Long-Buried New Jersey Seawall Spared Coastal Homes From Hurricane Sandy's Wrath
Built in 1882, then hidden by drifting sands, seawall mitigated 2012 hurricane's effects
Picture two residential beach communities on the New Jersey shore: Bay Head and Mantoloking. They sit side-by-side in Ocean County on a narrow barrier island that separates the Atlantic Ocean and Barnegat Bay.
Before Hurricane Sandy landed on Oct. 29, 2012, a motorist traveling north would pass through Mantoloking into Bay Head. He or she would note few changes in residential development, dunes, beaches or shoreline.
The difference, however, was hidden under the sand.
A long-forgotten, 4,134-feet-long seawall buried beneath the beach helped Bay Head weather Sandy's record storm surges and large waves, says geoscientist Jennifer Irish of Virginia Tech.
The stone structure dates to 1882. Its reappearance in 2012 surprised many area residents, underscoring the difficulties transient communities have in planning for future threats along their shores, Irish says.
"It's amazing that a seawall built nearly 150 years ago, then naturally hidden under beach sands and forgotten, would have a major effect under the conditions in which it was originally designed to perform," says H. Richard Lane, program director in the National Science Foundation's (NSF) Division of Earth Sciences.
NSF funded the research through a rapid response award following Hurricane Sandy.
"This finding should have major implications for coastal planning, as sea level rises and storms increase in intensity in response to global warming," says Lane.
The results, published online this week in the journal Coastal Engineering, illustrate the need for multi-levels of beach protection in coastal communities, Irish and colleagues say. Irish is the paper's lead author.
"Once we got to the site, we immediately saw the seawall," Irish says.
"The beach and dunes did their job to a certain point, then the seawall took over, providing significant dampening of the hurricane waves.
"It was the difference between houses that were flooded in Bay Head and houses that were reduced to piles of rubble in Mantoloking."
With recovery efforts underway and storms still circulating through the area, Irish and Robert Weiss, also a geoscientist at Virginia Tech, along with Patrick Lynett, a civil and environmental engineer at the University of Southern California, assessed the area, documenting high water marks, damage, overwash and breaches of the barrier island.
All oceanfront homes in the two boroughs were damaged, ranging from ground-floor flooding to complete destruction.
As measured by waterlines in the interiors of homes, flooding was similar in both boroughs.
The difference was the extent of the storm's effects.
In Mantoloking, an entire dune nearly vanished. Water washed over a barrier spit and opened three breaches of 541 feet, 194 feet and 115 feet, respectively, where the land was swept away.
In Bay Head, only the portion of the dune located seaward of the seawall was eroded. The section of dune behind the seawall received only minor local scouring.
Later, using Google Earth to evaluate aerial images taken two years before and immediately after Hurricane Sandy, the researchers looked at the area's houses.
They labeled a structure with a different roofline as damaged, one that no longer sits on its foundation as destroyed and the remaining houses as flooded.
The scientists classified 88 percent of the oceanfront homes in Bay Head as flooded, with just one oceanfront home destroyed.
In Mantoloking, more than half the oceanfront homes were classified as damaged or destroyed.
Despite the immense magnitude and duration of the storm, a relatively small coastal obstacle--the seawall--reduced potential wave loads by a factor of two.
The seawall was the difference between widespread destruction and minor structural effects, the researchers say.
"We are left with a clear, unintentional example," says Irish, "of the need for multiple levels of defense that include hard structures and beach nourishment to protect coastal communities."
Additional researchers include Wei Cheng and Stephanie Smallegan of Virginia Tech.
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