Bridge Sizing and Energy Grade Levels
When you have estimated relative approach discharges, you should have two, often contradictory objectives:
- Try to size the multiple structures so that they offer approximately the same relative carrying capacities as the relative flow distribution would indicate.
- To minimize cross flow, you need to obtain similar values of energy grade level at the approach section for all openings. Generally, if the relative velocity differentials are not approximately the same for all openings, head differentials develop, causing a redistribution of the approach flows.
Often, it is not possible to balance energy grade levels and conveyances simultaneously. Therefore, because of the importance of avoiding a redistribution of flow from natural conditions, place more emphasis on balancing energy grade levels by having velocity head differentials approximately the same for each of the openings.
Size the bridges in a multiple opening situation to avoid exceeding maximum allowable through-bridge velocities at any of the openings. Calculate backwater head for a multiple opening situation in the same manner as for single opening structures outlined in the
subsection and based on the appropriate floodplain subsection and flow apportionment. That is, consider each bridge separately using the flow apportionment and associated portion of cross section.