Cities 101: How Does Harbor Dredging Work in NYC?

In 1850, the depth of New York Harbor was between 10 and 20 feet. Today, the harbor is more than 50 feet deep. The change in the depth of one of the world’s largest natural harbors is the result of year-round harbor dredging that not only prevents the buildup of mud and sand at the bottom of the harbor, but actively and continually deepens the harbor.

According to Kate Ascher in The Works, maintenance of New York harbor began in the mid-nineteenth century. The ships of New York’s original Dutch settlers and British consquerers might have navigated through the harbor without problems, but ships were getting bigger. Clipper ships became steamships, which required deeper channels for safer navigation. The entrance to New York Harbor at Ambrose Channel was first taken down to a depth of 30 feet in 1884. This successful dredging initiated a program of regular deepening at the channel, and at other heavy-trafficked shipping channels throughout the harbor.

Ships have only grown larger since then, and harbor dredging has only intensified to accommodate them. New York Harbor’s dredging process involves up to eighty pieces of dredging machinery, making for the largest concentration of dredging equipment ever assembled in America. New York also houses the largest barge-mounted dredge in the world, which can drill down to 65 feet, has a bucket the approximate size of a garbage truck, and is appropriately named “T-Rex.”

Soft or loose material on the harbor floor is removed by a clamshell bucket attached to a barge-mounted crane, which places the sediment on an scow that is carried to a final disposal site. The removal of rock is entirely different–and much more exciting and destructive. Drill boats are used to make holes in rock, 6 feet deeper than the intended rock removal depth and approximately 10 feet apart. The holes are then filled with a liquid explosive called Porvex. The rocks loosened from the resulting blast are collected by an excavator dredge. Survey boats are brought in to certify the new depth of the harbor or waterway.

Where does this sediment go? For years, it was dumped into the Atlantic. Later, to minimize damage to navigation and sediments washing up on beaches, the U.S. Army Corps of Engineers designated the beaches off of Sandy Hook, New Jersey as a “Mud Dump.” Today, before haphazardly dumping on the New Jersey coastline, dredged materials are tested for contaminants to determine what its use will be elsewhere. Clean sediment is utilized for shoreline stabilization and fishing reef creation, or used to cap contaminated underwater dump sites.

The Army Corps of Engineers’ Harbor Deepening Program, which started in the mid-1990s and will end in 2014, is estimated to dredge a volume of sediment equalling thirty Empire State Buildings. Its goal is to give East Coast ports the same capacity for holding large ships as deeper West Coast ports have had. When the project concludes, the amount of sand dredged in the Ambrose Channel will drop from over 3 million cubic yards to just 423,000 cubic yards per year. But there is always more to dredge–the city’s next project may be the channels of Staten Island.

Read more from our Cities 101 series about how stuff works in the city. Additional reporting by Wesley Yiin.