The Corp's Hydrologic Engineering Center River Analysis System (HEC-RAS) is designed to simulate one-dimensional (1D) steady, unsteady flow. The latest release of HEC-RAS V6.0 also simulates unsteady two-dimensional horizontal (2D) sediment transport, and bed change, sorting, and layering. Sediment transport is computed with a non-equilibrium total-load formulation. The total-load transport equation is solved with implicit Finite-Volume methods on the same unstructured polygonal mesh as the flow solver. Sediment transport is coupled to the flow model at the time step level. One powerful feature of the 2D flow solvers is that they use 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. The sediment transport model is designed to work within the subgrid framework of the flow model, and computes subgrid erosion and deposition rates, bed elevations, gradations, and bed layering.
This document discusses how to utilize the 2D sediment user-interface, the model input and output, and how-to setup and run a 2D sediment transport model in HEC-RAS. The document is intended as supplemental to the 1D Sediment Users Manual as many concepts and features are covered in detail in that document. Most of the 1D sediment capabilities are supported in 2D sediment and many new features have been added to 2D sediment which are not available yet in 1D sediment. Some of the new sediment features include variable density bed sorting and layering model, flocculation, consolidation, hiding and exposure effects, multiple new transport potential formula. However, as a beta release, there are still several computational and user-interface limitations and known issues including the inability to hot-start sediment, the inability to couple 1D and 2D sediment, the inability to modify terrains based on computed bed change, inability to visualize subgrid output directly in HEC-RAS, the inability to specify avalanching parameters in the user-interface, and the inability to specify subsidense in 2D areas. Lastly, the 2D sediment transport feature in HEC-RAS V6.0 is a beta feature and should not be used for design purposes.