I have a floodplain layer with elevations assigned to it. Per scope of the project, I need to create a safety setback. The flood elevation is raised by 2 ft. and when the raised point extended horizontally to the ground becomes the setback. How do I create this setback/buffer?
You didn't specify a software requirement for your solution (although there is an ArcGIS tag on the question) so I've provided a method below based on the free and open-source GIS Whitebox Geospatial Analysis Tools, for which I am a developer.
Whitebox GAT has a tool called Elevation Above Stream (see image below):
For a given DEM and rasterized stream network...
...the tool will calculate the elevation difference between each location and its nearest downslope stream cell:
You can then threshold using the Raster Calculator, Reclass tool, Less Than tool, or Conditional Evaluation tool. In this example I've simply pulled out each cell in the elevation above stream raster with a value less than 2 ft and vectorized the resulting polygons:
There are your setbacks! I should also note that while I used a stream network as the target in this example, you could easily replace the required input streams file to the tool with a rasterized version of a floodplain polygon. It would work equally well, but would redefine the output as elevation above the floodplain. In fact, this is nearly the exact application that inspired me to develop this tool in the first place. Good luck and let me know if you need any further details.