RAINFALL.
The next consideration is the amount of rain-water for which
provision should be made. This depends on two factors: first,
the amount of rain which may be expected to fall; and,
secondly, the proportion of this rainfall which will reach the
sewers. The maximum rate at which the rain-water will reach the
outfall sewer will determine the size of the sewer and capacity
of the pumping plant, if any, while if the sewage is to be
stored during certain periods of the tide the capacity of the
reservoir will depend upon the total quantity of rain-water
entering it during such periods, irrespective of the rate of
flow.
Some very complete and valuable investigations of the flow of
rain-water in the Birmingham sewers were carried out between
1900 and 1904 by Mr. D. E. Lloyd-Davies, M.Inst.C. E., the
results of which are published in Vol. CLXIV., Min Proc.
Inst.C.E. He showed that the quantity reaching the sewer at any
point was proportional to the time of concentration at that
point and the percentage of impermeable area in the district.
The time of concentration was arrived at by calculating the
time which the rain-water would take to flow through the
longest line of sewers from the extreme boundaries of the
district to the point of observation, assuming the sewers to be
flowing half full; and adding to the time so obtained the
period required for the rain to get into the sewers, which
varied from one minute where the roofs were connected directly
with the sewers to three minutes where the rain had first to
flow along the road gutters. With an average velocity of 3 ft
per second the time of concentration will be thirty minutes for
each mile of sewer. The total volume of rain-water passing into
the sewers was found to bear the same relation to the total
volume of rain falling as the maximum flow in the sewers bore
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
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 -- --
10 " ............... 1.25 1.50 2.00 -- --
15 " ............... 1.05 1.25 1.50 -- --
20 " ............... 0.95 1.05 1.30 1.20 3.00
25 " ............... 0.85 0.95 1.15 -- --
30 " ............... 0.80 0.90 1.05 1.00 2.50
35 " ............... 0.75 0.85 0.95 -- --
40 " ............... 0.70 0.80 0.90 -- --
45 " ............... 0.65 0.75 0.85 -- --
1 hour .................. 0.50 0.60 0.70 0.75 1.80
1-1/2 " .................. 0.40 0.50 0.60 -- 1.40
2 " .................. 0.30 0.40 0.50 0.50 1.10
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
-------------------------------------------------------
0.4 inches and over 155 times each year
0.5 " 93 "
0.6 " 68 "
0.7 " 50 "
0.8 " 33 "
0.9 " 22 "
1.0 " 17 "
1.1 " Once each year
1.2 " Once in 17 months
1.25 " " 2 years
1.3 " " 2-1/2
1.4 " " 3-1/3
1.5 " " 5 years
1.6 " " 5 years
1.7 " " 5 years
1.8 " " 10 years
1.9 " " 10 years
2.0 " " 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 compare those taken in Birmingham with the mean of those
given in "Symons' Rainfall," taken at thirty-seven different
stations distributed over the rest of the country.
TABLE No. 10.
RAINFALL FOR 1903.
Mean of 37
stations in
Birmingham England and
Wales.
Daily Rainfall of 2 in and over ...... None 1 day
Daily Rainfall of 1 in and over ...... 3 days 6 days
Daily Rainfall of 1/2 in and over .... 17 days 25 days
Number of rainy days.................. 177 days 211 days
Total rainfall ...................... 33.86 in 44.89 in
Amount per rainy day ................ 0.19 in 0.21 in
The year 1903 was an exceptional one, but the difference
existing between the figures in the above table and the average
figures in Table 9 are very marked, and serve to emphasise the
necessity for close investigation in each individual case. It
must be further remembered that the wettest year is not
necessarily the year of the heaviest rainfalls, and it is the
heavy rainfalls only which affect the design of sewerage works.