Question

The hydraulic jump occurs in a channel when

• A. Depth of flow changes from sub critical to super critical
• B. Depth of flow changes from super critical to subcritical
• C. There is change of slope
• D. The flow occurs in an adverse channel

Hint:

Remember:
- Supercritical to Subcritical = Hydraulic Jump (rapid, turbulent, dissipates energy).
- Subcritical to Supercritical = Hydraulic Drop (smooth draw-down, accelerates flow).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
A hydraulic jump is a phenomenon in open channel flow characterized by a rapid, turbulent rise in the water surface.
It occurs when a high-velocity stream transitions to a low-velocity stream, converting kinetic energy into potential energy and turbulence.

Step 2: Detailed Explanation:
The state of open channel flow is defined by the Froude number (\(Fr\)):
- Supercritical flow (\(Fr > 1\)): High velocity, shallow depth (shooting flow).
- Subcritical flow (\(Fr < 1\)): Low velocity, deep water (tranquil flow).
When supercritical flow meets an obstruction, a change in channel slope, or a high tailwater condition:
- The flow is forced to slow down and transition to a subcritical state.
- This sudden transition cannot happen gradually; instead, the water depth increases abruptly, creating a highly turbulent zone known as a hydraulic jump.
- During this jump, substantial kinetic energy is dissipated as heat and sound.
Therefore, a hydraulic jump occurs when the depth of flow changes from supercritical (below critical depth) to subcritical (above critical depth).

Step 3: Final Answer:
A hydraulic jump occurs when the depth of flow changes from supercritical to subcritical.