Minimum cell size?

[alister.hood]

Trying to run a model with small cells I found that I get this error if I generate computation points with spacing of 0.401m or less:

Plan: ‘Existing pseudo steady’ (2370.p01)
Simulation started at: 10Nov2022 04:40:51 PM

Writing Geometry…
Computing 2D Flow Area ‘Perimeter 1’ tables: Property tables do not exist.
ERROR: Property Tables error: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index (Perimeter 1)
Geometry Writer Failed
Error Processing Geometry

The error message isn’t at all helpful, so maybe posting here will help someone else.
If I generate computation points with 0.402m spacing and then manually add extra points I don’t get an error. This seems bizarre.
Does anyone have any idea if this behaviour is expected? I guess I should report it to HEC.

[alister.hood]

OK, I found an old project where I used 200mm cell size, and 6.3.1 can still run it OK, so the problem obviously isn’t as simple as just the cell size. But my problematic geometry is very simple.

[Luis Partida]

I am not sure I know of any modeling scenario that would benefit from that small of cell sizes both resultant-wise and efficiency-wise

[alister.hood]

I use HEC-RAS for detailed modelling of urban overland flow paths – normally I wouldn’t use cells that small, but sometimes local government insist on it because they are more familiar with other modelling software where you need to use smaller cells to get good results.

[alister.hood]

For the record, on another project I’m getting this error with 500mm cells, but not with 600mm.

[Eric R]

You need to keep in mind that HEC-RAS 2D computes the hydraulic parameters not only individually on each face (a single point per face), but performs computations inside each face, which allows using cells with larger faces without losing fidelity of the results. This feature of HEC-RAS is called “sub-grid hydraulic model”.

A general idea of this property is presented in this topic: << kleinschmidtgroup.com/ras-post/hec-ras-subgrid-bathymetry-theory-and-application >>

I recommend that you increase the size of your model’s cells, which may eventually eliminate the error you are facing and even improve the calculation time.

Hope this help.

[alister.hood]

Yes, sub-grid bathymetry is what makes HEC-RAS great, and I don’t like to run models with such small cell sizes, but sometimes it is necessary.

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