What storm surge is and why residents of the Lowcountry coast should take it seriously

If you’ve been paying attention to Hurricane Irma, you’ve probably heard the word storm surge a lot. Maybe too much. Maybe you’ve dismissed it as something hyped and meaningless. That would be a potentially grave mistake.

According to the National Hurricane Center, storm surge is the biggest threat to life during a tropical storm or hurricane. Once you understand it, storm surge is downright terrifying.

“More people die from the water in a hurricane than the wind” said James Carpenter, meteorologist with the National Weather Service in Charleston.

The 1,500 people lost during Hurricane Katrina? Most of them died either directly or indirectly because of storm surge.

So, now you know to be afraid of it. But why? What is storm surge?

Simply put, storm surge is what happens when a dome of water forms under a hurricane or tropical storm, said Carpenter. An even easier way to think of it is that water under a hurricane is collecting, or pooling.

As an analogy, lets think something else that collects water, your kitchen sink. Imagine filling it all the way to the top, then keep going. The water has to go somewhere. In the analogy, that is your counter, but in reality, we are talking about vast swaths of coastal land.

Now, imagine that you are an ant on the counter next to that sink.

There are several factors that influence storm surge, among them the slope of the continental shelf where a surge might happen. The shallower that slope, the worse a storm surge can be, and the Lowcountry has an extremely shallow continental shelf.

For this reason, the same surge conditions that would cause an eight or nine foot surge in Miami Beach, which has a steeper slope to its shelf, would cause much higher surge on Hilton Head Island.

Another thing to keep in mind is that a surge is not a tide, it is a dome of water in the ocean literally overflowing, but there can be astronomical tides on top of the surge, so a not at all unusual eight foot tide on top of a 12 foot surge can put you under 20 feet of water.

These storm tides can be devastating according to the hurricane center. Think of them as battering rams made of water. A cubic yard of water weighs 1,700 pounds according to the hurricane center, and there are untold cubic yards in the average tide.

When you take the combined effects of surge and tides, and even normal wind driven waves on top of both of those, many structures just can’t withstand the abuse.

Houses are destroyed. Cars are swept away. Lives are lost.

Storm surge is a serious matter and worth your attention. Thankfully, the hurricane center has an interactive map that you can use to assess your risk of being affected by storm surge.

If your location is covered by any of the colors below, that is how much water you should expect to be under. Plus tides. Plus waves, which means it might be time to think about evacuating while you can.

This story was originally published September 10, 2017 at 3:33 PM with the headline "What storm surge is and why residents of the Lowcountry coast should take it seriously."