Technology & Science

How storm surges beach manatees and flood inland streets

With back-to-back hurricanes in the last few weeks, we've been bombarded with contrasting images of high water rushing over inland streets and manatees stranded on dry beaches. Here's the science behind those phenomena.

Hurricane-force winds can drastically change shorelines by pushing water toward or away from shore

This photo provided by Michael Sechler shows a beached manatee in Manatee County, stranded after waters receded from the Florida bay in a negative storm surge as Hurricane Irma approached. (Michael Sechler/Associated Press)

With back-to-back hurricanes in the last few weeks, we've been bombarded with contrasting images of high water rushing over inland streets and manatees stranded on dry beaches.

In the first case, where'd all that extra water come from? And in the second, where did the sea water go?

You can blame both on storm surge, what happens when powerful winds shove huge volumes of water toward — or away from — the shore.

Storm surge can be one of the most dangerous aspects of a hurricane. Massive amounts of water rush back towards shore and can drastically raise water levels, sometimes flooding streets and buildings far from shore. With Hurricane Irma, the National Hurricane Center issued warnings for storm surges of more than two metres.

Iconic Shem Creek is flooded as storm surge from tropical storm Irma hits Mt. Pleasant, S.C., on Sept. 11, 2017. (Mic Smith/Associated Press)

But there's also a strange phenomenon known as negative storm surge. That's what produced those extended, dry beaches in Florida that left some manatees stranded.

They were also an odd sight for residents used to water levels that don't deviate very much when the tide goes in or out, said Chris Fogarty, senior research meteorologist at the Canadian Hurricane Centre.

"It's wind pushing that water away," Fogarty said. "When wind is blowing off shore, you don't have the big waves at the beach because the wind is in the opposite direction."

On a smaller scale, Fogarty said you could watch how water in a puddle reacts to winds. Wind creates ripples based on the direction of the wind, which is essentially what happens in storm surge.

"It's all about wind direction," he said. 

"The strong wind shift blowing from one direction and then in the total opposite direction after the eye goes through ... pushes that same water right back in quite quickly behind the eye of the storm."

The impact of the storm surge depends on the speed of the wind, how low the pressure is in the eye of the storm and the shape of the coast, Fogarty said.

Residents inspect the extreme receding water in Tampa Bay ahead of Hurricane Irma on Sunday prior to the hurricane hitting the area. (Brian Blanco/Getty Images)

Harbours and bays are particularly affected by storm surge because water is funnelled into one place and pushed, which amplifies what happens to the water.

Because wind speeds vary depending on the part of the hurricane that hits a particular location, a small change in the hurricane's predicted path can cause a big change in the storm surge.

All these factors can make it difficult to predict how much storm surge will happen during a hurricane, Fogarty said. 

That's why it may end up being much bigger — or smaller — than predicted.

The water often returns very quickly. After a negative surge from Irma, more than 1.5 metres of water returned to one coast in less than an hour, he said.  

"If you see water get drawn out, it doesn't mean to go out and explore," Fogarty said. "It means something weird's going on. You don't want to be caught with the water coming back in."