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Canal lock

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Canal locks in England.
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Canal locks in England.

On navigable waterways, 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 inclined plane, it is the chamber itself which moves.

The two main uses of locks are to make a river more easily navigable, or to allow a canal to cross country that is not level.

The term airlock was coined for a device operating similarly to allow persons to pass to and from an air-filled underwater vessel.

Contents

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.

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, aqueduct or embankments, or to construct even more technical devices such as inclined plances 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.

Construction

All locks have three elements: A plan and side view of a generic, empty canal lock. A lock chamber separated from the rest of the canal by an upper pair and a lower pair of mitre gates. The gates in each pair close against each other at an 18° angle to approximate an arch against the water pressure on the "upstream" side of the gates when the water level on the "downstream" side is lower.

Operation

For a boat travelling upstream, the process is the logical reverse process - except that 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. Boaaters 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.) Operating locks can be tricky, and hard work. Gongoozlers is the English canals term for people who take entertainment from observing the fruitless endeavours of hapless narrow-boat crews struggling with locks.

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.

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).

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).

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 then 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".)

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.

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 then 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.

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 it socket, and so allows the gate to swing more freely.

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.

Windlass (\"lock key\")

A detachable crank used for opening lock paddles (NOT the winding mechanism itself). The simplest windlass ia an iron rod (circular section, about half an inch in diameter and three feet long) bent into an L-shape, with a square socket at one end for fitting onto the "stub" protruding from the lock winding gear. Most have two sizes of socket, for different locks standards. There may be refinements such as an extensible handle (for stiff paddles), or a free-rotating cylindrical sleeve around the handle (to prevent blisters). On canals with well-mantained (easy-to-lift) paddle gear, crews often prefer to carry a smaller, lighter windlass made of aluminium. Most boats carry three or four windlasses of different types.

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 hole of their windlass 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. After the boat has gone through the lock, the pawl must be disengaged before the paddle can be lowered.

\"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).

Variations

Not all locks work exactly as described above, and the terminology changes, too ...

Notes on operating a lock

Etiquette

There are few fixed rules (apart from things like "engines off in locks" on the Thames). Most rules are simply good manners. Nor are you likely to be penalised for breaches of etiquette apart from very serious ones. Remmeber, though, that amiable lockside chats are part of the pleasure of the canals, but once you name has been flashed along the "towpath telegraph" for inconsiderate behaviour, you may find the chat dries up! But don't be alarmed: making a newbie mistakes is not a crime on the canals, and all but the grumpiest people (in the shiniest boats?) remember their "favourite" embarrassing moments too well to hold slip-ups against a beginner (especially a beginner who knows how to ask advice, blush a bit, and apologise!).

If there is a queue of boats at a lock, then a skipper has no right to delay the queue by refusing to share the lock with as many boats as will fit in. If the skipper does not wish to share, then they should go to the back of the queue (though this is still a waste of water). However, most boaters are courteous and considerate, and may be happy to hold back on request from a nervous skipper, where they would refuse a demand.

When near the head of a queue for a boater-operated lock, it is normal etiquette (and common sense) for the boat's shore crew to go to forward to the lock to offer help to get boats through as quickly as possible. Even if help is not required (and it should not be imposed) the lockside chat is part of the sociable nature of a canal holiday, and a good way of finding out about good moorings, shops and pubs ahead.

This is a huge topic. At a lock, for instance, it means making sure that you are methodical, to avoid mistakes like having upper and lower paddles open at the same time, or having a gate open at one end of the chamber at the same time as a paddle at the other end. These examples would let water flow straight through the lock for no purpose. Such actions might be necessary when filling a lower pound that is short of water, but this would normally only be done with the permission of the waterway authority, and they would perhaps insist on doing it themselves, or on it being done in a paricular way (perhaps in a short burst, to releases a single wave to wash the boat over a short shallow patch).

On most English waterways, it is usually required that, before moving away from a lock, a boat's crew ensures that all paddles are lowered, and all gates closed, unless handing responsibility over to the crew of a following or oncoming boat. This is particularly important at times of water shortage. However, there are some local variations on this rule. Some boaters (except in times of water shortage) advocate always leaving the exit gates open, on the grounds that (a) It will always save you time (b) They will probably drift open anyway, if the other gates and paddles do not leak (c) If they DO stay shut, then 50% of the time it will save time for the next boat approaching the lock.

Risks

Illustrations

Pictures below depict various lock operations:

Image:Hiram M. Chittenden Locks-3.JPG|Tug and barge in lock when full. Image:Locks-1.jpg|Lock emptied for maintenance. Image:Locks-3.jpg|Lock emptied for maintenance. Image:Locks-2.jpg|Lock emptied for maintenance. Image:Hiram M. Chittenden Locks-8.jpg|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.

Operation of a canal lock
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Operation of a canal lock

Special Cases

Lock Flights

Loosely, a flight of locks is simply a series of locks in close-enough proximity to be identified as a separate group. For many reasons, a flight of locks is preferable to the same number of locks spread more widely: crews are put ashore and picked up once, rather than multiple times; transition involves a concentrated burst of effort, rather than a continually-interrupted journey; a lockkeeper may be stationed to help crews through the flight quickly; and where water is in short supply, a single pump can recycle water to the top of the whole flight. The need for a flight may be purely determined by the lie of the land, but it is possible to purposely group locks into flights by using cuttings or embankments to "postpone" the height change. Examples: Caen Hill, Bosley.

A lock flight should not be confused with a lock staircase. In a flight, each lock has its own upper and lower gates, there is a pound (however short) between each pair of locks, and the locks are operated in the conventional way.

Staircase locks

Barges at a lock on the Mississippi River
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Barges at a lock on the Mississippi River
When a very steep gradient has to be climbed, a lock staircase is used.  A staircase can be thought of as a "compressed" flight, where the  intermediate pounds have disappeared, and the upper gate of one lock is also the lower gate of the one above it. However,  it is incorrect to use the terms staircase and flight interchangeably: because of the the "loss" of the intermediate pounds, operating a staircase is very different from operating a flight.  It can be more useful to think of a staircase as a single lock with intermediate levels (the top gate is a normal top gate, and the intermediate gates are all as tall as the bottom gate).


Examples of famous large staircases in England include Foxton and Bingley.  Two-rise staircases are more common: Snakeholme Lock and Struncheon Hill Lock on the Driffield Navigation were converted to staircase locks after low water levels hindered navigation over the bottom cill at all but the higher tides - the new bottom chamber rises just far enough to get the boat over the original lock cill. In China, the recently completed Three Gorges Dam includes a double five-step staircase for large ships, and a ship lift for vessels of less than three thousand metric tons.  


Operating a staircase is to many inexperienced boaters the stuff of nightmares. 
The key worries (apart from simply being paralysed with indecision) are either sending down more water than the lower chambers can cope with (flooding the towpath, or sending a tidal wave along the canal) or completely emptying an intermediate chamber (although this shows that a staircase lock can be used as an emergency dry dock).  To avoid these mishaps, it is usual to have the whole staircase empty before starting to descend, or full before starting to ascend.


One striking difference in using a staircase (compared with a single lock, or a flight) is the best sequence for letting boats through.  In a single lock (or a flight with room for boats to pass) it is obvious that boats should ideally alternate in direction.  In a staircase, however, it is quicker (but less efficient in terms of water use) for a boat to follow a previous one going in the same direction. Because of this, some staircases have "up" and "down" time-slots.


As with a flight, it is possible for more than one boat to be in a staircase at the same time, but managing this without waste of water requires expertise.  On English canals, a staircase of more than two chambers is usually staffed: the locky at Bingley (looking after both the "5-rise" and the "3-rise") has worked there for more than 20 years and ensures that there are no untoward events and that boats are moved through as speedily and efficiently  as possible.  Such expertise permits miracles of boat balletics: it is possible for boats travelling in opposite directions to pass each other halfway up the staircase; or at peak times, to have all the chambers full simultaneously with boats travelling in the same direction.  



Doubled / Paired / Twinned Locks

Locks can be built in parallel (ie side by side). This can be called Doubling, Pairing, or Twinning.  There are several examples (in this case called "double locks") on one stretch of the Trent and Mersey Canal). Twinning gives advantages in speed : avoiding holdups at busy times; or increasing the chance of a boat finding a lock set in its favour.  Also, there can be water savings: the locks may be of different sizes, so that a small boat does not need to empty a large lock; or each lock may be able to act as a side pond for the other.


These terms can also (in different places or to different people) mean either a two-chamber staircase, or just a flight of two locks (as at Thornhill Double Locks on the Calder and Hebble Navigation).  Also, "double lock" (less often, "twin lock") is often used by novices on the English canals to mean a wide (14ft) lock, presumably because they are "double" the width of a narrow lock, and allow two narrow boats to "double up".



Stop locks

A "stop" lock is a (very) low-rise lock built at the junction of two (rival) canals to prevent water from passing between them.  During the competitive years of the English waterways system,
established companies would often refuse to allow a connection from a planned adjacent canal.  This situation created the Worcester Bar in Birmingham, where goods had to be transhipped between boats on rival canals only feet apart.


Where the older canal company saw an advantage in a connection, or where the new cmpany managed to insert a mandatory connection into its Act of Parliament, the old company would seek to protect (and even enhance) its water supply by insisting that at the junction the newer canal must be at a slightly higher level, often only a few inches.  Even though the rise was tiny, the difference in levels still required a lock (called a stop lock, because it was to stop water flowing between the canals).


Stop locks sometimes came in sequential pairs.  Where the water levels on either side of the junction could not be exactly controlled, each company would have its own lock with doors "pointing" towards its own canal.  It would close the doors whenever the other canal was low, to avoid donating water to it.  One example of a paired stop lock was at Hall Green Stop Lock at the junction of the southern terminus of the Macclesfield Canaland a branch of the Trent and Mersey Canal near Kidsgrove. This lock now, disappointingly, only has one lock again, mostly due to the lowering of the summit pound to improve tunnel height.  The T&M branch (from Hardings Wood junction by Harecastle tunnel north portal) is now often considered to be part of the Macclesfield (otherwise, the Macclesfield Canal has ALL 12 of its locks in one flight, at Bosley).


Many stop locks were removed or converted to single gates after nationalisation in 1948.



Flood locks

A flood lock is to prevent a river from flooding a connected waterway.  It is typically installed where a canal leaves a river. At normal river levels, the lock gates are left open, and the height of the canal is allowed to rise and fall with the height of the river.


However, if the river floods beyond a safe limit for the canal, then the gates are closed (and an extra lock created) until the river drops again.  Because this is a true lock, it is possible for boats to leave the canal for the flooded river despite the difference in water levels (though this is not likely to be wise) or (more sensibly) to allow boats caught out on the flood to gain refuge in the canal.


Note that if the canal is simply a navigation cut connecting two stretches of the same river, the flood lock will be at the upstream end of the cut (the downstream end will have a conventional lock).


Flood locks which have been used only as flood gates (see below) are often incapable of reverting to their former purpose without refurbishment.  That is, where only outer gates are ever closed (probably because a waterway is not a true commercial one, and therefore there is no financial imperative for a boat to venture out onto a flooded river) inner gates soon suffer from lack of maintenance.  A good example is on the Calder and Hebble Navigation, where structures referred to in the boating guides as "Flood Locks" are clearly only capable of being used for flood-prevention, not for "penning" boats to or from the river in flood.



Flood gates

A flood gate is a poor man's flood lock. Only one set of gates exist, and so when the river is higher than the canal, the gates are closed and navigation ceases. These are quite common in the French inland waterways system.  A Flood gates is also sometimes called a Stop Lock.



Bi-directional gates and locks

Where a lock is tidal (ie on side of the lock has water whose level varies with the tide), the water on the tidal side (the "downstream" side)may rise above the water on the normal "upper" side.  The "upstream" pointing doors will then fail to do their job, and will simply drift open.  To prevent water flowing the wrong way through the lock, there will need to be at least one set of gates pointing in the "wrong" direction.  If it is desirable that boats can use the lock in these cirumstances, then there  needs to be a full set of gates pointing towards the tidal side.  The  usual method is to have gates pointing in opposite directions at both ends of the chamber (alternatively, the "paired stop lock" arrangement of two separate sequential locks pointing in oppotise directions would work here - but would require an extra chamber).  If navigation is not required (or impossible) at one "extreme" (eg allow navigation above mid-tide, but just prevent the canal emptying at low tide) then it is only necessary to have one set of bi-directional gates.



Sea Locks

A lock connecting a canal or river directly with the estuary (or beach).  All sea-locks are tidal.



Tidal Locks

Loosely, any lock connecting tidal with non-tidal water.  This includes a lock between a tidal river and the non-tidal reaches; or between a tidal river and a canal; or a sea lock.  However, the term usually refers specifically to a lock whose method of operation is affected by the state of the tide.  Examples:


Very large locks

The world's largest canal lock is the "Berendrecht lock" and can be found in Antwerp, Belgium. The lock is 500 metres (1,640 feet) long, and 68 metres (223 feet) wide, and has four sliding lock gates. The size of locks cannot be compared without considering the difference in water level that they are designed to operate under. The total volume of water to be considered in any lock equals the product of its length, breadth and the difference in water levels. Lock staircases are used in an attempt to reduce the total volume of water required in relation to the amount of useful work done. The useful work done relates to the weight of the vessel and the height it is lifted. When a vessel is lowered the consumption of potential energy of the water consumed is considered. Alternatives to locks such as the Anderton Boat Lift do not rely on the consumption of water as the primary power source, are powered by motors and are designed to consume a minimum amount of water.


The 29 locks on the Mississippi River are typically 600 foot (180 m) long while tug and barge combinations are as much as 1200 feet (360 m) long consisting of as many as 15 barges and one tug. In these cases, some of the barges are locked through, using partially opened lock valves to create a current to pull the un-powered barges out of the lock where they are tied up to wait the rest of the barges and the tug to pass through the lock. It can take as much as an hour and a half to pass the lock.



History and Development




Top gate of a lock, showing the difference in water level.

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Top gate of a lock, showing the difference in water level.








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.



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, and the boat would be hauled through the opening against the water current with ropes, and when the boat was through, the opening would be quickly closed again.  This could also be used 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 however highly dangerous, and risky, and many boats were deluged by the torrent of water.  



Staunch

Another similar device was the staunch or water gate, consisting of one gate (or pair of mitred gates).  This was used on navigable rivers to enable vessels to get over shallows upstream of it, as closing it would increase the depth of water upstream.  However the whole head of water had to be drained from upstream before a boat could pass the water gate.  Accordingly they were not used where the obstacle to be passed was a mill weir.  



Pound Lock

The type of lock seen today is known as a pound lock as described earlier in this article, which work by raising or lowering the water level within a double gated pound. It is believed that this type of lock originated in Medieval Europe.



Use of Water




Canal lock in the Noordoostpolder, Netherlands.

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Canal lock in the Noordoostpolder, Netherlands.






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.



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 froma 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).



Pumping

Where it is clear that natural supply will not be sufficient to 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 to the summit from lower down. Such remedies may of course be installed later, when poor planning becomes apparent, or when there is an unforseeable 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).



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.



Alternatives




Three gorges dam model view. A pair of five locking steps is at center with a ship lift to the left

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Three gorges dam model view. A pair of five locking steps is at center with a ship lift to the left






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.  Here are three working examples...



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.



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.



Three Gorges Dam

Different solutions may be used in combination.  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.



Ship sizes

Locks restrict the maximum size of ship able to pass through, and this size is sometimes given a simple name:




The suez canal also has a maximum size of boat which may pass through it, but this is due to the size of the canal itself, as it has no locks:


See also



External links





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