U.S. flood risk is basically a wash thanks to changing weather patterns

Flood threats changing across US

University of Iowa study finds flood risk growing in the North, declining in the South

The risk of flooding in the United States is changing regionally, and the reasons could be shifting rainfall patterns and the amount of water in the ground.

us-flood-risk

A University of Iowa study has found that the risk of flooding is changing in the United States and varies regionally. The threat of moderate flooding is increasing generally in the northern US (red areas) and decreasing in the southern US (blue areas), while some regions remain mostly unchanged (gray areas). The findings come from comparing river heights at 2,042 locations with NASA satellite information showing the amount of water in the ground. The study was published in the journal Geophysical Research Letters. CREDIT American Geophysical Union

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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63 thoughts on “U.S. flood risk is basically a wash thanks to changing weather patterns

  1. Flood risk is just not that simple. Very dry soils, particularly in desert regions, resist infiltration. Instead of reducing flood risk, increasing dryness increases it, since rainfall is more likely to run off.

      • Certainly not, but the rate of water fall is the determining factor. Several inches of rain suddenly after a dry spell will most likely turn to run off and not soak in the soil. Dry soil needs a slow, steady rain for several hours so that the water has time to seep into the ground.

    • Steve & David. The term used in flood prediction and flood frequency analyses is the antecedent moisture index. This factor is calculated via antecedent precipitation indices of varying complexity, soil moisture measurements/estimates, and snow survey courses in higher/more norther locations.
      So in a sense this, factor of wet/dry soils preceding a rainfall/snowmelt event causing flooding is already built into current flood analyses procedures.
      And the rainfall intensity, duration, and areal extent usually dominate in flood peak production, although antecedent soil moisture is usually the second or third most important factor depending upon land use and management at the time of the flood.
      Seems to me like another study producing pretty maps of what we already know, and have known for decades or longer. Money might have been better spent IMHO.

  2. They seem to be contradicting themselves…
    They have Texas as dark blue….significant
    “Meanwhile, minor to moderate flood risk was decreasing in the southern portions of the U.S., where stored water has declined.”
    “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.”

    • One might argue since Texas is such a big centrally located state with normally relatively high (near SE US standards) rainfall in the east to desert amounts in the west, that it is either a good or bad example. Not only has it increased, but has the variation changed? The drought of the 50s, with neighbor state effects, is still the most severe, but from 1965 to 2010 the variability seems to be smaller than before. Wouldn’t bet money on it, but it would be interesting to predict what the next few decades bring. Texas is definitely wetter since the 50s, and where I live on the central coast the average has gone up something like 2 inches. The older recorded history has droughts and floods, but the extremes are the most likely to be reported.

      • I lived in Arizona (Scottdale and Tucson) and Texas (western Texas, no less!) and Death Valley itself in the 1950s to 1960s. The 1950s were very hot and dry. We were warned to not face the sun because our feet would be fried through our shoes, we cooked eggs on rocks for fun, it was blindingly hot.

        When my littlest brother witnessed his first rainstorm, he was scared because the rest of us ran outside screaming and he thought it was the end of the world.

        Then…in 1963 onwards, it was cold and wet. It snowed in Death Valley! Dry rivers ran so high bridges were washed out. I hauled people out of cars, using my horse and ropes, who were being swept down rivers that hitherto, never ran. It got colder and colder. Then, it reversed gears again, and became warm and dry in the 1980’s.

        My family lived in California since the Gold Rush and talked a great deal about the drought/wet/hot/cold cycles out there. Most people are newcomers and don’t have the faintest idea how the West can and does switch gears with no warning at all.

    • Another fact is that flooding is very seasonal depending on the location north/south and elevation. Something like the Koppen climate classification will show much of the seasonality of flood events. For example, much of the southern west coast is classified as Mediterranean with wet winters and dry summers. Expect flooding in winter.
      I think predictions of soil moisture would be most useful if regional and seasonal information via climate classification were used. Gross regional classifications such as long-term results for northern or southern US provide some information, but, not for flood prediction or flood frequency analyses.

  3. Much as this study is interesting, Grace reliance is sletchy. An example of how ground water retention supposedly found by GRACE in Australia ‘explained’ an apparent dip in SLR is given in essay PseudoPrecision. Junk NASA climate science from first principles and Australian meteorological data.
    Stream gauges are only crude measures because of runoff changes, precipitation intensity changes, and a host of other factors. Severe flooding in the Carolinas and Houston Texas in recent years is partly attributed to ground water saturation in those regions of the south. Water tables in the three water wells at my Wisconsin farm, several hundred feet above the river, have not changed in 30 years. Finally, groundwater is replenished by precipitation, and the NOAA regional precip indexes do not show an overall north/south difference. Covered that in essay Credibility Conundrums about the National Climate Report 2014.

    • In my SW Wisconsin county we had back to back 500 year floods in 07 and 08 and several lesser floods the last 3 years. Folks around here call that rainy weather.

  4. I am confused, all of the towns along the Humbolt River in Northern Nevada are in the above category. If there is sufficient snow in the Ruby mountains near Elko, then the river has water. Last year was the first time in 5 or 6 years that the river did not dry up before it got to Lovelock, NV., and many years since it ran past Winnemucca, NV. All of the red dots coincide with towns that lie within the floodplain of the Humbolt River. I have not seen widespread flooding on the river since the late 1980’s. It has been a wet year this year down there and if they get a good snowfall, I would expect that the river will be high, but no worse than it has ever been?
    Rye Patch reservoir near Lovelock was excellent stripped bass up until late 80’s when the water level got so low that fishing was no longer feasible.
    The area is considered high desert (5000’+) so flash floods do happen, but most of the time it is a high windstorm with eight or ten drops of rain.
    I think a bit more of boots on the ground would help this paper.

  5. Oh! FFS!…..They complain about no rain; they complain about rain; they complain about rain in the wrong place; the right place; too much rain; too little rain; too much rain; hard rain; soft rain; acid rain; alkaline rain; cold rain; warm rain; hard boiled rain; snow rain, sleet rain; vertical rain; horizontal rain; boring rain; and fun rain…….Sorry, I had to stop. So many possibilities. They would moan about upward rain.

    I was brought up on the West Coast of Scotland. If anyone wants to know what rain is, try spending half a lifetime there. There is nothing, any scientist can tell me about rain I can’t refute.

    • Good grief man! You’ve just provided them with a dozen new research projects! Vertical rain definitely needs further investigation.


    • Welcome Rain

      A cowboy and an Eastern dude,
      A most unlikely pair,
      Were flyin’ ‘cross the country.
      Assigned, by chance, to share
      The last two seats in tourist class
      On some big jet airplane,
      The conversation that they had
      Was mostly ’bout the RAIN.

      It’d been a pourin’ steadily
      For several days or so,
      Across a bunch of western states.
      And in New Mexico
      Where they were headin’ on their fight
      Most every pond and tank
      Was full, the creeks and rivers there
      Were runnin’ bank to bank.

      The dude had come to play some golf
      And tennis, don’t you know,
      The cowboy, to participate
      In some big rodeo.
      So each one was affected by
      Excess precipitation,
      And neither of ’em could adjust
      A pre-made reservation.

      The Eastern dude was most perturbed,
      And anxious to complain
      How his vacation would be spoiled
      By that confounded rain.
      He fumed and fussed and swore a lot
      And said it wasn’t fair;
      They’d advertised a desert
      And now a swamp was there.

      The cowboy’d be impacted too
      By breaks that he’d been given,
      ‘Cause ropin’ in arenas dry
      Was how he made his livin’.
      Yet he seemed sorta’ unconcerned,
      Not tryin’ to be rude,
      He set out to explain some facts
      To that frustrated dude.

      He said, “You ought to understand
      This country’s mostly dry.
      We need this moisture to survive;
      There’s lots of reasons why.
      So mister, don’t get too upset;
      Let’s try to be more lenient,
      ‘Cause RAIN is always welcome here,
      But seldom is convenient!”

      https://weberstudies.weber.edu/archive/archive%20D%20Vol.%2021.2-25.2/Vol.%2021.3/Tixier%20Poe.htm

  6. Next logical question.
    Have the researchers taken into account changing land use?
    If a town or city gets bigger, there is more concrete and more runoff , if you irrigate marginal farmland, it gets wetter etc.

    • I live in Saskatchewan, straight North of Montana and N.D. The 80’s were hot and dry and farmers went much more to zero till and standing stubble practices to help retain snow and reduce moisture loss. I’m sure it’s the same in the U.S. plains states. Additionally, spring runoff and land drainage is managed more aggressively. We have had adequate to excess soil moisture levels since at least 2000 with major flooding in 2012 and 2013.
      With short summers and flat land a little extra water retention goes a long way. I would think that land use is by far the most important factor in both increased moisture levels (zero till) and flood risk (drainage). I don’t think there’s any significant change in snow or rainfall in the last 40 years. The 60’s were very snowy, stormy and cold winters and we had flooding in the early 70’s. Drought in the mid to late 80’s (no snow). Just for a change.
      Another big factor for widespread spring flooding is how deep the ground freezes and how fast the thaw comes. This is an area with a lot of variables and a lot of range.

      • You hit on excellent points. Land use is one of the major components in flooding, and it is often pushed aside in the rush to blame “climate” change for the problems caused by improper land management/use. New Orleans and other cities in the lower Mississippi River valley system are perfect examples. Katrina flooded New Orleans in major part because several decades of political driven corruption and entrenched cronyism stopped ACoE and Louisiana state agencies from improving and revamping huge sections of the flood levee system. Blame was heaped on a President and Administration who had nothing to do with the problem, and the problem has yet to be fixed. Money is massively misappropriated, politically driven environmental lawsuits block work that is being done, and the same sets of suspects again, still, blame man caused globall warmining for everything from the ESA Mars Probe crash to grandma’s gout.

        Thus endeth the rant. As long as we continue to allow the same sets of a$$wipes to distort, dissemble, obfuscate and outright lie about “climate science” and the environment we will never be able to actually address the real solutions to real problems.

      • John, there been intensive tile drainage installed in the Red River valley in the last thirty years, yet somehow that not a factor in the recent floods. Add most the sloughs were drained in mu home county in Minnesota, which I believe why Ada Minnesota has been inundated several times in the last thirty years. Flooding and damage from is almost exclusive wrought but people doing stupid and repeating the same stupid thing over and over. The are letting/encouraging people to rebuild on the bottom land of the Mouse river in Minot, pure stupidity, Grand forks failure in 97 was due to overuse of dikes and not enough open bottom land to handle the water. Trying to protect home out on a peninsula with a thirty-foot dike is pure stupidity. They were the dike system first failed which lead to the whole town being inundated if not the land being covered but by the sewer system being filled up which then backup into homes. Improper land use is the primary reason we have far to much damage from flooding, thinking living in a flood plain and the heavy use of dikes to protect you is pure insanity. Somehow moving to higher ground and stay off flood plains is to simple of a plan.

  7. It’s not a uniform pattern

    Oh my. Flooding was previously uniform? Who knew?

    The river basins have a memory

    Oh.
    My.
    G_d.
    Do they also have emotions? Are they self actualized?

    So, if a river basin is getting wetter… your flood risk is also probably increasing

    Uhm… I can’t even come up with a sarcastic comment. No sh*t Sherlock doesn’t quite cover it.

    and aligns the results with National Weather Service terminology understood by the general public.

    Were they of the impression that the public is comprised of 5 year olds? See Johnny if this bowl is full of water, and we put more water into it, the extra water spills over the edges. Do you know why that is Johnny? Its because the bowl has a memory, and so it knows what to do when more water is put into it than it can hold. See, it makes the bowl angry Johnny, and that is your fault because carbon.

  8. ‘risk of flooding is changing’

    How many grams does a ‘risk of flooding’ weigh?

    ‘The researchers hope that their findings could revise how changing flood patterns are communicated.’

    How they are communicated. Surreal.

    • @Gamecock
      December 30, 2016 at 4:21 pm

      ‘risk of flooding is changing’: Yep, more social ‘science’. They’ve lost it all right.

  9. Pronouncement of conclusions based 1985 to 2015 data has no meaning. The rainfall presents a natural cyclic variation. In that a truncated part give misleading conclusions. For example Indian Southwest Monsoon Rainfall presented a 60-year cycle — a sine curve. When Indian Parliament raised a question on Indian rainfall, the concerned minister responded saying that the rainfall is decreasing. In fact he has chosen a truncated part of Sine Curve in which the decreasing arm was used to reach that conclusion. If the minister would have used the increasing arm of the Sine Curve, he would have arrived at “rainfall increasing”. In USA, they have data for more than 100 years, let the researchers use that type of data and present their conclusions, give meaningful conclusions.

    Dr. S. Jeevananda Reddy

    • Meaningful conclusions? They produce esoterica. Should the scientists publish, “There is a higher chance of flooding in the next ten years,” no one is going to do anything differently, even if they believe the prediction. Greater risk of a rare event motivates no one.

    • If we accept the WMO definition of climate as being weather over 30 years then all these ‘scientists’ have discovered is the climate as they are only looking at 30 years of data.

      To postulate change they need more than 30 years of data

    • @ Dr. S. Jeevananda Reddy
      December 30, 2016 at 4:31 pm: Hear, hear, and thanks for a perfectly clear short exposition on Cycles. And these two stupidities – 70’s ice age and recent/current false frying, are just one full cycle, as you say. Now, how do we get respected leaders to state that simply? Mr Trump, we are watching……

    • I was there on the campus the week the water rose. Students would go down to the bridge to watch the water go by. The Thursday afternoon that Coralville flooded, the project manager let us leave the hospital early, and I drove to the airport. By the time I got home to Dallas, Coralville was a 4′ deep lake.

  10. Parts of the Dakotas serve as a flood basin for Canada, so when central Canada gets above normal snowfall and it melts along with whatever compounding factors can occur, places like Minot, North Dakota get inundated.

    • Much of North Dakota’s watershed in the east all flows north, into Canada. the oddball river, the Red River (of the north) flows north. I live in Fargo. We always watch for the precip amounts south of us at the South Dakota border in the fall before freeze. If the ground saturates before freeze then we worry about snowfall. If we get the double wammy – saturated and frozen ground plus 50-70 inches of snowfall we know a flood is coming our way.

    • You want to see a flood plain.

      This is an absolutely amazing series of satellite images – a timelapse of the Ganges/Padma river in Bangaldesh changing course throughout the floodplain. Enormous changes in this enormous river since 1984.

      Here is your geography class about rivers changing course through a flood plain on steroids – and in real-time may one say.

      And nobody should be building a house or a city in this floodplain – the main city Dhaka is just on the north side of this zoom but doesn’t really show up. However, one of the other larger cities, Faridpur with close to 1 million people, is going to be washed away soon. Already one of the suburb developments is in the middle of the river.

      Zoom in closer and watch even small rivers nearby change by 2-3 kms even.

      https://earthengine.google.com/timelapse/#v=23.74754,89.99437,8.208,latLng&t=1.54

  11. The only science I see related to this paper is the take away that GRACE data is again at odds with reality.

    • Prior to 1957, the precipitation was measured in inches [1″ = 2.54 cm] and after 1956 onwards they are measured in mm.

      Dr. S. Jeevananda Reddy

    • Is it possible that all the above average rain in Oklahoma over the past 30 years could in anyway be affecting the slip zones in the earthquake faults? That is a lot of extra water that has to be accounted for. Might be interesting to track the rise in quake frequency against the constant wet conditions.

      pbh

  12. High school project. They didn’t engage the geology department or a USGS specialist. You need to know a lot more about locations of permeable formations giving a connection to aquifers from the surface and such things as river recharge zones. Those wetter areas in the north feed rivers flowing eastward to the Mississippi and these can be effective rechargers of the Ogallala aquifer that stretches from ND to TX. Apparently the aquifer drawdown in TX has slowed in recent years. Also the time scale for such an assessment is a way too short.

  13. And if more & more people move into flood risk areas there will of course be more humans impacted from these floods. How convenient for the ‘more severe weather’ green blob.
    BTW:
    FEMA sells below market rate flood insurance and then hands out ‘disaster’ cash to those who put themselves at risk …. ah.

  14. Been There,
    done flooding analysis for one river in the south of England,
    surprise result (sarc) :- the major factor was amount of rain
    even bigger surprise result (no sarc) :- more rain did not necessarily create more flood, the deciding factor appeared to be the direction of travel of the weather front …..

    Rain front traveling generally from mouth to head of river with rainfall data recorded , no flood up to a certain amount.
    Rain front traveling from head to mouth with similar and sometimes less rain recorded , flood
    we never did soil wetness checks those years ago though so maybe skewed results.

    I wonder if they looked at the direction of the weather/ timing of rainfall at gauges ?

  15. These people are just sooo clever..

    What, exactly, is GRACE measuring?

    To my mind, what determines whether a flood sweeps through a town/village or city is the water storage capacity of the top 2 or 3 feet of dirt.
    Is GRACE measuring that – pretty damn clever if she is.

    If the top layer of dirt is sandy/dry and porous, rain runs right through and almost immediatly into water courses & rivers.
    Likewise if the layer of dirt is thin & rocky underneath.
    In both cases its just like rain falling on the roof of your house and straight into the gutters and drains.
    That’s why deserts and city sidewalks/roads alike endure Flash Flooding – huge torrents of muddy water that rise quickly and subside just as fast.

    If the top layer of dirt contains a lot of organic matter, then it acts as a sponge/reservoir. It can ‘fill’ quickly and then let the water go slowly.
    Old plant material is basically cellulose, 5 carbon atoms and 5 water molecules and Water Sticks to Water. You really cannot emphasize that too much

    I really really doubt that GRACE is seeing that and meanwhile, does anyone fancy a nice vacation on the Aral Sea or Lake Bakhtegan?

    Nah, thought not

  16. And just to jump on the Texas weather bandwagon this morning, let us all note that Houston has experienced 2 >500-year flood events (tropical storm Allison and the Tax Day flood of 2016) in the past 15 years. I’m glad to see from the GRACE data that the overall chance of floods is decreasing. I’d hate to see what three 500-year flood events would look like here.

  17. Oh, Ambiguity, thy name be Climate Science! Was this done on a bet? Who could say the least using the most words, pretty pictures optional?

  18. 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.

  19. 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

  20. 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.

  21. 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.

  22. 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/

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