Lock (water navigation)

• 1. Introduction

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  A lock is a device for raising and lowering boats, ships and other watercraft between stretches of water that sit at different levels. It is the quiet workhorse of rivers and canals, the thing that lets a boat climb a hill or descend a slope without ever leaving the water. The defining feature is a chamber held in a permanently fixed position, a sealed room where the water level itself can be made to rise or fall. Fill it, and a boat floats upward. Drain it, and the same boat sinks gently down to meet the level below.

  That simple idea hides a long argument across centuries about how best to lift a boat. Who first solved the problem of moving watercraft between levels, and where did the breakthrough happen? Why would an engineer ever seal a barge inside a box and push the whole box through deep water? How does a single chamber become a staircase, and why do experienced boaters dread getting one wrong? What follows traces the lock from a frustrated tax administrator in medieval China to the largest sea gates on Earth, and explains why a passing boat is often the most welcome sight on a canal.

• 2. Eastern Origins

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  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, inventing the lock around 274-273 BC. The challenge there was not just lifting boats but holding back the sea.

  Qiao Weiyue, a high-ranking tax administrator, was losing imperial grain barges on the West River near Huai'an in Jiangsu. The barges were wrecked again and again at a double slipway, and Qiao discovered the cause was not bad luck. The soldiers stationed there had plotted with bandits to wreck the heavy barges so the spilled grain could be stolen. His answer in 984 changed waterways forever.

  In 984 Qiao installed a pair of sluice-gates two hundred and fifty feet apart, roofing the entire structure over like a building. By placing two staunch gates so close together, he created a short stretch of canal that could be filled from above and emptied below, a true pound lock. This design appeared during the Song dynasty, between 960 and 1279 CE, as the natural extension of the older flash lock, or staunch.

  The Chinese polymath Shen Kuo, who lived from 1031 to 1095, mentioned the pound lock in his book Dream Pool Essays, published in 1088. A fuller account survives in the historical text Song Shi, compiled in 1345, which records that 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 called hanging gates. When closed, the text says, the water accumulated like a tide until the required level was reached, and then when the time came it was allowed to flow out. On the Grand Canal, locks of this kind raised the level by 138 ft.

• 3. Europe Catches Up

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  In 1385 a sort of pound lock was built at Vreeswijk in the Netherlands, servicing many ships at once in a large basin. It was a crude beginning rather than the finished article. The first true European pound lock came eleven years later, built in 1396 at Damme near Bruges in Belgium.

  Bertola da Novate, the Italian engineer who lived from around 1410 to 1475, constructed 18 pound locks on the Naviglio di Bereguardo between 1452 and 1458. That canal formed part of the Milan system sponsored by Francesco Sforza. The technology was spreading fast through the waterways of medieval Europe.

  Leonardo da Vinci supplied the refinement that still defines locks today. Sometime around the late 15th century he invented the miter gate, the arrangement of two angled half-gates that meet in a shallow V pointing upstream. The water pressure pushes the halves together and seals them tight, and it remains the most common gate arrangement in the world.

• 4. How A Lock Works

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  All pound locks share three elements, and the whole operation usually takes between 10 and 20 minutes. The first is a watertight chamber connecting the upper and lower canals, fixed in place but with a water level that can vary. The second is a gate, often a pair of pointing half-gates, at each end. The third is a set of lock gear to empty or fill the chamber, traditionally a flat panel called a paddle lifted by manually winding a rack and pinion mechanism. Larger locks may use pumps instead.

  For a boat travelling downstream the sequence is straightforward. If the water in the chamber is low, the boat waits while the chamber fills through the upstream valve. The upstream gates open, the boat moves in, and the gates close behind it. The chamber then drains through the downstream valve until the levels match, the downstream gates open, and the boat moves out. For a boat going upstream, the whole process simply runs in reverse.

  Boaters approaching a lock are usually pleased to meet another boat coming towards them. That oncoming boat has just left the lock at their own level, which means the lock is already set in their favour and saves about 5 to 10 minutes. The friendly wave at the lock is, in part, gratitude for a chamber that does not need refilling.

• 5. Locks That Lift The Box

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  Around 1800 Robert Weldon proposed the caisson lock for the Somerset Coal Canal in England, a design that moved the boat without moving the water. The chamber was 80 ft long and 60 ft deep and held a completely enclosed wooden box large enough to take a barge. The boat entered the box, was sealed in by a closing door, and the box itself was moved up or down through the deep pool. Apart from leakage, the water never left the chamber, so the lock wasted no water. At the bottom of the chamber the box sat under almost 60 ft of water, at a pressure of 3 atm in total.

  The Prince Regent, later George IV, was given a demonstration of one of Weldon's locks. The engineering problems proved too great, and the design was never put into use on the Coal Canal. The idea of lifting the chamber itself, however, did not die with it.

  William Congreve, possibly inspired by Weldon, patented a hydro-pneumatic double balance lock in 1813. Two adjacent locks held pneumatic caissons that could be raised and lowered in counterbalance by moving compressed air from one to the other. Around 1817 the Regents Canal Company built one at the site of the present-day Camden Lock in north London, driven again by water supply problems. The company insisted on modifications to Congreve's design, the installation proved unsatisfactory, and conventional locks soon replaced it.

  The shaft lock took yet another approach, looking superficially similar to the caisson lock. It consists of a deep shaft with conventional upper gates, while the lower gates are reached through a short approach tunnel and need not reach the full height of the lock. The shaft lock at Minden has a fall of 12.7 m and uses eight tanks linked in pairs, refilling the chamber tank by tank to avoid wasting a complete lockful of water. An earlier attempt at Trollhattan in Sweden, on the line of the present Gota canal, aimed for a fall of 16 m, astonishing in 1749, but its approach tunnel proved unusable in floods and was replaced by a 2-rise staircase in 1768.

• 6. Stairs Made Of Water

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  Where a very steep gradient has to be climbed, a lock staircase is used, and it comes in two forms, real and apparent. A real staircase is a compressed flight in which the intermediate pounds have vanished and the upper gate of one lock serves as the lower gate of the one above it. The whole structure behaves like a single lock with intermediate levels. Because no pound separates the chambers, a chamber can only be filled by emptying the one above or emptied by filling the one below.

  Grindley Brook and Bingley are famous English examples of real staircases. Two-rise staircases are more common. Snakeholme Lock and Struncheon Hill Lock on the Driffield Navigation were converted into staircase locks after low water levels hindered navigation over the bottom cill at all but the higher tides. In an apparent staircase, by contrast, the chambers still share gates but the water passes through side ponds rather than directly from one chamber to the next, so the flight need not be full or empty before starting. Foxton Locks and Watford Locks on the Leicester Branch of the Grand Union are apparent staircases.

  Inexperienced boaters may find staircases difficult, and the dangers are real. Sending down more water than the lower chambers can hold floods the towpath or sends a wave along the canal, while completely emptying an intermediate chamber leaves a boat stranded, though that flaw doubles as an emergency dry dock. Staircases also reverse the usual etiquette. In a single lock boats should alternate direction, but in a staircase it is quicker for a boat to follow a previous one going the same way. For this reason staircases such as Grindley Brook, Foxton, Watford and Bratch are supervised by lockkeepers during the main cruising season.

  The lockkeepers at Bingley look after both the 5-rise and the 3-rise, and their expertise permits unusual feats. Boats travelling in opposite directions can pass each other halfway up the staircase by moving sideways around one another. At peak times every chamber can be full at once with boats all heading the same way. The Foxton flight consists entirely of two adjacent 5-chamber staircases, while on the Grand Union Leicester Canal the Watford flight pairs a four-chamber staircase with three separate locks.

• 7. Side By Side And Off The Line

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  Locks can be built side by side on the same waterway, a practice called doubling, pairing, or twinning. The Panama Canal has three sets of double locks. Doubling speeds traffic at busy times and raises the chance of finding a lock set in your favour, and it can save water too. Twinned locks can synchronise so that water from an emptying chamber helps fill the other, a facility long withdrawn on English canals though the disused paddle gear survives at Hillmorton on the Oxford Canal. Abroad it is still in use, with a pair of twinned locks opened in 2014 on the Dortmund-Ems Canal near Munster, Germany.

  The Soo Locks at Sault Ste. Marie show how varied paired locks can be. The American MacArthur Lock is 800 ft long, 80 ft wide and 29.5 ft deep, sized for ships that fit the smaller locks on the Welland Canal. The Poe lock is 1200 ft long, 110 ft wide and 32 ft deep, accommodating the larger lakers that operate only on the upper four Great Lakes. The Canadian lock, 77 m long, 15.4 m wide and 13.5 m deep, is used for recreational and tour boats.

  The staircase at Lockport, New York, was a doubled set, with five twinned locks letting east and west-bound boats climb or descend the 60 ft Niagara Escarpment, a considerable engineering feat in the nineteenth century. Today Lockport has two large steel locks, and half of the old twin stair acts as an emergency spillway, its original lock gates restored in early 2016.

  The Agde Round Lock on the Canal du Midi in France is the best known round lock, serving the main line of the canal while also allowing access to the Herault River. A second French round lock, the now-disused Ecluse des Lorraines, once connected the Canal lateral a la Loire with the River Allier. Other specialist forms range from the drop lock at Dalmuir on the Forth and Clyde Canal in Scotland, the only one of its kind ever built, to the diagonal lock, where boats moor to floating bollards that guide them along a shaft built on an incline.

• 8. Risers, Hazards And Giants

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  The rise is the change in water level inside a lock, and the figures stretch from modest to staggering. The two deepest locks on the English canal system are Bath deep lock on the Kennet and Avon Canal and Tuel Lane Lock on the Rochdale Canal, both with a rise of nearly 20 ft, each an amalgamation of two separate locks combined to suit changed road crossings. The Carrapatelo and Valeira locks on the Douro river in Portugal, 279 ft long and 39 ft wide, have maximum lifts of 115 and 108 ft. The two Ardnacrusha locks near Limerick on the Shannon navigation in Ireland have a rise of 100 ft, the upper chamber rising 60 ft and joined to the lower by a tunnel that stays hidden until the chamber is nearly empty.

  The cill is the great hidden danger of descending a lock, a narrow horizontal ledge protruding from below the upper gates. Allowing the rear of a boat to hang on the cill is the main risk, which is why its forward edge is usually marked by a white line on the lock side. Names vary by canal, called a babbie on the Oxford Canal and the cill bumper on the Grand Union, while operators in the United States and Canada call the ledge a miter sill.

  The paddles carried their own threat, especially on the old Erie Canal. Water reaching a certain position could push a paddle with enough force to tear the windlass out of a worker's hands, or knock a person standing in the wrong place into the canal, leading to injuries and drownings. To control a heavy boat, snubbing posts were used on horse-drawn and mule-drawn canals, since a 200-ton boat moving at a few miles an hour could destroy a lock gate. A rope about 60 ft long was wound around the post, and friction slowed the boat as it entered.

  In June 1873 on the Chesapeake and Ohio Canal, a drunk captain showed what happens when the rope stays loose. His already-leaking boat, the Henry C. Flagg, entered Lock 74 without being snubbed and crashed into the downstream gates, knocking them out. The outrush of water slammed the upstream gates shut and broke them too, sending a cascade over the boat and sinking it, suspending navigation for 48 hours. Far from such chaos, the modern giants are calm and vast. In 2016 the Kieldrecht Lock in the Port of Antwerp took the title of the world's largest lock from the nearby Berendrecht Lock, and in 2022 the IJmuiden sea lock serving the Port of Amsterdam became the world's largest by surface area, measuring 500 m long, 70 m wide and 18 m deep, with sliding gates holding back the sea.