Hello,
The Flow Distance tool in the Spatial Analyst Toolbox, Hydrology toolset can be used to compute HAND (Height Above Nearest Drainage). Please take a look at the help topic Flow Distance—Help | ArcGIS Desktop for ArcMAP and Flow Distance—Help | ArcGIS Desktop for ArcGIS Pro.
A quick summary: The flow distance tool computes, for each cell, the horizontal or vertical component of downslope distance, following the flow path(s), to cell(s) on a stream into which they flow. In case of multiple flow paths, minimum, weighted mean, or maximum flow distance can be computed. If an optional flow direction raster is provided, the down slope direction(s) will be limited to those defined by the input flow direction raster.
Once you create a HAND raster (using the Vertical option in Flow Distance tool), you may create multiple floodplain rasters for different flood depths to understand which regions might be flooded for a given depth. This can be achieved using the Con tool. For example a condition of 'HAND < 0.2' would return values for all cells where Height Above Nearest Drainage is less than 0.2.
Of course using a single flood depth for all streams in your study area might not be realistic as each stream segment or reaches within a stream segment could flood with a different depth. The accuracy of your results will also depend on how the surface raster is hydro-conditioned, how the surface raster aligns with stream data, and flood depth data that you are using, among other factors.
Hope this is helpful!
Thanks,
Neeraj
