MR. BLACKETT'S REPORT ON THE BULLER RIVER.

Westport Times, Volume V, Issue 844, 1 August 1871, Page 2

MR. BLACKETT'S REPORT ON THE BULLER RIVER.

A recent " Gazette" contains a long report by Mr Blackett, Marine Engineer, upon the various plans that have been proposed for the preservation of tho river banks, the improvement of the bar, and for staying encroachments by the sea upon the beach. Wo extract the following having reference to the effectiveness and practicability of the various plans proposed :

WOBKS PKOPOSED. 1. To open the channel behind Garden Island.

I have already, by telegram, given an opinion on this work, and have declined to recommend it, on the ground that I considered it very doubtful if it would have the desired effect, and that it might do harm in another direction, for which the Government would be responsible. I will now offer some further remarks on this scheme.

The action of the river has, for some time past, been steadily to close up the entrance of this side channel by delivering into it heavy drift timber and large masses of heavy shingle and boulders; and this would lead us to infer that a constant expenditure would be necessary to keep the en trance open. This inference will be rendered plainer by a glance at the map, which shows that it is essentially and literally a side channel, not in the direction of the main flow of the river. It will also be seen that, from its size, it could never have can ied off a very large proportion of the flood-water, and I may state also, on good evidence, that some eight or nine years ago this channel, except in floods, was dry from end to end, and that it was used constantly as a track by the early surveyors, all tending to show that its action must always have been very limited.

Further, it is possible that the success of the experiment might be too great, and chat too large a body of water might be made to flow off in this direction to the detriment of the main channel, and the possible destruction of land on the south bank. Again, supposing the exact desired amount of overflow to be attained, I consider its controlling action would not be so great as supposed, but that it would be dissipated m in the wide ex-pause-of water below the island, and influence, to a small extent only, the actual direction of the outflow of the river. Again, whatever the amount of this effect might be, it would only be exerted during floods, as the entrance to the side channel would be dry or nearly so during the ordinary state of the river. This fact would lead us to seek for some more permanent controlling power than the flood-water from this side channel.

That the opening of it would cause a deposit on the north beach is, I think, highly problematical, and a result, however much wished for, not to be depended on. It will be borne in mind that Mr Balfour recommended the opeuing of this channel, but it was for the purpose of reducing the velocity of the river in floods ; and it will be seen that we have already provided, by the use of stone groins, against the destructive effects of the velocity, which may now be usefully employed in preserving the depth of the channel.

2. To cut a new channel immediately below the town across a very narrow part of the spit. This proposal may, I think, be dismissed with very few remarks. The spit here is very narrow, the sea having reduced it to about a chain in width at high water. The channel would not be in the direction of the flo.v of the river, but nearly at right angles to it, and the action of both river and sea would be to fill up again any channel which might be cut in this place. Any money spent on this would, I think, be thrown away. 3. To cut a new channel across the north spit, a little lower down, in a straight line with the general course of the river. This channel would be longer than that described in No. 2, and would be cut obliquely across the spit. There would be a slightly greater chance of keeping such a chanuel open than the above, but, supposing the experiment to be successful and a channel established, we should then have two mouths to the river, and it needs little argument to show that, this being the case, the chances are that we should have less depth on the bar iD each than where only one mouth existed ; and the hope of the old mouth being entirely stopped up by the action of the sea is, I think, very small; the entrance no doubt would be shallowed, but the river would almost certainly seek the old exit in heavy floods, to the detriment of the newly formed channel.

There is the chance also of this experiment being too successful, and that the destruction of the lower part of the town, now sought to bo avoided, would bo hastened by such a process. I cannot recommend the adoption of this proposal. 4. To erect artificial works on the tail of the spit. This proposal is, T think, deserving of consideration, and might bo placed in the list of works necessary to improve the bar, and preserve the*harbour in a navigable state. It would have the effect of deflecting the current in the main channel, and giving it, according to tho form and position of the works, more or less a

mote northerly direction, which fe tn desired. w ; It will now be necessary to refer to the plana of" the river, where it will be soon that its course from a point about two miles from the sea, is that of a line nearly straight to commence with but curving gradually more and more as it approaches the sea, the Curvature tending southwards. The general plan of the river is very slightly altered since the place was first frequented tho principal change being an increaao in tho curvature opposite to the town of Wescpprt, caused by the destruction of the river banks to the extent of about three chains, the advance of the north spit near its extremity towards the south spit, which has been washed away for a distance of four or five chains.

The actual overflow of the river over the bar is not only increased in width, but points in a direction con. siderably more to the southward than before. The depth of the water on the bar is much less than during former years, but very little alteration in thia respect has taken place during the last twelve months. It will readily be understood, how. ever, that these changes, although very small and insignificant, compared with the size and volume of the river, and the magnitude of the forces at work .should appear to be of the greatest importance, and to affect materially the interest of those living on or near the spot.

Oil the north beach, however, the changes have been much greateraud, although, had proper measures been taken at a much earlier period, its destruction might have been par tially arrested, I cannot now recom mend that any works shall be under, taken on the beach itself, for the purpose of protecting the town, being of opinion that any expenditure for this purpose mubt necessarily be very large, and if contributed in part by the inhabitants themselves as pro. posed, their proportions would be better expended in procuring safetl sites, and in removing their proper. I ties. There is a prospect, also, that the damage has already reached its greatest extent, as the sea in placej has commenced to throw up large quantities of boulders, where formally was sand, forming a bank which is growing in extent seawards. The present movement may, in fact, be but one of those oscillations of the coast Hue which have evidently occurred, and not unfrequently, in former times. It now only remains to suggest such measures as may tend to the°im. provement of the river, and render it permanently safe and commodious. In the first place, I may state that any attempts to straighten its course will be attended with no advantage, the natural flow of all shingle bearing rivers for obvious reasons, is in a series of curves and in the present ease, m may, I think, ensure the permanence of deep water on or near the outer line of the curve, by protecting that line. This will be best done with stones and rocks as above described, and the greater irregularities of the curved line should be adjusted. As we approach the sea, it will be necessary, in order to prevent the wide dispersion of the current on the bar, to stop its tendency to flow sonthwards, and to concentrate its force by artificial works to be erected on the south spit. These works should be curved in the opposite direction, so as to turn the current more to the north, and give it force to deepen the passage over the bar. Works of a substantial character vvill be required here, as they will at times be subject to the action of the surf.

To assist the action of the above, I should adopt the recommendation in proposal No. .4, and erect artificial works on the tail of the spit. The order in which these works should be carried on, if not undertaken all at once, would be as follows, viz :- X. Curved groin on south spit, whica may cost when complete, say 20 chains at £SOO a chain, £IO,OOO, This is a work which, may be done gradually, by devoting a portion only of this sum to it annually. It is marked A on plans. 2. Stone training wall across shallow bay on north bank, where old snag lies, about 5 chains below Stanley Wharf. This is to regulate the curve. 7 chains, at £53 a chain, £371. This is marked B on plans. 3. Training wall or groin on tail i spit, say 17 chains at £53 a chain, £9Ol. It is mrked C on plans. 4. Protection of river banks from action of surf, by covering them ffi' D stones between the stone groins, say 17 chains, at £25 a chain, £425. Marked D on plans. 5. Same process carried downwards, along north bank of river, to preserve curve, to be done gradually, say 15£ chains, at £53 a chain, £B2l 10s. Marked B on plans. I have made Mr Dobson, Provincial Engineer, acquainted with the sub' stance of this report, with which,! may state, he fully concurs. I am indebted to him for the preparation of the plans, and for much valuable evidence and information ben ring on the subject, to which he has evidently given much careful observation. I have, &c, John Black ett, Acting Engineer-in-Chief-