146 



FLOODS AT THE MANCHESTER WATERWORKS 



operations were being carried on to render the reservoir water- 

 tight, which rendered it undesirable to impound water to a 

 greater depth than about 30 feet. 



At Rhodes Wood Reservoir, which it will be remembered 

 is the lowest in the valley, the embankment was sufficiently 

 advanced to allow water to be impounded 40 feet in depth. 

 Through the pipes of this reservoir all the water had to be 

 passed, and therefore by calculating the quantity which could 

 be discharged through them according as the pressure varied 

 during the progress of the flood, the whole quantity could 

 be precisely ascertained. 



It is well known that the velocity with which water is dis- 

 charged through pipes or through apertures, varies as the 

 square root of the pressure or head of water above the 

 opening. 



The proper co-efficient for finding the mean velocity of the 

 water discharged varies according to the character of the 

 opening. 



The theoretical velocity due to the height is the same as 

 that of falling bodies, which is ascertained by multiplying 

 the square root of the height by the co-efficient 8.0458. 



The co-efficient, however, for finding the actual velocity 

 varies from 5 to 7 ; that generally used for ordinary openings 

 being from 5.1 to 5.4. 



Should the water approach the opening with any velocity, 

 that must be taken into account, and a higher co-efficient 

 employed. 



To determine the quantity which will be discharged through 

 pipes, diffi3rent formulae have to be employed, for the friction 

 along the sides of the pipe forms a material element in 

 retarding the velocity of the water. Amongst many valuable 

 rules deduced from the experiments of various eminent 

 mathematicians and scientific observers, probably the simplest 

 for calculating this velocity, and perhaps that most generally 

 adopted, is one by Dr. Young from Eytelwein's Hydraulics, 

 and is as follows : — 



