Delft3D is described as 'This webportal facilitates the Delft3D Open Source Community to come together to share knowledge, brainstorm on new features and build working relationships'. There are eight alternatives to Delft3D for Windows, Linux and Wine. The best alternative is Storm Water Management Model, which is both free and Open Source. Other great apps like Delft3D are HEC-RAS (Free), MIKE 11 (Paid), GeoHECRAS (Paid) and SOBEK Suite (Paid).
GeoHECRAS speeds up HEC-RAS model creation using AutoCAD and GIS data sources. Work directly off of Google Maps, viewing and editing the model in 2D and 3D. Automated cross section cutting reduces time in building a model.
SOBEK is a powerful modelling suite for flood forecasting, optimisation of drainage systems, control of irrigation systems, sewer overflow design, river morphology, salt intrusion and surface water quality.
MIKE 21 is the ultimate Swiss army knife of coastal modelling. If you need to simulate physical, chemical or biological processes in coastal or marine areas, MIKE 21 almost certainly has the tool you need.