Getting Ghost flows in my 1D/2D model

[DownUnderNorth]

I’ve got an upstream 1D river connected to a downstream 2D flow area. fairly simple boundaries- 1 uppermost river section is inflow of 10 cumec, the lowermost cross section overlaps with the 2D perimeter (1D-2D exchange) and the 2D flow area has a downstream normal depth boundary.

As soon as I run the model, I get ghost initial depth throughout my river- so much so that it overtops all the lower boundaries and model becomes unstable within 2 seconds (the upstream hydrograph doesn’t start till a minute). I have no initial condition and all the Htab are set at lowest elevation of the cross section.

I tried setting this area the other way around- upstream 2D- directly connected to downstream river- But same issue. I get water on the river before the model starts running. What is going on?

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