The ratio of the hydraulic radius of a pipe running full to the hydraulic radius of a square section of a channel whose side is equal to the diameter of the pipe, is

.Let, circle of dia = d

So, the hydraulic radius of the circle,

For the square of the side, d,

Ratio :3/4

When the vertical plane surface is immersed deeper and deeper below the liquid surface, the centre of pressure becomes closer and closer to the centroid of the surface.