Lock (water navigation)
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. 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, the river-locks were probably part of the Canal of the Pharaohs. Ptolemy II is credited by some for being the first to solve the problem of keeping the Nile free of salt water when his engineers invented the lock around 274/273 BC.Ancient China
During 960–1279 CE, the natural extension of the flash lock, or staunch, was to provide an upper gate 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. 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 1373, 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 constructed 18 pound locks on the Naviglio di Bereguardo 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 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 which allows water to drain into or out of the chamber. Larger locks may use pumps.
- 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.
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 long and deep and contained a completely enclosed wooden box big enough to take a barge. This box moved up and down in the 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 of water – at a pressure of, in total. One of these "locks" was built and demonstrated to the Prince Regent, 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 CanalHydro-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.Inclined plane
The plane enabled wide-beam boats to bypass the flight of ten narrow locks, but failure to make improvements at the other end of the arm and high running costs led to its early demise. 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. There are plans to restore it, and some funding has been obtained.Shaft lock
Looking superficially similar to the caisson lock is the shaft lock. Shaft locks consist of a deep shaft with conventional upper gates. The lower gates are reached through a short tunnel. The gates only close off this approach tunnel so do not have to reach the full height of the lock. Notable examples have been built at Saint Denis, Horin and Anderten.The shaft lock at Minden has a fall of and has eight tanks linked in pairs to the lock chamber. As the lock is emptied water is run into each chamber in turn, for filling the water is released from the chambers thus saving the waste of a complete lockfull of water. An earlier attempt at a shaft lock had been made at Trollhättan in Sweden on the line of the present Göta canal. The fall would have been, astonishing in 1749. However the approach tunnel proved to be unusable in times of flood and the shaft lock was replaced by a 2-rise staircase in 1768.
Turf-sided lock
A turf-sided lock is an early form of canal lock design that uses earth banks to form the lock chamber, subsequently attracting grasses and other vegetation, instead of the now more familiar and widespread brick, stone, or concrete lock wall constructions. This early lock design was most often used on river navigations in the early 18th century before the advent of canals in Britain. The sides of the turf-lock are sloping so, when full, the lock is quite wide. Consequently, this type of lock needs more water to operate than vertical-sided brick- or stone-walled locks. On British canals and waterways most turf-sided locks have been subsequently rebuilt in brick or stone, and so only a few good examples survive, such as at Garston Lock, and Monkey Marsh Lock, on the Kennet and Avon Canal.Types of lock
Basic Pound lock
A pound lock is most commonly used on canals and rivers today. A pound lock has a chamber with gates at both ends that control the level of water in the pound. In contrast, an earlier design with a single gate was known as a flash lock.Pound locks were first used in China during the Song dynasty, having been pioneered by the Song politician and naval engineer Qiao Weiyue in 984. They replaced earlier double slipways that had caused trouble and are mentioned by the Chinese polymath Shen Kuo in his book Dream Pool Essays, and fully described in the Chinese historical text Song Shi :
The distance between the two locks was rather more than 50 paces, and the whole space was covered with a great roof like a shed. The gates were 'hanging gates'; when they were closed the water accumulated like a tide until the required level was reached, and then when the time came it was allowed to flow out.
The water level could differ by at each lock and in the Grand Canal the level was raised in this way by.