Discussion. Field and Model Investigation Into the Reasons for Siltation in the Mersey Estuary. (Includes Corrigenda).

Abstract

(formerly Chief Engineer, Manchester Ship Canal Co.) said that the part of the investigation in which his company was vitally concerned was the Eastham channel. The Eastham channel formed the approach to the Port of Manchester and it was absolutely essential that it should be maintained at a depth sufficient to deal with the class of traffic which the port handled.108. As Mr Price had said, since 1953, a great deal of difficulty had been experienced in dealing with the Eastham channel. The rate of siltation consistently exceeded the maximum possible dredging output, and consequently the channel slowly but surely silted up.109. When the Hydraulics Research Station took on the investigation, his company asked if it would examine certain schemes for training walls to see whether that type of construction would be of any use in dealing with the extremely difficult situation. The Research Station tested various schemes. All were fairly promising, but from an engineering point of view it was felt that none of them could be recommended to the company for the simple reason that the constructional difficulties were enormous. It would have been necessary to carry out the construction in a tidal estuary where the tidal range was 30 ft, where current-velocities ran up to a rate of 4-5 knots at half ebb, and where the work could be carried out with the least possible interference to traffic. The final objection to any project of that kind was the high cost of any of the schemes. The cost was not estimated accurately. but it was quite obvious that it would run into very big figures. That was expenditure of a kind which the company was not prepared to meet having regard to the uncertainty of the results likely to be achieved.110. The second proposal that the Wallingford station made was a very attractive one indeed from the company's point of view. The Research Station suggested that the company should stop dredging altogether and allow the channel to find its own depth. They said the depth would never become less than 5 ft and, on the model, it had been demonstrated very conclusively that that was so. But Mr Milne himself, with his day-to-day experience of that small part of the Mersey, could not agree with the Research Station that that was likely to occur in practice. His own view had been, and still was, that the channel could shoal up to 5 ft, and that the 5-ft soundings would develop into 4 ft, the 4 ft into 3 ft, and so on. That had been the case in the past, and he thought it would be the case in the future.111. Although, by stopping dredging altogether and adopting a channel with a depth of 5 ft below L.B.D., the company would have saved about S600 000 a year, DISCUSSION ON YIELD AND MODEL INVESTIGATION lNTOit was decided that it would not be possible for Manchester to continue as a first class port with an entrance channel of only 5 ft, and therefore they had to consider continuing with dredging. 112. About that time there was an alteration in the pattern of siltation which enabled his company to alter their dredgin...