Why does FEMA hate junctions in HEC-RAS?

[LanceMurphy]

I've been performing and managing flood studies for some time now but never questioned the methodology, until now. In a countywide study (or watershed for that matter) why are all the streams split up? Wouldn't it make more sense (and potentially be more accurate) to tie them all together with junctions and let HEC-RAS determine the tailwater conditions for each stream as opposed to figuring which streams are in backwater during mapping?

I've read the explanation in HEC-RAS Procedures for HEC-2 Modelers where it states "flooding in the main river and its tributary will not occur at the same time. Therefore, the flow-combination solution procedure available in the HEC-RAS may not be generally used for FISs." but I don't buy it. It would be more conservative to do so.

 

[cvg]

it might be more conservative, however, it might not correspond to a 1% or .2% flood. Every effort should be made to calculate flood levels to these regulatory flood levels.