io8 IMPROVED FISHERY HARBOUR ACCOMMODA TION 



level than that of the original bottom is so often attended 

 with great difficulty and expense. Where the deposit is 

 confined to the space between high- and low-water marks, 

 the scouring by means of salt or fresh water is comparatively 

 easy ; but where there is a bar outside the entrance, the 

 case becomes very different. The efficacy of the scour, so 

 long as it is not impeded by enlargements of the channel, 

 may be kept up for long distances, but it soon comes to an 

 end after it meets the sea." 



" When the volume of water liberated is great compared 

 with the alveus or channel through which it has to pass, the 

 stagnant water which originally occupied the channel does 

 not, to the same extent, destroy the momentum as where 

 the scouring has to be produced by a sudden finite impulse. 

 In the one case the scouring power depends c&teris paribus 

 simply on the relation subsisting between the quantity 

 liberated in a given space of time, and the sectional area 

 of the channel through which it has to pass ; while in the 

 other it depends on the propelling head, and the direction in 

 which the water leaves the sluice. 



Mr. Rendel's scheme for Birkenhead was on the former 

 principle, which it must be recollected is only applicable 

 where the soil is easily stirred up. In all cases of scouring 

 it is of course an essential condition that such a velocity be 

 generated as is necessary for acting upon the soil. The 

 large amount of back-water will be inoperative if it has less 

 than what may be called the effective velocity, or that required 

 for acting on the particular kind of soil of which the 

 bottom consists. 



If, for example, the discharge of a river be utilised 

 by the construction of regulating reservoirs, there will 

 be a diminution of scouring power, because a sudden 

 flood will remove what the same, or even a much greater, 



