80 SEWER DESIGN 



of impervious surface. The formula was first published in 

 this country in the report of James P. Kirkwood on the Water- 

 works of Brooklyn. It was used by Sir Joseph Bazelgette and 

 Mr. William Haywood in preparing the plans for the main 

 drainage- works of London, and has been much used elsewhere 

 both in this country and in England. 

 In its original form it was 



log rf- 



10 

 or, divested of its logarithmic form, 



rf 10 =^6309574; 



where d = diameter of sewer in inches; 

 A = number of acres drained; 



N = length in which the main falls one foot, which equals 

 1/5, where s is sine of slope, 



If 1/5 be substituted for N, and D in feet for d in inches 



or 



**- ** 



= 0.0001010 . 

 98135 y s 



This formula is still used by the Borough of Brooklyn 

 except that the term 6.8 in the formula is changed to 8, making 



Z) 10 = .001615. 



Since the rainfall is assumed to be i inch per hour, and 

 since half of it is assumed to enter the sewers, these two factors 

 are really understood, so that if r = the rainfall in inches per 

 hour reaching the sewers, which is equal to the actual rainfall 



