Engineers concentrated on passing flood flows without subjecting the highway to unreasonable risks. Biologists wanted upstream migrating fish passed through the drainage structures during critical times in the fishes' lifecycle. These objectives conflicted when the resource agencies recommended installing bridges instead of cheaper culverts. The cost difference between installing culverts and bridges was substantial. Arctic grayling, Thymallus arcticus Solution Properly designed culverts don't produce water velocities that exceed fish swimming abilities.
Fish have two different musculature systems for swimming.
A white muscle system generates power for short, vigorous swimming. A red muscle system furnishes power for long, sustained swimming. The culvert design must account for both swimming modes. Therefore, the engineer must know the hydraulic conditions where the fish swims. These conditions change throughout the culvert.