> As a native English speaker, would you say that a cul de sac is a passageway? Is it a passage if all you can do is return to the starting point?

Yes, "tunnel" can be used for features like that.  No one would think that https://en.wikipedia.org/wiki/Williamson_Tunnels are not tunnels because there is no exit at one end for some of them.

Rather than "writing a clarification proposal" I'd again suggest (as I mentioned before) just discussing it with other users on the forum.

I have to say, if this is a sewer / drain that drains into the stream to the north, surely there's some way for water (or, er, "other fluids") to get into it in the first place?  Presumably some sort of pipework?
