Mantoloking Seawall
Project Overview
Opportunity
In October 2012, a storm surge from Hurricane Sandy cut through the narrow section of the Barnegat Peninsula where the Borough of Mantoloking is located.
Mott MacDonald was retained to develop an emergency plan to restore New Jersey Route 35, which was breached in three places. All of the Borough’s 521 homes experienced varying degrees of damage from the storm, and scores of them were destroyed.
Mantoloking’s 11,000 feet (3.3 kilometers) of oceanfront retreated approximately 40 feet (12 meters), with a loss of about 500,000 cubic yards (382,000 cubic meters) of sand. By the summer of 2014, only 200,000 cubic yards (153,000 cubic meters) had been recovered from the streets, private properties, and the bay, leaving homes and businesses vulnerable.
Public works crews used bulldozers to create and maintain artificial sand dunes as a temporary protective measure against storm damage until a more permanent and reliable protective measure could be put in place.
Solution
We were retained to provide assistance with design, financing, and construction observation for a seawall to be built from steel sheet pilings along two miles (3.2 kilometers) of oceanfront of Mantoloking and one and one half miles in neighboring Brick Township.
The scope of the project included analysis of founding soil conditions, cross-island aquifer water transport, environmental impacts, anticipated wave conditions, end effects and materials to determine the optimum design of the sheet pile wall system.
Design and implementation of the wall required close work with the New Jersey Department of Environmental Protection to meet environmental constraints, development of detailed benefit to cost analysis (BCA) to acquire FHA funding, and detailed alignments to integrate into the proposed US Army Corps of Engineers project and the future redevelopment goals of Mantoloking’s and Brick’s oceanfront residents.
Construction began in July 2014 at the foot of the Mantoloking Bridge. (See video.) Two crews, one working to the north and one to the south, used specialized pile drivers to embed 45-foot-long (14-meter) steel sheets manufactured from A690 marine steel into the beach.
Crews constructed about 150 feet (46 meters) of seawall per day. The toe of each sheet was driven to 30 feet (9 meters) below sea level. The top elevation of the wall at +15 NAVD88 matched FEMA’s 100 Year Storm level. Any steel sheeting potential exposed above ground would later be covered with sand.
The project was completed in January 2015, although the installation of the final 400 feet (122 meters) was delayed by the investigation of historical timbers that were discovered and partially exposed along the alignment. Although unconfirmed at this time, these were suspected of being part of a Scottish shipwreck dating back to the 1800s.
Following the wall's completion, the oceanfront to the east of the seawall was covered by dunes in the Army Corps of Engineers’ long-planned coastal storm protection project. The project replenished the oceanfront with 200-foot-wide (61-meter) beaches and dunes with a crest elevation of 22 feet (6.7 meters) above sea level. Beach replenishment was needed due to the long-term historical erosion pattern for the area, which had diminished the protective value of the natural beach and dune system.
Outcome
Speaking when construction began in July, local resident Valerie McDowell said, “I’m thrilled. I’m so excited because we’re so totally exposed when the water breaches. There’s nothing protecting us.”
New Jersey Department of Environmental Protection Commissioner Bob Martin said at his on-site press conference on July 10, 2014, “This project will help protect a segment of coastline that was breached during Superstorm Sandy, especially offering protection to Route 35 and residents and businesses that were battered by Sandy. Coupled with the forthcoming coastal protection project by US Army Corps of Engineers, this particularly vulnerable section of the Jersey Shore will be more resilient for future severe weather events.”
As the project neared completion, Mayor John Ducey of Brick said, “Eight thousand of our homes not only here on the barrier island but on the mainland were flooded, and this wall will protect that from happening again.”
Robert Mainberger, PE, Mott MacDonald's project manager, said the marine-grade steel used in the wall would have a minimum lifespan of 75 years and could withstand another storm of the magnitude of Sandy. “Typically, we expect that if it stays buried in the sand, it should last for 100 to 150 years, and if it is exposed to the surf we’re looking at a lifespan of 50 years.”
In October 2015, the wall protected Mantoloking and Brick from heavy surf. According to Mantoloking police chief Stacy Ferris, “The wall did exactly what it was designed to do. Without that wall, we would have had breaches the length of the town.”
This particularly vulnerable section of the Jersey Shore will be more resilient for future severe weather events.local resident