SELECTION OF SITE FOR OUTFALL SEWER.
The selection of the site for the sea outfall sewer is a matter
requiring a most careful consideration of the many factors
bearing on the point, and the permanent success of any scheme
of sewage disposal depends primarily upon the skill shown in
this matter. The first step is to obtain a general idea of the
tidal conditions, and to examine the Admiralty charts of the
locality, which will show the general set of the main currents
into which it is desirable the sewage should get as quickly as
possible. The main currents may be at some considerable
distance from the shore, especially if the town is situated in
a bay, when the main current will probably be found running
across the mouth of it from headland to headland. The sea
outfall should not be in the vicinity of the bathing grounds,
the pier, or parts of the shore where visitors mostly
congregate; it should not be near oyster beds or lobster
grounds. The prosperity--in fact, the very existence--of most
seaside towns depends upon their capability of attracting
visitors, whose susceptibilities must be studied before
economic or engineering questions, and there are always
sentimental objections to sewage works, however well designed
and conducted they may be.
It is desirable that the sea outfall should be buried in the
shore for the greater part of its length, not only on account
of these sentimental feelings, but as a protection from the
force of the waves, and so that it should not interfere with
boating; and, further, where any part of the outfall between
high and low water mark is above the shore, scouring of the
beach will inevitably take place on each side of it. The
extreme end of the outfall should be below low-water mark of
equinoctial tides, as it is very objectionable to have sewage
running across the beach from the pipe to the water, and if the
foul matter is deposited at the edge of the water it will
probably be brought inland by the rising tide. Several possible
positions may present themselves for the sea outfall, and a few
trial current observations should be made in these localities
at various states of the tides and plotted on to a 1:2500
ordnance map. The results of these observations will probably
reduce the choice of sites very considerably.
Levels should be taken of the existing subsidiary sewers in the
town, or, if there are none, the proposed arrangement of
internal sewers should be sketched out with a view to their
discharging their contents at one or other of the points under
consideration. It may be that the levels of the sewers are such
that by the time they reach the shore they are below the level
of low water, when, obviously, pumping or other methods of
raising the sewage must be resorted to; if they are above low
water, but below high water, the sewage could be stored during
high water and run off at or near low water; or, if they are
above high water, the sewage could run off continuously, or at
any particular time that might be decided.
Observations of the currents should now be made from the
selected points, giving special attention to those periods
during which it is possible to discharge the sewage having
regard to the levels of the sewers. These should be made with
the greatest care and accuracy, as the final selection of the
type of scheme to be adopted will depend very largely on the
results obtained and the proper interpretation of them, by
estimating, and mentally eliminating, any disturbing
influences, such as wind, etc. Care must also be taken in
noting the height of the tide and the relative positions of the
sun, moon, and earth at the time of making the observations,
and in estimating from such information the extent to which the
tides and currents may vary at other times when those bodies
are differently situated.
It is obvious that if the levels of the sewers and other
circumstances are such that the sewage can safely be discharged
at low water, and the works are to be constructed accordingly,
it is most important to have accurate information as to the
level of the highest low water which may occur in any ordinary
circumstances. If the level of a single low water, given by a
casual observation, is adopted without consideration of the
governing conditions, it may easily be that the tide in
question is a low one, that may not be repeated for several
years, and the result would be that, instead of having a free
outlet at low water, the pipe would generally be submerged, and
its discharging capacity very greatly reduced.
The run of the currents will probably differ at each of the
points under consideration, so that if one point were selected
the best result would be obtained by discharging the sewage at
high water and at another point at low water, whereas at a
third point the results would show that to discharge there
would not be satisfactory at any stage of the tide unless the
sewage were first partially or even wholly purified. If these
results are considered in conjunction with the levels of the
sewers definite alternative schemes, each of which would work
satisfactory may be evolved, and after settling them in rough
outline, comparative approximate estimates should be prepared,
when a final scheme may be decided upon which, while giving the
most efficient result at the minimum cost, will not arouse
sentimental objections to a greater extent than is inherent to
all schemes of sewage disposal.
Having thus selected the exact position of the outfall, the
current observations from that point should be completed, so
that the engineer may be in a position to state definitely the
course which would be taken by sewage if discharged under any
conditions of time or tide. This information is not
particularly wanted by the engineer, but the scheme will have
to receive the sanction of the Local Government Board or of
Parliament, and probably considerable opposition will be raised
by interested parties, which must be met at all points and
overcome. In addition to this, it may be possible, and
necessary, when heavy rain occurs, to allow the diluted sewage
to escape into the sea at any stage of the tide; and, while it
is easy to contend that it will not then be more impure than
storm water which is permitted to be discharged into inland
streams during heavy rainfall, the aforesaid sentimentalists
may conjure up many possibilities of serious results. As far as
possible the records should indicate the course taken by floats
starting from the outfall, at high water, and at each regular
hour afterwards on the ebb tide, as well as at low water and
every hour on the flood tide. It is not, however, by any means
necessary that they should be taken in this or any particular
order, because as the height of the tide varies each day an
observation taken at high water one day is not directly
comparable with one taken an hour after high water the next
day, and while perhaps relatively the greatest amount of
information can be gleaned from a series of observations taken
at the same state of the tide, but on tides of differing
heights, still, every observation tells its own story and
serves a useful purpose.
Deep floats and surface floats should be used concurrently to
show the effect of the wind, the direction and force of which
should be noted. If it appears that with an on-shore wind
floating particles would drift to the shore, screening will be
necessary before the sewage is discharged. The floats should be
followed as long as possible, but at least until the turn of
the current--that is to say, a float put in at or near high
water should be followed until the current has turned at or
near low water, and one put in at low water should be followed
until after high water. In all references to low water the
height of the tide given is that of the preceding high water.
The time at which the current turns relative to high and low
water at any place will be found to vary with the height of the
tide, and all the information obtained on this point should be
plotted on squared paper as shown on Fig. 10, which represents
the result of observations taken near the estuary of a large
river where the conditions would be somewhat different from
those holding in the open sea. The vertical lines represent the
time before high or low water at which the current turned, and
the horizontal lines the height of the tide, but the data will,
of course, vary in different localities.
[Illustration: Hours before turn of tide. FIG 10]
It will be noticed that certain of the points thus obtained can
be joined up by a regular curve which can be utilised for
ascertaining the probable time at which the current will turn
on tides of height intermediate to those at which observations
were actually taken. For instance, from the diagram given it
can be seen that on a 20 ft tide the current will turn thirty
minutes before the tide, or on a 15 ft tide the current will
turn one hour before the tide. Some of the points lie at a
considerable distance from the regular curve, showing that the
currents on those occasions were affected by some disturbing
influence which the observer will probably be able to explain
by a reference to his notes, and therefore those particular
observations must be used with caution.
The rate of travel of the currents varies in accordance with
the time they have been running. Directly after the turn there
is scarcely any movement, but the speed increases until it
reaches a maximum about three hours later and then it decreases
until the next turn, when dead water occurs again.
Those observations which were started at the turn of the
current and continued through the whole tide should be plotted
as shown in Fig. 11, which gives the curves relating to three
different tides, but, provided a sufficiently large scale is
adopted, there is no reason why curves relating to the whole
range of the tides should not be plotted on one diagram. This
chart shows the total distance that would be covered by a float
according to the height of the tide; it also indicates the
velocity of the current from time to time. It can be used in
several ways, but as this necessitates the assumption that with
tides of the same height the flow of the currents is absolutely
identical along the coast in the vicinity of the outfall, the
diagram should be checked as far as possible by any
observations that may be taken at other states of tides of the
same heights. Suppose we require to know how far a float will
travel if started at two hours after high water on a 12 ft
tide. From Fig. 10 we see that on a tide of this height the
current turns two hours and a quarter before the tide;
therefore two hours after high water will be four hours and a
quarter after the turn of the current. If the float were
started with the current, we see from Fig. 11 that it would
have travelled three miles in four hours and a quarter; and
subtracting this from four miles, which is its full travel on a
whole tide, we see that it will only cover one mile in the two
hours and a quarter remaining before the current turns to run
back again.
Although sewage discharged into the sea rapidly becomes so
diffused as to lose its identity, still occasionally the
extraneous substances in it, such as wooden matches, banana
skins, etc., may be traced for a considerable distance; so
that, as the sewage continues to be discharged into the sea
moving past the outfall, there is formed what may be described
as a body or column of water having possibilities of sewage
contamination. If the time during which sewage is discharged is
limited to two hours, and starts, say, at the turn of the
current on a 12 ft tide, we see from Fig. 11 that the front of
this body of water will have reached a point five-eighths of a
mile away when the discharge ceases; so that there will be a
virtual column of water of a total length of five-eighths of a
mile, in which is contained all that remains of the noxious
matters, travelling through the sea along the course of the
current. We see, further, that at a distance of three miles
away this column would only take thirty minutes to pass a given
point. The extent of this column of water will vary
considerably according to the tide and the time of discharge;
for instance, on a 22 ft tide, if the discharge starts one hour
after the turn of the current and continues for two hours, as
in the previous example, it will form a column four miles long,
whereas if it started two hours after the current, and
continued for the same length of time, the column would be six
miles and a half long, but the percentage of sewage in the
water would be infinitesimal.
[Illustration: Hours after turn of current FIG. 11]
In some cases it may be essential that the sewage should be
borne past a certain point before the current turns in order to
ensure that it shall not be brought back on the return tide to
the shore near the starting point. In other words, the sewage
travelling along the line of a branch current must reach the
junction on the line of the main current by a certain time in
order to catch the connection. Assuming the period of discharge
will be two hours, and that the point which it is necessary to
clear is situated three miles and a half from the outfall, the
permissible time to discharge the sewage according to the
height of the tide can be obtained from Fig. 11. Taking the 22
ft tide first, it will be seen that if the float started with
the current it would travel twelve miles in the tide; three and
a half from twelve leaves eight and a half miles. A vertical line
dropped from the intersection of the eight miles and a half line
with the curve of the current gives the time two hours and a half
before the end, or four hours after the start of the current at which
the discharge of the sewage must cease at the outfall in order that
the rear part of the column can reach the required point before
the current turns. As on this tide high water is about fifteen
minutes after the current, the latest time for the two hours of
discharge must be from one hour and three-quarters to three
hours and three-quarters after high water. Similarly with the
12 ft tide having a total travel of four miles: three and a
half from four leaves half a mile, and a vertical line from the
half-mile intersection gives one hour and three-quarters after
the start of the current as the time for discharge to cease.
High water is two hours and a quarter after the current;
therefore the latest time for the period of discharge would be
from two hours and a half to half an hour before high water,
but, as during the first quarter of an hour the movement of the
current, though slight, would be in the opposite direction, it
would be advisable to curtail the time of discharge, and say
that it should be limited to between two hours and a quarter
and half an hour before high water. It is obvious that if
sewage is discharged about two hours after high water the
current will be nearing its maximum speed, but it will only
have about three hours to run before it turns; so that,
although the sewage may be removed with the maximum rapidity
from the vicinity of the sea outfall, it will not be carried to
any very great distance, and, of course, the greater the
distance it is carried the more it will be diffused. It must be
remembered that the foregoing data are only applicable to the
locality they relate to, although after obtaining the necessary
information similar diagrams can be made and used for other
places; but enough has been said to show that when it is
necessary to utilise the full effect of the currents the sewage
should be discharged at a varying time before high or low
water, as the case may be, according to the height of the tide.