Tide mill

A tide mill is a water mill driven by tidal rise and fall. A dam with a sluice is created across a suitable tidal inlet, or a section of river estuary is made into a reservoir. As the tide comes in, it enters the mill pond through a one way gate, and this gate closes automatically when the tide begins to fall. When the tide is low enough, the stored water can be released to turn a water wheel.

Tide mills are usually situated in river estuaries, away from the effects of waves but close enough to the sea to have a reasonable tidal range. These mills have existed since the Middle Ages, and some may go back to the Roman period.

A modern version of a tide mill is the electricity generating tidal barrage.

Possibly the earliest tide mill was located in London on the River Fleet, dating back to Roman times.

In recent years, a number of new archaeological finds has consecutively pushed back the date of the earliest tide mills, all of which were discovered on the Irish coast: A 6th century vertical-wheeled tide mill was located at Killoteran near Waterford. A twin flume horizontal-wheeled tide mill dating to c. 630 was excavated on Little Island. Alongside it, another tide mill was found which was powered by a vertical undershot wheel. The Nendrum Monastery mill from 787 was situated on an island in Strangford Lough in Northern Ireland. Its millstones are 830mm in diameter and the horizontal wheel is estimated to have developed 7/8HP at its peak. Remains of an earlier mill dated at 619 were also found at the site.

The earliest recorded tide mill in England was in Dover harbour, mentioned in the Domesday Book (1086). The book also mentioned the mills on the River Lea at Three Mills Island, now in London's docklands (see House Mill). By the 18th Century these had become the largest in England. There were about 76 tide mills in London, including two on London Bridge.

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