"Properly sized and placed culverts are important to accommodate water velocities and flows necessary for fish to swim through the culvert. Selecting culvert diameters to match the width of the stream at an average point is a basic first step.While culverts only cover small sections of any stream, their influence on fish and other aquatic species can be immense, especially when you consider the cumulative effects of many road crossings. Proper planning, design, and installation can protect roads while providing year-round fish passage and preserving healthy streams." - Fish Friendly Culverts by UW-Extensions
In addition to the standard traffic and hydraulic related criteria requisite to culvert selection and installation, the following additional criteria were followed to the extent practical:
• Crossing flow velocities passable to native fish species (i.e., generally two feet per second or less or as demonstrated by FishXing or another accepted method) must be achieved by each proposed design for all stream discharges conducive to fish passage within Program streams.
• Inlet and outlet perching and excessively shallow flows in the structure must be eliminated.
• Additional design considerations should include vehicular transport, sediment transport, riparian corridor continuity for herptiles and other wildlife, site topography, utility conflicts, safety, aesthetics, and other attendant issues.
• Identify and regrade any artificially maintained stream channel slopes immediately upstream and downstream of the stream crossing.
• All impediments necessitating leaping behavior will be removed. No cascading water conditions of any kind may exist.
• Water velocities must remain below two feet per second for structures exceeding 20 feet in length using flow rates typical of spring runoff.
• No structures should attain water velocities greater than four feet per second for any length and for a full array of expected flow events.
• In general, structures will either be installed well below bed grade, backfilled with coarse substrate, and have a slope of less than 1%.
• Deposits of natural or man-made debris will be removed and actions taken to help assure that debris does not accumulate in a short period of time.
• Care will be taken to assure adequate depth of flow during typical flow conditions. These actions include shaping and sizing structures adequately and assuring that water does not infiltrate bedding material thereby bypassing the water-filled portion of the structure.
• Hydraulic and hydrologic studies must demonstrate either no change or a decrease to the flood elevation as well as acceptable flow velocities through the proposed structure.
• Strong preference will be given to clear-span bridges or buried or open-bottom, single-barrel designs and structures infilled with appropriately-sized porous stone material.
Types of Culverts
Corrugated Aluminum Box Culvert with Headwalls and Wingwalls