HEC-RAS can perform two-dimensional (2D) hydrodynamic routing within the unsteady flow analysis portion of the software. Users can now perform one-dimensional (1D) unsteady-flow modeling, two-dimensional (2D) unsteady-flow modeling (Shallow Water equations (SWE) or Diffusion Wave equations (DWE)), as well as combined 1D and 2D unsteady-flow routing. The 2D flow areas in HEC-RAS can be used in number of ways. The following are some examples of how the 2D flow areas can be used to support modeling with HEC-RAS:

  • Detailed 2D channel modeling
  • Detailed 2D channel and floodplain modeling
  • Combined 1D channels with 2D floodplain areas
  • Combined 1D channels/floodplains with 2D flow areas behind levees
  • Directly connect 1D reaches into and out of 2D flow areas
  • Directly connect a 2D flow area to 1D Storage Area with a hydraulic structure
  • Multiple 2D flow areas in the same geometry
  • Directly connect multiple 2D flow areas with hydraulic structures
  • Simplified to very detailed Dam Breach analyses
  • Simplified to very detailed Levee Breach analyses
  • Mixed flow regime. The 2D capability (as well as the 1D) can handle supercritical and subcritical flow, as well as the flow transitions from subcritical to super critical and super critical to subcritical (hydraulic jumps).

2D flow modeling is accomplished by adding 2D flow area elements into the model in the same manner as adding a storage area. A 2D flow area is added by drawing a 2D flow area polygon; developing the 2D computational mesh; then linking the 2D flow areas to 1D model elements and/or directly connecting boundary conditions to the 2D areas.

Note: This document assumes that the reader already knows how to use HEC-RAS to perform 1D unsteady flow modeling. This document focuses on how to use the new 2D modeling capabilities and the new HEC-RAS Mapper features. For assistance with 1D unsteady flow modeling, and how to use the user interface, please review the main HEC-RAS User's Manual and Hydraulic Reference Manual.