Three recent natural disasters on U.S. soil have involved vast quantities of water. Hurricane Harvey turned Houston, Texas, into an “underwater metropolis,” CNBC reported Aug. 25, followed by Irma submerging the Florida Keys on Sept. 10, and Maria causing immense destruction in Puerto Rico on Sept. 20. The large vocabulary involving the action of water deserves clarification.
The waters submerging Houston are best termed overland flood, common in flat coastal plains when a severe storm turns up. The other type would be a flash flood, seen most often in mountainous or hilly terrain (like Hall County), when a single stream suddenly surges and overflows its banks.
The storm surge in a hurricane isn’t the water driven onshore by the wind. What the hurricane winds push along adds even more height to it. The storm surge is a large-scale rise of the ocean level, produced by the very low air pressure within the storm.
Put a straw into a glass of ice tea, suck gently and the tea inside the straw models the behavior of the ocean water. Storm surges, with the wind-driven waves on top of them, are the most dangerous aspect of hurricanes and tropical storms.
Riptides are dangerous to swimmers and don’t require a storm. Neither are they directly related to the tidal action caused by the moon. In a riptide, water is drawn away strongly from the beach, to compensate for the waves pushed onto it at some distance. The current can unexpectedly pull swimmers out into deeper water, which gets many in trouble.
Spring tides occur when the tidal range, or difference between high and low tide, is large. They are caused by alignment of Earth, moon and sun, or at least two of the three. When a tropical storm system makes landfall during spring tides, damage to oceanfront property is greater than during neap tides. A right-angle situation between Earth, moon and sun will cause the neap tides with their lower tidal range.
Overwash is a process becoming increasingly rare at barrier islands, including the Georgia ones. A natural, undeveloped barrier island gets washed over completely during large storms (hence the term). This distributes the sand and keeps the beach flat. Rows of buildings prevent this overwash, concentrating the wave force in front of the houses. This is the reason for beach erosion.
Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.