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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O.Op+ 00

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Figure III-tO--Roadway overtopping.

Equations (I) through (7) were devel-
oped for full barrel flow, shown in figure
III-7-A, The equations also apply to
the flow situations shown in figures
III-7-B and C, which are effectively
full flow conditions, Backwater calcu-
lations may be required for the partly
full flow conditions shown in figures
III-7-D and E, These calculations begin
at the water surface at the downstream
end of the culvert and proceed upstream
to the entrance of the culvert. The
downstream water surface is based on
critical depth at the culvert outlet or
on the tail water depth, whichever is
higher, If the calculated backwater
,profile intersects the top of the barrel,
as in figure III-7-D, a straight, full
flow hydraulic grade line extends from
that point upstream to the culvert en-
trance, From equation (4b), the full
flow friction slope is:

Hf 29 n2 V2
So = = -----------
L Rl.33 2g

In order to avoid tedious backwater
calculations, approximate methods have
been developed to analyze partly full
flow conditions. Based on numerous back-
water calculations performed by the FHWA

staff, it was found that a downstream
extension of the full flow hydraulic
grade line for the flow condition shown
in figure III-9-B pierces the plane of
the culvert outlet at a point one-half
way between critical depth and the top
of the barrel. Therefore, it is possible
to begin the hydraulic grade line at a
depth of (de + D)/2 above the outlet
invert and extend the straight, full
flow hydraulic grade line upstream to
the inlet of the culvert at a slope of
Sn' (figure III-9-D) If the tail water
exceeds (de + D)/2, the tailwater is
used to set the downstream end of the
extended full flow hydraulic grade line,
The inlet losses and the velocity head
are added to the elevation of the hydraulic
grade line at the inlet to obtain the
head water eleva tion.

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This approximate method works best
when the barrel flows full over at least
part of its length, (figure III-9-B)
When the barrel is partly full over its
entire length (figure III-9-C), the method
becomes increasingly inaccurate as the
headwater falls further below the top of
the barrel at the inlet. Adequate results
are obtained down to a headwater of O,75D,
For lower headwaters, backwater calcu-
lations are required to obtain accurate
headwater elevations,

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