Discussion. Changes in the Wash as Affected by Training Walls and Reclamation Works.

Abstract

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