Definitions for "Wetted Perimeter"
The length of the perimeter in contact with the water. For a circular pipe of inside diameter "d", flowing full, the wetted perimeter is the circumference, d. The same pipe flowing half full would have a wetted perimeter of d/2.
The area in which water touches a stream channel's walls.
the wetted surface of a stream (culvert) cross section which causes resistance to flow. The water to surface interface is a distance, usually expressed in feet.