When South Carolina officials said the state faced a 1,000-year storm, that meant the type of killer rain that lashed the state only comes around once every thousand years, right?
Well, not quite.
Then why is the South Carolina storm being called a 1,000-year event?
The terms “100-year storm,” “500-year storm” and “1,000-year storm” are standard measures used in weather circles to denote a rare event, like the severe rainfall in South Carolina that left nine people dead, about 40,000 needing clean water and 26,000 without power.
“We haven’t seen this level of rain in the Low Country in 1,000 years,” Gov. Nikki Haley told reporters during a news conference Sunday. “That’s how big this is.”
Workers and volunteers at the University of South Carolina bag bottled water for distribution to students on campus in Columbia, S.C., where flooding has made tap water unsuitable to drink without boiling.(Russ Bynum / Associated Press)
Flooding around homes in the Carolina Forest community in Horry County, between Conway and Myrtle Beach, S.C., on Oct. 6.(Janet Blackmon Morgan / Sun News)
In this image taken with a fisheye lens, Karen Whalen saves a picture from a trash pile in front of her flood-damaged home.(John Bazemore / Associated Press)
Trey McMillian looks over the damage done by floodwaters on a road in Eastover, S.C.(Sean Rayford / Getty Images)
A swimming pool is surround by flooding from the Waccamaw River in Horry County outside of Conway, S.C.(Janet Blackmon Morgan / Sun News)
Jerry and Tracey Hardy evacuate their family on Oct. 6.(Jason Lee / Sun News)
Eric Van Sant rescues possessions from a flooded home in the Forest Acres neighborhood of Columbia, S.C.(Sean Rayford / Getty Images)
Jeanni Adame uses her boat to check on neighbors near Summerville, S.C.(Mic Smith / Associated Press)
DNR officer Brett Irvin and Lexington Co. Deputy Dan Rusinyak carry June Loch to dry land after she was rescued from her home in the Pine Glen subdivision in the St. Andrews area of Columbia.(Tim Dominick / The State )
Five-month-old Jeremiah Odum, left, and his 2-year-old brother, Braxton, rest in a high school gymnasium being used as a Red Cross shelter for flood evacuees in Rowesville, S.C.(Russ Bynum / Associated Press)
Tara Saracina stands at her front door in the Ashborough subdivision near Summerville, S.C., after many of her neighbors left.(Mic Smith / Associated Press)
James Savage, left, and his girlfriend, Ianna Fincher, with her dog Lucy, kayak down Mayfield Street in the Ashborough subdivision near Summerville, S.C.(Mic Smith / Associated Press)
A vehicle floats in a small lake in the Forest Acres neighborhood of Columbia, S.C.(Sean Rayford / Getty Images)
Floodwaters close in on homes near Lake Katherine in Columbia, S.C.(Chuck Burton / Associated Press)
A business is destroyed by flooding near Gills Creek in Columbia, S.C.(Chuck Burton / Associated Press)
A vehicle and a home are swamped with floodwater from nearby Black Creek in Florence, S.C., on Oct. 5, 2015.(Gerry Broome / AP)
Children play in the floodwaters outside of Conway, S.C. on Oct. 4, 2015.(Janet Blackmon Morgan / AP)
Charlene Stennis takes her son Christian Hoo-Fong from a fireman after being stranded in a vehicle by Columbia, S.C., flooding.(Sean Rayford / Getty Images)
A boy tries to stay on dry by climbing along a fence on a flooded street in downtown Charleston, S.C.
(Mladen Antonov / AFP/Getty Images)
Farrell Rose and his fiancee Damita Trapp look away after floodwaters surrounded their home in Columbia, S.C.(Sean Rayford / Getty Images)
Firemen, from left, Norman Beauregard, Kevin Ettenger and Chris Rodgers, inspect the floodwaters at high tide in the historic downtown of Georgetown, S.C.(Mic Smith / Associated Press)
A dog tries to board a boat as two men row down a flooded street in Charleston, S.C.(Mladen Antonov / AFP/Getty Images)
The roof of a car peeks above the floodwaters in Columbia, S.C.(Sean Rayford / Getty Images)
James Atkinson attempts to fish in floodwaters near Garners Ferry Road in Columbia, S.C. after a record rainfall in the state.(Sean Rayford / Getty Images)
One of the more common mistakes is to assume that a 100-year flood means something that happens every 100 years, according to Federal Emergency Management Agency, which oversees the National Flood Insurance Program.
“Commonly, people interpret the 100-year flood definition to mean ‘once every 100 years,’” according to the agency. “This is wrong. You could experience a 100-year flood two times in the same year, two years in a row or four times over the course of 100 years. You could also not experience a 100-year flood over the course of 200 or more years.”
Then why even use the term?
One of the biggest uses for the measurement is to help determine the danger of flooding for insurance purposes and to help those who respond to disasters.
How is the measurement calculated?
Technically the measurement is a probability. A 100-year flood means that there is one chance in 100 of a flood occurring in each year. A 500-year flood means there is 0.2% chance of the flooding or rain event occurring each year and a 1,000-year event has a 0.1% chance of happening in any year.
The computation of the probability starts with data collected by gauges that measure rainfall or the flow of streams, explained Jayme Laber, a hydrologist with the Los Angeles office of the National Oceanic and Atmospheric Administration.
The gauges collect “tons of data” over years that can then be sorted into a variety of categories. Rain gauges collect water as it falls; flood or stream gauges collect data as the water flows past.
Using software, the data are then sorted to allow the meteorologist to determine what is typical rainfall in a five-minute burst, or in an one-hour period or any other duration, Laber said.
From those data, meteorologists then extrapolate the probability of a rainfall or flooding event and express it as a number: once in 50 years, 100 years and so on.
Have there been such events in Southern California?
For the Los Angeles area, a 1-in-1,000-year rain or flooding event occurs “pretty regularly,” roughly once a year somewhere in the fairly large region, Laber said. But that doesn’t mean every such event is a disaster.
For example, a 2011 spring storm in Palmdale was a 1-in-a-1,000-year event, he said. The area received about four times its usual daily rainfall, leading to flash flooding.
In December 2014, a “extremely rare rainfall” drenched Camarillo, at times dumping rain exceeding a 1,000-year event, Laber said. At one point, nearly an inch of rain fell over 14 minutes.
What are the details of the South Carolina event?
In South Carolina, the major storm began last week, part of a storm system that dumped more than a foot of rain across the state and soaked several others.
In general, most areas in South Carolina surpassed the threshold for a 1-in-a-1,000-year event, which in Charleston County is 17.1 inches over three days.
Boone Hall Plantation, just north of Mount Pleasant, in Charleston County, reported more than 24 inches of rain through Sunday morning.