Part 4 (1/2)
The intensity of the rainfall decreases as the period over which the rainfall is taken is increased For instance, a rainfall of linat 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 tohere autoes 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 Birham Observatory, shows the approxi 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 minutes175 200 300 -- -- 10 ”125 150 200 -- -- 15 ”105 125 150 -- -- 20 ”095 105 130 120 300 25 ”085 095 115 -- -- 30 ”080 090 105 100 250 35 ”075 085 095 -- -- 40 ”070 080 090 -- -- 45 ”065 075 085 -- -- 1 hour050 060 070 075 180 1-1/2 ”040 050 060 -- 140 2 ”030 040 050 050 110
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 Coluical Office, the rainfall given in colu described in their publication as ”falls too numerous to require insertion,” and those in column E as ”extreme falls rarely exceeded” It ical Office figures relate to records derived froh the falls mentioned may occur at several towns in any one year it ain visited by stornitude
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 perTable No 8, which is prepared froiven 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 ae has to be provided is a difficult matter to determine; it depends on the frequency and efficiency of the overflows and the length of ti 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 per 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 tih it is frequently found that the heaviest storh water Table No 9 shows the frequency of heavy rain recorded during a period of ten years at the Birland, e of the country
TABLE No 9
FREQUENCY OF HEAVY RAIN -------------------------------------------------------
Total Daily Rainfall Average Frequency of Rainfall
04 inches and over 155 times each year 05 ” 93 ”
06 ” 68 ”
07 ” 50 ”
08 ” 33 ”
09 ” 22 ”
10 ” 17 ”
11 ” Once each year 12 ” Once in 17 months 125 ” ” 2 years 13 ” ” 2-1/2 14 ” ” 3-1/3 15 ” ” 5 years 16 ” ” 5 years 17 ” ” 5 years 18 ” ” 10 years 19 ” ” 10 years 20 ” ” 10 years
It will be interesting and useful to consider the records for the year 1903, which was one of the wettest years on record, and to coiven in ”Symons' Rainfall,” taken at thirty-seven different stations distributed over the rest of the country
TABLE No 10
RAINFALL FOR 1903