Storm Waters Could Easily Flood Manhattan Due To Rising Seas
April 24, 2014

Storm Waters Could Easily Flood Manhattan Due To Rising Seas

[ Watch the Video: Odds of Storm Water Overflowing NY Increasing Significantly ]

April Flowers for - Your Universe Online

After Hurricane Sandy devastated New York, the fear of another flood has driven much concern in both the academic world and the political one.

A new study from Portland State University reveals that since the mid-1800s, maximum water levels in New York harbor during major storms have risen by nearly two and a half feet. This makes the possibility of water over-topping the Manhattan seawall 20 times greater than they were 170 years ago.

The study, to be published in Geophysical Research Letters, also demonstrated that global sea-level rise has raised the water levels along New York harbor by nearly one and half feet since the mid-19th century, and that the city's "once-in-10-years" storm tide maximum height has grown additionally by almost a foot in that same period.

Stefan Talke, an assistant professor of civil and environmental engineering at Portland State University, led the research team. He said that the newly recognized storm-tide increase indicates that New York is at risk of more frequent and extensive flooding that scientists expect from sea-level rise alone. For example, the storm-tide produced in the New York harbor by Hurricane Sandy was the largest since at least 1821.

Talke and a graduate student collected gauge data by photographing hundreds of handwritten hourly and daily tide gauge data going back to 1844--these documents are stored in the National Archives. His team created a spreadsheet of the data, adjusting where points were incorrect or missing, and using newspaper accounts of big storms to fill in the gaps. This allowed them to calculate storm tide levels and identify trends, which were then compared to climate data.

The results show that larger storm tides, and potentially more damage, are created by major, "10-year" storms hitting New York today than the identical storm in the mid-1800s would. The team calculated that there is a 10 percent chance that in any given year now, a storm tide in New York harbor will reach a maximum height of nearly six and a half feet, the so-called "10-year storm." The maximum height in the mid-1800s, however, was around 5.6 feet, nearly a foot lower than it is today.

"What we are finding is that the 10-year storm tide of your great-, great-grandparents is not the same as the 10-year storm tide of today," Talke said.

The amount that water levels rise during a storm—including  both storm surge (the abnormal rise above sea level caused by the storm) and the predicted astronomical tide—is called the storm tide. The calculated storm tide increase Talke and his team have uncovered is in addition to the nearly one and a half foot rise in local sea level that has been noted since the mid-19th century in New York harbor.

It is the combination of these two increases—storm tide and sea level—that leads Talke to claim that waters can now be expected to overtop the Manhattan seawall once every four or five years. The seawall is 5.74 feet high. Water was only expected to top the seawall once every 100 to 400 years in the 19th century when both storm tide and sea levels were lower.

Talke asserts that, although other studies have attempted to answer the question of rising storm tides in the region before, none of them have gone back as far in the historical record as his own. Agencies such as NOAA, for example, only have records that go back to the 1920s.

The team suggests that there could be multiple factors influencing the variability of the storm tides over the last 170 years in New York harbor. Decades-long variations in the North Atlantic Oscillation—an irregular fluctuation of atmospheric pressure over the North Atlantic Ocean and which exerts a strong influence on winter weather in Europe, Greenland, northeastern North America, North Africa, and northern Asia—accounts for approximately half the long-term change.

The researchers also suggest that longer-term trends, such as global warming and climate change, could also be influencing the increase in storm tides. Local factors, including the deepening of shipping channels around the harbor, could also be culprits, according to Talke.

Chris Zervas, a scientist at NOAA's Center for Operational Oceanographic Products and Services, commented on the study findings, saying that "they might indicate that "storm surges' interaction with New York harbor has gotten larger so that in addition to sea level rise, the storm surges may have been enhanced."

"For the latter part of the 1900s, [it shows] that the possibility of overtopping the seawall has increased quite a bit in addition" to sea-level rise, he added.

Having access to almost 200 years' worth of continuous data allowed the researchers to tease out decades-long cycles and long-term increases that might not have become visible with shorter data sets. The study findings could help in fine-tuning future predictions for storm tides in the coming decades, and could help New York and other cities mitigate future problems.

"If it turns out to be a local reason, as has been suggested in some cases, there could be local solutions as well," Talke said. "In some cases, we may be able to turn back the clock on that a bit."

Image Below: This NOAA graphic illustrates water level differences for storm surge, storm tide, and a normal high tide as compared to sea level. Storm surge is the rise in seawater level caused solely by a storm. Storm tide is the total observed seawater level during a storm, which is the combination of storm surge and normal high tide. Credit: NOAA