Introduction to the Design of Culvert Fishways

Fish passage through culverts is an important consideration in fish bearing streams. Just as adequate culvert capacity is required for safety, adequate provision for fish passage is required to maintain healthy fish populations. The main problem with improperly designed and installed culverts is that they form velocity barriers to fish migrants at the outlet, inlet or through the culvert barrel. If water depths are too low or water velocities at any of these three culvert locations exceed fish swimming ability, fish may be prevented from reaching their spawning grounds. Since hydraulic efficiency and optimum fish passage requirements are mutually exclusive objectives, compromises must be effected that permit adequate fish protection with maximum economy. Such compromises involve the matching of water velocities with fish swimming performance at design discharges that allow limited delay in fish migrations. Migrating fish must negotiate the culvert outlet, the culvert barrel, and the culvert inlet before successfully passing upstream. Hydraulic conditions, such as water velocities and depths, at each one of these three locations must be suitable for passage at the highest and lowest stream flows expected during fish migration. Fish need to swim continuously for the entire culvert length when no resting opportunities are available. Culvert length and velocities, as well as maximum distance that fish are able to swim, determine whether fish can pass through a culvert once they enter it