Defining the terms

PITTSBURGH (KDKA) – Recently we had some intense rainfall that caused intense flooding.  

That might even be an understatement.  

The rainfall was so heavy near Oakdale that swift water rescue crews were called out to rescue people and roads were closed because the Oakdale area had life-threatening flooding. 

When all is said and done, that area saw a 40 or 50-year flood! 

But what does the term “50-year flood” even mean?  To help explain, let’s bring in our friend Fred McMullen from the National Weather Service office in Pittsburgh.

You can read Elizabeth and I’s conversation with Fred below! 

Elizabeth: Hi Fred! What is a 50-year flood?

Fred: It really encapsulates a rainfall event within a certain time window. And so, when we say you know, a 50-year flood, we’re looking for a certain precipitation amount in a certain time window, and then over a location because it varies by location. One of the misnomers that people have is they think they can get a 100-year flood in back-to-back years. What we try to tell people is that a 100-year flood is just really a 1% chance of occurrence every single year. So, if you have a 100-year flood at your house, the next year, the probability is still 1%. But again, I’m giving you just one, you would think it’d be a little bit less than the previous year. 

Ray: What does this mean for people?

Fred: Say you take a 30-year mortgage on your house and a 1% chance of occurrence flood which is you know, essentially one in 50. You know, 150 to 100 years, has a 1% chance of occurrence, but over the life of your mortgage, if you weren’t in the 100-year floodplain, there’s a 26% chance that during the life of your mortgage of 30 years that you could see a 100-year flood. So, over time those numbers go up there based on the math calculations. 

Elizabeth: Can anything change the definition of these floods?

Fred: We have river stages, and then we associate certain impacts at this level, this road closes.  Stream channels can change over time, especially the ones that haven’t had a flood in say 20 to 30 years, and we see impacts that you know are not happening where they used to be, so those are some of the changes that we have. And then also with urbanization, you know, we put more houses in a location. There’s more runoff into the stream, and then we typically see higher stream flows, and then that could lead to more flooding potentially given more runoff.

Ray: How do more frequent and heavy rainfall events factor into this?

Fred:  We were going through a process now where we’re updating this atlas here, so we’ll have you know, more data, more recent data to allow us to kind of tweak those numbers so some places where you don’t see a lot of development. Heavier rain events will lead to, you know, higher frequency. It may not lead to the same amount of impact as you would in a place that has big urban sprawl but allows us to fine-tune the data to make sure we have an accurate representation because the datasets are always moving. Flooding occurs at random intervals. It could be that you’re gonna have two or three years of really wet years, then go through a dry spell.  California was really dry, and reservoirs really low. The last two winters we’ve had a lot of rain events. Now the reservoirs were back at, you know, at full levels. So it’s one of those things where we tried to kind of capsulate trends and trends and science trends in the atmosphere and trends and whether anything else,