The hydraulic radius (Rh) is an approximation of the average depth of the water in a flowing stream channel. The hydraulic radius controls the speed of flow in a channel, and thus its efficiency. Rh is calculated by dividing the cross-sectional area of the flowing channel by the length of the wetted perimeter. The wetted perimeter is the length of the channel boundary, measured across the stream, in direct contact with water.
Determine the cross-sectional area of the stream. For this example, assume a cross-sectional area A = 325m^2.
Determine the wetted perimeter of the stream channel. In this example, the wetted perimeter P = 71m.
Divide the cross-sectional area by the wetted perimeter. The result is the hydraulic radius of the stream. Using the example figures given, Rh = A/P = 325 m^2/71m = 4.58m.
The hydraulic radius is not half of the hydraulic diameter; it is one-fourth of the hydraulic diameter.