LIVINGSTON – The flooding Friday and Saturday that led to record-shattering crests at the Amite River and near-record highs along the Tickfaw at a staggering pace sent shockwaves throughout Louisiana and the rest of the nation – and it has puzzled meteorologists.
The unprecedented rainfall events in the area, in fact, have less than a 1 percent chance of occurring in any given year, according to the National Oceanic and Atmospheric Administration.
The 30-inch downpour that has devastated nearly 90 percent of Denham Springs and flooded more than 70 percent of Livingston Parish has led NOAA to classify the rain event as a once-in-every-500-years flood.
For Livingston Parish, it may have exceeded the statistics of even the 500-year event.
In a parish in which an estimated 40,000 homes were flood – and 90 percent of them considered possibly a “total loss” in Denham Springs – observers from NOAA believe the damage, based on population and statistics, could surpass the devastation New Orleans and the Mississippi Gulf Coast suffered in Hurricane Katrina.
According to NOAA, Louisiana is among six states – the others being Texas, West Virginia, Maryland, South Carolina and Oklahoma – that have endured record rainfall events.
The U.S. government, when it created its National Flood Insurance Program, devised a measure it calls "a 1-percent annual exceedance probability flood" (AEP) which sets an estimate on the chance of repeat flooding of a certain level in a certain area, according to NOAA.
The AEP defines floods that, based on statistics, have a 1-in-100 chance of being equaled or surpassed in any one year, which led to the term “100-year flood.”
The “500-year flood” equals the AEP of .2 percent, or a 1-in-500 chance an area will see the repeat of flooding at that level.
According to NOAA, the annual measurements are based on the strength of the flow of a body of water and the peak height of the water as recorded by steam gauges, which are placed along a river.
They collect numerical data over time, which determines the probability that a river will exceed those measurements in any given year.
The term “1,000-year flood” does not necessarily refer to an actual number. Instead, it’s a reference to statistics on the chance of a massive flood in a given river.
In other words, severe floods can occur two consecutive years, but the chances are extremely rare.
WAFB meteorologist Jeff Morrow, a resident of Livingston Parish, offered a different perspective. He said the many tributaries in the area will naturally bring more flooding to an area.
The mere geographical makeup of south Louisiana is waterways that include the Mississippi River – the largest river in North America – along with the Amite and Comite Rivers and hundreds of lakes, bayous and other tributaries, make the region much more susceptible for flooding, he said.
“Without question, this is the worst flooding we’ve seen in this area,” Morrow said. “But trying to put a perspective of a year event on this is foolish, based on where we live. It’s just the nature of where we are in Livingston Parish.
“The way we developed the areas, we’ve exploded the population and that makes it appear that much worse – and it is, because most people are in the impact zones,” he said.
“Think back on March and what we had in Livingston and Tangipahoa parishes, and the 1983 flood -- a historic event of 30 years -- and then go to the historic flood event in 1973 and 1977, and the one on the Mississippi River in the 1920s,” Morrow said. “In a one-hundred year span, we’ve had five.”
It remains unknown, however, why Livingston Parish has endured two severe floods this year – the other in March – while six other severe floods have occurred in less than a year.
Some experts attribute it to climate change and say that the gradually warmer temperatures on land and sea have led to an accumulation of moisture, which triggers much larger flooding events.
“We have been on an upward trend in terms of heavy rainfall events over the past two decades, which is likely related to the amount of water vapor going up in the atmosphere,” Dr. Kenneth Kunkel, of the Cooperative Institute for Climate and Satellites, told The Guardian.
“There’s a very tight loop – as surface temperatures of the oceans warm up, the immediate response is more water vapor in the atmosphere. We’re in a system inherently capable of producing more floods.”
David Easterling – a director at the National Centers for Environmental Information, which is operated by the NOAA – told The New York Times that the flooding “is consistent with what we expect to see in the future if you look at climate models. Not just in the U.S. but in many other parts of the world as well.”