NAPIER HARBOR WORKS.

Hawke's Bay Herald, Volume XXI, Issue 5279, 13 January 1879, Page 2

NAPIER HARBOR WORKS.

Sir,?— -Referring -to. the method proposed r by Mr Harding, of causing % "t^e' r travelling 1 shingle to be arrested in ite' course round the Bluff, by means'of W «« gabionnade," or fascine work; placed 1 a little higher than the surface ,of, low water, .and' thus to ! form a natural breakwater of shingle, I have ho doubt that such a plan, if con-'---stantly followed up by the erection of new fascine w6rk, ; as fast as the shingle accumulated would in time, cause a new spit to be formed, provided .that thef heavy seas rolling in from the south-east and east did not disperse and scatter the shingle over the bay. We have, however,' •■[ as Mr Harding observes, no reliable i data i of the quantity of shingle .passing the 1 Bluff annually, and as this quantity must' depend chiefly on the materials brought down in times of flood by. t^e Ngaruroro* and Tuki Tuki rivers, very little has been,deposited since the last flood of February^ > 1877, the greater part of which may nowrv be seen in the angle formed by the eastern jetty of the harbor works. ..;.,/."■. The great difficulty which the engineer has to contend with in harbor works is the travelling ahingle wherever it occur*/, and in a very able report by Mr Smeaton on Dover Harbor, that celebrated engineer's first enquiry was whether fixed piers or jetties run into the* sea would' permanently prevent the shingle from' getting round their ends, and he gave it' as his opinion that, the beneficial effeofc would only be temporary, and that the piers or jetties, would no.t stop theprw

gress' of the shingle from getting round their heads, when it would re-appear again on the inside, and continue its course. This opinion of Mr Smeaton (expressed more than 100 years ago) has been recently confirmed at Dover, from the fact that the Admiralty Pier, constructed there at a vast expense, and carried out to & depth of fully 60 feet . at high water, does not prevent the shingle from travelling round its head, and re-appearing on the inside, where it has formed a large bank, to the great trouble and annoyance of the engineer to the Board. : The late Mr Balfour, • also, who reported a few years ago on the subject of a proposed f harbor at Timaru, or its riciniiy, made some experiments on the travelling shingle at that locality, and erected a small experimental breakwater, which* although it was carried away by the heavy seas in about three months after ;; : its completion, yet, in that short period,' exercised such an effect in curtailing the passage of the shingle northwards, as almost to cover up the works, — the waves, as he describes it, " curling around , the end of the structure, and heaping up the shingle, on the inside, this action being continued until the breakwater was demolished, when the flhingleiresumed its travel noth wards." So far as the locality is concerned, therefore, the works proposed,, by. Mr Harding would be in mu6h ■■ the same category as the Admiralty pier at Dover, or Mr'Balfour's breakwater at Timaru, and although it is proposed by Mr Harding that when the requisite length was attained, the end might be turned abruptly sea-wards, so as to form a nucleus for the deposition of the shingle,— yet; supposing this were done, a time wouldsoon * come when the. elbow thus formed would be filled, in, and the, shingle 1 would then resume its travel, and be deposited on the inside, when it would form a bank as at Dover. For these reasons I think that in any proposed scheme for a breakwater at the bluff, the travel of the shingle northwards should not be interfered with, and that an opening near the shore should be left for its passage. This need only be of small .dimensions,, and a covering pier or breakwater, built of concrete blocks, with a hearting of shingle or rubble stones, if made sufficiently massive, would, I conBider, be the only work that would be able to resist the impact of the heavy seas that occasionally roll in from the east, and break with such force on the beach. Such a work would no doubt be costly, as all such works. necessarily are, but it would be effective and permanent, and as the question of a,, harbor of protection in Hawke's Bay has lately been reopened, I think it would be desirable that every possible information on so important a subject be collected, so that whatever scheme may ultimately be adopted, it may possess, all, the elements of success. — lam, &c, Ahukiri.