A cofferdam is a temporary water retaining (or excluding) construction aid. In this case, you build a ring around the breach by dumping a LOT of fill from barges, or from dump trucks if you have road access. Once it is higher than the waterlevel, you can pump it dry to unwater the worksite and repair the levee.
There has been a report that engineers are working to stem a breach near the 17th street canal in western NO (reported as memory serves, no guarantees as to accuracy). That's a different thing, an ongoing fight against time and moving water. If true, these guys are heroes, deliberately standing into harm's way to try and save NO.
As to method, you dump everything you have or can get your hand on into the hole and hope it clogs. At this point, I'd consider using demo charges to drop a building across the hole if there was one available. Point being, whatever it takes. Don't expect miracles, some reports have breaches in excess of 200 feet or yards, don't remember which.
How many cubic yards of fill would be requirewd to bring the city above water level permanently?
Thanks,,