Explanation:

In a free nappe, the pressure below the nappe is atmospheric. This can be explained using the concept of hydrostatic pressure and the behavior of fluids.

Hydrostatic Pressure:
Hydrostatic pressure is the pressure exerted by a fluid at a certain depth. It depends on the density of the fluid, the acceleration due to gravity, and the depth from the surface.

Free Nappe:
A free nappe refers to the flow of water over an open channel or a weir without any obstructions or control structures. In this case, the water flows freely and is not confined.

Pressure Distribution in a Free Nappe:
When water flows freely over a nappe, the pressure decreases with increasing elevation. This is because the velocity of the water increases as it flows down the nappe, resulting in a decrease in pressure according to Bernoulli's equation.

Pressure Below the Nappe:
The pressure below the nappe is atmospheric. This is because the water is not confined and is exposed to the atmosphere. At any point below the nappe, the pressure is equal to the atmospheric pressure. This can be explained by considering the hydrostatic pressure at that point, which is given by the equation:

Pressure = Atmospheric Pressure + (Density of Water * Acceleration due to Gravity * Depth)

Since the depth below the nappe is zero, the pressure below the nappe is equal to the atmospheric pressure.

Pressure Above the Nappe:
The pressure above the nappe is also atmospheric. Since the water is exposed to the atmosphere, the pressure at any point above the nappe is equal to the atmospheric pressure. This is because the water is not confined and is in direct contact with the atmosphere.

Conclusion:
In a free nappe, both the pressure below and above the nappe are atmospheric. This is because the water is not confined and is exposed to the atmosphere. The pressure distribution is determined by the hydrostatic pressure at each point, which decreases with increasing elevation.