Written for Progress. ...Nelson... Harbour Improvements. PROGRESS OF THE UNDERTAKING.

Progress, Volume I, Issue 4, 1 February 1906, Page 75

Written for Progress.

... Nelson ... Harbour Improvements. PROGRESS OF THE UNDERTAKING.

The Nelson harbour works now in progress embrace the cutting of a new entrance to the harbour through what is known as the boulder bank. The suggestion to open a new channel through the boulder bank emanated from Mr. Leslie H. Reynolds, m 1889, when he was called in by the Government to suggest the best method of improving the entrance to the harbour. Mr. Reynold's idea was considered at the time a fairly bold move, especially as the previously expressed opinion concerning the formation of the boulder bank presupposed a rock core. As it happens, subsequent operations m dredging through the cut have practically disproved such conclusions. For some years prior to 1899 the navigable depth on the bar lay well beyond the entrance proper ; and, as it continued to seriously decrease, it became absolutely necessary for the harbour authorities to move in the matter. The first investigations were made for the Admiralty by Captain J. L. Stoke R.N., in 1850, at which time the Waimea river discharged the bulk of its tidal and river waters through its eastern outlet shown upon plan No. 1, m a volume somewhat in excess of the outflow of the Nelson harbour. The combined flow of ihe Waimea river and haven waters made seaward in an almost northerly direction, and was sufficient to maintain a minimum depth of from nine to ten feet at low water. It appears that about the year 1875 the eastern outlet of the Waimea river commenced to shoal, and shortly following became entirely blocked, thus leaving the haven outflow to alone contend against the heavmg-up action of the sea, with the result that the wave forces speedily acted upon the seaward slope of the shoal, particularly to depths extending to 1 2 or 15 feet below low water, and drove the sand inshore, forming a crest, or bar, having a depth of 6 feet at low water, as shown on the chart prepared by the late Capt. Johnson in 1882. Since that date the sea forces have driven the bar in a direction inshore towards the boulder bank some 900 feet, and the two fathoms contour line an average distance of 700 feet, with the result that the low water depth on the bar crest has been reduced to about 5 feet during favourable conditions, and to 4 feet following north-west seas. This inshore travel of the bar has seriously interfered with the sailing course, and has made the incoming or outgoing of vessels a difficulty, to say nothing of the delay occasioned through waiting for the tides, as it is only towards high water that the larger vessels can negotiate the bar. Mr. Reynold's scheme of cutting a new channel through the boulder bank was favourably received by the Marine Department, but to make doubly sure the Nelson Chamber of Commerce, a few months later, obtained a report from the late Mr. Napier Bell, who was supplied with Mr. Reynold's plans and data ; and Mr. Bell evolved a scheme some-

what similar to that of Mr. Reynolds, the estimated cost of whose design was Up to this time the harbour was in the hands of the Marine Department, but 111 1901 the Nelson Harbour Board was formed, with the object of taking over the harbour and prosecuting the proposed work. The first of the contracts for machinery and plant were let in June, 1902, and the work of excavating the new cut was put m hand early m 1903. The works,

as designed by Mr. Reynolds, embrace the construction of north and south moles, as shown on No. 2 plan, flanking a cut seaward through the boulder bank and harbour shoals, which would give an entrance of 600 feet in width, and a depth of 15

ieet at low water spring tides, and at high water spring tides a total depth of 27 feet — the average rise of the spring tide in Nelson being 12 feet. With this depth the port could be worked by the largest vessels now trading there at any state of the tides. The Harbour Board, in carrying out the scheme, have for the present decided to construct the south mole only, and dredge the channel in the first instance some 600 feet in width ; but whether they are wise in omitting the north mole and narrowing the channel remains to be seen. The dredging is a fairly heavy work, especially through the boulder bank, which is composed almost entirely of grey syenite, ranging from sand and shingle overlain for some 10 or 12 feet by boulders up to nearly half a ton in weight, and operations are being carried out by a dredge of the combine type, with ladder and buckets, and centrifugal-pump discharge. The buckets empty the dredged material into a revolving screen, which is perforated with holes of about t.\ inches diameter, through which the finer material passes to the pump and is discharged on to the reclamation area shown on plan ; the heavier material being delivered from the screen into barges, and utilised in the formation of the walls. The dredge, designed by Mr. Reynolds, was constructed by the Otago

Foundry, Dunedin, and is a fairly heavy machine. The engines are compound condensing, and are capable of developing 375 h.p., while the boilers, of which there are two of the dry-back marine type, are designed to give the full head of steam required by the mam engine, the winch engines and the electric light plant, with which the dredge is supplied. The boulder bank extends from what is known as Mackays bluff for a distance of some twelve or fourteen miles, without a break, and it has always been an interesting feature m connection with Nelson haven, many theories having been advanced regarding the manner in which it was formed. Hochstetter, commenting on the matter, says :—": — " The boulders are entirely of syenite, and the same rock is found on the precipitous bluff which abuts upon the sea beyond Drumduan. The source is thus explained : fragments are constantly falling from the cliffs, and the action of the heavy northerly swell, combined with a strong current, takes them towards the south. The reason of their being deposited on the existing line is that in all probability a submarine reef underlies them, of which the Arrow rock, in the entrance of the Nelson harbour, may be regarded as the southern termination. This supposition is strengthened by the fact of the Arrow rock being of the same altered schists as occur immediately to the south of the syenite." Others again have maintained that a reef underlies the bank, and is the nucleus of its formation. Mr. Reynold's theory, which, as already explained, so far appears to be correct was given by him in his report of 1899, wherein he says: — "The question of the presence of a reef formation underlying the boulder bank was mentioned to me during my inspection of the port but I can see nothing which

points to or supports such a theory. After inspecting the boulder bank and Mackays bluffs, it appears to me that it was formed in a manner similar to the sand spits which, as a rule, are to be found at the entrance of the harbours on the New Zealand coasts. There is no question, to my mmd, that the material forming the bank came from the vast slips which have fallen from the bluffs, and that the harder portions have been driven along by the rip of the heavy seas, which, in the past, have been probably more frequent and of longer duration than at present. The direction of the prevailing seas m Tasman bay is from N.N.W , magnetic, and the general trend of the boulder bank is, approximately, S S.W., so that the seas impinge upon it at an angle of about 45 degrees, but the resulting rip or deflected force would be 111 a direction towards S.S.W., which corresponds to the general line of the bank. In nearly all the bays on the New Zealand coasts sand spits will be found extending from one of the flanking bluffs to the entrance, which is usually close to, or hugging, the other, and in many cases the seas impinge more at right angles to the axis of the spit than occurs m the case under consideration. A very good example of the formation of a bank against what might appear to be the forces of the sea is to be found in the Farewell Spit, and a similar action to that taking place there has, I consider, formed the bank at Nelson." Considerable interest has been aroused in the colony as to whether the cut will attain the object for which it was designed. Should it do so the benefit conferred upon Nelson and the travelling public will be inestimable.