Open-channel flow transition: The phenomenon in an open channel where a high-velocity (supercritical) stream abruptly converts to a low-velocity (subcritical) stream with a sudden rise in water surface is called what?

Introduction / Context:

A hydraulic jump is a classic rapidly varied open-channel flow transition used to dissipate energy downstream of spillways, sluices, and culverts. It occurs when supercritical flow (high velocity, low depth, Fr > 1) is forced to transition to subcritical flow (lower velocity, greater depth, Fr < 1), producing turbulence and a visible rise in the water surface.

Given Data / Assumptions:

• Open-channel flow under gravity.
• Upstream flow is supercritical; downstream control enforces subcritical flow.
• Idealized horizontal rectangular channel considered for simple explanation (general ideas still hold otherwise).

Concept / Approach:

The jump conserves mass and (approximately) momentum across the roller; energy is not conserved because substantial energy is dissipated by turbulence and air entrainment. Conjugate (sequent) depths y1 and y2 are related by momentum principles; the energy loss ΔE is a function of Froude number Fr1 of the incoming stream.

Step-by-Step Solution:

Verification / Alternative check:

Observation of roller, intense turbulence, and surface boil confirms a hydraulic jump. In contrast, “water hammer” is a pressure transient in closed conduits due to rapid valve action, not an open-channel phenomenon.

Why Other Options Are Wrong:

• Critical discharge refers to the condition at Fr = 1, not an abrupt surface rise with strong turbulence.
• Water hammer happens in pressurized pipes, not open channels.
• None of these is incorrect because “hydraulic jump” is the established term.

Common Pitfalls:

• Confusing sequent depth relationship (momentum-based) with gradually varied flow profiles (energy-based).

Final Answer:

Hydraulic jump