They've been doing work recently to Canning Dock next to Mann Island, think they were doing something with the gates. Anyway, I walk past almost daily and a couple of times it's been virtually empty as they opened the gates to the river at low tide and the water drained out. They just let the mersey fill it back up again afterwards.
You could do the same at BMD but they've chosen a different option and there's be sound engineering reasons for that no doubt. Jesse Hartley really was a genius.
Edit: found a picture
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Great pic that. Don't know why, I always thought they would be well deeper than that.