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Two 500-Year Floods Within 15 Years?

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We talk to Bob Holmes about some of the recent flooding events occuring in the Midwest, how does a 500-year flood occur twice in 15 years, and what do the recent events have in store for folks downriver.




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Scott Horvath: Welcome, and thanks for listening to the USGS CoreCast. I'm Scott Horvath. Obviously, in the news today there is a lot happening with the floods in the Midwest. And certainly the USGS is doing a lot to cover this and provide up to date and real time information and data regarding these floods and what has been happening. We want to talk today with Bob Holmes who is the National Flood Coordinator for the USGS. Bob thanks for joining us.

Bob Holmes: Oh you're welcome Scott.

Scott: So, one of the things that I know you've been getting a lot for questions about and one of the things that some of the public might be interested in knowing about is: as far as flooding goes, you know we hear this phrase "500 year flood." And supposedly this is the 2nd 500 year flood within fifteen years. Is that correct?

Bob: That's right, I mean, some of our numbers are showing that this, least on our frequency probability curves is about the second 500 year flood we've had in 15 years.

Scott: What exactly does that mean? How can you have two 500 year floods in 15 years?

Bob: Essentially, I think as hydrologists, we've done ourselves a disservice by calling it that. Essentially it's a probabilistic measure. A lot of people think "OK, if I've had a 500 year flood now, this year, we've got 499 years. We don't have to worry about it again." And that's simply not the case. Essentially, a 500 year flood is just that quantity of water that has the 1 in 500 chance in happening in any one year. Another way to say it would be, there's a .2% chance of a flood of this magnitude occurring in any one year.  So, it's essentially a probabilistic measure. We take our annual peak flow values from the USGS gages and we fit a probability model to those and come up with a number for a 100 year, a 500 year a 25 year whatever. You know the 100 year flood for example would be a chance of one in 100 of occurring in any one year.

Scott: So, I mean, will it ever go above 500 years? Is that even possible?

Bob: Well, Probabilistically yes sure, I mean you can have anything you want. But you have to remember this is the value of the long term gauging data. And this is why we really make every effort to keep gauges active for long periods of time because the more data you have... If we put a probability model to a gauging station with 25 years of record, that model is not as reliable as a gauging station with 150 years of record. Now, most of our gauges are well less than 100 years. Now In a current flooding area, we do have some gauges. Like on the Cedar Rapids, we've got a105 years of records there. So, those probability models are better than a gauge that's just been there for 20 or 30 years of records. 

Scott: With all this recent flooding, and actually current flooding, can you tell me a little more about maybe what's...Do we know what's in store for down river, for what's happening right now, lets say in Missouri or Mississippi? Do we know what's...?

Bob: Yeah, basically what we've got is: water is going to work its way down stream and so you get rain fall occurring in massive quantities, like we've had over the last couple weeks. And as it falls on that landscape it's going to run off into the smaller tributaries. And then that feeds the larger rivers like the Cedar and the Iowa, those have peeked. And the major metropolitan areas in Iowa, and they are moving on down river, those are coming and going down. And obviously the Mississippi is now, the big attention is on it. And we are seeing that flood pulse, that flood wave move down stream. And we're, you know, it's starting to crest in those areas in North Eastern Missouri, bordering the state of Illinois. And so we're starting to see the crest and peaks, say south of Quincy Illinois area. And we're going to be watching that.

We have USGS crews making observations of stage and making sure our stream flow gages are working properly and in some cases having to make repairs, emergency repairs on other things. And then we also have crews in boats using our hydro acoustic equipment making flow measurements where were actually looking at the volumetric flow rate and that's crucial because that data is fed into the national weather service's forecast model. Without that data they can't make up an accurate forecast. So we've got that, and we're anticipating that and we're watching that crest and we are monitoring that crest as it moves down the Mississippi River.

Scott: So we are on the ground right now well in advance of this, and anticipating whatever can happen and taking those measurements and gathering that data?

Bob: Yeah, I mean, we have one crew in Clarksville, Missouri above the lock and damn there at the special request of the St. Louis Army Corps of Engineers because they're really concerned that they have accurate flow information to make decisions from and emergency response stand point. And for their lock and dam operations and knowing what's coming down river. So we've had a crew measuring everyday from the USGS with the boat with our acoustic instruments measuring the volumetric flow rate. We essentially measure the velocity and the speed of the stream and the cross sectional area to be able to come up with these computations.

Scott: OK, and I just want to go ahead and mention while we are on the phone and while we are talking here that the USGS has a special page set up for all the recent flooding event and things that are occurring now and that will be occurring in the future here possibly. Hopefully not, but that page is at You can also get to that as well from the USGS home page at and clicking on the left hand side under the hazards section there's a link there for current mid-west flooding updates. Bob is there anything else that you want to add to this that I didn't ask you?

Bob: Well we're starting to dry out a little bit in the Midwest. I've looked at the five day quantitative precipitation forecast. And although we have rain coming we're not getting it in the quantities, or we're not expecting it in the quantities that we've had over the past several weeks. So I hope that we're going to start to dry out a little bit especially in those areas in Iowa and southern Wisconsin and South-Western Indiana that have been really hard hit over the past weeks. So our crews will stay vigilant and we'll probably have a lot of after response. We do a lot of flagging of high water marks and surveying where we can help the emergency managers prepare for the next one. We collect a lot of data after the fact on the flood high water that will be used. So just because the water recedes doesn't meant that our field crews get to come into the office.

Scott: Well Bob, we appreciate it. And keep up the good work with you and your teams out there.

Bob: OK, thank you.

Scott: OK

Bob: OK

Scott: And that does it for this episode for the USGS CoreCast. Thanks for listening and don't forget we did mention that you can go to home page and click on the Midwest flooding updates on the left hand side under the hazards section or you can go to the get all the recent information that the USGS has and the recent data and real time stream flow as well from that page. Again, USGS CoreCast is a product of the US Geological Survey Department of the Interior. Until next time I'm Scott Horvath saying, "Keep it cool."


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