In 2015, New Jersey pumped 30,000 cubic yards of dredged mud into a breach splitting Mordecai Island; the project rejoined the island and rebuilt nesting habitat

For a hundred years, wave by wave, Mordecai Island was slowly splitting in two. The barrier island off New Jersey's Barnegat Bay had been eroding bit by bit for decades. But then, in 2015, the engineers tried something simple. They placed dredged sediment back onto the island. As per the 2020 NOAA Technical Memorandum "Ecological Assessment of the Beneficial Use of Dredged Sediments for Island Restoration: Mordecai Island, Barnegat Bay, NJ," which observed the island over the following three years, the solution worked, although the same study pointed out an issue that might arise in the future.

A century of erosion split the island in two

Mordecai Island sits parallel to the shore of Beach Haven, which is a small beachside community located in Ocean County. For many years, waves generated inside the Barnegat Bay eroded it. Over a period of one hundred years, the erosion resulted in the loss of half the land of the island. Ultimately, it divided the island into two separate lobes with open water in between. That loss mattered for more than the map. The island serves as habitat for breeding birds and small estuarine animals. It also acts as a natural barrier, absorbing wave energy from the bay before it reaches the developed shoreline.


Engineers patched the gap with dredged mud

In 2015, the US Army Corps of Engineers' Philadelphia District, working under the Corps' Engineering With Nature initiative, came up with a practical fix. Instead of hauling in new material, the team used sediment dredged from the nearby New Jersey Intracoastal Waterway, mud that has to be cleared out periodically anyway to keep boat channels open. Crews pumped roughly 30,000 cubic yards of the material in such a way that a high elevation was created at the center where shorebirds could build their nests. The following year, crews planted salt marsh and upland species to help hold the sediment in place.

Three years of monitoring: what researchers found

To see whether the repair was actually working, the Philadelphia District partnered with NOAA's National Centers for Coastal Ocean Science and the Mordecai Land Trust. Between 2017 and 2019, the team monitored the island's surface elevation, shoreline position, and biological communities, as explained in the NOAA technical memorandum that was published in 2020. The findings showed the rejoined island managed to stay together during that period, which was an achievement by itself. The report, however, points out one challenge: sediments were moving around within the fill zone, and the researchers believe that without further materials and assistance from planting, another channel may eventually develop, thus separating the island once again.

There was reassuring news elsewhere in the report. Researchers did not find any long-term effects on the small bottom-dwelling creatures living in the mud or sand close to the island; these are organisms that scientists monitor each time dredged materials are used. The researchers also found that the island’s western shore, which is exposed to boat wakes, eroded at an average of 0.83 meters per year between 1970 and 2019. That amounted to about 15 acres of land lost over 49 years. On the southwestern side, though, engineers had installed shoreline stabilization structures, and the data show those structures have measurably reduced erosion on the shoreline next to them.

Why a small island's marsh matters so much

It is tempting to think of Mordecai Island as a minor patch job. It is not. Marsh islands like this one act as a buffer between open water and the actual coastline where people live. A widely cited wave-flume experiment published in Nature Geoscience found that marsh vegetation can reduce incoming wave energy by up to 60% even during simulated storm surge conditions when water levels are highest. Similar results turned up in a separate review of 75 scientific articles on the subject of coastal marshes, published in PLoS ONE. The marsh vegetation reduces erosion and decreases wave height. However, the researchers noted that the extent of this protective effect remains unknown at the moment. And so when an island like Mordecai erodes away, a coastal town isn’t just losing scenery. It’s shedding a natural buffer that would have otherwise absorbed some of the impact headed for its shoreline.

A model other coastal towns are borrowing

Mordecai Island has become somewhat of a demonstration project, an illustration of how dredged sediments could actually be used in order to rebuild habitats and simultaneously defend a shoreline rather than dumping them into the ocean or transporting them to other areas. Engineers now point to it as an example when planning similar sediment-reuse projects elsewhere along the Atlantic coast.

The story isn't finished yet. Continued monitoring will determine whether Mordecai needs another round of sediment before the gap reopens through natural erosion. Yet for a New Jersey shoreline that has been losing ground quietly for a century, three years of monitoring provide an important baseline.