Wetting perimeter, abbreviated as \(P_w\), is a hydraulic parameter used to describe the contact length between the fluid and the boundary of an open channel or pipe. It is the total length of the surface of the channel or pipe that is in contact with the fluid. The wetted perimeter depends on the shape and size of the channel or pipe and the water level. A larger wetted perimeter typically increases frictional losses, affecting the efficiency of fluid flow.
In a rectangular channel, the wetted perimeter is the sum of the length of the bottom and the two sides of the channel that are in contact with the fluid. In a circular pipe, the wetted perimeter is the circumference of the pipe.
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