Floating dock (impounded)

[2] Early docks were of simple construction: a single lock gate isolating them from the tidal water.

They need have no gate, but as the tide ebbs a raised sill or weir on the floor of the dock prevents the level dropping below a certain point, meaning that the ships in the dock remain afloat, although they still fall with the first ebb of the tide.

Half tide docks were only useful for ships of shallow draught, in areas with a large tidal range.

This allowed the dock's water level to be maintained and, more importantly, it increased the time for which tidal access was possible.

[4] This involved the diversion of the River Avon (Bristol) away from its previous route through the harbour and into a new channel at the New Cut.

At Bristol, Jessop controlled the height of the harbour water by a broad weir, built as a dam across the previous route of the river.

A small but significant example of this is the locking system on the River Darent in Dartford, now in restoration and which were fully operational between 1899 and 1983.

In practical use, the operation of Bow Locks is tightly controlled by the duty lock-keeper as every tide is different.