The hydraulic diameter is a term used in fluid mechanics and thermodynamics. It is a measure of the efficiency of a section to pass flow by relating the cross-sectional area of flow to the wetted perimeter of the cross-section.
The hydraulic diameter of a round pipe is simply its diameter. The equations for hydraulic diameter for other cross-sections are given in the table below.
|Square (L = side)|
|Rectangle (L1 and L2 = sides)|
|Flowing partially full|
|Half-filled circle (d = diameter)|
|Rectangle (h = height, w = width)|
|Wide, shallow stream (d = depth)|
|Triangle (d = depth, t = top, s = side length)|
|Trapezoid (d = depth, t = top, b = bottom, s = side length)|
The hydraulic diameter should not to be confused with the hydraulic radius term in the Manning equation, which is not half of the hydraulic diameter, but rather one quarter.