Anatomy of a Storm Surge

When a catastrophe like a major hurricane catches the collective attention of a nation, it isn’t surprising that citizens pick up part of the vocabulary from the experts and journalists who tell us about it. In 2005, when Katrina boiled onshore, we all heard terms like “Saffir-Simpson,” “Beaufort Scale” and “storm surge.” In the days surrounding a major event, such phrases become part of our consciousness for a time, and then we forget about them until the next disaster comes along.

Most people, when pressed, can come up with a fair definition of the term “storm surge.” We’ve heard it often enough that it’s almost part of our day-to-day language. However, though we all have a rough idea of what a storm surge is, we often get it confused with other phenomena. Strictly speaking, a storm surge is simply an onshore rush of water associated with a low-pressure weather system, usually a tropical cyclone (hurricane).

The most important force behind the formation of a storm surge is wind. As wind velocity increases, it simply pushes more water before it, much as your hand would if you swept it across the surface of a bathtub. The stronger the wind blows, the higher the “stack” of water grows.

A second important influence on the height of a storm surge is the weight of the air column (the barometric pressure) pressing down on the water’s surface. Since hurricanes are low-pressure weather systems, the water at the center of a hurricane tends to rise upwards. The more intense the hurricane is, the lower the barometric pressure at its center tends to be, resulting in a higher “bulge” on the water’s surface and a more pronounced storm surge.

Finally, the topography along the floor of the body of water (called “bathymetry”) influences the height of a storm surge as it gets pushed ashore. If the water is deep next to the shore, a storm surge tends to be lower; the energy of the surge can be dissipated downward into the deeper water, and not so much water gets pushed ashore. Storm surges that make landfall in shallower waters tend to be higher, and they reach farther inland because their energies cannot be dispersed. Conversely, deeper waters near shore produce higher and stronger waves than do shallow waters, due to the inherent mechanics of wave formation: circular wave energy patterns encountering steep shelves, as in deep coastal waters, direct their forces upward. Those same patterns coming ashore in shallow, sloping areas become elliptical and eventually flatten. Simplistically speaking, the place to be when a storm surge makes landfall in deep-water coastal areas is back from the beach. In shallow-water areas, a person wants to be WAY back from the beach.

A distinction should be made between “storm surges” and “storm tides.” A storm surge is the height of the water above the normal expected tide. A storm tide is the total height of the oncoming water, and includes the usual tide level added to the storm surge. Thus, if the normal tide level is two feet, and the storm surge adds an additional 15 feet, the storm tide will be 17 feet high.

As our planetary environment changes, we may all become more familiar with the effects of storm surges, abnormal tides, and other physical phenomena that are wrought by climatologic upheaval. For better or worse, we may become far more adept at discussing these meteorological events.