HEC-RAS

HEC-RAS (River Analysis System) is the US Army Corps of Engineers' hydraulic model and, by adoption, the global default. It computes steady and unsteady flow in 1D, 2D (full shallow-water or diffusion wave on flexible meshes) and coupled 1D-2D; bridge, culvert, weir and gate hydraulics; bridge scour computations; sediment transport and mobile-bed morphology; and basic water quality. RAS Mapper handles terrain, geometry and inundation mapping natively — no GIS licence required.

It is hard to overstate the ecosystem advantage: reviewer familiarity everywhere, training everywhere, and a fast-growing Python tooling scene for automation. If a project involves river hydraulics and there is no constraint forcing something else, HEC-RAS is the default for a reason.