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.

Most locks are closed by chevron or mitre gates rather than by 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.

The first caissons to be used to close docks in this way were 'ship caissons'. These are a floating hull which resembles that of a particularly tall boat.

This 'bateau-porte' is an seventeenth century French invention. In 1683, Peter Arnold constructed the first for the arsenal dockyard at Rochefort. The first ship caissons were constructed of wood, by traditional boatbuilding methods, but were later of wrought iron plates and later of steel.

Inside the caisson are ballast spaces, filled with water for stability. An upper space is sealed from the rest and this may contain either water, to sink the caisson firmly into its socket in the dock, or else pumped dry and allowing it to float free.

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Wikipedia