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I have a polygon layer that represents the max inundation extent along a stream for the one-year storm event (generated in hec ras).

If this floodplain was theoretically raised by one foot, what would the new floodplain extent look like?

This would be easier to do if the floodplain polygon was one elevation, but it's along a river so the elevation of the floodplain changes throughout the reach. I have ArcGIS Advanced Desktop and Spatial Analyst. I also have a high resolution DEM for the area.

How do I generate a new surface extent for this higher floodplain?

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    This is a basic capability of Spatial Analyst. All you need is a DEM. What have you tried?
    – Vince
    Dec 2, 2020 at 13:55
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    This cloudburst exercise might be useful for an example of SA capabilities.
    – Vince
    Dec 2, 2020 at 14:02
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    You could try detrending the surface model. Then generate contours from the detrended surface There are a number of different ArcToolbox tools for detrending here: github.com/LummiGIS/Detrend-Surface-Models. Each has its weaknesses and assumptions. You will need a Spatial Analyst license to use these tools. Disclaimer: I coded the tools. See this link for more information: gis.stackexchange.com/questions/11440/…
    – GBG
    Dec 2, 2020 at 17:58

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I ended up figuring this out myself. Here's what I did in case anyone is interested or has this problem. I had a water surface elevation raster for the year 1 flood extent (I didn't realize I had this, which is why I didn't include it in the original question). I used raster calculator to add 1 to every elevation value in the year 1 raster. Then I used a tool in the Arc Hydro toolbox called 'flood from stream WSE' on the new WSE raster and existing DEM. This tool produced a flood raster and polygon which seem to be spot on.

The flood from stream SWE tool requires a stream elevation raster, so I am curious how you could get the same result have only a flood polygon and terrain DEM.

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