it was found in the case previously worked out in detail that
in built-up roads a width of 36 ft + 2 (8 ft 7 in) = 53 ft 2
in, or, say, 160 sq. ft per lineal yard of road would drain to
the storm water sewer, and a width of 2 (6 ft 10 in) = 13 ft 8
in, or, say, 41 sq. ft per lineal yard of road to the foul
water sewer. This shows that even if the whole of the rain
which falls on the impervious areas flows off, only just under
80 per cent. of it would be intercepted by the special storm water
sewers. Taking an average annual rainfall of 30 in, of which 75 per
cent. flows off, the quantity reaching the storm water sewer in the
course of a year from each lineal

30 75
yard of road would be --- x 160 x --- = 300 cubic
12 100
feet = 1,875 gallons.

[Illustration: FIG. 17.--SECTION OF "LEAP WEIR" OVERFLOW]

The cost of constructing a separate surface water system will
vary, but may be taken at an average of, approximately, l5s.
0d. per lineal yard of road. To repay this amount in thirty years
at 4 per cent, would require a sum of 10.42d., say 10-1/2d. per annum;
that is to say, the cost of taking the surface water into special

10-1/2 d. x 1000
sewers is ---------------- = 5.6, say 6d. per 1,000
1875
gallons.