I was hoping someone could help with the following force/pressure problem.

I am trying to find the force due to water pressure acting on the green door under the tube as shown below:-

I work out the force as:-

Force = Density of fluid * gravity * Area * Height below water surface

Density of fluid = 1000

Gravity = 9.8

Area * Height below water surface = 0.2827

Height below water surface = 20

Force = 1000 * 9.8 * 0.2827 * 20 = 55409.2 Newtons

To be honest i am not sure if i should be taking the height below the water surface as 20 or 20 + 11.2 i.e. 31.2m.

I was hoping someone could help shed some light.

Thank you.