HY-8

HY-8 is the Federal Highway Administration's culvert hydraulics program and the closest thing culvert design has to a canonical tool. It implements the HDS-5 methodology — inlet and outlet control, performance curves, roadway overtopping, multiple barrels and improved inlets — plus energy-dissipator design from HEC-14 and basic aquatic-organism-passage checks.

Every US DOT accepts HY-8 output, and because HDS-5 nomographs underlie culvert practice in much of the world, it travels well beyond the US. For batch or scripted culvert work, the same methods appear inside HEC-RAS and various commercial drainage suites, but HY-8 remains the free reference.