An innovative $4 million restoration project along the Harlem River created a cost-effective, durable and ecologically friendly shoreline that serves as a model for future projects.
Along the Harlem River between 139th and 145th Streets in Manhattan, the steel sheet-pile wall securing the riverbank suffered extensive corrosion and developed holes. Watering entering the holes at high tide and exiting at low tide removed soil from behind the walls, creating large sinkholes that swallowed pavements and plantings along the neglected riverfront.
The New York City Department of Parks & Recreation enlisted Dewberry, New York, to design a new 2,000-ft. long riverfront structure that supports estuarine life, reduces wave energy, provides greenery and allows safe access.
After considering several alternatives Dewberry selected gabions for the restoration project. Gabions are wire mesh baskets filled with stones that are tied together to create a retaining wall.
Gabions were found to be cost-effective, durable and feasible to install. The porous and flexible structures provide habitat for marine creatures and encourage the growth of salt marsh grasses.
Waves coming into the shoreline vanish when they hit the gabions, explains Sufian Khondker, Dewberry’s head of water resource engineering. “The uniqueness of the design is that it absorbs the wave energy and does not reflect it back causing turbulence along the wall.”
In tidal rivers, where the gabions are exposed to alternate wetting and drying, the PVC is vulnerable to ultraviolet radiation and more likely to crack, exposing the steel wire to the salt water. The 316L is more suitable in saline environments and strong enough to sustain boat impacts.
Extremely soft mud on the river bottom complicated foundation construction for the gabion wall. Dewberry recommended removing as much mud as possible and then using large stones encapsulated with filter fabric to create a flexible foundation accommodating differential settling. A concrete leveling mat atop the stone provided a level surface for the gabions.
Dewatering was required to keep the construction area dry. Initially holes in the sheet-pile walls were patched to allow excavation behind the wall. But the dewatering system could not keep up with the large volumes of water entering through the patched walls and sheet-pile joints.
Instead large stones were placed underwater with tremie concrete filling the voids. Once the stones settled, the dewatering system was able to lower the water level to the top of the stones permitting dry construction of the leveling mat and gabions.
Along the shoreline two sections of the gabion wall were lowered to create tidal pools that fill with water during high-tide. Large stones surrounding the pools allow people to sit by the water’s edge.
An existing cove at the south end of the gabion wall was blocked by large chunks of concrete and junk. Redesign of the cove removed the concrete blocks above the shoreline, flattened the slope and placed large, flat, natural stones above the high water line to create a seating area.
Developer/Owner: New York City Department of Parks & Recreation
General Contractor: Phoenix Marine,Sayreville, N.J.
Construction Manager: URS Corporation,San Francisco, Calif.
Designer: Dewberry, Bloomfi eld, N.J.*
*Submitted Project to New York Construction