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Expansion and contraction are not used for unsteady flow. The unsteady flow solution is based on momentum.
However, bridge and culverts are preprocessed using steady flow to create a family of rating curves (so that the unsteady flow program can run faster). So I have the impression that the contraction and expansion is taken into account at bridges and culverts.
If you haven't already, download the RAS hydraulic reference manual. There is a wealth of information in it about how RAS does the computations (if you want to spend the time reading it all).
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