BLOG: What's A 1,000 Year Flood?

BLOG: What's A 1,000 Year Flood?

(WBTV) - When South Carolina Governor Nikki Haley said the state faced a 1,000-year storm, that meant the type of killer rain that lashed the Palmetto State this past weekend only comes around once every thousand years, right?

Well, not really.

So then, why is the South Carolina storm being called a 1,000-year event?

That's an obvious question, since none of us have been around for 1,000 years and reliable weather records for South Carolina go back no more than 150 years.

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 ten people dead, 40,000 needing clean water and thousands still without power Tuesday.

"We haven't seen this level of rain in the Low Country in 1,000 years," Governor Haley told reporters during a news conference Sunday. "That's how big this is."

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.

Often times, people interpret the 100-year flood definition to mean "once every 100 years".

It sounds as if that should be right, but in fact, it's wrong.  Any location could experience a 100-year flood two times in the same year, two years in a row or five times over the course of a century.  Or, more likely, as is the case in most communities,  a 100-year flood may never be experienced 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.

As I explained from the WBTV Morning News Alert Center Tuesday, 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 chance for event of this magnitude is so low, that virtually no meteorologist would make such a bold prediction in advance of a storm.  It's THAT rare of an event.

How is the measurement calculated?

The computation of the probability starts with data collected by gauges that measure rainfall or stream flow over a given river basin.

The gauges collect an incredible amount of data over many years, data 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 period of time.

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.

In general, most areas communities across the South Carolina Midlands and Low Country surpassed the threshold for a 1-in-a-1,000-year event, which in Charleston County is about 17 inches over a three day period.

Boone Hall Plantation, a popular tourist destination for Low Country visitors, just north of Mount Pleasant, reported more than two feet of rain over the weekend.

Some estimates from NASA suggest nearly 40 inches of rain fell on small portion of coastal South Carolina from September 26 through October 5, and rainfall between 27.5 and 37.5 inches fell over a large area of the state.

- Meteorologist Al Conklin

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