Non-embedded Culvert: 

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. 

Natural 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 structures.

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 be better.

Structure Options for Stream Crossings:

Two 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.

2.  Culverts:

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 Culverts:

Bottomless arch culvert installations are an excellent choice
from an environmental and fisheries viewpoint. 

The 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. 

In 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.

This 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:

Round 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 stream organisms.

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 here.

  The best design of a round culvert 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.

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 place. 

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 Arch” Culverts.

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 fish habitat. 

Each 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.