Lock (water navigation)

A lock is a specialized device used for water navigation. It allows boats, ships, and other watercraft to move between stretches of water at different levels.

This is essential for rivers and canals that cross land that is not level. By using a lock, engineers can make a river more easily navigable.

While most locks use a fixed chamber, some designs use a moving chamber. In a caisson lock, a boat lift, or a canal inclined plane, the chamber itself rises and falls.

These devices act as water elevators to overcome changes in elevation.

The most common design is the pound lock. A pound lock consists of three essential elements. First, it has a watertight chamber that is large enough to enclose one or more boats. Second, it features gates at each end to separate the chamber from the rest of the canal. These are often a pair of "pointing" half-gates called mitre gates.

Third, it uses lock gear to control the water level. This gear is usually a simple valve, such as a paddle. A paddle is a flat panel lifted by a rack and pinion mechanism. Larger locks may use pumps to move the water.

The mechanism of a pound lock follows a specific sequence depending on the boat's direction. For a boat traveling downstream, the process begins by entering the lock.

The lower gates are then closed. The chamber is filled with water from upstream using the upstream valve. Once the water level matches the downstream level, the lower gates are opened. The boat then exits the lock. For a boat traveling upstream, the entire process is reversed. The boat enters the lock at the lower level, the gates close, and the chamber is filled from upstream to lift the boat. This entire operation usually takes between 10 and 20 minutes.

History shows that humans have solved water level problems for millennia. In Ancient Egypt, river locks were likely part of the Canal of the Pharaohs. Around 274–273 BC, engineers for Ptolemy II invented locks to keep the Nile free of salt water.

Later, during the Song dynasty in China (960–1279 CE), pound locks were developed. In 984, a tax administrator named Qiao Weiyue installed a pair of sluice-gates 250 feet apart. He created a short stretch of canal to protect grain barges from being wrecked by bandits.

In Europe, a pound lock was built in Vreeswijk, Netherlands, in 1385. The first true European pound lock appeared in 1396 at Damme, Belgium. Between 1452 and 1458, Bertola da Novate constructed 18 pound locks in the Milan canal system.

Engineers have experimented with many different lock designs to save water or money. Robert Weldon proposed the caisson lock around 1800 for the Somerset Coal Canal in England.

This design used a deep pool and a completely enclosed wooden box. The boat entered the box, which then moved up or down through the water. Another design was the shaft lock, which uses a deep shaft and a short tunnel.

The shaft lock at Minden, Germany, uses eight tanks linked in pairs. As the lock empties, water runs into each tank in turn to save water. In the early 18th century, some locks used earth banks instead of stone. These were called turf-sided locks. Because their sides were sloping, they required more water to operate than modern vertical-walled locks.

Some specialized locks were built to handle very large differences in water height. An inclined plane allows wide-beam boats to bypass a long flight of narrow locks. While many are no longer in use, the remains of a famous one exist at Foxton in the UK.

There were also hydro-pneumatic locks, like the one built at Camden Lock in London around 1817. These used compressed air to move caissons in counterbalance. While interesting, these designs were often replaced by conventional locks due to technical problems. Most modern locks, however, rely on the proven and reliable pound lock system.

Understanding locks helps us see how humans manage large-scale water systems. They are vital for the transport of goods and the management of water resources. In the Grand Canal, the water level was raised by using locks to manage a total rise of 100 feet.

Boaters often coordinate their travel to save time. If a boat is traveling toward another, the first boat might exit the lock and leave the water at the level the second boat needs. This simple coordination can save between 5 and 10 minutes. Locks remain a fundamental part of how we connect different parts of our world through water.