Open Channel Flow MCQ with answer and solution

Concept

The total energy line (TEL) lies above the hydraulic grade line (HGL) by the velocity head.

Formula

TEL − HGL = V² / (2g)

Step-by-step solution

1. Total head = z + y + V²/(2g) 2. HGL (the water surface in an open channel) = z + y 3. Difference = TEL − HGL = V²/(2g) = velocity head

Explanation

The total head line includes the velocity head. The hydraulic grade line includes only the elevation head and the pressure (depth) head. So the gap between the two lines is the velocity head, V²/(2g).

Why the other options are wrong

B: elevation head is measured from the datum to the channel bed. C: pressure head is the flow depth (bed to HGL). D: total energy head is the full height of the TEL above the datum, not the gap.

Shortcut

TEL − HGL = velocity head, always (in pipes and in open channels).

Common mistake

Confusing the gap between TEL and HGL with pressure head. In open channels, the depth is the pressure head.