Hydraulic Design of Highway Culverts - DOT On-Line Publications

Figure VI-7--Fish Baffles in Culvert
Early in the planning process, fish migration data should be collected including pertinent field
data. If the stream crossing is located on a known, suspected, or potential fish migration route,
the following data are desirable: (38)
• Species of migrating fish.
• Size and swimming speed of fish.
• Locations of spawning beds, rearing habitat, and food-producing areas upstream
and downstream of the site.
• Description of fish habitat at the proposed crossing.
• Dates of start, peak, and end of migration.
• Average flow depths during periods of migration.
An understanding of some design inadequacies which will inhibit natural migration patterns is
desirable. Excessive velocities and shallow depths in the culvert or on paved aprons for the
migration design discharge should be avoided. High outlet elevations, often resulting from the
formation of a scour hole, may prevent fish from entering the culvert. High outlet velocities also
dislodge sediment which fills in small pools further downstream, smothering eggs and foodproducing
areas in the process. High upstream invert elevations produce a large unnatural pool
above the culvert which will trap sediment. Depressing the upstream invert elevation is also
harmful.
Simulating the natural stream bottom conditions in a culvert is the most desirable design option
to accommodate fish passage. Open bottom culverts, such as arches, have obvious
advantages if adequate foundation support exists for the culvert. Oversized depressed culverts
have the advantage of a natural bottom while overcoming the problem of poor foundation
material (Figure VI-8). However, on steep slopes, provisions may be necessary to hold bottom
material in place. Another option is to construct baffles in the bottom of culverts to help simulate
natural conditions. Figure VI-9 depicts a baffle arrangement used by several States in the
Pacific Northwest (30).
When the simulation of natural stream bottom conditions is unrealistic or unnecessary, criteria
for maintaining minimum depths and maximum velocities is most important. The high
roughness coefficient of corrugated metal may be all that is required at some locations to
maintain desirable depths and velocities. When maintaining a minimum depth in a culvert is a
problem, downstream weirs can be constructed. However, provisions must be made for fish to
bypass the weirs.
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