TIDAL WATERS AND WORKS
1. =Tides.=--The tides or “tidal waves” are caused by the attraction of
the moon and the sun. The phenomena are complex, and a full discussion
of their causes need not be given here. When the tide rises it is said
to “flow,” and it is called the flood tide; when it falls it is called
the ebb tide. The period between one tide and the next, _e.g._ from
high water to high water, is about twelve hours, twenty-five minutes.
At a spring tide the range of the tide is greater than usual; at a
neap tide less. Where there are channels, as, for instance, the seas
which surround the British Isles, the tidal waves run up them as the
tide rises in the neighbouring ocean, and run back as it falls. At some
places, as Southampton, the tide comes in from two directions, and
there is a double tide. The times and levels of high and low water at
various places have been ascertained by observation, and are recorded.
The levels are, however, liable to be affected by winds. A wind blowing
towards the shore raises the level of both high and low water; a wind
blowing off shore lowers both levels. A severe storm in the North Sea
has caused a double tide at London Docks, by accelerating the North Sea
tidal wave.
In a funnel-shaped estuary, especially if it faces the direction of the
tidal wave in the sea, the tide in going up the channel increases in
velocity, and the momentum of the water causes it to rise higher and
higher as the width decreases. At the upper end of the Bristol Channel
the range of the tide is double the range in the sea outside the
channel. The Bay of Fundy is another place where a similar phenomenon
occurs. When a river or estuary is shallow and the range of the tide
is great, so that its rise is rapid, the flood tide in some cases
advances in the form of a wave or “bore,” causing a sudden rise in the
water-level and a sudden reversal of the flow of the stream. A bore is
most pronounced at spring tides. That of the Severn is well known.
In the case of a tide running along a coast or up an estuary, the water
of the flood tide, after it has ceased to rise, continues for a short
time, owing to its momentum, to flow in the same direction as before.
The same thing happens when the ebb tide ceases to fall. The tide also
acquires special velocity, just as a river does, round any projecting
headland.
The rise and fall of the tide are least rapid near the turns of the
tide. If the time from the beginning to the end of the flow be divided
into six equal parts, the proportional rise of the water will be
approximately as follows. And similarly with the fall during the ebb.
Time 1 2 3 4 5 6
Rise of water ·067 ·25 ·5 ·75 ·933 1
Tidal waters are frequently charged, more or less, with silt, obtained
from the shore or from shallows near it, either by currents or tidal
waves sweeping along it, or by the action of ordinary waves. Tidal
waters flowing up and down the lower portions of rivers render them to
an enormous degree more capable of carrying navigation and, especially
if they become enlarged and form estuaries, more capable of being
altered by training works.
A tide-gauge is constructed on the same principle as a self-registering
stream-gauge. The rise and fall of the water are reduced, by mechanism,
to a convenient range, and are recorded on a band carried on a drum,
which is caused to revolve by clockwork. Another kind, which depends on
the use of an inverted syphon filled with air and a syphon of mercury,
is described in _Min. Proc. Inst. C.E._, vol. clxiv.
[Illustration: FIG. 66.]
2. =Tidal Rivers.=--Let A B (fig. 66) be the surface of the lower part
or mouth of a river, supposed to be of uniform width, and let B be the
mean sea-level. As the tide rises to D the water of the river is headed
up and assumes the line A D. When the tide falls to F there is a draw,
the river surface taking the line A F. If the rise of the tide B H is
so great that the discharge of the river cannot keep pace with it, so
as to fill up the whole space between A and H to the level of H, there
will be a flow of sea water from H to some point M, and of river water
from A to M. The point M will be lower than A and H. If the tide now
turns and the water-level H begins to fall, there will still be a flow
along H M. For a brief period it will be due to momentum, but it will
continue until, by the rise of the water-level at M and the fall at
H, the surface has assumed the form indicated by the dotted line A N
J. While this is happening, the point corresponding to M--where the
concave curve of the upland water meets the convex curve of the tidal
water--rises higher and shifts seaward. The character of the two curves
remains the same, but they become flatter and the surface N J nearly
level.
Thus the time of high tide at M is later than at H. It is later for
each point passed in going up the river from H towards A. Eventually a
point A is reached where there is no tide, that is, no rise or fall.
Far below this point, between A and B, there is a point above which
there is no upward current but only a slackening of the downstream
flow. At H a diagram showing the rise and fall of the tide is
symmetrical, at N the rises and falls are less than at H, and the
periods of their occurrence later. In going up the river the duration
of the flood tide decreases and that of the ebb tide increases. The
flood tide attains its greatest velocity soon after its commencement,
the ebb tide towards its close. The distances to which the tidal
influence extend are of course greater the greater the range of the
tide and the flatter the slope of the river. The discharge of the
river of course varies. The greater the discharge the more the rise
of the river tends to keep pace with that of the tide and the less
the distance to which the tidal influence extends. On a longitudinal
section of the river, the high-water line will be shown as A N H. This
is merely done for convenience. It is never high water at all points
simultaneously. To show the actual state of affairs at various stages
of the tide, series of lines must be drawn as in fig. 67, where the
firm lines show the flood, and the dotted lines the ebb tide.
The flow in the tidal reach of a river is the same as if the water
was alternately headed up by a movable weir and then allowed to flow
freely and be drawn down. If the water carries silt, the tendency for
deposit to occur is (CHAP. V., _Art. 2_) no greater than if there was
no heading up or drawing down. The tendency depends chiefly on whether
there is, on the average, any reduction in velocity or increase in
depth as compared with the non-tidal upstream reach, and whether the
water in that reach is fully charged with silt. If both the answers are
in the negative, no deposit due to river silt is likely to occur in the
tidal reach.
[Illustration: FIG. 67.]
If the sea water is charged with silt, it will of course carry silt
into the river as it flows up, but the whole volume of water which
enters has to flow out again. On the whole, the tendency for silt to be
deposited in the river is due only to the period of “slack tide” near
the time when the flow ceases. The tendency is seldom marked.
If the sea water carries silt and the river water is clear, the latter
assists of course in removing any deposit--that is, it tends to keep
the channel clear.
If the river channel is soft and if the sea water carries no silt, it
may, in passing up and down the river, become charged with silt and
return to sea still carrying it. It thus has a scouring effect on the
channel, and may deepen or widen it. If, owing, for instance, to the
flattening of the bed slope in its lower reaches, the river tends to
deposit its own silt in its tidal reach, the sea water may prevent this
deposit. Thus, as regards silting in the tidal reach of the river,
the tidal water of the sea has little prejudicial effect if it is
silt-laden, and a beneficial effect if it is not. Silt is likely to
deposit in the tidal reach of a river of uniform width, only in a case
in which the river water carries much silt, and the slope is flat or
cross-section great compared to that of the upper reach.
Sea water is heavier than fresh water by about 2·4 per cent., and this,
to some extent, prevents their mixing. At all stages of the flood tide
the tendency at the point where the fresh water meets the salt water is
for the fresh water to accumulate towards the surface and the sea water
towards the bottom. When the tide begins to flow up the river there
may be a low-level salt water current moving landward and a high-level
fresh water current moving seaward, but this is quite a temporary state
of affairs. The surface slope is landward, and the water moving seaward
is not moving in obedience to the surface slope. It is only moving as
a result of momentum previously acquired. The low-level current may
have some extra velocity and extra scouring power, but this cannot be
much, because the mean landward velocity of the whole stream must,
owing to the internal resistances caused by the two currents, be less
than it would be if there were not two currents. Moreover, the state of
affairs is temporary. The two kinds of water mix eventually, and their
temporary separation has no considerable effect on the general tendency
of the river in the tidal reach to scour or to silt.
A body of water included at any moment between any two cross-sections
of the tidal portion of a river may not reach the sea during the next
ebb tide. In this case it will flow back up the channel with the next
flood tide, and so be kept moving up and down, getting nearer, however,
to the sea at each tide.
De Franchimont has shown (_Min. Proc. Inst. C.E._, vol. clx.) how a
diagrammatic route-guide can be prepared for any tidal river to show
pilots or captains of vessels the best times for starting on voyages up
or down the river, and for passing each point on it.
3. =Works in Tidal Rivers.=--If any works are required in the tidal
portion of a river, the principles to be followed in designing them
are the same as if the river was non-tidal. All that has been said
in CHAP. VIII., _Arts. 1_ to _3_, applies to them. The river may be
straightened or trained or dredged. Generally training and dredging
are combined. Any dredging in the portion of the river nearest the sea
will not, of course, alter the water levels near the mouth, but it will
alter them further up. The tide will come up in greater volume and will
rise higher and extend further up. The ebb will be facilitated, and the
low-water level will be lowered. If any narrowing of the channel near
its mouth is effected by training walls for the purpose of lowering
the bed, the effect on the volume of tidal water entering the river
must be taken into consideration. If the narrowing is confined to a
reach near the mouth, and if the resulting deepening is not sufficient
to counteract the effect of the narrowing, the volume of tidal water
reaching the unnarrowed portions of the channel will be reduced, and
this may be injurious. Its scouring action may be insufficient. The
proper course may be to continue the narrowing upstream. If this is
done, then it is obvious that the width of channel in which deep water
is to be maintained at high water, or which is to be kept free from
deposit, is reduced in about the same proportion as the volume of tidal
water is reduced, and no harm is likely to result.
Any weir or similar structure which abruptly stops the flow of the tide
up a river checks it of course for a long distance back, perhaps to the
mouth. Old London Bridge used to obstruct the tide, and its removal
increased the range of the tide, and was beneficial.
Tidal rivers generally widen out to some extent near their mouths, and
are thus rather estuaries than rivers. The works in such rivers are
more fully discussed in _Art. 5_.
4. =Tidal Estuaries.=--If, instead of a river of uniform width, there
is an estuary whose width increases steadily towards the sea so that it
is funnel-shaped, the conditions described in _Art. 2_ are modified. An
estuary is formed first by the waves of the sea, which wear away the
angles at the mouth of the river and allow the tide to enter in greater
volume, and then by the flow and ebb of the tides. The slope of the
bed of the estuary is usually much flatter than that of the river, and
the water surface is as shown in fig. 67. The tidal movements extend
further upstream than in the case of a river, not only because of
the greater difficulty experienced by the upland water in filling up
the wide channel of the estuary, but because of the momentum of the
tidal water driving its way up the funnel-shaped channel (_Art. 1_).
The capacity of the estuary is of course much greater than is required
for the discharge of the upland water alone. If the sea-level remained
always at one height and if the upland water contained silt, it would
tend to deposit in the estuary and would certainly deposit in it to
some extent. The action of the sea water is the same as described in
_Art. 2_, scouring if it is clear when entering, of less account if
it is not clear. Owing to the funnel shape of the estuary, the tide
rises higher at its upper end than if the estuary were replaced by a
river channel, and the tide also extends further up. This may partly
or wholly compensate for the greater tendency of silt to deposit in an
estuary as compared with a river channel.
The ebb tide in an estuary does not always follow exactly the same
course as the flood tide. Of course the lowest parts of the estuary are
filled first and emptied last, but the channels are not all continuous.
A channel open at its lower end may have a dead end at its upper
termination, and _vice versa_. Also, at sharp bends in the channels,
the momentum of the water may cause differences in the paths traversed
by the flowing and ebbing currents. Wherever there is a deep channel
the water from the adjacent sandbanks tends, towards the close of the
ebb, to flow cross-wise into the channel, and in doing this it to some
extent washes down the banks into the channel.
5. =Works in Tidal Estuaries.=--Estuaries, when shallow, offer great
facilities for training. It used at one time to be said that any
change which reduces the volume of tidal flow must be injurious. It
would be injurious to restrict the mouth of the estuary, unless it
were exceptionally wide, and leave the rest untouched. If the whole
estuary is narrowed, and a suitable funnel shape preserved, the width
to be kept open is, relatively to the size of the mouth, no greater
than before, and the tide may flow up as far as before, and rise to as
high a level. The narrowing, if properly arranged, will improve the
shape of the estuary and cause an increased scour. The effect of the
upland water is also greater in the narrower channel. Improvements to
estuaries are not, however, restricted to training. There is always
one or more deep channels, and the best of these can be selected and
improved by dredging. The channel should be one along which both the
flood tide and the ebb tide will run. The above remarks as to training
do not apply to a case in which there is a bar outside the mouth of the
estuary. Training might check the scour at the bar. Bars are treated of
in CHAP. XV.
If an estuary is not funnel-shaped, if, for instance, it widens out
very rapidly, the tidal flow is much less effective in keeping the
channel open. In this case, training works, which would give the
necessary funnel shape, are indicated rather than dredging. If an
estuary is narrow at the entrance, the flow is much less powerful,
unless the narrow part is of greater depth, but even then the force of
the tide is reduced owing to the change in the shape of the channel.
The bed of an estuary may be of such soft or sandy material that a
dredged channel would be likely to be quickly filled up again by the
slipping in of material at the sides (_Art. 4_). In such a case an
untrained channel can only be kept open to its full depth by constant
dredging, and probably the best course is to construct a trained
channel, although it may be more expensive than in the case of a
harder channel, because of the depth to which the foundations of the
walls must be sunk into the soft bed. Also, if the bed of the estuary
is constantly shifting, a dredged channel alone will not succeed,
and training must be resorted to. Again, the bed may be of such hard
material that training walls would not cause it to scour. In this case
a channel should be dredged and need not be trained. For the great body
of intermediate cases in which the deep channel can be formed either by
dredging or training, both methods can be adopted. A common plan is to
train the upper part and to dredge the lower part where the estuary is
wider and the training walls would be more exposed to the waves.
[Illustration: FIG. 68.]
When an estuary is thus partly trained, the deepening due to the
training does not extend far beyond the point where the walls
terminate. The deposit of material along the sides of the estuary
may, however, extend some distance further down in places where the
tide can no longer have free play. This occurred in the Seine estuary
(fig. 68). The authorities of Havre, which lies at one side of the
estuary not far from its mouth, feared that if the training walls were
brought further down, the deposits might extend to their neighbourhood.
The reduction in the capacity of the estuary, due to the deposits,
caused it to become filled up more quickly, and the time of high water
at Havre was advanced. The dotted lines show a good arrangement of
training walls proposed by Harcourt.
There is no doubt that it is always feasible to carry training walls
right through an estuary, or at least down to a point where deep water
is reached, and if a proper funnel shape is given to the channel the
reduction of the tidal flow and silting up of the spaces behind the
walls need not cause any trouble. Training the complete estuary was
carried out in the case of the Tees, where, however, the estuary was
not of great length, and was not of a good shape for keeping itself
open. Any affluents entering the estuary can be provided with separate
trained channels. Difficulty may, however, arise if there are towns
which would be shut off from the estuary by the silt banks.
Generally the line selected for the trained or dredged channel should,
though it must be as short and direct as possible, coincide as nearly
as possible with that which the water naturally tends to keep open.
This may be toward one side of the estuary or the other, according to
the direction from which the tidal wave approaches. In the case of
the Dee, the best line was not adopted, attention having been chiefly
given to the question of silting up the spaces outside the walls and so
reclaiming land, a matter which should always be treated as of quite
secondary importance. Training walls in estuaries are generally built
only up to half-tide level. Were it not for the expense they might be
built up to high-water level. In the Seine estuary the walls were made
of blocks of chalk.
Whether a trained channel will keep itself open or will need periodical
dredging depends, of course, on the amount of silt in the water and on
its velocity and depth. The question must be worked out and calculated
as in the case of a non-tidal river.
The estuary of the Mersey differs from most others. Towards the mouth,
near Liverpool, it is narrow and it widens out further inland. The
tides, running through the narrow portion, to fill up the large inland
basin and to empty it again, keep the narrow part scoured to a great
depth. It was proposed to train the wide portion for the Manchester
Ship Canal. The training would, no doubt, have succeeded, but, owing to
the silting up of the greater part of the estuary, the scouring near
Liverpool would have been very greatly reduced and serious damage done
to that port.