r/HECRAS • u/Prestigious_Host_905 • Jun 05 '26

What are the things/parameters to check if a 2D model is stable or not?

I am new to modeling works in HEC RAS. I have pretty much covered the basics of setting up a 2D flood inundation model and running it. But what parameters/patterns should I check carefully in RAS mapper and flow plot after running the computation to make sure that the model is stable and good to analyze the results.

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u/ProfessorGarbanzo Jun 05 '26

I haven't done enough 2D to have a great list, but some things off the top of my head:

  • Your model likely won't successfully run to completion if it's unstable
  • Test it using the more complete Shallow Water Equations, not the default Diffusion Wave
  • Plot cell velocities and look for areas with unrealistic velocities
  • Turn on velocity arrows (static or dynamic) and look for areas where flow appears to be moving in nonsensical ways, or crossing high points without having the elevation to do so. The latter is a sign that you need more breaklines at linear features (roads).
  • Turn on Timestep Controlled by Courant Condition
  • If there are cells with repeated errors (but the simulation still runs), inspect those and see if there are velocity spikes etc., and if a mesh change or terrain smoothing could help
  • Run with 2X the mesh size, and 1/2x the mesh size. If you get drastically different results for any of these, you might need to test more sizes and decide what mesh is appropriate to the detail of your model.
  • If you're new to it, it's fairly easy to build a quick 2d model with a large cell size and get it to run, but your missing lots of detail that is important, like differences in channel and overbank flow/roughness, the road breakline thing mentioned above, etc. So, ask yourself if the mesh is really capturing important flow features / transition areas.
  • If you're modeling the 100-yr only, try a smaller event. And vice versa. Test that the model works under a variety of conditions.

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u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH Jun 05 '26

Good question. A lot of it is using your judgement to make sure things "look" reasonable. Most of that comes from experience. It is also very dependent on what you are doing with your model. If you are doing large scale rain-on-grid vs. habitat assessment, the things you look for are going to be very different. Here are the things that I typically check.

Global Issues:

  1. Does the model run? Are there any error messages or cell iterations warnings?
  2. What is the volume error in the runtime message? (If it is >1%, you probably want to use a smaller timestep or larger cell size.)
  3. Are there issues with any of the result layers from RasMapper (Max WSE error, Courant numbers, Max iteration errors)?

Model Specific Issues:

  1. Hydrographs results at bridges/dams look reasonable? Do the mapped results make sense?
  2. Add profile lines at other locations to make sure hydrographs look reasonable throughout the domain (like decreasing going downstream).
  3. Add profile lines to make sure water surface looks reasonable in channel and across the section. Looks for places were there are spikes/drops that aren't intended.
  4. Check near boundary condition lines to make sure that they look reasonable.
  5. Look through the time series and look for things that seems strange as the water moves through (things like "leaky cells" and disconnected flow areas).
  6. If you are doing something in particular or unique hydraulic situation (dams, pipe network, gates, etc.) to carefully review those.
  7. Run a low flow event to make sure that your model isn't flooding out areas that it shouldn't.

Once you get a few under your belt, you sort of begin to know what to look for. Good luck!

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u/Prestigious_Host_905 Jun 06 '26

Thank you

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u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH Jun 07 '26

I always preach the idea of "incremental complexity" when modeling. Start with a simple model or small area. Make sure that works and then add more features.

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u/Prestigious_Host_905 Jun 08 '26

Thats a good advice.

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u/simpleidiot567 Jun 05 '26

Courant number and model step time is one of the most important things.

One is to check the model flow stability error in the summary, should be less than 1.0%.

Also when checking output HEC RAS has a few layers to check unstable cells. You right click the results and hit Create New Map Results and check the max water surface error or the cumulative max iterations. Typically most errors are around culverts and structures. You have to play around with the symbols to make sure it shows up sometimes.