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Conditions Where Applicable
Culverts are the most commonly used method for providing access over a
watercourse, and particularly for small and medium sized streams. Several types
of culverts are used including; open bottom/bottomless arch, pipe arch, box, and
circular/cylindrical. Box type culverts are generally made from wood or concrete
while other types are made from plastic, concrete or, most commonly, corrugated
steel. Figure 1 identifies various culvert shapes.
Figure 1 Culvert Shapes
- Sufficient depth of flow and appropriate water velocities for fish passage
should be provided in culvert installations.
- Culvert size should be based on the capacity to handle peak flows. It may
be necessary to have a hydrologic and hydraulic analysis performed in order
to determine the correct size of the culvert to be used. The hydrologic
analysis is used to determine the peak flow and the hydraulic analysis is
used to calculate the capacity of the culvert to adequately pass the peak
The type of culvert selected and installed should minimize potential
impacts on fish habitat, maintain fish passage, and sufficiently accommodate
watercourse flows. To the extent possible, natural stream conditions (i.e.
widths, habitat, etc.) should be maintained. Figure 2 illustrates some
common terms associated with culvert crossings.
Figure 2 General Culvert Terms
- Natural bottom substrate and hydraulic capacity of watercourses are best
maintained using open bottom/bottomless arch culverts; these are the
preferred type of culvert crossings.
- Footings for open bottom culverts should be installed outside the normal
wetted perimeter of the watercourse and tied into the bedrock or
sufficiently stabilized to prevent erosion around the footing or
- For installation of cylindrical culverts in fish bearing streams, a
minimum culvert diameter of 1000mm should be provided and designed/sized
according to site specific considerations.
- Cylindrical culverts should be installed to simulate open bottom or pipe
arch culverts. Culverts up to 2000mm in diameter should be countersunk a
depth of 300mm below the streambed elevation. Culverts with diameters
exceeding 2000mm should be countersunk a minimum of 15% of the diameter
below the streambed elevation. Note: Countersinking reduces the hydraulic
capacity of the culvert, therefore the required diameter of the culvert must
be adjusted accordingly (Figure 3).
Figure 3 Countersunk Culvert
- Culverts should be aligned parallel to the existing natural channel and
located on a straight stream section of uniform gradient.
- The culvert should be placed on firm ground and be countersunk to the
appropriate depth. In sites where soft foundations are present the
unsuitable material should be removed and replaced by clean granular
material to prevent the culvert from sagging. Water movement under or around
a culvert installation should be prevented through the use of headwalls, or
other means, as necessary.
- A culvert should extend beyond the upstream and downstream toe of the fill
(e.g. a minimum of 300mm, see Figure 7).
- For multiple culvert installations the culvert intended to provide fish
passage should be placed in the deepest part of the channel and be
countersunk to the required depth. The remaining culvert(s) should be placed
a minimum of 300 mm above the invert of the fish passage culvert. (Figure
Figure 4 Multiple Culvert Installation
Figure 5 Perched Culvert Entrance
- A minimum water depth of 200 mm should be provided throughout the culvert
length. To maintain this water depth at low flow periods an
entrance/downstream pool can be constructed. In some cases, an upstream pool
may also be necessary.
- The invert of the pool outlet should be at an elevation that maintains a
minimum of 200 mm of water depth up to the inlet or upstream end of the
culvert (Figure 6).
- The culvert slope should follow the existing streambed slope where
possible. Excessive culvert slope, reduced culvert capacity due to
countersinking and maintenance of the 200 mm minimum depth of flow, and back
watering due to the creation of an outlet pool should be considered when
selecting the required culvert diameter to allow fish passage and pass peak
- Pools should be designed so that there is a smooth transition of flow from
the culvert to the natural stream width.
Figure 6 Outlet Pool
- The natural streambed elevation should be used
as the pool outlet invert; however, depending on site specific conditions, a
pool outlet may need to be constructed. It is essential that the invert
elevation of the pool outlet be stable and, if necessary, well maintained to
ensure a minimum water level in the culvert. Clean non-erodible riprap or
gabions should be used to stabilize the pool. The pool outlet may need to be
v-notched to enable fish passage at low flow periods. More than one pool may
- Pools should be pear shaped and sized such
that: pool length = 2 to 4 times culvert diameter; pool depth = 0.5 times
the culvert diameter, 1 meter minimum. (Figure 7). The culvert diameter
referred to in the above is that of the fish passage culvert.
Figure 7 Pool Sizing
- For stacked/multiple culverts, pools should be installed with the fish
passage culvert oriented to the center of the pool to allow for a smooth
transition of water from the culvert to the watercourse.
- Depending on site-specific conditions (e.g. steep slopes, long crossings,
constricted streams resulting in high water velocities, etc.), baffles/weirs
may need to be installed in the fish passage culvert. Baffles/weirs can
provide an adequate depth of flow and reduce the water velocity in the
culvert in order to facilitate fish passage. Baffle dimensions should be
provided as per Figure 8.
Figure 8 Baffle Sizing
- A minimum depth of flow of 200 mm should be provided throughout the
culvert and baffled sections. The drops between adjacent baffles should be a
maximum of 200 mm
- Baffles should be placed approximately 1 meter from the inlet and outlet
ends of the culvert, the next baffles should be 1/2 the baffle spacing. The
remaining baffle spacing should be determined by using the low flow (flow at
the time of fish migration, i.e. lesser of flow at 90% exceedance via flow
duration analysis or the 7 day, 10 year low flow) as a basis for meeting the
above depth of flow and drop between baffles criteria. Baffle spacing should
also provide a pool volume large enough to dissipate the kinetic energy
produced by the water falling over the weir; and consider high flows (i.e.
10% exceedance based on flow duration) during the fish migration period.
Baffle spacing is illustrated in Figure 9.
- The invert elevation of the outlet pool should be set to back water up to
the top of the outlet baffle.
- The upstream culvert invert, in some site specific situations, can be
countersunk to facilitate depth of flow provided that the head differential
is accounted for.
Figure 9 Culvert Baffle Spacing Requirements
Culvert installations should be suitably stabilized to prevent erosion,
seepage, and undermining and maintained in good repair and operating condition.
Modifications of the above criteria/guidance in consultation with the
Department of Fisheries and Oceans may be required to address the passage of
fish species other than salmon, brook trout, and brown trout in culvert
This factsheet concerning culvert installations is generic and has been
developed to apply to a variety of different circumstances. Some site specific
situations may warrant modification of the above guidance, as deemed appropriate
and in consultation with the appropriate Area Habitat Biologist. In some site
specific situations, a professional engineer and/or biologist should be
This Fact Sheet does not constitute DFO approval; other mitigative
strategies may be required.
The proponent is advised to contact all other appropriate regulatory agencies.
For more information contact the nearest
Department of Fisheries and Oceans office.