What flow condition should the design of a bridge be based on if upstream flow is subcritical and expected to change under the bridge?

In the scenario where the upstream flow is subcritical and is anticipated to change under the bridge, the most appropriate flow condition for the design of the bridge is based on critical flow.

Critical flow is significant because it marks the transition point between subcritical and supercritical flow regimes. At critical flow, the flow velocity is at its maximum for a given specific energy, and it effectively serves as a threshold that signals a potential change in flow conditions.

Designing for critical flow is essential for ensuring that the bridge can accommodate fluctuations and transitions in flow type due to changes in the hydraulic geometry or downstream conditions. This consideration is particularly important because it allows the structure to withstand the maximum forces and stresses that might occur during situations when flow shifts from subcritical to supercritical, which can happen rapidly due to variations in flow rates and channel configurations.

In summary, accounting for critical flow in bridge design helps manage the dynamic nature of waterways, particularly at locations where flow conditions may fluctuate significantly.