32 SEWER DESIGN 



interested in water-power and water-storage will continue to 

 refer to the amount of water falling per year as a basis for 

 their studies, such facts have no bearing on the needed capacity 

 of sewers. The fact that in New England the annual rainfall 

 is about 40 vertical inches and in Kansas only 20, does not 

 mean that the sewers in the former area must all be twice 

 the capacity of those in the latter. Nor is it necessary, because 

 the rainfall in Washington is 1 20 inches per year to build sewers 

 there three times as large as in New England. There is then 

 a difference between the amount of rain falling per year and 

 some other characteristic that determines the necessary size of 

 a sewer. 



In this country the first extended study of the subject was 

 made by Col. J. W. Adams in designing the early sewers for 

 Brooklyn. He noted the fact, since emphasized by A. J. 

 Henry in a special report of the Weather Bureau, that excessive 

 rains, or those that do damage, are naturally divided into two 

 broad classes: (a) rains of great intensity and short duration, 

 and (b) rains of light intensity and long duration; and that of 

 the two classes, the first are far more damaging and destructive. 

 Col. Adams, after consulting all the meagre rainfall records 

 available, chiefly those of 1849 to I ^s6, and noting that there 

 were but 19 days in .which the rainfall in 4 hours was an inch 

 or over, and but 15 days in which the rainfall for the entire 

 24 hours was as much as 2 inches, that the heaviest storms 

 reported were two of i\ inches in 4 hours, and that there was 

 no reported occurrence of as much as i inch within the hour, 

 concluded that if he made provision in his Brooklyn sewers 

 to carry off a rainfall of i inch per hour it would be sufficient. 



The two reports on the sewerage of Providence, one by 

 J. H. Shedd, published in 1874, and one by S. M. Gray, pub- 

 lished in 1884, represent the next advance in the study of the 

 question of rainfall. Mr. Shedd noted that of 185 storms 

 recorded for the 26 years before 1860 only 20 were at a rate 

 of over \ inch per hour, while 165 were of less, and that of 

 139 storms recorded in the 14 years, 1861-1875, 20 were over 



