Major Features

HEC-RAS 2025 Mesh Import to HEC-RAS version 6 - HEC-RAS 2025 has new meshing capabilities.  Users can create meshes in HEC-RAS 2025 and import them into HEC-RAS 6.6. Imported meshes cannot be edited (i.e. locked). After a mesh is imported, boundary conditions and hydraulic structures such as bridges can be added to the geometry. The figure below shows an example mesh and depth results in HEC-RAS 6.6 using an imported mesh from HEC-RAS 2025 (video).

Floodway Encroachment Zones - Floodway encroachment analysis for 2D modeling requires the use of encroachment Regions in HEC-RAS.  Encroachment Regions are created by user-defined polygons; however, there is a wizard to assist in generating polygons by creating contours from results maps.  The Zones layer allows you specify different contouring intervals for various areas (by "zones").

Floodway Evaluation Using Reference Lines - Reference Lines are used to evaluate 2D model results. For Floodway Encroachment Analysis, Reference Lines are set up to automatically compare Base plan to Encroached plan results.  The caveat to using Reference Lines is they must be create prior to running the model simulation as results are written out from the computation engine (rather than being evaluated during post-processing).  There are new tools to assist the user in automating the Generation of Reference Lines in RAS Mapper from results maps.  More discussion can be found in the links below.

You can interact with results maps and "track" water surface elevation contours or a line perpendicular to flow to  Create Reference Lines .

Porosity and Flow Drag - HEC-RAS 6.6 has the ability to simulate porosity and flow drag through porous media and subgrid features such as vegetation. The approach is similar to how the Manning's roughness coefficient is specified in HEC-RAS. A surface classification layer is created which can be a mix of raster and vector data. Each classification is then assigned specific porosity and flow drag parameters which are then interpolated onto the computational mesh cells and faces. The figure below shows example current velocity results for a flume with semi-circular vegetation regions. 

Wave Forcing - Wave forcing resulting from gradients in wave radiation stresses can be applied to all flow solvers in HEC-RAS 6.6. The wave forces are user-specified on grids similar to gridded meteorological variables such as precipitation. Supported input file formats are NetCDF and DSS-7. The example below shows a comparison of maximum water surface elevations with (cyan) and without (blue) in the Mississippi delta for hurricane Ida in 2021. Wave forcing increased the storm surge by more than 1 ft in some locations. 


Advanced Convergence Criteria - HEC-RAS 6.6 has the option to use additional convergence criteria for the 2D flow solvers aimed at reducing convergence stalling (i.e. iterating without reducing improving the solution). The new criteria reduce the number of iterations and thus simulation runtimes without impacting accuracy. As an example, the figure below shows a time series of the maximum number of iterations per timestep with (magenta) and without (black) the advanced convergence criteria. The advanced convergence criteria reduce the number of iterations and therefore the total simulation runtime.


Pipes (beta) - HEC-RAS has long had the capability to model pressurized pipe flow utilizing the Preissmann slot approach; however, the ability to perform integrated urban stormwater modeling using closed conduit conveyances, such as underground stormwater networks and tunnels, was lacking. In version HEC-RAS 6.6 functionality for pipe networks was released in beta form. The pipe flow computations are modeled with finite volume methodology similar to the 2D hydraulic computations.  The pipe network mesh is modeled independently of the surface mesh and interacts with the surface where connections are made between the 2D surface and the pipe network.

Pipe Network Simulation Results in HEC-RAS Mapper

Minor Improvements

Grain Class Filter for Floodplain Deposition - HEC-RAS can limit floodplain deposition by grain class, only depositing user-specified, grain-classes, that are transported in suspension and can contribute meaningfully to floodplain deposition.  This method provides a simple, helpful, middle approach between ignoring floodplain deposition, which often over predicts channel deposition (and can crash the model) or the veneer method which chronically over predicts floodplain deposition.

Flow Roughness Factors Curves and Calibration Regions - Calibration Regions are used in HEC-RAS to override the Manning's roughness coefficient. In order to facilitate calibrations, several output time-series variables have been added to HEC-RAS 6.6. The Calibration Region output variables are the same as for Reference Areas and include flows into and out of the region, water surface statistics, and water volume, and water volume error. The bottom roughness (i.e. Manning's coefficient) inside Calibration Regions can be adjusted by specifying roughness factors as a function of calibration region outflows. This is similar to how flow roughness factors work in 1D, except instead of using cross-section flows, the flow roughness is a function of the Calibration Region outflows. This feature allows user to adjust the roughness as a function of flow accounting for processes such as changing bedforms and bending vegetation. 

Improved Stability of Eulerian Shallow Water Solver - In previous versions of HEC-RAS, the Eulerian Shallow Water Equation solver (SWE-EM) was strictly limited in timesteps by Courant condition. This can often lead to long run times due to small timesteps or model crashes when not utilizing adaptive time stepping. In HEC-RAS 6.6, the SWE-EM advection scheme has been modified so that it is no longer strictly limited by the Courant number. The SWE-EM solver uses a semi-implicit advection scheme which has better stability properties and allows for much larger timesteps. 

Mixed Flow Quasi-Unsteady Sediment Transport - 1D Quasi-Unsteady sediment transport can now compute mixed flow profiles, including super-critical regions.

 

Holding Initial Water Surface Constant at Initial Condition Points - Added an option so that Initial Condition Points will try to maintain a constant water surface elevation during the initial condition and warmup period.  This can be useful for maintaining the known water surface at a reservoir for models that require a long initial condition time.  For a large reservoir, having multiple Initial Condition Points spread around the reservoir all set to the same elevation may give better results.

Bed Roughness Predictors - Sediment transport simulations can compute bed roughness dynamically based on the bed material and predicted bedform geometry. While in previous versions of HEC-RAS, this was a global setting, in HEC-RAS 6.6, the option is available to compute bed roughness by bed material type and utilize it for the hydraulic computations. In addition, the Karim (1995) bed roughness predictor was added as an option for 2D sediment transport. 

Fraction of Suspended Sediments - Added the option to choose the method to calculate the equilibrium fraction of suspended sediments when utilizing sediment transport functions that do not already provide a means of computing the value. 

Spatially Distributed Erosion Factor - Added the ability to specify an spatially distributed erosion factor which is assigned to sediment bed material types. The erosion factor is useful in calibrating sediment models. 

Separate 1D Floodplain Deposition Methods by Mass Partition and Distribution - The algorithms that determine the pattern of floodplain deposition (i.e. Distribution) were separated from the methods that determine the mass deposited in the floodplain.  This allows users to mix and match these methods, while providing HEC the flexibility to add several new methods that are under development into future version.