What an intriguing problem! You need xeric plants that can thrive in periodic flooding, sounds like. This article about someone who built a dry creek bed in a part of the US (Central Texas) apparently nicknamed "Flash Flood Alley" might have some information you could adapt to your circumstances:
http://www.penick.net/digging/...
I imagine there are irises, for instance, that could work, and though I couldn't be sure from the article it seems like there are numerous plants being used this way in this gardener's dry creek bed. Were it my project I would investigate some of the plants noted here as to their ability to thrive under such extremes of water/dryness.
Good luck!
