OAMARU HARBOUR.

Timaru Herald, Volume XIIC, Issue 13908, 20 May 1909, Page 3

OAMARU HARBOUR.

A REPORT ON DREDGING. Mr J. Blair Mason's report on the dredging requirements o! Lhe Oamaru ILariJJlir board was received at a special meeting of the Harbour Board on Aloud av. 'flu- lollowmg an; uie salient features of the report:— Entrance Depths.—The greatest natural depth in the entrance of channel shewn on the chart is 17ft. Bin. at low water, and reported as a hard lixed bottom. As it is outside the influence and protection of the breakwater, such depth becomes the limiting depth to the entrance to the port, and should govern the depths sought to be attained within the breakwater ant! liarhour. ...

Depth of Berthage and Swinging I3asin.—The depths required at the wharves and approaches and swinging basin are necessarily dependent upon the limiting entrance depth previously mentioned of 17ft. Bin. at low water, situated northerly from the end of the. breakwater. With a tide rise of oft.. vessels drawing up to 20ft. should be able to enter the harbour under ordinary weather conditions, with a margin o'f 2ft. Bin. to allow for the swell. A depth, therefore, of 21ft. for lUOft. wide maintained at the wharves would be ample to allow of perfect safety of vessels at all times. Over the approaches'and in swinging basin a depth of 20ft. low water is ample, affording, as it would, plenty of time for a vesrel of extreme draft to swing and berth on a falling tide: The deptn in the entrance channel within the influence of the breakwater is advised to be maintained at 18ft. low water. The amount of work required is set out, for dredging 1340 ft. of wharf berthage, bv 100 ft. wide to 24ft. low. water, total 3 acres, 23,710 e.yds; swinging basin and approaches, acres, to 20ft. 1.w., 260,440 e.yds; entrance channel, 2 acres, to 18ft, 3.300 c.vds; total 287,450yd5. in solid. Add for bulk increase 76,263; total 263,-18 e.yds. Air Ma.sou estimates the annual silting of the dredged area at 80,000 e.yds. , , , As to the character of. the ground borings nutde indicated that Jt would be all elav and silt. As. for the disposal of the spoil, Mr Blair recommends the use of pontoons or barges, the dredge Progress to be repaired and used as a fixed dredge only, not a hopper transport dredge. Ho recommends two wooden pontoons cf 120 yds capacity, to be moved about by means of wire ropes worked by power from 'the dredge and from the landing; the spoil to be pumped out of the barges for reclamations, or if too stiff to pump a Priestman grab to be used for discharging them. : Time of Completion.—On the'assumption.that the dredge works.eight hours per- dav, dredging for 44 weeks in each year, the.material to be excavated; neglecting accretionary silt, should : be removed within 26 months, and allowing for siltation 14 months, longer, or a total period of three years and four months to entirely complete the work. The employment of the dredge four to five months in each year thereafter is considered sufficient to remove the accumulating silt from the areas dc 7 lined and fully maintain the harbmtr depths, accompanied possibly by an improvement outside such areas. Total estimated cost £4757 for repairs and new plant; working cost £2922 ner annum; total for plant and 3 1-3 rd years' working £l4 498; esti-•,->iP+."d*eost nor cubic, vard, 6id. The. Board deferred consideration of the report till the 25th inst.