Lecture 21
Culvert Design & Analysis
Much of the following is based on the USBR publication:
“Design of Small Canal Structures” (1978)
I. Cross-Drainage Structures
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Cross-drainage is required when a canal will carry water across natural drainage
(runoff) channels, or across natural streams; otherwise, the canal may be
damaged
In some cases, cross-drainage flows are collected in a small channel paralleling
the canal, with periodic cross-drainage structures over or under the canal; this is
especially prevalent where there are poorly defined natural drainage channels
In culvert design for carrying runoff water, usually one of the big questions is
what the capacity should be
When the canal capacity is less than the natural channel capacity, it may be
economical to build an inverted siphon so the canal crosses the natural channel
With siphon crossings, it is not nearly as important to accurately estimate the
maximum flow in the natural channel because the structure is for the canal flow
In other cases, it may be more economical to provide cross-drainage by building
a culvert to accommodate natural flows after the canal is constructed
In these cases, the cross-drainage structure does one of the following:
1. Carry water under the canal
2. Carry water over the canal
3. Carry water into the canal
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Here are the common cross-drainage solutions:
1. Culverts
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These are often appropriate where natural flows cross a fill section of the
canal
Culverts may tend to clog with weeds, debris, rock, gravel, and or
sediments, especially at or near
the inlet
2. Over-chutes
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These are appropriate where
the bottom of the natural
channel is higher than the full
supply level of the canal
For example, over-chutes might
be used in a cut section of the
canal
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Canal over-chute & bridge
Gary P. Merkley
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Open-channel over-chutes can carry debris and sediment that might clog
a culvert, but pipe over-chutes may be equally susceptible to clogging
3. Drain Inlets
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With these structures, the flow of the natural channel is directed into the
canal
These may be appropriate where the natural flows are small compared
to the canal capacity, and or when the natural flows are infrequent
These may be appropriate when the canal traverses a steep slope, and
cross-drainage might cause excessive downhill erosion, compromising
the canal
These may be less expensive than over-chute or culvert structures, but
may require more frequent maintenance of the canal
Drain inlets may be problematic insofar as rocks, sediment and other
debris can clog the inlet and or fill the canal near the inlet, obstructing
the canal flow
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II. Alignment
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Align the culvert along natural open channels where possible so that the natural
runoff pattern is not disturbed any more than necessary
If the natural drainage channel is not perpendicular to the canal, it is best to have
a skewed alignment of the culvert
One or more bends in the culvert can be used to help follow the natural channel,
especially in longer culverts
If there is no apparent natural runoff channel, consider using the shortest straight
path from inlet to outlet
In some cases it may be unnecessary or undesirable to follow a natural channel
III. Barrel Profile
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Knowing the inlet and outlet locations will determine the length and slope of the
culvert
The invert of the inlet and outlet should correspond approximately to the natural
ground surface elevations at the two respective locations -- otherwise,
sedimentation and or erosion will likely occur, requiring maintenance
However, a compound slope may be needed if:
1. The culvert would not have enough vertical clearance
under a canal (about 2 ft for an earth canal, or 0.5 ft for a
concrete canal), road, etc.;
2. The slope of the culvert would cause supercritical openchannel flow, which might require a downstream energy
dissipation structure (making the design more costly); or,
3. You want to force a hydraulic jump to dissipate energy.
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BIE 5300/6300 Lectures
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The USBR recommends, in general, a minimum slope of 0.005 and a maximum
slope of somewhat less than the critical slope (maintain subcritical flow)
The minimum slope is imposed in an effort to prevent sediment deposition in the
culvert barrel
The barrel of the culvert is usually circular (perhaps corrugated pipe) or
rectangular
The maximum slope is imposed in an effort to avoid the additional cost of an
energy dissipation structure at the outlet (channels upstream and downstream of
culverts are typically unlined, although there may be some riprap)
With a compound slope, the upstream slope is steeper than critical, and the
downstream slope is mild, thereby forcing significant energy dissipation through a
hydraulic jump in the vicinity of the break in grade, inside the barrel
IV. Inlets and Outlets
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USBR Culvert Inlets
Type 1: “broken-back transition”, appropriate for natural channels with welldefined upstream cross-section (USBR Figs. 7-1 & 7-2)
Type 2: suitable for wide natural channels with poorly-defined upstream
cross section (USBR Fig. 7-4)
Type 3: “box inlet”, also for use in a poorly-defined natural channel, but
allows for a lower barrel invert at the inlet (USBR Fig. 7-5)
Type 4: similar to Type 3, but with a sloping invert, allowing for an even
lower barrel inlet (USBR Figs. 7-6 & 7-7)
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USBR Culvert Outlets
1. With energy
dissipation structure
2. Without energy
dissipation structure
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There are other USBR
standard inlet designs
(besides the above
four)
USBR-type culvert
Type 1 Transition (USBR)
inlets and outlets are
made almost exclusively of concrete
Some corrugated metal culverts have a circular or elliptical cross section with
smooth metalic inlet and outlet transitions
Use standard inlet & outlet designs if possible to save time and to avoid
operational and or maintenance problems
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Gary P. Merkley
Type 3 Transition (USBR)
Type 4 Transition (USBR)
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V. Pipe Collars
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Pipe collars are used to prevent “piping” along
the outside of the barrel and or damage by
burrowing animals
For culverts under canals, the typical USBR
design calls for three collars: one under the
center of the upstream canal bank, and two
under the downstream canal bank
A “short path” between two adjacent collars
means that the collars are too close together
and or their diameters are too small
The USBR recommends the following for minimum collar spacing:
Xmin = 1.2 Y
(1)
VI. Basic Design Hydraulics
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Culverts are typically designed for fullpipe flow in the barrel at the design
discharge value
This means that pressurized pipe flow is
impending at the design discharge, but
at lower flow rates open-channel flow
exists in the barrel
The upper limit on barrel velocity is
usually specified at about 10 fps, or
perhaps 12 fps with an energy
dissipation structure at the outlet
For full pipe flow without inlet and outlet
structures, in which case the culvert is
simply a buried pipe, you can use a limit
of 5 fps
Culvert with collars (USBR)
Knowing the design discharge and the
velocity limit, the diameter (circular
barrels) for full pipe flow can be directly
calculated
For rectangular barrel sections, you need to determine both width & height
Discharge capacity can be checked using the Manning (or Chezy) equation for a
circular section running full (again, impending pressurization)
For new pre-cast concrete pipe, the Manning “n” value is about 0.013, but for
design purposes you can use a higher value because the pipe won’t always be
new
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Gary P. Merkley
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You can also check the discharge using the Darcy-Weisbach equation, with
specified values for upstream and downstream water surface elevations in the
inlet and outlet structures, respectively
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The head loss through a typical inlet structure with inlet control can be estimated
as a “minor loss” by:
V2
hf = K
2g
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(2)
where the coefficient K may vary from 0.05 for a smooth, tapered inlet transition,
flush with the culvert barrel, to 0.90 for a projecting, sharp-edged barrel inlet
Note that the inlet and or outlet losses may or may not be “minor” losses when
dealing with culverts, especially when the barrel is short
For outlet control, the head loss is estimated as in the above equation for inlet
control, except that there will also be expansion losses downstream
For barrel control, the head loss is the sum of the inlet, barrel, and outlet losses
References & Bibliography
USBR. 1978. Design of small canal structures. U.S. Government Printing Office, Washington, D.C.
435 pp.
Gary P. Merkley
242
BIE 5300/6300 Lectures