StormSewer
Free, open-source storm sewer design: Rational, Manning, HGL, HEC-22
...HTML, CSV, LandXML and DXF export too.
The methods are public domain and the implementation is readable. Every number is worked by hand in VALIDATION.md, and the same network was run through Autodesk Hydraflow Storm Sewers, every difference explained. Compared, not certified.
Windows, macOS, Linux. Written by a practicing water resources PE. GPL-3.0.