Misconceptions of a 200 Year Recurrence Interval

Comments made by Salt Lake City Mayor Jackie Biskupski yesterday illustrate some common misconceptions about recurrence intervals of precipitation events.  As reported by the Salt Lake Tribune, the Mayor said regarding this weeks floods that "It was not a failure of infrastructure at all.  The system was truly overwhelmed within one hour of time.  A storm like this hasn't happened in probably 200 years.  It's really an unprecedented event."

The term recurrence interval is one that perhaps needs to be put out to pasture.  I see it misinterpreted all the time.  An event with a 200 year recurrence interval doesn't mean the event happens at 200 year intervals.  It means that the odds of it happening at a given location in any given year are 1 in 200.  It is possible to have another 200 year even this year or even next year.  The odds are low, but not zero.

Estimates of recurrence intervals are also problematic.  We have a relatively short record of good precipitation observations and the accuracy of recurrence intervals for extreme events are not precise.  In addition, these estimates are made using observations from the past, but the climate is changing and changing rapidly.  The likelihood of extreme precipitation events of the type experienced this week will likely increase during the 21st century, unless there is a significant change in the characteristics of the summer monsoon.  However, the odds are such that we will likely see an increase in the likelihood of the most extreme precipitation events in our part of the world.

So, my view is that the take away message from the event last week is that Salt Lake City remains vulnerable to events of this type and that we shouldn't treat it as something that is unlikely to happen in the future.  The recurrence interval of an event of that severity happening somewhere in Salt Lake City is shorter than 200 years, perhaps much shorter, due to the difference between a point and area frequency and the warming climate.

The good news is that the Mayor does indicate that they are looking at the city's storm drain system and stream channels to see what could be done.  Other municipalities in Salt Lake County should do the same and weigh the relative merits of increasing resiliency vs. cost.

Change is happening and systems designed for the climate of the 20th century are not going to be able to deal with extreme events in the climate of the 21st century (and beyond).