BEACH AND HARBOR PROTECTION IN HOKITIKA.

West Coast Times, Issue 683, 2 December 1867, Page 3

BEACH AND HARBOR PROTECTION IN HOKITIKA.

The following report of Mr Balfour, Colonial Marine Surveyor, has been sent to the Goldfields Secretary, by whom a copy of it was, on Wednesday 6th ult, forwarded to the Mayor: —

Hokitika, November 4, 1867

Sir — I have the honor to report that I have carefully inspected the Hokitika harbor aud river from the beach to the cuissoon dam above the town, exumined the records in the Engineer's office of tho various changes which the river has undergone since my last, vi-ifc in January, 1866, and informed myself by conversations with the District Engineer, the Harbor Master, and the Town Surveyor, .is to the effect of the various works which have been undertaken for the protection of the beach and river banks, and for confining the river in its usual channel, and the conclusions formed from the information thus collected may be summarised us follows : —

My deliberate opinion, formed two years ago, and only strengthened by lapse of time, is that practically nHliin^ can bo done to improve the bar or reader tue channel permanent. Certainly nothing can be done to the bar proper, which is of necessity out side the general coast line, and would only be extended furthor seaward by any works which might be attempted with the view of makiiif!: a permanent channel across it, and any work-i within the sand spits constructed with a view of confining and defining the river mouth would be uncertain in their action, and liable to be washed away by land floods or by heavy surf, besides being very costly. Even this latter method of benefitting the bar therefore though not impracticable, is not to be recommended, 'especially as such works would greatly increase the difficulty of relaunching stranded vessels iuto the river.

SA.XD SPIT.

Furthor experience bus only proved inoiv conclusively thut tho sandspit, though probably tolerably permanent in the long run is liable to many and great us well as sudden changes, and this would appear to justify nurl strengthen the recommendation of my former report, that sections should cither not be sold tliere at u.ll, or if disposed of, that they shoulil be so at tho purchaser's risk. At tho same time the valuo of tho property at stake in that portion of tho town may fairly be considered to justify even a very considerable expenditure in tho endeavor to protect it. Tho fnseiuo work and groynes appear to have acted well. In some places they have certainly been beneficial, in others probably so ; but it is dilficult to say which has been of most benefit or whether cither system would answer without the assistance of tho other. My own opinion is, that each is valuablo, aud that tho combination is more certain to be beneficial than either Bystem could be by itself. Were the the beach cut up imo lengths, as it were, by groynes, and each length protected by one or more fwoine or stake fences a» might bo re-

quired, tho maximum conceivable amount of protection would probably be attained. In this way the run of the sea either from the north or from the south would be checked, and the swell, with diminished force, would bo compelled by the groynes to run fair in on the beach in the manner best suited for carrying material (which would be deposited among the fascines) , and least liable to wash anything away. The system adopted by Mr Rochforfc of laying the groynes at right angles to the beach I am inclined, after mature consideration, to think the best. Were they inclined alternately to either side their action in moderate weather would be more powerful, but this would be more than counter-balanced by the impounding of the water likely to result when a high tido and heavy surf occurred together. The most perfect conceivable system of fascine work would be that which, while sufficiently elastic to yield to the waves, and sufficiently open to allow tho water to escape through the interstices, would be stiff enough to pxr-nit a fair amount of resistance to the sea, close enough to retain the major part of the gravel an 1 sand carried behind it, and at the same time durable. Fascines made of small scrub appear bost to fulfil all but the last requirement, bub they are of so perishable a nature (at least until buried in the sand) that I shall watch with great interest the action of the heavier stake work now being planted by the Town Surveyor. It has certainly answered well hitherto, aud should it be found equally effectual after it has, been tested by heavy surf, the greater durability of the material would justify the general adoption of stake, work in lieu of fascines for the future.

The Town Surveyor is planting the stakes with a considerable inclination to the shore lias acted judiciously in so far as the safety of his own work is concerned, but from their inclination landward, combined with their comparatively smooth surface, the stakes may not improbably have the effect of reflecting the water upwards, and causing it to fly over the top of the batik in greater volume than before, and in this respect they are inferior in pro-, tective power to the more yielding and elastic fascine work. This objection may, I think, be overcome at little if any extra cost by laying another row of stakes iv the trench inclining seawards — not close together, as the row inclined landwards is properly laid to intercept gravel and sand, but with a moderate opening, equal or nearly so to their average diameter, between each stake.

This seaward row, while sufficiently open to prevent the waives from getting too much hold of it, would certainly break up tho swell, and would, I believe, to a great extent — if not. completely — overcome the perhaps sole objection winch can be raised to the use of stake work, and make it as effectual as the fascine work, than which it will certainly be more durable. I must, however, as iv my former report, urge the importance of leaving intact all the protection provided by nature ; neither driftwood nor shingle should on any account be allowed to be removed from the beach, as they form the cheapest of all protections, the removal of which has been repeatedly found elsewhere to iuvolve the necessity of producing some more artificial and costly substitute.

HA.RBOK.

But little requires to be said as to the harbor. The wharfage has required strengthening and repair, as the scour of the river has proved more active than anticipated, especially when intensified by the numerous shipping so frequently made fast to it. This is a common occurrence, and experience ouly shows that any future extension of the wharfage must be constructed at first in a more substantial and costly manner than the earlier portions of the work. The bank protection seaward of the wharfage which has been ably carried out by Mr liochfort on the principles laid down in my former report, appears to have answered its purpose perfectly, and I can only regret that it has not been extended ere this. I should recommend an early extension of this work iv a good curve, of not less than ten chains, and one or two stages had better be constructed on the lino of this work for facilitating the landing of stock, as the present practice of lauding them on the unprotected spit is objectionable, from its breaking down the banks.

Above the wharfage tho bank will require similar protection at an early period as it is now eaten away very closely to the line on which such works had best be constructed ; a considerable portion or the whole if necessary, of this work can be very cheaply altpred, to form substantial wharfage at any time when additional accommodation may bo necessary.

WOItKS FOlt G-UIDIXG- THE HIVEE ABOVE TIIE

Owing to an error of -judgment on the part of the contractor, the dam recommended by me in the south channel has proved a failura. Haa it been constructed in tho cautious manner recommended by myself and ordered by Mr Rochfort, I have do doubt it would be still in existence, and would have answered its intended purpose.

I still feel satisfied that the river ran a very great risk of taking the south channel and deserting the town at the period of my former visit, and that it would probably have done so ere this, had no steps been taken to prevent it, and I believe that the c.tissoon-dam and other works, constructed by Mr Roehforb, have been of good service. Jfo doubt they have not hitherto had full effect, as Hie greater part of the river still flows in the channel, partially blocked, up by the dam, which requires repair, and perhaps renewal ; notwithstanding, the action of all the works taken have been in the right direction, and by careful watching, and judicious strengthening and extension of such p irts as may require it from time to time, tho river may, 1 think, continue to be successfully controlled. The river, purlfaps, is not in better form than it was before— at least it is not worse, as it threatened to become, and that result is really all that was hoped, and almost more than was expected (o be attained. Shinglebearing rivers, lil.e the Hokitika, are most capricious in their action, and most difficult to manage — even the he.iviejt works of masonry sometimes, in similar localities, fail to attain permanently the intended objects, and I think it is matter for congratulation that comparatively feeble and inexpensive works of timber and brushwood have in the present instauce been able to effect so much in the face of natural lorces, admittedly so powerful and so immeasurable as to cause all work* that can be attempted to assume the character rather of experiments, from the failure of which no discredit can accrue, while their even partial success is honorable to their designeoa, than of works which can be calculated to a nicety, and designed with a confidence that they must succeed.

The dam may, perhaps, do without repairs for a time. When it requires strengthening the best plan will bo to construct another tier of caissoons above, abutting ou and breaking joint with the present tier, thus it will be cheapier aud easier to bring into position, and stronger when finished than the original dam. It should not in any case, however, be carried above the surface of the ordinary low-water level of the stream, so that the overflow in floods may be a relief to the other channel. The new cut bids lair to be scoured out by

floods till its dimensions are such, that it will carry a large fraction of the river though the repuir and strengthening of the dam would certainly expodite the process. The trainingwall in the new cut has been beneficial in its action, but about two chains of additional works are required higher up to prevent the river from outting in behind the part nlready

made, and the lower end sh.ml 1 b_> i'\.;on led for about a chain in its present im •. It would be advantageous also to cut >i trench through the point of the Spit below to aid the water in straightening its channel.

Beneath I h.ive the honor to submit an estimate of tho cost of the works recommended aud which seem all that are at present required, though in combatting such tremendous natural forces as are here in question, with the means and materials at command nothing can be considtre.l final, a continuous struggle must be waged with the elements, and every new breach must be promptly stopped if the beach and river banks are to be maintained in anything approaching to a condition of permanence. ESTIMATE. 1. Beach Protection Works — £ s. d. Say five additional groins about five chains apart, at £180 900 0 0 Thirty chains of double stak- . ing as recommended, at £20 600 0 0 2. Jiiver Facings — Ten chains bolow wharfage, at £3000 3000 0 0 Allow for cattle landing stage 300 0 0 3. Additional Wharfage — Say four chains, at £100 ... 1600 0 0 •&. River Training Works — Three chains extension of training walls, at £50 ... 150 0 0 Cutting trench, say ... ... 80 0 0 K^pairs to Dam, say ... 700 0 0 £7330 0 0 In addition to this there will bo about thirty chains of river bank above the wharfage which may require protection sooner or later ab a cost of probably not less than L3OO per chain, but there is no present necessity for undertaking works on such an expensive scale. Indeed a considerable portion of the works included in the above estimate may properly be held over in the meantime, provided they are pushed forward with vigour as soon as the need for them arises. As to this I can only say that you may trust in the judgment of Mr liochfort, who will give timely notice when the time for undertaking each section of the work has arrived, and if they are undertaken with the necessary promptness on his recommendation, I feel confident that they will be completed iv sufficient time to avert any tlireatened danger. I have the honor to be, Sir, Your most obedient servant, (Signed) James Balfoub, Mem. Inst. C.E., Colonial Marine Engineer. The Goldfields Secretary, Hokitika.