Where does all the water for a flood come from?

er...in my care to double check my calculations, I forgot to remove one of the errors...scratch that 225 000 reference.

don't forget the city is located between two levee's... dangerous...

Storm surge. Basically, as this huge storm moves it pushes the ocean up in front of it (like if you moved heavy furniture across loose carpet)[1]. I think they are estimating a surge of 20 to 30 feet plus another 20 or 30 feet due to wave height[2]. This is more than enough to push the ocean/Lake Pontchartrain over the levees. It isn't all due to rain fall. In fact, most sites I've read today don't seem to be worried about the rain fall nearly as much as the levees breaking. [1] I got this from a graphic/video somewhere. The actual phenomenon is more complex. [2] There's been so many numbers thrown around today, but I think these are correct if not on the conservative side.

http://www.nola.com/hurricane/images/goingunder_jpg.jpg from the Mefi thread has some explanation of the bowl effect.

Hmm...ok...going to look up the links and videos, but my question is also more general since it seems every floor you hear about (including those not near the ocean) seem to have this discrepancy between rainfall and water levels. For example, there was flooding in Toronto a couple of weeks ago, and I don't think it rained enough to explain the amount of water. (Obviously not flooding on the magnitude NO is likely to see, but my point is about the discrepancy). What explains the discrepance when the ocean isn't involved?

New Orleans is also right next to Lake Pontchartrain, which will overflow into the city. But the main problem is that, under the best of circumstances, New Orleans needs to pump water constantly to avoid being flooded. It has one of the most sophisticated pumping systems in the world. This pumping system will not be able to deal with the expected flooding from Katrina, however, and so the flooding from Katrina will be added to the city's natural flooding. Insult to injury.

Well, Toronto is a far cry from being flat. I don't know the incident you're referring to, but I'd imagine the reporters meant "up to x feet" of flooding in some areas.

In Toronto, a lot of it was storm drains being unable to handle the amount of water in the short amount of time they had (that was a LOT of rain in a very short period), as well as rivers flooding (cops were rescuing people from the roofs of their nearly-underwater cars on the Don Valley Parkway, because of the Don River).

The Toronto flood downtown was caused by the heavy rainfall to the north of the city being concentrated by the Don Valley watershed. That's how a 10 cm rainfall ended up pushing the Don River a couple of metres higher than normal.