§ 10.3.4.2. Full Flow in Conduit - Type B  


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  • If the full flow condition exists within the length of the culvert then the hydraulic grade line will be at above the soffit. The hydraulic grade line at the culvert outlet is based on the outlet depth (H o ) being at or above the soffit at the outlet.

    Use the Equation 10.3.4.2.c on the following page to calculate the frictional slope of the culvert. If the frictional slope is less than the culvert slope, the hydraulic grade line may drop below the soffit of the culvert. If this condition exists then the culvert flow may be Type BA.

    The frictional loss through the culvert is determined by Equation 10.3.4.2.a.

    To determine the hydraulic grade line at the upstream end of the culvert, at the inlet use Equation 10.3.4.2.b. To obtain the headwater elevation the entrance loss will need to be calculated. See Energy Balance at Inlet section.

    35-H10-3-4-2.png

    Figure 10.3.4.2 - Outlet Control, Fully Submerged Flow
    (Source TxDOT, Hydraulic Design Manual, 2011)

    (Equation 10.3.4.2.a)

    h f = S f L

    h f = head loss due to friction in the culvert barrel, ft.
    S f = friction slope, ft. (See Equation 10.3.2.2.3)
    L = length of culvert containing full flow, ft.

    (Equation 10.3.4.2.b)

    H i = H o + h f − S o L

    H i = depth of hydraulic grade line at inlet (ft.)
    H f = friction head losses (ft.) (calculated using Equation 10.3.2.2.1)
    S o = culvert slope (ft./ft.)
    L = culvert length (ft.)
    H o = outlet depth (ft.)

    (Equation 10.3.4.2.c)

    35-H10-3-4-2c.png

    S f = friction slope (ft./ft.)
    Q = flow in pipe (cfs)
    n = Manning's 'n'-value
    A = Area of the pipe (ft )
    R = Hydraulic Radius (A/P W ) (ft.)