I recall reading somewhere that the second levee to go was the industrial canal. It is one that connects the Mississippi to the lake.
Here is a pic of the second breach:
http://us.news3.yimg.com/us.i2.yimg.com/p/ap/20050830/capt.ladp11108301631.hurricane_katrina_ladp111.jpg
Here is the pic of the levees:
http://www.nola.com/hurricane/popup/nolalevees_jpg.html
If the second breach is indeed at the industrial canal, then NOLA is lost unless they can stop it. Otherwise, the Mississippi is going to flow right in.
The question is, where exactly is this second breach? The water looks like it is flowing from the top right of the picture towards the middle. Is the middle the canal? The top right looks like a flooded neighbourhood of some sort.