paving.

The intensity of the rainfall decreases as the period over
which the rainfall is taken is increased. For instance, a
rainfall of lin may occur in a period of twenty minutes, being
at the rate of 3 in per hour, but if a period of one hour is
taken the fall during such lengthened time will be considerably
less than 3 in In towns where automatic rain gauges are
installed and records kept, the required data can be
abstracted, but in other cases it is necessary to estimate the
quantity of rain which may have to be dealt with.

It is impracticable to provide sewers to deal with the maximum
quantity of rain which may possibly fall either in the form of
waterspouts or abnormally heavy torrential rains, and the
amount of risk which it is desirable to run must be settled
after consideration of the details of each particular case. The
following table, based principally upon observations taken at
the Birmingham Observatory, shows the approximate rainfall
which may be taken according to the time of concentration.


TABLE No. 7.

INTENSITY OF RAINFALL DURING LIMITED PERIODS.
Equivalent rate in inches per hour
of aggregate rainfall during
Time of Concentration, period of concentration
A B C D E
5 minutes ............... 1.75 2.00 3.00 -- --