Protection of fish habitat is necessary for stream crossings where fisheries exist. The choice of crossing location is important in terms of both sedimentation effects and fish passage. For fish passage, preferred locations are those that do not cause large increases in velocity and have no abrupt changes in gradient or alignment of the channel. Sections of a stream with uniform alignment, good bank stability, and uniform gentle gradients are easier to cross with provisions for fish passage.

Figure 20. The three common types of metal culverts are classified by shape: (A) corrugated round, (B) corrugated pipe-arch, and (C) structural plate-arch. (Redrawn from Furniss et al. 1991, p. 316)

Road crossings can be barriers to fish migration, usually because of outfall barriers, excessive water velocity, and insufficient water depth in culverts (Yee and Roelofs 1980). Determine the type and extent of fish habitat before selecting drainage structure design. The incorporation of fish-passage facilities at stream crossings should be based on assessments of the life-cycle requirements of fish species, of habitat quality, and of the accessibility of sites to fish. Natural barriers downstream or immediately upstream from the site may eliminate the need to provide fish-passage facilities. Usually, a fisheries biologist must be consulted to assess the habitat. Contact your State or local fisheries biologist to determine the specific needs of the fish in your streams. Fords are sometimes used for low-water crossings where transportation requirements are seasonal and stream channel and slope configurations are suitable. Fords with concrete sills or grade-control structures can be barriers during low-flow conditions, but they can usually be mitigated in the design. Fords are preferable to culverts for fish passage because high-flow migration is unimpaired and low-water migration is easy to accommodate. See the section on Stream Crossing Methods for more information about using fords. Bridges and arch culverts are preferred for crossing streams with migratory fish. Bridges and arch culverts are beyond the scope of this guide, and appropriate assistance should be sought in designing and constructing them. Bridges are preferred because they usually cause less modification of the stream than do culverts, and are often the best way to ensure fish passage. Culverts are by far the most common type of crossing device and the most likely to cause barriers to fish migration. This section covers common metal culverts and avoiding barriers to fish passage. The three types of metal culverts commonly used are classified by shape (Figure 20): (A) standard corrugated-round, (B) standard corrugated pipe-arch, or (C) structural plate-arch. The first two may be prefabricated, as is usual for the smaller sizes up to 4.5 feet in diameter, or they may be of multiple design. Structural plate-arch culverts are always of multiple design because they are so large, and usually are fabricated on site.

Although the standard corrugated-round culvert (Figure 20A) is the type most commonly used, it is the least desirable for fish passage. The width constriction from stream channel to culvert is usually severe, and the gradient of the pipe should be less than 1 percent to keep water velocities within an acceptable range for fish passage. This type of culvert is also the most likely to be installed with its outlet end elevated above the tailwater level, producing an outfall barrier (Figure 21). Elevated outfalls must be avoided or mitigated. See Culvert Outfall Barriers for mitigation measures.

Figure 21. Improperly installed culverts can block fish passage: (A) water velocity too great, (B) water in culvert too shallow, (C) no resting pool below culvert, and (D) jump too high. (Redrawn from Evans and Johnston 1980)

The pipe-arch culvert (Figure 20B) is less desirable than the structural plate-arch, but can usually be installed to allow fish passage. Fabricated in smaller sizes, the pipe-arch culvert can be used in smaller, lower fills where structural steel arches would not fit. Wherever a pipe arch is used, the gradient must be kept below 1 percent to minimize water velocities. During periods of low flow, the water in this type of culvert can be spread so thinly across the bottom that fish passage is impossible. Baffles may be needed to increase the flow depth through the pipe arch. The structural plate-arch set in concrete footing (Figure 20C) is the most desirable culvert type for fish passage because the natural streambed is left mostly unchanged. Little narrowing of the flow occurs at either end of the culvert, and there is no significant change in water velocity. Where concrete footings are not practical, split, wide-flanged, buried steel footings have been used successfully. Many fisheries biologists believe the structural plate-arch is the only acceptable culvert type where fish passage is required (Furniss et al. 1991). Culvert crossings have been installed in thousands of streams with little or no thought to their effects on fish populations. A single, poorly installed culvert can eliminate the fish population of an entire stream system.

The following are some important considerations for culvert installation (Furniss et al. 1991, Yee and Roelofs 1980):
	The two most important considerations for fish passage through culverts are maximum acceptable water depth for the migrating species, and outfall.
	The culvert should be placed at or below the original streambed elevation, and water depth and velocity at low and high flows should be integrated into the design.
	Control scouring at culvert outlets with energy dissipators such as heavy rock riprap consistent with fish passage considerations.
	At stream crossings, avoid channel width changes and protect embankments with riprap.
	Align culverts with the natural course and gradient of the stream.
	Locate valley-bottom roads to provide a buffer strip of natural vegetation between the road and stream. For recommended buffer widths, see Table 1.
	Select periods of low flows for construction to limit disturbance.
	Design and construct a stream crossing so that if the culvert should fail, the stream flow will not be diverted out of the original channel (Figure 16).
	Ensure erosion-control measures are completed before the wet season in your area.
	Locate fuel storage areas away from the stream. Construct dikes to contain the largest possible spill.
	If gravel removal operations are permitted in the streams, coordinate the removal with a fisheries biologist who can give beneficial information to protect your fisheries.
	The diameter of culverts must be adequate to allow maximum flows and the expected debris to pass. Washing out of culverts and their earth fills damages the road and is a source of sedimentation. Channel bank stability upstream and downstream of culverts should be provided for. Road crossings alter the hydraulics of streams above and below the crossings for considerable distances, sometimes making streambanks more susceptible to erosion. Severe erosion can alter the configuration of the stream and crossing, and can eliminate the design components that provide for fish passage.
	A single large culvert is better than several small ones because it is less likely to become plugged, and carries water at low velocities.
	Where culverts are installed in stream sections with steep gradients, it is important to create or improve resting pools, cover, and bank protection along the stream above and below the culverts. Maintaining a stable stream bottom through the culvert-influenced area is essential.

Culverts can be insurmountable barriers to migrating fish when the outlet of the culvert is so far above the tailwater that fish cannot enter the pipe. This condition is an outfall barrier (Figure 21). When new culverts are to be installed on streams with migrating fish, every attempt should be made to avoid constructing outfall barriers. Putting a new culvert outlet below the tailwater elevation is sometimes not possible, and many existing culverts form outfall barriers. One way to correct an existing outfall barrier is to provide for one or a series of low-end dams below the culvert outfall. These dams may be nothing more than hand-placed rock “reefs,” wirebasket gabions filled with local rock, or concrete sills. These downstream dams raise the tailwater elevation and flood the culvert. Access by fish is not only enhanced, but water velocity in the culvert is decreased. The downstream dams should not create outfall barriers. In some streams, the range of flows is so great that it is impossible not to have the culvert outlet above the tailwater at some time. Also where severe fluctuations in flow require large culverts, fish passage may be impeded during low flows because of shallow flow over the broad culvert bottom. In such cases, stacked- or multiple-culvert installations can be used to provide fish passage. Placing the stacked culverts at different elevations ensures adequate discharge capacity as well as fish passage over a wider range of flows. Trash racks are detrimental to fish passage. The storms that often bring debris downstream are those in which many fish can move up to spawning areas. Although the protected culvert may not be a velocity or outfall barrier, a debris-laden trash rack can be impassable to fish. Therefore, debris-catching structures should be avoided on streams used by migrating fish, and crossings should be large enough to transmit debris downstream. However, increasing the culvert diameter may be impractical because of the increased cost. To compensate for the absence of culvert protection from avoiding debris-catching structures, the culvert should be large enough to allow debris to pass through it. Passing debris through the culvert is a valid alternative to intercepting debris above the culvert inlet, and should not be overlooked. Roads can have substantial adverse effects on fisheries. These effects can be greatly reduced if the protection of fish habitats is integrated into the planning, design, construction, and maintenance of roads.

Return to "A Landowner's Guide to Building Forest Access Roads" Table of Contents