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Caisson (lock gate)
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.
== Chevron gates ==
Most locks are closed by chevron or mitre gates rather than caissons. These are pairs of hinged gates that form a V shape, with the deeper water outside the V. Water pressure thus holds them closed. These gates can be opened and closed quickly, so they are used for canal locks, to change levels, and also for most freight docks. As the function of a freight dock is to ''enclose'' deep water, such gates point inwards. A graving or dry dock, in contrast, ''excludes'' water, and so their gates point outwards.
Hinged gates are relatively complicated, and so expensive, to construct. Large gates require powered machinery to operate them, machinery that must be provided for each set of gates.
Chevron gates can also only resist deep water on one side of the gate, which may be a drawback in some tidal areas where a high tide outside can exceed the depth inside the dock. Where such tides were encountered, sometimes a pair of opposed gates was used, opening outwards away from each other. Provided the water depth between them was kept low, these could resist high water from either direction. A pair of such gates was provided at Penarth Dock, owing to the exceptionally high tidal range of the Bristol Channel beyond.〔See TURNER(1882) Cardiff Harbour and Docks for a map of Penarth Dock, showing these gates.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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