Leat

A leat (also lete or leet) is the name, common in the south and west of England, for an artificial watercourse, or aqueduct, supplying water to a watermill or its mill pond. Leats may also deliver water for mineral washing and concentration, for irrigation, or to a dye or other industrial works.

According to the Oxford English Dictionary, "leat"' is cognate with "let" in the sense of "allow to pass through". Other names for the same thing include fleam (probably a leat supplying water to a mill that did not have a millpool). In parts of northern England, for example around Sheffield, the equivalent word is goit. In southern England, a leat used to supply water for water-meadow irrigation is often called a carrier, top carrier, or main.

[edit] Functions

Leats generally start some distance (a few hundred yards, or perhaps several miles) above the mill or other destination, where an offtake or sluice gate diverts a proportion of the water from a river or stream. A weir often serves to provide a reservoir of water adequate for diversion. The leat then runs along the edge or side of the valley, at a shallower slope than the main stream. The gradient determines the flow rate together with the quality of the wetted surface of the leat. The flow rate may be calculated using the Manning formula. By the time it arrives at the water mill the difference in levels between the leat and the main stream is great enough to provide a useful head of water – several metres (perhaps 5 to 15 feet) for a watermill, or a metre or less (perhaps one to four feet) for a water-meadow.

[edit] Mining

Map of the Roman gold mine

The aqueducts at Dolaucothi

In mining areas, leats were built to supply water for hushing mineral deposits, washing ore and powering mills. They were used extensively by the Romans, and can still be seen at many sites, such as the Dolaucothi gold mines. They used the aqueducts to prospect for ores by sluicing away the over burden of soil to reveal the bedrock in a method known as hushing. They could then attack the ore veins by fire-setting, quench with water from a tank above the workings, and remove the debris with waves of water, a method still used in hydraulic mining. The water supply could then be used for washing the ore after crushing using simple machines also driven by water.

The Romans also used them for supplying water the bath-houses or thermae and drive vertical water wheels.

Leats were built to work lead, tin and silver ores in Wales, Cornwall, Devon and the Pennines during the 17th century and later.

[edit] Dartmoor

• Drake's Leat, constructed in 1591 under the management of Sir Francis Drake as an agent of the Corporation of Plymouth to carry water from Dartmoor to Plymouth.

Devonport leat near Nun's cross farm

Devonport leat showing sluice gates

• Devonport Leat constructed in the late 18th century to carry water to the expanding naval dockyard at Devonport (now a part of Plymouth).
• Many leats on Dartmoor, mostly constructed to provide power for mining activities, although some were also sources of drinking water. The courses of many Dartmoor leats may still be followed ^[1][2]
• Many leats used for tin mining are marked on the 1/50000 and 1/25000 Ordnance Survey maps, such as the leat serving the now defunct Vitifer mine near the Warren House Inn.

[edit] References

[edit] See also

• Aqueducts
• Dolaucothi
• Flume
• Roman aqueducts
• Roman engineering
• Roman mining

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