previous sections we measured the bankfull width and slope of the stream
bed at the planned crossing.
Bankfull Width measurement determines the width of the
new culvert (all types) using
the following formula:
Culvert Width = Bankfull Width + 1 foot
We have also determined whether
important fish spawning habitat occurred on the site. Based on
these factors, we decided which of the four culvert types to install. The final dimension of the culvert,
length, is determined by measurements made on the site. The
measurements needed are the desired, final width of the road bed and the
slope of the embankments. The design width of the road bed (at the
culvert) should be at least the width of the existing
road. However, in some
cases it should be set at the width of any planned or expected
improvements that might occur during the life of the structure.
new culvert should be aligned with the existing stream channel. Sharp
turns in the channel above or below the structure will direct currents
into banks, eventually causing undercutting and collapse. However,
if stream bends adjacent to culverts can't be avoided, it is preferred
to locate culverts so they are aligned with the downstream channel.
Even if there is a sharp bend upstream.
of the culvert for any installation is based on the total width of the
plus the distance added by the slopes of the embankments on each side
of the road to the bottom of the culvert (see diagram below).
The road width is measured from the
point on each side of the road where slope meets level road shoulder.
The height of the fill is the vertical distance from the bottom of the
culvert at the middle of the road to the finished level of the center of
The actual slope of the embankments
should provide a stable configuration and may be based on
one-size-fits-all recommendations or requirements, or be determined by
knowledge and experience with local soil and fill characteristics.
General recommendations tend to be conservative and based on unstable
fill materials such as sand. This can result in longer slopes and
culverts than are really needed in applications where fill materials are
more cohesive and stable (such as clay).
While the general rule approach may
provide an extra measure of stability to slopes in some cases, it also
results in longer culverts, higher costs and more habitat and fish
passage problems. We recommend consultation with agencies with
local expertise such as U.S. Natural Resources Conservation Service
(NRCS), State natural resources agencies and experienced road
building contractors. This point in the project development is an
excellent time for consultation with regulatory
agencies for both advice and information regarding regulations and
In the example
above, the fill height needed to maintain a level road at the crossing
is 16 feet. Assume that we will use local fill materials that are
and capable of supporting strong plant growth for further
stabilization. Based on the observations made in this example, we have
assumed that a 2:1 bank slope is adequate for this site. We now have
the information needed to determine culvert length by using the
Culvert Length = Road Width + 2(Fill
Height X Slope)
Culvert Length = 32’ + 2(16’X2)
Culvert Length =96’
Establishing the elevations of the culvert inlet and outlet are
critical elements in the installation using the Stream Simulation
Determine the stream slope by
surveying the vertical drop in the stream bed from 100 feet above the
culvert (Point A) to 100 feet below (Point B). See image above.
Set culvert inlet and outlet depths (embed) to the appropriate amount
below the slope line. Round Culverts may be embedded from 20% to 40
%, ellipse or squashed arch culverts from 10% to 25%.
The method described above is the simplest method for finding stream slope.
However, a more
complete profile of the stream slope may be needed in some cases.
In so, several elevations are measured on the stream bed at points
above and below the structure to create an actual bed profile
When using either method you
will often note that the stream bed is elevated directly above the
existing culvert. This typically occurs when the initial
culvert placement is too high. Natural stream processes
deposit sediment, rock and debris above the culvert to a level
above the culvert inlet. When a new structure is placed at
the correct depth, the sediment deposit may be considerably higher
than the new culvert inlet. In most cases, the stream flow will
quickly cut through and redistribute these materials downstream to
normalize the channel.
The process is called “head
cutting” and will change the upstream channel shape and
configuration considerably. If the materials lodged above
the old installation are clean sand, wood or rock, downstream
deposition of these materials may not be a severe problem.
However, if large amounts of fine silt/sediment are involved, fish
habitat downstream may be damaged. In some cases, these
materials can be removed mechanically before the flow is released
through the new structure.
At this site, the
original culvert placement was too high above the natural stream slope.
Rocks and sand have accumulated at the upper end of the culvert blocking
more than 50% of the opening and creating a fish passage
Only a few hours
after installation of a new culvert (embedded 20% below the slope line),
head cutting is beginning to normalize the channel.
Culverts may be purchased with square or bevel cut ends that match the
bank slope. Bevel cut ends (see diagram) are thought to be less prone to
blockage by debris washing downstream. Placing rocks in the bottom
of culverts will also help to
natural stream bed.
With an outline diagram of the planned project, we are now ready to plan the
final steps before permits are requested. Requirements for construction
permits are highly variable among jurisdictions. In most cases however, the
project diagram will need to be supplemented with an erosion control plan
and a description of sediment control during construction. This image shows
one example of an erosion control plan for a small stream.
plan to control sedimentation of the stream during construction is often
required. This plan should describe how stream flow will be handled
during the construction period. In very low flow streams, simply
damming the stream with clean fill above the work site may provide time
for the installation. On streams where flows are too large for this
technique, water may be pumped around the work site, or a temporary
diversion channel may be required.
scheduling for stream work can be very difficult. Work must be done
during low stream flow periods to minimize sediment deposition and
transport, but must also consider the life stages of fish present in the
stream. Adult fish are usually not highly vulnerable to the type of
sediment depositions created by culvert projects. However, fish eggs and
fry developing in spawning beds are extremely vulnerable to being
smothered by sediment.
should be scheduled when no fish eggs or developing fry are in
substrates downstream of the project site. Most fish species spawn either in
spring or fall. These diagrams outline life cycles of some common
game fish. They also help to illustrate, that when spring and fall
spawning species are present in a stream, the construction window
may be limited to the months of July and August. In this situation,
some jurisdictions will only issue construction permits during that time
frame. Be sure to check with your local offices of the U.S. Fish and
Wildlife Service and state Natural Resource Departments to determine what
fish are present in the stream and permitting requirements.
Permits are usually
issued for a certain time period, such as 60 or 90 days. Within that
time it will be the builder’s responsibility to schedule the work when
streams are low and work can be completed under good conditions. Often
this means keeping some flexibility in work schedules to provide for
contingencies such as equipment problems or severe weather events.
Culvert Installation, At last!
Be Sure to Review Your
Contractor and machine operator are completely informed
on the final installation plan, elevation requirements and sediment
controls are in pace.
The proper permits have been issued and the responsible
natural resource agencies have been notified.
Electrical, gas and water utilities lines have been
located or cleared at the site.
Project supervisor will be on-site throughout the
All construction materials are present and ready.
No bad weather forecast for today/tomorrow.
Excavate culvert and
bypass channel. Elevations of inlet and outlet should be checked
often to ensure they are positioned correctly.
Appropriate use of a flow bypass channel during placement of a large,
multi-section box culvert. Note the clear water in the diversion channel
and the alignment of the new culvert with the natural stream channel.
Water flow bypass during
The fill should be clean mineral soil and free of
organic material, large rocks, tree roots or ice that will break down and
leave holes. Bedding and filling around culverts is an important step
in insuring that a culvert installation will not erode, settle or deform.
It also prevents “piping” or the leaking of water around the culvert.
Compact the soil carefully using the
Bedding and compacting fill
around the culvert.
When the culvert installation is
complete, trim and stabilize the work site as soon as possible.
Trim slopes to a stable
Apply grass seed or plantings
Apply mulch or erosion control
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