NEW PLYMOUTH HARBOUR.

Taranaki Daily News, Volume XXIII, Issue 23, 19 October 1901, Page 2

NEW PLYMOUTH HARBOUR.

MR NAPJEH BELL'S RKPORT ON THB i SSAND QUESTION. , THE HOLE-IN-THE-WALL OON- ' ] DEMNED. i At the Harbour Board meeting onp Friday afternoon Mr Napier Ball's re- i pjrS on the New Plymouth Harbour |' was read, as follows: j| To the Chairman of the Harbour i Board, — , Sir, —Acting on given instructions,! I had the plans and surveys showing the conditions of the port as regards the deposits of sand sent to me to Hobirt, where I made a study o f the •mbject. Subsequently the sacretary wrote me that it was the wish cf tho Boird that I should personally visit and inspect the port before reporting on the sand drift. I at once took st'ips to comply with this request, bu-, I fell ill and was. in cousequence, detained some months before I could at'end to given instructions. I have now carefully inspected tht* breakwater and the shore, aiad studied r,he various surveys and soundings made from time to time to show the progress of the sand drift. I have a'so carefully considered the proposal to cut a hole or gap in the breakwater with the view of letting the sand drift through naturally and so save the expanse of dredging. That all the drift-sacd can be disposed of in this way' in, I am told, firmly butertained by some people, who have for long insisted tbat the Boiro should try the experiment; but the ground on which they base their con tantion has given rise to much discussion, and it seems possible that nothing but actual experiment can convince tbem. It is said that the yearly quantity of sand which drifts along the coast towards the north is 70,000 cubic yards ; but this estimate must be very uncertain, as it is difficult, if not impossible, to measure it accurately, This assumed quantity amounts to 1 about 200 cubic yards a day, which is not a formidable figure to deal with; the sand, however, does not drif--1 regularly; in ffce weather it accumu--1 lates, but is carried away in great 1 quantities in rough weather. 1 The accumulation of sand has de- • posited all round the breakwater, which 1 in 1898 formed .a belt of sand abou* 12 chains wide, but soundings taken ! in 1900 show tbat the effect of dredg- ' ing in the port has been to contract tbe 1 width of the belt of sand deposited ' outside aa well as at the end of tha i breakwater, which is evidence that ' when the supply is checked tbe waves i gradually wash the sand inshore. Up to 1898 the port had silted s> ! much tbat its use was restricted to ' high water; by dredging, the navigable part of the port ij already restored to nearly the original depth before the 1 breakwater was constructed. Tbe i dredgd employed is very unall, but • lasl» year it removed 134,000 cubic b yards at a cost of 260 p<uca; if, there- ' fere, it is true that tbe yearly deposit 1 is 70,000 cubic yards a year, your 1 dredge seems to bo quits able to 1 remove it. ! The prevailing drift of the sand ia to ' the north, and is due to the oilique ' action of the prevalent west and south- : west waves on the general trend of the coast line, causing a current to flow • close along tbe beach within tbe action ' of the breakers; the waves keep the sand stirred up and the. current carries it forward. 1 Before the breakwatai was con- : structed all the beach was covered with 1 sand ; after it was finished the supply of of sand from the south being stopped ' the beach from the breikwater to New ' Plymouth, a distance of three miles, ! was swept clem of sand, and nothing • but rock was expose-*. The deposit ' from dredging and the natural ■ irif t getting around the breakwater, in 1 now covering the beach agiin with sand, which is yearly increasing. Under the lee of the breakwater the 1 direction of the wave action is altered, ■ and if the Bhelter were effective there would be no wave action and all drift : of sand woald cease. Many breakwaters have been rendered useless by : silting up under their sheltered lee, generally caused by stnd coming round the end and lodging under the shelter.; ' at other times occasional wind and : waves opposite to th 9 usual dirrction drift sand from the leeward beach into ' the shelter of the breakwater, and the shelter prevents its getting out again. Tbe difficulty caused by drifting i sand has often eugg«sted the method of ' connecting the shore with open viaduct of piles, but I know of ne cuse wbers c it Kat been sacestsful. Bsfore the ' present breakwater at Timaru was • built, Sir John Coode proposed to make > the breakwater off the coast and con- • nect it with the shore by 1200 feet or so of viaduct. Mr. John Oarruthers, who was Engineer-in-OhieF io New Zealand at the time, expressed tbe 3 opinion tbat this device would be a i failure; because, as he said, tho J shelter of the breakwater wou'd be as effectual in stopping the drift as tho breakwater itself, wbich seems quiti 1 reasonable, bscausu the average prep vailing roughness of tbe wave action is ' just equal to drift along the average quantity of material; but if tbat r wave action is checked by shelter then the same quantity cannot be carried past and accumulation must take ■ placs. ' Sir John Cooale may not have been • convinced by Mr. Ca'ruthers, con- ' tention, as some years afterwaids he designed for Warrnambool a concrete breakwater, separated from th« shore by 1200 feet of piled viaduct, with t e r object of letting the smd p'ss through. 1 When I saw it, a short bumpy sea ! and a strong current was parsing through the viaduct, and there was 16 ■ to 32 feet of water at tbe breakwai or and good berthage on the opp-si - ;" shore at the Warrnambool jettie*. f Last year the Inspector-General of ' Public works at Melbourne told m^ 3 that he was just going to Warmamf bool to mark out a berthage to re ■ dredged alongnide the breakwater, a< i the greatest depth now available wis 13 feet, and the whole bay was sil- od 3 up. At Port Elizabeth, in South Aftic-i, • tbe same thins; has t*ken place ; there j the br akwator w*s originally joine i 3 to the land," but as the rand comme' c<?d 3 to come roun-1 the end they cut rut i he shore end o f the b"e<k'-<-aier for 1000 3 fees, and replaced th» npe'iing with a piled viaduct. But the sand drift wok ' accumulated under the shelter of the 1 solid work, and Messrs. Coode and Mathews recommend that they must 1 depend on dredging to keep the, port; ■ open, and Warrnambool will have no option but to do the same. > In the case of New Plymouth, Sir i John Coode did no-, direct that an ! opening should be left to connect the solid breakwater with the shore, and it ■ is left to conjecture whether if that had ' been done the result might not be the j same as Warrnambool and Port Eliza-

beth. I know. of no other reason for , supposing that the same silting would ; happen here if an opening of, say 400 . feet, had been bridged by a viaduct, ] but that a considerable quantity of ; sand has been dredged and discharged at or near tke position of the end of the pipe of the fixed sand pump, and a small quantity has been pumped through the fixed sand pump and discharged at the same spot, all of which appears to have drifted away northward along the beach. But although | the feeble wave action at this spot may be competent in a long time to dispose of such a small quantity as has been deposited, it may not follow tlat it would sweep away as fast as it came the whole quantity of sand which', drifts round the breakwater in a year, | and if it did not remove all of it, then I of course the same silting would occur i which is seen at Port Elizabeth. Whether an opening of 400 feet ( would have solved the difficulty of the j drift-sand and obviated the necessity | for any dredging, no one can tell for ] certain, and even if it succeeded with i the present length of breakwater it might fail if the breakwater were extended. As for a smaller opening, that of 6 feet wide, which was proposed, would be to my mind absurd, as I imagine that it would be at once blocked up in heavy weather. Then an opening of 40ft has been proposed, regardiag which it appears to me that if it were not blocked up after a few gales had driven a quantity of sand i through, that the proportion of sand which was carried through ic would be i a very small proportion of the total quantity which drifts round the breakwater. Such an opening would probably be blocked up in fine woather and i be inoperuivo; it would only come into jc i"n in rough weither. Theee* ■ being tt'C sam ■ level on both sides, the only foic .vailnbltf to drive the smd >. tkrougu tho opeuiog would be the rise of each wave, and this force would be • dissipated a* Boon a? the wave , emerg d at I.h» hirbou;' end of ihe i 'openini.'. T'itt wav-8 would also causi j a current t flo"/thiough tb'< opining ; which w uVI al'tu he dissipated on com- ' ing ou*. at th'j harbour end. Thus tbe sand forced through the i opening would receive no further impetus from tbe sea outside as soon as . it had passed through the opening, and i it must therefore be assumtd that the feeble wave action at the inbide part i of the breakwater would suffice to . to carry away the stnd along tha beach i until it reached a position beyond tbe I sbel.er of t:m breakwatsr, where the , wave action would be of sufficient i strength to carry the sand onward. i If the great power and ceasele-s action of the sea is required to carry > along the stream of drift sand which , flo ws past tbe break water, then it seems 3 evident that the feeble action of the > little waves at the inside root of the » breakwater would hava power to carry , away only a very small part of this t great sand stream, and that if more 3 sand were forced through the opening . than tbe little waves could cope with it t would remain either to block up the ; . opaning or form a sand bank about it. > Beyond a distanca of a chain or two fi'om an opening, if there were any } power at the inner root of the break--3 water to shift any sand at all, it must . be a feeble power, and the quantity of , of sand it can deal with must be also . small. Tbe action of this force is also , quits as intermitent as the action of the 3 open 89a, or rather more so, because 3 when in fine weather the sea waves are still high enough to carry on some sand . drift, it is perfectly calm at the inner , root of the breakwater, and all forward r motion must be stopped for sand which [ has come through the opening. Oonj sequently the 40ft opening in the breakwater, if it acted at all, would , only relieve the necessity for dredging ■ to a very small extant. I The whole question at issue is whether the wave action at the inner , root of the breakwater can shift sand , placed there, and this might be tested j by taking out a joint of the piping of of tbe fixed sand pump and pumping I sand into the harbour at this spot. - To make such a test effectual, the bot- . torn round about this spot must be , carefully surveyed and sounded before and af ier the depone of sand, as tbe I motion of the water will spread it out . over a considerable area ; also, a con- | siderable quantity must be pumped in , to make sure of the result. j To make an opening iu the break- , water 40ft wide, the concrete must be cut out to low water level, piers of iron- , bark piles must be driven in 13 feet f spins, and the headway made good t with ironbark caps and floor beams, , and planked over with 4in planking, , Such an opaning would coat about , £7SO, and to make a similar opening of , 13ft spin would co3t about £260. There would be no d image to the t breakwater from making such an opening, and, if required, it could easily be \ closed with rubble stone. 3 The object of making an opening in x the breakwater being to get rid of the , dredging, I am of opinion that even if } the opening did some good work in j carrying away the sand it would have , little effect in diminishing the necessity for dredging, , It might, however have a bad effeot , in raising a doubt as to the ntcjssity [. for dredging, as it might be difficult to j prove how much sand was carried by [ hale in the wall and how much re--3 quired dredging, which might give rise to a popular cry to cease dredging. l The port is now of too much importance to have the safety of its cavigaj tion endangered by wavering and unj certain counsels, and if dredging were Ru«pend<'d to await the result of ex- , periments the safety of the port mi'ht be compromised amidst the disputes as j to what is best to be done. , One knows by experience how un- | popular is constant dredging, and what , clamourous outcry there usually is to ' try all sorts of experiments suggested by persons who are in no way respon- '• siole for the results. \ Seeing bow successfully this port has been restored from a hopeless condition , to its present success and prosperity by a very moJeratJ amount of dredging, , and how es ly and cheaply dreJging is I done by modern sand pumps, I can see nothing formidable in the outlook which has to be faced to keep this port ' open by dredging. If the Board thinks i it worth while to try and relitve thej dredging by cutting a gap in the break- , water, there is no difficulty in doing so, j and I have quoted the probable cost of ( making i-uch a hole. I cannot recom- , mend the Board to spend £7BO in ,' muking what I consider a usdess exI pariment, but I would suggest that tbe . pipe'f the fixed pump be cut a; tbe! . pjsition uf the proposed opaning, and I | that sand be pumpei into the huibour) at this place. Observation of what be- j . comes of this sand may lead to further, enlightenment as to what would hap- j ( pun if the bole were cut in the break-1 water. As things are, one can only conjecture from one's experience, and! | although I believe it is just possible It bat the sand drift might pass harm-

lessly throught a gap 400 feet wide with the present length of breakwater, I I do not belibve it could paBS through a gap of 40 or 50 feel; without forming a , bank, which would block up the opening and obstruct the harbour unless , removod by dredging. 5 There is no difficulty in keeping this J port open by dredging, and there is sufficient fine weather and smooth sea during any year to allow the work to be done. It' the depth be determined . to suit the traffic of such draught as the port requires, then there should he a resarve of depth dredged out to allow for siltiog up in stormy weather; and considering how many fine ports have to depend for their existence on dredging, I cannot see that New Plymouth lis at anv great disadvantage in this respect, I O. Napier Bell, M. Thst., 0.. E. Mr Sarten, in the course of some I vehement remark?, expressed the ! opinion that Mr Bell had not made out la fair report on the hole-in-the-wall 'question. His report was simply a 'repetition of what Mr Marchant had I already given the Board. Mr Sarten ' considered Mr Bell should have dealt more fully with the hole. He (Mr 'Sarten) was prepared to put down ,£SOO to-morrow against Mr Bell or I Mr Marcbant to prove that his conten'tiou was correct. They (Mr Sarten did not specify who) had, it seemed to him, only got Mr Bell to report against the opening, so that they would have more money to spend on dredging. He had proved Mr Bell's theory to be wrong, both at Napier and Wanganui < He (the speaker) had spent a lot of money in oonnection with this matter, and considered that Mr Bell, in making up his report, had not treated him ' fairly at all. Mr Oonnett said the Board had ' done all it could to assist Mr Sarten, i and had obtained the most qualified opiciun available o* his scheme. No matter what their own opinions might be members should abidu by t'ie report comptUd by Mr Bell. Mr Ward said he hal always op- , posed the hole-in-the-walt, but had [ given way in order that a final report should be obtained on the matter. Had the report been favourable, he would have felt it his duty to support that ' report. As it was, Mr Sarten should . accept the decision expressed, whioh the Board as a whole mubt support. Tho Chairman moved that Mr Bell ' be thanked for his able report, dealing with the sand drift in the harbour, ' and with the hole-in-the-wall; and < that the report be printed. He pointed ' out that Mr Bell had been most careful in making up his report, and had fully consulted Mr Sarten's ideas in the matter of making an opening in tho Breakwater, Mr Sarten moved aa an amendment, " That this Board has no confidence in s the report of Mr Bell." He said it seemed to him that Mr Bell was not wanted to report fully on his (Mr Sarten's) scheme. Mr Sarten went on to t state that on the previous day, when ' he was at the Breakwater, he had at- } tempted to go to show M.r B*ll something, wben the chairman and Mr " Marcbant both interrupted him with f "Don't take bim there; we have already done so." I . Tbe Chairman explained that he and I P Mr Marchant had simply intercepted Mr Sirten out of consideration for Mr Bell's health, Tbut gentleman was still suffering from a severe illness, and as he had already inspected the spot in question very closely, they did . not consider it advisable to cause him . any undue exertion, having just j noticed signs of a temporary weakness. Messrs Hignett and McLean spoke in appreciation of the report, which, I it bad been understood, was to be fina*. t Mr McLean added that he considered ' Mr Sarten should apol'gi'e to the gentlemen he had named. Mr Sarten : I won't apologise to any j man when he tries to sit on me! And L I won't even apologise to you I Mr Price also spoke in appreciation , of tbe report, and Mr Marcbant gave a ' personal explanation in connection with ' th'j incident complained of by Mr Sarten. t Mr Sarten's amendment was not , seconded, and the chairman's motion . was carried, Mr Sarten being the only \ dissentient. i Mr Napier Bell then attended the meeting, when the above resolution was communicated to him. A resolu- | tion tbat he should be asked to report on Mr Marchant's scheme for harbour . extension was xlso submitted. I Mr Bell slid he would look into the matter at once, ' In reply to Mr Connect, Mr Bell said additional Accommodation could of ! course be obtained by extendiog the mole. He would also repot ton the question of connecting the Moturoa and Mikotahi sugarlotves. t Mr Bell asked what atone was available for the work, Mr Marohant said there was not a I I great quantity of suitable stone handy, ; but he would have to inquire into all available sources. Some general discussian followed on the subject, Mr Sarten asked Mr Bell if he , would reoommend th tt tbe stnd pump , pipe should bj taken to tbe place where <he (Mr Sarten)advocated the opening, , to test its effect on the sand. Mr Bell replied in the terms of his recommendation in his report, which was favourable. Tbe discussion then ended.