Question

When the water flows over a rectangular suppressed weir, the pressure beneath the nappe is

• A. Very high
• B. Slightly above atmospheric
• C. Atmospheric
• D. Negative

Hint:

For open channel flow structures: - Suppressed Weir (No ventilation) \(\implies\) Trapped air is removed \(\implies\) Negative pressure under the nappe. - Contracted Weir \(\implies\) Air freely enters from sides \(\implies\) Atmospheric pressure under the nappe.

Answer 1

The Correct Option is D

Concept: A weir is an overflow structure built across an open channel to measure or control fluid flow. The sheet of water or liquid discharging over the crest of a weir is known as the "nappe". In a suppressed rectangular weir, the length of the weir crest matches the full width of the channel. Because of this structural alignment, there are no side contractions, and the water sheet (nappe) touches the channel side walls entirely during its discharge. As a result, air from the surrounding atmosphere cannot naturally enter beneath the underside of the cascading sheet of water.

Step 1: Understanding the phenomenon of air entrainment and trapment.
As water pours over the crest of a suppressed weir, it traps a specific volume of air directly beneath the underside of the nappe, bounded securely by the vertical weir face, the side walls of the channel, and the moving stream of water itself. As the water continues to flow downstream rapidly, it continuously drags and carries away the trapped air molecules beneath it through a process known as air entrainment.

Step 2: Analyzing the resulting pressure change.
Because the channel sides completely block access to the open atmosphere, the air carried away by the water cannot be replenished. This continuous removal of air creates a partial vacuum or low-pressure zone directly underneath the falling sheet of water. Consequently, the pressure drop beneath the nappe falls well below the local ambient atmospheric pressure, resulting in a negative pressure (or suction pressure) condition: \[ P_{\text{beneath nappe}} \lt P_{\text{atmospheric}} \implies P_{\text{gauge}} \lt 0 \text{ (Negative)} \]

Step 3: Consequences of unventilated negative pressure.
This negative pressure exerts a downward suction pull on the nappe, drawing it backward toward the face of the weir wall. This alters the discharge characteristics and leads to inaccurate flow measurements. To avoid this error, real-world systems use ventilation pipes to maintain atmospheric pressure beneath the nappe. However, in an unventilated suppressed weir, the pressure remains negative.