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 Katrina — you 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.