How Long-Forgotten Seawall Fended Off Sandy

A buried and forgotten seawall built in 1882 may have significantly weakened Hurricane Sandy's grip on one New Jersey town, new research shows.

Bay Head — a beach town located along the northeast shores of New Jersey — lay directly in the violent path of Hurricane Sandy when the storm barreled toward the Eastern Seaboard last October. And yet only one house from the town was lost to the storm. The neighboring town of Mantoloking, on the other hand, lost more than a quarter of its houses.

To figure out how Bay Head thwarted Sandy's blow, a team of coastal engineers from Virginia Tech visited the region within two weeks of the storm to survey the area. They found what they believe to be their answer in a 4,000-foot-long (1,200 meters) wall of rocks that many residents hadn't even known was there, they reported earlier this month in the journal Coastal Engineering. [Jersey Shore: Before & After Hurricane Sandy]

"Once we got there, we immediately saw the seawall," Jennifer Irish, an engineer at Virginia Tech and a co-author of the study, said in a statement. The team noted that dunes along the beaches of both Mantoloking and Bay Head likely helped beat back some waves, but that the seawall provided a clear advantage for Bay Head. "The beach and dunes did their job to a certain point, then the seawall took over, providing significant dampening of the waves. It was the difference between houses that were flooded in Bay Head and houses that were reduced to piles of rubble in Mantoloking."

Two towns

The team examined satellite imagery and beach data from the two regions to assess whether other factors could have played a role, but found nothing that stood out as strongly as the seawall.

"Because of [the towns'] close proximity, and based on our survey, I feel confident that the conditions that they were exposed to were virtually identical," Irish told Livescience.

The team believes that the combination of the hard seawall — which stands about 5 feet (1.5 m) above the sand — and overlying soft sand dune likely accounts for the structure's effectiveness.

"A seawall on its own is detrimental to the beach," said Patrick Lynett, an engineer at the University of Southern California who was a co-author on the study. By deflecting waves seaward, seawalls increase the amount of wave energy hitting beaches and cause more sand to wash away, he explained. "The seawall is good at protecting the town from being flooded, but for an extreme storm, it's not good."

The sand on top of the seawall provided extra cushioning, dampening the energy channeled back to the beach.

Extreme erosion

Other structures, like jetties that run perpendicular to beaches and breakwaters that sit underwater near shores, can also help prevent erosion, but usually not under extreme conditions like Hurricane Sandy. The team thinks that the combined seawall and dune could provide a good model for other beach towns looking to prevent erosion. But every beach is different and should be assessed on a case-by-case basis, Lynett said.

The team now plans to look more closely at their data to try to better understand how storms as large as Sandy affect erosion and other beach processes. 

"We really hope we can learn a lot from this terrible event, and improve our ability to recover and increase the resiliency of coastal communities," Irish said.

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