CHAPTER IV 



PROPORTION REACHING THE SEWERS 



THE maximum intensity of the rainfall to be cared for by 

 the sewer having been determined, either by carefully examining 

 the tabulated records or by making a diagram of the storms, 

 as indicated in the last chapter, the other part of the problem, 

 already stated, needs to be solved, viz., what proportion of the 

 amount of rain fallen reaches and is carried off by the sewer, 

 and at what rate of flow does the discharge take place? Evidently 

 these are variable quantities, depending on many unknown 

 conditions. The general slope of the surface, its geological 

 character, its physical condition, whether paved or unpaved, 

 the amount of roof and yard surface compared with lawn 

 and garden surface, the grade of the lateral sewers, and the 

 temperature of the air as affecting evaporation, will all influence 

 that proportion. Perhaps more than any other condition, 

 the previous state of the atmosphere will affect this amount. 

 If there has been for some time before the excessive rain a steady 

 drizzle, so that the ground has been well soaked and made 

 partially impervious, the amount afterward absorbed by the 

 soil is very small and the sewer receives a correspondingly larger 

 amount of water. It is therefore impossible to say, even with 

 a surface of known slope or known physical conditions, that 50 

 or 70 or 90 per cent of a rainfall will enter the sewer, because 

 no account can be taken of the soil permeability.* The only 

 absolute conditions occur when there is no exposed surface, 

 that is, when the district is entirely covered with roofs; then, 

 of course, all the rain is discharged at once into the sewers. 



One method suggested for determining the rate of discharge 

 is to compare the time required for discharge with that required 



* Jour. West. Soc. C. E., Vol. IV, p. 152. 



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