A while ago I posted a photo of a bridge we went through that had a little wooden door at the side. Someone kindly left a comment saying the doors in bridges are for storing "stop planks". We've seen many of them in little stands besides locks so it was interesting to see how they can be stored at bridges.
For non-boaters, stop planks are used to block off a canal if the water needs to be drained for maintenance work, or if there is a breach in the canal and they need to stop all the water running out. We've also seen them used for dry docks - where they pull the boat into the work area and then use stop planks to enable the dry dock to be drained.
The planks are dropped down narrow grooves in the stonework at the side of the canal. I tried to get a couple of photos of these but didn't encounter very good examples while I had camera in hand.