Jarvus,
That is a lot of good advice, we will start working through it. We are running it as a steady flow model (tried unsteady and it was not better). The instability I referred to was more of a model instability rather than flow, but the model may not be unstable.
Since the 36″ pipe is the limiting factor, we are considering submitting it as the existing conditions and ignoring the 4×6 that is upstream. A complete 36″ has the same effect in real life as half 36 and half 4×6, but not sure if FEMA will accept it.
Regarding the ‘spout of water’ i mean in the profile view, some of the models show water shooting up between the cross sections that delineate the 4×6 and the 36″. When the road over tops, the over topped water stops at that cross section and joints the water between sections.
I will post again when we get better results.