66 SEWER DESIGN 



of rain absorbed by grasses, and vegetation generally was an 

 important factor quite distinct from the amount of percola- 

 tion into the ground. 



Mr. Alvord, who has had a long practical experience in the 

 application of the theory of run-off to actual construction, 

 lays great stress on the absorption capacity of the soil.* He 

 believes that the earth becomes a great storage reservior after 

 a dry season and may entirely absorb the water from a short 

 storm. He shows that a certain district of Chicago, of an average 

 residential character, would require by theory a sewer 8| feet 

 diameter, but by reason of the soil conditions, the existing 

 sewer 4 feet diameter, J the capacity, is found to be large enough. 



Mr. J. H. Fuertes f has reported that by actual measure- 

 ments of rainfall and run-off, he found the percentage on an 

 open field, of hard clay soil, covered with grass, on a 5 per 

 cent slope to be 29 per cent, a high ratio for grass land. 



Perhaps no better summary can be had than that given 

 by Professor Mars ton, who says: { " The most important part 

 of this paper (by C. E. Gregory) seems to the writer to be the 

 general presentation of the following principles: 



" First. The water falling on the so-called impervious area 

 of an ordinary sewer watershed does not all run off as fast 

 as it falls, but part of it accumulates in increasing quantities 

 on the surface during downpours of moderate length, such as 

 cause the maximum discharges from sewer districts of ordinary 

 size. 



" Second. As the storm continues at the same rate, the ratio 

 of run-off from the impervious area to the rate of rainfall 

 increases, owing to the increased depth and velocity of the sur- 

 face flow toward the sewer, until, finally, if the storm lasts long 

 enough, the rate of run-off from the impervious areas becomes 

 equal to 100 per cent of the rate of rainfall. 



" Third. The pervious area becomes more and more 



* Jour. West. Soc. Engrs., Vol. IV, p. 154. 

 f Jour. West. Soc. Engrs., Vol. IV, p. 170. 

 } Trans. Am. Soc. C. E., Vol. LVIII, p. 498. 



