Flood threats changing across US
University of Iowa study finds flood risk growing in the North, declining in the South
In a new study, University of Iowa engineers determined that, in general, the threat of flooding is growing in the northern half of the U.S. and declining in the southern half. The American Southwest and West, meanwhile, are experiencing decreasing flood risk.
UI engineers Gabriele Villarini and Louise Slater compiled water-height information between 1985 and 2015 from 2,042 stream gauges operated by the U.S. Geological Survey. They then compared the data to satellite information gathered over more than a dozen years by NASA’s Gravity Recovery and Climate Experiment (GRACE) mission showing “basin wetness,” or the amount of water stored in the ground.
What they found was the northern sections of the country, generally, have an increased amount of water stored in the ground, and thus are at greater risk for minor and moderate flooding, two flood categories used by the National Weather Service. Meanwhile, minor to moderate flood risk was decreasing in the southern portions of the U.S., where stored water has declined. (See the above map.)
Not surprisingly, the NASA data showed decreased stored water–and reduced flood risk–in the Southwest and western U.S., in large part due to the prolonged drought gripping those regions.
“It’s almost like a separation where generally flood risk is increasing in the upper half of the U.S. and decreasing in the lower half,” says Villarini, associate professor in civil and environmental engineering and an author on the paper, published in the journal Geophysical Research Letters. “It’s not a uniform pattern, and we want to understand why we see this difference.”
Some of the regional variation can be attributed to changes in rainfall; a study led by Villarini published last year showed the Midwest and Plains states have experienced more frequent heavy rains in the past half-century. More rainfall leads to more groundwater, a “higher water base line,” Villarini explains.
“The river basins have a memory,” adds Slater, a post-doctoral researcher and the paper’s corresponding author. “So, if a river basin is getting wetter, in the Midwest for example, your flood risk is also probably increasing because there’s more water in the system.”
Why some sections of the nation are getting more, or less, rainfall is not entirely clear. The researchers say some causes could be the rains are being redistributed as regional climate changes.
The researchers hope that their findings could revise how changing flood patterns are communicated. In the past, flood risk trends have typically been discussed using stream flow, or the amount of water flowing per unit time. The UI study views flood risk through the lens of how it may affect people and property and aligns the results with National Weather Service terminology understood by the general public.
“The concept is simple,” says Villarini, whose primary appointment is in IIHR-Hydroscience, a branch of the College of Engineering. “We’re measuring what people really care about.”
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The paper: http://onlinelibrary.wiley.com/doi/10.1002/2016GL071199/abstract
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Seems like the typical alarmism of people focusing on the second derivative. Even if they are correct, the people at risk of flooding tomorrow, are the people at risk of flooding today and yesterday.
Yes, there will always be local factors, such as stupid building on flood plains. But the general answer still remains the same as it was before: Employ drainage engineers, not climate scientists.
Flooding is a broad brush. In my area, river flooding was, is, and will never be a problem. Flash flooding was, is and will always be a fact of life. And my area on that color contour plot? Deep blue.
Flooding of 2014 Oct 30 was epic. And of Memorial Day 2015 downright tragic. Yes, Wimberly Texas. Lies, damn lies and statistics. 11 dead is also a statistic
Well, flood risk might be neutral in the U.S., but in London things are much different. It “might” resemble Venice in the future. Includes a video of “Bill Nye’s Big Ideas to Combat Climate Change”, so it must be valid. Him being the Science Guy and all.
http://www.fastcoexist.com/3035948/postcards-from-the-future-show-what-london-will-look-like-after-climate-change
The interesting thing about this paper is the definition and reasoning they use for “flood risk” — basically their idea is that “existing ground water plus probability-based precipitation == flood risk”. Their view is true but trivial — of course if the water table is extremely high and and area gets a period of heavy rains and the topology is primed for flooding (mostly human caused) then flood risk is higher. But that’s not what they really measured for that map.
MOST areas are not prone to flooding — almost no matter how heavy the rains. SOME areas flood even with light rains. Much of the flooding in the Midwest (the “north” in the map) is heavy rains plus snow melt plus human-engineered river basins == disastrous flooding. Flooding in the Missouri-Mississippi river system is historically periodic (it has always happened) exacerbated by the Army Corps of Engineers attempts to “tame” the system (ibegun after the Great Mississippi Flood of 1927).
Almost no where does it flood in the SW US — except for flash flooding in arroyos — areas well known for danger and not built in.
I would say that New Orleans has the highest flood risk in the nation based upon the fact that it is -2 to -10 feet below sea level for about half the city area. And it is near the ocean/Gulf of Mexico
Yes, New Orleans is high on the list and the fix for would be to jack up the building and fill it in fill it in, it going to be in much deeper problem in a few centuries since we no longer al the river to flow through the delta and the delta is disappearing.
hmmm- Urban development in the US and the effects-
The relative increase in peak discharge is greater for frequent, small floods than infrequent, large floods.
“Increase in peak discharge because of urban development – 100 to 600 percent”
https://pubs.usgs.gov/fs/fs07603/