Vegas floods?
Thats what i was wondering lol
Flash floods in some cases. Storm water management is all about channeling water during rainfall. When you build "impermeable surfaces" like buildings and asphalt, you have less ground to absorb the water. The rain has to run off somewhere. It's prefereable for engineers to channel the rain in a predetermined direction rather than let if run off in all directions and cause flooding. Hence, these tunnels.
Yes it can flood just like the rest of the desert. Flash floods generally. When it does, these tunnels can fill to the top with high speed water. All the people could die, and everyone would blame Bush the next day.