HARBOUR IMPROVEMENT.

Timaru Herald, Volume XCVIII, Issue 18084, 30 June 1924, Page 4

HARBOUR IMPROVEMENT.

THE ROYAL COMMISSION’S REPORT.

Neither Scheme Approved.

FURTHER INVESTIGATIONS RECOMMENDED.

Further investigations recommended Axesrs C. J. x(- Williams, la. \ lcnerman, and J. D. (dimes, wnu were appomuxt a itoyal.tJommissj.oii ro ie|)uit on senexues of namour extension at' Timaru, nave submitted tneir repom. it begins Avnm a brief History oi me worKs earned out unit goes on to deal witii present conditions. i'rom tne evidence tuiten the Comnnssioneis conclude tiiali tub' heaviest seas roll iu towards 4 unaru Harbour from a soutneasteriy to easterly direction, and otten come witnout tne originating wind reaclung Ximaru. Sucli- seas strike tne Eastern Extension, dashing against and over it.- Theipart ot tlie wave passing tne breakwater runs on towards tne cliff ’ winch extends from the ‘•Dashing Rocks’? southwards, and en route,'. owing to the formation of the sea bed, swings.-towards the Waimataitai Jieacli and iCaroime Bay, and the entrance to the inner basin. Part oi the residual wave enters the basin in a southerly direction, running along the concrete breakwater behind No. t wharf, along which it runs without further diminution. . From observations made on May 13th., state the Commissioners, we estimate the ratio of reduction when the seas come from the.magnetic east at .244 from outside the. eastern extension to the entrance, of the inner basin; and .71 from the entrance to No. 1 wharf, or a total reduction of .173. The seas driving one behind the other against the cliff south of the “Dashing Rocks” result in the heaping up of the water in the neighbourhood, and this generates a southerly, current through the sheltered area past Caroline Day and along the North Mole, past the entrance of the inner basin. This current carries a considerable quantity of sand in suspension, which is deposited off the entrance of the inner basin, and between that point and the end of tho eastern extension. A further effect of the seas striking the “Dashing Rocks cliff has been described to us as a rebound, creating waves which can be traced across the bay and portions of which can be. seen to enter the inner The deposits of sand and silt appear to have accumulated during the last fourteen years at the following rate:—Caroline Bay, 96,500 tons per annum ;" ’in the dredged channel, 63,000 tons per annum; in the area between the dredged channel and the breakwater, 51,500 tons per. annum; total, 216,000 tons per annum. The shoaling in Caroline Bay is fairly distributed over the bay, in a north and south direction, but is more marked towards the shore than further out. The shoaling,, in the dredged channel follows, to a certain extent, the shoaling in the Bay, being more rapid at the inner end of the channel than at the outer end. The shoaling in the area between the dredged channel and the eastern extension is very marked immediately under the ee ;ot the mole, where it forms a definite mound, with from 2ft to 31t of water on it at low water, sloping off giadually towards the dredged channel, the greatest accumulation being at tho bend of the mole. Our statements as to the ratios and the phenomena of shoaling are based on the very complete records of soundings of the harbour made by . the Board’s engineers from time to time. THE EVIDENCE ANALYSED.

Analysing the evidence on which the übove description ot the present mute ot aitaira is based, tne Hoard. s engineer, harbourmaster, chairman, ami secretary, Mr Hibson, and Mi Bradley agree that they Have observed urn resulting current irom the Heaping of tlie seas at the Dashing Hocks" ciiir, passing the north mole at rates varying from li to k£ nnies per hour. „ , , With regard to the reflected waves from the "Dashing Hocks” cliff coming across tlie Hay anti entering the lniiei harbour, vve Have the evidence of the harbourmaster, who considers this the mam cause of the range, m the nmol basin during heavy seas, especially at high-water; of the engineer, who considers that this has a considerable effect on the range in the inner haibour; and of Mr Hobson, who states that lie has observed the reflected wavo and traced it across the Bay. On the other hand, Mr Bradley describes the reflected wave from the Dashing Rocks” as a “lazy little wave, which does no damage,” and other witnesses failed to observe this phenomenon. The other phenomena referred to in the evidence have been observed by members of the Commission, or check ed from office records, or are so "ell known, and so in accordance with oidlnary experience, as not to require special mention at this point. The matters now exercising tlie mmds of Hoard members in regard to the improvement of their harbour aio as follows, the more important being items (1) and (2) : . „ r (1) The silting occurring in Caroline Bav and in the dredged channel, with the consequent and considerable an nuai expense for dredging, which amounted to about £I2,OUU in 192 d. (2) The range experienced m tlie basin, amounting to 6 inches U 1 calnl weather, and stated to be from 3ft to 4ft 6in. in heavy seas, which at all limes necessitates special precautions and care in mooring vessels, thus involving considerable expense, including the sum of £SOO per annum m the purchase of hawsers alone. A reduction to a range of not more than hail that now obtaining is necessary to enable vessels to work continuously at No. 1 wharf. . , (3) The possibility of increasing the depth of water in the channel, which is now such that a vessel drawing _o feet 6 inches, recently entered the haibour during calm weather, a ‘ ta ° u S' the standard draught for the poit is 24ft Gin. , . , (-T) The possibility of increased accommodation for berthing vessels. Iho present accommodation amounts _ to some 3300 lineal feet, situated m a basin 50 acres in extent, m which a further wharfage of 2000 lineal feel can lie easily provided.

INLUENCES AFFECTING THE PROBLEM: The natural influence most vitally affecting the comfort and security oi the harbour is the commonly occurring ocean roll, which is seldom less than live feet in height, and is stated to bo from 18 feet to 20 feet during the heavy seas which occur from time to time, from E.S.E., and, less frequentjv from further cast round to ha magnetic E.N.E. This roll strikes l ie coast somewhat obliquely, and is “in. responsible for the continual northorlj travel of the shingle brought down by the various rivers debouching along the coast. It is also responsible for the range which formerly made lightoring impracticable in the open sea, as referred above. Works have boon built, and have from time to time boon os (ended trim the object of giving shelter iron) the rniigo, and preventing the shingle irom „.irking round and Ghouling tin: shin

ping berths. These works Bare been the means of improving the shelter so considerably that since their completion no vessels have had to be sent to sea for safety during storms. limy have also been fully effective in stem;ming the travelling shingle, r.ud though it is accumulating along the foreshore, it will be many years bcfoie it reaches out to and can travel round the end of the eastern extension. SECONDARY EFFECTS.

Besides producing the abovo desired results, certain secondary eflorts have followed which are having an infiuenco on the working and future of the port, and it is chiefly towards overcoming these that operations are now proposed. Tho secondary effects referred to are as follows: — . (1) The deposition of sand and mud water and the consequent shoaling ot the channel and Caroline Bay, and He ground under the lco of the eastern extension breakwater. As desciibed elsewhere in this report .the occurrences in Caroline Bay and tho dredged channel are compatible with the inateiiar having been thrown up directly from the ocean by wave action, as well as by deposit from the current .sweeping from Caroline Bay towards the entrance behind tho lee of the break water. The sand has; evidently been forced through and thrown over by tne heavy seas, and has dropped and formed a bank immediately on reaching calm water, the deposit being greatest where tho sea’s stroke is greatest. An examination of old plans and soundings shows a progressive growth of the :le-. posit at all the places mentioned, and does not appear to support the view that tho rate of deposition is increasing. The records of the deposits, of which the quantities have already been stated, show that during the 14 years, 1909-1923, roughly ono quarter of a million tons have come over the breakwater, and have so far—though ; that will not; continue-indefinitely—necessi-tated little or no dredging; and that the balance has conic in between tho end of the breakwater and the •Dashing Rocks.’ Approximately one third of this balance. has had to be removed by dredging in order to keep the channel clear. COUNTER CURRENTS.

(2) The counter current' from the ‘Dashing Rocks,’ via Caroline Bay and the north mole, towards the ,end of the oastern extension. As already described this current must be an active agent in , carrying sand and hit, and in depositing Them along the. Bay and in the dredging channel* $ (3) By reason of the waves, which outside havo their crests in a N.N.E. S.S.W. direction, expanding or wheel- , ing after passing‘ to the end of aho eastern extension of the breakwater sq,as. to follow along it. and ultimately break on the beach with their crests in a north-south direction, a wave, though much reduced in range, meets and runs into the basin entrance. Owing to tho angle of the entrance directing the incoming waves along the concrete wall, little further dispension can occur on the eastern side. It is this which so materially effects vessels lying at the southern (No.l) wliarf. (4) The rebounding wave from iho ‘Dashing Rocks.’ As described efore, the effect of this must be additional to that of the wave referred to under (3) above, but no figures as to its dimensions are available, and v g are unable to assess its true value. THE FINDINGS. The findings of the Commission as follow : The “Dashing Rocks” scheme includes a mole, commencing at a point about 400 feet southward ot r.ho ‘Dashing Rocks,’ and extending towards a point 450 feet inside , the termination of the eastern extension mole, and distant about 1600 ieet from the end of that mole. It would be constructed of large rubble in front of 1-20 ton blocks tipped from staging to a height of ten feet above high water, with a top width of 14 feet, and side slopes ot 1J to 1. At the end ol this mole would bp an entrance f M feet wide between the-end of it and a spur, extending on the same line frorn a point on the eastern extension T;0 feet inside the end of the latter. This spur would be 900 feet long, and would be constructed in the. same manner as tho ‘Dashing Rocks' mole, but tho stone would not require to be so heavy. A further spur, from die end of tho eastern extension, would be carried lor 1000 foot in a direction oi about N. 35 degrees E., and would be constructed in the same manner as tho ‘Dashing Rocks’ mole, only larger stono being used in it. Messrs Dobson and Clarke .suggest that, alter the “Dashing Rocks’’ mole has been completed, the necessity ;oi and the ultimate directions of the olivet spurs would be considered in the light of the results. ‘Wo propose, however, to consider tho complete scheme as shown on the plan, and to advise as to the results which might bo anticipated from it. These we find would be. as follows: (a) The swirl round Caroline Bay and the deposit of sand and mud in the dredged channel from this souroo would bi climi|nated. (b) The range in the inner harbour, due to the swing of the seas round the eastern extension, would be very considerably reduced. We would expect a reduction of fmnn 20 feet outside to about 1 foot 8 hnches at No. 1 wharf, (c) Whatever rango in the inner harbour is due to the rebound of the seas from ‘Dashing Rock-.;’ would be eliminated, (d) The dredg- , ing iu the channel between the two entrances would be reduced by at least ; one-lialf.

It has been thought in some quarters that the enclosure of such a considerable area would result in a tidal scour, which would itself keep the entrance open, blit calculation snows that so long as the entrance has to he kern the size necessary for navigation, t!in tidal current and scouring effect woubl bo comparatively small. hoars huvo also been expressed that the Ray v.dl l become a mud' flat, but we considoi that, whilst there certainly would bo u slow deposition of mud in the deejiei parts, the residual waves through tho entrance, arid those produced uy the wind inside the enclosure, should prove sufficient to keep a beach < I washed sand, and to confine the muddy deposits to the deeper wafer, As n means of providing deeper water there will lie no automatic improvement, but tho eastern extension spur will allow the dredge to operate further out than is now the case, and, if desired, to i increase tho depth to 28 feet at lew I water, I ACCOMMODATION, i In (lie matter of accommodation, •he j am minuted reduction in range n 11 i Kinder No. j wharf available at all I i:mev, and nothing neon bo done In i tlio way of providing further nocoiu- ; modal iihi ior many years. | With regard to cost, wo have exam | inod the ,|u;tii'.v nilos, and the aim-.., , i hereto, ami l<o, u also bail before u

particulars of the work previously. carried out, and, after carefully investigating thc.se, concur in the estimate ct cost made by Mr Clarke. Our examination of the quarry sites leads us to Unbelief that no difficulty will bo met with in obtaining the necessary stone. We-therefore find:. ' That the “Dashing Rocks” scheme, in its complete form, is sound from an engineering point of view. That it is quite practicable. ■... That it will effect the purposes foi . which is was devised. That the estimated cost is reasonable.

Wo would point out that in our opinion it will be found that the full height of the ‘Dashing Rocks’ mole provided by Mr Clarke will not be required throughout its full length, -lid we think that in this way the cost of the scheme may prove to be less than has been estimated. NORTH MOLE EXTENSION. This scheme comprises an extension of tiio north mole, starting from a point a little inside the curve in 'lie present mole, and running northwards for about SOU feet, then curving round to a lino parallel with tho north molo and back to the same point as it was to have been reached bv the ‘Dashing Rocks’ mole in scheme No. 1. The inner and outer spur walls, from the eastern extension, are also included, as in scheme No. 1, tho entrance being 800 feet wide. As before, we have carefully eon-' sidered this, scheme, and are of opinion that it would not be 'successful, as ■ ho north mole extension would not only prevent the lateral dispersion of live waves coming through the outer entrance, but would actually direct them towards and into tho inner basin. In other respects it would operato in the same manner as scheme No. 1, but for the reason stated, we cannot approve of it. If this wall, instead of turning towards- the north mole, ran towards Benvonue Point, the above objection would disappear. But, if the effects ol the seas striking the ‘Dashing Rocks,’ as described to us in the'evidence, are as important as that evidence indicates, an island mole, such as this would then be, would not be effective in preventing tho conveyance of silt into the channel, although it would prevent tho wave , reflection from • the “Dashing Rocks” reaching the inner harbour. It would not oven then - .reduce the range reaching the inner harbour to the same extent as would the “Dashing Rocks” scheme. In conclusion, while we have found ourselves able to advise that of the two schemes the “Dashing Rocks” proposal, as well as being practicable, v>i|) effect all the purposes for which it was devised, we are not satisfied that'tho relief desired by the Board cannot*, be obtained at much lass cost. We we also, in the absence of personal observation of the port under heavy weather conditions, unable to express an Opinion whether the drawbacks now existing are as acute as has been ie- ■ presented to us. _ We consider, too. that in a matter involving an expenditure of nearly £400,000, further , engineering investigations should be made with a l -view ‘to other 'schemes being devised which would give results more commensurate with their cost.

~For the above reasons we cannot give our approval to either of the schemes submitted to us.