SCOUR "ING AND SLUICING. 291 



" But notwithstanding the numerous drawbacks connected 

 with the system, it will be apparent that the sluicing was 

 capable of maintaining the low-water basin at its proper depth. 

 In fact, it could do more than that, for at the time the operations 

 were commenced a deposit of 32 inches in thickness had 

 accumulated, and at the end of ten months, when the scour 

 had operated with more or less regularity through fifteen sets 

 of spring tides, the deposit had been reduced to 14 inches, 

 leading to the conclusion that when the primary level had 

 once been reached a less amount of scouring power would suffice 

 to maintain the required depth. 



" It was not, therefore, on the ground of inability to maintain 

 it at its proper depth that the closing of the low-water basin 

 and its conversion into a wet dock was determined on. The 

 principal reasons for adopting this course were that the 

 operation of the sluices had been found to be dangerous to 

 the stability of the works and practically unsuited to the 

 proper and efficient working of the great float and low- water 

 basin for dock purposes." 



Among the causes which led to the failure of these sluicing 

 operations there appear to be the following : l 



The primary object was to set in motion the body of water 

 in the low-water basin. This was 1750 feet in length, and 

 varied from 300 to 400 feet in width, the depth being, when 

 sluicing commenced, 19 feet 4 inches. The weight of water 

 to be set in motion, or simply to be started, was therefore 

 338*275 tons ; and for this purpose twenty sluices were provided 

 having a combined area of 826 square feet, with a head of 

 water of 14 feet 3 inches, producing a total pressure of 336 

 tons ; or less than one-thousandth part of the load. 



This was a greater disproportion than was found to exist 

 in other instances where a load had to be moved by a com- 

 paratively small power. The 336 tons of power appear to have 

 been employed against the 338'275 tons of load at a velocity 

 of about 17 knots per hour, which had the effect of disturbing 

 portions of the water and throwing it into a state of great 

 confusion. 



The division of the sluices into two groups by an interval 

 of 1 20 feet led to eddies and foul currents. 



1 M.PJ.CK, vol xxix. p. 3. 



