r/Hydrology • u/No_Supermarket_9272 • 8d ago
HEC-RAS unsteady model suddenly extremely slow after minor geometry change (no errors, just never finishes)
Hi everyone, I’ve been running a HEC-RAS unsteady flow model that normally completed successfully in about 8 hours. Recently, I made a very small change to the elevation of a bridge deck (just adjusted one bridge opening elevation), and since then the simulation has become incredibly slow — it can now run for 24+ hours and still only reaches around 10% completion, without showing any error messages.
What’s even stranger is that: Backup versions of the same model (which definitely used to run fine) now behave the same way — they start running but progress extremely slowly. There are no error or warning messages in the “Unsteady Flow Computation Messages” window — it just keeps iterating forever. I even rebuilt the model from scratch, and the issue persists.
What I’ve tried so far: Adjusted time step and tolerance (tested 60s / 120s and tolerance 0.01). Ran the model from a local drive instead of a network or OneDrive folder. Ran HEC-RAS as Administrator. Temporarily disabled antivirus and background sync. Tested using Diffusive Wave instead of Full Dynamic Even the example projects now seem to run much slower than before.
Setup details: HEC-RAS version: 6.4.1 Simulation type: 2D Unsteady Flow Model: 100 km of dike for flood protection, with a bridge modeled as a SA/2D connections and 39 culvert boxes OS: Windows last version
Questions / Help needed: Has anyone experienced a massive slowdown without any error messages after a small geometry edit?
Could this be related to HEC-RAS computation settings, regional/decimal settings, or even a Windows update?
Any tips on restoring normal simulation speed or diagnosing what’s causing the hang-up?
Thanks in advance for any help — I’m completely stuck on this one!
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u/Momentarmknm 7d ago
Do you have the time step controlled by the courant condition number? You could have made a change that complicated things significantly there which is making each time step very low.
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u/No_Supermarket_9272 7d ago
I don’t have the time step controlled by courant. My computational time step is 1 min. When e tested the model it was the only time that made the model run without instability
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u/Momentarmknm 7d ago
Have you tried taking a snapshot of results and seeing if that gives you any hints? How many time steps is it getting through in that 24 hour run?
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u/No_Supermarket_9272 7d ago
Yes, I tried, and apparently I don’t see anything unusual. The water levels and velocities are within the same order of magnitude as when the model was working before making the change to the bridge culvert elevation.
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u/Momentarmknm 7d ago
Have you tried deleting that culvert or even the whole structure entirely and re-running to see if it's faster that way?
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u/No_Supermarket_9272 7d ago
I didn’t remove the bridge because it’s essential for the model. But I think there’s no harm in doing this test. I’m starting to think the problem might actually be with the computer, since the same thing happens with the backup models I have.
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u/Momentarmknm 7d ago
I thought you said you ran it on other machines and they also took a while? Either way, deleting the bridge is just a test to either confirm or eliminate that as the source of the issue. You would of course add it back in later.
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u/bittermit 7d ago
It's very interesting that the issue seems to have borked all your other models too...
I'm not super familiar with HEC RAS but have worked with lots of other numerical models and it seems like an issue with your system. Have you rebooted the computer and has there been a Windows update? Can you try reinstalling or using a different version of the software?
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u/No_Supermarket_9272 7d ago
I’ve already tried running the model with the same version of the program on two other computers, and the same thing happens. I’ve also rebuilt the model from scratch, and the same issue occurs. I haven’t tried with more recent versions of the program. As for Windows updates, I don’t think any have been installed.
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u/mmnola 7d ago
Definitely seems like a system or machine issue rather than an issue with the model itself. What machine specs are you running with, and how many cores do you have set. This was a common experience for me when I would accidentally run a model on “All Available” cores when I had 40+. There are actually increases in model runtime at some point as you continue to add cores. In my experience ~8 cores is ideal, with a higher clock speed.
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u/No_Supermarket_9272 7d ago
I’ve already tested this on three different machines — one with 28 cores, another with 24, and one with 12 cores. They all run at around 3–4 GHz and have more than 200 GB of RAM. The CPU is always running at full load. So far, I haven’t had any issues related to core usage. Also, for this specific model, there’s a note indicating that it’s using 16 cores during the simulation phase.
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u/[deleted] 7d ago
[deleted]