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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f. Determine the losscs through the
culvert barrel, H, using the outlct control
nomograph (figure 111-20) or equation
(S) or (6) if outside the range of the
nomograph.

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I) If the Manning n value given
in the outlet control nomograph is dif-
ferent than the Manning n for the culvert,
adjust the culvert length using the
formula:

L,' (";-)'

(9)

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Ll is the adjusted culvert length,
ft (m)
L is the actual culvcrt lcngth,
ft (m)
n1 is the desired Manning n value
n is the Manning n value from the
outlet control chart.

Then, use Ll rather than the actual
culvert length when using the outlet
control nomograph.

2) Using a straightedge, connect
the culvert size (point I) with the cul-
vert lcngth on the appropriate k. scale
(point 2). This dcfincs a point on the
turning line (po in t 3).

3) Again using the straight-
edgc, extcnd a linc from the discharge
(point 4) through the point on thc turning
line (point 3) to the Head Loss (H) scale.
Read H. H is the energy loss through
the culvert, including entrance, friction,
and outlet losscs.

Note: Carcful alignmcnt of the
straightcdge in neccssary to obtain good
results from the outlet control nomo-
graph.

g, Calculate thc rcquircd outlct
control headwatcr clevation,

ELho = ELo + H + ho (10)

where ELo is the invert elcvation at

the outlet, (If it is desired to include
the approach and downstream velocities
in the calculations, add the dowllStrcam
velocity head and subtract the approach
velocity head from the right side of
equation (10). Also, use equation (4c)
instead of equation (4d) to calculate
the exit losses and equation (1) to cal-
cula te total losses.)

h. If the outlet control headwater
elevation exceeds the design headwater
elevation, a new culvert configuration
must be sclected and the process repeated,
Generally, an enlarged barrel will be
necessary since inlet improvements are
of limitcd benefit in outlet control.

4. Evaluation of Results. Compare
the headwater elevations calculated for
inlet and outlet control. The higher of
the two is designated the controlling
headwater elevation. The culvert can be
expected to operate with that higher
headwater for at least part of the time.

The outlet velocity is calculated as
follows:

a, If the controlling headwater is
bascd on inlet control, determine the
normal depth and velocity in the culvert
barrel. The velocity at normal dcpth is
assumed to be the outlet velocity.

b. If the controlling headwater is
in outlet control, determine the area of
flow at the outlet based on the barrel
geometry and the fOllowing:

I) Critical depth if the tail-
water is below critical depth,

2) The tail water depth if the
tail wa ter is between critical depth and
the top of the barrcl.

3) The hcight of the barrel if
the tailwatcr is above the top of the
barrcl.

Repeat the design process until an
acceptable culvert configuration is deter-
mined. Once the barrel is selected it

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