The odds that New York City will be flooded by a storm, such as a winter nor'easter or a summer hurricane, is now at least 20 times greater than it was 170 years ago, a new study found. It pinned part of the blame on a rising sea level, but also found that other factors are playing a role in escalating the flood risk.

The study, accepted for publication in the journal Geophysical Research Letters, found that a combination of long-term sea level rise, the destruction of wetlands and dredging of waterways as well as possible shifts in storm tracks and strength may all be sharply boosting the risk of dangerously high storm tides.

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In a sign of the shift to a riskier coastal flood environment in New York, one which is mirrored in many low-lying coastal cities worldwide, the study found that three of the nine highest recorded water levels in the New York Harbor region since 1844 have all occurred since 2010, and eight of the largest 20 such events have occurred since 1990.

This is no coincidence, says Stefan Talke, a civil engineer at Portland State University and lead author of the study.

The annual probability that a storm, such as a winter nor’easter or a summer hurricane, will overtop the typical Manhattan seawall has increased about 20-fold since 1844, Talke told Mashable, going from less than 1% a year in the 19th century to about 20-25% today. That means that waters can be expected to top the lower Manhattan seawall, which is about 5.7 feet high — once every four to five years, according to a summary from the American Geophysical Union (AGU). That's up from once every 100-400 years back in the late 19th century.

In large part because of a 1.4-foot increase in local sea level since 1856, the height of the 10-year flood level has increased by 2.4 feet during that period, the study found. A 10-year flood is one that has a 10% probability of occurring in any given year.

Storm tide heights of the highest events in lower Manhattan, showing that Sandy was the highest since 1900. Image: UCAR

The issue of how sea level rise and other trends are combining to create a far more perilous coastal existence for cities like New York has taken on a new urgency since Hurricane Sandy struck in 2012, bringing the highest storm tide, which is a measure of the storm surge plus tide level, on record since at least 1821, the study found.

Hurricane Sandy cost at least $65 billion, and claimed more than 100 lives. The vast majority of the deaths in New York City were due to drowning, as the Atlantic Ocean temporarily reclaimed land in every borough of the city, especially on Staten Island.

In the wake of Sandy, former New York Mayor Michael Bloomberg proposed a $19.5 billion plan to bolster New York's defenses against future sea level rise and storm surges. Whether Mayor Bill de Blasio will follow through on that plan is not yet clear.

A steady drumbeat of studies have been published in recent years warning that coastal cities will have to adapt to frequent assaults from the sea, with growing coastal megacities in the developing world facing the biggest challenge.

For example, a 2012 study found that an additional 3.3 feet of sea level rise would cause the current 100-year flood event to occur far more frequently along the East Coast, at about once every 20 years. Engineers typically design infrastructure to withstand a 100-year storm, which means that if 100-year events become more frequent, buildings, railways, and bridges may not be strong enough.

Talke told Mashable that while sea level rise likely accounts for a significant amount of the added coastal flooding risk in New York, other dynamics are also at work. Over the past 150 years, he said, there were other major manmade changes to New York Harbor as the city grew.

For example, New York lost 85% of its wetlands during that period, and these helped to blunt the impact of a major storm surge. In addition, the shipping channel between Sandy Hook, New Jersey and Raritan Bay used to be just 24 feet deep in the 1800s, whereas they have been doubled in order to accommodate large container and cruise ships.

The deeper water can increase wave heights during a storm event, Talke says. “[It] basically decreases the drag on an incoming wave, and is as if you had switched from a VW bus to an aerodynamic car,” Talke said. “You can go faster, and lose less energy. From the point of view of a wave, that means you are getting higher.”

500,000 data points

For the study, Talke and his coauthors at the Stevens Institute of Technology in New Jersey undertook a painstaking examination of nearly 500,000 data points from tide gauge records to find the maximum water level reached during each year. Some time periods required correcting for data biases known to affect certain types of tide gauges, and the scientists checked their data against the New York Times archives to make sure they did not miss any major storm events.

According to Talke, water level measurements in the late 1800s were nearly as accurate as those taken today. Water levels used to be measured using gauges that employed a stilling well, which is “basically a pipe with holes at the bottom.” This pipe would act to remove the effects of wind and waves.

“Until at least the early 1990s in the U.S., a float on the water moved up and down, and was connected to a pencil via a chain and some gearing. The pencil moved up and down on a scroll as the tide moved up and down, and a continuous trace of the tide was measured by moving the scroll forward in time with a clock,” Talke said. “These days the tide can be measured by radar, pneumatically, or [through] other means, but one still needs a method of removing the effect of waves.”

Talke says that one of the problems that plagued early tide gauge measurements in New York was the tendency of the harbor to freeze up much more widely and frequently than it does today, in another reflection of the warming the region has experienced.