Homeland Security Watch

News and analysis of critical issues in homeland security

October 5, 2015

Carolina flooding and exceedance probabilities

Filed under: Catastrophes,Recovery,Resilience,Risk Assessment — by Philip J. Palin on October 5, 2015

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 been “just” 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 worked — 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 is 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.


Yikes!  Here are the two opening sentences and the closing sentence of the lead editorial in the October 6 Charleston Post and Courier:

Catastrophic rainstorms like this one happen only every thousand years, climate and weather experts explained as unprecedented flooding forced people out of their homes, closed roads and submerged stalled vehicles. One can only hope that assessment holds true… We must work to prepare for the next disaster, even if it doesn’t happen for a thousand years.

The P&C is a much better than average daily newspaper.  When they get this so very wrong, we can be sure most other folks will too.

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Pingback by Prepper News Watch for October 5, 2015 | The Preparedness Podcast

October 5, 2015 @ 12:36 pm

[…] Carolina flooding and exceedance probabilities […]

Comment by Arnold Bogis

October 5, 2015 @ 9:02 pm


You win the award for most subtle argument that climate change is occurring:

“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.”

I hope I didn’t ruin the effort. In your experience, do the folks you work with in at risk areas take the idea seriously?

Comment by William R. Cumming

October 5, 2015 @ 11:31 pm

Thanks Phil! And apparently 39 S. Carolina dams failed or failing. FEMA does not map the flood hazard of failed dams nor do many below dams buy flood insurance, and some including me might argue for mapping the storm not the flood.

Over the course of the last week nowhere inn S.C. has rasinfall exceeded 20 inches or more.

Hurricane Camille dropped 39 inches in 36 hours in Nelson Country, VA., 1969!

Comment by Philip J. Palin

October 6, 2015 @ 4:04 am

Arnold: “Sea-rise” and subsidence are widely accepted. The cause of sea-rise can be contentious to discuss. Some are very serious about climate change. Others are deeply skeptical. There is an increasing consensus regarding consequences, regardless of cause. In my experience this disagreement regarding cause is less an impediment to meaningful engagement with mitigation or resilience than other factors. There is almost a competition between “growth” and “resilience” for mind-share. “Sustainability” seems to be promising approach for finding some common ground.

Comment by William R. Cumming

October 6, 2015 @ 7:52 am

Long long ago in a galaxy far far away the National Flood Insurance Maps under the National Flood Insurance Act of 1968, as amended, should have been additionally amended to require all NFIP maps to reflect sea level rise, whatever the cause.

And this was NOT the 1000 year flood or storm for S.C.!

Comment by William R. Cumming

October 6, 2015 @ 12:12 pm

Supposedly all disasters local but responses not necessarily so. Within 6 months a Presidential Primary in S.C. and wondering how candidates will address this event?

A Presidential Disaster declaration for this event issued.

Comment by William R. Cumming

October 6, 2015 @ 3:50 pm

Excellent discussion of the 1,000 year flood and the 1,000 year storm linked on today’s RECOVERY DIVA post and provided by the former head of the ASFPM [Association of State Flood Plain Managers]!

Thanks Claire and Chad!

Comment by Vicki Campbell

October 7, 2015 @ 2:43 am

This comment should really be posted on the Recovery Diva blog in relation to this topic, but I’m tired of the bizarre, school marmish censorship, for the most superficial reasons, that goes on behind the scenes there around posting comments. So I’ll post it here instead because I think it ties in well with what Phil has posted.

Bill, I agree that the discussion by Brad, via Chad, was mostly good – but he (they) got one thing wrong, IMHO, that is a classic and major example of risk miscommunication that it seems to me has caused a lot of mitigation and preparedness planning errors on the part of both the public and many managers and policy makers about the most deadly and destructive natural disaster there is, floods.

To use the 500 year flood example instead of the more dubious (as Brad points out) 1,000 year example – to tell the public that a 500-year event means there is a one in 500 chance of a flood, or storm, or whatever, of happening in any given year is not only fairly meaningless but misleading in the abstract. Communicating probability within a theoretically infinite time-frame is neither a very useful or accurate way of conveying actual risk to the public, or thinking about it as a policy or planning decision-maker. It is also not any way that the public should be expected to think about or comprehend the information either. Because of its general irrelevance in the abstract, the human mind will try to ground the statement in time more clearly – usually within a timeframe beginning with the last 500-year event if no other is defined – which they will then misinterpret unless they have training in probability analysis, because the human mind is not inherently well designed for the purposes of grasping probabilities very well, especially when dealing with extreme events, or events on the margins.

More importantly however, as I understand it, is that once the one in 500 chance of an event is grounded within any timeframe, and the clock starts ticking, the probability statement itself becomes inaccurate, and begins misconveying the actual risk involved, which actually changes from year to year – it does not stay the same. It is neither wholly random or constant from one year to the next for every consecutive year that another 500 year event does not occur.

So I guess I would have to disagree with you, Phil, in that I think the real mistake in understanding risk begins not with the public, or the Charleston Post, but with the traditionally crude way in which risk has been communicated, as well as calculated. So whether we’re talking about a mortgage or the time it takes to plan and complete a structural mitigation project, risk analysis and communication both, to my mind, needs to be much more competent, specific, rigorous, and more than anything, timely than offered by the almost useless barometer of the one-in-pick-your-number year event rating process currently used.

Comment by William R. Cumming

October 7, 2015 @ 9:21 am

Vicki! Agree with your excellent comment. And IMO much of S.C. will be in recovery next two decades. Will be interesting to watch Congressional action, and in particular with a Presidential Candidate still in for the Republicans from S. Carolina- a current US Senator.

As a start a special LIDAR survey of the entire state of S.C. post flood should be paid for by Uncle Sugar and detailed research on the event also conducted and paid for by the FEDS.

Comment by Vicki Campbell

October 7, 2015 @ 1:27 pm

Thanks, Bill. One of the best example I’ve run across of the fundamental and potential deadly confusion that flows from poor risk probability analysis and communication is when the then Commander of the Army Corp of Engineers, Carl Strock, stated a few days after Katrina struck New Orleans that “we figured we had a two-humdred to three-hundred year level of protection. That means that an event that we were protecting from might be exceeded every two-hundred or three-hundred years. So we had an assurance that, 99.5%, that this would be okay. We, unfortunately, have had that 0.5% activity here.”

In reality, as the actual head of the Army Corp of Engineers was thinking and making such a strikingly wrong assertion, when Katrina side-swiped New Orleans, the chances of that specific category of storm doing so that year was actually 1-in-6, otherwise known as Russian roulette.

There is of course much more to the story of Katrina, the levees, and the Army corp’s overall incompetence. But for the purposes of this conversation, the above comments by the Commander was definitely a “Houston we have a problem” moment, IMHO.

Comment by William R. Cumming

October 7, 2015 @ 1:42 pm

A footnote–now the USACOE uniformed chief of the Lower Mississippi River Division no longer a shoo-in to head USACOE!

Comment by William R. Cumming

October 7, 2015 @ 1:43 pm

A new CRS report on disaster recovery for bridges and transit systems dated 10/02/2015:


Comment by Tom Russo

October 8, 2015 @ 4:09 pm

I live in Myrtle Beach and my relatives in Chicago believe I am under water. It great effort to describe the reality, the geography and the excessive hype.

It was nice to check here at the hlswatch blog and find some reality!

Comment by William R. Cumming

October 9, 2015 @ 10:23 am

Tom! Dr. Susan Cutter lives in Columbia!

Comment by Erik Rau

October 12, 2015 @ 2:44 pm

I think Vicki made some excellent points, especially this one: ‘…the actual risk involved, which actually changes from year to year… is neither wholly random or constant from one year to the next…’

Most emergency managers of my acquaintance (myself included) do not have a strong background in or even appreciation for the statistical analysis of risk. In preparation for or response to an event, we focus on subject-matter-expert statements, but very few emergency management agencies that I’ve seen track events in a meaningful way, much less apply any decision theory practices like Bayesian analysis.

Sadly, I’m just parroting noises I’ve heard, I don’t really understand enough to recommend specific approaches. I will say that some of the work by Charles Twardy in the realm of wildland SAR seems very interesting. SAR in general has a nearly thirty-year history of keeping and using statistics, however, that I’m not sure emergency management has yet developed. Perhaps we can stand on the shoulders of the SAR community, however, and become quick learners.

Comment by William R. Cumming

October 13, 2015 @ 7:04 am

Erik! Great comment! Many other disciplines also do not understand RISK–often ignored by ECONMISTS, e.g., when doing benefit cost analysis.

And only crudely understood by the MILITARY worldwide.

Comment by William R. Cumming

October 24, 2015 @ 9:26 am

N.B. {note Bene] that the September 2015 NPG [National Preparedness Goal] makes almost no mention of PREPAREDNESS in the traditional sense and has incorporataed under the word PROTECTION many traditional PREPAREDNESS items.


IMO PREPAREDNESS consists of planning [what is the risk analysis and planning basis]; logistics and mobilization beyond the planning basis; funding 24/7/365; training, personnel and resources; systems and communications; and in general PREPAREDNESS equals CAPABILITY.

Thus the NPG undermines PREPAREDNESS in my opinion.

The real question? How fast can the day job assets of federal departments and agencies be turning into CRISIS MANAGEMENT and RESPONSE ASSETS?


Check out the PREPAREDNESS budget of the ARC and you can see that this federally charter and statutorily refernced NGO has almost no PREPAREDNESS BUDGET.

Hoping that FEMA refugee Brad Keiserman is reading this comment.

Comment by William R. Cumming

November 19, 2015 @ 10:16 am

Phil! Question?

I keep getting this post and thread first when I click on HLSWatch.com! Am I the lone ranger?

Comment by William R. Cumming

December 5, 2015 @ 10:45 am

Am hoping that more details can be provided on the S.C. event?

Pingback by 1000-year storms in South Carolina - James Howard

December 16, 2015 @ 3:04 pm

[…] Homeland Security Watch has an excellent post on the recent claims that South Carolina has faced at 1000-year storm. Focusing on the annual exceedance probability (AEP), the story post throws a bit of a damper on it. […]

Comment by William R. Cumming

December 22, 2015 @ 5:15 am

The Congress mandated a report now releases by the NAS on affordability and the risk premiums for NFIP insurance:


Comment by William R. Cumming

December 29, 2015 @ 7:57 am

Great Britain is about to launch a comprehensive review of its flood control structural policies. Wondering if USACOE will be helping?

And Samatha Medlock will soon be departing the CEQ as a staffer in the Obama Administration. IMO she is an important player on water and flood control issues.

Comment by William R. Cumming

February 27, 2016 @ 5:23 am

A period for open comment has occurred for the world of hydrology on Publication 17!

Comment by William R. Cumming

February 27, 2016 @ 5:36 am

For background info and history of Pub 17 go to:


Comment by William R. Cumming

March 2, 2016 @ 6:09 am

In a move I missed Sam Clovis became a national Campaign Co-Chair for Donald Trump! This comment is not a political recommendation but if you don’t know Sam you should know and read what I believe is the Most Outstanding Article yet published on HS and federalism since 9/11/01! See link below:


Comment by William R. Cumming

March 9, 2016 @ 9:31 am

How is recovery going in S.C.?

Comment by William R. Cumming

March 21, 2016 @ 2:40 am

OIG DHS and GAO have released a highly critical report of the NFIP’s WYO [Write-your-Own] program.

Comment by plasticpyrolysisplants.com

July 24, 2017 @ 3:00 am

Perhaps the natural disasters are unavoidable, but we can prevent it in advance.

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