hydraulic radius[hi′drȯ·lik ′rād·ē·əs]
a hydraulic characteristic of the cross section of a fluid flow, expressed by the ratio of the area of this cross section to its so-called wetted perimeter (that is, that part of the perimeter along which the flow comes in contact with solid walls). The value of the hydraulic radius changes according to the dimensions and shape of the channel’s cross section. For a filled pipe of round cross section, the hydraulic radius is equal to one-fourth of the diameter, and for open channels of large width the radius is assumed to be equal to the mean depth of flow. The hydraulic radius is widely used in hydraulic calculations.