76 SEWER DESIGN 



By reference to Table VIII it is plainly seen that if it were 

 residential area, the roof surface and road area would accord 

 with a population of less than 15 per acre, and that therefore 

 by Table X there should be somewhat less than 15 per cent 

 the intensity of rain found in the rate of run-off. The max- 

 imum storm measured had a flow of thirty-five minutes at the 

 rate of 0.78 inch per hour, the duration being that for 

 concentration. The rate of rainfall was 46.0 cubic feet per 

 second for the entire area and the maximum run-off was 2.5 

 cubic feet or 5.4 per cent. The entire storm lasted 3^ hours, and 

 the ratio of the entire volume of run-off to the entire rainfall 

 was 19.3, the run-off, however, lasting sixteen hours. 



In New York City in 1888, gagings * by Rudolph Hering 

 were made of the run-off from a district in the lower part of the 

 city containing 221 acres. The time of concentration was 

 probably about forty-five minutes. The percentages of rela- 

 tively impervious area were as follows: 



That is, assuming 70 per cent of the paved area and 20 

 per cent of the grass area to be relatively impervious, 78 per 

 cent of the rainfall should reach the sewer. The actual per- 

 centages reported range from less than 10 to 75 per cent. A 

 percentage of 65 was common, and one rain following a snow 

 storm appeared to discharge even more than 75 per cent. It 

 is, however, worth noting that the greatest flow per second 

 in the sewer was due to a very heavy thunder shower, lasting 

 thirteen minutes, 38 per cent of which gave the maximum 

 sewage flow recorded. 



In applying the relation to cases where greater densities 

 of population occur than are given in Table X, it must be 

 remembered that the rate of increase of the reduced imper- 



* Trans. Am. Soc. C. E., Vol. LVIII, p. 464- 



