This PBS interview (PDF) with John M. Barry, author of Rising Tide, on the Mississippi flood of 1927, has an interesting quote that explains why engineers couldn't close the breach in the levee protecting the city from Lake Pontchartrain:
Well, you've got to understand that when there's a crevasse, it's not simply the water flowing over the top of the levee as if it were overflowing a bath tub. What you get is tremendous turbulence, unbelievable forces at work, and in a great crevasse the river will gouge out a hole in the earth and the greatest crevasse on record, which was in 1927 about 15 miles north of Greenville, Mississippi, you know, the hole in the levee was about two-thirds of a mile wide. And they sounded it with a hundred-foot line and found no bottom. It was later they figured out that it was 130 feet deep.No wonder the 3,000 pound sand bags disappeared as fast as they could drop them in the hole.