EXCESSIVE RAINS 39 



farther than any single drainage-area within ordinary municipal 

 limits. 



The relation between the intensity of a rain and the duration 

 of that intensity, shown by Table III above, was brought out by 

 Kuichling very clearly, by means of which he finds a method 

 of determining the duration of any rain of a given assumed 

 intensity. A similar method is generally applicable. The 

 exact relation is unreliable, as it varies in different localities, 

 and, the data being uncertain, it is probable that for some time 

 to come conclusions will be only approximate. The method 

 outlined is, however, the best available for gaining this first 

 step in determining the amount of rain to be considered in the 

 sewer design. 



The method may be reduced to the following: First collect 

 all the rainfall statistics that are available for the city in ques- 

 tion and for any other places that are in the same locality and 

 under the same meteorological conditions. Unfortunately 

 such data are usually defective in accuracy and in the time 

 covered, but no other method will ever give as good results 

 as a study of past records. With all the available data at hand, 

 compute the intensities of all rainfalls whose rate of fall is 

 greater than | inch per hour, regardless of the duration of the 

 storm, and for every recorded storm, plot a point on cross- 

 section paper with the intensity as ordinate and the duration 

 of the storm as abscissa. A number of points, each correspond- 

 ing to a storm, are thus obtained. The rainfalls of low intensities 

 are, of course, most frequent, so that that part of the diagram 

 will be well studded with points; but the isolated points repre- 

 senting the heavier rains will usually be sufficient in number 

 to show that the shorter rains and heavier intensities correspond, 

 and that there is some proportionate relation between the two. 

 By joining the points by a series of broken lines, selecting those 

 points which represent the greatest recognized intensities for 

 that time, an irregular envelope is found, the ordinates of which 

 give the probable maximum intensities for that locality for the 

 corresponding period of time. This envelope is only located 



