THE PROTECTION OF THE BANKS OF THE BULLER RIVER.

Colonist, Volume XII, Issue 1175, 29 December 1868, Page 3

THE PROTECTION OF THE BANKS OF THE BULLER RIVER.

REPORT BY THE PROVINCIAL ENGINEER,

(From the Provincial Government Gazette.) To His Honor the Superintendent. Sib, —I have the honor to forward, for the information of His Honor the Superintendent, the following report on the damage done to the banks of the river Bailer, and on the moans pr'opospd for protecting them against further encroachments of the river. In accordance with instructions received from His Honor, I proceeded to Westporfc on the 7th November, and remained at that place for a period of eight;, days, during which I was employed in taking a number of necessaiy observations. I found that the jetties, lately constructed, have been almost entirely destroyed; a portion of the lower one yet remains, considerably out of level, to which has been fitted a temporary stage, so that it can be used as a landing wharf. * number of piles of No. 3 jetty are still standing; this structure was in progress on the occasion of the last heavy flood of 14th October, and a temporary staging has ilso been fixed to it, where goods can also be landed. The tracing forwarded by Mr. Lowe very correctly shows the nature and extent of the damage done by the last floods, and it is evident that the damage is gradually extending along the river bank upwards. The greatest amount of destruction h>is taken place a!"mg a length of about 30 chains above GHadstonestreet, and we may expect still further ravages along the lower 15 to 20 chains of this length, where the river will form a still deeper curve; the rate of destruction will not, I apprehend, be so great as it has hitherto been; nor do I share'the apprehensions said to be entertained by the inhabitants of Westport that the river will cut a new channel through the town and North. Spit, and thus destroy the town. In order to arrive at any just conclusion as to the nature of any future protective works, it will be necos3ary to examine and consider carefully the manner in which the destruction of the river banks is accomplished, which I will briefly explain:—During . a heavy flood, the river bed. is scoured out to a great depth, the scouring action being greatest close to the bank, which is thus cut away to an abnost perpendicular face, and to a depth (as explained in Mr. Lowe's report) of 30 feet below low water line. As the flood subsides the scojtw'ng action becomes less and less, and the river gradually deposits gravel in the deepest parts, hejng unable to carry it forward ; the result of this being that a set of soundings, taken sometime after a heavy flood, will give a greatest depth of 17 or 18 feet, in place of 30 feet as above, stated, shewing a deposit of loose material of> depth of 12. feet. As the flood subsides tb,fc upper, parts of the bank fall in, a process wiuchis immensely accelerated by the aption of a,&w,ell>froni the, sea. whicji extends a considerable. cliatance.up, the river, '; and causes, at every flow of tl^eJiide, a, tolerably heavy surf to beat against the iqjrighi bank. The. action of this is to. und^mine. thp. bank about 6 to, 7 ieelj ; below, its su^ce. apd; to cause large masses of it to, i fall in, which are gradually crumbled; a,w,ay, by : the. • sui'f; or carried away bodily by, the. next i flood. This action of-the siys, if'left tp, itself; wns3 ;be to. form a fla,ttish ; shore, tp, the, river bank,,, fta . destructive effect (apsu'fc from. flpods) oauld be 1 arrested, by, simple. a,nd; comparatively- inexpensive niea,ns. llt i;ein^iTj,s then, to. point out some means by which j th,e. scow during floods can also be arrested, and this hpmg accomplished, the safety of the banks will be scoured. The ordinary piled and plankocl breastwqrl?, w.olt filled in behind, would appear to be (ja^iMed to. rcsiit both, destroying agencies^ $ '^ fowa

deep Enoughs but it would require to be cawiccl oufi in sucli a-"way and on such a scale, as would render it very Costly; for, although I have stated 30 feet as Ihe greatest depth of scour at low water, as yet experienced/this by no means points out the extreme limit which should be provided for, judging from the extremely treacherous and shifting nature of the ■river bottom, which appears to be little better than quicksand during floods. { The peculiar form of breastwork I should recommend, were this plan of protection adopted, would be to drive piles, at least 60 feet long, on a moderate inclination, all along the face of the work at a distance of about 3 feet from centre to centre, the top ends being surmounted by a capping piece, and stvatigthened by land ties. Behind these I should fill in large pieces of rock, forming, in fact, a rough stone wail, say of 4 feet in thickness, and in front of the piles woiikl also lay in an angle or footing of rocks, to resist the down ward scour of the water. These, as well as the rocks behind the piles, would gradually settle during the first few floods till they became firmly bedded and immovaeble. The permanence of this work would be limited only by the decay of the timber used in its construction., ■ ■ , I have not yet ascertained, but search is now being made to determine, whether rock can be obtained at an available distance from West-port; I have hope 3 that it may be found in a spur near the Orowliti, and within say two miles of the povt. Assuming that a supply can be got from this place, I should estimate the cost of this breastwork at about' ■ £500 t per chain. - Another plan, and one, I think, most worthy of consideration, would be to throw out from the river bank, at a safe distance above .the^own, a projecting groin of such a character, as would withstand any Hood, however severe, and of such a size and shape f-as would deflect the current sufficiently to protect the line of bank below from scour, and render the water below it comparatively still. This being accomplished, we Bhould then have to deal only with the destroying action of the surf, already described, from which the bank might be protected by a low breastwork of plies and planks, backed with stones and rock, which would also form an enduring edge to the bank, and protect it as well from the effects of traffic, &c. The cost of this would be about £30 to £40 per chain. As regards the construction of the groin itself, I have beenforcedto the conclusion, from careful observations, that any work depending on the use of piles, however well strengthened and braced, will be fouud .insufficient to withstand the forces brought to bear against it iv heavy floods, and we must look fo the use of some other material, in order to resist-successfully . the power of the river. I can confidently recommend heavy rock-work for this purpose as a certain means , of attuining the desired object—and with it a solid substantial groin could be formed, which would resist the action of the stream, and dellecfc the current to any desired extent. The manner of constructing it would be simple and safe; the tramway laid for the supply of stone would be continued to the edge of the river bank, and the stone itself in large masses would be tipped out of the trucks aud left to find its own bed ; the tramway being extended along the groin itself as the work progressed. We have examples of groins thus formed naturally, both in the Buller and the Grey, by .the action of tributary streams forcing m;isses of stone and roc-lcs of all sizes into the main stream during floods, and which successfully withstand the heaviest floods in the main river. A frequent inspection of these natural groins, and the manner in which they resist and divert the stream, has convinced me of the fitness of such a structure for the purpose intended. I estimate the cost of such a groin, projecting say 2£ chaius from tho,edge of the bank, at a proper angle at about £4000. Its position would be opposite Henley-street, that is, about 33 ,_chajns.abQve..,G:ludstone.^U»et—^Should ixn^vp^mP^^ stilling the water below it not rea^lfTwTtxr jh Glarl-Btone-street, a smaller groin of the same character might be run out about half-way between the two places; and were Hat any time thought desirable to control the action of the surf rolling into the river, it could be accomplished by constructing another immediately below Gladstone-street—where the channel becomes much shallower. % I will now summarize the works which, with a view to the greatest economy, .combined with pefhianence and certainty of action, I should recommend to be undertaken for the defence of the river banks, viz:— '..'.. '■■■ ■ ■ Large stone Groin at Henley-street ... £4,000 Say 30 chains of Breastwork ap£4o ... 1,200 ! Formation of Wooden Tramway, say ! 2 miles at £500 ... 1,000 ■ £0,200 Middle Groin, if required, say 1,500 Lower do. do. say ... ... 2,000 While on the Wost Const I took on opporrunitv of examining the protective works iv progress at G/eymouth, and also at Hokitika. The former consists of about 51 chains of, wop-len breastwork, formed of piles about 32 feet long, driven 6 feet apart, capped, and planked over, about 20 feet down, to the water line; the whole being strengthened at intervals by land tie 3 and braces. Between •- this and the edge of the bank, about 30 feet, there exists a triangular space,; the riyer beacti ,or shoreshelving from the street level to a depth of about 23 feet at the line of piles, giving about 13 feet of wafer* on this line. This space it is intended to'fill up entirely with brushwood, tree tops, branches and logs, without any admixture of earth or gravel ; and* it is expected that this will effectually prevent the scour of the river banks, if not at once, gradually, asj the mass of brushwood settles down firmly on the bottom. There is a reasonable hope for the success of this plan, if, aa I have been informed, all the piles have been driven to a hard or solid bottom. The entire Co3t of the work will be about £600i),'a like sum having been expended on former-proboctiVe works, all of which have been carried away. ,< , At Hokitika, I observed a great variety q£ protective works, some completed, and some (S in/pr ( 'gress; the wharf itself is protected in front by-sjfeet pilinw' about 20 feet long, and is yet standii^ while below" it, and above it, wing-dams, or .groins, o f character have been erected,, Bom& <?fvfhieh'liave' not. yet been tested by floods. . Breastworks of different, kinds have also -beera erected, the most successfaji of which, I wasjinforaicd,. being made with pileadjeepty.-driven in two rows on" different levels, baleen, which a mass of brushwood' has been' forood.down to,the sloping bantf of theriver, and has as ; yet not been destroyed, nor-■ from it§. appearance does, it seem likely that it'will be. ■^he amount spent in Hokitika in these works, bus i I believe, exceeded £40,000 I examined these river works with interest, as lifting likely to. exhibit- amctirpt. them some principle which might be favorably'adapfed to the Buller. Jn all, howevei-> are points of weakness, even in the,iy- present situations, and as they s(and, riot ono ©f them would resist the action of a sin"lo flociA hi the Buller, in which river tho volumo °of " water, is immeasurably greater than in the Grey orHokitika. The use of fcfeo timber- also, nx»st of it of a very perishable JSfttu.ro, will lead in the com'se of a ver'v few- JOSiS* to, a large outlay, for necessary repairs and maintenance. On, tins account I should strongly recommend tho use of stones and rocks in the formation of permanent protective- works wherever practicable. As a temporary protection of the river bank adjust the surf I authorised the erection of a low breastwork, not exceeding 15 chains in length', and i guaranteed the payment of £3 per chain towards its cost, on conditions o( the work being executed to a given specification, aud: to the satisfaction of the District Engineer., This will prevent, further destruction of tho baJiks,-, unless against very high floods, and may lnsfc untilr aqmstlyng more permanent shall be decided en. Meanwhile I must point out, in the most fombb--jfnanner, the danger to which a great portion of the . ' Reserve is liable, and the certainty that a consider-. I able amount of further dama^e/ift^t,.ensue i unlesa s .

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