May 24 2011

100 Year Flood

The record flooding along the Mississippi River has been front page news for the past few weeks.  There have been several news articles, blog posts, photographs, and tweets reporting on the historic levels,  damage, road closures, levee failures, levee breaches, and evacuated communities.  There have also been several articles discussing changes that need to happen in how we manage floodplains.


So far most of the articles I have seen describe it as a ‘historic’ flood.  At some point it will probably be given a more detailed designation such as a 100, 500, or ___ year flood.  When a large natural disaster occurs this is what we do.  Tornadoes get an F-1-5 rating, earthquakes have the Richter scale, and floods get a rating based on their statistical frequency.  It is necessary, and helpful, to assign these ratings to floods.  However, confusion can result without properly understanding the context of how this rating is determined and used.

There has been an intentional shift away from talking about the 100 year flood to a more precise definition.  Instead of  ___ year flood, the new terminology is annual exceedance probability (AEP).  So the 100 year flood is now called the 1% AEP.  While this is a more correct way to define the rating I don’t think it really helps clear up the confusion surrounding the flood rating, especially not for the general public.

Understanding the context here is really important.  There are generally three groups involved with floods.  The people who are directly effected by the flooding, the insurance players like FEMA, and the engineers and planners who are making decisions about what happens in and around a river.


The engineers and planners are interested in where it is safe to build, and how high, if any protection against flooding is needed.  If a community is experiencing some flooding they can either move houses and businesses out of the floodplain, or they can build some type of flood control system in the form of dams, levees, floodwalls, or some combination of them.  The questions they are interested in are:

  • How far or high from the normal river channel is it safe to build?
  • How high should we build our flood protection to?
  • What level of flooding should we design for?

These are some of the questions that have to be answered, and they are not easy ones.  The answers to the first two questions get complex because of current land ownership and land use, but they are directly tied to the last question.  The data and tools to accurately determine flood levels do exist today, but the amount of flow, usually expressed in cubic feet per second (cfs), to design to is more approximate.  Ultimately we don’t know how big flooding will be on a given river, nor how often it will occur.  The amount of flow is based on a combination of historical records and statistics.  While not a shot in the dark, it is still approximate, and even changes as we learn more about the climate and what effects flood levels.


Insurance is all about chance.  The question an insurance company will ask is:

  • At what level of flooding will we insure someone for damages?

No one would buy insurance for the 10,000 year flood, and no insurance company would cover someone in the 1 year floodplain.  In the 1960’s the United States government decided to use the 1% AEP flood as the basis for the National Flood Insurance Program (NFIP).  This line in the sand was thought to be a good balance between protecting people without being too restrictive.


The range of questions that the general public is interested in depends largely on their current understanding of flood risk and where they are located in relation to water.  They generally want to know what kind of impact that potential, or eminent, flooding will have on them and their property.  They are asking questions like:

  • Where will it flood?
  • How deep will it get?
  • Is my house at risk?
  • How does this flood compare to previous floods?

The public often correlates flooding information to known historic events.  The biggest event in recent history is often seen as the largest flood that could happen, although this is often not the case.

So What Now

So the 100 year flood is really just an approximate level of flooding originally set by the NFIP that has been used for design standards all over the country.  It does represent a specific level of flooding, but is not the end all when talking about, managing, and living in floodplains.  While engineers and planners often take into account other flood levels such as the 50 year and 500 year flood, the need to think beyond 100 year flood from a design, planning, insurance, and public standpoint is needed.

We have the tools to model a range of flows so that we can have a better understanding of what could happen during larger floods. There are also tools being developed to better communicate to and educate the public on possible flood risks.

I don’t think we should get rid of the “100 year flood” as a way to communicate flood levels, but I think it would be helpful to start thinking about it as a medium size flood and start managing rivers, floodplains, and communities with a view towards the full range of possible flooding.

What are your thoughts?

Some content taken from USGS Poster explaining more about the 100-year flood.

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