Flash lock

Early locks were designed with a single gate, known as a flash lock or staunch lock. The earliest European references to what were clearly flash locks were in Roman times.

In England the "gate" was similar to a temporary needle dam: a set of boards, called paddles, supported against the current by upright timbers called rymers which normally kept the level of water above it to navigable levels. Boats moving downstream would wait above the lock until the paddles were removed, which would allow a "flash" of water to pass through, carrying the boats with it. Upstream boats would be winched or towed through the lock with the paddles removed. Considerable skill was involved both in removing the paddles in a timely manner and navigating the boat through the lock. Flash locks of this type have been documented since at least 1295 C.E.

Flash locks were commonly built into small dams or weirs where a head of water was used for powering a mill. The lock allowed boats to pass the weir while still allowing the mill to operate when the gate was closed. However it could take up to a day or even more to restore the water levels after a boat had passed, so their use was unpopular with the millers.

As navigation increased in importance, improvements were made to the basic design, and they came to be known by various names according to where they were located. Thus on the Thames they were called navigation weirs, on the East Anglian rivers they were called staunches, those on the River Avon, Warwickshire were called water gates, and in a number of instances they were called half locks. On the River Nene and some of the tributaries of the River Great Ouse, a design using a guillotine gate in a wooden frame was used from the early seventeenth century onwards. The gate was opened by operating a large spoked wheel, connected by chains to a toothed drum.

Although the pound lock was considerably easier to navigate, and pound locks had been built in China since 983, in the Netherlands from 1065 and in Britain from the 1560s, a few flash locks remained. Those on the Nene continued to be used until they were replaced in a programme of modernisation, which included building new locks, carried out between 1936 and 1941. The last flash lock on the Thames was Hart's Lock, which lasted until 1937, while on the Lower Avon, the structure of Cropthorne Water Gate lasted until the reopening of the river to navigation in 1961, although it had not been used for navigation for many years before that. Although slightly different in concept, Thames Lock at Weybridge on the Wey Navigation had an additional single gate some 100 yards (91 m) below the lock, which when closed raises the water level above it, allowing boats which would normally foul the cill to work through the lock. It continued in use until 1969, to enable grain barges to use the lock, and although grain traffic stopped in that year, it recommenced in 1981, when the gate was again used. The gate is still used at the lock-keeper's discretion for boats which have a draught exceeding 1.75 feet (0.53 m).

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