FLOOD01360

• County: Statewide
• Title: Hydraulic Design of Highway Culverts
• Date: 9/1/1998
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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EL';s the elevation of the
streambed at the face, ft (m)

HWiis the required headwater
depth, ft (m)

Possible results and consequences of
this calculation are:

I) If the FALL is negative or
zero, set FALL equal to zero and proceed
to step f.

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2) If the FALL is positive, the
inlet control section invert must be
depressed below the streambed at the
face by that amount. If the FALL is
acceptable, proceed to step f.

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3) If the FALL is positive and
greater than is judged to be acceptable,
select another culvert configuration and
begin again at step a.

f. Calculate the inlet control
section invert elevation as follows:

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ELi = ELof - FALL

where ELi is the invert elevation at
the face of a culvert (ELf) or at the
throat of a culvert with a tapered inlet
(EL,).

3. Outlet Control. The outlet control
calculations result in the headwater
elevation required to convey the design
discharge through the selected culvert
in outlet control. The approach and
downstream velocities may be included
in the design process, if desired. The
critical depth charts and outlet control
nomographs of appendix D are used in the
design process" For illustration, refer
to the schematic critical depth chart
and outlet control nomograph shown in
figures III-19 and III-20, respectively,

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a. Determine the tailwater depth
above the outlet invert (TW) at the design
flow rate. This is obtained from backwater
or normal depth calculations, or from
field observations.

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600

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1000

FLOW RATE (Q)

Figure III-19--Crltlcal depth
chart (schematic).

b. Enter the appropriate critical
depth chart (figure III-I 9) with the
flow rate and read the critical depth
(de)' de cannot exceed D!

(Note: The de curves are truncated
for convenience when they converge. If
an accurate de is required for de > .9D
consult the Handbook of Hvdraulics or
other hydraulic references. (24))

c. Calculate (de + D)/2

d. Determine the depth from the
culvert outlet invert to the hydraulic
grade line (ho)'

ho = TW or (de + D/2), whichever is
larger.

e. From table 12, appendix D, obtain
the appropriate entrance loss coefficient,
k., for the culvert inlet configuration.

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