Inundation limits within flat floodplains

[Jonathon McDonald]

As explained within the FEMA report “2D Watershed Modeling in HEC-RAS Recommended Practices” there may be significant “cupping” errors when mapping inundation limits. I am experiencing this within my current model with large amounts of water being shown along slopes (1%-10%).

In this FEMA report, they mention there are ” several different external mapping techniques being used throughout the country to address cupping issues”. What is everyone doing to adjust inundation limits to be “correct”?

[Jonathon McDonald]

I should mention my model is a several square mile watershed in 2D.

[Eric R]

I have never used external methods to resolve this. For my models and purposes, I have always found satisfactory results by decreasing the local grid (as suggested in the FEMA manual itself).
To me, determining the optimal cell size seems to require an iterative process, optimizing computational time with the desired level of result.
In some specific cases, you can perform auxiliary analyzes using a 1D model.
In your case, where it is a large watershed, I think that locally reducing the mesh can partially help, depending on the degree of cupping you are experiencing.

[Luis Partida]

I had a discussion with some colleagues regarding cupping specifically. Basically, that document was written prior to the release of HEC-RAS V6.2 which introduced a new render mode called Depth Weighted faces or “precip mode”. This render mode eliminates most issues related to cupping, so I would try this before any GIS related external mapping efforts.

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