The Sewerage of Sea Coast Towns

de. This depends on two factors: first, the amount of rain which may be expected to

the pumping plant, if any, while if the sewage is to be stored during certain periods of the tide the capacity of the res

s, 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 distri

h 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 addit

eing 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

ential 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,

E No

.. 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

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 t

or 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 minut

E No

LOWS OF S

---+------

storm wa

eet per m

perviou

tration. +---+

B | C

--+---+---+

106 | 121

| 91 | 1

| 75 | 9

| 64 | 79

| 57 | 7

| 54 | 64

| 51 | 5

| 48 | 5

| 45 | 5

0 | 36 | 4

24 | 30 |

| 24 | 30

--+---+---+

= 3,630 cub.

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

E No

Y RAIN ----------

all. Average Freq

---------

" 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

--------

ettest years on record, and to compare those taken in Birmingham with the mean of those given in "S

E No

LL FOR

n o

tio

ham Eng

le

f 2 in and over

1 in and over ..

1/2 in and over ..

ys................

..................

day .............

e very marked, and serve to emphasise the necessity for close investigation in each individual case. It must be further remembered that the we