What Is Storm Surge and Why Is It Dangerous?

Hurricanes can move a lot of water from coastal areas during storm surges. Pictured here is Apalachicola Bay near the Florida Panhandle during Hurricane Irma. The area's normal seaside view is now more of a mudside view. Mark Wallheiser/Getty Images

Hurricanes bring to mind torrential rains and powerful winds, but the destruction hurricanes can cause may happen before they even make landfall.

Storm surges are the cause of that destruction. These are influxes of water that are essentially pushed inland by hurricanes, and they are generally responsible for much of the damage associated with hurricanes.

Water where it doesn't belong

An illustration of a storm surge wave
This illustration shows how a storm surge tide functions: with a wall of water flowing higher than water typically goes in an area. Robert Simmon/NASA/NOAA/Wikimedia Commons

The intensity of storm surges depends on a number of factors, but the cause is the same. As explained by the National Hurricane Center [PDF], the circulation of the wind around a hurricane's eye creates creates a vertical circulation on the ocean water below. When the hurricane is just out over the ocean, this vertical circulation isn't disturbed, and there weren't be any sign of a surge.

However, as the hurricane approaches land and the water becomes shallower, the ocean bottom disrupts that circulation. The water cannot travel down as it did when the storm was over the ocean. The water has no place to go but toward the land. We're used to the normal flow of the tides along coastlines, but all this extra water, pushed up by the storm, ends up on top of the usual tide. The result is the storm tide.

A visualization of a storm surge can be seen in the video below, a sort of a tempest in a tumbler.

The soy-sauce-and-water combination getting pulled into the glass is a good representation of water being sucked up by a hurricane. The fellow on the video cites a low pressure system as a cause for a storm surge, but it isn't the sole or even the most important factor, according to the National Hurricane Center. Other factors, like the storm's speed, size and angle of approach matter, too. Similarly, the coast's features, from barriers that may disrupt the flow of the water or the depth of the ocean floor, also play an important part in a storm surge.

For instance, a faster storm will create a higher surge than a slower one along the open ocean, but that same slower storm would create a higher surge in water that's more enclosed, like in a bay or on a sound. Gradually deepening shorelines, like those found along the Gulf Coast, result in higher surge tides.

You may be wondering how this is different from a tsunami wave, since both are large walls of water that sweep onto land. The key difference is that tsunami waves are triggered by a geological event, like an earthquake, while storm surges are meteorological events in and of themselves.

Surge effects

As the Vox video above explains, storm surges often occur before a hurricane even makes landfall. The resulting flooding can make evacuations dangerous and difficult, compounding the risks once the hurricane actually arrives.

Surges are also the cause of most of the damage and the death associated with hurricanes. This report from the American Meteorological Society says that almost 50 percent of hurricane-related deaths between 1963 and 2012 were storm surge-related, and the National Hurricane Center credits storm surges, directly and indirectly with many of the deaths during Hurricane Katrina in 2005.

Given how surges sweep onto land, the damage they do is often twofold. There's the damage done by the flooding, and there's damage from floating debris that functions as battering rams during the surge.

Storm surges also can create land where there's typically water. The photo at the top of this file shows a muddy bay along Florida's Panhandle during Hurricane Irma. Tampa Bay itself become a dog park during Irma.

Even though the water in these areas was drained by Irma, the water returned as a very serious risk to everyone there. The National Weather Service even issued a flash flood warning for areas in Florida so residents would understand the dangers.

Preparing for the storm surge

First and foremost, it's important to pay attention to evacuation orders provided by weather services. Sometimes they can come too late, as they did with Katrina, but in other instances, there's enough warning, and it's best to get to some place safe. Along those lines, it's best to not underestimate a storm surge, either. As Weather Underground points out, storm surges are often a rapid rise in water over the course of a few minutes, meaning you may not have enough time to get out if you stayed, and that blocking the door with a towel isn't going to do you much good.

Apart from that, the tips involved in preparing for a hurricane still apply: Know your evacuation route and destination, have extra cash, make plans for pets and board up your home the best that you can.

A more long-term plan for storm surge prep is to encourage the conservation of wetlands in your area. Swamps, estuaries and the like promote growth of vegetation that can absorb some of the surge and prevent them from reaching developed parts of the land.