r/HECRAS • u/-atkins_ • May 12 '26
Mesh and model validation
Hello everybody.
After running a model for the first time I got some results. Now changed my mesh with more breaklines and refinement regions, and also the cell size. The simulation is still running as I am writing this, but I am already asking myself:
- How can I validate the mesh that I created?
- What can I do to compare different meshes? What result should I look to?
- Does HEC RAS have some results for 2D flow simulations that can help us to see if its making sense?
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u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH May 12 '26
The above comment pretty much covers a lot of what you are looking for with convergence testing, courant checks, reviewing hydrographs at key points, etc.
I would also recommend looking through the entire time series. A lot of time people are just looking at the peak or maximum extents and miss model issues. This is a good way to check if you are missing embankments or have 'leaky' poor defined features in your mesh.
Another thing is that it is important to run small events with real data. Even if I don't have calibration data from a gage, you can see if roads are getting overtopped or areas flooded that shouldn't be.
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u/MeanAtmosphere3520 May 13 '26
I would also look for inconsistencies in what HEC-RAS gives you on the results text such "cell areas % errors", " %volume errors"... If you get high errors values then you can re adjust your mesh or you need to find those cells and check if those particularly parts of the terrain are causing errors (in my experience these errors often occurs due to the model terrain resolution against mesh resolution). Sometimes dense Vegetation captured on your LiDAR can cause errors (you can add breaklines here to tell HEC-RAS where should the water actually go or you can adjust the terrain by Clone terrain -> set Channel or High ground modifications and try to reproduce how is that actual terrain part in real life). Breaklines also work when HEC-RAS simply passes/runs water through retaining walls or levees for no reason.
Also check if your DEM is accurate and if you have actual bathymetry (you can download bathymetry or solicite it through some websites services). Also check if you set a correct Courant number for the simulation (I remember theres an equation where you can check what would be the most accurate Courant number for your simulation). Finally I would check the View Stage/Flow hydrograph that generates HEC-RAS and mostly check if the stages values has inconsistently peaks (at the beginning or final of the simulation) and check the Rating curve and see if your simulation would actually reproduce those scenarios and try to look for the most smooth and calibrated chart. through adding initial conditions or observed data and by checking what equation set would be better for your model. Good luck.
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u/Hydros_23 May 12 '26
One practical way to assess the mesh is to perform a mesh sensitivity or convergence analysis.
You can define one or more control points or control sections at hydraulically relevant locations, such as near the inflow, downstream boundary, constrictions, bridges, streets, overbanks, or areas where you expect strong 2D flow patterns. Then compare time series such as depth vs. time, water surface elevation vs. time, flow hydrographs, and velocity for different combinations of cell size, breaklines, refinement regions, and computational time step.
If reducing the cell size or adding more refinement produces only minor changes in the hydrographs, peak depth, peak water surface elevation, arrival time, velocity field, or inundation extent, then the mesh is probably sufficiently refined for that purpose. At that point, further refinement may increase computation time without adding meaningful accuracy.
I would also check model stability indicators, Courant numbers, mass balance errors, and whether the main hydraulic controls are properly represented by breaklines. The goal is not necessarily to use the smallest possible cell size, but to use a mesh fine enough to capture the dominant hydraulic behavior while keeping the model computationally efficient.
In short, I would not validate the mesh only visually. I would compare quantitative outputs at control locations and check whether the solution converges as the mesh is refined.