Lock
Lock
A lock or water lock is an enclosed, rectangular chamber with gates at each end, within which water is raised or lowered to allow boats or ships to overcome differences in water level. Locks have been used over 2,000 years, and although they are most often used by boats on canals, they also are used to transport massive ships between seas. All locks operate on the simple buoyancy principle that any vessel, no matter what size, will float atop a large enough volume of water. By raising or lowering the level of a body of water, the vessel itself goes up or down accordingly. Locks are used to connect two bodies of water that are at different ground levels as well as to “walk” a vessel up or down a river’s more turbulent parts. This is done by a series of connecting or “staircase” locks. Locks contributed significantly to the Industrial Revolution by making possible the interconnection of canals and rivers, thus broadening commerce. They still play a major role in industrial society.
History
The ancestor of the modern lock is the flash lock, also called a navigation weir or stanch. It originated in China and is believed to have been used as early as 50 BC. The flash lock was a navigable gap in a masonry dam or weir that could be opened or closed by a single wooden gate. Opening the gate or sluice very quickly would release a sudden surge of water that was supposed to assist a vessel downstream through shallow water—often a very dangerous process. Using the flash lock to go upstream was usually safe, but extremely slow since the gap in the dam was used to winch or drag a vessel through.
At some point, what now seems a very obvious improvement was made, and a second gate was added to the flash lock, thus giving birth to the pound lock. The first known example of a pound lock (whose dual gates “impound” or capture the water) is in China in AD 984. Supposedly built by Chiao Wei-Yo on the West River section of the Grand Canal near Huai-yin, it consisted of two flash locks about 250 feet (76.2 m) apart. By raising or lowering guillotine gates at each end, water was captured or released. The space between the two gates thus acted as an equalizing chamber that elevated or lowered a vessel to meet the next water level. This new method was entirely controllable and had none of the hazards or surges of the old flash lock.
Although a primitive form of lock was used in Belgium as early as 1180, the first pound lock in Europe was built at Vreeswijk, Holland, in 1373. Like its Chinese ancestor, it also had guillotine/up-and-down gates. The pound lock system spread quickly throughout Europe during the next century and was eventually replaced by an improved system that formed the basis of the modern lock system. During the fifteenth century, the multitalented Italian artist Leonardo da Vinci (1452-1519) served the Duke of Milan as engineer and devised an improved form of pound lock whose gates formed a V-shape when closed. In 1487, da Vinci built six locks with gates of this type. These gates turned on hinges, like doors, and when closed they formed a V shape pointing upstream— thus giving them their name of miter gates. Da Vinci realized that one great advantage of miter gates is that they were self-sealing by the water pressure (since they point upstream). Also when there is a difference in water level between one side and the other, the pressure holding the gates together is at its greatest.
Most of the great canals of Europe use locks. In France, the Briare Canal, completed in 1642, included forty locks, one series of which was a staircase of six locks that handled a fall of 65 feet (20 m). The famous Canal du Midi that leads to the Mediterranean was finished in 1692 and used 26 locks to surmount the 206-foot (61 m) difference from Garonne to Toulouse. It then descended 620 feet (189 m) through 74 locks. The first lock in England was built in 1566, but it was not until 1783 that a lock was completed in North America at Lake St. Francis in Canada.
Construction and operation
The earliest locks were built entirely of wood, with stone and then brick eventually becoming standard materials. The gates themselves were always wooden, with some lasting as long as 50 years. Filling or emptying these early locks was often accomplished by hand-operated sluices built in the gates. On later and larger locks, conduits or culverts built into the lock wall itself were not only more efficient, but let the water enter in a smoother, more controlled manner.
Nearly all locks operate in the following manner: (1) A vessel going downstream to shallower water enters a lock with the front gate closed. (2) The rear gate is then closed and the water level in the lock is lowered by opening a valve. The vessel goes down as the water escapes. (3) When the water level inside the lock is as low as that downstream, the front gate is opened and the vessel continues on its way. To go upstream, the process is reversed, with the water level being raised inside the lock. What the operators always strive for is to fill or empty the lock in the fastest time possible with a minimum of turbulence.
In modern locks, concrete and steel have replaced wood and brick, and hydraulic power or electricity is used to open and close the gates and side sluices. Movable gates are the most important part of a lock, and they must be strong enough to withstand the water pressure arising from the often great difference in water levels. They are mostly a variation of da Vinci’s miter gates, except now they usually are designed to be stored inside the lock’s wall recesses.
Probably the best known locks in the world are those used in the Panama Canal—the most-used canal in the world. Completed in 1914, the Panama Canal is an interoceanic waterway 51 miles (82 km) long that connects the Atlantic and Pacific Oceans through the Isthmus of Panama. It has three major sets of locks, each of which is built in tandem to allow vessels to move in either direction, like a separated, two-way street. Each lock gate has two leaves, 65 feet (20 m) wide by 7 feet (2 m) thick, set on hinges. The gates range in height from 47-82 feet (14-25 m), and are powered by large motors built in the lock walls. The chambers are 1,000 feet (305 m) long, 110 feet (33.6 m) wide, and 41 feet (12.5 m) deep. Most large vessels are towed through the locks. As with all modern locks, they are operated from a control tower and use visual signals and radio communications. Any future major changes or improvements in the canal or its locks must consider the fact that ocean-going vessels are simply becoming too large to pass through. The future may see the construction of a sea-level canal 10 miles (16 km) west of the existing canal. If so, it, like the Suez Canal, will contain no locks of any kind.
KEY TERMS
Flash lock —A simple wooden gate that was placed across a moving body of water to hold it back until it had become deep; the sudden withdrawal of which would cause a “flash” of water downstream that would carry a boat over the shallows below.
Guillotine gate —An early wooden gate on a lock that was operated by being raised or lowered.
Miter gate —An improved type of lock gate that turns on hinges, like a door, and meets to form a vee pointing upstream.
Pound lock —A name for an early form of two-gate lock in which a chamber is enclosed at either end by vertically-rising gates and into which water is admitted or released to change its water level and allow a vessel to do the same.
Staircase lock —Two or more locks sharing a common gate that form a series of water steps and allow a vessel to negotiate a steep rise.
Resources
BOOKS
Paget-Tomlinson, Edward. The Illustrated History of Canal & River Navigations. Sheffield, England: Sheffield Academic Press, 1993.
OTHER
Rideau Canal Waterway (Ontario, Canada). “History of the Rideau Lockstations: The Basics of a Rideau Lock” <http://www.rideau-info.com/canal/history/locks/lock-basics.html> (accessed December 2, 2006).
Leonard C. Bruno