72 



SEWER DESIGN 



that 1 8 per cent of the acre would probably be roof -surf ace. 

 To this was added the impervious surface of the streets, which, 

 with due allowance for the future, was taken as 16 per cent 

 of the acre, making in all 34 per cent impervious, the rest being 

 well-compacted earthen surfaces of back yards and courts 

 which are specially drained. These last were taken to be of 

 such a character and amount as to discharge rain-water from 

 an area equal to 25 per cent of the whole. For a density of 50 

 persons per acre it was assumed that there were no vacant 

 lots, that both the dwellings and the business and apartment 

 buildings were more crowded together, since the land is more 

 valuable, and that therefore the roof-surface amounts to 28 

 per cent of the acre. The amount of street-surface will not 

 differ materially from the amount previously estimated, but 

 nine-tenths of it, or 25 per cent of the whole, may be regarded 

 as impervious. Since the yards are more likely to be paved, 

 they may be considered to discharge an amount equal to 28 

 per cent of the whole. Similar analyses were made for other 

 densities, and the final relations determined on are as follows: 



TABLE VIII 



If the population-densities increase beyond 50 persons 

 per acre, the roof and impervious street-surface will also increase 

 up to a maximum, while the hard-earth surface will increase 

 up to a certain point and then rapidly decrease, being replaced 

 by a larger value of the other two factors, the open-earth space 

 being taken up entirely with paved yards. The amount of 



