When Hurricane Sandy's record-setting storm surge washed into coastal New Jersey, the upscale borough of Mantaloking took a body blow. Almost every house in the borough was damaged, and many were totaled.
Just up the beach, the adjacent borough of Bay Head took the same wave on board. But in Bay Head, most houses survived.
The difference? According to new research findings from Virginia Tech storm surge experts, Bay Head's houses were saved by a long-forgotten stone sea wall, built in the mid-1800s and buried long ago by drifting natural sand dunes. Kansas City InfoZine is carrying the story, based on a release from Virginia Tech ("Long-buried New Jersey Seawall Spared Coastal Homes from Hurricane Sandy's Wrath").
In Bay Head, "The beach and dunes did their job to a certain point, then the seawall took over, providing significant dampening of the hurricane waves," said Virginia Tech professor Jennifer Irish. "It was the difference between houses that were flooded in Bay Head and houses that were reduced to piles of rubble in Mantoloking."
"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."
The researchers used computer simulations to compare the expected effect of the Sandy storm surge on a dune front with and without reinforcement from the buried rock structure, concluding: "Examining the Bay Head simulations, it is clear that behind the seawall, the wave force potential is reduced by greater than a factor of two — if the seawall did not exist, wave-averaged forces on these oceanfront homes would have been twice as large as experienced. Similarly, erosive flow velocities over the dunes in Bay Head were reduced, with respect to those in Mantoloking. Additionally, as pre-storm beach profiles and hydrodynamic conditions during the storm were very similar from Mantoloking to Bay Head, had the seawall not existed, forces on the oceanfront homes would have been equally similar, and the damage observed in Mantoloking would have likely been found in Bay Head as well."
Irish and her colleagues conclude that, going forward, engineers should be thinking about hybrid shore-protection structures composed of hard seawalls, buried in soft sand. "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."
Irish's original paper in the Elsevier scientific journal Coastal Engineering, "Buried relic seawall mitigates Hurricane Sandy's impacts," by Jennifer L. Irish, Patrick J. Lynett, Robert Weiss, Stephanie M. Smallegan, Wei Cheng, can be downloaded in PDF format for $34.95.