Wait. You first said that "Troyano's book states that two arches at the Alconetar site are original (spans 6m and 7m)", but now you write that "his text and captions imply that the arch on the left is an original Roman arch and the other two are more recent". In that case, the bridge would not be a Roman segmental arch bridge at all, wouldn't it? 

 Did Troyano somewhere write that the wooden low span/rise ratio bridges of the Romans also constitute segmental arches (despite their superstructure being out of wood, not stone)?