Sunday the Governor of South Carolina explained that the state has not seen the current level of flooding in 1000 years. I doubt that was what she was told. But I understand how she might have heard it.
Monday morning’s headline in USA Today was 1,000-YEAR STORM SLAMS S.C. The millennial reference was not explained.
‘1,000-year’ rain floods South Carolina, and it’s not over yet is how the CNN website headlined their video-and-text reporting. The text-based version includes, “A “1,000-year rainfall” means that the amount of rainfall in South Carolina has a 1-in-1,000 chance of happening in any given year, CNN meteorologist Taylor Ward said.”
A thousand-year-flood or a hundred-year-flood are shorthand descriptions for the annual exceedance probability for such a flood-event. Too often this is heard, as by the governor, as suggesting it has been a very long time since such an event and — of even more concern from a public understanding and public policy perspective — it will be a very long time until such an event recurs.
Here’s how the U.S. Geological Survey explains exceedance probabilities:
In the 1960’s, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program. The 1-percent AEP flood was thought to be a fair balance between protecting the public and overly stringent regulation. Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or exceeded in any 1 year, and it has an average recurrence interval of 100 years, it often is referred to as the “100-year flood”. Scientists and engineers frequently use statistical probability (chance) to put a context to floods and their occurrence. If the probability of a particular flood magnitude being equaled or exceeded is known, then risk can be assessed. To determine these probabilities all the annual peak streamflow values measured at a streamgage are examined. A streamgage is a location on a river where the height of the water and the quantity of flow (streamflow) are recorded. The U.S. Geological Survey (USGS) operates more than 7,500 streamgages nationwide (see map) that allow for assessment of the probability of floods. Examining all the annual peak streamflow values that occurred at a streamgage with time allows us to estimate the AEP for various flood magnitudes. For example, we can say there is a 1 in 100 chance that next year’s flood will equal or exceed the 1-percent AEP flood. More recently, people talk about larger floods, such as the “500- year flood,” as tolerance for risk is reduced and increased protection from flooding is desired. The “500-year flood” corresponds to an AEP of 0.2 percent, which means a flood of that size or greater has a 0.2-percent chance (or 1 in 500 chance) of occurring in a given year.
I have done some modest work with the Charleston, South Carolina region on community resilience. They have been concerned about flooding and especially the potential impact of a strong hurricane. The current storm has “just” been a rain event, but it has exposed — again — many of the vulnerabilities of concern to the community. Even without strong winds and storm surge, massive volume and high tides have been destructive enough.
The greatest challenge in Charleston — and everywhere else I have ever been — is a strong tendency to diminish attention to what are perceived as rare risks. Too often I have seen individuals and organizations decide that since they just experienced something exceedingly rare they have essentially been indemnified from future losses.
Rather, at least in many coastal communities, there may be good cause — data-driven and statistically sound — to reasonably assume that this weekend’s extremes will recur with increasing frequency in the next hundred years. Instead of indemnifying, it is just a down payment.
Late Monday USA Today reported:
The biblical flooding in South Carolina is at least the sixth so-called 1-in-1,000 year rain event in the U.S. since 2010, a trend that may be linked to factors ranging from the natural, such as a strong El Niño, to the man-made, namely climate change.
So many “1-in-1,000 year” rainfalls is unprecedented, said meteorologist Steve Bowen of Aon Benfield, a global reinsurance firm. “We have certainly had our fair share in the United States in recent years, and any increasing trend in these type of rainfall events is highly concerning,” Bowen said.