Full Flow at Outlet and Free Surface Flow at Inlet (Type BA)

If the friction slope is flatter than the conduit slope, it is possible that full flow may not occur along the entire length of the culvert (see the Table 8-5 on
Entrance Loss Coefficients
). The following steps should be followed:
  1. Determine the length over which full flow occurs (L
    f
    ) is using the geometric relationship shown in Equation 8-14 (refer to Table 8-5 on
    Entrance Loss Coefficients
    ):
    EquationObject246271
    Equation 8-14.
    where:
    • L
      f
      = length over which full flow occurs (ft. or m)
    • S
      o
      = culvert slope (ft./ft. or m/m)
    • S
      f
      = friction slope (ft./ft. or m/m)
    • H
      o
      = outlet depth (ft. or m)
    • D
       = Conduit barrel height (ft. or m).
    Use the following table to determine how to proceed considering a conduit length L.
    Table 8-4: Conduit Length (L) Procedure Determination
    If…
    Then proceed to…
    Comment
    If S
    f
     ≥ S
    o
    Entire length of culvert is full
    If L
    f
     ≥ L
    Type B energy loss calculations
    Entire length of culvert is full
    If L
    f
     < L
    Step 2.
    Outlet is full but free surface flow at inlet
  2. Determine Type BA free surface losses, if applicable. Free surface flow begins at the point of intersection of the hydraulic grade line and the soffit of the culvert barrel as shown in Figure 8‑11. If this condition occurs, determine the depth of flow at the inlet using the Direct Step Method with the starting depth (d
    1
    ) equal to the barrel rise (D) and starting at the location along the barrel at which free surface flow begins.
  3. Determine Type BA hydraulic grade line at inlet, if applicable. When the inlet end of the conduit is reached using the direct step method, set the calculated depth at the inlet as Hi and refer to Energy Balance at Inlet to determine the headwater.
    Point at Which Free Surface Flow Begins (click in image to see full-size image)
    Figure 8-11. Point at Which Free Surface Flow Begins