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