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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. To
identify implementation projects for this grant, the Authority conducted
assessments of streams throughout the Spruce Run Reservoir watershed.
Three stream restoration projects were conducted as part of the
grant:
·
Crystal Springs, Spruce Run (see Fall 2006 newsletter for more
details);
·
Old Farm Road, Mulhockaway Creek; and
·
Hoffman Park, Mulhockaway Creek.
| The Hoffman
Park Stream Restoration Project is located on a branch of the
Mulhockaway at the eastern end of the park, off Mechlins Corner
Road. Hunterdon
County owns and manages the park.
Prior to implementation of the Authority’s project,
this reach of Mulhockaway Creek followed a relatively
straight course through forested upland and wetland areas. A
deteriorated and undersized culvert was present under the access
road. An accumulation
of sediment had occurred on the upstream side of the culvert.
Streambank erosion was evident upstream of the culvert and scour
on the downstream side of the culvert resulted in an approximately
five-foot drop in elevation from above the culvert to below.
The stream channel had incised approximately five feet
below the top of the bank downstream of the culvert, leading to
banks that were approximately six feet in height, eroding and
unstable. |
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Downstream face of
culvert prior to construction |
The Authority hired
the Louis Berger Group, Inc. to design a stream restoration project that
would replace the culvert, better connect the stream with its natural
flood plain and riparian wetlands and reduce sediment movement to Spruce
Run Reservoir. We worked with a team of technical experts, including
Hunterdon County Parks, the Delaware River Basin Commission and the
Natural Resources Conservation Service to select a design that would best
accomplish our goals. The
project is located within the Highlands Preservation Area;
we received the first Highlands permit issued for such a project in
June 2006.
The project design
included the following:
·
Adjust stream pattern and profile:
The stream sinuosity, or pattern, was adjusted to establish a
stream slope that can transport the sediment load without degrading or
aggrading the channel.
·
Adjust stream dimension:
A bankfull bench was established along the stream channel to
provide a place for energy dissipation of water and sediment during high
flow events.
·
Install instream structures to stabilize the stream:
Several types of in-stream structures – log vanes, cross vanes
and root wads - were
installed to stabilize the stream bed, to reduce streambank erosion and
reduce the energy of water during high flow events.
·
Replace the culvert system:
The culvert was replaced with a concrete bridge that spans the
stream to provide fish passage and improve flow and sediment transport.
·
Plant native vegetation:
An herbaceous seed mix and annual cover crop were planted at the
end of construction in August and September.
More than 400 trees and shrubs and approximately 1,400 willow and
dogwood stakes were planted
in November.
One interesting
condition in our Highlands permit was the requirement to have a qualified
herpetologist monitor the site during active construction.
What’s a herpetologist? A
biologist who specializes in reptiles and amphibians - frogs, toads,
turtles salamanders, to name a few. At
Hoffman Park, the species of concern were bog turtle (endangered) and wood turtle (threatened).
There is habitat for both species near the project, and both have
been sighted in other areas of the park.
Hunterdon County Parks
provided qualified herpetologists who did 2 “turtle trainings” for
Authority staff and staff from Vollers and Berger.
In addition, County Parks staff were on-site every day during
active construction. While we didn’t find any turtles during construction,
we did move many, many frogs and salamanders out of the
construction zone – most of
them more than once!
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Construction
began at the end of June 2006.
Vollers Excavating & Construction of North Branch, NJ
served as the project contractor and Berger provided full-time
construction management.
In order to complete the work, the stream was diverted around the
project reach, through an adjacent borrow pit and back into the
channel at the downstream end of the project reach. This enabled Vollers to work in dry conditions as they carved
the new channel and bankfull bench and installed the structures.
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Construction
Mobilization, June 2006 |
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On September
14th, the big day arrived – time to re-divert the
stream back into its new channel.
As the berm between the new channel and the diversion
channel was removed, the “stream paparazzi” followed the
movement of water through the new channel, over and around the
in-stream structures, through the new bridge and to the end of the
project reach.
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New
arch concrete bridge, September 2006 |
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The project
area experienced several significant storms following completion
of construction – one on the day after we re-diverted the
stream. These storms
gave us an indication of just how dynamic the Mulhockaway stream
system is and reminded us that the computer models that help us
design such projects can’t predict everything.
Stream systems are not static – we expect them to change
over time, but our project reach experienced many changes very
quickly, leading to the need for adaptive management.
Authority staff have been working with Berger to design
modifications to three of the structures that were installed.
The Authority’s Grounds Maintenance staff installed the modifications
in December 2006.
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Installation
of cross vane |
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The Authority
is monitoring the success of the project in several ways.
First, we’ll be continuing our macroinvertebrate
monitoring at Hoffman Park and our other two restoration sites.
Macroinvertebrates are the little organisms that live in
the substrate of the stream – worms, mayflies, stoneflies and
others. We have several sets of pre-construction samples to use as
comparison. This will
enable us to see how quickly the stream is recolonized by
macroinvertebrates and if the project affected the diversity of
species present. We’ll
also be monitoring the success of the vegetation that was planted.
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Visitors
to Hoffman Park Site |
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The third type of
monitoring is of what are called geomorphological characteristics.
By surveying the locations of the stream bed, the banks, the
meanders of the stream, we can tell how the stream is moving within
its new pattern and determine if it is stable.
We expect the stream to move within a certain range of these
characteristics.
Watershed staff spent much of the
summer at the project site, observing the construction, monitoring
for turtles and giving tours to a variety of guests.
These included representatives of the D&R Canal
Commission, NJDEP Commissioner Lisa Jackson and several of our board
members. Many Authority
staff visited the project throughout construction.
If you haven’t been out to the project yet, or would like
to see how the stream is progressing, please contact Kathy Hale in
the Watershed Unit (908) 685-0315, x28 or khale@raritanbasin.org
for a tour. |
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Completed
cross vane |
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Installation
of log vane |
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Completed
Project, October 2006 |
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