64 SEWER DESIGN 



of increase in the said percentages of discharge changes abruptly 

 from a high to a low figure, agrees closely with the computed 

 lengths of time required for the concentration of the storm- 

 waters from the whole tributary area; and hence the said 

 percentages at such times may be taken as the proportion of 

 impervious surface upon the respective areas." 



Since the early work of Kuichling in Rochester no similar 

 work on a generous scale has been reported to the engineering 

 world. In 1908, a committee of the Boston Society of Civil 

 Engineers was appointed * to compile available data and to 

 direct experiments to be made by members of the society, on 

 this subject. Some work has been done, but no report published. 

 Similarly, the Sanitary and Municipal Section of the Western 

 Society of Engineers has undertaken to collect data from their 

 members. The engineers in charge of sewer work in Phila- 

 delphia, in St. Louis, in Cleveland, and doubtless in many 

 other cities, have on hand experimental data giving valuable 

 comparisons for those particular cities and is much to be hoped 

 that they will some day allow them all to be published. The 

 time may come when the uncertainties of the percentage of 

 run-off to rainfall will be recognized as so great as to make 

 values based on it useless. Then if accumulated records 

 will allow, storm-water flow can be referred directly to area and 

 surface conditions. Until such data are available, at all events, 

 a percentage of the rainfall is the only known method of approx- 

 imating to the truth. 



Three large districts of Chicago have been studied, the 

 sewage measured and the rainfall gaged. L. K. Sherman 

 presented the results f to the Western Society of Engineers, 

 Jan. 15, 1912. The unit of time used however was the day 

 so that the effect of short storms of high intensity is not shown. 

 It was demonstrated that while the maximum rainfall was 

 such as to deliver water to the area at rates of 95, 47, and 48 

 cubic feet per second per square mile, the run-off was 50, 15, 



* Engineering News, Vol. LIX, p. 219. 



t Jour. West. Soc. Engrs., Vol. XVII, p. 361. 



