Venturi Flume — Pressure Regime During Flow In a Venturi flume operating correctly in an open channel, the water through the throat flows at which pressure condition?

Introduction:
A Venturi/Parshall flume accelerates open-channel flow through a contraction to control discharge without creating enclosed pressurization. Knowing the pressure regime clarifies why head measurements are taken relative to the free surface and why coefficients are close to unity.

Given Data / Assumptions:

• Open channel, free surface present.
• Flume properly aerated and not submerged beyond recommended ratio.
• Gages referenced to the free surface elevations.

Concept / Approach:
Open-channel flows communicate with the atmosphere; thus the static pressure at the free surface is atmospheric. In a Venturi flume, even at the throat where velocity is highest, the flow is still at atmospheric pressure because the water surface remains open and aerated. The energy change manifests as velocity head variation and a depression of water depth, not as a sealed internal pressure drop.

Step-by-Step Solution:
Identify the boundary condition: free surface ⇒ p = p_atm.At the throat, depth reduces and velocity increases, but the free surface still enforces p = p_atm.Therefore the correct statement is ‘‘atmospheric pressure’’.

Verification / Alternative check:
Calibration equations use water depth (head) rather than enclosed pressure; submerged flow corrections account for tailwater effects but still reference atmospheric conditions.

Why Other Options Are Wrong:
‘‘Gauge’’ or ‘‘absolute’’ here are distractions; the operative fact is that static pressure equals atmospheric in open-channel sections.

Common Pitfalls:
Confusing closed-conduit Venturi meters (pressurized) with open-channel Venturi flumes.

Final Answer:
atmospheric pressure