Bridge geometry is entered as deck/roadway, piers, and sloping abutments (as applicable). Deck data are entered as station and elevation to provide the opportunity to define any shape. You’ll need to define the entirety of the high and low chord of the bridge deck, including the arches, using station and elevation data. Station = distance along the bridge.
With long culverts, we have modeled the capacity of the culvert separately using a software like StormCAD and then modeled the overland flow path using HEC-RAS with the flow contained the culvert removed from the HEC-RAS reach.
Adding that I don’t have any interpolated cross sections in this geometry, which is the only known issue causing an unhandled exception listed in the 6.1 Release Notes.