HOUSTON, TX - Hurricane Harvey is on its way toward becoming one of the costliest hurricanes of all time, with damage being compared to Hurricane Katrina in 2005.

Katrina was a hurricane with a lot of preparation, but also a missed forecast. Katrina was originally expected to make landfall in Florida and continue into the US mainland. Instead, it turned east, headed back into the Gulf of Mexico and made landfall again as a Category 3 hurricane two days later.

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Harvey became a hurricane, then rose to a Category 4 and made landfall in a day and a half, giving people a lot less time to prepare.

But in both storms, the hardest-hit areas were coastal cities very close to sea level. New Orleans is built below sea level, using a levee system to keep water at bay. Houston, however, is only about 40 feet above sea level and is almost entirely flat.

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The levee system in New Orleans failed, causing the flooding of the entire city. By contrast, Harvey has simply dumped so much rain on the city of Houston that there's nowhere for it all to go.

Days after Katrina, tens of thousands of people were loaded onto busses and taken to refugee centers in Houston. Many never left. Now those same people may find themselves refugees again as Harvey continues to flood the city.