A new research on the increasing risk of 'compound flooding' for major US cities was conducted by the scientists at the University of South Florida's College of Marine Science. Analysis of numerous datasets shows that US cities situated along the Atlantic and Gulf coasts are at highest risk of flooding when the strong storm surge and high amount of rainfall events occur simultaneously.
Almost 40% of the U.S. population is situated in the coastal regions of the State, and, consequently, strong flooding events can have devastating social, economic and environmental consequences on the affected areas, said lead author of study, Thomas Wahl of the University of South Florida College of Marine Science and the University of Siegen in Germany.
The focus of the research was the combined occurrence of storm surge and high precipitation events, which can result in direct water run-off and increased river discharge. Data sets for storm surges and rainfall was collected and analyzed for more than 30 American cities along the Atlantic, Gulf and Pacific coasts. Some of the data was collected during 1950s, and from some sites, the data was gathered from the beginning of the 20th century.
Elevated water levels in estuarine regions, storm surge flooding event worsening with heavy rainfall and a moderate storm surge blocking or slowing down drainage were established as the key flooding mechanisms: "The complex interplay between storm surge and precipitation can lead to, or exacerbate, the impacts of flooding in coastal zones through multiple mechanisms.Whether or not all of these mechanisms are relevant at a particular site strongly depends on the local setting", explained Wahl.
Analyzes of the historical data revealed the risk for compound flooding was higher for cities along the Atlantic and Gulf coasts than for the Pacific coast regions. The detailed look on the New York City data showed that storm surges in 'the Big Apple' were combined by a heavy precipitation events in a case when a high pressure system stretched from Newfoundland south over the North Atlantic from where the moist air was transported into the low pressure system responsible for the storm surge. The severe flooding in the area during the "Hurricane Sandy" event in 2012, turned out to be a consequence of extreme storm surge, as the rainfall was very small to cause such an event.
In average, the research showed the number of compound flooding events was constantly increasing along a number of places along the coastline, during the last century.
"Our results demonstrate the importance of assessing compound flooding and its links to weather and climate, but we need more research at local scales to determine impacts," concluded Wahl and his colleagues. "That research will require complex, integrated modeling experiments that investigate surface and drainage flows and include storm surge, rainfall and river discharge. In light of climate variability and change, it will be important to develop a detailed understanding of future patterns of storm surge and high precipitation amounts occurring in tandem."
- "Increasing risk of compound flooding from storm surge and rainfall for major US cities" – Thomas Wahl, Shaleen Jain, Jens Bender, Steven D. Meyers, Mark E. Luther – Nature Climate Change (2015) – doi:10.1038/nclimate2736
Featured image: New York streets flooded during Sandy event, October, 2012. Image credit: PM Breakfast.
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