July 17, 2013
Buried Seawall Helped Protect New Jersey From Hurricane Sandy
April Flowers for redOrbit.com - Your Universe Online
Bay Head and Mantoloking are residential beach communities that sit side-by-side on a narrow barrier island that separates the Atlantic Ocean and Barnegat Bay in Ocean County, New Jersey.
Someone traveling between Mantoloking into Bay Head before Hurricane Sandy landed on October 29, 2012, would have noticed few changes in residential development, dunes, beaches and shoreline between the two towns. The important difference was hidden beneath the sand.
Jennifer L. Irish, an associate professor of civil and environmental engineering at Virginia Tech, led a team of engineers and geoscientists who investigated the forgotten, 4,133-foot seawall buried beneath the beach. The seawall helped Bay Head to weather Sandy's record storm surges and large waves over multiple high tides, according to Irish -- an authority on storm surge, tsunami inundation and erosion.
The reappearance of the seawall, built in 1882, surprised many area residents and underscored the difficulties transient communities have in planning for future threats at their shores.
"It's amazing that a seawall built nearly 150 years ago, naturally hidden under beach sands, and forgotten, should have a major positive effect under the conditions in which it was originally designed to perform," said H. Richard Lane, program director in the National Science Foundation's (NSF) Division of Earth Sciences, which funded the research. "This finding should have major implications for planning, as sea level rises and storms increase in intensity in response to global warming."
The discovery of the seawall illustrates the need for multi-levels of beach protection in oceanfront communities, according to the study published online in the journal Coastal Engineering.
"Once we got there, we immediately saw the seawall," Irish said. "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."
Irish worked with Robert Weiss, assistant professor of geosciences in the College of Science at Virginia Tech, and Patrick Lynett, an associate professor of civil and environmental engineering at the University of Southern California. The team documented high water marks, damage, overwash and breaches of the barrier island.
Ranging from ground-floor flooding to complete destruction, all the oceanfront homes in both boroughs were damaged. Flooding, according to water lines on the interior of homes, was similar in both locations. The difference, according to the researchers, was the extent of the storm's impact.
The entire dune almost vanished in Mantoloking when water washed over the barrier spit and opened three breaches of 541 feet, 193 feet, and 114 feet where the land was swept away. In contrast, only the portion of the dune located seaward of the seawall was eroded in Bay Head, and the section of dune behind the seawall received only minor local scouring.
The research team used Google Earth to evaluate aerial images taken two years before and immediately after Hurricane Sandy. The team looked at houses, labeling a structure with a different roof-line as damaged, one that no longer sits on its foundation as destroyed, and the remaining houses as flooded.
Only one oceanfront home in Bay Head was destroyed, while the researchers classified 88 percent of the homes as being flooded. By contrast, in Mantoloking more than half the oceanfront homes were classified as damaged or destroyed.
Hurricane Sandy had an immense magnitude and long duration. Despite that, a relatively small coastal obstacle reduced the potential wave loads by a factor of two and was the difference between widespread destruction and minor structural impacts, the study found.
"We have a great deal of compassion for the people who have had to endure the devastation of Hurricane Sandy in Bay Head and Mantoloking," Irish said. "It will have little solace, but we are left with a clear, unintentional example of the need for multiple levels of defense that include hard structures and beach nourishment to protect coastal communities."