CHAPTER III 

 EXCESSIVE RAINS 



IF storm- water drains are to be constructed, it becomes 

 necessary to determine, as closely as possible, the amount 

 of storm-water likely to enter the sewer. Evidently it will 

 be due to two influencing conditions: the actual amount of 

 ram falling in a given time, and the proportion of that amount 

 reaching and carried off by the sewer. Only recently has 

 careful observation been brought to bear on these points, 

 and even now only an approximate estimate is possible, as 

 the conditions are continually changing. 



It should here be pointed out that the run-off for sewers differs 

 in this respect from the run-off for storage and for power pur- 

 poses. As long-time stream-gagings become more available, 

 there is a growing tendency to disregard altogether studies of 

 the rainfall over the watershed, as" being valueless if not 

 misleading. Thus Clemens Herschel has declared * that in 

 forming a judgment as to the discharge of a river, a knowledge 

 of rainfall is of no importance and that therefore rainfall 

 records should cease to have the attention given them as in the 

 illogical reports on stream flow of the past. 



Mr. J. C. Hoyt, of the U. S. Geological Survey, also says f 

 that it is difficult to understand how engineers can continue 

 to estimate run-off from rainfall data. 



For maximum rates of run-off, however, a knowledge of 

 maximum rates of rainfall is essential and the local conditions 

 known before any indication of the probable run-off is obtained. 



It may also be asserted that, while the annual rainfall 

 is an interesting meteorological study and while engineers 



* Trans. Am. Soc. C. E., Vol. LVIII, p. 30. 

 t Trans. Am. Soc. C. E., Vol. LVHI, p. 34. 



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