The figures in column A will not probably be exceeded more than once in each year, those in column B will not probably be exceeded more than once in three years, while those in column C will rarely be exceeded at all. Columns D and E refer to the records tabulated by the Meteorological Office, the rainfall given in column D being described in their publication as "falls too numerous to require insertion," and those in column E as "extreme falls rarely exceeded." It must, however, be borne in mind that the Meteorological Office figures relate to records derived from all parts of the country, and although the falls mentioned may occur at several towns in any one year it may be many years before the same towns are again visited by storms of equal magnitude.
While it is convenient to consider the quantity of rainfall for which provision is to be made in terms of the rate of fall in inches per hour, it will be useful for the practical application of the figures to know the actual rate of flow of the storm water in the sewers at the point of concentration in cubic feet per minute per acre. This information is given in the following Table No. 8, which is prepared from the figures given in Table No. 7, and is applicable in the same manner.
TABLE No. 8.
MAXIMUM FLOWS OF STORM WATER.
—————————————+—————————————————
| Maximum storm water flow in
| cubic feet per min per acre
| of impervious area.
Time of Concentration. +———+———+———+———+———
| A | B | C | D | E
—————————————+———+———+———+———+———
5 minutes | 106 | 121 | 181 | — | —
10 " | 75 | 91 | 121 | — | —
15 " | 64 | 75 | 91 | — | —
20 " | 57 | 64 | 79 | 73 | 181
25 " | 51 | 57 | 70 | — | —
30 " | 48 | 54 | 64 | 61 | 151
35 " | 45 | 51 | 57 | — | —
40 " | 42 | 48 | 54 | — | —
45 " | 39 | 45 | 51 | — | —
1 hour | 30 | 36 | 42 | 45 | 109
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.
FREQUENCY OF HEAVY RAIN ———————————————————————————-
Total Daily Rainfall. Average Frequency of Rainfall
———————————————————————————-