COOK STRAIT.

Taranaki Central Press, Volume IV, Issue 319, 29 December 1936, Page 2

COOK STRAIT.

SURVEY OF ITS DEPTHS

Revelations Of The Soundings

l A considerable addition to the knowledge of Cook Strait has been made through the series of over two hundred soundings taken by H.M.S. “Wakakura” recently. The surface of this stormy' piece of water is familiar to many thousands of people and they will be interested to know something of the depths between Lyall Bay and the month of Blind River, Marlborough, about forty miles south-south-west of the northern point. The existence of several “deeps” had been proved by previous experience in cable-laying, and the Post Office obtained the services of the Navy to make a very detailed examination of the area over which it is proposed to lay a modern coaxial cable now being manufactured in England. This will be an important, link between the North and South Islands for it will be capable of providing up to twentyfour channels of telephone communication simultaneously, when worked to full capacity, thus providing ample scope to meet future expansion of telephone toll business across Cook Strait.

The greatest care is necessary in fixing the position for such an important cable so that it will lie on the sea-floor without risk of damage from chafing. Soundings were therefore taken at half-mile intervals along the whole route, and where there was uncertainty regarding an i ideal bed for the cable, double and even treble lines of soundings were made on parallel lines. Samples of the material on the sea-bed were also secured, and the result of a fortnight’s work under difficult weather conditions has been the selection of a thoroughly satisfactory' route on which the cable should lie secure for at least twenty-five years without risk of interruption to the traffic it will carry. Progressive deterioration must be expected, but with a carefully make British cable for which a good bed has been surveyed, it is anticipated that 25 years should elapse before, a definite fault is likely to develop.

A 2,500 Feet "Deep.” The survey disclosed an immense hole- about half-way' between Byall Bay and Blind River, the bottom of this depression being 2,500 feet below the surface. Starting from Lyall Bay in the North Island and proceeding south-west towards the proposed landing point of the cable in the South Island, 'the downward slope was found to be fairly uniform, and at two and a-half milejs the depth was about 140 feet. Six miles out the soundings -disclosed a gully across the proposed cable route. The bottom dropped sharply within a quarter of a mile from 400 feet, to 800 feet, rising again to 300 feet within three-quarters of a mile. As a result of additional soundings on either side-, the edges of the gully were surveyed and a deviation in the route slightly' to' the north will enable this depression to be avoided. For another 10 miles the depth ranged from 480 to 600 feet.

In this broken country half way across the -Strait, the soundings which had ' for seven miles been -about 540 feet suddenly increased to 900 feet and later to 1,320 feet. This “deep” was about three miles ■in extent, and towards the South Island the bottom was found to rise sharply to 600 feet. Beyond this point the water shoaled gradually, providing an ideal route for the proposed landing at Blind River.

Samples of the Sea Bed. Utilising a “snapper” dredge constructed in the Post Office -workshops, samples of the material on the bottom were secured along the whole route. These proved that the future cable will generally be resting on a good bottom where it will not be chafed by rocks. Near Ly'all Bay there are rocky patches, but in previous cable operations the engineers had obtained a good deal of knowledge of the area and will be able to avoid rough places by making slight deviations from the direct route.

Soundings were taken by means of a line piano wire only a little larger in thickness than a pin. At the end of the wire was the “snapper” dredge with a mechanism causing two grabs to close on impact with the bottom, thus picking up a little of the material. Following the sandy' conditions for a few miles from Lyall Bay came a. series of small stones, fine gravel and some sand for a considerable distance across the Strait until 20 miles from Blind River, where the depth commenced to steadily decrease, the bottom being found to be principally mud with occasional small stones and shells. The only coral brought up was from a point midway in the Straits, and •the best “catch” was well out i

deep water, the dredge bringing up from 1,200 feet a live crab, which had been existing under a pressure of 6001 b to the square inch. An oyster-like crustacean was dredged up a few miles off the mouth of the Awatere River. Very careful observations were made during th© soundings to fix exact positions for the guidance of the cable-layng steamer, and work was only undertaken when three land positions were clearly visible to enable angles to be taken by sextant. A future 'task will be to secure a continuous profile of the bottom along the whole route by means of the echo-sounding apparatus which records depth as the ship moves along. This will be done when a suitable vessel is available, and it is expected that all preparations for the new cable will be completed in good time! before i'ts arrival about mext April.