Exit Loss Considerations

The traditional assumption in the design of typical highway culverts is continuity of the hydraulic grade line. At the outlet, this implies that when the tailwater is higher than critical depth and subcritical flow exists, the hydraulic grade line immediately inside the barrel is equal to the tailwater level. This is reasonable for most normal culvert designs for TxDOT application. However, by inference there can be no accommodation of exit losses because the energy grade line immediately inside the culvert can only be the hydraulic grade line plus the velocity head, no matter what the velocity is in the outfall.
Occasionally, an explicit exit loss may need to be accommodated. Some examples are as follows:
  • conformance with another agency’s procedures
  • comparison with computer programs (such as HEC-RAS)
  • design of detention pond control structures in which storage volumes are sensitive to small changes in elevation.
If such a need arises, base the starting hydraulic grade level (H
o
) is based on balancing
Equation 8-29
between the outside and inside of the culvert face at the outlet. A common expression for exit loss appears in Equation 8-30. This assumes that the tailwater velocity (v
TW
) is lower than the culvert outlet velocity (v
o
) and the tailwater is open to the atmosphere. If the above approach is used, it is most likely that the outlet depth (H
o
) will be lower than the tailwater. This conforms to basic one-dimensional hydrostatic principles.
EquationObject261299
Equation 8-29.
where:
  • H
    o
    = outlet depth - depth from the culvert flow line to the hydraulic grade line inside the culvert at the outlet (ft. or m)
  • v
    o
    = culvert outlet velocity (ft./s or m/s)
  • v
    TW
    = velocity in outfall (tailwater velocity) (ft./s or m/s)
  • h
    o
    = exit loss (ft. or m).
EquationObject262300
Equation 8-30.
where:
  • K
     = loss coefficient which typically varies from 0.5 to 1.