A lock is a device used for raising and lowering boats, ships and other watercraft between stretches of water of different levels on river and canal waterways. The distinguishing feature of a lock is a chamber in a permanently fixed position in which the water level can be varied. In a caisson lock, a boat lift, or on a canal inclined plane, it is the chamber itself (usually then called a caisson) that rises and falls. Locks are used to make a river more easily navigable, or to allow a canal to cross land that is not level. Over time, more and larger locks have been used in canals to allow a more direct route to be taken.
History
Ancient Egypt In ancient Egypt, river locks were probably part of the Canal of the Pharaohs. Engineers of Ptolemy II are credited by Diodorus Siculus with being the first to solve the problem of keeping the Nile free of salt water when they invented the lock around 274–273 BC.
Ancient China Between 960 and 1279 CE, the natural extension of the flash lock, or 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 type of lock, called a pound lock, was first used in medieval China during the Song dynasty (960–1279 CE). 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 to 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.
Medieval Europe In 1385, in medieval Europe a sort of pound lock was built at Vreeswijk, Netherlands. This pound lock serviced many ships at once in a large basin. Yet the first true European pound lock was built in 1396 at Damme near Bruges, Belgium. The Italian Bertola da Novate (c. 1410–1475) constructed 18 pound locks on the Naviglio di Bereguardo (part of the Milan canal system sponsored by Francesco Sforza) between 1452 and 1458.
Basic operation
All pound 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 each 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 (paddle) 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 a boat travelling downstream, the process is:
If the water in the chamber is low, the boat waits while the chamber is filled by the upstream valve. The upstream gates are opened and the boat moves in. The upstream gates are closed. The chamber is drained through the downstream valve until the water level matches the downstream water level. The downstream gates are opened and the boat moves out. For a boat travelling upstream, the process is reversed. The whole operation will usually take between 10 and 20 minutes, depending on the size of the lock and whether the water in the lock chamber was at the boat's level or the other level when it arrived at the lock. 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 about 5 to 10 minutes. However, this is not true for staircase locks, where it is quicker for boats to go through in convoy, and it also uses less water.
Historic lock designs Historic lock designs, which are no longer used for constructing new locks as these have now been replaced by newer and better designs, are as follows:
Caisson lock
Around 1800 the use of caisson locks was proposed by Robert Weldon for the Somerset Coal Canal in England. In this underwater lift, the chamber was 80 ft (24.4 m) long and 60 ft (18.3 m) deep and contained a completely enclosed wooden box big enough to take a barge. This box moved up and down in the 60 ft (18.3 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 (18.3 m) of water – at a pressure of three atm (304 kPa; 44.1 psi), 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.
Composite material locks To economise, especially where good stone would be prohibitively expensive or difficult to obtain, composite locks were made, i.e. they were constructed using rubble or inferior stone, dressing the inside walls of the lock with wood, so as not to abrade the boats. This was done, for instance, on the Chesapeake and Ohio Canal with the locks near the Paw Paw Tunnel. and also the Chenango Canal
Hydro-pneumatic canal lift Possibly inspired by Weldon's caisson lock, William Congreve in 1813 patented a "hydro-pneumatic double balance lock" in which two adjacent locks containing pneumatic caissons could be raised and lowered in counterbalance by the movement of compressed air from one caisson to the other. 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. The company insisted on various modifications to Congreve's design; the resulting installation proved to be unsatisfactory, and was soon replaced by conventional locks.
… excerpt ends here. Continue reading the full article.
