Two 500-year floods in 15 years

By: Dr. Jeff Masters , 2:48 PM GMT on June 19, 2008

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The U.S. Geological Survey has preliminary data showing that this month's floods on four of Iowa's rivers--the Cedar, Iowa, Shell Rock, and Wapsipinicon--were 500-year floods. Back in 1993, many rivers in the Midwest also experienced 500-year floods, so the region has endured two 500-year floods in the past 15 years. How can this be? First of all a definition--a 500-year flood is an event that has only a 0.2% chance of occurring in a given year, based on available river flow data. Of course, reliable data only goes back a century at most, so designation of a 500-year flood event is somewhat subjective. Still, it seems rather improbable that two such huge floods should occur within such a short time span, raising the question of whether the floods were, in part, human-caused.

In a provocative story in the Washington Post today, it was pointed out that part of the flooding is due to the draining of wetlands for farming purposes. As nature's natural buffers against flooding are drained and filled to provide room for more farmland, run-off and flooding are bound to increase. Furthermore, as more levees are built to protect more valuable farmland and new developments, flood waters are pushed out of the former areas they were allowed to spread out in and forced into river channels behind the new levees. Even higher levees must then be constructed to hold back the increased volume of water they are asked to contain.

Climate change contributing to flooding?
The heaviest types of rains--those likely to cause flooding--have increased in recent years (see my February blog, "The future of flooding", for more detail). According to the United Nations' Intergovernmental Panel on Climate Change (IPCC) 2007 report, "The frequency of heavy precipitation events has increased over most land areas". Indeed, global warming theory has long predicted an increase in heavy precipitation events. As the climate warms, evaporation of moisture from the oceans increases, resulting in more water vapor in the air. According to the 2007 IPCC report, water vapor in the global atmosphere has increased by about 5% over the 20th century, and 4% since 1970.

Over the U.S., where we have very good precipitation records, annual average precipitation has increased 7% over the past century (Groisman et al., 2004). The same study also found a 14% increase in heavy (top 5%) and 20% increase in very heavy (top 1%) precipitation events over the U.S. in the past century. Kunkel et al. (2003) also found an increase in heavy precipitation events over the U.S. in recent decades, but noted that heavy precipitation events were nearly as frequent at the end of the 19th century and beginning of the 20th century, though the data is not as reliable back then. Thus, climate change is likely partly to blame for increased flooding in the U.S., although we cannot rule out long-term natural variations in precipitation.


Figure 1. Forecast change in precipitation and runoff for the period 2080 to 2099 compared to 1980 to 1999. The forecasts come from the A1B scenario from multiple climate models used for the Intergovernmental Panel on Climate Change (IPCC) 2007 report.

The forecast
According to a multi-model consensus of the climate models run for the Intergovernmental Panel on Climate Change (IPCC) 2007 report, precipitation and river runoff for the Mississippi River drainage basin are expected to increase only slightly by the end of this century (Figure 1). However, more of this rain is expected to fall in heavy precipitation events, the ones most likely to cause flooding. As a result, the U.S. needs to prepare for an increase in the number and severity of 100-year and 500-year flooding events in the coming century.

References
Kunkel, K. E., D. R. Easterling, K. Redmond, and K. Hubbard, 2003, "Temporal variations of extreme precipitation events in the United States: 1895.2000", Geophys. Res. Lett., 30(17), 1900, doi:10.1029/2003GL018052.

Groisman, P.Y., R.W. Knight, T.R. Karl, D.R. Easterling, B. Sun, and J.H. Lawrimore, 2004, "Contemporary Changes of the Hydrological Cycle over the Contiguous United States: Trends Derived from In Situ Observations," J. Hydrometeor., 5, 64.85.

Tropics
It's quiet in the tropics. There are no threat areas to discuss, and none of the models are forecasting tropical storm formation in the next seven days.

Jeff Masters

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53. MonkeeInDaTrunk
10:08 AM PDT on June 19, 2008
"man-made" global warming....it's NOT man-made, it's "Clara-bell-the-cow-made"....animal flatulance is a major cause, don't ya know....
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52. moonlightcowboy
4:55 PM GMT on June 19, 2008
Climate change...blah, blah, blah!

Furthermore, as more levees are built to protect more valuable farmland and new developments, flood waters are pushed out of the former areas they were allowed to spread out in and forced into river channels behind the new levees. Even higher levees must then be constructed to hold back the increased volume of water they are asked to contain.

Hhhhhhhmmmmmm, it seems to me that farm subsidies are supporting flooding happening in otherwise non-flood areas. I mean how else can one really read this? The problem isn't with dounpours - we've always had those. The way this reads it's more of a problem with government taking lobby dollars, using subsidies, and re-routing water where the flow would more normally and naturally move. That, and poor choices for development.

Climate change? Naaaahh, it's man trying to control and manipulate his environment, again. And, mostly out of graft, greed and corruption. Gosh, we are really just so smart aren't we?
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51. pearlandaggie
4:56 PM GMT on June 19, 2008
i agree the USGS should avoid using the "hundred year" probabilities when describing flooding potential. changes in drainage, land use and development occur more rapidly than flood plain maps can be updated, resulting in a false sense of security that is attached to a 100- or 500- year flood plain. i'm not sure how one would compensate for the change, but a range of percentages might be a good way to go (1-5%, 5-10%,25-50%, etc.).....just a thought
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50. IKE
11:49 AM CDT on June 19, 2008
456...you may be right about being clipped with showers from that wave.........
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49. IKE
11:44 AM CDT on June 19, 2008
SWEEP! LOL. Go Rays...fans are starting to support the team...funny how winning changes things........
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48. DocBen
4:41 PM GMT on June 19, 2008
43. Floodman 4:37 PM GMT on June 19, 2008
39. tigertail

Dr Master's forum, Dr Master's right to say what he will.

To which I would add that Dr. Masters is absolutely correct in his comments.

I would also note the areas of reduced precipitation in Dr. Masters' maps. Western Kansas is in a multi-year "drought". Perhaps it is more than that; and if so they better start adjusting quickly.
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47. TampaSpin
12:43 PM EDT on June 19, 2008
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46. IKE
11:39 AM CDT on June 19, 2008
TampaSpin....I'll get you another broom for the Astros/Rays series coming up...you'll need it after you sweep my team that sucks in 2008. They've lost 16 of 19....
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45. MonkeeInDaTrunk
9:39 AM PDT on June 19, 2008
yo Jerry!
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44. TampaSpin
12:38 PM EDT on June 19, 2008
Floodman i have my own broom but, i do appreciate the offer......LMAO
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43. Floodman
11:35 AM CDT on June 19, 2008
39. tigertail

Dr Master's forum, Dr Master's right to say what he will.
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42. Floodman
11:33 AM CDT on June 19, 2008
40. TampaSpin

I've got a broom right here for the Rays!
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41. pottery
12:28 PM AST on June 19, 2008
Good Morning.
Interesting Blog, Dr. Masters. Thank you.
From my own experience, individuals are generally very good at preparing for severe weather events. It is National Governments that fail miserably in this regard. Sad but true.
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40. TampaSpin
12:31 PM EDT on June 19, 2008
All right all im out for a while....gotta go trim the palm trees, mow, and edge...anybody wanna help can come over....lol..also going to the RAYS vs. Cubs game tonite.....SWEEP!
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39. tigertail
4:28 PM GMT on June 19, 2008
First, As an insurance guy I wish the USGS would stop using "XXX Year Flood" declarations. It puts people too much at ease, too complacent, unwilling to buy flood and insurance and/or build their property in safer places leaving our society with a mess that we all have to pay for in terms of FEMA and SBA loans and grants, relief funds, National Guardsmen, rebuilding of infrastructure, etc. Second, Jeff, please go easy with the Global Warming stuff. Those of us who think it's a pretty weak theory and it dims your otherwise sensible credibility. Stick with the forecasting please!
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38. TampaSpin
12:29 PM EDT on June 19, 2008
FSU model
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37. TampaSpin
12:16 PM EDT on June 19, 2008
The CMC has the CATl system with a low in the Carribean just off S. Am.
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36. Drakoen
4:12 PM GMT on June 19, 2008
The global models show the EPAC getting very active so at least we have those storms to track rather than staring at the dry air and unfavorable upper level winds in our basin.
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35. DocBen
4:08 PM GMT on June 19, 2008
We had a pretty big tornado back in the 90s - across McConnel AFB and some other areas. Have had a bunch all around but not here this year.
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34. nrtiwlnvragn
12:07 PM EDT on June 19, 2008
The TWD referencing 3 storms and the MJO was for the Eastern Pacific
Member Since: September 23, 2005 Posts: 14 Comments: 11032
33. stormlvr
3:44 PM GMT on June 19, 2008
The level of enthusiasm over blobs and invests on this site is pretty amazing. So yea, July should be a lot more interesting. 1-2 named storms is possible with this next MJO pulse. Maybe something close to home early in the pulse with even the possibility of development east of 60w toward the mid and later portion of the pulse. We will need to see some moderation of the overall atmospheric conditions prevalent so far in June though and that is also reasonable to expect.
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32. Nolehead
3:53 PM GMT on June 19, 2008
makes you really wonder who's doing the estimating in our gov't....
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31. pearlandaggie
3:47 PM GMT on June 19, 2008
29. exactly. if the baseline data is not good or there is a lot of uncertainty in the dataset, then the frequency may be underestimated.
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30. OSUWXGUY
3:47 PM GMT on June 19, 2008
It would be hard enough considering you only have an accurate history of ~100 years in most areas...

But then the actual river system changes...with changes in wetlands...development and associated increased runoff...and of course dams and dikes that channel streams differently!
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29. OSUWXGUY
3:44 PM GMT on June 19, 2008
Aggie (27)

Yeah. Estimating the tail probability for any extreme event (hurricanes, floods, earthquakes) deals with a great deal of uncertainty.
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28. Nolehead
3:43 PM GMT on June 19, 2008
i think FEMA better get ready big time and to make sure that if and when they are needed that they would actually send materials to the needed people this time!! that CNN report still just gripes me...that still is just unreal..
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27. pearlandaggie
3:41 PM GMT on June 19, 2008
i wonder how good our disaster probability estimates are and how they are determined....
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26. sonofagunn
3:38 PM GMT on June 19, 2008
Another factor to consider in Midwest flooding/stormwater runoff is that floods are responsible for renourishing the soil in many areas. We would be better off if runoff and nutrients were redeposited over farmland instead of being dumped into the GOM's deadzone.
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25. pearlandaggie
3:37 PM GMT on June 19, 2008
14. Hurricane Jig? Hurricane Easy? LOL

I think I like Hurricane Dog the best...probably would be spelled Dawg today!
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23. OSUWXGUY
3:34 PM GMT on June 19, 2008
LOL - I was thinking that if I said 1-3 someone would focus on the high end of that range.

Point is none of us knows for sure...but I WOULD be somewhat surprised if we have a more unstable/moister environment associated with positive MJO and have ZERO storms...
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22. guygee
3:33 PM GMT on June 19, 2008
Great scholarly blog update by Dr. Masters.
Et Tu, 03?

Repost from the end of the last blog,

Here is a fun site:

Generate soundings from MAPS or RUC Analyses and Forecasts, RAOBS, Profilers, Radiometers...

You can generate an interactive java upper air sounding diagram for an arbitrary lat-long with global coverage by choosing GFS as the data source.

Enter a latitude and longitude (in decimal, with west and south being negative), separated by a comma.

For example, in the vicinity of the CAtl wave, say roughly 13.5N, 56.0W (13.5,-56.0):
Upper Air Plot from GFS latest 3 hr forecast

Have a good day all!
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21. cchsweatherman
11:35 AM EDT on June 19, 2008
I'll be back sometime after 7:30 tonight.
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20. Inyo
3:26 PM GMT on June 19, 2008
Interesting climate map, but I've seen a lot of different ones. California tends to fall through the cracks... on that one it indicates drier, but just barely... others predict 50% decreases or even 100% increases in rainfall. weird...

One thing is for sure.. it is HOT here.
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19. sporteguy03
3:23 PM GMT on June 19, 2008
Eye,
Any thoughts on the TWD?
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18. sporteguy03
3:17 PM GMT on June 19, 2008
Taz,
Thank you for the updates!
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17. all4hurricanes
3:22 PM GMT on June 19, 2008
I still think three storms now would be surprising
considering the only really favorable places for the past month have been upper gulf and occasionally a portion of the sea near the Bahamas
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16. all4hurricanes
3:21 PM GMT on June 19, 2008
Wichita Kansas?
didn't that get hit by a tornado in the 90s
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15. Nolehead
3:19 PM GMT on June 19, 2008
3 storms in a couple of weeks would not suprise me in the least bit...it would just be par for the course the way this years weather has gone..
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14. all4hurricanes
3:19 PM GMT on June 19, 2008

what if we have a season like this lots of cape verde storms
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13. DocBen
3:12 PM GMT on June 19, 2008
Having lived here in Wichita for over 1000 years (based on floods) I am familiar with the effects of messing up the floodplain. When a 50-year rain produces a 500-year flood (as happened here on several creeks) something is VERY wrong. I'd like to know if the Mississippi basin has had two '500-year rain events'
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12. ajcamsmom2
10:18 AM CDT on June 19, 2008
Thanks for info Dr. Masters. Living here in South Louisiana flooding along the Mississippi River is always a concern.
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11. weathermanwannabe
11:08 AM EDT on June 19, 2008
Thanks Dr. M; hopefully, StormW will weigh in on the MJO issue raised by the TWD and the impact on the Tropical Atlantic season...Not too sure (I do not have a clue) as to how this works (yet); don't know about that many storms just a few weeks out, but, could it increase the total numbers for the whole season (as suggested in the UKMET report?)........Who Knows Right?......BBL
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10. KEEPEROFTHEGATE (Mod)
3:06 PM GMT on June 19, 2008
nice update doc gives us something new to ponder
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9. franck
3:09 PM GMT on June 19, 2008
They will feel hungry again in a half hour anyway.
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8. franck
3:05 PM GMT on June 19, 2008
Will hogs eat rice?
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7. all4hurricanes
3:03 PM GMT on June 19, 2008
"rain could come to Japan" from weather channel
the only model forecasting Fengshen takes it by Japan
if this storm goes slightly more westwardly than predicted it might end up over Japan
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6. LS1redline
2:58 PM GMT on June 19, 2008
Nice info on the flooding Dr. Masters. The levee effect is definitely a big problem when the river is bottle-necked upward instead of flooding into the plains. I grew in Hannibal, MO during '93 and can recall the river dropping several feet when 2 of the big levees (West Quincy, MO and the sny levee across the river in IL) broke. The same can be seen in the flood of '08, just look at the river level peaks and valleys in the NWS AHPS. Those changes are due to levee failures. Here is an example:
Link
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5. all4hurricanes
2:57 PM GMT on June 19, 2008
Three named storms in Early july thats insane do you raelly think three storms will form in the next 15 days thats september active were still in june
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4. OSUWXGUY
2:52 PM GMT on June 19, 2008
Agreed Taz-

I think late June/Early July could see 1-3 named storms when that positive MJO pulse crosses into the Atlantic...

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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