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A half tide dock is a partially tidal dock. Typically the dock is entered at high tide. As the tide ebbs a sill or weir 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 tables 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, or where the dock had disappeared entirely, such as Old Dock Sill in Liverpool. [1]
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, Padstow, Douglas, and Peel (the latter two in the Isle of Man).
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and are also caused by the Earth and Moon orbiting one another.
The River Weaver is a river, navigable in its lower reaches, running in a curving route anti-clockwise across west Cheshire, northern England. Improvements to the river to make it navigable were authorised in 1720 and the work, which included eleven locks, was completed in 1732. An unusual clause in the enabling Act of Parliament stipulated that profits should be given to the County of Cheshire for the improvement of roads and bridges, but the navigation was not initially profitable, and it was 1775 before the first payments were made. Trade continued to rise, and by 1845, over £500,000 had been given to the county.
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. In British English, the term is not used the same way as in American English, it is used to mean the area of water that is next to or around a wharf or quay. The exact meaning varies among different variants of the English language.
HMS Conway was a naval training school or "school ship", founded in 1859 and housed for most of her life aboard a 19th-century wooden ship of the line. The ship was originally stationed on the Mersey near Liverpool, then moved to the Menai Strait during World War II. While being towed back to Birkenhead for a refit in 1953, she ran aground and was wrecked, and later burned. The school moved to purpose-built premises on Anglesey where it continued for another twenty years.
Bow Creek is a 2.25-mile (3.6 km) long tidal estuary of the English River Lea and is part of the Bow Back Rivers. Below Bow Locks the creek forms the boundary between the London Boroughs of Newham and Tower Hamlets, in East London.
Richmond Lock and Footbridge is a lock, rising and falling low-tide barrage integrating controlled sluices and pair of pedestrian bridges on the River Thames in southwest London, England, and is a Grade II* listed structure. It is the furthest downstream of the forty-five Thames locks and the only one owned and operated by the Port of London Authority. It was opened in 1894 and is north-west of the centre of Richmond in a semi-urban part of southwest London. Downstream are Syon Park and Kew Gardens on opposite banks. It connects the promenade at Richmond with the neighbouring district of St. Margarets on the west bank during the day and is closed at night to pedestrians – after 19:30 GMT or after 21:30 when BST is in use. At high tide the sluice gates are raised and partly hidden behind metal arches forming twin footbridges.
The Old Dock, originally known as Thomas Steers' dock, was the world's first commercial wet dock. The 3+1⁄2 acres (1.4 ha) dock was built on the River Mersey in Liverpool, England, starting in 1710 and completed in 1716. A natural tidal pool off the River Mersey, which probably gave its name to Liverpool centuries earlier, was partially filled and locked in from the river with quay walls erected.
An ordnance datum (OD) is a vertical datum used by an ordnance survey as the basis for deriving altitudes on maps. A spot height may be expressed as above ordnance datum (AOD). Usually mean sea level (MSL) at a particular place is used for the datum.
Port of Kolkata or Kolkata Port, officially known as Syama Prasad Mookerjee Port, is the only riverine major port in India, in the city of Kolkata, West Bengal, around 203 kilometres (126 mi) from the sea. It is the oldest operating port in India and was constructed by the British East India Company. Kolkata is a freshwater port with no variation in salinity. The port has two distinct dock systems – Kolkata Dock at Kolkata and a deep water dock at Haldia Dock Complex, Haldia.
A linkspan or link-span is a type of drawbridge used mainly in the operation of moving vehicles on and off a roll-on/roll-off (RO-RO) vessel or ferry, particularly to allow for tidal changes in water level.
The 1928 Thames flood was a disastrous flood of the River Thames that affected much of riverside London on 7 January 1928, as well as places further downriver. Fourteen people died and thousands were made homeless when floodwaters poured over the top of the Thames Embankment and part of the Chelsea Embankment collapsed. It was the last major flood to affect central London, and, along with the disastrous North Sea flood of 1953, helped lead to the implementation of new flood control measures that culminated in the construction of the Thames Barrier in the 1970s.
The Haringvlietdam, incorporating the Haringvliet sluices, are hydraulic engineering structures which closed off the estuary of the Haringvliet, Netherlands, as part of the Delta Works. The structure consists of 17 sluices, several kilometres of dam and a shipping lock, and formed the sixth project of the Delta Works.
An acqua alta is an exceptional tide peak that occurs periodically in the northern Adriatic Sea. The term is applied to such tides in the Italian region of Veneto. The peaks reach their maximum in the Venetian Lagoon, where they cause partial flooding of Venice and Chioggia; flooding also occurs elsewhere around the northern Adriatic, for instance at Grado and Trieste, but much less often and to a lesser degree.
Estuarine water circulation is controlled by the inflow of rivers, the tides, rainfall and evaporation, the wind, and other oceanic events such as an upwelling, an eddy, and storms. Estuarine water circulation patterns are influenced by vertical mixing and stratification, and can affect residence time and exposure time.
A tidal barrage is a dam-like structure used to capture the energy from masses of water moving in and out of a bay or river due to tidal forces.
A floating dock, floating harbour or wet dock is a dock alongside a tidal waterway which maintains a 'constant' level, despite the changing tides.
A branch dock is a dock that forms part of a large harbour system of interlinked docks.
A caisson is a form of lock gate. It consists of a large floating iron or steel box. This can be flooded to seat the caisson in the opening of the dock to close it, or pumped dry to float it and allow it to be towed clear of the dock.
Tidal flooding, also known as sunny day flooding or nuisance flooding, is the temporary inundation of low-lying areas, especially streets, during exceptionally high tide events, such as at full and new moons. The highest tides of the year may be known as the king tide, with the month varying by location. These kinds of floods tend not to be a high risk to property or human safety, but further stress coastal infrastructure in low lying areas.
In coastal and environmental engineering, the closure of tidal inlets entails the deliberate prevention of the entry of seawater into inland areas through the use of fill material and the construction of barriers. The aim of such closures is usually to safeguard inland regions from flooding, thereby protecting ecological integrity and reducing potential harm to human settlements and agricultural areas.