384 SEWERAGE 



The quantity r, though commonly stated as the rate of rain- 

 fall during the greatest downpour, has been shown to be 

 scarcely more than an arbitrary coefficient. In climates 

 where the intensity of rainfall varies greatly with the dura- 

 tion of the storm, it is necessary, in using r, to fix on a definite 

 length of time as representing the duration of a typical storm, 

 and this is equivalent to arbitrarily fixing the value of r. In 

 using the Buerkli formula, the European practice is to give r 

 values ranging from 1.75 to 2.5 in. per hr., but recent American 

 practice gives r values of from 2 to 3.5, and even higher, for 

 sewers designed to carry all the storm water. In St. Louis, 

 Mo., a value of .75 for/ and values for r varying from 3.02, for a 

 district containing 100 A. to 3.51, for a district containing 2,000 

 A., were used. Observations taken in Rochester, N. Y., of 

 rainstorms lasting less than 1 hr. indicate that, for the condi- 

 tions in that city, storms lasting 51 minutes give the greatest 

 flow. For storms of this duration, the value of r will be about 2. 

 A value of 2.75 is about the mean of American practice. 



Capacity Required for a Separate System. The design of the 

 sewers of a separate system is based on the quantity of sewage 

 delivered, and provision must also be made for carrying subsoil 

 and ground water. No rule can be given for determining the 

 amount of subsoil water that may be added to the flow. For 

 8-in. pipes, it runs from 5,000 gal. per mi. per da. to 25,000 

 gal. or more. It can only be determined approximately from 

 previous experience in similar cases and from what knowledge 

 can be secured as to the subsurface conditions. 



Sewage Discharge and Water Supply. The available records 

 of sewer gaugings for American cities are not sufficient to indi- 

 cate accurately the quantity of sewage per capita that must be 

 provided for. Records of water supply, however, are abunc 

 and, since the sewer gaugings that have been made indicate 

 the quantity of sewage from a given district is somewhat 

 than the quantity of water consumed by its inhabitants, 

 statistics of water supply are useful and are the main fact 

 in estimating the sewage discharge. 



In using the records of a public water supply for this pur- 

 pose, it must be remembered that often there are facto- 

 ries that have a private water-supply, and these may often 



