RIVER BANK PROTECTION.

Clutha Leader, Volume V, Issue 226, 8 November 1878, Page 5

RIVER BANK PROTECTION.

On Wednesday; evening tho River Bank "* Protection Committee received an elaborate report by Mr Higginson, C.E., upon the above subject. After mentioning his examination of the river, the. rapidity of the current, nature of bottom, &c, th© report proceeds : — ■ Upon ascertaining theso facts T did not see my way to recommend that any protective work should be at once put in hand in order to shield the loose banks now above water, however much the residents and owners of land naturally wished it, because it is evident to me that it would be throwing money away.' I however left my plan for fascine work protection in the hands of the secretary, so that should you think it worth while experimenting upon a "portion of the bank ' for a chain or two in length you can make what use of it you pleaseThere appear to be two ways in whioh works mis-lit be carried out with some hope of success, both of wliich act upon the main stream, by diverting which off the bank the latter may be protected with fascine work. The first method is to construct a massive groin, projecting from the last rocky spur of the hills sufficiently far out into the river to throw the stream on to the opposite shingle spit, thereby restoring the stream to its old channel. The second method is to construct a series of less expensive groins of shorter length at intervals along the bank, commencing near the hills and continuing as far as the clay bottom appears to exist, namely, where the flood channel broke across. These groins would be sufficiently near to eaoh other to receive and deflect the stream as it is thrown off each in turn, allowing the river to keep its present curved course which seems natural to it in turning such a sharp bend.The first-mentioned method I do not recommend, because of its probably enormous cost, for in order to act successfully on a river of such magnitude the groin would assume the form and dimensions of a marine breakwater, costing say LIOO per lineal foot if' rock of sufficiently good quality and size could be procured in proximity, which I doubt. Such a work would also require supplementing by other works lower down the river, as well as the protection to the bank. Its success would also probably depend upon the nature of tha foundation, which I have had no means of ascertaining. ' I however do not think it probable that the rock would be within reach sufficiently far out from the bank to give a foundation. It seems to dip very rapidly, so that it is almost impossible to assume the depth to wliich a rock groin might sink into the soft shingle before reaching the lowest point of scour — probibly 50 feet. The second method, whioh I think affords some hope of. being carried out with success and at a reasonable oost, is what I would recommend, as it appears to me to be a more rational proceeding to train a river of tbis description in its existing course than to attempt to foroe it out of it. In making the following proposals or suggestions as to the manner in which this should be carried out, I beg that it may be borne in mind, that I have only been able to examine the river in its present flooded state, and have only obtained such information as to the nature of ita bed and bank as could be gleaned from an examination of the portion now above water-mark and by sounding and probing the bottom when within reach by a pole. Supposing, therefore, that the bank and bed for some distance out is of stiff clay within re*ch of piles, I would advise that a series of groins be constructed at an ancrle of about 45 degrees down sti*eam, projecting 100 feet or "more into the river according to the nature of the bottom^ tbeir distance apart being in proportion to the length it is found possible to construct them.the result of whioh would be to push the main stream, awiy from the bank sufficiently far to maintain either dead water or a much reduced velocity alongside the bank between them, consequently causing a deposit to form inshore, or at all events allowing the banks to be efficiently protected by well constructed fascine work much in tho manner of the plan left in your hands. These groins might behest and cheapest constructed of pile work, well braced and strutted behind, but not too close together, in a double or treble row in order to obtain stability. Behind these groins I would securely attach by mooring •chains a row oi trees, inoluding branches, with their ste.ms up stream, and placed as close together as their branches would allow. The effect of this work would be, I think, to so far offer obstruction to the current as to gradually deflect it off the bank and cause comparatively still water below eaoh groin. In course of time during a fresh the branohes of the trees would . accumulate rubbish and floating suags, thereby gradually helping to obstruct the stream and as the flood subsided causing a deposit to form for gome distanco below, in which probably the fcroas would be partially buried. As the river subsided in the winter the holes probably formed by eddies between the trees and pile-work and at the back of the latter could ba filled up with layers of fascine, mattrasses and the best stone* procurable in alternate layers inorder to impart weight toit ; this would eventually render the whole groin a compact mass, tlie end of wliich would be the only weak point to be looked after. These groins would all be of the same nature as far as the ground seems firm, by which time the stream would have sufficient direction imparted to -it to shoot it across the bight (now cut out where the ground is bad and rotten) ou to the gravel point, which would, with pro. tectfon, act as a groin ; the length of soft bank could then also be protected so soon aa the velocity bf the stream acting on it is reduced. T find it impossible to give a reliable estimate of what such work would cost, but conld do soafter obtaining a plan. of tho exisMng river banks and survey of tne main stream, obtained either by soundings or otherwise, without which I can form no idea 'of the number of groins required, the length of the piles, or the length of the banks reqiiiring protection. I must here call attention to the inevitable result which must sooner or later ensue from taking no precautions to deal with this river in the manner and on the scale it requires ; it will, without donbt, cut a straight course through between the township and the hills, abandoning tbe old channel, and rendering the present r'aiiwfV" bridge useless. There is a decided channel now cut in the position indicated, -which with every successive high flood must be increased in size, and unless a reef of rock is found to exist below the superstratum of .alluvial soil, nothing ca.i prevent the river breaking through if left to its own devices as hitherto. Should the bank be permanently proteoted,* it might be advisable not to attempt to embank' high floods out of this channel, hut allow them to, flow over, as by turning the whole of these extraordinary floods through the railway bridge might be fatal to that structure, or at all events to the embankment at its southern end. The damage to the low ground by these floods passing over could, be almost entirely prevented by planting tbe whole thickly with oder willows, which would become invaluable for the repairs to the works. , ...,'".' I am. however confident that the river now seriously threatens the whole of the Government railway works in the valley, and that unless de« cided steps are at once taken to prevent it, an 7 enormous expenditure will shortly be necessary in order to build a new bridge. I am afraid that' the metho 1 suggested hy me for dealing with the river "will bo too costly for. the town to undertake, "unless assisted; by the Government, whose interests are if possible more seriously threatened; than those of the inhabitants 7 of the townshi-r themselves, as should the'river "-'. change "its course ; the del-i^and loss cause^by a.v : 7stoppaae of the rail way -traffic wpuld?^iffect'{th^97 4::-4 interests of nearly the wholep^pula'tion^so'uth ; of - ;; ■ : ; Dunedin. should ib occur A^ef.tli&Mned^^aeh;yy;y opened through !f or. sorne little; .tiuio?^ft|^:^vft^fc7 ..The only .tempi) rary measure; thatLlxcan s^ggest;?:^?? as likely .to lessen the .effpct."of7tlie^^^ : . the. -bat_k,7 and: ; once? is (as I stated . yesterdaylfflpstin^whd trees dbwii the:* river, and-"thn>i?n\oSm : .par-illeltpithgn^rMnkon .and Binlring?j-ii£H^ l thinkl.thUtrth^ ; floating them migh^eyovMoovS^^^ttoi^^i'li^^B^

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