CANALS AND CONDUITS
1. =Banks.=--All banks which have to hold up water should be carefully
made. The earth should be deposited in layers and all clods broken
up. In high banks the layers should be moistened and rammed. The
dotted lines in fig. 29 show two possible courses of percolation
water. The vertical height--from the water-level to the ground outside
the bank,--divided by the length of the line of percolation is the
hydraulic gradient, as in the case of a pipe, and this gradient is
more or less a measure of the tendency to leakage. A bank which has
water constantly against it nearly always becomes almost water-tight in
time. The time is less or greater according as the soil is better, and
according to the amount of care with which the bank is made.
[Illustration: FIG. 29.]
The side slopes of banks vary with the soil. Generally they are 1½ to
1, but they are sometimes 2 to 1 or even 3 to 1 if the soil is bad or
sandy, or if great precautions against breaches are to be taken.
Leakage can sometimes be stopped by throwing chaff or other finely
divided substances into the water at the site of the leak. In other
cases it is necessary to dig up part of the bank, find the channel
by which the water is escaping, and fill it up by adding earth and
ramming. On some navigable canals in France it was at one time the
custom to lay the reach dry, when a bad leak occurred, and to dig away
the bank and lay slabs of concrete or puddle over the place. This plan
was abandoned, and instead of it sheet piles are driven in. They are
then withdrawn one at a time and, if any leakage occurs, the space is
filled with concrete.
The dimensions of a bank should depend to some extent on the head of
water against it and on the volume of the stream whose water it holds
up. A breach is obviously more serious the greater the volume of the
escaping water. This volume depends on the size of the stream and on
its velocity. In navigation canals in England the bank on the side
opposite the towing-path is usually 4 to 6 feet wide and 1½ feet above
the water. In irrigation canals in India the bank of a very large canal
is 2 feet above the water and 20 feet wide, while that of a small canal
with 6 feet of water is 8 or 10 feet wide and 1½ feet above the water,
and that of a small distributary channel with 3 feet of water is 4 feet
wide and 1 foot above the water. The soil is often poor.
Further remarks, which apply to banks of special height or special
importance, are given under Embankments (CHAP. XII., _Art. 6_).
2. =Navigation Canals.=--A navigation canal is sometimes all on one
level, but generally different reaches are at different levels, the
change being made by means of locks. A “lateral” canal--the most common
kind--runs along a river valley more or less parallel to the river. It
is frequently cheaper to construct such a canal than to canalise the
river. A “summit” canal crosses over a ridge and connects two valleys.
A navigation canal requires a supply of water to make good the losses
which occur by lockage, leakage, or absorption and evaporation. A canal
may be of any size, according to the size of the boats which are to be
used. There is always room, except in short reaches where the expense
of construction has to be kept down, for two boats to pass one another.
A lateral canal obtains water from the river or from the small
affluents which it crosses. For a summit canal it may be necessary to
provide storage reservoirs. The canal crosses the ridge where it is
low, and the reservoirs are made on higher ground. Reservoirs may be
required also for other canals to hold water for use in dry seasons or
in order to fill the canal quickly when laid dry for repairs.
In tropical countries weeds grow profusely in canals which have still
or nearly still water. Traffic tends to keep them down, but they have
to be cleared periodically.
In designing a barge canal the chief considerations generally are that
it shall not be in such low ground or so near a river as to be liable
to damage by floods, that it shall not traverse very permeable soil or
gravel--this is often found near a river,--that the material excavated
shall be as nearly as possible equal to that required, at the same
place, for embankment, and that as far as possible high embankments,
which are very expensive to construct and are more or less a source of
danger, shall be avoided. The side slopes of the banks of a navigation
canal depend on the nature of the soil. They are generally 1½ to 1, but
the inner slope may be 2 to 1. The banks are generally 1½ or 2 feet
above the water-level, the width of the bank on the towing-path side
ranging from 8 to 16 feet, but being generally 12 feet and the width
of the other bank 4 to 6 feet. The width of a canal is made sufficient
for two boats to pass, and the depth is 1½ to 2 feet greater than the
draught of the boats used. In some cases the banks are protected by
pitching for short lengths, but generally they are merely turfed. The
sides near the water surface wear away, so that the side slope becomes
steeper in the upper part and flatter in the lower part. The resistance
of a boat to traction in a canal is given by the formula
R = _r_(8·46)/(2 + (A/_a_)),
where _r_ is the resistance in a large body of water and A and _a_ are
the areas of the cross-sections of the canal and of the immersed part
of the boat. When A is six times _a_, R is only 6 per cent. more than
_r_. In practice A is never less than six times _a_.
Regarding methods of protecting banks, see CHAP. VI.
A ship canal is a barge canal on a large scale. The speed of ships has
to be strictly limited to avoid damage to the banks.
The Manchester Ship Canal takes in the waters of the Irwell and the
Mersey, and conveys them for several miles. It is thus a canalised
river for part of its course. Below that it is a tidal stream, the tide
being admitted at its lower end where it joins the estuary of the
Mersey, and passing out higher up where it leaves the estuary after
skirting it. This circulation of water is beneficial to the estuary.
The Panama Canal might have been constructed at one level, but the cost
of this, and the time occupied, would have been double that of making
it a summit canal. The water of the river Chagres is to be impounded to
form a lake of great extent that will not only supply water for lockage
but will itself form part of the high-level reach of the canal, and
ships will be able to traverse it at greater speed than in the rest of
the canal.
Some Indian irrigation canals have been constructed so as to be
navigable. The increase in cost has usually been enormously in excess
of any resulting benefits.
3. =Locks.=--An ordinary lock is shown in fig. 29A. The space above the
head gates is called the “head bay,” and that below the tail gates the
“tail bay.” The floor of the lock is often an inverted arch. Sometimes
the floor is of cast-iron. The “lift wall” is generally a horizontal
arch. The gates when closed press at their lower ends against the
“mitre sills”; and the vertical “mitre posts” at the edges of the
gates meet and are pressed together. The gate, in opening and closing,
revolves above the cylindrical “heel post”--which stands in the “hollow
quoin” of the lock wall--and when fully open is contained in the “gate
recess.”
A lock is always strongly built, of masonry or concrete. The walls have
to withstand the earth pressure when the lock is laid dry for repairs.
The floor has to withstand the scouring action from the sluices.
Regarding the upward pressure of the water when the lock is empty,
see CHAP. X., _Art. 3_. The lift or difference in the water-levels of
the two reaches of a barge canal is generally from 4 to 9 feet, but
occasionally it is much more.
[Illustration: FIG. 29A.]
The gates of small locks are generally of wood and are counterbalanced.
Those of large locks are of wood or steel, and the weight is generally
taken by rollers. Ordinary wood should not be used if the _Teredo
navalis_ exists in the waters. An iron gate, if enclosed on all sides
by plating, is buoyant, and the rollers and anchor straps which hold
the upper ends of the heel posts are thus relieved of much weight. The
gates of the Panama Canal locks are 110 feet long and 7 feet thick, and
the height ranges from 48 feet to 82 feet.
The sluices for filling and emptying a lock are placed in the gates or
in the walls. The gates and sluices are generally worked by hydraulic
power or by electricity.
Locks are frequently arranged in flights. There are, in a few
instances, 20 to 30 locks in a flight, the total lift being 150 to 200
feet. By this means the number of gates is reduced, the tail gates of
one lock being the head gates of the rest, and there is a saving in
labour in working the locks.
Let L be the volume of water contained in a lock between the levels of
the upper and lower reaches, and let B be the submerged volume of a
boat. The “lockage” or volume of water withdrawn from the upper reach
of the canal is shown in the following statement:--
+---------+---------+------------+-----------+---------+-------------------------+
| | | | | | Lockage. |
|Reference| Number | Direction | Lock or | Lock or +-----------+-------------+
| Number |of Boats.| of Travel. | Locks | Locks | Single | Flight of |
| of Case.| | | Found. | Left. | Lock. | _m_ Locks. |
+---------+---------+------------+-----------+---------+-----------+-------------+
| 1 | 1 | Down. | Empty. | Empty. | L - B | L - B |
| | | | | | | |
| 2 | 1 | ” | Full. | ” | - B | - B |
| | | | | | | |
| 3 | 1 | Up. | Empty. | Full. | L + B | _m_L + B |
| | | | | | | |
| 4 | 1 | ” | Full. | ” | L + B | L + B |
| | | | | | | |
| 5 | 2_n_ | Up and | Going | Going | _n_L | _mn_L |
| | | down |down, full.| down, | | |
| | |alternately.| Going up, | empty. | | |
| | | | empty. |Going up,| | |
| | | | | full. | | |
| | | | | | | |
| 6 | _n_ | Down. | Empty. | Empty. | _n_L-_n_B | _n_L - _n_B |
| | | | | | | |
| 7 | _n_ | ” | Full. | ” | (_n_-1)L | (_n_ - 1)L |
| | | | | | - _n_B | -_n_B |
| | | | | | | |
| 8 | _n_ | Up. | Empty. | Full. | _n_L+_n_B |(_m_+_n_-1)L |
| | | | | | | +_n_B |
| | | | | | | |
| 9 | _n_ | ” | Full. | ” | _n_L+_n_B | _n_L + _n_B |
| | | | | | | |
| 10 | { _n_ | Down.} | | | | |
| | { _n_ | Up. } | ” | ” | (2_n_-1)L |(_m_+2_n_-2)L|
+---------+---------+------------+-----------+---------+-----------+-------------+
In the case of a single lock, if two boats are to pass through, one
descending and one ascending (cases 2 and 3), the descending boat would
be passed through first if the lock were full, and the ascending boat
first if empty; in either case, the total lockage is L, or L/2 for
each boat. This also appears from case 5. Cases 6 to 10 show that if
a long train of boats descends, even though the lock is full for the
first boat or if a long train ascends even the lock is empty for the
first boat, the total lockage is nearly L per boat. Thus in a single
lock, boats should pass up and down alternately so far as this may be
possible.
In the case of a flight of _m_ locks, a single boat in descending uses
no more water than if there were only one lock, the same water passing
from lock to lock, but in ascending it uses more. In the case of a
number (2_n_) of boats going up and down alternately (case 5), the
lockage is _m_ _n_ L, the lockage per lock per boat being L/2, but in
the case of a long train of boats descending followed by an equal train
ascending (cases 7 and 8), the lockage is less. If _n_ is supposed to
be equal to _m_, the average lockage per boat is as follows:--
_m_ = 1 2 3 4 5 6 Infinity
Lockage = L/2 L 7L/6 5L/4 13L/10 4L/3 3L/2
per boat
Thus in a case where _n_ and _m_ are very large, the average lockage
per boat, when the boats pass up and down in trains, is to the lockage
per boat, when the single boats pass up and down alternately through
_m_ single locks all at different places, as 3 is to _m_. The reason
for the difference, which may appear puzzling, is that when the locks
are at different places they are worked independently of one another.
Sometimes a lock is provided with intermediate gates which provide a
short lock for short vessels. In the Manchester Ship Canal, alongside
each lock there is another of smaller size to be used for small vessels
and thus save lockage. At the Eastham lock, where the Manchester Ship
Canal descends into the estuary of the Mersey, there is, below the tail
gates, an extra pair of gates opening towards the estuary, so that the
lock can be worked when the water of the estuary is higher than that
in the canal. Water can be economised by means of a “side-pond,” into
which the upper portion of the water from a lock can be discharged and
utilised again when the lock has to be filled. If two locks are built
side by side, each acts as a side-pond to the other. Two flights of
locks can be built side by side.
Sometimes instead of a lock there is an inclined plane, up or down
which are drawn on rails caissons containing water in which the boats
float. The rails extend below the water-levels of the two reaches, and
the caissons can thus be run under the boats. “Lifts” have also been
constructed by which the boats can be lifted bodily and swung over from
one reach to the other.
4. =Other Artificial Channels.=--The method of calculating the
discharges of channels in which water is to flow is a question of
hydraulics. The principles and rules to be followed, in the design of
earthen channels, have been stated in CHAP. IV., _Art. 6_, and in CHAP.
VIII., _Art. 5_. The design of banks has been dealt with in _Art. 1_
of this Chapter. For conveying water for the supply of towns, or for
other purposes, masonry conduits are often used. A usual form is shown
in fig. 30. The curving of the profile of the cross-section gives an
increased sectional area and hydraulic radius, and hence an increased
discharge.
[Illustration: FIG. 30.]