Slug Flow

When the flow becomes unstable, a phenomenon termed slug flow may occur. In this condition the flow oscillates between inlet control and outlet control due to the following instances:
  • Flow is indicated as supercritical, but the tailwater level is relatively high.
  • Uniform depth and critical depth are relatively high with respect to the culvert barrel depth.
  • Uniform depth and critical depth are within about 5% of each other.
The methods recommended in this chapter accommodate the potential for slug flow to occur by assuming the higher of inlet and outlet control headwater.