CHAPTER VIIL 



Deposits Scouring power of water Primary object in sluicing Bottom velocity 

 Duration of scouring Fresh water for scouring purposes Avonmouth 

 entrance Tilbury Dock entrance Barry Dock entrance Thames Embank- 

 ment landing-places Calais entrance channel Dunkirk entrance channel 

 St. George's Stage, Liverpool Removal of shoals by propeller sluicing 

 Honfleur sluicing basin Canada Basin, Liverpool Low-water Basin, Birken- 

 head. 



DEPOSITS arise from two essentially opposite causes, viz. com- 

 motion and stagnation. They are also of different kinds of 

 material. First, those lighter matters such as mud, silt, and 

 fine sand which are held for a time in mechanical suspension, 

 and which, as the water approaches stagnation, are gradually 

 dropped in the order of their specific gravity and mass. 

 Secondly, those grosser matters, such as the different kinds of 

 sand and gravel which are rolled along the bottom or drawn 

 forward with the water by the action of the waves and currents. 



The first-named class are deposited within enclosed areas, 

 and on the lee side of piers and groynes, which obstruct the 

 progress of the waves and currents. The second class of deposits 

 accumulate on the weather side of piers and groynes ; the amount 

 and position assumed will depend on the direction and force of 

 the waves or currents in relation to the foreshore, and to the 

 obstructions. 



Scouring Power of Water. There appears to be some difference 

 of opinion as to the scouring action of water, as will be seen 

 from the following data taken from different authorities, as to 

 the bottom velocities required to move different materials. 



Stevenson. 1 



15 feet per minute will just begin to work on fine clay. 



30 lift fine sand. 



40 lift sand as coarse as linseed. 



60 sweep along fine gravel 



120 roll along rounded pebbles 1 inch diameter. 



180 sweep along slippery angular stones the size 



of an egg. 

 393-60 required at Havre and Fecamp to scour gravel. 



1 Stevenson, ** Construction of Harbours," 2nd ecL p. 238. 



