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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For the partly full flow situations, the
flow in the barrcl is subcritical.

Condition III-7-A represents the clas-
sic full flow condition, with both inlct
and outlet submerged. The barrel is in
pressure flow throughout its length,
This condition is often assumed in calcula-
tions, but seldom actually exists,

Condition IlI-7-B depicts th~ outlct
submerged with the inlet ullsubmerged. For
this case, the headwatcr is shallow so
tha t the inlet crown is exposed as the
flow contracts into the culvert.

Condition III-7-C shows the entrance
submerged to such a degree that the cul-
vert flows full throughout its entire
length while the exit is unsubmergcd.
This is a rare condition. It requires
an extremely high headwater to maintain
full barrel flow with no tail water. The
outlet velocities are usually high under
this condition.

Condition III-7-D is more typical. The
culvert entrance is submcrged by the
headwater and the outlet end flows frccly
with a low tail water. For this condition,
the barrel flows partly full over at
least part of its length (subcritical
flow) and the flow passes through critical
depth just upstream of the outlet.

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Condition III-7-E is also typical,
with ncither the inlet nor the outlet
end of the culvert submcrgcd, Thc, barrel
flows partly full over its entire length,
and the flow profile is subcritical.

2) Factors Influencing Outlet
Control. All of the factors influencing
the performance of a culvcrt in inlet
control also influence culverts in outlct
control. In addition, the barrcl charac-
teristics (roughncss, area, shapc, length,
and slope) and the tailwatcr clcvation
affect culvert performance in outlct
control. (table I)

The barrel rou2hness is a function of
the material used to fabricate the bar-
rel. Typical materials include concrcte

and corrugated metal. The roughness is
represented by a hydraulic resistance
coefficient such as the Manning n value.
Typical Manning n values for culverts
are presented in table 4. Additional
discussion on the sources and derivations
of the Manning n values are contained in
appendix B.

The barrel area and barr,<;.L:'!:t~V_l; are
self explanatory.

The barrel len2th is the total culvert
length from the entrance to the exit of
the culvert. Because the design height
of the barrel and the slope influence
the actual length, an approximation of
barrcl length is usually necessary to
begin the design process.

The barrel slODe is the actual slope
of the culvert barrel. The barrel slope
is often the same as the natural stream
slopc. However, when the culvert inlet
is raised or lowered, the barrel slope
is different from the stream slope.

The tailwater elevation is based on
the downstream water surface elevation.
Backwater calculations from a downstream
control, a normal depth approximation,
or field observations are used to define
the tail water elevation.

3) Hydraulics of Outlet Control.
Full flow in the culvert barrel, as
depicted in figure III-7-A, is the best
type of flow for describing outlet control
hydraulics.

Outlet control flow conditions can be
calculated based on energy balance. The
total energy (HJ required to pass the
flow through the culvert barrel is made
up of the entrance loss (He)' the friction
!osscs through the barrel (Hr), and the
exit loss (HO>. Other losses, ir.cluding
bend losses (Hb), losses a t junctions
(H), and loses at grates (H~ should be
includcd as appropriate. Thcse losses
are discussed in chapter VI.

HL = He + Hr + Ho + Hb + Hj+ Hg (I)

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