RIVER PROTECTION WORKS.

Westport Times, Volume III, Issue 432, 18 December 1868, Page 2

RIVER PROTECTION WORKS.

MR. BLACKETT'S REPORT.

The report of Mr Blackett, the Provincial Engineer, upon the protective works required at Westport, has been published in the Provincial Gazette of the 12th insfc. It is addressed to the Provincial Secretary and is as follows :

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 Buller, and )n the means proposed for protecting them against further encroachments of the river.

I found that the jetiies, 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. A number of piles of No. 3 jetty are still standing ; this structure was in progress on the occasion of the last heavy flood of the 14th October, and a temporary staging has also been fixed to it, where goods can also be landed. The tracing forwarded by Mr Lowe very cor; jctly shows the nature and extent ui the damage done by the last and it is evident that the damage is gradually extending along the river bank upwards. The greatest destruction has taken place along a length of about 30 chains above Gladstoue street, 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 necessary 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 almost 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 scouring action becomes less and less, and the river gradually deposits gravel in the deepest parts, being unable to carry it forward ; the result of this being that a set of soundings, taken some time after a heavy flood, will give a greatest .depth of 17 or 18 feet, in place of 30 feet as above stated, showing a deposit of loose material of a depth of 12 feet. As the flood subsides the upper parts of the bank fall in, a process which is immensely accelerated by the action of a swell from the sea, which extends a considerable distance up the river, and causes, at every flow of the tide, a tolerably heavy surf to beat against the upright bank. The action of this is to undermine the bank about G to 7 feet below its surface, and to cause large masses of it to fall in, which are gradually crumbled away by the surf, or carried away bodily by the next flood. This action of the surf, if left to itself, would be to form a flattish shore to the river bank, and its destructive effect (apart from floods) could be arrested by simple and comparatively inexpensive means. It remains then to point out some means by which the scour during floods can also be arrested, and this being accomplished, the safety of the banks will be secured.

The ordinary piled and planked breastwork, well filled in behind, would appear to be calculated to resist both destroying agencies, were it put down deep enough, but it would require to be carried out in such a way and on such a scale, as would render it very costly ; for, although I have stated 30 feet as the 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 duringfloods. 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 strengthened by land ties. Behind these I should fill in large pieces of rock, forming, in fact, a rough stone wall, say 4 feet in thickness, and in front of the piles would also lay in an angle or footing of rocks, to resist the downward 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 immovable. 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 l'ock can be obtained at an available distance from "Westport; I have hopes that it may be found in a spur near the Orowaiti, and within say two miles of the port. Assuming that a supply can be got from this place, I should estimate the cost of this breastwork at about £SOO 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 town, a projecting groin of such a character as would withstand any flood, however severe, and of such a size and shape 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 should 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 piles and planks, backed with stone 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 £3O to £4O per chain. As regards the construction of the groin itself, I have been forced to the conclusion, from careful observations, that any work depending on the use of piles, however well strengthened and braced, will be found insufficient to withstand the forces brought to bear against it in heavy floods, and we must look to the use of some other material, in order to resist successfully the power of the river. I can confidently recommend heavy rockwork for this purpose as a certain means of attaining the desired object—and with it a solid substantial groin could be formed, which would resist the action of the stream, and deflect 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 truck and 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 masses of stone and rocks of all sizes into the main stream daring 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 \ chains from the edge of bank, at a proper angle, at about £4OOO. Its position would be opposite Henley-street, that is, about 33 chains above Grladstone-street. Should its action in stilling the water below it not reach as far as Grladstone-street, a smaller groin of the same character might bo run out about half-way between the two places ■ and were it at 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 shallower.

1 will now summarise the works which, with a view to the greatest economy, combined with permanence 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 at £4O ... 1,000 Formation of Wooden Tramway, say, 2 miles at ,£SOO 1,000 _„,., _ £6,200 Middle Groin, if required, say, ... 1,500 Lower do. do. say, ... 2,500 The conclusion of Mr Blackett's report, and a postscript in which he calculates the tramway at six miles, costing £3OOO, win be published tomorrow.