Caisson (lock gate)

These gates can be opened and closed quickly, so they are used for canal locks, to change levels, and also for most freight docks.

[1] A pair of such gates was provided at Penarth Dock, owing to the exceptionally high tidal range of the Bristol Channel beyond.

The first ship caissons were constructed of wood, by traditional boatbuilding methods, but were later of wrought iron plates and later of steel.

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.

To seal the gate the enlarged 'keel' of the ship caisson fits closely into a groove in the stonework of the lock opening.

To make use of the pressure of water acting on the caisson, the seal is not placed on the base of the keel, but on the dry side.

As the narrowness and high tides of the River Wye limited the size of the ships that could be built there, they came to specialise in caisson work and supplied most of the docks along the Bristol Channel and beyond.

When closing the gate, the track guides it automatically back into place, avoiding the slow manoeuvering with tugs or winches necessary to align a ship caisson.

Electrically-operated sliding caissons, installed around 1900, on the entrance of the Zeebrugge Canal to the North Sea could operate within two minutes.

[iii] Busy gates, such as for a canal lock, may have dock sides high enough that the caisson can run beneath them.

[8] A further advantage of the floating caisson is that, unlike the hinged gate, it does not need space within the dock to allow it to swing.

[3] Floating caissons were sometimes used in places where a high tidal range made conventional chevron gates impractical.