Hi, 

If in both cases the intial condition you impose is normal flow and the discharge remains steady, I see no reason for finding strong erosion in one case and not in the other one. If the discharge is unsteady (it is wise to first make a run with steady discharge to check that boundary conditions behave as you want), does erosion occur before the wave gets to the dowsntream end? If yes, it seems that the initial condition is not normal flow in the case of extreme erosion. Check the bed level of the most dowsntream location. You have to adapt this to both cases.