New Jersey Water Supply Authority

Hoffman Park Stream Restoration Project

Union Township, Hunterdon County, New Jersey

In 2003, the New Jersey Water Supply Authority (NJWSA) received a Targeted Watersheds Grant from the United States Environmental Protection Agency (EPA) to protect surface water quality in the Raritan River Basin. The Raritan Basin Watershed Management Plan (NJWSA, 2002) identified the restoration of streams and riparian areas that have been physically damaged by harmful land uses and the protection of high-quality streams and riparian areas as implementation priorities.

As part of the EPA-TW grant, NJWSA identified three riparian restoration projects:

1.      Old Farm Road Riparian Buffer Improvement (click here for a factsheet on the project);

2.      Crystal Springs Stream Restoration Project (click here); and

3.      Mulhockaway Creek/Hoffman Park Stream Restoration Project

The Hoffman Park Project is located on a branch of the Mulhockaway Creek at the eastern end of Hoffman Park in Union Township, Hunterdon County.  The Mulhockaway is one of the two main streams that feed Spruce Run Reservoir.  The 353-acre park is owned by Hunterdon County and Union Township and managed by the County. 

Prior to the restoration project, this reach of Mulhockaway Creek followed a relatively straight course through forested upland and wetland areas.  The stream was conveyed under a dirt access road via a deteriorating and undersized piped culvert system consisting of four pipes, at least two of which are completely blocked.  An accumulation of sediment had occurred on the upstream side of the culvert as a result of the pipe impedance.  Bank erosion was evident upstream of the culvert system, likely resulting from the hydraulic conditions created by the structurally deficient culvert system.  Scour on the downstream side of the culvert system resulted in an approximately five-foot drop in elevation and the development of a large scour pool.  The channel bed had incised approximately five feet below the historical floodplain downstream of the culvert system, and the banks within this reach were actively eroding and exceptionally unstable.  Overland flow from adjacent wetland areas into the stream severely eroded the bank in concentrated areas, resulting in headcuts. 

NJWSA engaged The Louis Berger Group to design a stream restoration project that would replace the crossing, restore stream sinuosity, better connect the stream with its natural flood plain and riparian wetlands and reduce sediment movement to Spruce Run Reservoir.   The design team worked with NJWSA and Hunterdon County Parks to develop a design that met the project goals and park needs.  NJDEP-Land Use Regulation provided valuable input from the regulatory perspective. 

The project is located within the Highlands Preservation Area; a Highlands Preservation Area Approval Permit Application was submitted in January 2006, with NJDEP-Division of Watershed Management as a co-applicant.  In June 2006, we received the first Highlands Preservation Area Permit approval to be issued in the state of New Jersey.  Construction of the project began in late June 2006, with Vollers Excavating & Construction serving as the contractor.

The final stream restoration design included the following:

·            Adjust stream pattern and profile:  The stream sinuosity, or pattern, was slightly adjusted to establish an appropriate channel slope that will effectively transport the sediment load without degrading or aggrading the channel.  The stream bed elevation was slightly increased to bring the stream closer to the historical floodplain, to partially alleviate the substantial grade difference between the top of bank and toe of bank, and to bring the stream closer to the water table of the adjacent wetlands.

·            Adjust stream dimension:  A bankfull bench was established contiguous to the stream to provide energy dissipation during high flow events, to provide area for riparian habitat establishment and to provide floodplain storage of sediment.

·            Installing instream structures to stabilize the stream:  Cross vanes were installed within the project reach to stabilize the channel bed and log vanes were used to reduce near bank shear stress and reduce bank erosion.

·            Manage actively migrating headcuts:  The headcuts were treated by grading the vertical, eroded banks to a gentler slope and further stabilizing the banks by seeding and planting woody vegetation.  In addition, rock was buried at the headcut locations to prevent them from continuing to migrate.

·            Replacing the existing deteriorated culvert system with a bridge to provide fish passage and improve flow and sediment conveyance:  The existing hydraulically inadequate culvert system was replaced with a concrete bridge that spans the stream and bankfull bench, allowing unobstructed flow during flood events.

Project mobilization began June 24, 2006.  We were working within a very narrow construction window due to the classification of the Mulhockaway as a C1-TP stream.  Construction had to be completed by September 15^th.  Most work was completed by late August, at which time significant precipitation events delayed installation of the new bridge until September 13^th.  The stream was diverted into the new channel on September 14^th, meeting the permit deadline.

NJWSA will be monitoring the site for the next several years, including vegetation surveys, macroinvertebrate sampling and geomorphological (longitudinal profile and cross-sections) surveys. 

Bridge Installation Day Pictures

Stream Opening Pictures