Why levees don’t work

Experts have warned about New Orleans’ vulnerability for years, chiefly because Louisiana has lost more than a million acres of coastal wetlands in the past seven decades. The vast patchwork of swamps and bayous south of the city serves as a buffer, partially absorbing the surge of water that a hurricane pushes ashore.

Experts have also warned that the ring of high levees around New Orleans, designed to protect the city from floodwaters coming down the Mississippi, will only make things worse in a powerful hurricane. Katrina is expected to push a 28-foot storm surge against the levees. Even if they hold, water will pour over their tops and begin filling the city as if it were a sinking canoe.

After the storm passes, the water will have nowhere to go.

Before the levees were built, the river would top its banks during floods and wash through a maze of bayous and swamps, dropping fine-grained silt that nourished plants and kept the land just above sea level.

The levees “have literally starved our wetlands to death” by directing all of that precious silt out into the Gulf of Mexico, van Heerden said.