Castle Park, where the barges were moored until recently was flattened during World War II. The rubble was exported to the USA, and was used as landfill to create Bristol Basin, New York.
There's a transatlantic trade in rubble? I've certainly seen rubble from demolished historic structures wind up in unusual places, but it almost always nearby
During the war, ships were arriving from the US packed with munitions. They needed something to take back, just for ballast, so they used rubble from the destroyed city.
Well, this was an incredibly thorough deep dive. I worked along Bristol Harbor for 10 years and never consciously noticed these barges. Lots of interesting tech companies have come and gone along that harborside.