38 SEWER DESIGN 



Mr. Mitchell very properly points out that while the 

 occurrence of very heavy rains lasting a few moments only is 

 interesting, such rains have but little practical significance, both 

 because they occur at such infrequent intervals and because they 

 affect so limited an area. He also recalls that such extraordinary 

 occurrences are considered by. the courts before whom suits 

 for damage have come * as beyond the need for considera- 

 tion in engineering work, being thus classed with earth- 

 quakes, fires, strikes and other calamities impossible to 

 foresee. 



Mr. J. N. Hazlehurst, in studying the question of excessive 

 rains for the purpose of designing sewers for Mobile, Ala., 

 found f that the maximum intensity for a storm lasting for thirty 

 minutes was 3.1 inches, for sixty minutes, 2.7 inches and for 

 120 minutes 1.5 inches. He concluded, however, that such 

 storms were so infrequent as to be not worth considering and 

 he assumed an intensity of 1.5 inches per hour as a practical 

 maximum to be cared for in designing the sewers. 



The first extended and detailed study of the excessive 

 storms for a single locality was made in 1889 by Emil Kuichling, 

 C. E.,. who included in his elaborate report to the city of Rochester 

 on the East Side Sewer a discussion of the probable rainfall, 

 and the amount of storm-water to be expected. His work, 

 based on the records of the Weather Bureau at Rochester, 

 Oswego, and Buffalo, and on other records kept at Cornell 

 University, Mt. Hope Reservoir, Hemlock Lake, and by two 

 special employes of the city, emphasizes the fact, as just given, 

 that the rate of rainfall varies with the unit of time chosen 

 for the rain-measurement, and that for the greatest intensities 

 a shorter period than an hour must be chosen for a unit. He 

 also points out that the area covered by a storm is of limited 

 extent, and that the heavier the rate of fall, the less the area 

 affected. From his observations, however, he finds that gen- 

 erally the clouds which furnish the rainfalls of large rate extend 



*i 33 N. W. Rep. p. 835; 32 N. Y. p. 489. 

 f Engineering News, Vol. XL VI, p. 26. 



