WAIRAU RIVER COMMISSION

Marlborough Express, Volume LI, Issue 251, 23 October 1917, Page 6

WAIRAU RIVER COMMISSION

(Concluded.)

About the time the Tua Manna banks were, erected one bmale, occupying the Peninsula, with assistance trom the Spring Creek River Board, erected a bank extending trom the railway line along the right bank of the Spring Creek to a point beyond its confluence with die Wairau Rived. This work had the eheut ol stopping the flowing of the fioodwaters of tho Wairau River across the Peninsula, as they had been wont to do, reducing very largely the effective waterway in the river. Ihe combination of this work with tlie banking of the Tua Marina River Board caused a constriction of the waterway, raising the flood-level and resulting in breaches of the banks in many places, the banking-up of tho water in the Spring Creek, and materially increasing the liood damage. Realising tho danger likely to arise from this Peninsula bank, the Spring Creek River Board had in May, i9lO, arranged to purchase the whole of the Peninsula in order that they might remove the banks. However, the flood of July arrived before the work was carried out, although'a small cut had been made, which probably had some effect in lessening the floodlevel. This bank is still in existence, and a source, of danger. The small triangle below the Peninsula (Sections 84, 85, and 86) across the \Vairau River is not included in any river district. ■ i . Banks erected by the owners on this land have materially contributed to the restriction of the waterway and the raising of the flood-level and consequent damage. Although.' the various Boards have spent large sums in maintaining their respective works, we not© that they have allowed the waterways to be choked by the growth of willows, manuka, gorse, ♦ broom, and other vegetation, so that in many places, especially in the vicinity of the Peninsula, the waterway has been, reduced, and in many places rendered entirely inadequate to carry off .the. flood-water. One tributary, of the Wairau River which is causing considerable anxiety is the Waihopai River. It is the. largest tributary of the Wairau, and brings down a very large body of water, with a very sharp fall, heavily charged with detritus. It is buildin"1, a tan at its junction with the Wairau River, which is causing it to deviate to tho east, threatening-to break into Gibson Creek and to flood the low-lying lands about Renwick, and thence to pour its waters into the Opawa. In 1911 a considerable ..part ol the water had actually found its way into Gibson's Creek. 'The Lower "Wairau River Board, acting on the advice of the Government Engineer,' erected several groynes at the confluence, which had entirely cut it off. line lowest and most important; of-these groynes was in 39:6 maliciously cut by some persons unknown. Although this was immediately brought under the notice of tho Board, no repairs were attempted until recently, after several minor floods had considerably enlarged the cut. The Board, now thoroughly awakened to the. .danger, made several unsuccessful attempts to repair the breach. Recently tihe stonework was vevr nearly completed, but, owing to failure to take ' reasonable precautions, a small flood has again undone their work, and the Renwick lands have been flooded. This we regard as a matter of extreme gravity, as at{ any time the whole'river might change its course through this breach and flow via Renwick into the Opawa ; and we doubt whether the present channel and bridges could carry the water of this river in flood, even if there were no flood in the other streams—viz., tho Fairhall, Omaka, and Taylor. . ■Shortly -after the construction of Ihe groynes at the confluence of the Waihopai River, and avowedly in purHianoe of the same object, and with a view to protecting the Board's resetve situate abreast of the breach, tlie Board erected further groynes in the vicinity of what is us.ua.lly known as Condcr's Bond. These, in atlditoin to tho objects stiated by the .Board, were said by various witnesses to have prevented the inflow of the Wairau into the Opawa to a marked extent, consequently throwing an increased .amount, of water into the Wairau. We are--satisfied that this was the case in food-time. Before the last of these grownes was completed the- Spring Creek River Board sought to enforce the old injunction on the ground that the object of the work was to close the breath. The matter went as far as'tho Court of Appeal, and the rase was dismissed. The Lower Wairau River Board ceased operations pending the de-

(•isi.Hi of the Courts, nnd have nob yet completed tihe work. AVo mast draw attention to tlic characteristic of all shingle rivers, which bend to flow alternately on their right and left banks as the shingle piles up and diverts their . courses by gradually raising the whole river-bed. The result is that > the Wairau may at any time flow ; against its left bank, following the old main channel, which must therefore be constructed and regulated to a size adequate to carry the whole of its water. On the other hand, it may swing against the right bank, as it threatens to do in more than one place at present. This tendency may have the effect of forcing the whole of the water into the Opawa, | which would result in serious disaster Ito Blenheim and the rich lands adjacent. As the Wairau has for long tended to the\ left of the delta, the time must be approaching when the direction would be reversed in the absence of works to prevent it. Another cause of flood to Blenheim and the lands adjacent is the Taylor River and the Fairhall, Mill Creek, and Doctor's Creek, which join the Taylor above Blenheim and run in the- old course of the Omaka. The old course <of [the Omaka was insufficient to carry this water, and even now, with the stop-banks which have been erected, it is doubtful whether it is largo enough. The 1911 flood demonstrated this inadequacy, and the town was flooded 4 feet deep in Market Square. It is to be noted that the banks have been raised since this flood, but the waterway, especially under the bridges, is insufficient. A proposal has been made to divert the waters of the Fairhall and Mill Creek into the Opawa at or about the confluence of these streams, a course which we consider necessary, though the most favorable point can only be determined after further investigation. The Tua'Marina River is the only tributary of the Wairau of aiiy importance on the left bank. It flows down the Waitohi Valley, through large swamps which fill up in flood time, and is the cause of much flooding in its own valley and at its confluence with the Wairau. There is a small system of banks, and further work is required before the flat lands can be cultivated. It flows into the Wairau very close to the point where the channel is badly constructed, and a very small rise in the Wairau causes backing up in the Tua Marina. RAINFALL. Unfortunately our rainfall records are somewhat inadequate. Around Blenheim the normal rainfall for the last twenty years is abouti 30 inches, for the last four years only 24m., and as low as 19in. in one year. Nevertheless the Wairau district has suffered more from floods—notably the floods of July and November, 1916—than, for the last eighteen years This shows how little information as to floods can be obtained from the rainfall records, as floods are due to special storms. Probably the rainfall at the headwaters of the rivers in the ranges is very much greater and more liable to sudden j JUrFIbOD CHARACTERISTICS. From the information before us, which is anything but ample, it appeal's that the amount of water carried by this river in proportion to its drainage area is very groat. This may be accounted for by the steepness of the sides of the valleys of the river and its tributaries, together with the regular and steep fall, j straightness and open nature of the channels, alloAving the flood-waters to reach the head of the delta in the shortest possible time. The scantiness of the soil on the hills and the imperviousness of the rocks and the absence of forest are also contributory causes. • I The high waters encircling the watershed carry more or less snow in winter, the melting of which on a sudden change of temperature may cause a flood out of all proportion to the recorded rainfall. In addition to the floods in the main Wairau caused by northwesterly rains, southerly rains cause Hoods in the lower tributaries. Floods from the mountains take longer to reach the plains than do those from the lower tributaries, .so that in the event of southerly rain following northerly rain, by twelve to eighteen hours the flood-waters reach the plain ! simultaneously. When this occurs !we have what is called an "old man" ! flood. .'.aJJJ HARBOR, In the early days loth the Wairau and the Opawa were navigable—the Wairau up to the Ferry Bridge and the Opawa up to Blenheim. The tidal range is 3ft 6in to 4ft 6in at the entrance, and its effect is felt ,at

.Blenheim in tho Opawa River, and at ! tho Peninsula on the Wairau. Under tho influence of the weather tho entranco used to shift up and : down the Bouldorbank within a range ij of two to three miles, and as a result jj the bar was often unworkable. "When tho bar was in its extreme northerly position, the course of tho river being longer with no more fall, the floods, especially in the lower reaches, were intensified. Tho Lower Wairau River jioard and those interested in shipping ma.de attempts, more or less successfully, to open a more direct channel through the Boukleruauk, somotimes at the very place where the present channel runs. The Wairau Harbor Board (constituted in 1907), acting on engineering advice, constructed a training-wall to confine the river to a new cut which was subsequently made. This work is not yet completed, but for the past j two years has had a beneficial effect j on the entrance, which is now workable except in cases of very heavy weather. Witnesses state that the effect of this straight and open channel has been to reduce the flood-level by 4ft at Dillon's Point (the confluence of the Wairau and the Opawa) and by a lessening amount to a distance of five miles up the course of both rivers. Probably the beneficial j effect is felt higher than this. The Wairau has been gradually j shoaling, and this lias been accen- ; tuated by the opening of Hose's over- • flow, and it is no longer navigable to the Ferry Bridge. This may be due to tho decreased amount of water flowing down the stream as a result of the diversion of part into the Opawa. The navigable part of the Opawa has been silting up, but it is still open for small Boats' up to 100 tons. The- tidal flow to and from tho lagoons lying between the mouth and the Vernon Bluff- assists materially in keeping the entrance open, although the fullest effect is not obtained owing to constriction in the connecting channels. There is a danger of Rose's ovex*flow scouring out so as to absorb all the Opawa water. It would probably then become tho navigable channel, but this would cut off access to the existing wharves, upon which a large expenditure has been made by the Harbor Board. The policy of the Board is to improve the harbor and channels with a view to accommodating vessels up to 200 tons burthern. If this were done a large trade in coal and produce might be developed, to the great advantage of the district. The effect of these harbor improvements would be markedly beneficial to drainage and flood-prevention. REFERENCE CLAUSE I.— Control and improvement of Wairau River and Tributaries. Without extensive surveys and tho preparation of, proper engineering plans it is impossible for us to indicate in detail the works necessary to give effect to the general scheme which we propose to recommend. It is to he noted that none of the local bodies are in possession of plans of the works which they have constructed, many of which works may be incorporated into the new scheme. We consider that the channel of the Wairau should be improved so^ as to carry all the waters of the Wairau, as no scheme which could be devised within practical limits would ensure a proportional division of these waters between the Wairau and the Opawa. The tendency referred to before, of the river oscillating from north to south, makes this imperative. More than this, from such data as is obtainable we find that the old Opawa i Channel and Rose's overflow channel are not capable of carrying safely more than the high-flood waters of the southern tributaries —viz., the Omaka, Fairhall, Mill Creek, and Taylor River. Having regard to the necessity of making the Wairau capable of carrying all the water, there remains no valid reason why the breach should remain open. Further, in view of the extreme danger, amounting to disaster, which would ensue if the Wairau changed its course to the Opawa, as it undoubtedly might under the influence of a southerly trend, the^ breach must be closed at all events. The work required presents, no difficulties. The steps we recommend to improve the main channel are:— (a) The clearing of all vegetation in tho beds. Tho willows are at present causing untold damage. In view of the protection of the banks a fringe of willow may be necessary, but this must be strictly regulated. (b) The extension of the works of the Hnrbor Board at the mouth and tih.o making- of the same permanent. The channels connecting with the lagoon should be improved so that the full benefit of the tidal scour should be obtained. (c) Tho enlarging .r>f the channel to a, uniform discharging capacity, which should be capable of carrying the largest flood hitherto observed with a- roa- . >sonablo margin of safety. (d) The. work under clause (v) will necessitate the .straightening of tho river at the Maori Point, at the point immediately abovo 'the Ferry Bridge (Sections 84, 85, 86), and ai> the Peninsula.. (c) Portion of the 'existing stopbanks will require to be moved so a.s to allow adequate waterway, and others must be strengthened, and some raised. Some flood-gates may bo necessary. (f) The removal of the damaged banks at Leslie's breach and its vi :inity is neeessaiy. (g) Upon the fixing of the lines of b inking, all lands lying outside ■of them sihould be acquired, and all obstructions removed therefrom (h) Protective measures, including groynes, may be required to deal with erosions. j FURTHER RECOMMENDATIONS. (j) Protective works of a- durable and su.bstaaiti.il character should be ' erected to provent the waters of the Waihopai .from breaking into Gibson's Creek. (k) From the junction of tho Omaka with the Opawa to Dillon's Point tho Opawa must be treated in a way similar to that recommended for the Wairau, based on the amount of water which it will have to carry. (1) The Fairhi'.ll and Mill Creoles should bo led into the Opawa at a :sito to be determined after survey,- preferably below their confluence, and the necessary stop-banks and protection provided. (R.i) Tho present position of tho Taylor seems to us satisfactory, but eaivful investigation mustbe made to ascertain whethor in all parts its discharging oapfii'itv is acitHiM.'ite, :si)d if ioui;^ n.; I :i, by ~o JL must bo

enlarged and all obstructions removed. it' tho Omaka and. Opawa are relieved as proposed, the dangers rf the flood to Blenheim will be obviated. | (n) The co-operative drain should i be improved so that it may bo \ adequate to deal with all the. j water which enters it. < (o) Tho Pukaka main drain should be cleared and enlarged if necessary, and the stop-banks ■ continued to its outlet. Neces- i sary flood-gates should be pro- ' vided. (p) Tho banking on the sides oi the Tua Marina must bo made to conform to the amended levels and design of the works on tho main river. (q) We found an opinion prevailing among many of the witnesses that an overflow channel from Tua Marina to the sea would provide adequate flood relief. Wo cannot recommend this, as from rough levels taken we are .satisfied taht an adequate channel could not bo provided under an expenditure of £o(),.0O0. An examination of The ground traversed leads us to believe that the difficulties inherent to the' project and tho keeping of tlie sea mouth open are insuperable. Further, we consider it unnecessary, and deti imental to tho present harbor entranco. There is a- j ossibility of Rose s; Overflow becoming a serious source of expense. The channel should be protected in such manner as may be doomed advisable from the scoui'ing action of tho flood waters, which may lower the levol of its bottom until the whole of the Opawa flows down this channel. ' If, however, the Harbor Board wishes at any time to< open. Rose's Overflow to navigation, they should not <lo .so without the consent of the Governor. REFERENCE CLAUSE 2.— CONTROLLING BODY. We have no hesitation in recommending that' the control of the river should be vested in one central authority. With regard to the Harbor Board, we consider that it should not be interfered with, except that the central authority should, with the consent of the Governor, have the right to do such works-as they consider necessary in and -about the rivers under the jurisdiction of the Harbor Board. The provisions of the Harbors Act,. 1908, Section 148, will safeguard the interest of all parties. RKFEtRENCE CLAUSE 3.—AREA OF CENTRAL DISTRICT. Wo recommend that the area of the central district shall include the whole of the watershed of the Wairau River and its tributaries. REFERENCE CLAUSE 4.— Powers, of Central Authority. W'e recommend that the powers set out in the Land Drainage Acts should be conferred upon the central authority in addition to the statutory powers of River Boards, and that the election of the Board should be conducted on the system of proportional representation, end that one member of the Board be appointed by the Harbor' Board. ' Your Excellency's . Commissioners are further of opinion that there is no Board now existing which could deal satisfactorily with this matter, nor is it likely t'that the proposed central j authority could bo expected to carry ; (tut the large and varied works recommended by us. Such works in- \ volve complicated surveys, and call for thy best enginering skill. Further, works of this kind must be j canned out in accordance, with a definite* programme., which musti be controlled by the authority responsible for tho design, if leconomy, safety, and satisfaction are to be secured. We therefore recommend that the design, of tho scheme and the execution thereof be carried out under thedirection of the Minister for Public Works, : and on completion the woi'Vs be lv/nded over to the newly constituted local! body. Further, tntit legislation ixvj introduced providing for the raising of a sum of money sufficient to comtho works and to give effect to ' our recommendations; that repayment- of such sum (less such contribution as you may decide iio make from | the general funds of the Dominion) be ! .secured by a rate levied on the ratable property of tho new district, classified in proportion to the benefit derived from the worts. REFERENCE CLAUSE 5. In the absence of plans and thenecessary information, we are una.blo to make an estimate of the cost of the proposed works. We are quite satisfied "that the cost will compare very favorably with tho enormous losses tho settlers, have suffered in the past, and against a repetition of j which they have no adequate protection. We also recommend that, in vie-w of the necessary delays incident to: the preparing of tho scheme and the i passing ■of the i-ecessary legislation,.] 'tho surveys and investigations be! pressed on* and put in hand without I delay. j We are pleased to report that the ] finances of nil tho Boards are m a ] very satisfactory condition, and j there wil' be a very small liability for i the new Board to take over. j Further, the large and expensive .< works now existing have, been carried j out with but very little assistance ] from tl\e general funds of the j Dominion. i (The Commission adds that its report was unanimously adopted).