The 100-Year Floodplain: Explained.

Floodplain maps.  This topic alone drives more confusion and misunderstanding than nearly any other one I encounter.  There are people who sincerely believe that because they do not reside within a mapped 100-year floodplain that they will not experience flooding or have a near zero chance of it….seriously.  To dig into why this assumption is flawed on many levels we have to first understand where these maps come from and what exactly they represent.

A mapped 100-year floodplain or Special Flood Hazard Area is, as per FEMA, defined as:

Flood hazard areas identified on the Flood Insurance Rate Map are identified as a Special Flood Hazard Area (SFHA). SFHA are defined as the area that will be inundated by the flood event having a 1-percent chance of being equaled or exceeded in any given year. The 1-percent annual chance flood is also referred to as the base flood or 100-year flood.

So first to address the naming of this flow event determined to be the base flood or 100 year flood.  As mentioned in FEMA’s definition, it is based on probability.  100 year event equates to an annual probability of 1%.  That does NOT mean that it happens only once every 100 years (give or take).  It can happen multiple times in one year, and over consecutive years.  A very important caveat to mapped floodplains and their associated probabilities…the mapping goes out of date, and very soon after creation in some cases.  The faster an area is developing the faster the mapping is obsolete.  Mapping is revised periodically, but accurate “boundaries” are constantly being chased.  That is to say nothing of the factors that are NOT taken into consideration in the mapping process.  It is a considerably imperfect science at best.

It may help to better understand why they are generated in the first place.  I feel like this quote from an article on the Myth of the 100 year Floodplain is rather eye-opening:

“We have basically taken a condition for insurance, which is based on probability, and set it up as a de facto standard for floodplain management, all because of the flood insurance program,” said Knight, who is now a senior research engineer at the University of Maryland. “But it doesn’t reflect the risk, it reflects the probability.” (read full article here)

That’s really the flaw in the whole system, as Knight says, “it doesn’t reflect the risk, it reflects the probability”…and often an inaccurate one at that.

So why is this expensive, arduous task undertaken approximately every 8-10 years?  It’s about assessing the need for insurance.  Everyone is at risk, even the FEMA website states this:

Everyone lives in an area with some flood risk—it’s just a question of whether you live in a low-, moderate-, or high-risk area.

I am personally aware of homes as businesses well outside of a mapped floodplain, both 100 and 500 year that have experienced flooding.  That’s a whole different conversation though!

Back to the maps.  They are built off of modeling results.  If you use or know anything about models, they are described as a “black box” and what you get out is only as good as what is put in to them.  As an observer and researcher of natural systems for many years, I must confess that my faith in the results generated by hydrologic-hydraulic models is wanting.  They are necessary however, and it is important to understand the basics of how the models are created, how they work, and their limitations.  Overall, they utilize topographic data in the form of cross sections for the waterway (either physically surveyed on the ground or from aerial surveys), estimated channel roughness, presence of culverts and/or bridges, and flow data (based on nearby USGS gage data if available or indirectly calculated from rainfall data).  There are generalizations and estimations at every step in the process.   The biggest issue of all as far as I can tell, however, is how quickly the maps themselves are out of date, yet still utilized.

As equally disconcerting as the lack of applicability for the data in real time, is the misconception by the public that they (the lines on the map delineating 100 yr, 500 year, etc) actually mean something solid.  If you don’t want to believe what the hydrologist is telling you, just check for yourself.  Check a local USGS gage for the past five years and compare the return periods for various flow events (1, 5, 10, 25, 50, 100 year flows) to that of the past thirty years (thirty year – or more – data sets are ideal for calculating return periods, but not always available) .  You will see the differences.  Of course, it varies based on local climate conditions, where exactly you live, how developed you watershed is, how quickly it is developing, and various other watershed characteristics.  I have seen it myself, and it’s a rather disturbing trend, especially when your “business” is designing restoration projects based on data sets that are adjusting as we speak…this new normal is a moving target for those of us aiming for it.

I know there is a lot of jargon in this post and I may have done little to clear up the confusing issue, or answer the question of “do I need flood insurance even though I am well outside of the high risk zone”, but this is a difficult topic to simplify much more.  My goal in trying the break it all down is to say, do NOT trust the current maps if your area is growing, and know that your are never at zero risk.

More specific question on the topic?  Let me know!