STORM WATER IN SEWERS.
If the whole area of the district is not impermeable the
percentage which is so must be carefully estimated, and will
naturally vary in each case. The means of arriving at an
estimate will also probably vary considerably according to
circumstances, but the following figures, which relate to
investigations recently made by the writer, may be of interest.
In the town, which has a population of 10,000 and an area of
2,037 acres, the total length of roads constructed was 74,550
lineal feet, and their average width was 36 ft, including two
footpaths. The average density of the population was 4.9 people
per acre. Houses were erected adjoining a length of 43,784
lineal feet of roads, leaving 30,766 lineal feet, which for
distinction may be called "undeveloped"--that is, the land
adjoining them was not built over. Dividing the length of road
occupied by houses by the total number of the inhabitants of
the town, the average length of road per head was 4.37 ft, and
assuming five people per house and one house on each side of
the road we get ten people per two houses opposite each other.
Then 10 x 4.37 = 43.7 lineal feet of road frontage to each pair
of opposite houses. After a very careful inspection of the
whole town, the average area of the impermeable surfaces
appertaining to each house was estimated at 675 sq. ft, of
which 300 sq. ft was apportioned to the front roof and garden
paths and 375 sq. ft to the back roof and paved yards. Dividing
these figures by 43.71 in ft of road frontage per house, we
find that the effective width of the impermeable roadway is
increased by 6 ft 10 in for the front portions of each house,
and by a width of 8 ft 7 in, for the back portions, making a
total width of 36 ft + 2(6 ft 10 in) + 2(8 ft 7 in) = 66 ft 10
in, say 67 ft On this basis the impermeable area in the town
therefore equals: 43,7841 in ft x 67 ft =2,933,528; and 30,766
lin ft x 36 ft = 1,107,576.
Total, 4,041,104 sq. ft, or 92.77 acres. As the population is
10,000 the impermeable area equals 404, say, 400 sq. ft per
head, or ~ (92.77 x 100) / 2037 = 4.5 per cent, of the whole
area of the town.
It must be remembered that when rain continues for long
periods, ground which in the ordinary way would generally be
considered permeable becomes soaked and eventually becomes more
or less impermeable. Mr. D. E. Lloyd-Davies, M.Inst.C.E., gives
two very interesting diagrams in the paper previously referred
to, which show the average percentage of effective impermeable
area according to the population per acre. This information,
which is applicable more to large towns, has been embodied in
Fig. 16, from which it will be seen that, for storms of short
duration, the proportion of impervious areas equals 5 per cent.
with a population of 4.9 per acre, which is a very close
approximation to the 4.5 per cent. obtained in the example just
described.
Where the houses are scattered at long intervals along a road
the better way to arrive at an estimate of the quantity of
storm water which may be expected is to ascertain the average
impervious area of, or appertaining to, each house, and divide
it by five, so as to get the area per head. Then the flow off
from any section of road is directly obtained from the sum of
the impervious area due to the length of the road, and that due
to the population distributed along it.
[Illustration: FIG. 16.--VARIATION IN AVERAGE PERCENTAGE OF
EFFECTIVE IMPERMEABLE AREA ACCORDING TO DENSITY OF POPULATION.]
In addition to being undesirable from a sanitary point of view,
it is rarely economical to construct special storm water
drains, but in all cases where they exist, allowance must be
made for any rain that may be intercepted by them. Short branch
sewers constructed for the conveyance of foul water alone are
usually 9in or 12 in in diameter, not because those sizes are
necessary to convey the quantity of liquid which may be
expected, but because it is frequently undesirable to provide
smaller public sewers, and there is generally sufficient room
for the storm water without increasing the size of the sewer.
If this storm water were conveyed in separate sewers the cost
would be double, as two sewers would be required in the place
of one. In the main sewers the difference is not so great, but
generally one large sewer will be more economical than two
smaller ones. Where duplicate sewers are provided and arranged,
so that the storm water sewer takes the rain-water from the
roads, front roofs and gardens of the houses, and the foul
water sewer takes the rain-water from the back roofs and paved
yards,
it was found in the case previously worked out in detail that
in built-up roads a width of 36 ft + 2 (8 ft 7 in) = 53 ft 2
in, or, say, 160 sq. ft per lineal yard of road would drain to
the storm water sewer, and a width of 2 (6 ft 10 in) = 13 ft 8
in, or, say, 41 sq. ft per lineal yard of road to the foul
water sewer. This shows that even if the whole of the rain
which falls on the impervious areas flows off, only just under
80 per cent. of it would be intercepted by the special storm water
sewers. Taking an average annual rainfall of 30 in, of which 75 per
cent. flows off, the quantity reaching the storm water sewer in the
course of a year from each lineal
30 75
yard of road would be --- x 160 x --- = 300 cubic
12 100
feet = 1,875 gallons.
[Illustration: FIG. 17.--SECTION OF "LEAP WEIR" OVERFLOW]
The cost of constructing a separate surface water system will
vary, but may be taken at an average of, approximately, l5s.
0d. per lineal yard of road. To repay this amount in thirty years
at 4 per cent, would require a sum of 10.42d., say 10-1/2d. per annum;
that is to say, the cost of taking the surface water into special
10-1/2 d. x 1000
sewers is ---------------- = 5.6, say 6d. per 1,000
1875
gallons.
If the sewage has to be pumped, the extra cost of pumping by
reason of the increased quantity of surface water can be looked
at from two different points of view:--
1. The net cost of the gas or other fuel or electric current
consumed in lifting the water.
2. The cost of the fuel consumed plus wages, stores, etc., and
a proportion of the sum required to repay the capital cost of
the pumping station and machinery.
The extra cost of the sewers to carry the additional quantity
of storm water might also be taken into account by working out
and preparing estimates for the alternative schemes.
The actual cost of the fuel may be taken at approximately 1/4
d. per 1,000 gallons. The annual works and capital charges,
exclusive of fuel, should be divided by the normal quantity of
sewage pumped per annum, rather than by the maximum quantity
which the pumps would lift if they were able to run
continuously during the whole time. For a town of about 10,000
inhabitants these charges may be taken at 1-1/4 d. per 1,000
gallons, which makes the total cost of pumping, inclusive of
capital charges, 1-1/2 d. per 1,000 gallons. Even if the extra
cost of enlarging the sewers is added to this sum it will still
be considerably below the sum of 6 d., which represents the
cost of providing a separate system for the surface water.
Unless it is permissible for the sewage to have a free outlet
to the sea at all states of the tide, the provision of
effective storm overflows is a matter of supreme importance.
Not only is it necessary for them to be constructed in well-
considered positions, but they must be effective in action. A
weir constructed along one side of a manhole and parallel to
the sewer is rarely efficient, as in times of storm the liquid
in the sewer travels at a considerable velocity, and the
greater portion of it, which should be diverted, rushes past
the weir and continues to flow in the sewer; and if, as is
frequently the case, it is desirable that the overflowing
liquid should be screened, and vertical bars are fixed on the
weir for the purpose, they block the outlet and render the
overflow practically useless.
Leap weir overflows are theoretically most suitable for
separating the excess flow during times of storm, but in
practice they rarely prove satisfactory. This is not the fault
of the system, but is, in the majority of the cases, if not
all, due to defective designing. The general arrangement of a
leap weir overflow is shown in Fig. 17. In normal circumstances
the sewage flowing along the pipe A falls down the ramp, and
thence along the sewer B; when the flow is increased during
storms the sewage from A shoots out from the end of the pipe
into the trough C, and thence along the storm-water sewer D. In
order that it should be effective the first step is to
ascertain accurately the gradient of the sewer above the
proposed overflow, then, the size being known, it is easy to
calculate the velocity of flow for the varying depths of sewage
corresponding with minimum flow, average dry weather flow,
maximum dry weather flow, and six times the dry weather flow.
The natural curve which the sewage would follow in its downward
path as it flowed out from the end of the sewer can then be
drawn out for the various depths, taking into account the fact
that the velocity at the invert and sides of the sewer is less
than the average velocity of flow. The ramp should be built in
accordance with the calculated curves so as to avoid splashing
as far as possible, and the level of the trough C fixed so that
when it is placed sufficiently far from A to allow the dry
weather flow to pass down the ramp it will at the same time
catch the storm water when the required dilution has taken
place. Due regard must be had to the altered circumstances
which will arise when the growth of population occurs, for
which provision is made in the scheme, so that the overflow
will remain efficient. The trough C is movable, so that the
width of the leap weir may be adjusted from time to time as
required. The overflow should be frequently inspected, and the
accumulated rubbish removed from the trough, because sticks and
similar matters brought down by the sewer will probably leap
the weir instead of flowing down the ramp with the sewage. It
is undesirable to fix a screen in conjunction with this
overflow, but if screening is essential the operation should be
carried out in a special manhole built lower down the course of
the storm-water sewer. Considerable wear takes place on the
ramp, which should, therefore, be constructed of blue
Staffordshire or other hard bricks. The ramp should terminate
in a stone block to resist the impact of the falling water, and
the stones which may be brought with it, which would crack
stoneware pipes if such were used.
In cases where it is not convenient to arrange a sudden drop in
the invert of the sewer as is required for a leap weir
overflow, the excess flow of storm-water may be diverted by an
arrangement similar to that shown in Fig. 18. [Footnote: PLATE
IV] In this case calculations must be made to ascertain the
depth at which the sewage will flow in the pipes at the time it
is diluted to the required extent; this gives the level of the
lip of the diverting plate. The ordinary sewage flow will pass
steadily along the invert of the sewer under the plate until it
rises up to that height, when the opening becomes a submerged
orifice, and its discharging capacity becomes less than when
the sewage was flowing freely. This restricts the flow of the
sewage, and causes it to head up on the upper side of the
overflow in an endeavour to force through the orifice the same
quantity as is flowing in the sewer, but as it rises the
velocity carries the upper layer of the water forward up the
diverting plate and thence into the storm overflow drain A deep
channel is desirable, so as to govern the direction of flow at
the time the overflow is in action. The diverting trough is
movable, and its height above the invert can be increased
easily, as may be necessary from time to time. With this
arrangement the storm-water can easily be screened before it is
allowed to pass out by fixing an inclined screen in the
position shown in Fig. 18. [Footnote: PLATE IV] It is loose, as
is the trough, and both can be lifted out when it is desired to
have access to the invert of the sewer. The screen is self-
cleansing, as any floating matter which may be washed against
it does not stop on it and reduce its discharging capacity, but
is gradually drawn down by the flow of the sewage towards the
diverting plate under which it will be carried. The heavier
matter in the sewage which flows along the invert will pass
under the plate and be carried through to the outfall works,
instead of escaping by the overflow, and perhaps creating a
nuisance at that point.