Drawing on historical storm and flooding records, combined with computer simulations, Stevens researchers analyze a little-known natural phenomenon called a continental shelf seiche that can cause unanticipated flooding hours after storms pass, and had been mistaken for a tsunami in the past. The findings can better inform flood forecasting and bear important safety implications for New York and New Jersey coastal communities.
A seiche is a wave that forms when water in a lake, bay or harbor begins sloshing back and forth between its coasts, after being disturbed by strong winds. “The motion is similar to water rocking inside a bathtub after being pushed,” explains Research Associate Professor Philip Orton, who studies storm surges, flood forecasting and coastal engineering and adaptation at Stevens School of Engineering and Science’s Department of Civil, Environmental and Ocean Engineering. “Instead of passing through only once and dissipating, the wave bounces off or reflects between the sides of the tub until it peters out.” In nature, this wave reflects off the coastlines that bound the body of water until it loses its energy.
A different type of seiche called continental shelf seiche can occur at the continental shelf locations, where the sloping edges of a continent submerge in the ocean. In such places, the ocean tends to be so vast and deep that it acts like a solid border or a wall, pushing the wave back to the shore. “Continental shelf seiches are much less widely known than the classic type of seiches that occur in enclosed or semi-closed bodies of water,” explains Orton, “largely because there is no obvious reflecting boundary on the deep ocean side.”
The New York Bight — a roughly triangular, coastal stretch that spans from Cape May, New Jersey, to Montauk Point on Long Island, New York — sits directly on the continental shelf. “So when the wave goes to the edge of the continental shelf, about 100 miles offshore, and hits this stationary deep ocean that doesn’t want to move, the wave bounces off it and comes back to shore,” Orton explains.
A recent study by Orton and PhD candidate Tam Trinh found that the New York Bight is more susceptible to continental shelf seiches than other coastal areas within the larger, Mid-Atlantic Bight. When researchers compared 17 coastal tidal gauges across the Mid-Atlantic Bight, they found that water level oscillations in the New York Bight were anomalously large compared to other regions. “In our study, we found that these oscillations are triggered by abrupt changes in wind forcing,” explains Trinh. “Strong onshore winds can push large volumes of water toward the coast as a storm surge, and when the wind weakens, changes direction, or the storm moves away, the water can be released and then oscillate.”
Researchers outlined their findings in the new paper, titled Historical resurgences after tropical cyclones in the Mid-Atlantic Bight: A primary mechanism and hotspot, published in the journal Continental Shelf Research.
Historical data analysis shows that the New York Bight is a hotspot for such “resurgence,” including two occurrences that were so intense they were misidentified as tsunamis.
In 1938 and 1944, two major hurricanes struck Long Island and, after the initial winds subsided, the surges came back unexpectedly hours later, leading observers to mistakenly believe they had witnessed a tsunami. More recently, smaller post-storm resurgences happened in 2020, hours after Hurricane Isaias abated, causing some coastal flooding in the New York Harbor and surrounding areas.
“A continental shelf seiche can occur after a hurricane has passed and can cause a surprise onset of dangerous currents and flooding if it coincides with high tide,” Orton says. “Where mistaken for tsunamis, there was likely a coincidence of high tide, the continental shelf seiche, and very large storm swells all superimposed, causing an additional period of dangerous conditions,” he clarifies.
“In New York Harbor and the neighboring coastal areas, the seiche period is approximately 7 to 8 hours, meaning that resurgences of high-water levels arrive every 7 to 8 hours after the initial storm surge,” Trinh explains. “Due to this timing, continental shelf seiches may coincide with high tide and amplify coastal water levels, potentially leading to secondary coastal flooding many hours after a storm has passed. And, unlike short-duration storm-surge peaks, these seiche oscillations may persist for several tidal cycles.”
The seemingly improving post-storm weather conditions can catch residents and first responders off guard. People may decide that because the storm passed, the worst flooding is over — only to experience a renewed rise in water levels afterwards. These secondary surges can also generate strong currents in harbors, bays and inlets, damaging docks and boats and flooding roads, tunnels, subway systems and low-lying neighborhoods that may be in the middle of recovery operations.
In the era of accelerating sea level rise, these findings have a growing importance. Higher baseline water levels mean that even moderate resurgence waves are more likely to exceed flood thresholds. Infrastructure that previously flooded only during the main storm surge may flood again when the crest of the seiche arrives.
Orton cautions that for the densely urbanized environment of the New York Bight — with its interconnected tunnels, transit systems, ports, electrical infrastructure and millions of residents near the shoreline — these secondary surges can have severe consequences. “Continental shelf seiches are less studied and harder to forecast than ordinary storm surges,” explains Orton. “Our research aims to better understand the risks and improve flood forecasting and emergency management.”
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