During Katrina, the storm surge coming up the Intracoastal Waterway destroyed a 4, 000-foot long floodwall.

FORBES: Billions In New Defenses Mean Isaac Will Not Rival Hurricane Katrina In Damage

Those errors led to levee and floodwall collapses that destroyed much of New Orleans when Katrina hit seven years ago.

FORBES: Hurricane Isaac Will Test the New Orleans Levees

Days ago bused-in prison inmates worked shoulder to shoulder with the National Guard and local volunteers to build a makeshift floodwall of sand and gravel.

NPR: More Rain Expected For Already Swollen Rivers

The MTA took extensive measures before the storm to limit the amount of water flooding into tunnels, including by boarding up ventilation grates in low-lying stations, piling sandbags at station entrances, inflating a rubber dam in the Long Island Rail Road's West Side yards in Manhattan and hastily erecting a floodwall across the mouth of the No. 3 train tunnel near East 148th Street in Harlem.

WSJ: Salt Water Puts City's Subway System 'in Jeopardy'