1-1/2 " | 24 | 30 | 36 | -- | 85
2 " | 18 | 24 | 30 | 30 | 67
--------------------------+------+------+------+------+-------
l inch of rain = 3,630 cub. feet per acre.



The amount of rainfall for which storage has to be provided is
a difficult matter to determine; it depends on the frequency
and efficiency of the overflows and the length of time during
which the storm water has to be held up for tidal reasons. It
is found that on the average the whole of the rain on a rainy
day falls within a period of 2-1/2 hours; therefore, ignoring
the relief which may be afforded by overflows, if the sewers
are tide-locked for a period of 2-1/2 hours or over it would
appear to be necessary to provide storage for the rainfall of a
whole day; but in this case again it is permissible to run a
certain amount of risk, varying with the length of time the
sewers are tide-locked, because, first of all, it only rains on
the average on about 160 days in the year, and, secondly, when
it does rain, it may not be at the time when the sewers are
tide-locked, although it is frequently found that the heaviest
storms occur just at the most inconvenient time, namely, about
high water. Table No. 9 shows the frequency of heavy rain
recorded during a period of ten years at the Birmingham
Observatory, which, being in the centre of England, may be
taken as an approximate average of the country.

TABLE No. 9.