CHAPTER VI 

 MATHEMATICAL FORMULA 



WE have seen how the amount of rainfall to be provided 

 for in a sewer depends on the rate of rainfall and on the dura- 

 tion of the storm; that this amount is an uncertain quantity, 

 and that its value is generally made to depend more on a long- 

 established custom than on any experimental certainty. We 

 have seen that the rate of rainfall may vary from the least 

 dampness through rates of an inch per hour, which is the rate 

 usually given in the text-books, up to 4, 5, or even 6 inches per 

 hour. We have further seen that the maximum rate is a func- 

 tion of the length of the storm, and that it is not possible to 

 make a determination of a rain rate unless the length of the 

 storm considered is also known. It has been pointed out that 

 while high rates are usually only for short periods, they may 

 nevertheless be more troublesome than a more moderate rain 

 lasting a longer time and yielding a larger volume. We have 

 seen that the period of time adopted as a unit is of importance 

 for calculating the intensity of the storm, and that the size of 

 the district from which the run-off is to be determined governs 

 the choice of this period. The fact that the condition of the 

 ground, its slope, porosity, degree of saturation, all have an 

 influence on the proportion of rainfall furnished to the sewers 

 has also been pointed out. And the evident conclusion is 

 that there is a wide latitude for judgment, that it is not possible 

 to make a design with the precision used in other engineering 

 constructions, but that the size of the storm-sewer can only 

 be properly designed by carefully considering the district to 

 be served, and by basing the judgment, which must be used, 

 on as thorough an acquaintance with the district as possible. 



In spite of all the uncertainty as to the data of the problem, 



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