ArcHydro Adjoint Catchment - multiple drainage lines in one catchment

[RichHammond]

I am using ArcHydro for Pro (2.0.15) and ArcHydro for ArcGIS (10.6.0.40) - but I found the same issue on other recent versions. I have a very large area with 58,000+ catchments and drainage line segments. About 20-30 of these catchments have two drainage lines within the same catchment (same gridid identified) - essentially where a side stream flows into the catchment. Running the Adjoint Catchment process works, but fails to create adjoint catchments for some (but not all) of those catchments with more than one drainage line. The attached image shows an example with a catchment containing two drainage lines, and then a side line in a different catchment that enters from the bottom. The drainage lines are selected. The other picture shows the adjoint catchment in green and you can see how it left our the catchment with the two lines - and the upstream catchment with the one line. 

Here is what I know and the hack I used to solve it. I am looking to understand the cause and then to know the correct solution.

• The drainage lines all show the correct from, to, nextdownID - and the gridid for both lines is the same catchment. 
• This problem DOES NOT occur with all of the catchments that have more than one drainage line. About 20-30 catchments have two lines, but only 12 of the catchments result in a missing adjoint catchment. I cannot see anything different between the ones with and without the problem.
• I can solve it using two different techniques: (1) for the example when the second line is a terminal upstream segment I just deleted that upstream segment; (2) for catchments where the two lines are part of a river I split the catchment so that each line has a separate catchment.  Both solutions work to produce a complete adjoint catchment. 

Is this a bug? Is there an ArcHydro tool designed to fix/avoid the problem of multiple drainage lines within each catchment? Should a catchment be allowed to have two drainage lines?

[ChristineDartiguenave]

There can be only one drainage line in each catchment (exception would be nhdplus with very small lines).

In your case, the issue seems to be the source data. How did you create the drainage lines? The input streams need to go from confluence to confluence. A stream cannot connect to the middle of another stream as it seems to be the case on your picture. You need to split the long stream at the location where the smaller stream connects. You will end up with 3 drainage lines and 3 catchments.

Christine Dartiguenave

Esri Water Resources Team

[ShawnLandry]

I wrote the original question but had a colleague submit before my ERSI login was sorted out. 

The symbology is bad, but what appears to be one long line is actually two - split right at the confluence. The drainage line layer seems accurate. Based on your reply - that every drainage line should correspond to one catchment - it seems like the Catchment layer is wrong. What might cause the stream segmentation - catchment grid - catchment polygon - drainage line processes to result in a single catchment with two drainage lines? The underlying data is a LiDAR-derived 10ft pixel DEM for a mountainous area. 

Thanks,

Shawn