An increased storm tide flowing high enough to exceed Manhattan's seawall defenses is 20 times greater today than it was 150 years ago, according to a new study published in a journal of the American Geophysical Union.
The increased risk comes in large part to rising sea levels in New York Harbor, bringing with it the risk of extensive flooding on a more frequent basis, co-author Stefan Talke said.
"The most surprising finding in our research was that we noticed this large increase in storm tides in New York Harbor-nearly 1 foot for the [once in] '10-year storm' over the past 150 years," Talke said.
"This result was dependent on sea level rise. When local sea level rise is included, the 10-year storm tide has increased by more than 2.2 feet since the 1800s."
This is a significant long-term shift in risk, he said.
Storm tide is the combination of increased water levels built by a storm, or storm surge, and the astronomical tide.
"Our paper shows that storm tide and storm surge risk in Manhattan has been increasing over time, for any given level of storm," Talke said. "Results suggest that storm tides and flooding similar to Hurricane Irene in 2011 have about a 10 percent chance of occurring in any given year, even with no further increase in sea-level."
However, Talke said it is important not to confuse 'over-topping' a seawall with massive flooding of Manhattan.
"The area flooded depends on many factors, including the length of time that a storm tide stays over a threshold and whether the land is rising behind the seawall or not," he said. "The seawall height itself is not consistent and varies from place to place."
Talke said a nominal height of 1.75 meters (5.74 feet) was chosen for the study. The National Weather Service states that a threshold of 1.8 meters produces moderate flooding in New York.
According to the study, the increased risk of water levels reaching this height and topping the lower seawall is likely to occur once every four to five years. In the 1800s, this risk was substantially lower as water was only expected to exceed the lower seawall once every 100 to 400 years.
"Another confusion concerns climate change-we cannot yet attribute our results to climate change," he said.
Talke said the reason behind the one and a half foot increase in sea level could be due to a variety of factors.
"The primary factor influencing increased water levels in New York Harbor is a nearly 1.5 foot increase in sea level, which in New York is both due to sinking of the land (subsidence) and global sea level rise. Over the last 150 years, those processes have had an equal effect."
Talke said the reason for the overall increase in storm tides is being investigated further.
"The cause of the increased storm tides needs to be found so that we can figure out whether they will increase in the future," Talke said. "We do know, based on the paper results, that storm tides are influenced by large-scale climate patterns in the North Atlantic, such as atmospheric pressure difference between the Azores Islands and Iceland."
This is known as North Atlantic Oscillation, he said, adding that during decades when the atmospheric pressure difference is smaller than average, there is an observed greater probability of large-scale storm tide events and greater variability overall year to year.
Above-normal averages are also dependent on onshore flow and the duration of the storm, which allows the storm surge to build, AccuWeather.com Senior Meteorologist Dave Dombek said, adding that the phase of the moon will compound this for even greater water levels.
"Astronomical high tides are higher than normal and if a particular storm arrives at the time of high tide there will certainly be a higher storm tide," AccuWeather.com Senior Meteorologist Brett Anderson said.
"The actual track and intensity of a storm is critical. A strengthening storm tracking inland into the New Jersey shore [similar to Sandy] can direct the highest storm surge into New York City."
Rising sea levels globally are a result of melting glacial ice, warming oceans and the loss of ice on the Greenland and Antarctic ice cap, Anderson said.
"About half of the past century's rise in sea level is due to warming oceans (thermal expansion)," Anderson said, citing a University of Colorado Sea Level Research Group study, which indicates that global sea level is currently rising at a rate of 3.2 mm (0.13 inches) per year.
"Not all locations in the world are rising, but when you look at the globe as a whole, there is no doubt that sea levels are rising," Anderson said.
Talke said in theory, storm-tide averages over time should stay the same if there are no changes to the tides and the statistics of storm magnitudes or tracks.
Increase in storm tides over time can also be independent of the North Atlantic Oscillation, according to Talke.
"The reason for this is unknown, but we hypothesize that it is some combination of local changes to the harbor and climate-induced changes to storm tracks or storm magnitudes," he said. "Over the past century, harbors and estuaries the world over have been deepened and streamlined, and wetlands have been drained."
This has decreased the frictional resistance to both tides and storm surge, he added.
"Basically, more water can enter and exit a harbor," Talke said. "More subtle effects such as resonance can also change when the geometry of a harbor is altered."
In New York Harbor, there have been small, but significant changes to the tides over time, indicating that the physics of the system have been changing in response to local processes, according to Talke.
"An effect on storm tides and storm surge is therefore also plausible, but the exact mechanisms remain to be investigated," he said.
According to the study, Sandy produced the largest storm tide on record since 1821.
Talke became interested in historical storm tide four years ago during his participation in a project investigating the change in tides along the West Coast. During that time, he found evidence of earlier measurements through Google not recorded in the NOAA database.
Talke noticed that documents from the 1800s were not always reflected in any modern documents.
"Even people that I contacted at NOAA did not know anything about the data, and it was presumed lost," he said. "However, with persistence, I found someone at NOAA who knew of an old box in a storage room, and an archivist with the National Archives who knew of additional documents."
Talke continued to dig for the lost information, which would eventually provide him with some of the data he was looking for.
"Little by little, through a combination of luck and pluck, I was able to find more and more of the data, including eventually the ‘mother lode'-a series of 26 boxes that included tabulations of all the major U.S. tide measurements in the nineteenth century, going back to the 1830s and 1840s," he said, adding he is still sorting through a large portion of the documents.
His motivation in New York was primarily Hurricane Sandy's impact in October 2012. In 1893 and 1821 hurricanes also struck New York, but their storm tide heights were unknown.
However, because of an experience he had months earlier, he knew that the 1893 hurricane did not produce the highest storm tide of the year. With his results, he knew data stretching as far back as 1835 was available in the archives.
"The motivation for the study was to get a better idea of the return period for hurricane Sandy, using more data," he said. "What we found is that one really needs to include the 1821 event to get a decent return period; we currently have a manuscript in preparation on that issue."
Talke, an assistant professor of civil and environmental engineering at Portland State University, said the ‘jury is still out' on methods to mitigate and reduce storm tides in the future; however, it would likely be wise to include the possibility of increased storm tides and rising sea levels into regional planning.
"If there are local causes to increased flooding, there could be local solutions," he said. "However, initial modeling efforts by my co-author, Philip Orton, suggest that restoring wetlands and reducing channel depths in Raritan Bay (a side embayment of New York Harbor) can significantly diminish the storm surge wave."
Talke added that more work is needed to explore such ‘green' alternatives to traditional civil engineering structures.