1) A channel cut to convey surplus water away from a reservoir or aqueduct, designed to prevent flooding or overflowing (OED).

1861 The byewash, which was a circular chimney about four yards in diameter, was on the south or right-hand side of the reservoir ... and communicated with a tunnel emptying itself on the lower side of the embankment, Holmfirth.

places Holmfirth
dates 1861

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