490 Sir H. T. De La Beche on Estuaries and their Tides. 



Much mischief has been found to be occasioned in estuaries 

 by embanking, producing sensible effects even when portions 

 only covered at spring-tides have been taken for agricultural 

 purposes. When it is considered that the weight, for friction 

 purposes, arising from volume of water combined with the re- 

 quisite velocity, causes any given amount of scour or mecha- 

 nical action on the bed of an estuary, it follows, that if we stop 

 out the tide, diminishing the volume of water, even supposing, 

 for convenience, that the velocity remained the same, which 

 would not be the case, the scour or mechanical action cannot 

 be the same as previously, and that, this action being dimi- 

 nished, detritus which would have been swept onwards under 

 the former conditions would now remain and accumulate. 



It may be here observed, that, in estimating the power of 

 the combined action of volume and velocity of water to produce 

 scour or mechanical effects on the bottom of an estuary, great 

 care should be taken to ascertain the real velocity where the 

 waters come into contact with such bottom, shoving and push- 

 ing detritus onwards, otherwise very erroneous inferences may 

 be drawn. Certain mechanical effects are considered to be 

 produced by the action of given velocities of water. Without 

 entering upon the question of how far the experiments made 

 may appear to justify the conclusions drawn, it will be obvious 

 that, whatever action may be due to the velocity of water alone, 

 if the velocities on the surface of the main stream of tide at 

 certain times are only known, and those at the bottom, from 

 considerable friction, are much less, calculations as to scour, 

 founded on the surface velocities (those usually alone known 

 to mariners and others engaged in the navigation of estuaries), 

 cannot be accurate. 



Although much injury would be occasioned by stopping 

 out the entrance of effective tidal waters in estuaries by em- 

 bankments, it does not follow that judicious arrangements 

 may not be made with them, so that one greatly improved 

 general course may be obtained for the waters, and a main 

 channel formed of increased value for the purposes of naviga- 

 tion ; the loss of water occasioned in one place being compen- 

 sated by contrivances for allowing a greater volume to come 

 into another, so that the total effective water remains the same ; 

 a provision usually considered when improvements in estuaries 

 are under investigation. 



Again, upon the principle of letting out a volume of water 

 to scour when it can have the greatest useful velocity, and pro- 

 duce the greatest mechanical effects on the bottom, — a prin- 

 ciple, however, which requires great care and consideration in 

 its application when the channels to be scoured are long, — 

 waters may be retained behind embankments employed for 



