the north side of the channel and parallel to the Great South Wall, constnictod during the eighteenth century in order the better to confine the river and tidal waters on the lower reaches within fixed limits, and

thereby induce an artificial .scour. Sir Thomas, in addition, proposed the formation of an island on the bar in order, as I suppose, the bettor to confine and agitate the water in in that locality, and prolong the velocity of the outflow beyond the bar. It has been observed that floating ob-

structions on the margin of tideways

rapidly work a local change on sand banks and the bottom ; thus large guide-buoys moored on the edge of shallows in time deepen the water there ; and old hulks laid up in the stream, and- light-ships moored in tidal currants cause a considerable deepening of the water in their respective localities. Eel alive fevan improvement of the channel by the Sputh Pass of the Fiver Mississipfeq.-it pi ay bo stated that Captain Eads (filtered into a contract with the United States Government, whereby he bound and obliged himself, in consideration of the sum ijgf 5.] million dollars to be paid and maintain a depth of 30 ffeeFof water over the bar, and of 26 feet through the bank at the head of the Pass, for a period of 20 years. The South Pass is reckoned to be 12 miles in length, with an average width of TOO feet, and 20 feet deefr; but previous to deepening the head and mouth of this Pass the only track i'o r shipping lay through the South-west Pass, where a depth of water was only acquired and maintained at from 16 to 18 feet by constant dredging. One? the required depth is secured (and it is nearly so) by operative works, vessels drawing as much as 26 feet of water will be enabled- to navigate an almost direct channel never less than 300 feet wide, from the Gulf of Mexico to New Orleans. Captain Eads began operations in the summer of 1175, and at that time the depth of water on the bar at the mouth of the South Pass, was only 7 feet, and over the hank at the head of the Pass something like 14 feet. In June last, exactly four years after he had started the works, the depth of water through the scoured channel across the barwas recorded at 28 feet, and over the shoal at the head of the Pass at 24jj feet, but such works had just.been completed there as would ensure 27 or 28 feet being obtained by the scour of the first flood in the river. The system here adopted to effect the required amount of induced tidal scour at the mouth so as to act efficiently upon the bar, consists of jetties upwards of two miles in length, and spaced about a thousand feet apart. These are composed of rafts cf willow branches, termed “ mattresses,” sunk on the liver bed by rough stones. After settlement the whole is levelled up with additional willows, sand, and gravel, and then capped over with concrete blocks formed in position, and ranging from 23 to 70 tons each, which brings the erection from 2 to 3 feet above the zero plane of average flood tide. Upon this, after the whole is consolidated and subsided, a concrete parapet is proposed to be erected.

It has recently been officially reported that these jetties, after frequent trials , by heavy river floods and severe sea storms, show no signs of weakness ; that the concrete work has every indication ot durability, is free from all appearance of fracture, and the settlement of each jetty throughout has been remarkably uniform. As an indication of the great faith that Captain Eads and his confreres entertained relative to the utility, practicability, and ultimate resuit of his operative works upon the tidal waters of the Mississippi in the South Pass, it may be stated that until ho Lad obtained a navigable channel 20 feet deep from the main river to the sea, he received no payment from the Government whatever, binder any circumstances he is ■ not entitled to a full payment of .million dollars till the contract is fulfilled, and which implicitly binds him to maintain a depth of 30 feet at the mouth, and of 26 feet at the head of the pass for 20 years. Once these depths are secured a -yearly sura of 100,000 dollars will be allowed him for maintenance, but nothing before that great desideratum is accom•.plished. In dealing with bars and shallowslocated at the mouths of tidal inlets and arms of , the sea., entirely - uninfluenced by ;river \actions, consideration of the principle to r. 1 'pled in order to obtain..a force and

outflow in that situation, is somewhat distinctly different from the modus operandi successful!y practised in river tideways. In the ease of tidal rivers the more the current is confined to, and trained into, a fixed and straight course within the lower roaches, so much the more will the strength and duration of the outflow of the head waters he feit at the the mouth during the ebb discharge. On the other hand, in estuaries of the ocean where currents are uninfluenced by any diischarge from the watershed of the mainland, and are to be treated pure and simph, the strength and duration of the discharge ever the bar will be regulated by the extent and situation of the tidal area within, and in proportion to the value of concern rated resistance brought to bear upon the water at the mouth, in order to induce a racing through the channel on the bar, and from an impediment to the influence and action of wind and sea from without, so must the amount and range of an artificial scour be thereby regulated and determined.

In most, hut certainly not all instances, the reclamation of hays and foreshores of estuaries has a most decided influence on the growth of banks and bars at the outie', and even altho.gu dredTng may to some extent compensate for the loss of tidal area, still the fact remains that the quantity of elevated water is in consequence reduced, and the outgoing current rendered the more sluggish and enfeebled. In long estuaries having an irregular and broken coastline, in which the tide meets with resistance either from a contraction of the passage, or from islands in the fainvayfreclamation be 1 ow the narrow is i:; excess of that leading to the bar ; but every attempt at reclamation below the marrows, curtailing the tidal area within the bar, will have a corresponding effect in reducing the strengdx and efficiency of the outflow for scouring purposes, and the tendency with such a result must be to have the detritus from the ocean driven further in and piled higher up by the combined agency of wiud and sea.

Numerous are the instances ■where water-courses on the coastline have been cut on from all communication with the sea thaough storms driving in and piling n;? sand mp shingle across the outlet, and until such time as a sufficient head of water has accumulated to force the obstruction, these water-beds become isolated fresh-water lochs, or brackish lagoons. The tendency of artifical works, contracted seaward, of a sand-bar situated at the mouth of a river or inlet, is in tne direction of shifting the bar further out, just in proportion to the amount of shelter afforded the efflux from the action of wind aud waves ; and whether the screen take the form of a breakwater, pier, embankment, or foreshore reclamations, the like result will follow.

Although the rapid development of mechanical appliances his in these later tunes placed within the easy reach of the engineer a means by which, in a measure, lie may overcome, in a safe and sure wry, obstacles to maritimatenavigation, even in most exposed situations, and granted that in an endeavour to work antagonistically against the action and influence of natural laws, and thereby remove, bit by bit, the obstructions piled up by primary forces, he may utilise such means to the best advantage possible ; still the fact remains that unless auch endeavors tend to counterat the disturbing and transportiong power of the ocean locally, and lead to an establishing of works of a lasting description, the result cannot be considered satisfactory, and may be looked upon in the light of only a makeshift' at the best.

Much valuable plant has been built, and is still building, for the purpose of operating upon harbour bars in all parts of the world, but experience has shown that dredging in such a case is an expensive and ever-reccurring necessity, and to be successful must be constant where it is unaided by judicious works of a permanent character.

So long as harbor bare are open to th e full force and action of the elements of their formation, and local influences remain undisturbed by the works of man, the dredgings of to-day are liable to be replaced with fresh material by the storms of to-morrow, and the whole once more restored to its natural and primeval state.

Wherever practicable, the agency of an induced tidal sdcur is perferable to dredging on a bar, earned out even under the most favorable circumstances. and it must be in this direction that the engineer of the future should look for the means of relief. When financial considerations are to be met and balanced by commercial gain to a port, and circumstances and situation are favourable to works of utility being done, the more these partake of the nature and character of solidity and permanency the betler ; and whenever the influx of the sea current can be protected by artifical embanking from the pernicious influence of the elements from without, and the efflux of tidal waters be strengthened and accelerated thereby from within, such work should be accomplished with as little delay as possible and according to the situation and extent thereof so will the effect of the local change proportionately operate beneficially upon the bar by partly removing, or wholly dispersing, the material of its formation. [Concluded.]