THE IMPROVEMENT OF THE GREY RIVER.

Grey River Argus, Volume XV, Issue 1887, 24 August 1874, Page 2

THE IMPROVEMENT OF THE GREY RIVER.

The following is the report of Mr Moriarty on the improvement of the bar and entrance of the Grey River, which arrived by the p.s. Charles Edward yesterday : — To his Worship W. S. Smith, Esq., Mayor of Greymouth.

Sir, — Tn attention to your request that I should " Examine and report upon the best plan of improving the bar and entrance of the Grey River, and fixing the channel in a permanent position, arid to show by what means a maximum depth of water can be maintained from the entrance, from deep sea water to the full extent of the present wharf accommodation—the report to be accompanied by plans and estimates of the cost," I now do myself the honor to report as follows: — ;:!.'■■'.

I have made a very careful examination of the channel and banks of the river from the " Gorge," above the town j to its embouchure ; and have caused soundings to be taken to ascertain the depths of water available for vessels lying at ! the wharves of navigating the channel ' approaching it.

I have also examined the bar and; the beaches for some distance on either side of the river ; and have caused numerous soundings to be taken on the bar, and as close into the surf on either side of it, as it was considered safe to venture with the steamer. 1 have likewise made myself acquainted with the various changes which have taken place from time to time on the bar, and at the river' 3 mouth, since the settlement of the place — as they have been described by Captain Allardyce, the harbor-master, and" other persons who would seem to have given close attention to the subject — and are shown on the very interesting plan recording those changes, which has been compiled in the Survey Office, and which, taken in conjunction with the reports as , to the condition of the bar at the different periods at which those surveys were made, justify me, I think, in stating that I have acquired sufficient acquaintance with; the natural features and peculiarities of the place, to warrant my arriving at an' opinion as to the measurea that should be undertaken for the improvement of the Porfe, ■-■ ' ■■"'■■'•■• ■■'■ ■•- '■•' • : -" i -- : ''■•''■ !

The soundings on the bar and on the I adjacent banks, as well as those inside the river from the entrance to the upper part of the wharf, are shown on the accompanying general plan. The changes which have taken place on the banks within the river and on the shingle spits on either side of its entrance from 1865 to 1873, are also shown nn the accompanying plan, which has been partly compiled from one in the office of the Survey Department. The Grey is a torrent river which, taking its rise in the lofty and precipitous ranges of mountains cf the Southern Alps, has a very rapid descent to the aea, and is subject after heavy rains, or from the melting of the snows, or from both combined, to sudden floods of great height and force, and the velocity, of the ■ stream in its descent is so great, that it sweeps alone; in its course,: vast masses of boulders, shingle and sand. From the place called the Gorge, at which the river breaks through the limestone hills, the shingle and boulders, ejected with great force,' and meeting ths counteracting influence of the sea waves, have spread out on either side, forming extensive beds, which in the progress of ages have gradually encroached on the sea. On these shingle banks the town of Greymouth now stands, and the contest between the encroachments of shingle on the sea, and the resisting action of, the waves which tends to throw it back again on the beach, is now transferred to the present mouth of the river and the bar. A marked difference maybe observed in. the character of the material forming the north and south, beaches at the entrance cf the river. On. the latter, and extending from the spit for many miles in a southerly direction, the beach >is composed mainly of sand from high-water mark downwards, but with a belt of shingle about high-water mark, and occasional patches of shingle strewn here and there, the sand deposit, however,' largely predominating ; about low-water mark, the beach continues flattened out, and is composed of a dark colored fine sand. Extending out in a .westerly 1 direction from the present sand-spit tliere -is a long stretch of shallow water on which a heavy surf is . , constantly breaking ; . and the signalman, who has been at his present station for about mine /yearsjistates that within his observation this. -bank; on the south side has never :beeti absent, while there has been frequently bold water off the north beach and spit. : ' j On the north beach, and extending as far as Elizabeth Head, the character cjf the beach is different from that last described, being composed mainly of boulders and large shingle, mixed with but. a small proportion of sand. ..-.■-.,! This difference , in the character of the deposit on the north and southern beaches is due to the prevalence of the southwesterly swell, which for the greater part of the year rolls. ia on the coast, and which is aided by the prevailing current setting along shore from south to north. The sand coming from the bed of the ocean is thrown up on the beach by the action of the waves, but is partly held in check close in shore, by the overflow of the river, hence the formation of the eitensive banks on the south-western side of the "entrance. But the shingle, which is i of river ■ origin; after its ejection , jis thrown up by the south-westerly swell and the current, along the north beach. •Thus there is always a tendency of the sand-spit to grow across the mouth of "the river, and force it in a northerly direction. But there' are times : when, other forces .come into operation to. counteract tbjis tendency : , the melting of the : snows br heavy rains send down floods, which, . taking the most direct course to sea, forbe their way through the sand spit, a^id form for themselves new channels, to be again filled up and again reformed, as oil c force on the other, the south-west swell, or the-land floods, prevail. ■- j These ever-recurring, changes, of course, render the navigation uncertain aid dangerous, and while the channel jia shifting from south ; to north, or babk from north to south, "the bar is always bad.. . . ... : . . I On this part of the subject, : Admiral Richards, the Hydrographer, "who surveyed :this coast, remarks :of the Gr^ey River :— ;'lt has a bar at the. entrance constantly shifting ; ,the ; navigation, depending on local pilotage. After a heayy fresh, when the channel breaks out straight, in a westerly direction, it is safe and easy, of access for vessels drawing Bft or 9ft water ; but in the absence of any fresh in, the rivery the channel makes either jto the north or to the south, but usually I to the. north, running for a short distance: parallel with the coast line, and at such ■ times the entrance to the Grey is dangerous." He also observes, " There is a constaut heavy westerly swell, rolling in on this portion of the coast." i An illustration of the frequency of the changes in the direction of the channel over the bar has been furnished since my arrival here. Coming in on Friday, the 20th instant, we took the 'Northern Channel over the bar, which has beeii in use for some short time back, but by Saturday (the following day) it had changed,. and ; the steamer entering was directed by the signal man to take another, channel which, in the interim, had opened considerably to. the southward of that by • 'which we entered. ;. This mere, element of uncertainty, : even in fine weather,-; would be of itself a source. of considerable danger' to vessels trading to the ; Port ; of T coiirse it is greatly aggravated by bad weather, ' and when the .normal depth, , due: to a continuance of the channel in one direction, has; not. been attained.- , In, general it is found that the depth' of water on the bars of tidal harbors is maintained, by the tidal action ; the ceaseless fle w and reflux of the tide tending to keep down the accumulation of sand thrown in by the action of .the waves ; here, however, it is not .so, there; is no. alternating action of the tide, for= on ; the .third day.; after the spring, I found that at half flood, and after a long continuance' of dry weather, . when the river was unusually low, that there was still an. outward set between the heads. Towards high tide there would probably be a slight indraught of sea : ■] water flowing beneath the fresh, but it ' cannot be very great, for I am informed 1 that the river is nearly always drinkable i at the town. It is obvious therefore that i .there can be no beneficial influence exer- 1 cised on : the deepening of the channel by c the indraught of the flood tide, the Only, i forces: acting .on it, tending. to keep open \ the mouth of the river and counteract the t ■tendency of .the surf to throw back the. c sand and .shingle, on the beach and bar c being that due to the ebb, when thereia a afceady afi& considerable disoharge o£ fresh a

water, and to the' action of the land floods and freshets. This will be made more apparent on reference to the longitudinal section of the river. The section Bhows that the fall of the surface of the river in times of flood from the Gorge to the embouchure is sft, and from thence outwards to mean tide level the fall is over :9ft; At an ordinary^ time when there is no, fresh in the river, the fall of surface at lo»w water springs was nearly 18in from the. Gorge to the entrance/, and^at half-flood tide it was about 2in higher at the former than at the latter place, but at the top. of high water it rises one foot higher at the heads than at the Gorge, and there would then be, withoat doubt, a slight indraught from the sea, but it can only be for a very short time at high water, and must be so trifling as 'to have no influence , on the channel, i At no stage of tide is there any upward current at the wharves. -" Were tYie entrance to be^' sufficiently widened and deepened to -admit i the free flow of the flood tide, into the harbor, there is rib. doubt but • that the level of high water at the Gorge : would -become equal to that at the heads, and the w.Hole area of the inner part of the harbor and • the lagoons, on the rsouthrwest of .'"the town, being filled to the extent, of one foot : higher than at present, .riot only would raifscour on the bar bebansed 7 to some perceptible extent by the indraught of this large body of water ; but on the ebb the full benefit of the scour resulting from the larger body of water to Tie discharged would materially increase the action on the bar, and consequently tend to deepen the channel through it. . The Harbormaster states, and in this he is supported by the qbncuVrerit testimony of the masters of steamers and all other persons with whomThayejpoken on the subject that, the bar is always 1 at its best state when the .channel through, it is open in a W;N.' or N^W. direction, and that -: it continues coperi ; for a; /longer time, and with a moresiinif6rm. I de]gth while iri this direction that' infaby. other ; it is also said to be more easily and'safely . taken by ''vessels entering or leaving the port. But all -are agreed that : tlie:>bar"!3'T bad,, and the" eutrance ' difficult s and:dangerous—wheri'theriver opens;iriia.direction anything to the south of west; (.and -'rriust' r be_sb ; is obvious: fr.qm,a,n inspection of : the' localities^" whichrsnowsjfix- , tensive banks lying out in a westerly direction frioiri the southern spitjKon' which a heaVy rsurl . is : constantly - breaking. These facts have to be carefully bornd in • mind; when devising works .for tjie^improvement of the entriirice,jand J fiiing the direction of any proposed chanrierbyeishe bar, the: object of which . shoiild : !be to direct the channel by artificial mean's^ and maintain it permanently in that; course, which experience has shown to have been most tia"- 1 consonance with the results of the natural forces operating at this place. The evidence of Captain Allardyce and other persons with whom I have spoken, quite bears out the statement of 'Admiral Richards, to the effect thai;- for nearly nine months of there is a south- • westerly ocean iiweirlle.tting •in on the coast and bar^ ari&i.hat *he heaviest gales blew from S f Wl,"W:fa m d N.W.^the two former driving in ; 6n. the bar ''a tieaVyiaW long continued -swell ; but with NvW. " winds the seas .are short, /^comparatively harmless, and soon rnn down,; .these circumstances at once account for the observed fact to which Ihave-freferreU of the entrance : to the- river,~;an& tKe bar being 'riaiqfe - permanent and in their best states wheri the river opens in a N.W. or W.N.W,. direction., ■ r r •-.- I arii informed that- in the year £l:B64, during-a heavy flood, the river broke'-out in a westerly direction; cutting through : the /south, sp^t ., near, the slaughter-house, 1 1 and discharging across the 1 Ibflgl|atbank, a which; may /be -seen, to i extend s oHit"fbr a 3 considerable distance at this place. And , at that time, and so long „as the outlet continued in the' sairie direction, .the bar , was bad, but as the channel began toieork 1 to the northward through the joint action 3 of the ! s6uth-west swell j arid northerly cur- : rent which packed the sand in on the 3 southern side, the bar improved arid continued.; to. do so till the -channel got to I about N.W., when it was at its best state. - The progressive movement's bt the channel r northward, in the years 1865 66-71-73-74, I are shown on the accompanying) plan. - Latterlythe: channel , had; gone itoo far to r the northward, and a long, spit extended i . from the south shore to the 1 'northward j '.. overlapping the entrancej^'and^Jrtirining ; parallel with the bach. /' 'This r sp'ifr forms l, the ; , inner, arid shallower, bar; : v ati'd f tll^ • navigable channel ran for, some distance • to the northward, between "it and the •: shore Hne,tiU it ultimately ;turned^ out 1;; ■seaward.;-;;-.;: ■•■■■■- ,~:Y'' ;■ 1 When the; channel opens top far, 3 , t0 the . northward, as it. has recently" "done, the : navigation becomes and dangerous ; as Vessels, after crossing the bar, 1. have *o haul -round;; broadside to the : waves, . and run in on a course parallel and ■ close to the beach, ;till; they come Jto the 1 entrance of the i river, when, ; ihey/; have 1 again, to alter their course nearly at right • arigles'to enter the : river jvthis! is at all times a difficult manoeuvre,, and.:sometimep would be impracticable^ when^there ; happened tb' be a strong out-run of current „ fro «i. the. river. / , -- -.. ; V^ If the channel across the bar 'rah. fair in and .out,- in >, continuation, of '-,' theTptesent channel at the entrance of the^Hyer, the iriavigation.iWould;. be,. rendered-^ easy and : safe to 'the extent (of its available -depth of water } but,. :at? present, the-shaiiftwness ; ; fdrms ; britoneeiement of the danger ; the tortuosity of the. channel^beingj anoiher ■arid greater;' ; ' ';';' Ariy works, therefore, which may be undertaken,. for the -improvement of the entrance to the Grey, mrisVhaVe primarily for their object, ;the fixing of the^ channel at the entrance of the river and across the bar in one : direction, and maintaining it therein. This"tlifection to be such 'as experience has shown to have beeri^ most permanent, under the normal condition! df the river, the currents, a%d the, waves. Another object should be, whileleading the mouth of the river into deep water, to protect it from the influence of the southwest swell, so that; vessels -entering or leaving would ; have the 'protectibn the works afforded^ while crossing the inner bar. It has been stated atbat, after southerly^ and south - westerly .gales, which send in a heavy sea, accompanied by a strong northerly currerit, . the,* south spit rapidly extends, and the nortfi beach is washed , away ; while, after a northj westerly, gale, the south spit recedes, and the north- beach follows it... ... A^lurther object, therefore, should be to arrest this oscillatary movement of the shingle and - graveLaCTOßs* the entrance^ andiobtain the game condition of things*-by 3 Artificial

means— as would have existed had nature placed a rocky headland on the southerr aide. Whenever rivers discharge into the sea between rocky headlands the bars are rarely bad, but on the other hand, when rivers discharge on low sandy shores, the bars are always bad, and the channels uncertain. The engineer, therefore, when designing works for the improvement of the entrance of a river, endeavors to imitate nature in this respect, and to produce artificially a similar condition of things 10 that which is observed to follow from natural causes. All bar improvements have this broad principle for their basis, but success, or failure, in the attainment of the desired results, in each individual case, depends on the skill and judgment with which it is applied. As -regards the improvement of the inner portion of the river at the town, and the maintenance of deep water alongside of the wharves, much will depend on the curve to which the line of the wharf shall be formed, and I would recommend that it be as flat as possible. When a river sets into a bend, the greatest fores of the water is exercised on the concave side, and the sharper the bend the greater the force with which it acts on the bottom and sides of the channel, deepening the water close inshore, and washing down the bank— while on the convex side, the water travels with less velocity, or may even have a reverse current ; hence the shingle or sand brought by the river in its course is always deposited in banks on the convex side. But the flatter the curve and the more nearly it approaches to a straight line, the less is the tendency of the river to seek either shore, and the greater the force of the current towards the centre of the channel where the friction is least. In the instance under consideration, we flatter ourselves we make the curve of the line of wharfs, the less will be the tendency to cut away the bank on the-.Greymouth side. The river will discharge more freely in times. of flpod, and there will be less danger of inundation. A perfectly straight line, however, would not be desirable in this instance, as it will always bB an object to preserve a sufficiently strong run of current along the face of the wharf to prevent accumulation of shingle, and maintain a sufficient depth of water for vessels of moderate draught. The improvement of the entrance by widening, straightening, and deepening the channel will also tend towards the prevention of inundation of the town by the land floods, and the deposition of shingle in the channel near the wharfs. ■ .'>'■■■ ■ The works which I would recommend for improving the bar and fixing the channel in a permanent position, are delineated on the accompaningplan which is taken from a survey made by your Town Surveyor, Mr Johnston. They consist of an internal training wall, commencing at the wharf and continued in a gentle curve to the south spit, being a length of 3300 ft, and: a breakwater extending thence outwards in a north-westerly direction fora distance of 2100 ft which will bring the head of the breakwater into 12ft at low tide. The effect of the breakwater would be to protect the entrance from the break of the S.W, swell, and arrest the northerly trend of the sand which would .be retained in the groin at the base of the breakwater. I would propose to carry the head of tie breakwater outwards in a r more westerly direction than the body of .the; work, so as to prevent greater resistance to the breaking waves which will roll in on its end, and thus diminish the danger of its being washed across the channel: this would also have the effect of giving the northerly coast currents a direction more .off Bhore at this point, and therefore more in the direction of the currents Betting out from the river, which would 'tend to prevent the formation of a spit at the end of the breakwater. The stones for the breakwater should be in as large blocks as can be conveniently, handled, and should average not less than ten tons weight. Stones of this size would not be shifted much by the waves, and with proper tackle and plant, and careful - quarrying, there should be no difficulty in 'obtaining them ot this size from the limestone quarries at the Gorge ; the fcmaller material produced in quarrying the larger blocks for the breakwater can be used in the construction of the inner training wall ; but in this the larger stones, weighing from one ton to half a ton, should be placed on the river side; . the inferior materials being deposited on the inner or land side. The reclamation of the space between the training wall v and the present bank may be done with gravel, quarry rubbish, or any other material which may come cheapest.. If a large coal trade is to be done ak this port (and from what I have myselr seen of the seam at the pit and read of it In the Parliamentary papers as to the excellence of the quality of the coals, and its practically inexhaustible quantity, I believe that there is. every reasonable prospect of a great and rapidly increasing trade), it will be necessary to provide at the wharf, and about the cranes, or whatever other appliances be adopted for loading, a. considerable extent of standage room for the trains of full and empty coal trucks — the present area abutting on the Btreets would be too limited. It will be necessary, therefore, to widen out the space below Chapman street on which to form sidings^ but this, I have pointed out in another place, will be rather beneficial than otherwise to the navigable channel as tending to flatten the curve at this point, and I have little doubt but it will also be followed by the recession of the shingle on the opposite side. I would propose leaving an opening in the training wall to admit of the lagoon being filled and emptied every tide (so. as to act, in some measure, as a sluicing reservoir), conveying the stone for the breakwater across it on a pile bridge— the area of the lagoon is about 650 acres, and the water received and discharged from it every tide must increase in some measure the intensity of the ebb scour on the bar. I would not propose at present carrying out any works on the north side of the entrance— they may not be required at . all, but if they be, it would be better tc defer them till the effect of those on the south side shall have been seen — unti] the river shall be allowed to establish its : regimen under the altered conditions which will result from the construction ot the southern breakwater. I think, that after the works I have in dicated have, been in operation some few years, you may confidently, look to havt an available depth for navigation — o about 15ft or 16ft at H. W. neaps, on thi bar and: in the channel leading to thi

wharves. This would not of course be sufficient for very large vessels ; but, such would not be necessary, to command o very large coal trade, which is, in the present day, carried on for the most part in screw ; steam colliers, or small class sailing vessels. The natural capabilities of Greymouth are not great ; and too much must not be expected from any works which may be undertaken for its improvement, but if those which 1 have suggested be carried out, I have no doubt of the port being rendered safe, and comparatively easy of access, for vessels of the claß3 I have, named. At present it cannot be considered as safe or easy of access to any vessel. The plan which accompanies this report is a general plan .of the river from the Gorge to the bar, showing the present wharves and their proposed extension, the training walls and breakwater, with soundings in the river and on tho bar and adjoining banks. The cost of the works I have proposed would be about L 94,998, made up as follows : — 124,500 tons second class stone in inner tracing wall, at 4s 6d ... £28,012 175,000 ditto in breakwater, large blocks averaging 10 tons each, at 5s 43,750 1000 feet-lineal of wharf, at £8 ... 8,000 Bridge at entrance to lagoon ... 1,600 Railway and sidings to breakwater 3,000 Pitching inner face of training wall 2,000 Add for contingencies 10 per cent. 8,636 £94,998 I have the honor to be, Sir, Your most obedient servant, E. Moriarty.

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