270 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



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

 other case it depends on the propelling head and the direction 

 in which the water leaves the sluice." * 



Duration of Scouring. At Dunkirk, sections of the channel 

 taken before and during the process of scouring, went to show 

 that the whole effect is produced in the course of the first 

 quarter of an hour, there being no alteration in the sectional 

 area of the channel after that time. 2 



Mr. Keller, Government Engineer of Berlin, remarks with 

 regard to artificial scouring that the flushing water should not 

 be permitted to discharge suddenly, but be distributed over a 

 period of some time. By such means, the dangerous rapidity 

 of flushing would be prevented, and the scour, though less violent, 

 would exercise a more lasting and beneficial influence ; 3 as the 

 issuing stream has to put in motion the whole of the water in 

 the basin or channel, it is some time before it reaches its maximum 

 velocity, therefore this period should be made to coincide with 

 dead low water. 4 



When artificial scour is employed, it generally takes place 

 only at spring tides. The sluices are opened a little before low 

 water, and the scouring lasts from one and a half to two hours. 

 This rarity of action has a bad effect, as compared with con- 

 tinuous natural scour, owing to the opportunity given to the 

 silt to settle and harden. Artificial scour should, therefore, be 

 more frequent, begin earlier and continue till the turn of the 

 tide. 5 



Fresh Water for Scouring Purposes. It has been frequently 

 observed that salt and fresh water flow in opposite directions, 

 the salt water flowing in as an undercurrent, while the fresh 

 water is running out over the tidal water. 6 Observations made 

 on the currents of the River Dee, at Aberdeen, showed that on 

 certain occasions there was a perceptible outward current at the 

 surface during the whole period of the flood tide, and that simul- 

 taneously there was a current of sea water running inwards. The 

 salt water flowed in underneath, rising up the fresh water which 

 in reality ran out over the top of the salt water. It was thought 

 that these results were perhaps due to the steepness of the river 

 bed and the velocity of the fresh water, but they tended to show 



1 Stevenson, Construction of Hnrbours," 2nd ed. p. 235. 



9 Ibid., p. 237. M.P.LC.K, vol. Ixx. p. 67. 4 //</(?., vol. Ixvii. p. 460. 



Hid., p. 462. 8 Ibid., vol. xxi. p. 336 ; vol. xx. p. 325. 



