[jinman]

Tom, when I looked at your first picture, I could just see water going over the culverts about 1' deep. Your single culvert picture looks like it had that in mind and was designed to let floods top it. I suspect that even two culverts will not take the flow from a flash flood. If there are developments in the area, all those lawns, streets, and concrete shed water. I'd start from zero like you are planning and build culverts for the 90% flow and the entire bridge to withstand being topped in a flood for the final 10%. I think you have to design the top so water won't erode from the top down as well as the sides of the culverts. If you use a single culvert, you only have two sides to deal with. If you stick with two culverts, you have that center section between the culverts to stabilize and that makes a much more complex structure in my opinion.