EXTENSION OF BREAKWATER

Gisborne Times, Volume XXVII, Issue 2653, 8 November 1909, Page 3

EXTENSION OF BREAKWATER

REPORT FROM MR. MARCHANT.

Mr. Marchant’s report on the proposed extension of the breakwater was received as follows at the Harbor Board office on Saturday:— /

Proposed extension of breakwater for improvement of tbe entrance to the Turanganui river, by F. W. Marchant, AX. Inst.. C.E.. . The entrance to the Turanganui river has a concrete set blockwork breakwater, built on its eastern, or weather side, and the river channel is trained and defined on its western side by means of a wall constructed of timber framing filled in with rubble.' The clear distance between the breakwater and the wall is 250 feet.

In order to sand drift into the entrance channel, the Reside training wall was extended, so that it projected 50 feet past the end sf the breakwater measured at a* right angle.. The storm wave runs from a S.E. direction only as shown on plan attached. Its angle relatively to the line of the entrance channel is such, that a length of more than 150 feet of the leeside wall is exposed to the full stiength-of the’storm wave without the least protection; from. r the concrete breakwater..

The result is, that during, storms or quite moderate: swells from seaward, a furious sea runs in this channel. First of all it sweeps along the leeside wall, then cannons off that diagonally across the channel on to the leeside smooth face of the breakwater, is reflected at an angle olf that again oil to the training, wall once more, and SO fMI.

The channel is positively dangerous under these circumstances, and it says a great deal for those navigating it that no serious accident, lias so far occurred. In order to improve the navigation of the entrance channel, it is proposed to build an extension of tbe breakwater so that the leeside training wall would be completely under its shelter. 'lMie minimum length sufficient to properly effect this, is about 300 feet set in a position shown on accompanying plan. The breakwater would be built on tlie same principle as that used in the New Plymouth breakwater extension, which is proved to be absolutely stable, is a cheap method’ of construction-, and moreover- lias the faculty of breaking; uf) the seas in a manner which no set blockwork could effect.

Instead of carrying on the extension in the same direction as the existing work it would be canted to seaward so as to give a wider entrance for one thing, and allow any future extension in a better direction than lias the original 1 work.. The finances of tlie Board are such that the expenditure on this most necessary work must be kept down to tlie lowest possible amount. For this reason tlie height and width of the breakwater have been kept to a; minimum coi/sistent with sufficient strength. The strength of tbe sea at this point is much reduced by shoal water and foul ground to seaward, so that a very heavy structure is not required to withstand its attack. The extension is to be built from a staging as shown on plan. The bulk ol tlie mound is to be of papa rubble obtained from Pa Hi'll, 1J miles seaward from tlie entrance. It is proposed to be conveyed in drays and waggons, as the woi> is of small extent, and it is desired to avoid tlie cost of a tramway. The top-of the rubble mound is to be tevelled off with bags of plastic concrete on which are set one layer of concrete blocks, these capped by a concrete monolith 18 feet wide to 3 feet above high water mark.The weather side and outer end of the rubble mound is to be covered with the random blockwork of 15-ton concrete blocks, as a wave-breaker and protection to the underlying rubble. The Board-,,has very little plant available for the "work, and as it is necessary, owing to its financial position, that the smallest nossible quantity of machinery should be purchased, it is proposed to obtain only a 20-ton quarry crane for lifting the concrete blocks on to trucks and lowering them into their places. The staging would be built as the main work proceeded. The estimated cost of the work_ is £3G per foot on this system, plus £2500 for the purchase of a crane, construction of block -moulds, etc. If the Board could afford to expend another £2OOO on tbe work a tramway could be laid to Pa Hill, using secondhand rails’ and cheap -sleepers for the work. Tlie advantage of doing so would be that much larger blocks of rubble could be used for the work, and this would decrease the risk of tbe mound being scattered during construction by a heavy sea-before it could be defended bv tbe concrete blockwork.