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DiscussionSir Claude Inglls, introducing the Paper, said that everything in it was in agreement with, and followed directly from, the regime concept and its laws. An estuary was said to be in regime when the silt and sand carried into it by its rivers and from the sea during flood tides, was washed out again during ebb tides; so that no overall change took place. This tacitly assumed that the level of the sea relative to the land, and the materials exposed on the bed of the estuary, showed no progressive change-that, in fact, the whole of the material coming down the rivers was washed out to sea. This could not occur in an estuary with a loose boundary of silt and sand unless it had developed a regime shape-that was to say, a shape which tapered gradually from its seaward end-so that the loss of energy, due to friction as the tidal wave progressed upstream, was a minimum for the material in the estuary. 68. Where the sea level was rising relative to the land-as was the case at the mouths of the Thames* and the Wash-not merely did the depth increase because of the relative rise of water level, but the velocity/depth relation also increased; so, where the bed of the estuary was mobile, it scoured, increasing the depth still further.69. In an estuary which differed widely in shape from the tapering regime outlineas in the Wash-little if any of the silt reached the sea, most of it depositing in the shoaler parts of the estuary near the mouths of the rivers and around the margin.70. In the case of the Wash, however, the marginal accretion could not have been wholly, or even mainly, due to rivers, for two reasons; first, because the volume of accretion in the reclaimed areas shown in Fig. 2a, Plate 1, and Fig. 16, Plate 2, far exceeded the volume of silt carried by the rivers; and secondly, because the greatly accelerated and long-continued accretion which resulted from building training walls and reclamation embankments could only be explained by assuming that, as in the case of the Lune and the Mersey, only part of the silt carried in from the sea by the flood tides was washed out again by the ebb tides, the quantity of silt retained increasing as the rate of accretion increased. This silt from the sea then progressed upstream; because in an estuary which was not in regime the suspended silt tended to concentrate near the bed where the upstream flow during flood tides exceeded the downstream flow during the ebb tides. The rate of this upstream progression of silt was observed to exceed 1 mile/day in the Thames6 and was much greater in the Mersey.71. Fig. 28a showed the margin of the Wash and the M.L.W.S. contour in 1917-18, when the last complete Admiralty survey had been carried out. This contour could be considered as the approximate dividing line between the sandy bed and the silty foreshore; and when compared with the margin, it would be seen that there WBS a marked tendency for the Wash to develop a more natural outline than its present margin. This tendency was still more clearly seen in Fig. 28b, which

(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...

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