The hydraulic mean depth of a liquid flowing through a pipe is equal to the sectional area of liquid divided by the wetted perimeter. The sectional area of liquid is equal to The wetted perimeter equals ; hydraulic mean depth = sectional area= wetted perimeter .

Knowing the diameter of the pipe and the depth of the liquid, the angleӨ may he found from the equation tan where y equals The hydraulic mean depth for pipes running full or half full is a/4.

Diagram of Hydraulic Mean Depth.