What happened before? Have boats built to similar dimensions passed through this bridge unimpeded? And what are the engineering tolerances on the dimensions?
Based on the width reported by the owner - 4m - and the Fraenkel Report* quoted dimensions from Napton to the top of Bordesley - 3.8m - we would appear to have a negative proximity coefficient here. Now the Fraenkel Report dimensions have to be treated with a degree of circumspection, from memory it gives Hillmorton as maximum length 70ft when 71'6" GUCCCo boats had been using the locks for years, but they were presumably based on some sort of survey of pinch points.
I'll say it again: The structure of the bridge you can see might be wider than the boat but below water level it's not that the channel isn't maintained, it's the structure of the bridge itself which is narrow because of the batter on the retaining walls. (I've just had a look at some notes on civil engineering structures, 1:8 is typical for brick retaining walls)
We were sat down at teatime yesterday with someone who was on the boats when the cement traffic was running from Southam to Sampon Road and asked him if they ever ran through this bridge breasted up,"no it's too narrow" was the reply which would suggest the bridge has been less than 14ft wide since at least the early 60s.
*Someone fished it out, also at teatime yesterday - we drink a lot of tea.