CHAPTER V 

 RELATION OF DENSITY TO PERCENTAGE 



WHILE it is evident that more rain will be discharged into 

 a sewer from a closely built-up territory than from an open and 

 agricultural district, yet so far as the data of the last chapter 

 go, no light has been thrown on the proper variation of the 

 percentage as determined by the relative amounts of pervious 

 and impervious surface. Our knowledge on this subject is 

 due to Mr. Kuichling. 



If it is assumed, as indeed seems reasonable, that the 

 density of population bears a direct ratio to the percentage of 

 impervious area in a given district, and if that ratio is once 

 determined under general conditions, the determination for 

 other places of their population-densities will serve approx- 

 imately, at least, by means of the same ratio, to determine 

 the percentage of impervious surface also, and so the percentage 

 of the rainfall discharged through the sewers. The relation 

 between the population and the impervious surface was found 

 by a laborious compilation of the amount and character of 

 street-surface, roofs, lawns, gardens, etc., and of the population, 

 all in typical districts, and by a reduction of all the areas of 

 semi-impervious nature to the areas of impervious surface, 

 equivalent in discharging power. It was assumed that the 

 duration of the storms was such that even from impervious 

 pavements not all the rain was discharged an assumption 

 only justifiable in dealing with storms of great magnitude, 

 whose duration is expressed in minutes. For long rains even 

 garden or lawn surfaces may reduce the losses due to evapora- 

 tion and surface inequalities, so that if the duration of the 

 storm is sufficient, the surface becomes practically impervious; 

 but in general such soils will absorb nearly all the rain falling. 



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