Tropical Storm Lee's flood in Binghamton: was global warming the final straw?

With one of the wildest weather years in U.S. history drawing to a close, it's time to look back at some of this year's unprecedented onslaught of billion-dollar weather disasters--and the lessons we should have learned. One of these disasters was the approximately $1 billion in damage due to flooding from Tropical Storm Lee, which brought torrential rains along a swath from Louisiana to New York in early September. Among the hardest hit cities was Binghamton, New York (population 47,000), where record rains due to the remnants of Tropical Storm Lee on September 8 brought a 1-in-200 to 1-in-500 year flood to the city's Susquehanna River. A flood 8.5 inches higher than the city's flood walls spilled over into the city that day, damaging or destroying over 7,300 buildings in Greater Binghamton, and causing hundreds of millions of dollars in damage. Damage to Binghamton's sewage treatment plant and city infrastructure alone are estimated at $26 million. Damage to one elementary school is estimated at $11 - 19 million. The total damage to the county Binghamton lies in (Broome) and the downstream Tioga County is estimated at $1 billion. I argue that there is strong evidence that the extra moisture that global warming has added to the atmosphere over the past 40 years could have been "the straw that broke the camel's back" which allowed Binghamton's flood walls to be overtopped, causing tens of millions in damages. Had this event occurred 40 years ago, before global warming added an extra 4% moisture to the atmosphere, the Susquehanna flood would have likely stayed within the city's flood walls.


Figure 1. Front Street Bridge on the Susquehanna River in Vestal, NY, immediately following the flood of September 8, 2011. Image credit: USGS, New York.


Figure 2. The Susquehanna River at Binghamton crested on September 8, 2011, at the highest flood height on record, 25.71'. The previous record flood was 25', set June 28, 2006. Flood records in Binghamton go back to 1846. Image credit: NOAA/AHPS.


Figure 3. Damage survey of Binghamton, New York after rains from the remains of Tropical Storm Lee sent the Susquehanna River over the city's flood walls on September 8, 2011. Image credit: City of Binghamton.

Binghamton's 2nd 1-in-200-year+ flood in five years
This year's flood is the second 1-in-200 to 1-in-500 year flood in the past five years to hit Binghamton. On June 26 - 29, 2006, tropical moisture streaming northwards over a front stalled out over New York state brought over thirteen inches of rain to portions of southern New York. The Susquehanna River swelled to record levels, triggering devastating flooding that cost at least $227 million. In Binghamton, the Susquehanna River crested eleven feet over flood stage, the greatest flood since records began in 1846. The flood walls protecting Binghamton were overtopped by a few inches, allowing water to pour into the city and cause tens of millions of dollars in damage. This flood is another example of a case where global warming may have been "the straw that broke the camel's back", allowing the flood walls to be overtopped by a few inches. While it is not impossible that the 2006 flood and the 2011 flood could have occurred naturally so close together in time, such a rare double flood has been made more likely by the extra moisture added to the atmosphere due to global warming.


Figure 4. Susquehanna River floodwaters overtop a flood wall along North Shore Drive, Binghamton, NY, on June 28, 2006. Photo courtesy of Alan A. Katz, and available in the USGS report, Flood of June 26 - 29, 2006, Mohawk, Delaware, and Susquehanna River Basins, New York.

The 2011 Tropical Storm Lee flood event on the Susquehanna: a convergence of rare events
Near-record rains fell over much of New York, Pennsylvania, and surrounding states during the first four weeks of August 2011, thanks to an active weather pattern that brought numerous thunderstorms. By August 27, Binghamton, New York had already received nearly double its normal total of 3.45" of rain for the month. When Hurricane Irene swept northwards along the mid-Atlantic coast on August 28, the storm dumped record rains that triggered billions of dollars in flood damage. The Susquehanna River Valley and Binghamton were spared the heaviest of Irene's rains and suffered only minor flooding, but the region received 3 - 5 inches of rain, saturating the soils. The 2.72 inches of rain that fell on Binghamton brought the total rainfall for August 2011 to 8.90", making it the rainiest August in city history (weather records go back to 1890.) Irene's rains helped give New York, New Hampshire, New Jersey, and Vermont their wettest Augusts since record keeping began in 1895.


Figure 5. Rainfall amounts from Hurricane Irene ranged from 3 - 5 inches over Binghamton and the Susquehanna River Valley upstream (northeast) of the city. Image credit: David Roth, NOAA/HPC.


Figure 6. Rainfall amounts from Hurricane Lee ranged from 5 - 10 inches over Binghamton and the Susquehanna River Valley upstream (northeast) of the city. Image credit: David Roth, NOAA/HPC.

Irene set the stage for what was to become the greatest flood in recorded history on the Susquehanna River. On September 5, a front stalled out over Pennsylvania and New York. Tropical moisture streaming northwards in advance of Tropical Storm Lee was lifted up over the front, and heavy downpours resulted. The rains continued for four days, and were amplified by the arrival of Tropical Storm Lee's remnants on September 7, plus a stream of moisture emanating from far-away Hurricane Katia, 1,000 miles to the south-southeast. Binghamton, New York received 8.70" of rain in 24 hours September 7 - 8, the greatest 24-hour rainfall in city history. This was nearly double the city's previous all-time record (4.68" on Sep 30 - Oct. 1, 2010.) The record rains falling on soils still saturated from Hurricane Irene's rains ran off rapidly into the Susquehanna River, which rose an astonishing twenty feet in just 24 hours. By noon on September 8, the rampaging Susquehanna River crested in Binghamton at 25.71', the highest level since records began in 1846. The river would have risen higher had the city's flood walls been higher, but since the water was overtopping the flood walls and spreading out over the city, the river was limited to how high it could rise. By month's end, precipitation in Binghamton for September 2011 totaled 16.58", more than thirteen inches above normal, making it Binghamton's wettest month since records began in 1890.

We can thus see how the record Susquehanna River flood of September 8, 2011 was due to a convergence of rare events, which included moisture from three tropical cyclones:

1) The unusually heavy rains during the first four weeks of August, before the arrival of Hurricane Irene.

2) Hurricane Irene's 3 - 5 inches of rain.

3) The extreme rains from Tropical Storm Lee's remnants.

4) The enhanced rainfall on September 7 - 8 due to a moisture plume from Hurricane Katia.

Had any one of these events not occurred, it is questionable whether the flood walls in Binghamton would have been overtopped. One could also argue that the flood walls would not have been overtopped had there been less development in the Susquehanna's floodplain. Dr. Peter Knuepfer, Associate Professor of Geology and director of the Environmental Studies Program at Binghamton University, and Dr. Burrell Montz, who is now Professor and Chair of Geography at East Carolina University, wrote in a 2007 essay titled, Flooding and Watershed Management, "the 2006 flood might be considered a land use flood, due to the levels of development in floodplains in Conklin and elsewhere in the Binghamton area." They argued that development on the Susquehanna's floodplain has been driven by economics, without enough thought to how development increases flood heights downstream. "It can hardly be argued that we need to reacquaint the river with its floodplain," they concluded. In an email I received from Dr. Knuepfer, he indicated that some positive steps have been taken to reduce flood vulnerability in the Binghamton area before this year's flood: "There's still more development in the floodplain than should be, though there is a little more awareness (but only a little!) about the downstream implications of raising levees and walls (and certainly this seems to be true at the Federal level). From Binghamton downstream--the Susquehanna River had a 200+ year flood (the number one chooses depends on how one treats the historic flood record, but it was clearly an event well beyond the historical record.) Some areas flooded by the river in 2006--houses, specifically--no longer exist due to FEMA buy-outs. Yet there is still development in flood-prone areas, so there is still a degree of floodplain development that contributes significantly to the disaster. On the other hand, this flood overtopped levees and flood walls precisely because it was a bigger natural event than these were designed to withstand. So there's still more exposure than I'd like to see, but this was a natural disaster." To illustrate how development in a flood plain can increase flood height, consider this stat from nrdc.org: a 1-inch rainstorm falling on a 1-acre natural meadow produces about 28 bathtubs full of runoff into local rivers. However, a 1-inch rainstorm falling on a 1-acre parking lot produces sixteen times as much runoff--448 bathtubs full. We obviously can't convert our parking lots into meadows, but we can create permeable pavement, planted swales around parking lots, rain gardens planted along sidewalks, green roofs, and more trees to help absorb rainwater like a sponge. The city of Philadelphia has recently started an ambitious effort to reduce flood through such green infrastructure efforts.


Figure 7. Water vapor satellite image taken at 2:45 pm EDT September 7, 2011, during the height of the heavy rainstorm affecting the Susquehanna River Valley near Binghamton, NY. Moisture came from the remains of Tropical Storm Lee, tropical moisture streaming northwards and lifting over a stalled front, and from Hurricane Katia, located 1,000 miles to the south-southeast, between Florida and Bermuda. White and blue colors show where copious atmospheric moisture lies, while brown colors show dry air. Image credit: NOAA/NESDIS.

The global warming connection
Finally, I'll add one more "straw that broke the camel's back" that contributed to the overtopping of the flood walls in Binghamton: global warming. Had the flood of September 8, 2011 occurred in the atmosphere of the 1970s or earlier, the flood walls would have been less likely to be overtopped. There is a well-established relationship in atmospheric physics called the Clausius-Clapeyron equation, which says that atmospheric moisture will increase by 6% - 7% for every degree Centigrade increase in Earth's temperature. Global sea surface temperatures in the regions where hurricanes form, between 30°S and 30°N latitude, warmed 0.9°F (0.5°C) between 1970 - 2004, due to global warming (Trenberth et. al, 2007.) Satellite observations show that atmospheric moisture over the oceans increased by 1.3% per decade between 1988 - 2003 (Trenberth, 2006), so we can expect that the amount of moisture storms have to work with has increased by 4% since 1970 and 5% since 1900 (IPCC, 2007.) The amount of rainfall a hurricane can now drop as a result of this increase in moisture can be much more than 4 - 5%, though. The extra moisture in the atmosphere helps intensify storms by releasing "latent heat" energy when it condenses into rain. Latent heat is the extra energy that is required to convert liquid water to gaseous water vapor, and this energy is liberated when the vapor condenses back to rain. The released latent heat energy invigorates the updrafts in a storm, allowing it to draw in moisture from an area greater than usual (a typical storm draws in moisture from an area 3 - 5 times the radius of the precipitating region, according to Trenberth et.al, 2003.) This effect is thought to be the main reason why heavy precipitation events--the ones most likely to cause floods--have been increasing over the past 50 years, in general agreement with the predictions of climate models (Figure 8.) A 2008 study in the Netherlands by Lenderink and Meijgaard called "Increase in hourly precipitation extremes beyond expectations from temperature changes," found that "one-hour precipitation extremes increase twice as fast with rising temperatures as expected from the Clausius–Clapeyron relation when daily mean temperatures exceed 12°C. In addition, simulations with a high-resolution regional climate model show that one-hour precipitation extremes increase at a rate close to 14% per degree of warming in large parts of Europe." A 2007 study led by Dr. Kevin Trenberth of the National Center for Atmospheric Research, "Water and energy budgets of hurricanes: Case studies of Ivan and Katrina", looked at how much additional rainfall hurricanes might be dropping as a result of global warming. The researchers found that global warming likely increased the amount of rain dropped Hurricane Ivan and Hurricane Katrina by 6 - 8%. The authors wrote, "We conclude that the environmental changes related to human influences on climate have very likely changed the odds in favor of heavier rainfalls and here we suggest that this can be quantified to date to be of order 6 to 8% since 1970. It probably also results in more intense storms. The key point is that the value is not negligible, and nor is it large enough to dominate over the natural processes already in place. In the case of Katrina and New Orleans, where rainfalls locally exceeded 12 inches (305 mm), this would mean an enhancement of about 0.75 to 1 inch (19 to 25 mm). Although incremental, such changes can cause thresholds to be exceeded (the straw that breaks the camel's back.) Small differences of a few percent in rainfall can matter a great deal when that extra water is concentrated by a river drainage system to create a flood. For example, observations of flooding events in the Pennsylvania's 7.2 square km Mahantango Creek watershed (Troch et al., 1993) showed one case where two rainfall events with the same maximum precipitation rate generated flow rates in the creek a factor of seven different, even though the difference in total precipitation between the two events was about a factor of two. A modeling study by Jha et al. (2004) predicted that climate change would cause a 21% annual increase in precipitation over the Upper Mississippi River basin by 2040. However, their model predicted that streamflow would increase much more than this--51%. This occurred as a result of rain falling on saturated soils, which creates disproportionately large runoff. Much of the rain falling on dry soils takes time to infilrate the soil, and the arrival of this water into a river is delayed. But if soils are saturated, a greater percentage of the rain runs off immediately into the river, resulting in higher stream flows and higher flood potential. The largest increases in streamflow in their model occurred in spring and summer, when flood danger is at its highest.


Figure 8. Percent increase in the amount falling in heavy precipitation events (defined as the heaviest 1% of all daily events) from 1958 to 2007, for each region of the U.S. There are clear trends toward more very heavy precipitation events for the nation as a whole, and particularly in the Northeast and Midwest. Climate models predict that precipitation will increasingly fall in very heavy events in coming decades. Image credit: United States Global Change Research Program. Figure updated from Groisman, P.Ya., R.W. Knight, T.R. Karl, D.R. Easterling, B. Sun, and J.H. Lawrimore, 2004: Contemporary changes of the hydro-logical cycle over the contiguous United States, trends derived from in situ observations. Journal of Hydrometeorology, 5(1), 64-85.

Conclusion
There is strong evidence that the extra moisture that global warming has added to the atmosphere over the past 40 years could have been "the straw that broke the camel's back" in the case of the Susquehanna River floods of June 2006 and September 8, 2011, which overtopped the flood walls in Binghamton, New York, causing tens of millions of dollars in damages. During September 8, 2011 flood, the Susquehanna River rose twenty feet in 24 hours and topped the flood walls in Binghamton by 8.5 inches, so just a 6% reduction in the flood height would have led to no overtopping of the flood walls and a huge decrease in damage. Extra moisture in the air due to global warming could have easily contributed this 6% of extra flood height. It is possible that detailed computer modeling studies of the event may conclude that global warming was not a significant factor in this particular case, but we will see an increasing number of these back-breaking extreme flooding events in the future as the climate continues to warm and we increasingly load the dice in favor of greater extreme rainfall events. It is wildly improbable that two 1-in-200 to 1-in-500 year floods could have occurred on the same river within five years of each other naturally. Increased moisture in the atmosphere due to global warming and increased flood plain development are shifting the odds in favor of more extreme floods occurring more often. Our flood control system, which is designed for the climate of the 20th century and a lesser degree of flood plain development, is bound to be increasingly overwhelmed if we continue to put more structures into flood plains and continue to pump more heat-trapping carbon dioxide into the atmosphere. Unfortunately, we are not dealing well with the "new normal" for extreme floods. The National Flood Insurance Program, which charges unrealistically low insurance premiums, is $18 billion in debt. A government shut-down was narrowly avoided in September over disputes on how to pay for the damages from this year's 1-in-100 to 1-in-500 year floods on the Mississippi, Missouri, Ohio, Souris, Susquehanna, and hundreds of smaller rivers. Federal funding to operate 321 USGS stream gauges critical for issuing accurate and timely flood warnings was eliminated this year, and funding for an additional 69 gauges is threatened, including gauges on the Susquehanna River where this year's extreme flooding occurred. Eliminating funding for stream gauges in an era of increasing floods is like being too cheap to replace your cracked windshield that's hard to see out of, when you're about to drive the most difficult and dangerous road your car has ever attempted, at night, in a heavy rainstorm. You'll be unaware of the coming danger until it's too late to avoid it. Flood damages are going to grow much worse and potentially cause serious harm to the American economy in the coming decades, and our politicians need to adopt intelligent policies that don't cater to special interests in order to deal with the increasingly frequent and larger extreme floods that a warmer climate will bring.

References
IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Jha, M., Z. Pan, E. S. Takle, and R. Gu (2004), Impacts of climate change on streamflow in the Upper Mississippi River Basin: A regional climate model perspective, J. Geophys. Res., 109, D09105, doi:10.1029/2003JD003686.

Lenderink, G., and E. van Meijgaard (2008), Increase in hourly precipitation extremes beyond expectations from temperature changes,, Nature Geoscience 1, 511 - 514 (2008)
Published online: 20 July 2008 | doi:10.1038/ngeo262

Suro, T.P., G.D. Firda, and C.O. Szabo, 2009, Flood of June 26 - 29, 2006, Mohawk, Delaware, and Susquehanna River Basins, New York, USGS Open-File Report 2009-94-1063.

Trenberth, K. E., A. Dai, R. M. Rasmussen and D. B. Parsons, 2003: The changing character of precipitation", Bull. Amer. Meteor. Soc., 84, 1205-1217.

Trenberth, K. E., C. A. Davis and J. Fasullo, 2007: "Water and energy budgets of hurricanes: Case studies of Ivan and Katrina," J. Geophys. Res., 112, D23106, doi:10.1029/2006JD008303.

Trenberth, K.E., J. Fasullo, and L. Smith. 2005. "Trends and variability in column-integrated atmospheric water vapor," Climate Dynamics 24:741-758.

Trenberth, K. E., 2011: Changes in precipitation with climate change. Climate Research, 47, 123-138,
doi:10.3354/cr00953.

Troch, P.A., J.A. Smith, E.F. Wood, and F.P. de Troch, "Hydrologic Controls of Large Floods in a Small Basin: Central Appalachian Case Study", Journal of Hydrology, 156:285-309, 1994.

Other posts looking back at the remarkable weather events of 2011
Wettest year on record in Philadelphia; 2011 sets record for wet/dry extremes in U.S.
Hurricane Irene: New York City dodges a potential storm surge mega-disaster

Jeff Masters