Principles of training Rivers through Tidal Estuaries. 505 



of depth is attained. In very winding rivers, the additional precaution 

 has to be taken of somewhat reducing the width where the deepest 

 channel shifts over from the concave bank on one side to the concave 

 bank on the opposite side at the next bend lower down, so as to reduce 

 the shoal which is found near the point of contrary flexure by con- 

 centrating the current at this place. 



The training of the outlets of sediment-bearing rivers into tideless 1 

 seas is determined by the same principles; for a definite discharge is 

 directed and concentrated between training walls or piers, so as to 

 scour a channel across the bar formed, in front of the outlet, by the 

 accumulation of deposit dropped by the enfeebled issuing current. 

 The increased velocity of the current through the contracted outlet 

 carries the silt into deeper water, where it is either borne away by 

 any littoral current, or again forms a bar, after a lapse of time 

 depending on the depth, which can be removed by an extension of the 

 training works. 



The training also of the upper part of the tidal portion of rivers 

 has been effected on similar principles to the non-tidal portion, with 

 satisfactory results, even though the problem is, in this case, com- 

 plicated by the changes in the direction of the current, and the 

 requisite maintenance of the tidal capacity. 



In the lower parts, however, of tidal rivers, where the tidal flow 

 predominates, it is difficult to determine the proper width for a trained 

 channel, which, whilst narrow enough to secure an adequate depth, 

 should not very materially check the tidal flow to the detriment of 

 the outlet. Moreover, where the estuary is large, considerable doubt 

 may exist as to the best direction for the training walls ; and the 

 establishment of training walls in a wide estuary, where the flood tide 

 is charged with silt, has resulted in extensive accretions,* and corre- 

 sponding reduction of tidal capacity, by the concentration of the tidal 

 flow and ebb in the trained channel, and a consequent enfeeblement 

 of the currents ab the sides, favouring deposit. The principles, 

 indeed, upon which the training of tidal rivers should be based, are in 

 a very undefined and unsatisfactory condition, as exemplified by the 

 conflicting opinions of engineers whenever important training works 

 through estuaries are proposed, as exhibited with reference to the 

 schemes for training works in the upper estuary of the Mersey,f for 

 which the Manchester Ship Canal promoters sought powers in 1883 

 and 1884, and as at present exist about the extension of the training 

 works in the Ribble estuary. J This is due to the various conditions 



* ' Instit. Civ. Engin. Proe.,' vol. 84, pp. 246 and 295, and Plates 4 and 5. 



t Evidence before Select Committees of Lords and Commons on the Manchester 

 Ship Canal Bills, Sessions 1883 and 1884, and ' Instit. Civ. Engin. Proc.,' vol. 84, 

 p. 309, fig. 7. 



' Instit. Civ. Engin. Proc./ vol. 84, p. 260, fig. 1. 



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