268 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



Hiscox. 1 



24 feet per minute moves fine clay. 



30 



48 



96 

 210 

 270 

 360 

 600 



Beardmore. 2 



loam and fine sand, 

 fine gravel, 

 gravel size of beans, 

 shingle 1 inch diameter, 

 stones 1 incli diameter. 



3 to 6 inch diameter, 

 boulders and rocks. 



30 feet per minute will not disturb clay with sand and stones. 



40 sweep along coarse sand. 



60 fine gravel. 



120 rounded pebbles. 



ISO angular stones. 



For the velocity required to move stones or shingle, Sir J. 

 Leslie gives the following formula : v = 4 Vet. Where a denotes 

 in feet the side of a cubic block of stone, or the diameter of 

 a boulder, and v the velocity of the water in miles per hour 

 which is capable of moving it along the bottom. 8 



Lentz gives 2^ feet per second as the lowest velocity that 

 will scour silt, and 5 feet to 6J feet as the lowest that will 

 scour sand. 4 



These velocities greatly exceed those given by other authori- 

 ties. The explanation of this is that Lentz's figures refer 

 to the power of raising and scouring away, while others refer 

 to transporting only ; that is, that it takes ten times the velocity 

 of water to scour up silt that has once become settled and firm 

 than is required to prevent the silt from settling. 



Rankine gives of the top velocity as the bottom velocity 

 of ordinary currents, and in the case of very slow currents. 5 



The primary object in sluicing is to set in motion the whole 

 body of water in the channel or basin at a velocity sufficient 

 to remove the material to be dealt with. The sluicing should, 

 as a rule, be frequent, so as to prevent any great deposit and 

 aggregation taking place and establishing a condition which 



Scientific American Supplement, vol. xxxi. p. 12,608. 



" Manual of Hydrology," p. 3'J. 



Stevenson's " Construction of Harbours," 2nd ed. p. 238. 



M.r.LC.E., vol. Ixvii. p. 461; vol. Ixx. p. 36. 



" Manual of Civil Engineering," p. 674. 



