storm water which may be expected is to ascertain the average
impervious area of, or appertaining to, each house, and divide
it by five, so as to get the area per head. Then the flow off
from any section of road is directly obtained from the sum of
the impervious area due to the length of the road, and that due
to the population distributed along it.

[Illustration: FIG. 16.--VARIATION IN AVERAGE PERCENTAGE OF
EFFECTIVE IMPERMEABLE AREA ACCORDING TO DENSITY OF POPULATION.]

In addition to being undesirable from a sanitary point of view,
it is rarely economical to construct special storm water
drains, but in all cases where they exist, allowance must be
made for any rain that may be intercepted by them. Short branch
sewers constructed for the conveyance of foul water alone are
usually 9in or 12 in in diameter, not because those sizes are
necessary to convey the quantity of liquid which may be
expected, but because it is frequently undesirable to provide
smaller public sewers, and there is generally sufficient room
for the storm water without increasing the size of the sewer.
If this storm water were conveyed in separate sewers the cost
would be double, as two sewers would be required in the place
of one. In the main sewers the difference is not so great, but
generally one large sewer will be more economical than two
smaller ones. Where duplicate sewers are provided and arranged,
so that the storm water sewer takes the rain-water from the
roads, front roofs and gardens of the houses, and the foul
water sewer takes the rain-water from the back roofs and paved
yards,