To His Worship W. F. Smith, Esq., Mayor of Grey Mouth.
In 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, and 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, to its embouchure; and have caused soundings to be taken to ascertain the depths of water available for vessels lying at the wharves, or navigating the channel approaching it.
I have also examined the bar and the beaches for some distance on each side of the entrance 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. I have likewise made myself acquainted with the various changes which have taken place from time to page 4 time on the bar, and at the river's mouth, since the settlement of the place—as they have been described by Capt. Allardice, the Harbour 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 wore 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 measures that should be undertaken for the improvement of the Port.
The soundings on the bar and on the 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 on 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 of the Southern Alps, has a very rapid descent to the sea, 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 along in its course, vast masses of boulders, shingle and sand.
From the place called the Gorge, at which the page 5 river breaks through the limestone hills, the shingle and boulders, ejected with great force, and meeting the 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 the 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 may be observed in the character of the material forming the north and south beaches at the entrance of 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 becomes flattened out, and is composed of a dark coloured fine sand. Extending, out in a westerly direction from the present sand spit there is a long stretch of shallow water on which a heavy surf is constantly breaking; and the signal man, who has been at his present station for about nine years, states that within his observation this bank on the south side has never been absent, while there has been frequently bold water off the north beach and spit.
On the north beach, and extending as far as Elizabeth Head, the character of the beach is different from that last described, being composed mainly of page 6 boulders and large shingle, mixed with but a small proportion of sand.
This difference in the character of the deposit on the northern and southern beaches is due to the prevalence of the south-westerly swell, which for the greater part of the year rolls in 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 outflow of the river, hence the formation of the extensive banks on the south-western side of the entrance. But the shingle, which is of river origin, after its ejection is 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 this tendency: the melting of the snows or heavy rains send down floods, which taking the most direct course to the sea, force their way through the sand spit, and form for themselves new channels, to be again filled up and again reformed, as one force on the other, the south-west swell, or the land floods, prevail.
These ever recurring changes, of course, render the navigation uncertain and dangerous, and while the channel is shifting from south to north, or back from north to south, the bar is always bad.
On this part of the subject, Admiral Richards, the Hydrographer, who surveyed this coast, remarks of the Grey River: "It has a bar at the entrance page 7 constantly shifting; the navigation depending on local pilotage. After a heavy fresh, when the channel breaks out straight in a westerly direction, it is safe and easy of access for vessels drawing 8 or 9 feet water; but in the absence of any fresh in the river, the channel makes either to the north or to the south, but usually 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 constant heavy westerly swell, rolling in on this portion of the coast."
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 been 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 course 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 harbours is maintained by the tidal action; the ceaseless flow 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 page 8 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 that the river is nearly always drinkable at the town. It is obvious therefore that there can be no beneficial influence exercised on the deepening of the channel by the indraught of the flood tide, the only forces acting on it, tending to keep open the mouth of the river and counteract the tendency of the surf to throw back the sand and shingle on the beach and bar being that due to the ebb, when there is a steady and considerable discharge of fresh 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 shows that the fall of the surface of the river in times of flood from the Gorge to the embouchure is 5 feet, and from thence outwards to mean tide level the fall is over 9 feet. At an ordinary time when there is no fresh in the river, the fall of surface at low water springs was nearly 18 inches from the Gorge to the entrance, and at half-flood tide it was about 2 inches 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, without 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 page 9 influence on the channel. At no stage of tide is there any upward current at the wharves.
Were the entrance to be sufficiently widened and deepened to admit the free flow of the flood tide into the harbour, there is no doubt but that the level of high water at the Gorge would become equal to that at the heads, and the whole area of the inner part of the harbour and the lagoons, on the south-west of the town, being filled to the extent of one foot higher than at present, not only would a scour on the bar be caused 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 be discharged would materially increase the action on the bar and consequently tend to deepen the channel through it.
The Harbour Master states, and in this he is supported by the concurrent testimony of the masters of steamers, and all other persons with whom I have spoken on the subject that, the bar is always at its best state when the channel through it is open in a W.N.W. or N.W. direction, and that it continues open for a longer time, and with a more uniform depth while in this direction than in any other; it is also said to be more easily and safely taken by vessels entering or leaving the port. But all are agreed that the bar is bad, and the entrance difficult and dangerous—when the river opens in a direction anything to the south of west; and that it must be so is obvious from an inspection of the localities, which shows extensive banks lying out in a westerly direction from the southern spit, on which a heavy surf is constantly page 10 breaking. These facts have to be carefully borne in mind when devising works for the improvement of the entrance, and fixing the direction of any proposed channel over the bar, the object of which should be to direct the channel by artificial means, and maintain it permanently in that course, which experience has shown to have been most in consonance with the resultants of the natural forces operating at this place.
The evidence of Captain Allardice and other persons with whom I have spoken, quite bears out the statement of Admiral Richards, to the effect that for nearly nine months of the year there is a south-westerly ocean swell setting in on the coast and bar, and that the heaviest gales blew from S.W., W., and N. W., the two former driving in on the bar a heavy and long continued swell, but with N.W. winds the seas are short, comparatively harmless, and soon run down; these circumstances at once amount for the observed fact to which I have referred of the entrance to the river, and the bar being more permanent and in their best states when the river opens in a N.W. or W.N.W. direction.
I have been informed that in the year 1864 during a heavy flood the river broke out in a westerly direction, cutting through the south spit near the slaughterhouse and discharging across the long flat bank, which may be seen to extend out for a considerable distance at this place. And at that time and so long as the outlet continued in the same direction, the bar was bad, but as the channel began to work to the northward through the joint action of the south-west swell, and northerly current which packed the sand in on the page 11 southern side, the bar improved and continued to do so till the channel got to about N.W., when it was at its best state. The progressive movements of the channel northward, in the years 1865-66-68-71-73-74, are shown on the accompanying plan. Latterly the channel had gone too far to the northward, and a long spit extended from the south shore to the northward, overlapping the entrance, and running parallel with the beach. This spit forms the inner and shallower bar; and the navigable channel ran for some distance to the northward, between it and the shore line, till it ultimately turned out seaward.
When the channel opens too far to the northward, as it has recently done, the navigation becomes difficult and dangerous; as vessels, after crossing the bar, have to haul round broadside to the waves, and run in on a course parallel and close to the beach, till they come to the entrance of the river, when they have again to alter their course nearly at right angles to enter the river; this is at all times a difficult manoeuvre, and sometimes would be impracticable, when there happened to be a strong out-run of current from the river.
If the channel across the bar ran fair in and out, in continuation of the present channel at the entrance of the river, the navigation would be rendered easy and safe to the extent of its available depth of water; but, at present the shallowness forms but one element of the danger; the tortuosity of the channel being another and greater.
Any works, therefore, which may be undertaken, for the improvement of the entrance to the Grey, must have primarily for their object, the fixing of the channel at the entrance of the river and across the bar in one direction, page 12 and maintaining it therein. This direction to be such as experience has shown to have been most permanent, under the normal conditions of the river, the currents, and the waves. Another object should be, while leading the mouth of the river into deep water, to protect it from the influence of the south west swell, so that vessels entering or leaving would have the protection the works afforded while crossing the inner bar. It is has been stated that, after southerly, and south-westerly gales, which send in a heavy sea accompanied by a strong northerly current, the south spit rapidly extends, and the north beach is washed away; while, after a north-westerly gale, the south spit recedes, and the north beach follows it. A further object therefore should be to arrest this oscillatary movement of the shingle and gravel across the entrance, and obtain the same condition of things—by artificial means—as would have existed had nature placed a rocky headland on the southern side.
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, endeavours to imitate nature in this respect, and to produce artificially a similar condition of things to 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.page 13
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 force 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 down 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, the flatter we make the curve of the line of wharfs, the less will be the tendency to cut away the bank on the Grey-mouth side. The river will discharge more freely in times of flood, and there will be less danger of inundation. A perfectly straight line, however, would not be desirable in this instance, as it will always be an object to preserve a sufficiently strong run of current along the face of the wharf to prevent the 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 page 14 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 accompaning plan 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 3,300 feet, and a breakwater extending thence outwards in a north-westerly direction for a further distance of 2,100 feet, which will bring the head of the breakwater into 12 feet 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 the breakwater outwards in a more westerly direction than the body of the work, so as to present greater resistance to the breaking waves which will roll in on its end, and thus diminish the danger of its being washed down across the channel: this would also have the effect of giving the northerly coast currents a direction more off shore at this point, and therefore more in the direction of the currents setting 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 page 15 proper tackle and plant, and careful quarrying, there should be no difficulty in obtaining them of this size from the limestone quarries at the Gorge; the smaller 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 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 at this port (and from what I have myself 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 streets 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 page 16 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 to defer them till the effect of those on the south side shall have been seen—until the river shall be allowed to establish its regimen under the altered conditions which will result from the construction of the southern breakwater.
I think, that after the works I have indicated have been in operation some few years, you may confidently look to have an available depth for navigation—of about 15 or 16 feet at H.W. neaps, on the bar and in the channel leading to the wharves. This would not of course be sufficient for very large vessels; but, such would not be necessary, to command a 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 Grey mouth are not great; and too much must not be expected from any works which may be undertaken for its improvement, but if those which I have suggested be carried out, I have no doubt of the port being rendered safe, and comparatively easy of access, for vessels of the class I have named. At present it cannot be considered as safe or easy of access to any vessel, page 17 The plan which accompanies this report is a general plan of the river from the Gorge to the bar, showing the present wharfs and their proposed extension, the training walls and breakwater, with soundings in the river and on the bar and adjoining banks.
|124,500 tons second class stone in inner tracing wall, at 4s. 6d.||28,012||0||0|
|175,000 ditto in breakwater, large blocks averaging 10 tons each, at 5s. 0d||43,750||0||0|
|1000 feet lineal of wharf, at £8||8,000||0||0|
|Bridge at entrance to lagoon||1,600||0||0|
|Railway and sidings to breakwater||3,000||0||0|
|Pitching inner face of training wall||2,000||0||0|
|Add for contingencies 10 per cent.||8,636||0||0|
|£94,998 0 0||0||0|
Your most obedient Servant,
S.T. Leight & Co., Hunter-street, Sydney.