I would say a description of a "tunnel" is reasonable enough. And what the Artists impression shows is much preferable to a full-blown enclosed tunnel as far as I am concerned as well. If this is also a cheaper solution, looks like a win-win to me.
I think having "debris flow shelter" would be meaningless as a headline, but "Tunnel" is a descriptive term that makes sense.
Out of curiosity, I looked up the definition of a "Tunnel". According to Mirriam-Webster, which is one respected source, it can be "a covered passageway" - so that can be legitimately referred to as a "Tunnel".