The hydraulic radius (Rh​) is a fundamental parameter used in fluid mechanics, particularly in the analysis of open channel flow. It represents the ratio of the cross-sectional area ($A$) of the flow to the wetted perimeter ($P$) of the conduit. Mathematically, it is expressed as:

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Where:

• Rh​ is the hydraulic radius, typically measured in meters (m) or any unit of length.
• $A$ is the cross-sectional area of the flow, typically measured in square meters.
• $P$ is the wetted perimeter, which is the total length of the perimeter of the cross-section that is in contact with the flowing fluid.