Design multiple structures so that each structure’s carrying capacity (or conveyance) is approximately the same as the predicted discharge approaching the structure. Poorly sized structures could result in a reapportionment of the approach discharges. Reapportionment of flow, in turn, may cause excessive backwaters, unacceptable along-embankment velocities, and excessive velocities through some structures.

In addition to striving for balance in proportion (discussed in the Carrying Capacity Guidelines subsection above), satisfy average through-bridge velocity requirements. Unfortunately, widely disparate through-bridge velocities cause uneven backwaters that will likely redistribute of flow, upsetting the originally designed balance of structure conveyances. The goal is to balance conveyances and simultaneously try to assure that the resulting energy grade levels at the approach cross section (Section 4) are about the same for each bridge in the multiple opening facility. (See for more information.)