WEIRS AND SLUICES
1. =Preliminary Remarks.=--Every structure which interferes at all
with a stream causes an abrupt change in the stream (CHAP. IV., _Art.
1_). At an abrupt change there are always eddies, and these have a
peculiar scouring effect. This effect is greatest where the velocity
of the stream is abruptly reduced as where, for instance, after being
contracted by an obstruction, it expands again or where it falls over
a weir or issues from a sluice opening. In all cases of this kind the
protection of the structure from scour is of primary importance.
The site of a weir or other permanent structure should, if the stream
is unstable, be in a fairly straight reach, or at least not be
immediately downstream of a bend. This is because of the tendency of
bends to shift downstream (CHAP. IV., _Art. 8_). There is no particular
advantage in selecting a narrow place. A narrow place is likely to be
deep or it may be liable to widen. In a hard and stable stream there is
no restriction as to site.
Weirs are frequently constructed for purposes of navigation, as
mentioned in CHAP. VIII. They are also used in streams which are not
navigable in order that the gradient may not be too steep, and in
irrigation canals for the same reason. They are used both in rivers
and canals in order that the water-level may be raised and water drawn
off by branch channels for purposes of manufactures, water-power or
irrigation.
Upstream of a weir there is more or less tendency for silt to deposit,
but it by no means follows that there will be deposit (CHAP. IV., _Art.
2_, last par., and _Art. 3_, last par.). When deposit of sand or mud is
feared, small horizontal passages, known as “weep holes,” may be left
in the weir at the level of the upstream bed. In the old Nile barrages
iron gratings were provided, but they were needlessly large.
[Illustration: FIG. 31.]
An inherent defect of an ordinary weir is that it obstructs the passage
of floods. The obstruction may or may not be of consequence. Sometimes
it is of great consequence. Attempts have been made to partially remedy
the evil by placing the weir obliquely to the stream, thus giving it
a greater length. At ordinary water-levels the flow over the crest of
the weir is normal to its length, or nearly so. Supposing that the
water has to be held up to a given level, the crest of the weir must be
higher, because of its greater length, than if it were normal to the
stream. In a flood the water has a high velocity and flows over the
weir in a direction nearly parallel to the axis of the stream, so that
the effective length of the weir is not much greater than if it were
normal to the stream, and, its crest being higher, it obstructs the
flood as much. Oblique weirs are usually made as in fig. 31. If made in
one straight line, there might be excessive action on the bank at the
lower end.
If the weir is lengthened, not by being built obliquely but by a
widening of the stream at the site, the crest has to be raised and
nothing is gained.
The only arrangement by which a weir can be made to hold up water when
a stream is low and to let floods pass freely, consists in having part
of the weir movable, _i.e._ consisting of gates, shutters or horizontal
or vertical timbers, which can be withdrawn to let floods pass, and
can be manipulated to any extent so as to regulate the amount of
water passing. A familiar instance of a movable weir is the one which
is usually placed across a mill stream, the wooden gates working in
grooves in the masonry.
Above a weir in Java, 162 feet long, there was a great accumulation of
shingle in the bed of the river, and the head of a canal taking off
above the weir became choked. The crest of the weir on the side away
from the canal was raised 5¼ feet and the crest sloped gradually down,
a length of 43 feet on the side next the canal remaining as it was.
This was quite successful. It was practically a contraction of the
river near the canal off-take, and this must have caused scour, so that
the bed became lower than the floor of the canal head and the shingle
was not carried in. The shingle, however, is said to have been carried
over the weir (_Min. Proc. Inst. C.E._, vol. clxv.).
A lock is an adjunct to a weir, used when navigation has to be provided
for. The lock may be placed close to the weir or it may be in a side
channel, the upstream end of the lock being about in a line with the
weir. Locks have already been discussed in CHAP. IX., _Art. 3_.
Frequently a “salmon ladder” has to be provided. It consists of a
series of steps or a zigzag arrangement so that the velocity of the
water is not too great for the fish to ascend.
2. =General Design of a Weir.=--Unless the bed and sides of the channel
are of rock, a weir has side walls and rests on a strong floor or
“apron.” These need not extend far upstream, but must extend some way
downstream because of the scouring action of the water.[11] A common
type of weir is shown in fig. 32. The downstream face is made sloping,
so that the water may not fall vertically and strike the floor below
the weir. The thickness and length of the floor depend on the volume
of water to be passed and on the height which it will fall and on the
nature of the soil, and are generally matters of judgment, though rules
regarding them, applicable to certain special cases, are given in the
next article.
[Illustration: FIG. 32.]
The upper corners of the weir should be rounded. This prevents their
being worn away; but the rounding of the upstream corner has another
advantage. If the corner is sharp, the stream springs clear from it and
the weir holds up the water higher, especially in floods. With small
depths of water the difference is less, and it vanishes when there is
only a trickle of water. Thus a crest rounded on the upstream side
holds up low-water nearly as well as a sharp-edged crest, but lets
floods pass more freely. Any batter given to the upstream face has a
similar advantage. The rounding is of more importance as the batter is
less. For similar reasons, the upstream wing walls should be splayed
or even curved so as to be tangential to the side wall, and not built
normally to the stream. These advantages are sometimes lost sight of.
The downstream walls are splayed to reduce the swirl.
The body of the weir may be of rubble and the face-work of dressed
stone. In large weirs the stones are sometimes dowelled together.
Where, as in many parts of India, stone is expensive, brick is used for
small weirs, the crest and faces being brick on edge.
Downstream of the floor, unless the channel is of very hard material,
there is paving or pitching of the bed and pitching of the sides, and
these may terminate in a curtain wall. The bank pitching may be of any
of the kinds described in CHAP. VI., _Art. 3_, and the bed paving as
described in CHAP. V., _Art. 6_, but downstream of a weir the eddying
is continuous and the lap of the water on the bank is ceaseless, and
good methods are necessary. Sometimes planking, laid over a wooden
framing or attached to piles, is used instead of paving and pitching.
In case the height of a weir is great relatively to its thickness, the
danger of its being overturned must be considered. To be safe against
overturning, the resultant of the pressure on the weir must pass
through the middle third of its base (see fig. 62, CHAP. XIII.).
[Illustration: FIG. 33.]
3. =Weirs on Sandy or Porous Soil.=--If the channel is very soft or
sandy the weir may be built on one or more lines of wells. The wells
are not so much to support the weir as to form a curtain and prevent
streams, due to the hydraulic gradient A E (fig. 33), from forming
under the structure and gradually removing the soil. It is assumed
in the case represented by the figure that the maximum head occurs
when the downstream channel is dry. Any removal of soil from under
the weir may cause its destruction. The wells should be as close
together as possible, and the spaces between them carefully filled up
with brickwork or concrete to as great a depth as possible, and below
that by piles. Instead of wells, lines of sheet piling--cast-iron or
wood--can be used. A good fit should be made, but it is not necessary
that the joints should be absolutely water-tight. The object is to
flatten the hydraulic gradient by increasing the length travelled
by the water from B E to B L G H E. Of course, no flattening occurs
at a point where the curtain is not water-tight, but if only small
interstices exist, none but small trickles of water can pass, and
the interstices will probably soon be choked up, just as the sand in
a filter bed becomes clogged and has to be washed. In any case, no
important stream could develop otherwise than round the toe of the
curtain. It has been stated that when a curtain is water-tight the
water follows the line B L M G H K E, but this requires proof. Another
plan is to cover the bed and sides of the channel with a continuous
sheet of concrete extending upstream of the weir from B to D--thus
flattening the hydraulic gradient from A E to F E. Instead of concrete,
clay puddle can be used with pitching over it. The choice between the
different methods depends largely on questions of cost and facility of
construction. It has been said that a certain amount of leakage occurs
under structures such as the Okla weir (_Art. 4_), which nevertheless
remains undamaged. There have, however, been cases in which failures of
works have occurred, especially when there has been a great difference
between the water-levels of the upstream and downstream reaches, from
no other apparent cause than the passage of water underneath the works.
Weirs in porous soils have been discussed by Bligh (_Engineering
News_, 29th December 1910), who gives the following as safe hydraulic
gradients (_s_) or ratio of the greatest head A B to the length B E:--
Fine silt and sand as in the Nile 1 in 18
Fine micaceous sand as in Colorado
and Himalayan rivers 1 in 15
Ordinary coarse sand 1 in 12
Gravel and sand 1 in 9
Boulders, gravel and sand 1 in 4 to 1 in 6
These figures are probably quite safe enough even for the most
important works and for those where the heading up is constant. For
small works or for regulators (_Art. 5_) where the heading up is not
constant, steeper gradients are permissible. Much also depends on
the condition of the water. If it contains much silt, all interstices
will probably become choked up. The hydraulic gradient in the case of
the Narora weir across the Ganges was 1 in 11. The weir failed after
working for twenty years. It was rebuilt with a gradient of 1 in 16. In
the Zifta and Assiut regulators on the Nile the gradients are 1 in 16·4
and 1 in 21.
[Illustration: NARORA WEIR AS ORIGINALLY BUILT.]
[Illustration: NARORA WEIR AS RECONSTRUCTED.]
[Illustration: FOUNDATION OF THE ZIFTA REGULATOR, RIVER NILE.]
Regarding the upward pressure on the floor due to the hydrostatic
pressure from the head A B, there is a theory that the weight of a
portion of the floor at any point P should be able to balance the
pressure due to a head of water P R. This, supposing the masonry to be
twice as heavy as water, would give a thickness of floor equal to half
P R. According to Bligh, the theoretical thickness ought, for safety,
to be increased by one-third. Practically the thickness need not, in
most cases, be made even so great as is given by the theoretical rule.
On canals in the Punjab it is certainly less. Water passing through
soil or fine sand does not exert anything like the pressure which it
exerts when passing through a pipe. It acts in the same manner as in a
capillary tube. It is only in coarse sand or gravel or boulders that
water flows as in a pipe.[12] If the tail water covers the floor, the
weight of a portion of floor is reduced by the weight of an equal
volume of water. If the foundation of any part of the floor is higher
than B E, the upward pressure on it is reduced because the water has to
force its way upwards through the soil.
Bligh also states as an empirical rule that in order to provide
efficiently against scour the length of floor B E should be 4/_s_
√(H/13), where H is the maximum head A B; and he points out that in a
case where this length is less--as it usually is--than that necessary
to give a hydraulic gradient of the requisite flatness, according to
the rule previously quoted, it is better to add an upstream floor B
D, which may be of puddle and therefore cheap, than to add to the
downstream floor a length E C which must be of masonry or concrete, and
that this arrangement, by shifting the line of hydraulic gradient from
A E to F E, gives a reduced upward pressure on the downstream floor.
The length E N to which pitching, if of “rip-rap” type, should extend
is given by Bligh as 10/_s_ √(H/10) √(_q_/75), where _q_ is the maximum
discharge in cubic feet per second passing over a 1-foot length of the
weir, and H is the head A B.
4. =Various Types of Weirs.=--The type of weir shown in fig. 32 may
be varied by steepening or flattening the slopes of one or both
faces. Flattening increases the cost but gives a greater spread for
the foundations. It may, however, be combined with a decrease in the
width of the crest. Flattening of the downstream slope reduces the
shock of the water on the floor, but the slope itself, especially
the lower portion, has to stand a good deal of wear, and the length
exposed to this is increased. Flattening the upstream slope facilitates
the passage of floods. The same result is obtained by making the
crest slope upwards (fig. 34). In a small stream or in an irrigation
distributing channel, a weir may be a simple brick wall with both faces
vertical and corners rounded.
[Illustration: FIG. 34.]
Weirs in America are often built of crib-work filled with stones. Weirs
are also made of sheet piling filled in with rubble, and the top may be
protected by sheet iron. A weir made on the Mersey in connection with
the Manchester Ship Canal works was so made. There were three rows of
piles and the filling in the back part was of clay.
Sometimes the downstream faces of weirs used to be made curved (figs.
35 and 36), the object being to reduce the shock of the falling water,
but the advantage gained is not very appreciable, and this type of weir
is not very common.
[Illustration: FIG. 35.]
[Illustration: FIG. 36.]
The Okla weir (fig. 37) across the river Jumna near Delhi was built
about thirty-eight years ago on the river bed, which consisted of fine
sand. The depth of water over the crest in floods is 6 to 10 feet. The
material, except the face-work and the three walls, is dry rubble.
[Illustration: FIG. 37.]
When the reach of channel downstream of a weir has a bed-level much
lower than that of the upstream reach--this is often the case in
irrigation canals,--the work is known as a “fall” or “rapid.” At a
fall the water generally drops vertically, and a cistern (fig. 38) is
provided. The falling water strikes that in the cistern and the shock
on the floor is greatly reduced. An empirical rule for the depth of the
cistern, measured from the bed of the downstream reach, is
K = H + ∛H √D,
where H is the depth of the crest of the fall below the upstream
water-level, and D is the difference between the upstream and
downstream water-levels. At some old falls on Indian canals the water,
as it begins to fall into the cistern, is made to pass through a
grating which projects with an upward inclination from the crest of the
weir at the downstream angle. This splits up the water and reduces the
shock, but rubbish is liable to collect.
[Illustration: FIG. 38.]
In the usual modern type of canal fall in India the weir has no raised
crest, and the water is held up by lateral contraction of the waterway
just above the fall. The opening through which the water passes is
trapezoidal (fig. 39), being wide at the water-level and narrow at
the bed-level. In a small channel there is only one opening, but in a
large canal there are several side by side, so that the water falls in
several distinct streams. The curved lip shown in the plan is added
to make the water spread out and cause less shock to the floor. The
dimensions of the openings are calculated so that however the supply
in the canal may vary, there is never any heading up or drawing down.
The detailed method of calculation for finding C F and the ratio of
A B to B C is given in _Hydraulics_, CHAP. IV. In cases where it is
only necessary for the notch to be accurate when the depth of water
ranges from B C to three-fourths B C, it will suffice to calculate
as follows:--Let _b_ be the bed width of the canal, and let Q be the
discharge and B the mean width of the stream when the depth of water is
B C. Decide on the number of notches, and let W be the width of a notch
calculated as if it were to be rectangular, _i.e._ by the ordinary
weir formula. Increase the width to W´ = 1·05 W. Then make the notch
trapezoidal, keeping the mean width W´, and making the bottom width _w_
(or C F), such that _w_/W´ = _b_/B. The top width of the notch is of
course increased as much as the bottom width is reduced.
[Illustration: FIG. 39.]
A rapid has a long downstream slope, which is expensive to construct
and difficult to keep in repair, especially as the canals can only be
closed for short periods. Rapids exist in large numbers on the Bari
Doab Canal in India, the face-work consisting in many cases of rounded
undressed boulders--with the interstices filled up by spawls and
concrete--which stand the wear well. Rapids have again been used on the
more modern canals in places where boulders are obtainable, and where
deep foundations would have given trouble in unwatering. The upstream
face of a rapid is vertical, or has a steep slope.
[Illustration: FIG. 40.]
5. =Weirs with Sluices.=--The long weirs built across Indian rivers
below the heads of irrigation canals generally extend across the
greater part of the river bed. In the remaining part--generally the
part nearest the canal head--there is, instead of the weir, a set of
openings or “under-sluices” (fig. 40) with piers having iron grooves in
which gates can slide vertically. The piers may be twenty feet apart
and five feet thick. The gates are worked by one or more “travellers,”
which run on rails on the arched roadway. The traveller is provided
with screw gearing to start a gate which sticks. When once started it
is easily lifted by the ordinary gears. The gates descend by their own
weight. The gate in each opening is usually in two halves, upper and
lower, each in its own grooves, and both can be lifted clear of the
floods. In intermediate stages of the river these gates have to be
worked a good deal. (See also CHAP. V., _Art. 5_.) Usually the weir
has, all along its crest, a set of hinged shutters, which lie flat at
all seasons, except that of low water in the river.
[Illustration: FIG. 41.]
The canal head consists of smaller arched openings, provided with gates
working in vertical grooves and lifted by a light traveller. If the
floor of the canal head is higher than the beds of the river and the
canal, it may be said to be a weir, but otherwise the canal head is
merely a set of sluices without a weir.
The barrage of the Nile at Assiut (fig. 41), and the old barrages of
the Rosetta and Damietta branches, consist of sets of sluices without
weirs. At Assiut there are piers five metres apart and gates working in
grooves like those, above described, at Indian headworks.
[Illustration: FIG. 42.]
The “dam” across the Ravi, at the head of the Sidhnai Canal in the
Punjab, also consists of sluice openings without a weir. The piers are
connected by horizontal beams (fig. 42), against which, and against
a sill at their lower ends, rest a number of nearly vertical timber
“needles,” fitting close together, which can be removed when necessary
by men standing on a foot-bridge. In floods the needles are all removed
and laid on the high-level bridge (not shown in the drawing), the
foot-bridge being then submerged. With needles the span between two
piers can be greater than would be possible with a gate. Needles can
be used up to a length of 12 or 14 feet, excluding the handle which
projects above the horizontal beam. They can be of pine, about 5 inches
deep in the direction of the stream, and 4 inches thick.
Where a branch takes off from a canal in India there are usually no
fixed weirs but two sets of piers--one in the canal and one in the
branch,--with openings and gates like those at the canal heads, or else
with wider openings and needles. These works are called regulators.
The gates are worked by travellers or by fixed windlasses or racks and
pinions. Very small gates for distributaries are often worked entirely
by screw gearing. For the smaller branches the gates are replaced by
sets of planks or timbers lying one above another and removed by means
of hooks. They are replaced by means of the hooks or by being held in
position some little height above the water, and dropped. They are
finally closed up by ramming.
In the case of either planks or needles, leakage can be much reduced by
throwing shavings or chopped straw into the water upstream of them.
Needles can be provided on their downstream sides with eye-bolts just
above the level of the beam against which their upper ends rest. They
can then be attached by chains or cords to the beam or to the next
pier, and cannot be lost when released. They can be released by a lever
which can be inserted under the eye-bolt. By pushing the head of a
needle forward and inserting a piece of wood under it, a little water
can be let through. In this way, or by removing needles here and there,
the discharge can be adjusted with exactness.
At a needle weir in an Indian canal all the needles in one opening are
reported to have broken simultaneously. A possible explanation is that
one needle broke and that the velocity thus set up in the approaching
stream caused the others to break. On another occasion when a canal was
dry all the needles were blown down.
Sometimes the beam or bar against which the upper ends of the needles
rest is itself movable. At Ravenna, in Italy, the bar between any two
piers has a vertical pivot at one pier and can swing horizontally. Its
other end is held by a prolongation of the next bar, near to its pivot.
If the end bar of the weir is released, each bar in turn is released
automatically.
At Teddington on the Thames the oblique weir, 480 feet long, has
thirty-five gates, which extend over half the length of the weir. They
are worked by travellers which run on a foot-bridge. The openings do
not extend down to the river bed, but are placed on the top of a low
weir. The other half of the weir is fixed. The gates are raised to let
floods pass.
At Richmond on the Thames the arrangements are similar, the gates being
counterbalanced to admit of easy and rapid raising. When raised they
are tilted into a horizontal position so as not to obstruct the view.
In Stoney’s sluice gates a set of rollers is interposed between the
gate and the groove. The rollers are suspended from a chain, one end
of which is attached to the top of the gate and the other end to the
groove. The rollers thus move up or down at half the rate of the gate,
and some of them are always in the proper position for taking the
pressure. Escape of water between the gate and the groove is prevented
by a rod which is suspended on the upstream side of the gate close
to its end, and is pressed by the water against the pier. Stoney’s
sluice gates, with spans ranging up to 30 feet, have been used on the
Manchester Ship Canal for the sluices by which the water of the river
Weaver is passed across the canal, and at locks for passing the flood
waters of the Irwell and Mersey down the canal. The gates are balanced
by counterweights.
Frame weirs,[13] used chiefly on rivers in France but also in Belgium
and Germany, are a modification of the needle and plank arrangements
above described. For the masonry piers there are substituted iron
frames or trestles, which are hinged at the floor-level so that, when
the timbers have been removed, the frame can be turned over sideways
and lie flat on the floor, thus leaving the waterway absolutely clear
from side to side of the stream. The foot-bridge which rests on the
frames is removed piece by piece. The frames are raised again by means
of chains attached to them. In order that the frames may not be too
heavy they are spaced 3 to 4 feet apart, or very much nearer than when
masonry piers are used. Horizontal planks can thus be used of shorter
lengths than the needles, and they can be made up into greater widths
so that the leakage is less.
A further modification consists in placing the bridge platform above
flood-level, and in hinging the frames to it instead of to the floor.
The frame turns about a horizontal axis parallel to the length of the
weir. A weir of this kind can be used for greater depths of water than
the ordinary frame weir.
In some cases the horizontal planks are connected together by hinges so
that they form a “curtain.” The curtain is raised by rolling it up by
means of a traveller. It admits of rapid and accurate adjustment of the
water-level, but there is considerable scouring action below a curtain
when it is somewhat raised.
[Illustration: FIG. 43.]
6. =Falling Shutters.=--In Thénard’s system, first used in France,
a shutter (fig. 43) is hinged at its lower edge and is held up by
a strut. When the lower end of the strut is pushed aside it slides
downstream and the shutter falls flat. To enable the shutter to
be raised again an upstream shutter, which ordinarily lies flat
and is held down by a bolt, is released, and it is then raised by
the current to the extent permitted by a chain attached to it. The
downstream shutter is then raised. Thénard’s system was not much used
in France because the river had to fall to a level somewhat too low
for navigation before the shutters could be raised. The sudden jerk
on the chain of the upstream shutter is also liable to do damage. The
system has been adopted on some of the long weirs which cross Indian
rivers downstream of the heads of irrigation canals. To prevent damage
by shock, a hydraulic brake was designed by Fouracres. It consists of
a piston which travels along a cylinder and drives water out through
small holes. The shutters are placed on the top of the fixed weir,
where they usually lie flat, except in the low water season, any
adjustments of the river discharge being effected by means of the
under-sluices.
In the Chanoine system of falling shutters (fig. 44), used first in
France, the shutter is hinged at a point rather higher than the centre
of pressure. The hinge is supported by a vertical trestle, which is
hinged at its lower end and is supported by a strut which slides in
a groove and rests against a stop. When the water rises to a certain
height above the top of the shutter, it is turned by the force of
the water into a horizontal position. The struts can then be pushed
sideways out of the stops by means of a “tripping bar,” which lies
along the floor parallel to the line of shutters and is worked from the
bank. The struts, trestles, and shutters then fall flat. To close the
weir the shutters are first raised into the horizontal position which
they occupied before falling, by means of a hook worked from a boat or
by chains attached to a foot-bridge running across the river upstream
of the weir. They can then be easily closed by a boat-hook. The water
closes them of itself if it falls low enough.
[Illustration: FIG. 44.]
When the shutters fall a great rush of water occurs. To obviate this
a valve is made in the upper half of the shutter. It consists of a
miniature shutter on the same principle as the main shutter. The pivot
of the main shutter is made at such a height that the shutter will not
turn over when only a small depth of water flows over it. Instead of
this the valve comes into operation. The valve also facilitates the
raising of the shutter. Again, instead of the tripping bar, which
would sometimes have to be of great length or be liable to damage owing
to stones jamming in its teeth, the shutter can be released by pulling
the strut upstream so that it falls into a second groove, down which
it slides. When a tripping bar is used, its teeth can be so arranged
that the shutters are released a few at a time, first singly, then
in twos and threes. Sometimes there are gaps of a few inches between
one shutter and the next, and the gaps can be closed by needles if
necessary.
Chanoine shutters can be very rapidly lowered, and they are used in
France and in the U.S.A. in places where sudden floods occur. They
are also used for navigation “passes” where most of the heavy traffic
is downstream and where it is too heavy to be dealt with in a lock. A
foot-bridge across the stream or across the navigation pass is always
an assistance, but sometimes it cannot be used when there is much
floating rubbish or ice. With a foot-bridge the cost is greater than
that of a needle weir.[14]
[Illustration: FIG. 45.]
In the Bear Trap weir (fig. 45) the upstream shutter rests against
the downstream one. Both are raised by admitting water from the upper
reach, by means of a culvert, through an opening in the side wall, and
they are made to fall by placing this opening in communication with
the downstream instead of the upstream reach. This kind of shutter is
only suitable for passes of moderate width, and it is rather expensive
on account of the culverts.[15]
Shutters with fixed supports are used on the Irwell and Mersey. A fixed
frame is built across the stream (fig. 46) and the shutters are hinged
to it. When the water rises to a certain height above its top, the
shutter turns into a horizontal position, but as this causes a severe
rush of water the shutter is usually raised by a chain attached to its
lower end and worked from the bank. When in a horizontal position,
it is held there by a ratchet. When the stream falls the ratchet is
released and the shutter is closed by the stream. This kind of shutter
cannot be used where there is navigation.
On the weir 4000 feet long across the river Chenab at Khanki in the
Punjab, the falling shutters, 6 feet high and 3 feet wide, are hinged
at the base and held up by a tie-rod on the upstream side. The trigger
which releases the rod is actuated by means of a wire rope carrying a
steel ball, and worked by a winch from the abutment of the weir or from
one of the piers, which are 500 feet apart. A winch is fixed on the top
of each pier, and communication with the piers is effected by means of
a cradle slung from a steel wire rope, which rests on standards and
runs across the weir. The wire rope which carries the steel ball passes
over a series of forks, one on each shutter. When one trigger has been
released, that shutter falls and the ball hangs loose. A further haul
on the rope causes it to actuate the trigger of the next shutter, and
so on. If it is desired to drop only some of the shutters, the rope
is passed over the forks of those shutters only. The shutters can
be raised by means of a crane which runs along the weir on rails
downstream of the shutters or, if the water is too high to allow of
this, by a crane in the stern of a boat which is moored upstream of the
weir and allowed to drop down.
[Illustration: FIG. 46.]
[Illustration: FIG. 47.]
7. =Adjustable Weirs.=--Drum weirs, invented by Desfontaines, have
been used in France and Germany. Two paddles (fig. 47) are fixed on a
horizontal axis and can turn through about 90°, the lower paddle, which
should be slightly the larger, working in a “drum,” which is roofed
over and can, by means of sluices, be placed in communication with
either the upper or lower reach of the stream. According as the upper
paddle is to be raised or lowered, water is admitted from the upper
reach above or below the lower paddle, the water on its other side
being at the same time placed in communication with the lower reach.
On the weirs first made on the Marne, the height of the upper paddle
was 3 feet 7½ inches, and there were, in a weir, a number of pairs of
paddles, each being 4 feet 11 inches wide. By having sluices at both
abutments communicating with both reaches, and by opening or closing
each of them more or less, the various paddles can be made to take up
different positions, and thus perfect control over the discharge is
obtained by simply turning a handle to control a sluice gate. A weir
has since been made with a single pair of paddles extending right
across the opening (33 feet), and the height of the upper paddle is
over 9 feet.[16]
The chief objection to drum weirs is the necessity for the hollow or
drum, which renders the work very expensive, except when only a small
depth of water is held up.
[Illustration: FIG. 48.]
The old sluice gates of the Nile barrages were made segmental (fig.
48), turned on pivots in the piers, and were raised by chains.
In some factories in Bavaria and Switzerland there are self-acting
shutters which revolve on a horizontal axis at the lower edge, and
are counterbalanced by cylindrical weights which roll on ways in the
side wall. This arrangement is suitable when there is only one span,
which can, however, be as great as 30 feet. An adjustable weir used
at Schweinfurt on the Maine, consists of a hollow iron cylinder, 59
feet long and 10 feet in diameter, running across the stream. The
cylinder is pear-shaped in cross-sections, and can be made, by means of
mechanism, to revolve, the water passing over it. Another kind used at
Mulhausen on the Rhine consists of a hollow iron cylinder 85 feet long
and 9·8 feet in diameter. The whole cylinder can be raised by winches
(_Min. Proc. Inst. C.E._, vols. cliii. and clvi.).
8. =Remarks on Sluices.=--In all kinds of sluice openings or
regulators, the principles of design as regards protection of the bed
and sides, splaying and curving of walls and piers, thickness of floor,
and prevention of the formation of streams under the structure are the
same as laid down for weirs.
In order that a pier may be safe from being overturned by the pressure
of the water when the gates or timbers are down, the resultant of its
weight, including that of anything resting on it, and of the water
pressure on it, must pass through the middle third of its length. This
generally occurs when there is an arched roadway. Otherwise it must be
arranged for by prolonging the base of the piers downstream, and giving
the downstream side a batter or steps.
The floor should usually be placed at a level somewhat lower than the
mean bed-level of the stream. The bed may possibly be lowered in course
of time. Lowering the floor also gives a greater thickness of water
cushion to take the shock of water falling over the gates or planks. It
is convenient to build, on the floor, a low wall or sill, reaching up
to the level of the bed or thereabouts, and running across from pier
to pier under the line of gates or needles. The height of the gates
or needles can thus be reduced, and there is little chance of silt or
stones collecting and interfering with them. In the case of needles the
wall must be strong enough to resist their horizontal pressure. If ever
the bed is lowered, the wall can easily be cut down or removed.
Sluices with gates are, of course, used in connection with works other
than weirs or regulators, as, for instance, in reservoirs or locks, or
generally for communication between any two bodies of water. The gate
may or may not be wholly submerged. If it is not wholly submerged,
planks can be used. Needles can be used if the flow is always in one
direction and never in the reverse direction. In all cases protection
downstream of the opening is required.
In designing a set of sluice openings or regulators, it is sometimes
the custom to make the total area of waterway the same as that of the
stream in its unobstructed condition. There is no particular reason why
it should be the same. In a description of the Assiut Barrage (_Min.
Proc. Inst. C.E._, vol. clviii., p. 30), it is mentioned that one of
the reasons for placing the floor lower than the river bed was that
the width of the waterway of the barrage was less than that of the
river. The bed has to be heavily protected in any case, and the proper
principle is to fix a velocity which is considered to be safe and, the
maximum discharge being known, to determine the area of the waterway
accordingly. In the case of a very wide river like the Nile, with a
well-defined channel, it is inconvenient to make the distance between
the abutments of a work much less than the width of the channel, but
so far as velocity is concerned, the floor need not usually be lower
than the bed. The protection given to the channel on the upstream side
of the barrage (fig. 41) seems to be rather greater than necessary.
The thickness of the floor (9 feet 10 inches) seems excessive. The
thickness originally proposed was much less.
Of the many kinds of apparatus described in this chapter each
possesses some advantages and disadvantages. Gates require a bridge
with powerful lifting apparatus, and are suitable for large bodies of
water and great depths. Comparing needles with planks, the former can
be worked by one man and admit of rapid removal, and require far fewer
piers. Planks require two men, and are sometimes liable to jam, but
obstruct floating rubbish less than needles, and in shallow water give
rise to less leakage. Whether needles or planks are used, masonry piers
are most suitable where sand or gravel are liable to accumulate on the
floor, or where there is much floating rubbish. The hinged frames are
suitable in other cases. Falling shutters of the Chanoine type admit of
very rapid lowering, and can be used without a foot-bridge. The drum
weir is perfect in action, but its cost is high.
At any system of sluices the regulation should be so arranged as to
minimise the chances of damage to the bed and banks where this is at
all likely to occur. If the gates are opened only near one side of the
structure, there will be a rush of water on that side, and serious
damage may occur. The opening should be done symmetrically and, as far
as possible, distributed along the whole length.
Until experience has shown it to be unnecessary, soundings should be
taken at regular periods of time downstream of every important work
where scour can occur. When scour is found to have occurred at any
particular part of the work, the rush of water at such places should,
as far as possible, be prevented, and a chance given for silting to
occur.
Unless experience shows that damage is not likely to occur, a stock
of concrete blocks, sandbags, or other suitable materials should be
kept on the spot ready for use. Life-buoys should be provided on any
work where large volumes of water are dealt with, especially if it is
unfenced in any part, or if any of the men employed are casual workers.
Regarding works for preventing a river from shifting its course so as
to damage or destroy a weir or similar work, see CHAP. XI., _Art. 3_.