One-dimensional analysis of an existing bridge involves the same concepts employed for designing a new bridge: assume that the flood flow will distribute itself to attain a constant energy grade at the approach section. The existing bridge will likely redistribute flow from what the approach channel conditions might otherwise imply. The stagnation points become functions of the bridge openings and the channel conditions. Until the computed energy levels at the approach section are approximately equal, you need considerable trial and error may be needed to adjust stagnation points, determine conveyance apportionment, and analyze each opening.