This is a very good question that I also wondered about.That's really cool. When you say it's a culvert do you mean a previously open stream was redirected into a tunnel through rock? I'm wondering why, that seems like a lot of work. It kind of looks like a mine. Are there remains of door / sluice gate in pics 6-7?
To be fair I'm not quite sure of the history of why this stream was culverted as it just runs below what is a field above. Both the infall and the outfall sections are brick and stone built but the section in the middle is rock. It must have been a lot of work to have done this but I can't work out where the stream would have flowed if this culvert had not been built. Naturally the watercourse runs in the lowest lying land but without this culvert, a lot of this land would have probably been under water so perhaps it was built to provide more land for agriculture.
Yes, at some point in time there appears to have been a sluice gate / penstock in the rock section which must have been operated from a form of control in the field above. Definitely one of the more unusual culverts!