Combined 1D and 2D Hydraulic Modeling within HEC-RAS

Abstract

HEC has added the ability to perform two-dimensional (2D) hydrodynamic flow routing within the unsteady flow analysis portion of HEC-RAS. This new capability within HEC-RAS has the following attributes: 1. The ability to perform combined 1D and 2D modeling within the same unsteady flow model will allow users to work on larger river systems, utilizing 1D modeling where appropriate and 2D modeling in areas that require a higher level of hydrodynamic fidelity. 2. The program solves either the full 2D Saint Venant equations or the 2D Diffusion Wave equations. This is user selectable, giving modelers more flexibility. 3. The 2D flow solver uses an Implicit Finite Volume algorithm. The implicit solution algorithm allows for larger computational time steps than explicit methods. The finite volume approach provides a measure of improved stability and robustness over traditional finite difference and finite element techniques. 4. The 1D and 2D solution algorithms are tightly coupled on a time step by time step basis (optionally, iteration by iteration within a time step). 5. The software was designed to use structured or unstructured computational meshes. 6. Each cell, and cell face, of the computational mesh is pre-processed in order to develop detailed hydraulic property tables based on the underlying terrain used in the modeling process. Additionally, each face of a computational cell is pre-processed into detailed hydraulic property tables (elevation versus, wetted perimeter, area, roughness, etc…). This allows the user to use larger computational cells, without losing the detail of the underlying terrain. 7. Mapping of the inundated area, as well as animations of the flooding can be accomplished right inside of RAS using RAS Mapper. 8. The 2D Flow Area computational solution has been programmed to take advantage of multi-processors on a computer (referred to as parallelization). This paper will discuss the details of the new 2D modeling capabilities that have been added to HEC-RAS. Additionally, the new capabilities of combining 1D and 2D modeling elements within the same unsteady flow model will be demonstrated with the application of real world data sets.