Wetted Perimeter

The wetted perimeter refers to the length of the perimeter of the conduit which is in contact with the fluid. The wetted perimeter is used along with the cross sectional area of flow to determine the hydraulic radius which is a measure of the efficiency with which the conduit is able to transport the fluid.

The amount of the conduit perimeter which is wetted obviously depends on the height of the fluid in the conduit. For a circular cross section pipe the wetted perimeter increases quickly when the flow in the pipe is low. Then the rate of increase in the wetted perimeter decreases until the pipe is  50% full before the rate of increase starts to increase again. When the pipe is nearly full the rate of increase is very high again.

The wetted perimeter directly affects the velocity of the flow and the flow rate achieved through the conduit. The fluid loses energy due to the friction between the pipe wall and the fluid. Therefore the smaller the wetted perimeter, the higher the velocity and flow rate. This effect can most clearly be seen as a circular cross section pipe begins to run full. As the pipe begins to run at around 93% height, the pipe actually starts to lose capacity. This happens because the wetted perimeter is increasing much quicker than the cross sectional area of the flow. This has the effect of reducing  the velocity of the flow and the overall flow rate begins to decrease. This effect means that a circular cross section drainage pipe cannot run 100% full until it is already overloaded, as the maximum flow is achieved at around 93% of the pipe height. This can be seen in the below graph showing the increases in wetted perimeter, hydraulic radius, average velocity and flow rate.