The Corp's Hydrologic Engineering Center River Analysis System (HEC-RAS) is designed to simulate one-dimensional (1D) steady, unsteady flow and sediment transport. The HEC-RAS release 5.0 and newer also simulates unsteady two-dimensional horizontal (2DH) flow by solving either the diffusion-wave equation or non-conservative shallow water equations using an implicit Finite-Volume and Finite-Difference methods on an unstructured orthogonal mesh. One powerful feature of the 2DH flow solvers is that they implement the subgrid topographic variations directly into the model thus improving the accuracy of the solution and permitting the use of relatively coarse meshes resulting in reduced computational times.
HEC-RAS 6.0 includes a beta release of 2D sediment transport and morphology change. Despite being a beta release, the sediment model includes many features including multiple grain classes, mixed cohesive/noncohesive transport, and a novel approach to subgrid sediment transport and morphology change. The model is designed for short to mid-term simulations primarily due to limitations in the subgrid bed change calculations. HEC-RAS 2D sediment transport solves a bed-material load transport equation but separates the bed-material load into bedload and suspended-loads with empirical formulas. The transport equation is solved with an implicit Finite-Volume scheme on the same mesh as the hydrodynamics. The bed is can be modeled as a single layer or with a user-defined number of bed layers.