Open Channel Flow or Pressure Flow

When a conduit is not submerged, the principles of
open channel flow
apply. When the conduit is submerged, pressure flow exists because the water surface is not open to the atmosphere, and the principles of conduit flow apply. For circular pipes flowing full, Equation 6‑3 becomes:
EquationObject210231
Equation 6-16.
where:
  • Q = discharge (cfs or m
    3
    /s)
  • z = 0.4644 for English measurement or 0.3116 for metric.
  • n = Manning’s roughness coefficient
  • D = pipe diameter, ft. or m
  • S = slope of the energy gradeline (ft./ft. or m/m) (For uniform, steady flow, S = channel
  • slope, ft./ft. or m/m).