88 SEWER DESIGN 



McMath's formula for St. Louis gives: 



C = 8.2i for built-up areas; 



C = 3-39 for rural and suburban areas. 



On the N. Y. diagrams the values are: 



C = 10.59 f r built-up areas; 

 C= 8.97 for average areas; 

 C= 6.59 for rural areas. 



In the design of the Walworth Run Sewer in Cleveland, 

 use was made of an original formula: 



or in the form of p. 85 



a somewhat lower value of the exponent of s, but otherwise 

 very similar to the others. The values of c-r chosen range 

 from 3 to 7, the latter being for the most densely built-up part 

 of the city. Fig. 13 already referred to shows that the form 

 of the equation is not the important factor but rather the value 

 assigned to the constant C. This has already been shown to 

 vary not only with the amount of impervious surface, but 

 with other factors, so that an engineer in one city might properly 

 assume 75 per cent of the rainfall to enter the sewers while in 

 another city, with the same rain intensity and impervious area, 

 50 per cent might be ample. It is impossible to reduce a ques- 

 tion of judgment and experience, such as the assignment of a 

 proper value to C becomes, to the fixity of a mathematical 

 table. The opinion of an experienced engineer on this point 

 should always be sought by a young engineer with no previous 

 experience who is designing storm-sewers. 



As a further example of work done in this direction, Fig. 



