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, Netherlands .]] , The Netherlands .]] 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 . 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 Tunnel s, cuttings, Aqueduct s 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. BASIC CONSTRUCTION AND OPERATION . 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.]] All locks have three elements:
The principle of operating a lock is simple. For instance, if a boat travelling downstream finds the lock already full of water:
Notes
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 two deepest locks on the English canal system are Bath Deep Lock 12 on the Kennet And Avon Canal and Tuel Lane Lock on the Rochdale Canal which both have a rise of nearly 20ft (sources vary as to the exact rises, so it is not possible to guarantee which is the deeper of the two). Both locks are amalgamations of two separate locks, which were combined when the canals were restored. 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 manoeuvring 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".) Cill , Paris]] , Austria]] ]] 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 lock side 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 gate, or pair of half-gates, made of oak or elm (or now sometimes steel). When closed, a pair 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.'' 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. 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. 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 disengaged and the paddle wound down with the windlass. Dropping paddles by knocking the pawl off can cause damage to the mechanism - the paddles are typically made of cast iron and can shatter or crack when dropped from a height. In areas where water-wastage due to vandalism is a problem, for example the Birmingham Canal Navigations ), paddle mechanisms are commonly fitted with vandal-proof locks (nowadays called "water conservation devices", which the authorities believes sounds nicer) which require the boater to employ a key called a "handcuff key" before the paddle can be lifted. These have caused at least one fatal incident. On the Calder And Hebble Canal in Yorkshire , a lock crew secured the "upstream" paddles before the boat had left the lock: they were unable to quickly refil the lock when the boat hung up on the cill, resulting in the boat sinking in the lock. Several wheelchair-users inside were unable to escape. 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.
"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 ...
ILLUSTRATIONS Pictures below depict various lock operations: |
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