Karin:
With regard to your question on how much increasing base flow affects the downstream water surface elevations, it all depends upon how much additional base flow you are adding.
If the percentage of additional base flow is much less than the peak discharge from the routing of the flood wave, I would not be concerned about it.
Sincerely,
Chris E. Maeder, M.S., P.E.
Senior Technical Engineer
chris.maeder@bossintl.com