IMPROVEMENT OF THE BAR.

Otago Witness, Issue 1639, 21 April 1883, Page 13

IMPROVEMENT OF THE BAR.

As our readers are aware, the Harbour' Board have finally decided to carry out the plans proposed by their engineer (Mr G. M. Barr) for increasing the depth of water at Otago Heads ; and when the Government notify their approval, tenders will be invited in Australia and throughout New Zealand for the coiMruction of the whole of the work in one contract. Mr Barr's proposal may be described generally to be the formation of a breakwater starting from high-water mark on the northern side of the entrance to the port, and extending for a distance of one mile right across the north channel, until it reaches a point on the outer side of the bar where there is a depth of 27ft at low water. The construction of such a breakwater forms the most essential part of Sir John Coode's plan, and would, we are informed, probably be undertaken firSt, supposing it were resolved to carry out that eminent engineer's design in its entirety. Sir John Coode's plan also included the construction of an eastern breakwater starting from Taiaroa Head ; but Mr Barr's ! opinion is that that work is not necessary, at any rate at the present time. The following details of the' works have been kindly furnished to us by the Board's engineer, and will doubtless be read with interest.

The first work will be the construction of a tram or railway leading from the northern cliffs (where hard and heavy stone is obtainable in abundance) to close upon high- water mark. At that point the training-wall proper, with its staging, will commence, and extend right across the north channel, as already described.

The total length of the staging and wall will be exactly one mile ; and, includiug the railway, the total length of the works will be 130 chains. The first part of the training-wall to be undertaken will be the construction of the staging, which will be formed of three piles in each pier, placed 20ft apart. The piera will be 22ft apart, thus giving 240 spans in the total length. The lowest part of the cross-beams will be 12ft above high-water mark, se as to secure that they will not be struck by the full force of any sea. No bracing of any large dimensions will be below these beams, so that the whole resistance offered to the sea by the structure will consist of the timber of the piles. It is recognised by engineers that it is better to avoid heavy bracing, as the amount of resistance which it offers to the waveß imperils the structure. The piles used will be as far as possible native timber ; but when the work is taken into deep water, ironbark or jarrah will be used. The staging will carry three lines of rails, and the atone to form the training- wall will be tipped from waggons running on those rails. The stone can thus be deposited over a total breadth of 50ft. The piles will range from 23ft to 57ft in length. The training- wall proper will be constructed of etone obtained from the cliffs already referred to, and the blocks will vary in size from a quarter of a ton up to ssven tons. The smaller blocks will be used in the "hearting "of the work, and the larger ones in the outer parts, and especially where the work gets into deep water. The width of the top of the training-wall will range from 24ft up to 35ft, and there will be a mole-head at the extreme end of 100 ft in diameter. The waves will naturally give tho rubble such slopes as will ensure stability, and they are expected to range from 1 in 1$ t ■ about 1 in 3, according to the force of the waves and the size of the material. The crest will be 3ft above low-water level, and as the tide rises 6ft 6in, this will allow fully the second half of the tide to flow into the harbour without impediment ; while the second half of the ebb— which is the most powerful for scour— will be entirely confined and directed across the bar, where deep water is wanted. The outer extremity, besides being increased in size to a diameter of 100 ft, will be protected by a number of concrete blocks, each weighing not lobs than 22 tons. On the outer extremity of the mole a cone? efc foundation will also be pro.-

pared for a lighthouse. T?oy this foundation the concrete will be deposited ia mass to ft length of 3ft above high-water mark. The work now proposed to bo carried out will leave a width of 30 chains at the narrowest part of the entrance, which is half as wide again as the Narrows at Harrington Point. •_ This would allow an ultimate width of 1500 ft if Sir John Coode's eastern breakwater were after wards carried out. That is very wide for harbour entrance, many of the entrances to harbours in Great Britain not being more than 500 ft or 600 ft.

The eatiinated cont of the works now proposed to be carried out is given at from £86,000 to £90,000. From recent observations it is found that the ebb current is ia all cases stronger than tho flood. The greatest yelocity observed at the second quarter of the ebb tide is 3& knots per hour ; and the greatest velocity of the flood tide— also at its second quarter—is 2'Z knots per hour. This, of course, will be of much advantage in giving greater scour on the bar outwards. About 40 borings have been put down upon the bar and in its neighbourhood, at depths below low water of from 30ft to 44ft. s They disclosed in all cases sand of but slight cohesive character. This affords reasonable ground for concluding that the deepening of the bar will be effected by scour alone when the wall is constructed. The borings were executed by putting down 3-in pipes and forcing a strong current of water through them into the Band, caumng the sand to Wash away, and the pipea to sink at the mean rate of 10ft per minute.