Why Should I Care?

Having witnessed firsthand the devastation of a river flood this past month, it seemed an appropriate time to look into an area of science that can sometimes be very confusing – probability of prediction and its fluid nature.  In this case we will focus on what a 100 year flood translates into, but another common example you may be familiar with is the XX% chance of precipitation in a weather forecast.  In chatting with folks after the flood event the two most common questions I heard were – ‘Does a 100 year flood only happen every 100 years?’ and ‘What does term 100 year flood really mean?’  What I found particularly interesting was almost everyone from which I heard these inquiries had some idea of what they thought it meant but also had a meaningful level of doubt that their interpretation was correct.  So, what does it mean?

Technically the term refers to the recurrence interval for which waters on the given river will reach a particular height (USGS), however it is sometimes easier to think of it as the chance or probability that an event will occur in a time period.  For instance, if considering just 100 years, there would be a 1% chance of that event happening in a given year (FEMA).  OK, all that sounds great, but then we get into the complexities of can a 100 year flood occur two years in a row, or even twice in the same year? and does the 100 year flood level change over time?  The answer to both of these questions is YES, and this points to the real complexities with trying to understand the meaning of what a 100 year flood really is.  Also, how prediction can change over time and might vary in given circumstances.

Actually creating a 100 year flood level for a given area involves one part Las Vegas, two parts science and one part engineering.  Traditionally the level is determined by looking at historic rain fall amounts in an area (science), incorporating that into what that should do to the level of a river in an area (engineering), and rolling the dice a bunch of times (Vegas) to determine how likely it is that those rainfall levels will occur again (science).  Whatever the rainfall level is that is not reached 99% of time, therefore the remaining 1% or 1 out of 100 gives you the water inflow quantity for a river.  The corresponding translation of water into river levels results in a level that is reached 1% of the time or 1 out of every 100 chances, hence 100 year flood level.

OK, seems straight forward enough, right?  So how is it you can have two 100 year floods in less than that 100 year recurrence interval or for that matter even in back-to-back years or the same year.  Let’s utilize a simple example – imagine two hurricanes tracked through roughly the same region one behind another and slammed into the gulf coast of the US and then moved almost on the same path inland bringing huge amounts of rain to the same areas less than a week apart.  Now, is that likely, again odds would say no and over time those chances would tend to spread out, but reality is it could happen like in the 2004 hurricane season (NHC).  Multiple storms took relatively the same track over the SE US, particularly hurricanes Ivan and Jeanne.  This example also shows a bit of a complexity in the calculation process.  Imagine how the first storm passing through saturated everything in the area, now the second passes through and it will take a lot less rain to raise the river level to the same height again.  Now, not having seen the equations used by the flood map creators, I have no idea if they account for this type of event or for that matter that if two events occurring so close together might be considered one flood.  End result is two significant events back to back do not change the chances of an event, but stack their occurrence.  Just like two blackjacks can happen back-to-back in Las Vegas, odds are they won’t but it can, however over time they will occur XX times out of XX hands.

Now our other challenge with this issue was the fluid nature of flood levels.  The particular area I was in had the level changed 3 times in less than 25 years.  Why would this happen, well there are a variety of things that might drive this including:

• Changes in climatology – an area becomes more prone to stronger rain events or hurricane activity
• Changes in basin terrain – an drainage area might become structural different even changed by a previous flood
• Changes in human activity – particularly altering drainage patterns for a river basin with buildings and roads

So these items can lead to a shift in how the weather in a region will impact river levels and those probability calculations.

This personal experience ended up being a good opportunity to discuss probabilities as it relates to the science that impacts on our daily lives.  In the near future we will take a look at some of the uncertainty associated with trying to make these predictions and how the level of uncertainty should also influence your decision making process.  If you want to learn more about flood level calculations, the USGS site referenced earlier has some additional details about some items discussed in this post.