The engineers re-opening the Forth and Clyde canal used various techniques to re-instate the canal where it was crossed by roads.
But where the canal crosses the busy A814 in Clydebank, none of these were possible so a "drop lock" was used.
This view looking north shows the canal only a few feet below the road level (in the distance, in line with the white van)
At each end there is a protective boom, shown closed which lifts up to allow boats to enter the lock. Also shown are the lock gates, their normal state is open allowing free flow of water.
When a boat enters (moving west in this case) the gates are closed at both ends of the lock (bottom left of picture), and the lock keeper (right of picture) activates pumps to empty the lock back into the canal. There is also a sluce to empty water into a local burn, depending on whether you want to conseve water or electricity.
The lock keeper has access to either side of the drop-lock by this walkway under the road.
From the walkway you can see the water under the road with insufficient clearance. You can also see another boom stopping the boats in the lock progressing under the bridge until the water level has reached minimum.
The lock has now been pumped out to half-way…
Still going down. The concrete bollard at the left of the picture has a steel rod on the lock side of it. This boatman in red has tied-up to the rod, and the boat remains securely moored as it drops. Also notice the sill is about to become visible under the lock gates.
Minimum water level, the boats are about to move forward under the bridge.
Before they move the protective boom is raised (here shown half way up) to allow them through.
The boats sail through under the A814, the whole process is reversed on the other side, with the lock being refilled from the canal.
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Photographs: James Gentles