Erroneous Inundation Found In RFC FIM

[derekgiardino]

HUC8: 01070006
NWM ID: 6745646
Merrimack River
RFC FIM 2023-12-18 15Z Adopted from CONN3 RFC Forecast

Noticing an erroneous overestimation from Branch 0 extending out into the town of Mancester. Doesn't seem to follow a gap in 3DEP but is finding a way out of bank and then filling a large area away from the Merrimack River.

[RyanSpies-NOAA]

I noticed that there doesn't appear to be an underlying NWM flowline showing up in the area of inundation to the east of the Merrimack River.

Does appear that the inundation is coming from branch 0, so I guess the question is why is there not a NWM flowline here and where is it being crosswalked?

So... after digging deeper it appears there is an orphaned NWM flowline north of interstate 93 draining into Steven's Pond.

That NWM headwater point seems to be allowing the DEM derived flowline to create a flowpath to the southwest where it joins up with the Merrimack River.

That means there is a DEM derived flowline that doesn't have an overlapping NWM flowline so the crosswalk is mapping flows in that DEM derived reach to the mainstem Merrimack.

Tagging @mluck on this issue since he's been exploring crosswalk issues recently.

[mluck]

Nice analysis, @RyanSpies-NOAA. Some follow-on information:

• The root of the issue seems to be that the orphaned NWM stream incorrectly flows to the north away from Steven's Pond. I've put the headwater point on the image below as well as an image of the DEM.
• That's why the DEM-derived reaches have no NWM counterpart, and as a result, the NWM catchments are also incorrect which leads to the crosswalking error to the NWM mainstem highlighted in yellow.
• In addition, there seems to be an issue with the NWM catchment for the orphaned NWM flowlines, as they are part of the mainstem catchment which also has a flowline, so it appears that there should be a separate NWM catchment for the orphaned segment. Because of this, the DEM-derived streams coming out of Steven's Pond are also crosswalked to the mainstem in the red NWM catchment.

[mluck]

Verifying with 10-year recurrence flows that #1205 resolves this issue since DEM-derived reaches whose midpoints aren't within 100 meters of a NWM stream (colored lines) are eliminated from inundation (top: v4.5.2.3, bottom: #1205) @derekgiardino