Storm surges amped by rain increase US flood risk

Sea storm surges amplified by heavy rain are a greater flood threat to US coastal cities than previously understood, and are occurring more frequently according to a study published Monday.  

The number of such "compound" events "has increased significantly over the past century," said a paper in the peer-reviewed journal Nature Climate Change.

In New York City, for example, the weather conditions that typically cause the combined conditions are twice as likely to occur today than in the mid-20th century, the researchers found.

Forty percent of the US population and more than half of the nation's economic productivity are in low-lying coastal counties.

To date, risk assessment for flooding in seaboard cities has calculated threats from ocean storm surges and from heavy rainfall separately.

But such projections "may result in an underestimation of the flood risk," the study's lead author, Thomas Wahl of the University of South Florida, told AFP by email.

"Our study shows that the two main drivers for flooding in coastal areas are not independent from each other, and often occur simultaneously."

Jointly, these forces can have a "multiplier effect" on coastal infrastructure, he added, with consequences ranging from washed-out bridges to crippled sanitation plants or compromised levees.

Storm surge is an abnormal sea level rise generated by a storm, over and above the predictable astronomical tides.

Hurricane Katrina in 2005, which left 1,500 people dead and caused some $75 billion (68 billion euros) in damage, exemplified the destructive potential of such surges.

By examining historical records of rainfall, tide gauge readings, and hurricane tracks, Wahl and colleagues documented a growing overlap between surges and heavy rain since the early 20th century.

A likely driver of this increase would be climate change effects -- especially sea level rise, which can boost the destructive power of surges by adding centimetres or inches to flood waters.

But even without rising sea levels, there was increasing confluence between storm surges and downpours, the team found.

"Why those weather situations occurred more often is one of the things we want to look at in the future," Wahl said.

The US coastal areas most affected are the southeastern seaboard, and the Gulf Coast from the southern tip of Texas to western edge of Florida -- much of it less than three meters (10 feet) above mean sea level.

But the paper said there was no direct equation between the size of a storm or volume of rain, on the one hand, and the destructive threat for cities, on the other.

For one thing, the shape of the continental shelf can dramatically change outcomes.

Drainage systems, flood barriers and soil conditions can also influence the severity of flooding.