Depressed Nappe Pressure Analysis

A 'depressed nappe' refers to a sheet of liquid (like water flowing over a weir or spillway) that curves downwards significantly below its theoretical trajectory.

This downward curvature occurs when the air pressure beneath the nappe is lower than the external atmospheric pressure. The higher atmospheric pressure acting on the top and sides pushes the nappe downwards more than the lower pressure beneath it pushes it upwards, causing the depression.

Therefore, in the case of a depressed nappe, the pressure of the air below the nappe is less than atmospheric.

Key Points:

• Depressed Nappe: Downward curvature of the liquid sheet.
• Cause: Pressure difference between the exterior and the space below the nappe.
• Result: Lower air pressure below the nappe compared to the atmosphere.

This condition is often associated with insufficient ventilation under the nappe.