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  • In late August 2005, Hurricane Katrina was moving its way across the Gulf coast.

  • It was classified as a Category 3 storm.

  • Dangerous, but in a region with a long history of Category 4 and 5 hurricanes, it wasn't

  • expected to be the most intense.

  • Katrina was only one day away from the Louisiana coast when the mayor of New Orleans issued

  • an evacuation order.

  • Unfortunately, it was too late.

  • The sea level had already risen in a phenomenon called storm surge.

  • Water rapidly rose up against the city's levees, a series of walls designed to keep the area

  • from flooding.

  • Before Katrina made landfall, the levees broke.

  • A wall of water rushed into the city, trapping thousands.

  • What followed was one of the worst natural disasters in US history.

  • "We're expecting storm surge of 20 to 30 feet high"

  • "Flood waters are drenching city streets

  • "A levee broke during the height of the storm"

  • "There are bodies...uh... floating in the water there."

  • Storm surge was the main cause of death during Hurricane Katrina.

  • In fact it can be the most dangerous part of any hurricane.

  • It occurs when strong winds from an approaching hurricane push water into the shore.

  • As the sea rises, a bulge of water sweeps over coastal areas, causing destruction along

  • the way.

  • These are the normal astronomical tides at Dauphin Island to the east of New Orleans.

  • When you compare them to the water levels during hurricane Katrinayou can see the

  • dramatic rise.

  • That's the storm surge.

  • What makes this rise dangerous is that it starts to build up before the hurricane makes

  • landfall

  • So the coastal flooding from it can make evacuation procedures and the impact of a hurricane much

  • worse.

  • In 2008, hurricane Ike caused a big storm surge around Galveston, TX a day before landfall.

  • The rising water cut off evacuation routes stranding hundreds.

  • More recently, the National Hurricane Center issued dire warnings for the storm surge accompanying

  • Hurricane Irma, the most powerful storm to ever rage through the Atlantic Ocean.

  • A storm surge can also be unpredictable.

  • Rising sea levels caused by global warming increase the risk of storm surge.

  • But there are many factors that can change its size and strength.

  • For example, the largest one recorded before Katrina was about 22 feet, during Hurricane

  • Camille in 1969.

  • But Camille was a category 5 storm with 190 mph winds.

  • Katrina was only a category 3 with 130 mph winds, but it had a storm surge of about 28

  • Katrina had slower winds than Camille but it was twice as wide, which dramatically increased

  • the storm surge.

  • These calculations help forecasters warn communities at risk.

  • But they're often not enough to prevent damage on the ground.

  • This map shows the highest storm surge events worldwide since 1880.

  • In the US, the eastern and southeastern coastlines are among the most vulnerable areas for storm

  • surges in the world.

  • Along the east coast, hurricane Sandy got its strength from a massive storm surge in

  • 2012.

  • On the Gulf Coast side, places like Texas and South Florida have seen multiple hurricanes

  • so they've built infrastructure to help defend against excessive flooding.

  • Levees and seawalls are designed to stop or redirect rising water away from cities.

  • But even those can be inadequate, as seen during hurricanes Katrina and more recently,

  • Harvey.

  • What really concerns experts, though, are places that don't experience a lot of hurricanes

  • but are still vulnerable to storm surge.

  • This map shows that in the event of a big hurricane, based on the characteristics of

  • the shoreline, the coasts of Northwest Florida and Georgia would be at comparable risk to

  • the Gulf Coast.

  • These areas have shallow water, which means sea level can rise faster and water can reach

  • further inland making the flooding worse.

  • But they've seen fewer hurricanes than the Gulf Coast and they are likely to be less

  • prepared.

  • So when a major hurricane like Irma hits low-lying areas like these, the storm surge can be the

  • first and deadliest thing headed their way.

In late August 2005, Hurricane Katrina was moving its way across the Gulf coast.

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