Real threat is storm surge

Sea rise in the Naples-Fort Myers area was expected to be 10 to 12 feet

ORLANDO — Hurricanes are often defined and categorized by their wind speed, but the real danger comes not from furious winds but from the sudden, often fatal rise of the sea.

Storm surge is the wind blowing water toward the shore at a rate at which it overwhelms levees and seawalls designed to protect the shore. If the area close to shore is shallow, as it is in Florida, the water builds quickly and at some point overwhelms the surrounding coastal area, because it has no place else to go.

Other factors besides geography can make this dangerous. The size of the storm, barometric pressure and the direction the storm is coming from are factors in causing the sea to surge.

Large storms, such as Irma, are bad because water can start to build well in advance of the storm, in a phenomenon sometimes called a “bubble” or “bulge” of water. Most coastal communities on the Atlantic Ocean and Gulf of Mexico are no more than 10 feet above sea level, so the ability to take a big storm surge is not great.

The real danger for the people of Naples, Fort Myers and Tampa, on the west side of the state and the storm, will come after the eye has passed. As the storm approached, water was being drawn from the shore, creating an illusion of a very low tide. As soon as the eye passes and the winds switch, the water that had receded will be blown back, as well as the reserve of water that had built up.

The storm surge in the Naples-Fort Myers area was expected to be 10 to 12 feet.

The other problem with a storm surge is that they are fast: A 10-foot storm surge at sea level can submerge the first floor of a building in a matter of minutes.