HAND Based Flood Depth and Extent not running in entire watershed

Question

I have been running the HAND-based Flood Depth and Extent From Table tool for two adjoining watersheds. However, I consistently receive output—both flood depth raster and flood extent polygon—for only one watershed, and even that appears in fragmented sections. Despite rerunning the tool multiple times, the result remains the same. When I previously ran the basic HAND-based Flood Depth and Extent tool, I obtained flood depth outputs for the entire watershed as expected. I generated the catchment and flow direction rasters using the Arc Hydro tools, and calculated the HAND raster using the Flow Distance tool from the Spatial Analyst toolbox.

To prepare the Input Flood Depth Table, I first classified stream order using the Strahler method. The resulting stream order raster was converted to vector format using the Stream to Feature tool. After exporting the attribute table, I added a new field named "H" to represent vertical height values. Additionally, I renamed the ArcID field to COMID to align with the naming conventions used in the input requirements of the flood depth analysis.

DeanDjokic !

DeanDjokic · Answer

If the tool and data you used on a single watershed worked, then the most likely problem you are seeing now is related to how you are combining the two watersheds together.  if you processed each watershed independently (defined streams, catchments, and HAND), then each one will most likely have the VALUE of the catchment raster starting from 1.  if you then combine them into a single raster, there will not be a uniqueness of VALUES across the landscape and the H table and you will get strange results. so the key elements to maintain:

1. COMID (Long integer) in the table needs to match the VALUE field for the catchment you are assigning depth for.  there should be no duplicate entries (rows with same COMID) in the table.

2. H (Double) in the table - depth that you want to "flood" stream/catchment with.  that should really be coming from your hydrologic analysis, not something like stream order (stream order should work as a number, but is really meaningless from hydrologic point of view).

3. VALUE in the input catchment raster must match the COMID in the table (and be unique across all catchments being processed).

Hope this helps.

Dean