Grab your boots, New Yorkers|500 years flood now come every quarter-century!

Grab your boots, New Yorkers: The inundation of Hurricane Sandy might have been billed as a 3,000 year flood, but according to new research, the recurrence interval for Sandy-sized flood events has shrunk. By a factor of 23.

“Flood heights are increasing and have increased since the pre-anthropogenic era, not only because of rising sea levels but also because of the impact that climate change is having on tropical cyclones,” said lead author Andra Reed of Penn State University.

Reed and colleagues made their conclusions based on climate models that simulated tropical storms and subsequent flooding for the region beginning in 850. They found that average flood height increased by more than 4 feet from 850 to 2005.

When Hurricane Sandy hit New York in 2012, it caused an estimated $50 billion in damage and destroyed at least 650,000 houses. Flooding from the storm reached a height of 9.2 feet in Battery Park. Prior to 1800, such a flood could be expected to hit the city every 3,000 years. Today, a flood of that magnitude will occur on average every 130 years, Reed said. By comparison, storms that are now anticipated roughly every 24 years will have a height of 7.4 feet in Battery Park.

“We wanted to make sure we had two time periods that were really distinct: one before we saw a lot of anthropogenic influence on the climate, and one where we were sure anthropogenic forcing would be dominant,” says Horton’s colleague Andra Reed at Pennsylvania State University.

Proxy sampling

As a proxy for sea-level rise, the researchers took sediment samples from the salt marshes on the New Jersey shore, where fossilised, salt-loving microorganisms called foraminifera gave a record of sea height. After estimating the ages of sediments they put together the past sea-level record.

“For the first time, we can look back beyond the observational record, which is very short, and look at how hurricanes and sea-level rise have changed through time,” says Horton. This allows us to see what effect these factors have had on flooding in New York City and the New Jersey region, he adds.

The researchers then used several global climate models to generate a set of hypothetical cyclones consistent with what winds and the atmosphere would have been doing at different times.

Bigger surges

The main reason for higher storm surges is an increased sea level. But the models also showed a rise in the most extreme kinds of storm – those with stronger maximum wind speeds and large ones like Sandy that move slowly.

“Think of it like a snowplough,” says Andrea Hawkes of the University of North Carolina Wilmington, who was not involved in the study. “The size of the shovel at the front gets bigger, so you’re shovelling more water. The slow-moving storms mean that for days you have all this water getting shoved in front, and you’re more likely to have it hit [the coast] at high tide.”

Reed says it’s hard to generalise to other cities, because sea-level rise can vary a lot locally. But she thinks we could see an increase in flooding at other points along the east coast.

“There is plenty of time for the water to penetrate inland and also there is a very good chance that the water will still be around when high tide comes by,” Holland said.

The current study did not project future flooding but with sea levels anticipated to continue rising, coastal areas will experience increasingly high waters during storms.

“There is a consistent message appearing and at the very minimum planning and consideration for future infrastructure needs to take it into account,” Holland said.

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