Definition - What does Hydraulic Radius mean?
Hydraulic radius is the area of the water prism in a pipe or channel divided by the wetted perimeter. Thus, for a round conduit flowing full or half full, the hydraulic radius is d/4. Hydraulic radius measures the flow efficiency of a pipe. In trenchless technology, it is a function of the shape of the pipe in which the liquid is flowing. It does not indicate half of the diameter as the name suggests.
Another term sometimes used for this quantity is hydraulic mean depth.
Trenchlesspedia explains Hydraulic Radius
In the designing of sewers, the following parameters have to be calculated: Sewer diameter and slope, the roughness coefficient of the pipe, the runoff flow rate of water, and the flow velocity of water. The equation used to derive the hydraulic radius for a circular sewer flowing full is:
R = A / Pw or R = (ΠD² / 4) / ΠD = D / 4
R = Hydraulic radius
A = Cross sectional area
Pw = Wetted perimeter
D = diameter of pipe