A half tide dock is a partially tidal

dock The word dock () in American English refers to one or a group of human-made structures that are involved in the handling of boats or ships (usually on or near a shore). In British English, the term is not used the same way as in American Engl ...

. Typically the dock is entered at high tide. As the tide ebbs a sill or

weir A weir or low-head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the water level. Weirs are also used to control the flow of water for outlets of l ...

prevents the level dropping below a certain point, meaning that the ships in the dock remain afloat, although they still rise and fall with the tides above this. Half tide docks are particularly useful in areas with a large tidal range. The sill of a half tide dock must be set sufficiently far below the daily high tide mark to allow ships to pass over it. Obviously this was easier to achieve with small ships of shallow draught, or in areas with a large tidal range. Inside the dock, the depth of water beneath the sill's level depends only on the depth to which the dock was excavated, although this obviously increases construction costs. The importance of the sill, and the tide's height above it, is reflected by these dock sills becoming an important local datum level and for

tide table Tide tables, sometimes called tide charts, are used for tidal prediction and show the daily times and levels of high and low tides, usually for a particular location. Tide heights at intermediate times (between high and low water) can be approxi ...

s being calculated for heights above it (i.e. clear draught in and out of the dock). This importance has often continued for many years after the original sill had been replaced, as at Sharpness on the

Severn Estuary The Severn Estuary () is the estuary of the River Severn, flowing into the Bristol Channel between South West England (from North Somerset, Bristol and South Gloucestershire) and South Wales (from Cardiff, Newport to Monmouthshire). Its very h ...

, or where the dock had disappeared entirely, such as Old Dock Sill in

Liverpool Liverpool is a port City status in the United Kingdom, city and metropolitan borough in Merseyside, England. It is situated on the eastern side of the River Mersey, Mersey Estuary, near the Irish Sea, north-west of London. With a population ...

Princes Half-Tide Dock entrance interior

The

River Mersey The River Mersey () is a major river in North West England. Its name derives from Old English and means "boundary river", possibly referring to its having been a border between the ancient kingdoms of Mercia and Northumbria. For centuries it h ...

has a large tidal range, and many of the docks in Liverpool and

Birkenhead Birkenhead () is a town in the Metropolitan Borough of Wirral, Merseyside, England. The town is on the Wirral Peninsula, along the west bank of the River Mersey, opposite Liverpool. It lies within the Historic counties of England, historic co ...

are half-tide docks. One of these, Canning Dock, is undergoing significant development of its half-tide locks. ITV repor

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A "gate-flap" half-tide dock

A modern refinement of the half-tide dock uses a "gate-flap", which replaces a fixed sill with a hinged flap that opens as the tide rises, and closes as the tide ebbs. When open, the gate-flap rests horizontally at the seabed level. Operation is usually hydraulic, with automatic control; and warning lights or alarms will indicate when the gate-flap is about to be lowered or raised. The advantage over the older fixed sill is that, when the gate-flap is down, there is greater draught available to boats as there is no sill providing an obstacle. Such gate-flap systems are in place in

Swansea Swansea ( ; ) is a coastal City status in the United Kingdom, city and the List of urban areas in the United Kingdom, second-largest city of Wales. It forms a Principal areas of Wales, principal area, officially known as the City and County of ...

Padstow Padstow (; ) is a town, civil parishes in England, civil parish and fishing port on the north coast of Cornwall, England, United Kingdom. The town is situated on the west bank of the River Camel estuary, approximately northwest of Wadebridge, ...

, Douglas, and

Peel Peel or Peeling may refer to: Places Australia * Peel (Western Australia) * Peel, New South Wales * Peel River (New South Wales) Canada * Peel Parish, New Brunswick * Peel, New Brunswick, an unincorporated community in Peel Parish * Pee ...

(the latter two in the

Isle of Man The Isle of Man ( , also ), or Mann ( ), is a self-governing British Crown Dependency in the Irish Sea, between Great Britain and Ireland. As head of state, Charles III holds the title Lord of Mann and is represented by a Lieutenant Govern ...

). File:Peel - geograph.org.uk - 24245.jpg, Peel inner harbour at high water (the gate-flap is at the extreme left).

References

{{Reflist Coastal construction Docks (maritime)

A "gate-flap" half-tide dock

See also

References