Hydraulic Design of Highway Culverts - DOT On-Line Publications

2. Outlet Control.
a. Partly Full Flow. Large conduits, such as long span culverts, usually flow partly full
throughout their lengths. In addition, the invert of the culvert is often unlined. In these situations
it is advisable to perform backwater calculations to determine the headwater elevation.
The backwater calculations begin at the tailwater level or at critical depth at the culvert exit,
whichever is higher. Hydraulic resistance values for the backwater calculations are contained in
Hydraulic Flow Resistance Factors for Corrugated Metal Conduit. (25) Data from that reference
are included in Appendix B. Selected resistance values for natural channels are found in Table
11 of Appendix D. Note that when the perimeter of the conduit is constructed of two or more
materials, a composite resistance value should be used. Methods of calculating composite
resistance values are discussed in Appendix B.
b. Full Flow. If the conduit flows full or nearly full throughout its length, Equation (7) may be
used to calculate the outlet control headwater depth.
HW = TW + H
(7)
o
L
HL is the total loss through the culvert barrel which is calculated using Equation (1) or Equation
(5). TW is either the tailwater depth or (dc + D)/2, whichever is larger. Values of critical depth for
most conduits are provided in the manufacturers' information. In Equation (5), the hydraulic
radius and velocity are full flow values. The Manning’s n value is a composite value when more
than one material is used in the perimeter of the conduit.
3. Discussion of Results
The inlet control headwater obtained from Figures III-21 or III-22 includes the approach velocity
head. Therefore, credit may be taken for the approach velocity head in determining the required
headwater pool depth.
In outlet control, the same limitations on use of the approximate backwater method apply as for
culverts with design charts. That is, if the headwater (referenced to the inlet invert) falls between
1.2D and 0.75D, use the results with caution. For large, expensive installations, check the
results using backwater calculations. If the headwater falls below 0.75D do not use the
approximate method. Perform backwater calculations as illustrated in the following example
problem.
4. Example Problem
Problem No. 5: Design of a long span structural plate corrugated metal elliptical culvert.
Use a long span culvert to pass the 25-year flood of 155.744 m 3 /s (5,500 ft³/s) under a high
roadway fill. The design flow should be below the crown of the conduit at the inlet, but the check
flow (100-year flow) of 212.378 m 3 /s (7,500 ft³/s) may exceed the crown by not more than 1.524
m (5 feet). Use the following site conditions:
ELhd: 73.152 m (240 ft)
Elevation of Stream Bed at Culvert Face (ELsf): 67.056 m (220 ft)
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