Wetted Perimeter and Hydraulic Depth

Wetted Perimeter

In Fluid dynamics, wetted perimeter refers to the perimeter of the cross sectional area of the pipe in contact with the medium.  If the pipe is circular, the wetted perimeter will be 

If the pipe has a square cross-section, the wetted perimeter will be 4a.

Hydraulic depth

Hydraulic depth refers to the length through which the liquid flows through the pipe.

Hydraulic depth = Area of the flow / wetted perimeter.