TIMARU HARBOR WORKS.

Temuka Leader, Issue 2811, 4 May 1895, Page 2

TIMARU HARBOR WORKS.

The following report by Mr J. P. Maxwell, Consulting Engineer to the Timaru Harbour Board, has bceu distributed to the members of the Board, and will be read at their meeting on the 14th May : " Tiraarn, April 25th, 1895. " The Chairman, Timaru Harbour Board. " Sir, —I have the honour to report that I arrived in Timaru on the I9th inst., and since that date have been occupied in examining various report and plans handed to me by the secretarys and in examining the harbour works,, plant, etc. The travel of detritus along this coast is now so well recognised that 1" need not offer any proofs, but it is essential to my report to allude to its character. The detritus ranges from heavy shingle, mixed with coarse gravel or sand, requiring the impact of the waves to drive it along the sea's bottom and foreshore, to fine silt held in suspension, and carried by light currents. Of the latter it is not my intention to speak further now. The belt of travelling detritus varies much in breadth, the coarse shingle and heavier parts most occupying the higher portion of the beach, and retaining a steeper slope above water than the talus of finer material, which extends under water sometimes many hundreds of feet. Though modified by special conditions, as a rule, in the indentations and, where the average travel is slow, the belt is widest, at the headlands and where the travel is quick, it is narrow. Off rocky headlands, where the water is deep, all the belt may practically travel below water. Mr O'Connor estimated the average amount to eventually pass "the breakwater at 135,000 tons annually. Mr Goodall and Mr Marchart have estimated it at less. Messrs Wilson and Napier |Bell support Mr O'Connor. 1 am disposed to think that Mr O'Connor may be under the mark, judging from the extensive deposits in Caroline Bay. The top of the beach distant from the base of the breakwater recorded in your office was : January, 1888 550 feet January, 1891 880 feet November, 1894 ... 910 feet April, 1895 230 feet The toe of the steep beach in November, 1894, is stated to have been about 1120 feet out, the commencement of the curve being at 1260 feet. It is probable that the belt of travelling detritus will be very narrow along the outer arm of the breakwater, possibly it may all pass below water. Experience shows us that shingle drifts very freely in heavybreaking seas up to 8 fathoms of water; in south-easterly seas the detritus may be expected to travel rapidly along the outer arm, spreading at the end, and being partly driven onwards and across the entrance, and partly deposited under the shelter of the end. In easterly seas, deposits brought by the south-easterly seas will be ddveu more directly across" Caroline Fay and into theentrance. The question of the action of the detritus at the entrance is one which, so far as I can learn, does not seem to hare been closely discussed by the engiueers who have previously reported on the harbour. My observations for some time past have led me to conclude that the finer detritus was pssing the end and crossing Caroline Bay in appreciable quantities G or 7 years ago. It has been gradually increasing, and ! latterly the shingle and coarse sand have also been passing. On the 23rd and 24th I insts. I examined the material outside the I etitrauco. At 800 feat off the end of the breakwater the grab brought up coarse sand and gravel and a littlo shingle; at 300 feet oil" the bend of the north mole the grab brought up shingle; at 400 feet off the end of the breakwater the Timaru pumped about 150 tons of shingle, gravel, and sand in an hour. The position of the south beach is now such that the maximum movement vouud the end will soou be reached. Observations go to show that very large quantities of detritus are intermittently moved in storms. A movement of 20,000 or 30,000 tons along the outer eud during a storm might, at any time, bo expected. It is not possible to predict what area this may spread over, or the form the deposit may take, but wo must expect it to occupy some acres around and in the entrance. A quantity of 10,000 cubic yards, about. Ift. deep would occupy about 0 acres ; a deplh of 2 it over 0 acro3 would represent about 20,000 cubic yards, or say 30.000 tons. The latter quantity roughly represents the amount which the entrance and the vicinity has shoaled. The inference we may draw is, that if about the entrance the bottom is kept dredged to 22 or 23 fejt, the depth it formerly was, that 30,000 or 40,000 tons arriving in one storm will not interfere with navigation up to 19 or 20 feet at low water. But as there would be an active tendency for a spit to form, dredging is needed to maintain the proper depth. Of late years I have come to the conclusion that there will be no sudden or overwhelming stoppage of the entrance if the dredging is properly attended to. The position of the south beach is such that very large quantities of detritus must be pa-sing the end of the breakwater in storms ; probably there is more going past than is being retained south. The beach-line will not bo quite stable because S.E. seas drive the detritus along, while easterly seas pile it up from 12 to 15 feet above h.w. mark. During easterly seas a large quantity of material may be thrown over the breakwater end, which will hive to be dredged from inside. The low water mark iu Caroline Bay is now outside the 2-fathoni lino of a few yoars back. Carolino Bay was denuded of detritus 3oon aftor the works were begun, and the sea broke under the railway bridges at low water. Wo should probably find on getting fresh soundings that there is not. far short of half a million cubic yards of sand and shinglo deposited there, which has practically ill passed round the breakwater. Sn long as there is room for the detritus passing the tin! of the breakwater to get away, the heavy S.E. and E. seas will drive tho greater part onwards, and across the harbor entrance. T see no reason to suppose that, for a long time to come, the quantity to bo dredged will be much in exe.a i "f juceit accumulations, but, r.iilnx i'i ' uiueh has been allowed to aeeuiii'.iiato ..'. .a:el •'.. , '<<!ind the entrance, ■i:i,i ilia, ;.i.:i- r :imved at once. As (.!;,:.>'! e '•••: ::'.•■; more lillt'd up i'i-:. v.'.W a ■ .!■! -i •<.■;• for a shingla beaah ro :.. ■ , ,■•■<: -a a c;i, v,-d lino we i "!■!•,• :' * ' :■- ' ■■' bend of tht! north :i:■,!;■ ! w. ' ' ■'! ' o between Whales Ci\v ! : .!)■' I'ut this process wil! b - a '..':'.. -on •. ;qid it nm.-.t bo coi- ;■■•■ ■:■.-.: :•-.■ : a ; .\ Who: her the .jii-'-.o ■> .■' :» : i • ' '.ti'oliou Hay be ;n'i'. i ' : :.. ::' '.iliKt tons a year, it will, o;i ; ::■• ' •■■ ■!■. : t; [.eolaibly as eonv;: .■ :i: ' • ■ ■ '■ •• i' tha-e as. ( ,a the , ..i'i .-i i ; .i ,oe s;.r:>' which thine;., iiavo ,:.,.-.■ ■. ■;,.> o. I have considered Mr M ;.'<- ; >.i:i;' ■ ;: -Da ad; Very closely, and a! • > Mr !'■■ . i i! > in' thi.-.k that e.'h'T of ;':• a: w.aibi inert, the else fully. ,\ .i;!i ■■" '.i •■ .1 i :. •.'• lnieacepi, more than a ;> . ;i".. .:..'. i liing dotmus, and t:x-ten.-:ivo dredging ut the entrance. woulduo\

be avoided. I, therefore, recoinmeu your board, for the present, to cou centrate its operations on dredgiu, at the entrance, about the are shaded on the general plan at tached. A careful series of soundiugs shoul be taken over Caroline Bay, extondiug be yond the entrance, and same on the soutl Bide. They should be accurately plottei on a general plan with soundings taker before the works were begun, so that i more accurate a3timate of the change which have occurred may be arrived a than ia at present available, and so that i may be used for information and guidauci in dealing with future operations. It i of the greatest importance that the board should have this information for futun guidance. A proposal has been made foi extending the breakwater at the bend. 1 do not think this would serve any uaefu purpose. The protection of the eutranc< has occupied my attention. It is risky foi large steamers entering and leaving ii heavy weather, and is so situated as tc admit a heavy Bwell with easterly seaa causing a severe send in the harbor. Tht send in the harbor is such as to compe. large steamers to leave at a time when il is least desirable to do so. An extension of the breakwater would improve tht entrance and diminish the send. It should be in a direction that will not interfere with the course of ships as they now entet the port, but it should not be more easterly than is necessary for that purpose. It should not extend more than 600 ft beyond the present end, as it is undesirable to place the bend of the North Mole too much under shelter. I have marked such an extension on the lithographed plan compiled by Mr Mar chant in 1887. The extension may lead to shingle piling up along the outer arm of the breakwater, which would be washed over in storms, and it would, therefore, be necessary to raise it. I have considered constructing this with random rubble blocks, carried to 15 feet above h.w., which would be cheaper than concrete, if the stone is obtainable on reasonable terms. There is some difficulty in altering the system of extension, as the rubble must be deposited from a staging to get the proper base and slopes. 1 estimate this work to cost about £50,000. The dredge Timaru did not work well at the trial I witnessed, but this was due to a defect in the winch, and not to the pump, which pumped the shingle in great volumes when the suction pipe was properly trained. With a little experience in placing the pipe and handling the apparatus there can be no difficulty in lifting and removing 1500 to 2000 tons a day. A dredge lifting 1500 tons a day for 150 days a year means a total of 225,000 tons, which is roughly represented by an area of 30 acres, 3 feet deep. This is an immense dredging power, and there should be no difficulty with it in providing room for all the detritus passing the end to distribute itself over, without any encumbrance to the navigation of the harbour. Arrangements should be made for working the Timaru systematically at and around the entrance for the next six months. If the material is unsuitable in places for the pump the grabs should be used. In case the Timaru has to be laid up for repairs at any time, I should recommend having the Tauiwha overhauled and put in order, so that she may be put in commission if required. It is desirable to obtain two additional grab buckets suitable for sand dredging. The breakwater structure seems iu good condition, but some slight repairs to the monolith should be effected where the seas have torn the surface. " I have the honour to be, Sir, "ionr most obedient servant, "J. P. Maxwell, M. Inst. C.E " "I have to thank Mr Marchant, M Inst. C.E., for information on various points."