TIMARU HARBOR WORKS.

Temuka Leader, Issue 2188, 14 April 1891, Page 3

TIMARU HARBOR WORKS.

(Continued ) " Eeport of Commissioners dated 2nd April, 189 L, in compliance with Chairman's letter dated 21st March, 1891. " N.B. This is signed by Mr O'Connor only, Mr G-oodall sending in separate memorandum.

" Timaru, 2nd April, 1891, Be Timaru Harbor Works. Alternative No 2.— Dredging.

" Coming now to the second alternative, namely, the removal of shingle by meanß of di-jdeing, and conveying it to the northward, entirely clear of the harbor, say to the vicinity of the Dashing Rocks, it appears, after a very eyeful study of the question, that this is by far the best means to adopt, and there Heems to be no reason why it should not give satisfactory results for a moderate annual expenditure, which in view of the interests at stake, may be looked upon as moderate.

,l lt U only right to say, however, that the probability of being thus able to deal satisfactorily! and at moderate cost, with ihia shingle accumulation, is due to a oreat measure to the recent invention and progressive improvement nf pump dredgars, of large power and capacity, and that if such machines as have recently become obtainable were still in the wornl) of futurity, the existing and imm nent danger from shingio accumulation wouM be made more serious. There is also the advanaage inj}thi3 case thU the bed

of the harbor affords good holdiug ground for piles of staging, which will probably be an absolutely necessary appliance towards the performance of the requisite dredging.

"The nmoui't of annual shingle accumulation which has to be dealt with is not in itself an appalling quantity. The total retained between Patiti Point and tbe breakwater during the last 12 years, shows the average annual increment to be under 120,000. To dredge that quantity during th*» 300 working d«va in a year would oniy iuvolve lifting 400 tons per day. "It is probable, however, that some of the annuil accumulation of shingle, in the immediate vicinity of the breakwater, which is now giound into sand, and carried round the harbor in suspension, would be dredged up before being so converted into sand. It is impossible of course to say how much this would amount to, but, judging by the accumulation in Caroline Bay, and all other data available, it would probably We an excessive estimate to put it at 15,003 tons per annum. Taking it at that, however, so as to be on the safe side, the total annual quantity to be dredged would thus be 130 000 tons. Th<u for 300 days would be equivalent to 450 tons per day. " To dredge 460 tons of sand in a day would be a very small matter, and reqnir? but a very small power of dredger, if i* were possible to get at it conveniently evpry day ; but in this case such cannot be expected, and it will consequently be necessary to provide appliances and machinery of ample power to do the year's work in less than a year. " Taking all the circumstances into account, however, it is probable, that by adopting the pl*n which will presently ba detailed, it would be possible to dredge at Timaru, to a hk'h percentage of efficiency, and in effective situations, during asy about half the yeir ; and, if this were achieved, the dredge need only bo ctpab'e of lifting Bay 1000 tons in eight hours.

• " A dredge capable of lifting 1000 tons in 8 hours might therefore possibly be sufficient, but allowing for interruptions for repairs, and other contingencies, and in view ot the magnitude and importance of the interests at stake, it would be desirable to provide machinery and appliances, in this instanoe, capable, if occasion requires, of lifting and disposing of as much as 1500 tons in eight hours. Details of Dredqino Plant.

" Having very carefully considered, and calculated the coßt of, many methods which suggested themselves, or were suggested by your engineer, Mr Marchant, by which the requisite dredging and conveying of the stuff might be carried out, it appears that the following is the most advantageous one, in view of the various and manifold requirements of the case, and it is therefore recommended for your approval and adoption, namely : " A pump-hopper-dredger, capable of lifting, under favorable circumstances, 400 tons of sand or shingle in an hour, and having hopper capacity for 308 tons of sand or shingle ; with compound surface condensing engines indicating about 200 horse power. This vessel, while dredging, to lie alongside the breakwater wharf, in the vicinity of the tail of the shingle bank, within the harbor, aa indioated on a plan therewith, and to do its dredging through a pipe and nozzle of the usual form, but supported on a light timber piled etaglng, outside the breakwater, as indicated on the plan aod longitudinal section herewith. The pipe and staging to follow up the outer margin of shingle accumulation, to such extent as may be necessary to render the dredging effective. That is to say, in order to dredge only what will be restored again by accrning accumulation, and not merely to make excavations in the interior of the existing shingle bank, which might not be restored, as that would not be contending at all with the currant accumulation.

" In addition still further to secure the result of only dredging what is necessary, namely, -what would otherwise add to the area of accumulation—and also in order to dredge at cheapest rate practN cable, it would be desirable that the dredging, in the first instance at any rate, should be confined to the face of the shingle bank, and be only carried down to s+y 6 feat below low water, as the shingle below that, when the backing is rem ived, would probably be caßt up by the sea. If so, it could then be dredged at less cost, or, if oast up very high, it mi(»ht not require dredging at all. "The nozzle of supply pipe would be guided from staging by means of a hand crane.

" When hopper is filled the vessel wou'd, of course, go to sea and discharge its load, say in the vicinity of the Dashing Rocks.

"In addition to one or more pipes nod nozz'es for working qh staging, the ship should also be provided with sufficient pipes aod nozzles for dredging at sea, or within the harbor, and also with steam orane jibs or other suitable appliances for guiding such nozzles wheu at work. "The cost of this vessel and sraping, etc., vould probably be about £10,009. "The interest, and depreciation in value of machinery, due to that expenditure would amount to say, ll£ per cent., namely, to £llsO per snnum. •'The coat of the dredging, and depositing the stuff dredged, after m«king due allowance for contingencies, and including repair* of machinery nnd all oth'r expenses, would be about 4d par ton, and this for 1§5,00Q tons would coins to £2250 per annum.

" The total cost for dredging an average of 135,000 tons in each ye;»r would therefore be about £3400 per annum, and that would he equivalent to 6.04 pence per ton, including interest aod depreciation on value of plant. "To describe herein all the other systems of dredging which have beep inquired into would make th'B report entirely too cumbrous.

"As it may be desirable, however, that a list of the most favorable ones among them should be furnished, with the reasons for their rejection, this has been done in an appendix hereto attached. "If the machine which is recomnr ndeH to bo procured is approved of, it woa'd be very desirable, in view of its being likely to be required to work continuously end permanently, that the best class of dredge procurable of its kind, nnd having all the latest improvementp, should be obtained; and, in order thi& may bo done, it is suggested that the board should have A completo drawings and specifications prepared, defining exactly the existing conditions and the character and the extent of I the] work required to be performed,

and eend lha aiDiu to some qualified expert in London to do ihs best he <«m for them under the circumstances, or what would be the more satisfactory pi in, for the board to sent some qualified person from .here whoiiv you can 'horoughly rely upon, and who is conV: r&«nt with all the circumstances of the o-.se, to yieit some of the principal mauut dories in England and Holland aod -• i«fy himself «s to which is likely to . coduca the most. Sdiißfactory eiticle

■ Ming a contract for its construction. Joe reason, among others, wh*y this latter < an wonld probably ba the most satisfictory one to Bdoptis that no matter how complete such plans aud specifications may t>e it in impossible in practice fco predicate <1 t,-:e questions that may arise, and to -upply answers to them io anticipation. 'Co a person unacquainted with the locality -iiso a machine might appear to be quite suitable which to one acquainted with the ucnlity would be evident at a glance to be unsuitable. •'Another reason is the rapid ioprovementa now continually beiog made id i heso machines which could not be predicated in any specification sent from here rt.id which might or might not be neoes.'•iry or suitable for the Timarti work, and which an expert in England would not consequently know whether to accept or reject. Altogether, therefore, it is proh.ble that the cost of seeding someone acquainted with the locality to Eogland or Holland would be more than saved ; >f not in the procuring of a higher class machine possibly in the rejection of some altogether unnecessary redundancy. Dredging Recommended in the Meantime Pending Procuring the Plant Herein Descbibeb.

" Whichever way (he question is decided, however, it i 8 evident, at any rate, that the procuring of the necessary machinery for the efficient peiforinance of the work now required to ba done, will take a considerable time, and, as no time should be lost in meeting the shingle accumulation danger, it is suggested that in the meantime the board'e exisiing Priestman grab dredge should be utilised, to t-uch aD extant os this can economic >lly be done ; first by working it on the edge of the breakwater, and afterwards, un'ess th 6 i-hingle accumulation keeps continually progressing forward, on a staging of the character indicated on Ihe section herewith, following the face of the shingle back to such extent as may be requisite, For,some time, at any rate, it is probable that this proeeap, though inadequate to keep pace with the annual accumulation, will not be more expensive per ton dredged thaß any other plan would be; in fact, while the circumstances remain suitable, it iniy possibly be the cheapest plan of all, bo that nothing can be lost by edoptiog it bo fur 8B it will uc'. No dredging, howeyer, should be done where it will not be filled op again by the accruing accumulations, as this would not help to meet the difficulty in any way, and would merely make holes in the area which might otherwise be utilised for various purposes. Surveys. "To refer now, for a moment, to a subject which is of minor importance, but which we think should nevertheless receive attention, we would wish to take this opportunity of recommending that periodica! and systematic records should be kept of the progress of the shingle bank, and of the sea bottom foe some considerable distance out therefrom (where sand deposit is liable to take place) in order to be able to satisfactorily define and realize from time to time the changes which are gradually occurring, and thus if possible trace their causes, and if poesible counteract them, or some of them, when they are detrimental to the harbor. To do this satisfactorily will require the establishment of permanent mirks;*defining suitable base lines for soundings, etc., which would henceforward always be adhered to ; and this will, in the first instance, entail some little expense, but we feel satisfied that such expense is justiiable, and will probably lead to economy in the long run, even if only in the future cost of making the soundings and measurements, which will probably be made from time to time in some way in any case. Conclusion. •' In conclusion we would w'sh to exexpress our thinks to the chairman and members of the board, for the facilities which they have placed at our disposal towards acquiring the information neoes sary for this report, and for explaining so definitely as they have done the exact questions which they wished to have answered. Also to the officers of the board for their willing and kindly assistance. This is especially so as regards your engineer, Mr Merchant, whose care* ful and long continued study of the questions referred to us has given him must valuable and exhaustive knowledge and ideas on the aubjeot, and these he most willingly, and cordially placed at our disposal. " We have the honor, etc., " Your obedient servants, "C. Y. O'Connor, M. Inst. O.E, (To be Contiflaed.)