THE LOWER HARBOUR.

Otago Daily Times, Issue 11189, 11 August 1898, Page 3

THE LOWER HARBOUR.

MR LESLIE RETNOLDS'S REPORT. Tho following repori by Mr Leslie Reynolds was yesterday handed to the secretary of'the Harbour Board: — The Chairman of the Works Committee, Otago Harbour Board. u"i~7 ll'- accordaßce with instructions recerved from your board's secretary on the 27th June, 1898, I have duly examined the Lower Harbour, and have now the honour to submit' my report -upon the beat' method of permanently straightening that portion of the chan-' pel extending between Harrington Point «>■,<! inn lightship. i Asyour board is doubtless familiar with lh" general conditions of this portion of the channel, it is, I judge, unnecessary for me to enter nto extended remarks or, the subject. 1 shall DnmuOrev Gu' bno% Wlth m[y the fading points which may. tn conjunction with the acco,np an}nng p , ans Nos , and 2> ag9 . Ht .c Z ifch mLinLv y r 0"8 '? nd the wnicn i uave to make P i^X aD. ex*mination °'" the plans and data, ex ending back for years, it is evident that the o? P nnTl I 0"" mV waterway are in a state of unstable equilibrium, and continually changing their positions. From Z 27 Should beUTcUd" 0113 Ch"B"» m°^ which is formed by a ] ong \ 0 ,« e. \ ti™ tending from the north Bp S following the channel as it cuts over towards the Concave or peninsular side. This fretting of the channel « duo to the centrifugal force! of the ebb and flood currents principally the former. When the bend in the channel becomes very pronounced the flood tide refußea to take the lons tortuous course, and forces a new cut through the neck of the spit. This accomplished it can do no more, and the ebb tide immediately commences an eastward fretting on the co^ cave side of the newly-formed line of channel and thus short cuts and bends follow each other- in succession. From the conditions govern mg the direction of the flood and ebb currents, it is impossible that a dredged channel through the .pit ca* have other than a shor. existence unless aided by works desired so as to bring the directions of the ingoin"°and outgoing currents more in harmony with°each other, and the line of desired fairway than naturally exists. From an engineering pomt of view, the training of both currents into one line is really a very simple raattei, and there are two patent methods of accomplishing this Ihese methods consist, in the one case of training by means of a wall projecting from the shore, and thence following a direction parallel to tho line of channel desired to be established. The second method is by means of constructing spurs or groynes extending from the Kaik beach as far out as the line of proposed channel. Upon purely scientific principles the former of the two methods is to be favoured. As regards a matter of cost, however, the system of groyning will be found to be considerably loss than that of constructing a wall, and this difference in the works, which shall be presently recommended, in my opinion, much more than compensates for the slight advantages with regard to regularity of channel, etc., which a wall would ensure, beyond that which v/ould be brought about by

groynes.. TJponplan No. 1 1 nave shown a lino of wall, and the groynes A, B, C, together with the secondary groynes D and E, and, were ! either system adopted, the result to the chan- ; nel would be practically the same. A wall of shorter length than shown would not, I consider, bring about the improvements aimed at, nor would a less number of groynes meet the case. As has already been explained, the fretting of the channel towards the peninsular side is due to the natural cweep and the centrifugal forces of the in and out-flowing currents, the mean velocity of which is practically at the rate of 1| knots per hour, although at the maximum their velocity will doubtless reach to say 3 knots per hour. All that is required, in my opinion, to straighten up the line of fairway is to slacken, or kill, the velocity of the currents running, with centrifugal force, into the bend. There can r be no question that large quantities of sand traverse the channel with incoming and outgoing waters. This was demonstrated by observations taken by myself during very calm conditions, and I found that fine sand; in this instance, was in suspension at a depth of only 38ft below the surface. During times of rough weather on the coast, or even under normal conditions, a very considerable travel of sand m suspension takes place. This was the case during the rough weather which .occurred last : week, when the water in the Lower Harbour : appeared quite milky. Were the currents which flow round the bend reduced, as abovo alluded, to, the sand traversing over the area affected would settle and cause natural shoaling of the bend, which would be accompanied • by a corresponding deepening of the opposite side of the channel, and fretting of the toe cf the spit. To obtain this through deflection of the currents is the object sought to be accom- : plished, by either training walls.or groynes. : i io construct either a training wall or groynes ! as laid down on the plan, of rubble, or of. rubble and clay, as has been suggested, would entail an expenditure "which I do not consider necessary, as, in my opinion, all that is required is to . nullify those portions of the currents which ' ; cling to the concave side of the bend, and this ! could be effected by the construction of groynes consisting of netting of strong wire, specially ' woven for the purpose, located as indicated by letters A, B, C, on plan No. 1. The effect of ! wire netting in a waterway is very similar to ; that of scrub brcakwinds on a sandy beach. In the latter case the wind force, and in the former that of tire current is killed, and where this happens and there is sand in suspension, deposit takes place. I have calculated the approx>mate result, to the flow ,of current, were netting of IJid mesh stretched across the waterway, and find that it would be reduced to from one-fourth to one-fifth of its original velocity. This is assuming that the netting. remains free from seaweed, which in all probability would not be the case.and the velocity from . this cause might be still further reduced. There j are instances where very troublesome river curI rents have been wholly deflected .by methods ; similar to the above, and recently I advised r works of v like Tiature, which, I understand, have proved entirely successful. I recommend that the spurs shown on the t plan be con-, shucted in .the following manner:—A/ carpeting of fascine.mattress weighted with' stones,''or old rails, be first laid .along tho bottom, on the'line of proposed groyne; following this piles be , driven at intervals of from 25ft to 30ft, and the. j netting be stretched between "these, looped to j the piles, and suitably weighted so as to. drop | to the bottom of the channel. The upright po- ! sition of the netting would be maintained by | 3in by 2in battens stapled to it. The heights of j these groynes could be raised by adding other i widths of netting as required. Assuming that, / in the first instance, the width of netting used I was Bft or 9ft. This would be left."until' the | shoaling reached nearly to the top. when a seconi strip.of netting would be slipped over thepiles, in the same manner as the first, till the required height of groyning was attained. Referring to plan No. 2, it will be seen that I recommend that the groynes be constructed to a height of 10ft below low water, at their extreme ! ends, and at their roots to lft or 2ft below low • water level Were the groynes constructed to j the. length shown on the'plans., and to the ! above-mentioned heights, the area of present I waterway, which- would be shut out by them, ~ would represent about one-third of the whole, j and Ido not. consider it necessary nor advisable ; to recommend that a greater proportion of the sectional area of the channel be interfered with.' One of the principal points to be guarded against in the carrying out of any works to straighten the channel, is to carry out the work in such a manner as would not interfere with the navigation course. In order to avoid this, the groynes, in the first instance, would stop short of their most suitable lengths until the deflected currents had edged the channel over sufficiently 'to admit of t heir J being extended. For instance, commencing with groyne A on plan, this would be constructed channelward frohr the' Katie jetty in the first case for a distance of 13 chains; groynes B and C each for 10 chains. In the next extension the first groyne would be extended its full distance of 18 chains,, while an additional five chains "blight be added simultaneously to the other two; and the remaining distance to complete these added when the channel had further cut'across to the west , side.. It trill'be necessary to protect frmn scour, by substantial'rubble, mounds, the temporary, , as also the finished ends of the groynes. It will also be necessary when silting has occurred to the full height of. the groynes to protect.the top of the groynes ngainst the; scouring action, of the currents. -This would bo accomplished by the fascine carpeting and rubble, as shown in section of groynes, plan No. 2. ' . By adopting this' system pf extending the works, piece by piece, the necessity for dredging_ a new channel through the spit is obviated. This would be a- very costly matter." entailing a probable expenditure of £10,000. When considering the question of recommending works, such as the above explained. I made full inquiries with regard to the effect which might be expected from seas in the locality! On this point I am assured by the board's inspector. (Mr Stephens) that their range is not such as would be likely to in any degree affect such a structure, especially at the depth to which I propose they should be built. If your board should have any doubt as to the efficacy of this method, it would be a simple and inexpensive experiment to construct some five chains on the Kaik beach, and by utilising old rails—which I believe your board have in hand—the experiment could be made for the sum of £60. As to the probable time it would take for the natural agencies' to bring about the shoaling desired around these groynes, it would bo impossible to way with any degree of certainty; but should a year or two elapse before a deposit of Bft or 20ffc ;took place against the works, as they wouW be constructed in the first instance, the result to the channel meanwhile could riot, in my opinion, fail to be satisfactory, as the netting itself, irrespective of the sand deposit, would deflect a large body of the water against tho toe of the spit, and thereby considerably lessen the bend so detrimental to navigation. The groynes are shown on plan to extend chanhelward, so as to brins the channel into what may be termed an ideal line; but I question if it Would be fotind necessary to extend them much beyond the first atagre- aS ■ shown upon plan No. ?.. When a desired distance has been attained it would-be a decided advantage to. aubdivicis- the stretches between, groynes A B C by the secondary groynes D and X . . . . ■ I estimate tin' cost of the works which I have described as follows.--' • . ■■■■■■■ Groynes A, B, and C, including opening of quarry and supervision, constructed as far as stage lon plan .. £2,550 To complete above groynes- to full .length .. 2J160 Secondary groynes, D, B ... .. 600 Total .. ~ .... £5,300 In the above estimates are included the cost of-hardwood piles throughout the full extent of the work; but as your board has a quantity of old rails in hand, were these used, where suitable, the cost. would be. appreciable reduced. The above works, if constructed of rubble and quarry stripping, would 7 ] estimate-, cost about .. ' ..r £18,500 A longitudinal wall of the Same ma- v terial about : • .. £22,000 so that tho advantages in the way of cost, by the system which I recommend, are so obvious that no further comment is necessary. I wish to record my thanks to."Mr- Stephens, who. has most willingly assisted'me in obtaining the necessary data.—l have, &c, Leslie H. Reynolds. Dunedin, August S, 1898.