Increased Risk of Compound Urban Flooding May Sink Cities

Why is there increased urban flooding over time? That's a good question. Now, scientists have taken a closer look at the increasing risk of "compound flooding" for major U.S. cities to better understand the conditions that cause these events.

"Nearly 40 percent of the U.S. population resides in coastal counties," said Thomas Wahl, lead author of the new study, in a news release. "Flooding can have devastating impacts for these low-lying, densely populated and heavily developed regions and have wide-ranging social, economic and environmental consequences."

The researchers actually focused on the joint occurrence of two distinct flooding sources in coastal regions: storm surge and high precipitation. The scientists also identified three compound flooding mechanisms: elevated water levels in estuarine regions, storm surge flooding that worsens with heavy rainfall, and moderate storm surge that blocks or slows down drainage.

Using data that reached back to the 1950s and some data from the beginning of the 20^th century, the researchers determined that the risk for compound flooding was higher for cities along the Atlantic and Gulf coasts than for those on the Pacific coast.

The researchers also took a look at New York City in greater detail. They found that storm surges in New York City are accompanied by heavy precipitation when a high pressure system stretches from Newfoundland south over the North Atlantic from where moist air is transported into the low-pressure system causing the storm surge.

So what does this all mean? In the end, the scientists found that overall the number of compound flooding events has been steadily increasing over the past century along many stretches of coastline.

"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," said Wahl. "That research will require complex, integrated modeling experiments that investigate surface and drainage flows and include storm surge, rainfall and river discharge."

The findings are published in the journal Nature Climate Change.

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