CHAPTER X
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.
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.
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.).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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]
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.
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.
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_.