C.S.
I'm not really good with this stuff, but I was thinking she could use a sand table, a pail of water and some lego houses. She could make the sand into a dam and show what happens when the dam breaks. She could also show what happens when water goes over a seawall, or heavy rains affect the homes. Perhaps a drainage area could lead the water away from the homes into a "lake".
After teaching the principles, she could even ask the class to design a city where torrential rains would not leave the homes flooded (they could put the homes on high ground, or create drainage into a lake or ditches).
I hope that helps!