A Brief Summary in the Basic Steps for Choosing, Sizing
and Installing Culverts at Stream Crossings
Please use this guide only as a quick reference to the material presented.
Refer to the documentation presented throughout this web site for more detailed information.
1. We recommend a
road crossing design process called the Stream Simulation Design Method.
2. Generally, when flow
velocities through a culvert exceed .6 meters (or 2.0 feet) per
second, some sizes and species of fish will be blocked.
3. Measure bankfull width of the stream
(average of 10 measurements).
4. Establishing the elevations of the
culvert inlet and outlet are critical elements in the installation using
the Stream Simulation Method.
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 images below.
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%.
Determining the Bankfull Stage is the most important measurement for
establishing culvert size in handling peak stream flows (refer to image).
6. If the Bankfull Width is more than
20 feet or if stream slope is more than 6%, use a span
If the Bankfull Width is less than 20 feet and stream slope is less
than 6%, a span bridge may be used or one of the four culvert types may
8. If slope is 3% to 6%, a span bridge is
preferred but a bottomless
arch culvert may also be used.
9. If slope is from 0% to 3%, bottomless
arch, round, box or ellipse culverts may be used. If using a round
culvert, the best design
will include natural stream materials in the bottom of the culvert that help to
minimize and alter water flows, thus allowing for easy fish passage.
10. Choice of the structure type is an important consideration, but
the placement and installation are just as critical for the success of
11. For all culverts, the width (D) of
the opening will be the Bankfull Width of the stream, plus 1 foot.
12. Length of the culvert will be the total
road width plus 2 times (fill height X slope).
13. Survey and establish the desired height of the culvert inlet and
14. Culvert should be embedded to
correct for elevations at inlet and outlet.
15. Stream bank erosion and stream
sedimentation at culvert crossing can be a major problem for fish
habitat if the appropriate control measures are not adopted.
16. Soil erosion
problems at fish crossings can be minimized if not completely
eliminated, with advance planning. The 1st step is to protect the work area long before the heavy construction begins.
17. Another very important consideration is the potential for
geomorphic effects, in particular, channel incision. Channel incision or
down-cutting of the streambed tends to propagate upstream by means of
migration of a headcut. Often, this headcut migration is arrested when
it encounters a culvert. If the culvert is replaced, the headcut may
then proceed onward upstream, spreading channel incision, and inducing a
period of channel destabilization and habitat degradation.
Click here to review
the document (pdf) Geomorphologic Impacts of Culvert Replacement and
Removal: Avoiding Channel Incision.
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November 19, 2008