Suppose that the are of the plates is 6.35 m sq and height is negligible. Both the plates could be made to slide. In the initial case the whole apparatus was empty. Door 2 was in closed position and door 1 was open such that when the water from the top of the tunnel was poured no water could leak from the door 2 side. After filling the apparatus the top was sealed and the door 1 was closed. After that door 2 was opened.
Pressure at the top surface of the door 1 is indicated as green and that to the bottom as red.
So the pressure green will be :
Pressure= Density of water X Acceleration due to gravity X height
= 1000 X 10 X 15 = 150000 N/m sq
But what height should be considered while finding the pressure red?