StormSewer
Free, open-source storm sewer design: Rational, Manning, HGL, HEC-22
StormSewer is a free, open-source desktop program for gravity storm-drain design and analysis. Draw the network on a scaled aerial or import it from a Hydraflow / Civil 3D Storm Sewers .stm file, LandXML or DXF, then run the Rational method, Manning capacity for circular, box, elliptical and arch conduits, and a standard-step HGL/EGL backwater pass with junction losses and tailwater.