As mentioned previously, HEC-RAS solves the Diffusion-Wave Equation (DWE) and Shallow-Water Equations (SWE). There are two methods for solving the SWE in HEC-RAS: the Eulerian-Lagrangian Method (ELM-SWE), and Eulerian Method (EM-SWE). The solvers are similar except in how they treat the acceleration and pressure gradient terms. This section described in detail the numerical methods applied. In general, all three solvers use a combination of finite-difference and finite-volume methods on an unstructured polygonal mesh with subgrid bathymetry.