to the maximum intensity of rainfall during a period equal to
the time of concentration. He stated further that while the
flow in the sewers was proportional to the aggregate rainfall
during the time of concentration, it was also directly
proportional to the impermeable area. Putting this into
figures, we see that in a district where the whole area is
impermeable, if a point is taken on the main sewers which is so
placed that rain falling at the head of the branch sewer
furthest removed takes ten minutes to reach it, then the
maximum flow of storm water past that point will be
approximately equal to the total quantity of rain falling over
the whole drainage area during a period of ten minutes, and
further, that the total quantity of rainfall reaching the
sewers will approximately equal the total quantity falling. If,
however, the impermeable area is 25 per cent. of the whole,
then the maximum flow of storm water will be 25 per cent. of
the rain falling during the time of concentration, viz., ten
minutes, and the total quantity of storm water will be 25 per
cent. of the total rainfall.

If the quantity of storm water is gauged throughout the year it
will probably be found that, on the average, only from 70 per
cent. to 80 per cent. of the rain falling on the impermeable
areas will reach the sewers instead of 100 per cent., as
suggested by Mr. Lloyd-Davies, the difference being accounted
for by the rain which is required to wet the surfaces before
any flow off can take place, in addition to the rain-water
collected in tanks for domestic use, rain required to fill up
gullies the water level of which has been lowered by
evaporation, and rain-water absorbed in the joints of the