Full Flow in Conduit (Type B)

If full flow is occurring in the conduit, rate of energy losses through the barrel is constant (for steady flow) as seen in Figure 8‑10. The hydraulic grade line is calculated based on outlet depth, H
o
, at the outlet.
Outlet Control, Fully Submerged Flow (click in image to see full-size image)
Figure 8-10. Outlet Control, Fully Submerged Flow
Full flow at the outlet occurs when the outlet depth (H
o
) equals or exceeds barrel depth D. Full flow is maintained throughout the conduit if friction slope is steeper than conduit slope, or if friction slope is flatter than conduit slope but conduit is not long enough for the hydraulic grade line to get lower than the top of the barrel.
NOTE: Refer to Type BA – Submerged Exit, Free flow at Inlet to determine whether the entire conduit flows full.
Equation 8-11determines the energy loss (friction loss) through the conduit:
EquationObject243268
Equation 8-11.
where:
  • h
    f
    = head loss due to friction in the culvert barrel (ft. or m)
  • S
    f
    = friction slope (ft. or m) (See Equation 8-13.)
  • L
     = length of culvert containing full flow (ft. or m).
Equation 8-12 is used to compute the depth of the hydraulic grade line at the inside of the inlet end of the conduit. Refer to Energy Balance at Inlet to determine the headwater.
EquationObject244269
Equation 8-12.
where:
  • H
    i
    = depth of hydraulic grade line at inlet (ft. or m)
  • h
    f
    = friction head losses (ft. or m) as calculated using Equation 8-11.
  • S
    o
    = culvert slope (ft./ft. or m/m)
  • L
     = culvert length (ft. or m)
  • Ho
    = outlet depth (ft. or m).
Equation 8-13 is used to calculate friction slope. If friction slope is flatter than the conduit slope, the hydraulic grade line may drop below the top of the barrel. If this occurs, refer to Type BA - Full Flow at the outlet and free surface flow at the inlet.
EquationObject245270
Equation 8-13.
where:
S
f
= friction slope (ft./ft. or m/m)
  • z
     = 1.486 for English measurements and 1.0 for metric.