Lots of people are reacting to these two words as they relate to the ‘Katrina Warning’ video released yesterday. While some may argue semantically, I think there is a very real difference between being warned that levees in New Orleans might be topped and warning that the levees would be breached.
Overtopped means much less water comes in, and once the surge recedes the water remains behind the levee system. Breached means a barrier no longer exists, and that means flooding that stays.
Patterico covers this subject quite nicely.
To expand on your distinction, if the levees had only been overtopped, the pumping systems would have minimized the damage, while if breached, the pumping systems became useless.