It was a brick-lined Victorian storm outfall. Somewhere on this thread is a photo of the canal work, taken around October last year which shows the exposed tunnel. Unfortunately by the time I saw the photo and went down to have a look, it had been capped off.
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I think the drain was made redundant by more modern storm drains further along the dock. It was only short anyway, starting at an overflow chamber about 30m further inland, and exiting out of the dockside into the river. The purpose was so that if the combined sewers got overloaded, they'd overflow into the river rather than into the streets.
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