To determine the water surface profile for subcritical flow, where must flow calculations start?

For determining the water surface profile in subcritical flow conditions, flow calculations should start from downstream. This approach is necessary because subcritical flow is characterized by a relatively low flow velocity compared to gravitational forces, and the flow conditions downstream influence the water surface elevation upstream.

In subcritical flow, the Froude number is less than one, indicating that gravitational effects dominate the flow behavior, resulting in a gradual adjustment of water surface profiles. Beginning the calculations at the downstream end allows for the establishment of boundary conditions and the influence of hydraulic structures or other factors on the water surface profile as it moves upstream. This method ensures that all potential influences on the water surface elevation are accounted for as one moves upstream, leading to an accurate calculation of the flow profile.

Starting from upstream or from midway would not provide the necessary conditions to correctly evaluate the flow profile, as these approaches could overlook important downstream boundary conditions essential for accurately modeling subcritical flow behavior. Therefore, beginning from the downstream end is the most effective approach for this type of analysis.