Editing Leat (section)

==Design and functions==

===Water mills===
Leats generally start some distance (a few hundred metres/yards, or perhaps several miles/kilometres) 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]] in the source stream 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, together with the quality of the wetted surface of the leat, determines the [[Volumetric flow rate|flow rate]]. 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 [[hydraulic head|head]] of water &ndash; several metres (perhaps 5 to 15 feet) for a watermill, or a metre or less (perhaps one to four feet) for the controlled irrigation of a [[water-meadow]].{{Citation needed|date=May 2024}}

===Water supply===
Leats are used to increase the yield of a reservoir by trapping streams in nearby catchments by means of a contour leat. This captures part or all of the stream flow and transports it along the contour to the reservoir. Such leats are common around reservoirs in the uplands of Wales.{{Citation needed|date=May 2024}}

===Mining===
[[File:Dolaucothimap2.jpg|thumb|upright=1.2|Map of the Roman gold mine]]
[[File:Aqueduct1.jpg|thumb|left|upright=1.2|The aqueducts at Dolaucothi]]
Leats were built to work [[lead]], [[tin]] and [[silver]] ores in mining areas of Wales, [[Cornwall]], [[Devon]], the [[Pennines]] and the [[Leadhills]]/[[Wanlockhead]] area of Southern Scotland from the 17th century onwards. They were used to supply water for [[hushing]] mineral deposits, washing ore and powering mills.{{Citation needed|date=May 2024}}