ORD Stormwater Management Model (aka "SWMM")
This is the open source SWMM source code repository maintained by the Open Water Analytics group.
SWMM is a dynamic hydrology-hydraulic water quality simulation model. It is used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas. SWMM source code is written in the C Programming Language and released in the Public Domain.