Ran into a couple of culverts today that had these same characteristics. Looking at the reference photos in the protocol, it did not appear to fit as an open bottom arch. However, there were no defined abutments for parameter "C" as shown in the diagram.
Wondering which it should be classified under if we run into more similar
I would ignore the slight curve to the underside of this bridge, and classify it as a Bridge with Abutments (despite it not having perfectly obvious abutments). The curve there is slight enough that it can be ignored (compared to the curve of the arch-bridges in the protocol examples) and so measurement C could be skipped.
Once we get into sharper and more drastically curved bridges (on the underside), then it would be appropriate to call them an arch-bridge and collect measurement C, which for that structure is the wall rise.