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

2
l\g
r:sz:: - - ~,
. 1\'
. -- --

\ /
HW HWo "'-J

j

.

<l
'~
.
'~
,
J

-- -----_kG.L h I
11.'" -T~------__ __ ~H' HL y~
"'-~ ---
____ Ho - __ ZQ__

-------

SECTION (0

.

-

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J
I
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A
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1
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TW

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SECTION 2

.

.

A

Figure III-S--Full flow energy and hydraulic grade lines.

Figurc III-S dcpicts thc energy grade
line and the hydraulic grade line for
full flow in a cui vert barrel. The energy
grade line represents the total energy
at any point along the culvert barrel.
HW is the depth from the inlet invert to
the energy grade line.The hydraulic grade
line is the depth to which water would
rise in vertical tubes connected to the
sides of the culvcrt barrel. In full
flow, the energy grade linc and the hyd-
raulic grade line are parallel straight
lines separated by the velocity hcad lines
cxcept in the vicinity of the inlet where
the flow passes through a contraction.

The headwater and tail water condi-
tions as well as the entrance, friction,
and exit losses are also shown in figure
III-S. Equating the total energy at
scctions I and 2, upstream and downstream
of the culvert barrel in figure III-S,
the fOllowing relationship results:

V 2
u
HW 0 + -----.-
2g

V d2
= TW +--.-- + HL
2g

(6)

HW 0 is the hcadwater dcpth above
the outlet in vert, ft (m)
V u is the approach velocity, ft/s
(m/s)
TW is thc tailwater dcpth above
thc outlet in vcrt, ft (m)
V d is thc downstream velocity,
[t/s (m/s)

4

HL is the sum of all losses in-
cluding entrance (He)' friction
(HI)' exit (Ho) and other
losses, (HJ, (H), etc.,
ft (m)

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Note that the total available upstream
energy (HW) includes the depth of the
upstream water surface above the outlet
invert and the approach velocity head.
In most instances, the approach velocity
is low, and the approach velocity head is
neglected. However, it can be considered
to be a part of the available headwater
and used to convey the flow through the
cui vert.

.

Likewise, the velocity downstream of
the culvert (V d) is usually neglected.
When both approach and downstream veloci-
tics are neglected, equation (6) becomes:

~
.
.

HW 0 = TW + HL

(7)

.
.
1
.
.
.

In this case, HL is the difference in
elevation between the water surface eleva.
tion at the outlet (tail water elevation)
and the water surface elevation at the
inlet (headwater elevation). If it is
desired to include the approach and/or
downstream velocities, use cquation (4c)
for exit losses and equation (6) instead
of equation (7) to calculate the headwater.

36