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CULVERTS
Description
A culvert is a closed conduit used to convey water from one area to another, usually from one
side of a road to the other side.
Importance to Maintenance & Water Quality
Disposal of runoff from roadway ditches will help preserve the road bed, ditches, and banks.
Strategically placed culverts, along with road ditch turn-outs, will help maintain a stable velocity
and the proper flow capacity for the road ditches by timely outleting water from them. This will
help alleviate roadway flooding, reduce erosion, and thus reduce maintenance problems. In
addition, strategically placed culverts help distribute roadway runoff over a larger riparian
filtering area. Culverts preserve the road base by draining water from ditches along the road,
keeping the sub-base dry.
Culvert Profile
General
Culverts can be divided into two functional types: Stream Crossing and Runoff Management.
The first culvert type, stream crossing culvert, is self defining. A culvert is required where the
roadway crosses a stream channel to allow water to pass downstream. The second type culvert,
runoff management culvert, is one which is strategically placed to manage and route roadway
runoff along, under, and away from the roadway. Many times these culverts are used to transport
upland runoff, accumulated in road ditches on the upland side of the roadway, to the lower side
for disposal. These culverts are commonly called cross-drains.
Installation, modification, and improvements of culverts should be done when stream flows and
expectancy of rain are low. Ideally, the entire installation process, from beginning to end, should
be completed before the next rain event. All existing and/or reasonable potential stream flows
should be diverted while the culvert is being installed. This will help reduce or avoid
sedimentation below the installation site.
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- Culverts For Stream Crossings
When installing culverts (and bridges) for stream crossings, seek to maintain the original and
natural full bank capacity (cross-sectional area) of the channel. Constrictions at these points are
contributing factors in costly bridge and culvert “blow-outs” which generate a large volume of
sediment deposited directly into the stream. Align and center the culvert with the existing
stream channel whenever possible. As a minimum, align the culvert with the center of the
channel immediately downstream of the outlet. If channel excavation is required to help align
the culvert, it is frequently best to excavate the upstream channel to fit the culvert entrance and
align the outlet with the existing natural channel. Minimal disturbance of the channel at the
culvert outlet should be the priority consideration. Inasmuch as possible, the grade of culverts
should be determined by the grade of the existing channel, but usually not less than 0.5% nor
more than 1%. The outlet should discharge at the existing channel bottom. A professional
engineer, experienced in hydrology and culvert hydraulics, should be consulted for determination
of actual culvert grades when dealing with peculiar alignment or laying conditions, and upon any
deviation from normal and usual installation procedures. Keep disturbance of the channel
bottom, sides, adjacent land, and surrounding natural landscape to a minimum during
installation. Install energy dissipating structures and/or armor at the outlet where scour and
erosion are likely to occur from high exit velocity due to steep culvert installation, near proximity
to channel banks, drops at the end of the culvert, etc. (See Chapter 4 on OUTLET
STRUCTURES). Establish and maintain at least one foot of road bed cover over all culverts.
Two feet or more cover is the desired optimum.
- Culverts For Runoff Management
Where cross-drains are needed in conjunction with “turn-outs”, it is ideal to place culverts no
more than 500 feet apart along the roadway to control the volume and velocity of flow within
road ditches. Steeper road slopes may require closer spacing to discharge accumulated runoff
in excess of ditch capacity and/or to keep velocities down. Inasmuch as possible in non-stream
crossing locations, a “turn-out” (“tail-ditch”) should coincide with the outlet location of a crossdrain
culvert to “dump” transported and accumulated water from the receiving ditch. Where
private roads and driveways intersect public roadways, install culverts to maintain continuity of
flow within the ditch while allowing access across the ditch. In cases of no head wall, install
enough culvert to extend each end at least two (2) feet past the toes of the road bank slopes.
Install energy dissipating structures and/or armor at the outlet where scour and erosion are likely
to occur from high exit velocity due to steep culvert installation, near proximity to ditch banks,
drops at the end of the culvert, etc. (See Chapter 4 on OUTLET STRUCTURES). Establish and
maintain at least one foot of road bed cover over all culverts. Two feet or more cover is the
desired optimum.
Exhibit 3.1a - Culverts for Stream Crossing
Exhibit 3.1b - Culverts for Crossing Natural Drains
Exhibit 3.1 - Culverts for Crossing Natural Streams and Water Courses
Major Cross
Drains
These usually facilitate large drainage areas which may include a network of roadway ditches, field drains, etc.
and often discharge at a point in or near a natural stream or drainage.
Intermittent
Cross Drains
These usually connect road ditches on the upland side of a roadway to road ditches on the opposite side,
or convey water to discharge points on the opposite side of a roadway.
Miscellaneous
Cross Drains
These maintain continuity of flow by connecting or re-connecting road ditches which have been crossed by an
intersecting road or driveway, or simply provide for drainage under roads and driveways. Also, note the need
for scour protection at the outlets. This is a significant source of sedimentation.
Exhibit 3.2 - Runoff Management Culverts
Maintenance At Sensitive Aquatic Environment Crossings
Inasmuch as possible, maintain pre-existing conditions in streams when performing maintenance
on culverts for stream crossings. Avoid impedances to fish and other aquatic fauna by adhering
to the following:
a. Contact the state fisheries biologist or the U.S. Fish and Wildlife Service (USFWS) for
assistance.
b. Do not substantially alter water velocities. Especially do not create excessive velocities.
Keep in-pipe velocities within those navigable by fish.
c. Do not create vertical barriers.
d. Do not create adverse water depths. Keep in-pipe flow depths comparable to those of the
associated stream channel.
e. Do not create flows outside the range of flows normally encountered throughout the year,
or at least those flows which may negatively impact the aquatic life in the stream.
f. Make sure the culvert design accommodates the size and species of fish passing through
it.
g. Provide resting pools at culvert inlet and outlet for culverts installed across streams with
high channel gradients.
h. Use corrugated culverts to decrease water velocities through the culvert and supply
resting areas for migrating fish.
i. Use bridges, bottomless arches, partially buried culverts, or other similar structures in
areas where fish passage and specie habitat is an important consideration.
j. At stream crossings, select a culvert site where there will be no abrupt change in gradient
and the upstream and downstream channel alignments are as straight as possible for 50
feet in either direction.
k. Consider maximum design flows which will not create adverse stream conditions.
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FISH FRIENDLY DESIGNS
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Figure 3-1. Examples of Fish Friendly Designs
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Culvert Installation/Replacement
In live (flowing) streams install sandbags, silt fences, earthen dikes, or other appropriate
measures to inhibit flow when possible. Use a pump to convey water around the
excavation/work site. Discharge pumped water onto a stable outlet to prevent scour. With live
stream flows which cannot be impounded, divert the flow to one side of the culvert alignment.
Enough room should be allowed to properly excavate the entire pipe trench and bed the entire
culvert. Minimize disturbance of the surrounding soil and vegetation.
Excavate trench side slopes on a safe grade to prevent caving. Inasmuch as possible, the bottom
of the trench should be at least twice the width of the culvert to be installed and graded as near
to designed culvert grade as possible.
One method of properly installing a culvert is to start at the outlet end, lay the culvert up-slope,
properly bedding each joint as installation proceeds. The first section or “joint” is critical, and
special attention should be given to proper installation, grade, and alignment to reduce the
potential for scour and erosion from water discharge, and, to ensure the whole culvert is aligned
properly. Pipe joints should be wrapped 1-1/2 times around with geotextile filter fabric. The
fabric should extend at least two feet either side of the joint or edges of the connecting band if
one is used. This is especially true with concrete or other types of pipes which may not have
rubber or mastic seals at the connecting joints. Once the culvert is installed and secured in place,
divert the flow through the culvert and commence filling the by-pass channel, if any, and
complete the backfilling around the culvert. Backfill around and over the culvert should be
placed evenly and level in maximum 12" loose lifts and thoroughly compacted before adding
successive lifts. Scarify (roughen) the top two inches of compacted surfaces which have a slick,
smooth, or glossy finish after compaction. Six (6) inch loose lifts should be used below the
midpoint of the culvert. Do not use or operate machinery closer than two feet of the culvert.
Mulch and vegetate all disturbed areas. Use silt fences or other appropriate erosion control
measures to prevent or reduce erosion and sedimentation until stabilizing vegetation is
established.
Head Walls (Headers)
Head walls may be used when hydraulic capacity needs to be increased, and/or when installing
a head wall will be more efficient than culvert replacement. Head walls should be flush with the
end of the culvert. Head wall “wings” (extensions) help mold and direct channel flow into the
culvert and protect the area around the inlet from scour. Head walls may be of poured concrete,
bagged concrete, concrete blocks, bricks, logs, cut wood, or may be shaped loose rock riprap, etc.
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Cleaning and Maintenance
One method to account for all culverts is to maintain an inventory of culverts and under-drains
and use a checklist from this inventory to account for culverts during inspections. Inspect
culverts often, especially in the spring and autumn, and after storm events, checking them for
signs of corrosion, joint separation, bottom sag, pipe blockage, piping, fill settling, cavitation of
fill (sinkhole), sediment buildup within the culvert, effectiveness of the present inlet/outlet
inverts, etc. Check inlet and outlet channels for signs of scour, degradation, agradation, debris,
channel blockage, diversion of flow, bank and other erosion, flooding, etc.
Practice preventive maintenance to avoid clogging of pipes and other situations which may
damage the culvert or diminish its design function. If a culvert is plugged with sediment, flush
it from the outlet end with a high pressure water hose. Take measures to reduce downstream
sedimentation and clean debris and sediment from the outlet ditch afterwards.
When replacing damaged culverts which handle the flow adequately, use the same size, shape,
and type of pipe. Changing any of these criteria may adversely effect the established stability
of the ditch, stream, and/or roadway.
Culvert head walls come in many shapes, sizes, and materials.
Head walls can stabilize culvert outlets and entrances and improve flow efficiency at inlets.
Exhibit 3.3 - Head Walls (Headers)

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