Culvert Hydraulic Analysis and HY-8

The Federal Highway Administration's HY-8 software automates culvert hydraulic computations for culvert analysis and design.

HY-8 hydraulic computations

The HY-8 User Manual is included with the software installation and is accessible through the Help button on the top toolbar. FHWA describes the manual as providing information on the program's technical features, capabilities and limitations.

March 2025 corrections

FHWA's HY-8 release notes give Version 8.0.1.2 a build date of March 5, 2025.

• Roadway overtopping. An error related to the first roadway station caused overtopping calculations to fail. FHWA reports that the error was fixed in this version.
• Decreasing station. Water Surface Profile Data for culverts with decreasing station are computed correctly in this release.
• High headwater. The release corrects the length of the headwater line in high-headwater plots.
• User-defined culverts. Front-view plots normalize user-defined culverts by the invert. FHWA explicitly states that the calculations did not need to be changed for this plotting correction.

Calculation responsibility

FHWA places responsibility for application of the software with the user. Its guidance calls for the responsible designer to understand the potential accuracy limitations of the program results.

The designer must independently cross-check the results using other methods and examine their reasonableness using engineering knowledge and experience.