THE RETAINING WALL

Otago Daily Times, Issue 21390, 18 July 1931, Page 8

THE RETAINING WALL

REASONS FOR REMOVAL HARBOUR BOARD ENGINEER INTERVIEWED. Some speculation has been aroused as to the reasons which induced the. Harbour Board to come to a decision to remove a large portion of the upper section of the half-tide retaining wall. The general opinion seems to have been that the action was dictated by the necessity for securing more swinging room at the entrance to the basin, and possibly also to provide for a greater scour by tidal action in the Victoria channel. In the course of a conversation with a Daily Times reporter yesterday the Harbour Board engineer (Mr J. M‘G. Wilkie) explained that this was only half the truth, and that a number of other factors had influenced the board in arriving at its decision. Mr Wilkie stated that primarily the rt roval of the wall had been necessitated by the gradual increase in the tonnage and length of ships coming up to Dunedin and by the fact that the wharves had been extended further down towards Logan Point. It was a well-known fact that a ship passing through a current must be moving faster than the current itself in order that she should answer to her helm, otherwise, her steering way being lost, she was liable to get out of control. The wall caused a fairly strong tidal current to run up the Victoria channel past the ton end, which was almost opposite the far end of the Victoria wharf. Ships of the size of those which visited Dunedin in the past were able to reduce speed at a point much nearer the basin than was the case with the largest vessels to-day, and before they had lost steerage way they were free of the current and had entered the practically motionless water of the basin. Complete control of the ship was thus retained until she had reached her berthage. A large overseas vessel of the type which now visited this port, however. coming up to Dunedin on the flood tide, was compelled to reduce speed while still well down the channel, with the result that before she had reached the basin she was travelling at practically the same speed as the current. As a result it was found that there was a tendency for the ship to give some difficulty in handling during the strong flood tide. By removing the wall to a point almost opposite Ravensbourne it was considered that the distance which a ship would travel through still water would be very materially increased, giving a large vessel much more room in which to reduce speed without losing steerage way. It would also be possible by dredging to increase the size of the basin and afford more swinging room for ships when berthing and leaving the wharfs. It had been stated in some quarters, Mr Wilkie said, that by removing the wall as far aS Ravensbourne the amount of water passing down the channel with the ebhing tide would be reduced. He himself, however, was of the opinion that the opposite would be the case, and that when the work was completed it would be found that the volume would be actually greater and that the scouring effect of the current in the channel would thus be increased. Silting up bad never been rapid at the top end of the channel, and he believed that when the wall had been demolished the comparatively still water which would exist down to Ravensbourne would largely prevent the silting up process, making the work of keeping a clear channel to the basin easier than in the past. The task of removing the wall, which is composed of loose rock, has been in progress for a short time. A grab, mounted on a punt, is being used to bring the material to the surface, the spoil then being conveyed by floats down to the Port Chalmers end of the wall, where it is being used to extend the lower section of the wall. The work is not being pushed ahead as a matter Of urgency, and it is anticipated that it will not be completed for another 12 months.