The cross section of a water conveyance has three key parameters that affect the flow:
Area, A
This is the area perpendicular to the direction of flow, and is measured in square metres (m²). For a circular conduit of diameter D,
A = 𝛑D²/4
The average velocity of flow at any point in a water conveyance is determined by the flow rate and the cross sectional area of flow at that point:
Velocity (V) = Flow (Q) / Area (A)
Wetted Perimeter, P
This is the distance around the perimeter of the cross section, measured in metres (m). For a circular conduit of diameter D,
P = 𝛑D.
Hydraulic radius, R
The hydraulic radius of a water conveyance is the ratio of its cross sectional area, A to its wetted perimeter, P:
R = A / P
For a circular conduit of diameter D, the hydraulic radius is:
P = (𝛑D²/4) / (𝛑D) = D/4