In this program, we recommend the
Simulation Design Method because it provides maximum benefits
to stream habitat and is a practical and effective method to design and
build stable road crossings.
installed using the Stream Simulation Design Method include
natural stream substrate bottoms
that will maintain habitat values and provide for passage of all fish.
Preferred Alternatives And Crossings
Before design work begins, the
following alternatives and structure types
should be considered. In order of preference they are:
1. No Action: Is this stream crossing really necessary? Can the road be
realigned to avoid crossing the stream?
2. Bridge: Spanning the entire stream is best for the stream environment,
but is it practical?
3. Streambed Simulation Strategies: Bottomless arch or embedded
At some sites, it may be impractical or cost prohibitive to employ
Stream Simulation Design Method.
In those cases other techniques may be
required such as:
This is often referred as a hydraulic design, associated with more
traditional culvert design approaches and is limited to low slopes for
fish passage. Non-embedded culverts must be set deep enough into
the stream bed (at least 10% of their diameter) to provide adequate
water depth for fish to swim and to keep flow velocities down.
The culvert in this photo
is set too high, resulting in excessive flow velocity and depth too
shallow for fish to swim.
stream substrates like gravel, rock, sand and woody debris, provide
habitat features that support plant and animal stream life; including
shelter, food and spawning areas. These substrate types are just
as valuable when contained within culverts.
Baffled Culvert, or Structure Designed with a Fishway:
Used for steeper slopes and not usually recommended for new
installations, this technique may be used to retrofit existing
When first installed, this box culvert was a velocity barrier to fish
passage on an important brook trout, steelhead and coho salmon spawning
stream. The recent installation of a series of jumping pools, (essentially
a fish ladder-on the left of the picture) now provides fish passage to 4 miles of quality
habitat upstream. The cost of this retrofit fish passage project was about $35,000. If the
original installation had been a bottomless culvert, the extra cost of the
retrofit would not have been necessary and habitat values at the site would
for Stream Crossings:
types of structures are generally used for stream crossings, they are:
1. Span Bridges:
Span bridges are recommended for sites on
streams that are more than 20 feet in width (bankfull stage width
measurement) or with slopes more than 3%, or where
important spawning habitat is present.
This inexpensive, prefabricated bridge is a good
solution for low traffic
(one lane) crossing and maintains high
quality fish habitat in the trout stream it spans.
For sites on streams that are less than 20 feet
in width (bankfull
stage width measurement) or with slopes less than 6%, and where important
spawning habitat is not present, culverts may be considered. Four
general types of culverts may be used; round, ellipse, box or bottomless
arch. All are readily available from manufacturers in a wide range of
sizes and lengths.
A. Bottomless Arch
arch culvert installations are an excellent choice
from an environmental and fisheries viewpoint.
typical design for installation of a pre-cast concrete, bottomless arch
culvert is shown in this line drawing. It is mounted on footings
(poured-in-place) that extend below the scour line. Like a span
bridge, they allow for natural stream channel processes that will
maintain favorable habitat and fish passage under the structure.
Bottomless arch culvert installations also have the low profile
advantage of an ellipse type culvert. Bottomless arch structures
may be used at sites with slopes ranging fro 0% to 6%, but bottom
materials inside the culvert should include boulders large enough to
withstand current flows. On sites with slopes from 3% to 6% this
means utilizing D90* size rock (D90 size material refers to the largest 10% of
naturally occurring boulders in the stream). In some installations special preparations (like rock
placement) are needed to address possible scour erosion of the stream bed that
could undermine the culvert footing.
this photo, the flange footing type, bottomless arch culvert is being
lowered onto the prepared streambed. Note the placement of D90
(or larger) boulders
in the channel to provide resting places for migrating fish.
The term D90
refers to the size of the boulders in the vicinity
of the crossing.
The size of a D90 boulder is equal to the average of the largest 10%
of the boulders in the stream bed.
corrugated metal, bottomless arch culvert is provided with a metal
flange footing and rests on a cement base. It is a highly
functional installation, does a great job of maintaining fish habitat
and is competitive in price with a round culvert installation.
B. Round Culverts:
culverts of corrugated metal, pre-cast concrete or plastic are by far
the most commonly used structures at road crossings in the United
States. They are suitable for use where slopes range from 0% to
3%. At slopes > 3%, it is very difficult to maintain natural substrates
on the metal or concrete bottom.
When properly installed and embedded, they can
be both fish friendly and the least expensive option.
Unfortunately, many existing culverts are not properly sized or
installed correctly and cause damage to stream habitats, fish and other
These round culverts
are set too high or “perched”
and makes for a difficult jump, even for mature fish. Assuming
that fish could leap into these culverts, excessive flow
velocities will stop their upstream spawning migration
The best design of a round culvert will include
materials in the bottom of the culvert that help to minimize and alter
water flows, thus allowing for easy fish passage.
C. Box Culverts:
Box culverts are commonly used where traffic
loads or higher fill levels place heavy stresses on the structure.
They are suitable for use where slopes range from 0% to 3%. At slopes >
3%, it is very difficult to maintain natural substrates on the metal or
concrete bottom. They are usually made of concrete and may
be purchased as pre-fabricated units of various lengths, or poured in
This poured-in-place box culvert has a number of
problems: A vertical jump, water too shallow for fish to swim, and at
higher flow levels, it may generate water velocities that act as a fish
barrier. Replacing this structure under a major highway would be an
extremely expensive project. However, retrofitting the structure with
current deflectors and by raising the level of the plunge pool may be
possible to enhance the fish passage function of this culvert.
D. Ellipse or “Squashed
Ellipse culverts are a viable choice where a
lower profile (and less fill cover) is required. They are suitable
for use where slopes range from 0% to 3%. At slopes > 3%, it is very
difficult to maintain natural substrates on the metal or concrete
bottom. This ellipse culvert handles water flows well, but
should have been sized larger and set lower (embedded) in the stream
to allow natural stream bed materials to line the bottom and provide
of the culvert types described above have advantages and disadvantages
in terms of cost,
ease of installation and its effects on stream habitat
and fish passage. Choice of structure type is an important
consideration, but the placement and installation are just as critical
for the success of the project.