Hurricane Laura could bring an ‘unsurvivable’ wall of water up to 20 feet high. Here’s how storm surges get so dangerous.

A partially submerged car on a flooded street after Hurricane Florence struck Piney Green, North Carolina, September 16, 2018. Carlo Allegri/Reuters
  • Hurricane Laura is forecast to bring an “unsurvivable” surge of up to 20 feet of water to parts of coastal Louisiana and Texas.
  • Storm surges like this happen when hurricanes or tropical storms push coastal water levels above the expected tides.
  • Storm surges can be more deadly than winds, since they cut off evacuation routes and sweep away vehicles.
  • Rising sea levels make storm surges more devastating.
  • Visit Insider’s homepage for more stories.

Hurricane Laura is gathering strength in the Gulf of Mexico as it churns toward Louisiana and Texas.

The storm reached Category 4 status, with sustained wind speeds of 140 mph, on Wednesday. Before the cyclone even makes landfall, forecasters expect a dangerous flood of ocean water.

“Little time remains to protect life and property before water levels begin to rise and winds increase in the warning areas,” the National Hurricane Centre tweeted on Wednesday afternoon.

The NHC forecast warned that an “unsurvivable storm surge with large and destructive waves will cause catastrophic damage from Sea Rim State Park, Texas, to Intracoastal City, Louisiana.” The agency expects water to rise up to 20 feet above the ground along much of the Louisiana coast, with up to 15 feet in some parts of Texas.

Storm surges happen when hurricanes push water inland from the ocean. This is where most of the flooding – and much of the danger – comes from.

The NHC has issued a storm surge warning for the coast from Freeport, Texas, to the mouth of the Mississippi River, indicating that forecasters anticipate a life-threatening surge of water there.

Hurricane laura potential destruction infographic

“This surge could penetrate up to 40 miles inland from the immediate coastline, and flood waters will not fully recede for several days after the storm,” the forecasters wrote.

Here’s how a storm surge happens and why it’s so dangerous.

Hurricanes stir up ocean water, then push it onto land

Hurricanes are large low-pressure systems that create a cyclonic wind effect. Those winds force ocean water to spin down into the water column, creating vertical circulation in the ocean below. Once the hurricane reaches shallow coastal waters, the ocean bottom interrupts this cycling. Unable to go down, the water pushes out onto land.

These water levels can rise rapidly – a few feet in just one minute, according to the Coastal Emergency Risks Assessment.

How a hurricane forms inforgraphic

Storm surge can be deadlier than hurricane winds

While hurricane-force winds can rip roofs off of homes and take down trees and power lines, the surge of ocean water rushing inland often causes more damage. It doesn’t take much, either: According to the NHC, just 6 inches of fast-moving water can knock over an adult, and it only takes 2 feet to carry away an SUV.

Hurricane Katrina’s 20-foot storm surge in New Orleans breached the city’s levees 15 years ago. Elsewhere, Katrina’s storm surge reached 30 feet. More than 1,800 people died in that storm and the ensuing floods, which also caused $US108 billion in damage.

Hurricane Katrina
Hurricane Katrina floods New Orleans, August 29, 2005. AP Photo/Eric Gay

Storm surge often arrives before a hurricane’s winds, closing off roads, cutting evacuation routes, and leaving people stranded in flood zones. That’s part of what makes the phenomenon so dangerous. Surges can also continue after the storm’s centre passes, preventing emergency responders from reaching flooded areas.

The Saffir-Simpson scale, which ranks hurricanes in categories 1 through 5 based on wind speeds, does not account for storm surge, so even cyclones weaker than Katrina or Laura can produce huge walls of ocean water.

If a surge coincides with high tides, it gets an extra boost. That’s what happened when Hurricane Sandy flooded New York City in 2012.

Heavy rainfall can also compound the effects of a storm surge, dumping more water on top of the ocean flood. That was especially true during Hurricane Florence, which drenched the Carolinas in 2018, and Hurricane Harvey, which dumped more than 60 inches of rain on parts of Texas in 2017 and killed at least 68 people.

Forecasters don’t think Hurricane Laura’s rainfall will be as heavy as that of either of those cyclones – the NHC expects up to 15 inches of rain – but its storm surge is set to surpass theirs by several feet.

Warming, rising oceans can make storm surges more devastating

Larger, stronger, faster cyclones generally produce higher storm surge. And as Earth’s oceans and air get warmer, tropical storms overall are getting stronger, wetter, and slower.

Hurricane Dorian flood Nassau bahamas
Houses line a flooded street after the effects of Hurricane Dorian arrived in Nassau, Bahamas, September 2, 2019. John Marc Nutt / Reuters

Rising sea levels can make storm surges even more devastating by giving them a higher starting point and allowing them to reach further inland.

“Our confidence continues to grow that storms have become stronger, and it is linked to climate change, and they will continue to get stronger as the world continues to warm,” James Kossin, an atmospheric scientist at NOAA who studies how climate change affects tropical cyclones, told the Washington Post.