Lock (water transport)
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On waterways (navigable rivers and canals) a lock is a particular type of device for raising or lowering boats between stretches of water at different levels. The distinguishing feature of a lock is a fixed chamber whose water level can be varied; whereas in a boat lift or canal inclined plane, it is the chamber itself which moves.
Locks are used to make a river more easily navigable, or to allow a canal to take a reasonably direct line across country that is not level.
The term airlock was coined for a similar device used to allow persons to pass to and from a location in which a particular atmosphere is maintained, such as underwater, in space, or in a clean room.
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[edit] Use of locks in river navigations
A lock is required when a stretch of river is made navigable by bypassing an obstruction such as a rapid, dam, or mill weir — because of the change in river level across the obstacle.
In large scale river navigation improvements, weirs and locks are used together. A weir will increase the depth of a shallow stretch, and the required lock will either be built in a gap in the weir, or at the downstream end of an artificial cut which bypasses the weir and perhaps a shallow stretch of river below it. A river improved by these means is often called a Waterway or River Navigation (see example Calder and Hebble Navigation).
The lowest lock on a navigable river separates the tidal and non-tidal stretches. Sometimes a river is made entirely non-tidal by constructing a Sea Lock directly into the estuary.
In more advanced river navigations, more locks are required.
- Where a longer cut bypasses a circuitous stretch of river, the upstream end of the cut will often be protected by a flood lock.
- The longer the cut, the greater the difference in river level between start and end of the cut, so that a very long cut will need additional locks along its length. At this point, the cut is, in effect, a canal.
[edit] Use of locks in canals
Early completely artificial canals, across fairly flat countryside, would get round a small hill or depression by simply detouring (contouring) around it. As engineers became more ambitious in the types of country they felt they could overcome, locks became essential to effect the necessary changes in water level without detours that would be completely uneconomic both in building costs and journey time. Later still, as construction techniques improved, engineers became more willing to barge directly through and across obstacles by constructing long tunnels, cuttings, aqueducts or embankments, or to construct even more technical devices such as inclined planes or boat lifts. However, locks continued to be built to supplement these solutions, and are an essential part of even the most modern navigable waterways.
[edit] Basic construction and operation
All locks have three elements:
- A watertight chamber connecting the upper and lower canals, and large enough to enclose one or more boats. The position of the chamber is fixed, but its water level can vary.
- A gate (often a pair of "pointing" half-gates) at either end of the chamber. A gate is opened to allow a boat to enter or leave the chamber; when closed, the gate is watertight.
- A set of lock gear to empty or fill the chamber as required. This is usually a simple valve (traditionally, a flat panel lifted by manually winding a rack and pinion mechanism) which allows water to drain into or out of the chamber; larger locks may use pumps.
The principle of operating a lock is simple. For instance, if a boat travelling downstream finds the lock already full of water:
- The entrance gates are opened and the boat sails in.
- The entrance gates are closed.
- A valve is opened, this lowers the boat by draining water from the chamber.
- The exit gates are opened and the boat sails out.
Notes
- If the lock was empty, the boat would have had to wait 5-10 minutes while the lock was filled.
- For a boat travelling upstream, the process is reversed: for instance, the chamber is filled by opening a different valve which allows water to enter the chamber from the upper level.
- The whole operation will usually take between 10 and 20 minutes, depending on the size of the lock, and whether it was originally set "for" the boat.
- Boaters approaching a lock are usually pleased to meet another boat coming towards them, because this boat will have just exited the lock on their level and therefore set the lock in their favour — saving some work and some 5-10 minutes. (This is not true for staircase locks, where it is quicker for boats to go through in convoy.)
[edit] Details and terminology
For simplicity, this section describes a basic type of lock, with a pair of gates at each end of the chamber and simple rack and pinion paddles raised manually by means of a detachable windlass operated by the boat's shore crew. This type can be found all over the world, but the terminology here is that used on the British canals. A subsequent section explains common variations.
[edit] Rise
The change in water-level effected by the lock. The deepest lock on the English canals is Tuel Lane Lock on the Rochdale Canal with rise of about 20 feet. A more typical (English) rise would be 8-12 feet (though even shallower ones can be encountered).
[edit] Pound
The level stretch of water between two locks (on a river, the corresponding term is commonly reach). The lock allows a boat to move between the pound above it (upper pound) and the pound below it (lower pound).
[edit] Chamber
The main feature of a lock. It is a watertight (masonry, brick, or concrete) enclosure which can be sealed off from the pounds at either end by means of gates. The chamber may be the same size (plus a little manouevering room) as the largest vessel for which the waterway was designed; sometimes larger, to allow more than one such vessel at a time to use the lock. The chamber is said to be "full" when the water level is the same as in the upper pound; and "empty" when the level is the same as in the lower pound. (If the lock has no water in it at all, perhaps for maintenance work, it might also be said to be empty, but a less-confusing term for this is "drained".)
[edit] Cill
A narrow horizontal ledge protruding a short way into the chamber from below the upper gates. Allowing the rear of the boat to "hang" on the cill is the main danger one is warned to guard against when descending a lock, and the position of the forward edge of the cill is usually marked on the lockside by a white line. The edge of the cill is usually curved, protruding less in the centre than at the edges.
[edit] Gates
The watertight doors which seal off the chamber from the upper and lower pounds. Each end of the chamber is equipped with a pair of swinging oak, elm (or now sometimes steel) half-gates. When closed they meet at an angle like a chevron pointing upstream (this arrangement is often called pointing doors) and a very small difference in water-level squeezes the closed gates securely together. This reduces any leaks from between them and prevents their being opened until water levels have equalised. If the chamber is not completely full, the top gate is secure; and if the chamber is not completely empty, the bottom gate is secure (in normal operation, therefore, the chamber cannot be open at both ends). A lower gate is taller than an upper gate, because the upper gate only has to be tall enough to close off the upper pound, while the lower gate has to be able to seal off a full chamber. The upper gate is as tall as the canal is deep, plus a little more for the balance beam, winding mechanism, etc; the lower gate's height equals the upper gate plus the lock's rise.
[edit] Balance beam
A long arm projecting from the landward side of the gate over the towpath. As well as providing leverage to open and close the heavy gate, the beam also balances the weight of the gate in its socket, and so allows the gate to swing more freely.
[edit] Paddle
The simple valves by which the lock chamber is filled or emptied. A paddle is simply a sliding wooden panel which when "lifted" (slid up) out of the way allows water to either enter the chamber from the upper pound or flow out to the lower pound. A gate paddle simply covers a hole in the lower part of a gate; a more sophisticated ground paddle blocks an underground culvert. There can be up to 8 paddles (two gate paddles and two ground paddles at both upper and lower ends of the chamber) but there will often be fewer.
[edit] Winding gear / paddle gear
The mechanism which allows paddles to be lifted (opened) or lowered (closed). Typically, a square-section stub emerges from the housing of the winding gear. This is the axle of a sprocket ("pinion") which engages with a toothed bar ("rack") protruding from the top of the paddle. A member of the boat's shore crew engages the square socket of their windlass (see below) onto the end of the axle and turns the windlass perhaps a dozen times. This rotates the pinion and lifts the paddle. A pawl engages with the rack to prevent the paddle from dropping inadvertently while being raised, and to keep it raised when the windlass is removed, so that the operator can attend to other paddles (it is considered discourteous and wasteful of water to leave a paddle open after a boat has left the lock). To lower a paddle the pawl must be dissengaged and the paddle wound down with the windlass. Dropping paddles by knocking the pawl off can cause dammage to the mechanism.
[edit] Windlass ("lock key")
A windlass (also known as a 'lock handle', 'iron' or simply 'key') is a detachable crank used for opening lock paddles (the word does not refer to the winding mechanism itself).
The simplest windlass is made from an iron rod of circular section, about half an inch in diameter and two feet long, bent to make an L-shape with legs of slightly different length. The longer leg is called the handle, and the shorter leg is called the arm. Welded to the end of the arm is a square, sometimes tapered, socket of the correct size to fit onto the spindle protruding from lock winding gear.
- Socket: Traditionally, windlasses had a single socket, designed for a particular canal. When undertaking a journey through several canals with different lock-gear spindle sizes it was necessary to carry several different windlasses. A modern windlass usually has two sockets for use on different canals : in fact some canals - and even some locks - now require both sizes.
- Handle: The handle is long enough for a two-handed grip and is far enough from the socket to give enough leverage to wind the paddle up or down. There may be a freely-rotating sleeve around the handle to protect the tender hands of a novice boater from the blisters which can be caused by the friction of a rough iron handle turning against soft skin.
- Arm: A "long throw" windlass has a longer arm so that the handle is further from the socket to give a greater leverage on stiffer paddles. If the throw is too long then the user, winding a gate paddle, risks barking their knuckles against the balance beam when the handle is at the lowest point of its arc. A sophisticated modern windlass may have an adjustable-length arm.
- Materials : Early windlasses were individually hand forged from a single piece of wrought iron by a blacksmith. More modern techniques include casting of iron or bronze, drop forging and (the most common technique) welding. Some boatmen had their windlasses 'silvered' (or chrome plated) for increased comfort and to prevent rusting. Windlasses are now only rarely plated, but a popular modern choice of metal is aluminium, whose smooth and rustproof surface has the same advantages of longevity and blister-reduction, and is also very light.
[edit] "Turning" a lock
This can simply mean emptying a full lock or filling an empty one (We entered the lock, and it only took us five minutes to turn it). It is used more often to refer to a lock being filled or emptied while you are not in it (The lock was turned for us by a boat coming the other way) and particularly when there is no boat in it at all (The lock was set for us, but the crew of the boat coming the other way turned it before we got there).
[edit] Variations
Not all locks work exactly as described above, and the terminology changes, too ...
- Single gates on narrow canals (locks approx. 7 feet / 2.1 m wide)
- A few narrow locks imitate wide locks in having paired gates at both ends (eg Bosley, on the Macclesfield canal)
- On most English narrow canals however, the upper end of the chamber is closed by a single gate the full width of the lock. This was cheaper to construct and is quicker to operate, as only one gate needs to be opened.
- Some narrow locks (eg on Birmingham Canal Navigations) go even further. They have single gates at the lower end also. This speeds up passage, even though single lower gates are heavy (heavier than a single upper gate, because the lower gate is taller) and the lock has to be longer (a lower gate opens INTO the lock, it has to pass the bow or stern of an enclosed boat, and a single gate has a wider arc than two half-gates).
- Steel Gates. Some locks (particularly modern commercial ones) use gates made of steel.
- Swinging Gates: Even very large steel-gated locks still can use essentially the same swinging gate design as small 250-year-old locks on the English canals.
- Sliding Gates: Some low-head locks use sliding steel gates (see Kiel Canal).
- Guillotine Gates: Some locks have vertically moving steel gates — these are quite common on river navigations in East Anglia. Sometimes just one of the pairs of swinging gates is replaced by a guillotine: for instance at Salterhebble Locks, where space to swing the balance beams of bottom gates of the lowest lock was restricted by bridge widening.
- Vertically-Rotating Gates: Gates which, when open, lie flat on the canal bed and which close by lifting (London Flood Barrier).
- Rotating-Sector Gates. These work very like traditional swinging gates, but each gate is in the form of a sector of a cylinder. They close by rotating out from the lock wall and meeting in the centre of the chamber. A small example is at the sea lock on the Ribble Link. A dramatically-large one can be seen at the Rotterdam flood defences (huge flood gates).
- Alternate paddle gear
- Some manually-operated paddles do not require a detachable handle (windlass) because they have their handles ready-attached.
- On the Leeds and Liverpool Canal, paddles are raised by turning what is in effect a large horizontal wingnut (butterfly nut) lifting a screw-threaded bar attached to the top of the paddle.
- On the Calder and Hebble Navigation, some paddle gear is operated by repeatedly inserting a Calder and Hebble Handspike (length of 4" by 2" hardwood) into a ground-level slotted wheel and pushing down on the handspike to rotate the wheel on its horizontal axis.
- Lock Keepers. Some locks are operated (or at least supervised) by professional lock keepers. This is particularly true on commercial waterways, or where locks are large or have complicated features that the average leisure boater may not be able to operate successfully. For instance, although the upper Thames (England) is almost entirely a leisure waterway, the locks are usually staffed.
- Powered operation. On large modern canals, especially VERY large ones such as ship canals, the gates and paddles are too large to be hand operated, and are operated by hydraulic or electrical equipment. Even on smaller canals, some gates and paddles are electrically operated, particularly if the lock is regularly staffed by professional lockkeepers. Powered locks are usually still filled by gravity, though some very large locks use pumps to speed things up.
- Fish Ladders. The construction of locks on rivers obstructs the passage of fish such as trout going upstream to spawn. Measures such as a fish ladder are often taken to counteract this.
[edit] Illustrations
Pictures below depict various lock operations:
 Tug and barge in lock when full. |
 Lock emptied for maintenance. |
 Lock emptied for maintenance. |
 Lock emptied for maintenance. |
 Gate opening mechanism. |
In November 2004 the Hiram M. Chittenden Locks was emptied for maintenance. This provided an opportunity to visualize how a lock works without water obscuring the bottom of the lock. For reference the picture above on the far left shows the lock in operation with a tug and barge, loaded with sand and gravel bound for a nearby concrete mixing plant, waiting for the gates to open. The cutout in the side wall in the bottom left corner of the picture contains the gate when open.
The lock has three pairs of gates, one pair at each end and one pair in the middle so that half the length of the lock can be used when whole length of the lock is not required thus saving water. The last three pictures show from left to right, the low water end of the lock, the center pair of gates and the high water end of the lock. The person walking on the bottom near the middle of the lock in the second picture from the left gives a measure of the size of the lock. In the pictures of both ends of the lock the string of penstock openings are visible along the sides at the bottom. The water entering and leaving the lock flows by gravity through these openings. It requires around 15 minutes to fill or empty the lock.
[edit] Special cases
[edit] Lock flights
[edit] History and development
[edit] Dams and weirs
In ancient times river transport was common, but rivers were often too shallow to carry anything but the smallest boats. Ancient people discovered that rivers could be made to carry larger boats by making dams to raise the water level. The water behind the dam deepened until it spilled over the top creating a weir. The water was then deep enough to carry larger boats. This dam building was repeated along the river, until there were "steps" of deep water.
[edit] Flash locks
This however created the problem of how to get the boats between these "steps" of water. An early and crude way of doing this was by means of a flash lock. A flash lock consisted essentially of a small opening in the dam, which could be quickly opened and closed. On the Thames in England, this was closed with vertical posts (known as rimers) against which boards were placed to block the gap.
When the gap was opened, a torrent of water would spill out, cattying a "downstream" boat with it, or allowing an "upstream" boat to be manhauled through against the flow. When the boat was through, the opening would be quickly closed again. The "gate" could also be opened to release a 'flash' downstream to enable grounded boats to get off shoals, hence the name.
This system was used extensively in Ancient China and in many other parts of the world. But this method was dangerous, many boats were sunk by the torrent of water.
[edit] Staunch
A more sophisticated device was the staunch or water gate, consisting of a gate (or pair of mitred gates) which could be closed (and held shut by water pressure) when the river was low, in order to float vessels over upstream shallows at times of low water. However, the whole upstream head of water had to be drained (by some auxiliary method approaching modern sluices) before the a boat could pass. Accordingly they were not used where the obstacle to be passed was a mill weir.
[edit] Pound lock
The natural extension of the Staunch was to provide an upper gate (or pair of gates) to form an intermediate "pound" which was all that need be emptied when a boat passed through. This is a pound lock, the type of lock seen today: known in Medieval Europe, but first used on the Grand Canal in China long before that...
The Songshi or History of the Song Dynasty, volume 307, biography 66, records how Qiao Weiyue, a high-ranking tax administrator, was frustrated at the frequent losses incurred when his grain barges were wrecked on the West River near Huai’an in Jiangsu. The soldiers at one double slipway, he discovered, had plotted with bandits to wreck heavy imperial barges so that they could steal the spilled grain. In 984 Qiao installed a pair of sluice-gates two hundred and fifty feet apart, the entire structure roofed over like a building. By siting two staunch gates so close by one another, Qiao had created a short stretch of canal, effectively a pound-lock, filled from the canal above by raising individual wooden baulks in the top gate and emptied into the canal below by lowering baulks in the top gate and raising ones in the lower.
[edit] Use of water
The main problem caused by locks is that, each time a lock goes through one fill-empty cycle, a lockful of water (tens or hundreds of thousands of gallons) is released to the lower pound. In over-simplistic terms: on a canal where only one boat will fit into a lock, a boat travelling from the summit pound to the lowest pound is accompanied on its journey by one 'personal' lockful of water. To prevent the canal from running dry, some method must be used to ensure that the water supply at the canal summit is constantly replenished at the rate that the water is being drained downwards. This is, of course much more of a problem on an artificial canal crossing a watershed than on a river navigation.
[edit] Design
When planning a canal, the designer will attempt to build a summit level with a large reservoir, or one supplied by an artificial watercourse from a distant source, or one as long as possible (to act as its own reservoir) or which cuts across as many springs or rivers as possible (or all of these).
[edit] Pumping
Where it is clear that natural supply will not be sufficient to replenish the summit level at the rate that water will be used (or to allow for unexpected periods of drought) the designer may plan for water to be back-pumped back up to the summit from lower down. Such remedies may of course be installed later, when poor planning becomes apparent, or when there is an unforeseeable increase in traffic or dearth of rain. On a smaller scale, some local pumping may be required at particular points (water is continually recycled through some locks on the Kennet and Avon canal).
[edit] Side ponds
A way of reducing the water used by a lock is to give it a reservoir whose level is intermediate between the upper and lower pounds. This reservoir can store the water drained from the upper 1/3rd of the lock as a boat descends, and release it to fill the lower 1/3rd next time a boat ascends. This saves the total amount of water lost downhill in each fill-empty cycle. On English canals, these reservoirs are called side ponds, and the gear controlling them is sometimes coloured red. This has given rise to the famous mnemonic "Red before white, you're alright; white before red, you're dead" (referring to the danger of incurring the wrath of the locky, rather than any inherent physical risk in the mechanisms themselves). On some flights of locks with short intermediate pounds, the pounds are extended sideways — in effect to provide a reservoir to ensure that the pound does not run dry (in case, for instance, the lock below leaks more than the lock above). These extended intermediate pounds are sometimes confused with side ponds.
[edit] Alternatives
As well as the 'static' approaches mentioned earlier (various types of contouring, excavating, and spanning), there were many ingenious dynamic solutions, mostly variations on the boat lift or the inclined plane. These tend to be more expensive to install and operate, but offer faster transit and waste less water.
[edit] Inclined Planes
An inclined plane consists of a cradle (to hold a barge) or caisson (a box full of water in which a barge can float) which moves on rails sideways up a slope from one waterway to the other. There may be two caissons balancing each other's weight via a connecting cable. The motive power may be steam or hydraulic, or may come from overbalancing the top caisson with extra water from the upper waterway. There are no working waterway inclined planes in the UK at the moment, but the remains of a famous one can be seen at Foxton in Leicestershire on the Leicester arm of the Grand Union Canal. The plane was replaced by a flight of locks, but there are plans to restore it.
[edit] Boat Lifts
Here are three examples of boat lifts (the last two are currently in operation)
[edit] Somerset and Camden
Around 1800 the use of Caisson locks (actually a type of boat lift) was proposed by Robert Weldon for the Somerset Coal Canal in England. In this underwater lift, the chamber was 80 ft long and 60 ft deep and contained a completely enclosed wooden box big enough to take a barge. This box moved up and down in the 60 ft (18.2 m) deep pool of water. Apart from inevitable leakage, the water never left the chamber, and using the lock wasted no water. Instead, the boat entered the box and was sealed in by the door closing behind it, and the box itself was moved up or down through the water. When the box was at the bottom of the chamber, it was under almost 60 feet of water—at a pressure of three atmospheres, in total. One of these "locks" was built and demonstrated to the Prince Regent (later George IV), but it had various engineering problems and the design was not put into use on the Coal Canal.[1][2] However, in about 1817 the Regents Canal Company built one of these locks at the site of the present-day Camden Lock, north London. Here the motivation was, again, water supply problems. Even though the change in level is much lower than that would have been the case in Somerset, the system was soon replaced by conventional locks.[3] No commercially successful example has ever been built.
[edit] Anderton
The Victorian Anderton Boat Lift, the world's first vertical boat lift, linking the Trent and Mersey Canal and the River Weaver in Cheshire has recently been restored.
[edit] Falkirk
The Falkirk Wheel, the world's first rotating boat lift, acts as the centrepiece of the restoration of the Forth and Clyde and Union Canals. The spectacular "Wheel" presents the 21st century's solution to replacing a flight of locks which formerly connected the canals and which were filled in 1930. The Falkirk Wheel was the winning design in a competition to design a new lock. Visitors can now take a boat trip on the Wheel and be lifted over 100 feet in a few minutes compared to the time it took when the original lock staircase operated.
[edit] A combined system - the Three Gorges Dam
At the Three Gorges Dam on the Yangtze River (Chang Jiang) in China there are two stair-steps of five large ship locks. In addition to this there is a ship lift (a large elevator) capable of moving a three thousand ton ship vertically in one motion.
[edit] Ship sizes named after Locks
Locks restrict the maximum size of ship able to pass through, some key canals have given rise to the name of standard ship sizes:
[edit] References
- ^ The Somerset Coal Canal. Bath Royal Literary and Scientific Institution. Retrieved on 2006-10-06.
- ^ History of the Caisson Lock On the Somersetshire Coal Canal. The Somersetshire Coal Canal (Society). Retrieved on 2006-10-06.
- ^ Faulkner, Alan (2005): The Regent’s Canal: London’s Hidden Waterway. Waterways World Ltd. ISBN 1-870002-59-8.
[edit] See also
- Boat lift
- Canal inclined plane
- Canal pound
- Canals of the United Kingdom
- Erie Canal — New York State
- Flash lock
- Gongoozler
- Grand Union Canal
- Kiel Canal This low head single lock uses sliding gates.
- Murray River — Australia
- Panama Canal
- Peterborough Lift Lock — Canada (Ontario), an example of boat lift
- Pound lock
- Rideau Canal — Canada (Ontario)
- Saint Lawrence Seaway — Canada
- Sault Ste. Marie Canal — Sault Ste. Marie, Ontario
- Soo Locks — Sault Ste. Marie, Michigan/Ontario
- Trent-Severn Waterway — Canada (Ontario)
- Trollhättan Municipality - lock system from lake to North Sea in Sweden
- Welland Canal — Canada (Ontario)
[edit] External links
- Lock and dam on the Mississippi
- Deltaworks.Org — Philipsdam Locks, separates sweet and salt water while giving free passage to ships
- The River Wey and Wey Navigations Community Site — Wey Navigations, information and images related to canals, narrowboats and lock operation
- Diagonal Lock — A new concept technology for navigating canal networks, designed to enable the conservation of water and the preservation of delicate ecosystems.
- Sault Ste. Marie Canal National Historic Site
- Locks on the River Thames
- Interactive Lock River and Rowing Museum
