52 SEWER DESIGN 



were affected by rain of a different intensity from nearer points, 

 which can discharge their share of the run-off only after the 

 crest of the rainstorm has passed. By assuming a number 

 of zones around the outfall, an additive method has been 

 developed * by which each zone has its approximate duration 

 and intensity. The resulting average time of concentration 

 is thus shown to be only about one-half that if the rain were 

 assumed to fall uniformly for the entire period. It is believed, 

 however, that such a refinement is not justified. There are 

 rains falling with a maximum intensity almost uniformly for 

 the short period involved in these studies. The rate of rain- 

 fall is only one factor in determining the run-off and the other 

 factors, such as slope, form of the tributory area, and character 

 of the surface introduce such uncertainties that any attempt 

 to subdivide the time of concentration into the time by zones 

 Is more of theoretical mathematical interest than of real practical 

 advantage. Further, if another rate of rainfall than that 

 shown by the observed time elapsing from the beginning of the 

 storm to the time of the maximum flow in the sewer, checked 

 in the case of Rochester at least by the actual time required for 

 water to reach the outfall from the most distant point, is taken, 

 then other values than those found in Rochester for the per- 

 centage of the rainfall reaching the sewer must be taken. 



For an example of the use of the diagrams described above 

 see Chapter XIV. 



PROBLEMS 



14. Using the data of Table II plot a curve similar to that shown in 

 Fig. 4 and derive its equation in the form y = 



15. Plot the data of Table III, using logarithmic paper, and derive 

 the equation of the curve passing approximately through those points 



of the form y = ~^- 



* Engineering News, Vol. LXI, p. 265; Engineering Record, Vol. LIX, p. 265; 

 Trans. Am. Soc. C. E., Vol. LVIII, p. 458; Vol. LXV, p. 321; Jour. Ass'n Eng. 

 Soc., Vol. XX, p. 204. 



