CHAPTER VIII.

STORM WATER IN SEWERS.


If the whole area of the district is not impermeable the
percentage which is so must be carefully estimated, and will
naturally vary in each case. The means of arriving at an
estimate will also probably vary considerably according to
circumstances, but the following figures, which relate to
investigations recently made by the writer, may be of interest.
In the town, which has a population of 10,000 and an area of
2,037 acres, the total length of roads constructed was 74,550
lineal feet, and their average width was 36 ft, including two
footpaths. The average density of the population was 4.9 people
per acre. Houses were erected adjoining a length of 43,784
lineal feet of roads, leaving 30,766 lineal feet, which for
distinction may be called "undeveloped"--that is, the land
adjoining them was not built over. Dividing the length of road
occupied by houses by the total number of the inhabitants of
the town, the average length of road per head was 4.37 ft, and
assuming five people per house and one house on each side of
the road we get ten people per two houses opposite each other.
Then 10 x 4.37 = 43.7 lineal feet of road frontage to each pair
of opposite houses. After a very careful inspection of the
whole town, the average area of the impermeable surfaces
appertaining to each house was estimated at 675 sq. ft, of
which 300 sq. ft was apportioned to the front roof and garden
paths and 375 sq. ft to the back roof and paved yards. Dividing