The extensive damage to an amusement park roller coaster in the aftermath of Hurricane Sandy is seen in Seaside Heights, New Jersey, November 13, 2012. REUTERS/Tom Mihalek
Thomson Reuters
Hurricane Sandy was a freak storm.

The massive system was the costliest storm to strike the US since Katrina.

When it made landfall four years ago on October 29, 2012, it impacted a region from Florida to Ontario, Canada, flooded the New York subway system and wreaked havoc on coastal New Jersey. The total damage to New York City was worth $19 billion and to New Jersey $29 billion.




Calling Sandy a 260-year event doesn't mean it'll take 260 years to see another storm like it.

What that 260-year-storm label means is that in any given year there is a 1/260 chance of a Sandy-like storm flood happening. That's about the same chance that the 2016 presidential election will end in a tie, according to the website Fivethirtyeight.

To get this result, Orton fed data on past storms into a flood model of the area from Maryland to Cape Cod, then simulated thousands of storms to see what kind of flooding they would cause.

Severe hurricanes in the New York City area are pretty rare, so to make sure he had enough data he dug into the histories of storms dating back to 1700. In the process, he showed that no storm has caused as much flooding as Sandy in recorded history.

Two Category 3 hurricanes, in 1788 and 1821, packed more of a wind punch than Sandy. But they were smaller and faster moving, so they caused less flooding. Sandy stands out because it remained in the area for about three days, pushing waving after unrelenting wave onto land to cause historic floods.

Orton set the model to generate results for conditions as they exist today, not accounting for future sea level changes due to climate change. The goal, he said, was to get a baseline for major flood risks in the area in the next decade

Future studies can run the same model with adjusted sea levels to see how bad things might get in our changing world.

His results, he said, show that people should take the threat of major storms like Sandy seriously, and work to protect against them in a changing world. But they also show that people shouldn't panic, and that smaller, more common floods are the more immediate concern.