522 Mr. L. F. Vernon-Harcourt. 



It wonld be impossible to determine by experiment the time any 

 changes in an estuary would occupy. The figures, in fact, giving- 

 the number of tides during which each experiment was worked, are 

 not even intended as an indication of the rate of change in the model, 

 and much less as any measure of the period required for such changes 

 in an estuary, but merely as a record of the comparative duration of 

 each experiment. It was observed, however, that the changes were 

 most rapid where the modifications effected by the lines of walls 

 inserted in the model were greatest (Plate 2), and slowest where the 

 lines in the model produced the least alterations. (Plate 3, fig. 1, 

 Plate 4, fig. 3.) 



Principles for Training Tidal Rivers deduced from Experiments. 



The foregoing investigations, viewed merely as experiments, with- 

 out any reference to their bearing on the Seine, may serve for 

 indicating some general principles applicable in training tidal rivers- 

 through wide estuaries. Direct experiment for each estuary is 

 undoubtedly preferable to abstract reasoning, where such experiment 

 is possible, as it reproduces the special conditions of the estuary to be 

 investigated. Nevertheless, general principles may be of value in 

 guiding the choice of designs to be investigated, so as to avoid waste 

 of time in testing unfavourable schemes, and also in cases where the 

 conditions of an estuary are not sufficiently known to afford a correct 

 basis for experiment. 



The experiments may be divided into* three classes, namely : 



(1.) Outlet of estuary considerably restricted, and channel trained 

 inside towards outlet. (Plate 2.) 



(2.) Channel trained in sinuous line, expanding towards outlet, 

 but kept somewhat narrow at changes of curvature. (Plate 3, figs. 2 

 and 3, and Plate 4, figs. 1 and 2.) 



(3.) Channel trained in as direct a course as practicable, and 

 expanding regularly to outlet. (Plate 3, fig. 1, and Plate 4, fig. 3.) 



The experiments of the first class exhibited a deep outlet, and a 

 fairly continuous channel inside, where the training works were pro- 

 longed to the outlet. The channel, however, was irregular in depth 



tidal flow in and out of the lower estuary would favour the natural heaping-up 

 action of the sea on the sands outside ; so that eventually, not only would the initial 

 deepening of the narrowed outlet be lost, but the good depths in the bay outside the 

 estuary would be imperilled." 



Compare also Plate 3, fig. 1, with the following extract from ' Instit. Civ. Engin. 

 Proc.,' vol. 84, p. 250 : " The continuously concave southern training wall, whilst 

 very favourable to Honfleur, will unduly keep the ebb current to that side, and 

 therefore away from Havre. Also, the extension of the wall along the Katier Bank 

 will act like a groyne, and, arresting the silt-bearing southern current, will connect 

 Trouville Bank with the shore, and lead to a large accumulation of deposit in front 

 of Trouville .... and also the low walls proposed will not prevent accretion." 



