Thousand year storm? How do you prove?
CLEMSON – The monumental rainfall that inundated most areas of South Carolina over a five-day period in early October has been described as a “thousand-year storm.” Certainly, historic amounts of rain fell from the sky – mostly on Oct. 4, but spread out from Oct. 1 to 5, with estimates that six trillion gallons of water fell on South Carolina.
Destructive volumes of water blasted through cities and suburbs. Power went out. Dams breached. Highways shut down. Roads crumbled. Bridges collapsed. And damage to crops cost South Carolina’s already-beleaguered farmers hundreds of millions of dollars. But was this truly a “thousand-year storm”? Is such a concept even provable?
“There is a high probability that certain portions of our state – those that exceeded 20 inches of rain – did experience a thousand-year event, and it is a fact that the majority of the state received a huge amount of rainfall during one of our historically driest months,” said Cal Sawyer, water resources specialist for Clemson Cooperative Extension.
“But since some of our oldest record-keeping is less than 125 years, it makes the ‘thousand-year’ concept impossible to prove.” Some areas of the state, especially on its western borders, received relatively little rainfall. For example, one National Weather Service rain gauge in McCormick County received only 0.05 inches.
Contrast that to several eastern counties, such as Georgetown, that received as much as 23 inches. “It is accurate to say that this was an enormous event, a devastating event,” said Sawyer, who is an associate professor in Clemson’s agricultural and environmental sciences department. “But when we call it a ‘thousand-year’ event, we are describing the statistical rarity of it as opposed to being scientifically accurate.
This is not meant to diminish the destruction that occurred. It is simply an exercise in probability.” Sawyer says it would be a mistake to write off October’s storm as a meteorological fluke that won’t happen again for another thousand years. “I hope that this will be viewed as a teaching moment, something that serves as an impetus to mitigate the financial and environmental impacts of severe flooding in the future,” Sawyer said.