Switching between the normal flow and dynamic flow equation for a conduit is controlled automatically by EXTRAN. The Froude number or the Vedernikov number of the conduit determines the use of the normal flow equation or the dynamic flow equation. Froude numbers > 1.0 (supercritical flow) cause the conduit flow to be calculated from Manning’s equation using the upstream cross-sectional area and hydraulic radius of the conduit. Vedernikov numbers < 1.0 (roll wave formation) also trigger the use of Manning’s equation.
Automatic switching based on the Froude number or Vedernikov number is new to EXTRAN 5. All versions of EXTRAN had automatic switching to the normal flow equation when all of the following three conditions occur in a conduit:
When all three conditions are met the flow is considered "normal". Normal flow is labeled with an asterisk in the intermediate printout, and the conduit summary lists the number of minutes the normal flow assumption was used for each conduit.
The equation used to calculate the normal flow, Qnorm, is:
The Froude number (F#) is calculated as:
D = the depth of water in the conduit, and the Vedernikov number (V#) for turbulent flow is calculated as:
Rf = Full flow hydraulic radius, and
Af = Full flow cross sectional area.